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fall 1997 logo1997 MRS Fall Meeting & Exhibit

December 1 - 5, 1997 | Boston
Meeting Chairs:
 Harry A. Atwater, Peter F. Green, Dean W. Face, A. Lindsay Greer 

Symposium D—Nitride Semiconductors



Steven DenBaars,Univ of California-S Barbara
Bruno Meyer, Justus-Leibig-Univ-Giessen
Shuji Nakamura, Nichia Chemical Industries Ltd
Fernando Ponce, Xerox Palo Alto Research Center
Toby Strite, IBM Zurich Research Laboratory 

Symposium Support 

  • AIXTRON Semiconductor Technologies GmbH
  • Bede Scientific Inc.
  • EMCORE Corporation
  • EPI Chorus
  • Hewlett Packard Company
  • MMR Technologies Inc.
  • Morton International, Inc.
  • Nichia Chemical Industries, Ltd.
  • Oxford Instruments America, Inc.
  • Renishaw PLC
  • Rockwell International
  • Siemens AG
  • SULA Technologies
  • SVT Associates Inc.
  • Thomas Swan & Co. Ltd.
  • Toyoda Gosei Co., Ltd.

1997 Fall Exhibitor

Proceedings published as Volume 482 
of the Materials Research Society 
Symposium Proceedings Series.

* Invited paper

Chair: Fernando A. Ponce 
Monday Morning, December 1, 1997 
Salon F (M)

8:30 AM *D1.1 
THE EVOLUTION OF THE NITRIDE SEMICONDUCTORS. I. Akasaki, Dept. of Electrical and Electronic Engineering, Meijo University, Nagoya, JAPAN.

The current state of the group III nitrides is the result of persistent pioneering research over three decades. From the first demonstration of a single crydtalline GaN by hydride vapor phase epitaxy by Maruska and Titjen to the current commercially available high efficiency devices, the trajectory of the nitrides has been marked by significant contributions. One of the foremost pioneers was Prof. Jacques I. Pankove, who developed the first GaN LED while working at RCA Laboratories in 1971, and to whom this years MRS Fall Meeting Symposium on Nitride Semiconductors is dedicated Prof. Pankove made singular and significant contributions during the 1970's, far ahead of his time. He carried the flag when no one thought much of the nitrides as can be verified by his numerous publications in the field, exceeding 30 in number between 1970 and 1977. In this talk, the evolution of the group III nitrides will be briefly sketched with emphasis on the key developments that led to the current successes, namely: (a) the achievement of high-quality nitrides with specular surfaces free from cracks and pits, (b) control of impurities for effective doping, (c) p-type activation of Mg-doped GaN, and (d) development of LED, LD and transistor technologies. A summary of the main scientific and technological challenges will also be presented.

9:00 AM *D1.2 
MOCVD GROWTH AND CHARACTERIZATION OF HIGH Al CONTENT AlGaN. Oliver Briot, Sandra Ruffenach-Clur, Bernard Gil and Roger Aulombard, Universite Montpellier II, Montpellier, FRANCE.

AlGaN alloys are important materials for the design of nitride devices, since their bandgap can be tuned between 3.4 eV (GaN) and 6.2 eV (AlN). They can be used for confinement purposes (quantum structures and optical confinement), for light filters and Bragg reflectors, in surface acoustic wave devices and UV photodetectors. In this paper, the applications, MOCVD growth and characterization of AlGaN alloys will be reviewed. The MOCVD growth of the alloy on the whole range of composition will be detailled. In particular we discuss the problems of premature reactions between ammonia and the aluminum precursors, the mechanism of incorporation of aluminum and its dependency versus temperature, pressure and flow rates. The crystalline quality of AlGaN will be analyzed through TEM and X ray diffraction results, and Raman and optical characterization of the layers will be reported.

9:30 AM D1.3 
MOCVD GROWTH OF GAN ON BULK ALN SUBSTRATES. Hongqiang Lu, Ishwara Bhat, Byung-Chan Lee, Glen Slack, and Leo Schowalter, Rensselaer Polytechnic Institute, Troy, NY.

The commonly used substrates for GaN and AlGaN epitaxy have been either sapphire or SiC. Sapphire is not lattice matched to nitrides, but is chemically and thermally compatible. SiC (eg: c-plane 6H SiC) has a better lattice match with nitrides parallel to the c-plane, but the presence of different poly-types of SiC gives rise to a large lattice mismatched condition in the a-plane direction. AlN, on the other hand, can be grown in 2H-polytypes and hence closely lattice matched to GaN in all directions. In this paper, we report on the growth of single crystal epitaxial layers of GaN on c-plane and a-plane bulk AlN substrates by metalorganic vapor phase epitaxy. The AlN boules were grown by the sublimation-condensation techniques. Two types of wafers were cut from this boule, one being the c-plane substrate of 3mm x 3mm in size and other being a a-plane substrate of 4mm x 4mm in size. Growths were carried out by first growing a buffer layer of AlN at 1100C, followed by a 1.5m thick GaN at 1050C. As grown epilayer on c-plane AlN was smooth, replicating the substrate morphology. However, the epilayer had a high density of cross-hatch defect lines, presumably caused by the mechanical polishing damage. Photoluminescence and x-ray diffraction measurements were carried out on these layers to determine the film quality. PL measurements show that the layer grown with 1000A thick AlN buffer layer had a band edge PL FWHM of 13 meV at 16K, and the yellow luminescence peak was very weak (about 100 times weaker) compared to the BE peak. X-ray measurements show that the epilayer is single crystal. Epitaxial growth on a-plane AlN substrate is being carried out and the detail results on these will be presented at the conference.

9:45 AM D1.4 
SIMULATIONS OF AlGaN AND InGaN GROWTH FOR MOCVD REACTOR DESIGN AND OPTIMIZATION. T.G. Mihopoulos, V. Gupta, H. Simka, K.F. Jensen, Massachusetts Institute of Technology, Dept. of Chemical Engineering, Cambridge, MA.

Organometallic Chemical Vapor Deposition (OMCVD) of AlGaInN involves complex chemistry and transport phenomena which determine the quality of the deposited layers. Large temperature gradients create complex mixed convection flow fields that influence the precursor distribution and availability through thermal diffusion. We have undertaken a modelling approach that helps identify the relative importance of different transport and reaction processes to the overall growth performance in several reactor configurations. Interactions of precursors leading to Lewis acid-base adducts, (TMGa:NH3, TMAl:NH3) and subsequent elimination of CH4 were investigated using the hybrid density functional theory (B3LYP) method. This pathway leads to the formation of stable dimer and trimer ring species containing Ga, and/or Al and N which affect growth behavior in the reactor. Thermodynamic and kinetic information for these processes are determined and used in macroscopic reactor modelling studies. A kinetic mechanism for InGaN growth that builds upon reported data for gas-phase decomposition of TMGa and TMIn is also presented. Very good agreement with several experimental results for GaN, AlGaN and InGaN growth rates is demonstrated over a wide range of temperature and pressure in several reactor geometries. Different temperature and flow distributions lead to varying degrees of parasitic reactions in the reactors studied.

Chair: Richard J. Molnar 
Monday Morning, December 1, 1997 
Salon F (M)

10:30 AM *D2.1 
HIGH INDIUM CONTENT INGAN FILMS AND QUANTUM WELLS. Wim Van der Stricht, Ingrid Moerman, Piet Demeester, Univ of Gent-IMEC, Dept of Information Technology, Gent, BELGIUM; Laurence Considine, Edward J. Thrush, John A. Crawley, Thomas Swan & Co. Ltd., Harston, Cambridge, UNITED KINGDOM.

Since the demonstration of a CW room temperature operated nitride based laser diode by Nichia Chemicals the ternary alloy InGaN has received an increased attention. In light emitting diodes and laser diodes InGaN is used as an active layer because it exhibits the possibly of controlling the emission wavelength from the visible to the near UV. However, the exact underlying physics of these devices (for example then role of InGaN quantum dots in the emission mechanism) are not fully understood yet. Similarly the OMVPE growth of InGaN needs to be explored more thoroughly to be able to improve device efficiencies. The main problem with InGaN growth is that low growth temperatures are required to achieve a high indium incorporation, because of the high indium volatility at temperatures above 1000 ƒC. Lowering the growth temperature however usually results in an inferior material quality, because at temperatures below 1000 ƒC the decomposition rate of NH3 is very low. InGaN films and InGaN/GaN quantum wells with high indium content have been grown by MOVPE and characterised to evaluate the growth process and the indium incorporation efficiency. The characterisation techniques include photoluminescence, DC X-ray and TEM. The closed spaced vertical rotating disk reactor configuration results in a very high growth efficiency for InGaN material, compared to other configurations. InGaN layers with an indium composition up to 56 % have been deposited which still exhibit very good optical properties (intense PL emission). The influence of various growth conditions on the InGaN composition and quality have been investigated to optimize the layer quality. The influence of growth temperature, V/III ratio, growth rate and rotation rate on the indium incorporation efficiency has been examined.

11:00 AM D2.2 
PROCESS PRESSURE EFFECTS ON GAN MATERIAL QUALITY BY LOW PRESSURE MOCVD. B.T. McDermott, R. Pittman, E.R. Gertner, Rockwell Science Center, Thousand Oaks, CA; J. Krueger, C. Kisielowski, Z. Lilienthal-Weber, and E. Weber, University of California, Berkeley, CA.

Low pressure MOCVD growth of GaN was studied over the pressure range of 50 to 200 torr. Hall effect, x-ray, photoluminescence, and TEM were used to characterize the material. The electron mobility was used to optimize the material growth. Initial GaN nucleation layer studies were performed at 50 torr, and resulted in material with dislocation densities as low as 3-5 x108 cm -2. For constant GaN nucleation conditions on sapphire, increasing the growth pressure for the high temperature GaN strongly influenced the mobility. Experiments were performed that show this mobility enhancement is more an effect on the nucleation layer, as opposed to a bulk effect. Best results were obtained at 150 torr, as a compromise between enhanced mobility and uniformity of electrical properties. Final optimization of the GaN growth sequence resulted in electron mobilities in excess of 500cm2/Vs for an intentially Si-doped (5x1017cm-3) 5 um GaN film. To our knowledge, this is one of the best mobilities attained in a low-pressure, high speed rotating disk reactor. Other growth properties to be discussed are nucleation layer thickness and V/III ratio influences on the mobility.

11:15 AM D2.3 

We developed horizontal atmospheric MOCVD system with three layered laminar flow gas injections for the growth of III-nitrides. Using this newly developed MOCVD system, we have made a parametric study on p type GaN film. Hall measurement, double crystal x-ray diffraction (DCXRD) rocking curve measurement and photoluminescence (PL) measurement were performed to characterize Mg-doped GaN film. Secondary ion mass spectrometry (SIMS) measurement was performed to evaluate Mg and residual impurity concentration. Mg concentration increases linearly with increasing bis-cyclopentadienyl magnesium (CP2Mg) flow rate from 0.05 to 0.5 mol/min and saturates at about 2X1020/cm3 at higher Cp2Mg flow rate. Room temperature PL of the Mg-doped GaN film whose Mg concentration was 4X1019 atoms/cm3, shows very strong and broad single peak at 390nm. The film showed p-type conductivity by post-growth thermal annealing at 800C for 20 min in N2 ambient and the Hall carrier density was 8X1017/cm3. After the post-growth thermal annealing the 390nm PL emission intensity decreased about two order of magnitude. Hall carrier density of the Mg-doped GaN films grown at the temperature ranging from 1000-1075C showed maximum at 1025-1050C, while Hall mobility increased linearly from 6cm2/Vs to 10cm2/Vs with increasing growth temperature. SIMS depth profile of the p-type GaN film (Hall carrier density; 3X1017/cm3) shows uniform Mg doping distribution with abrupt Mg concentration change at the interface of the Mg-doped layer and the non doped GaN under layer. Increase of oxygen concentration associated with Mg doping was observed with increasing Mg concentration. Details about other residual impurities such as carbon and silicon will be presented in the paper.

11:30 AM D2.4 
INFLUENCE OF THE GaN GROWTH PARAMETERS ON FREE CARRIER CONCENTRATION AND TOTAL COMPENSATION. A.E. Wickenden, D.D. Koleske, R.L. Henry, W.J. DeSisto, and R.J. Gorman, Naval Research Laboratory, Washington, DC.

Gallium nitride films grown without intentionally introduced dopants can have resultant transport properties which range from n-type to highly resistive, with varying degrees of overall compensation. In addition, the compensation ratio of the film directly impacts carrier mobility in intentionally doped GaN. The transport characteristics of the films, typically measured by the van der Pauw Hall technique, are directly related to the GaN growth process conditions. Here, the effects of process parameters such as nucleation layer thickness, growth temperature, and nitridation schedule on the electrical properties will be presented. For each growth parameter studied the resulting the free carrier concentration and mobility were used to estimate the electrical compensation as a function of the growth parameter. From this analysis the growth parameters fall into three general catagories: 1) parameters which show a large increase in compensation as the number of free carriers decreases, 2) parameters which show a moderate change in both compensation and number of free carriers, and 3) parameters which show no change in compensation but a large change in the number of free carriers. Data for the analysis has been obtained from our MOVPE grown films as well as values reported in the open literature. The possible factors limiting the achievement of high mobility and low background carrier concentration will be discussed as well as the growth parameters which contribute to highly resistive GaN.

11:45 AM D2.5 

Despite its large lattice mismatch with GaN and AlN, sapphire has been successfully used as a substrate for heteroepitaxial growth of nitride films. However, the microscopic details of the interface composition and thin film structure are still largely uncertain. We have performed total energy and force calculations for thin films of AlN on Al2O3(0001) with different interface structures and different surface stoichiometries. 
The equilibrium structure of the bare sapphire (0001) surface is terminated by a ML of Al atoms, independently of the Al chemical potential. The terminating Al atoms are relaxed inwards with respect to the bulk configuration. A stoichiometric AlN bilayer on such a surface turns out to be unstable and very weakly bound to the substrate, independently of the Al chemical potential and of the polarity of the overlayer. Other interface structures can be more conducive to the adhesion of the film to the substrate. In particular, the lowest energy structure we have found is an AlN film with Al termination and ML of Al vacancies, on a sapphire surface terminated by ML of Al atoms. In this structure, the film is bonded to the substrate in a continuation of the oxide geometry. 
Based on our still preliminary calculations for the energies of thin films of AlN on Al2O3(0001), and on the established results for thin films of AlN on SiC(0001)[1] and on SiC(100)[2], we expect that two-dimensional growth of AlN is far easier to achieve on SiC. This behavior is not only a consequence of the huge lattice mismatch between AlN and sapphire, but also arises from the topological constraints imposed at the interface between the wurtzite structure of AlN and the corundum structure of Al2O3.

Chair: Kathy Doverspike 
Monday Afternoon, December 1, 1997 
Salon F (M)

1:30 PM *D3.1 
DEFECT GENERATION AND MORPHOLOGICAL EVOLUTION IN GaN HETEROEPITAXY. J.S. Speck*, X.H. Wu*, P. Fini*, E.J. Tarsa*, B. Heying*, S. Keller, U.K. Mishra, and S. P. DenBaars*, *Materials Dept. and ECE Dept., Univ. of California, Santa Barbara, CA.

Experimental results and models are presented for the mechanisms of threading dislocation (TD) generation and reduction in GaN growth on sapphire. The optimal nucleation layers (NLs) for achieving low TD density material (with high electron mobility, >700 cm2/Vs at 298 K, and strong PL) consist of mixed cubic/hexagonal GaN, which sustains its stacking disorder and island-like morphology after heating to HT. The HT growth proceeds by the formation of coarse islands which then coalesce. The HT island coalescence is accompanied by rapid surface smoothing. Isolated HT islands generally contain a small number of mixed or pure screw character TDs, whereas the majority of the edge TDs are generated at island coalescence. Under optimal conditions, a total TD density of 5x108 cm-2 can be achieved. Using identical NLs controlling the HT growth conditions can markedly change the HT island nucleation density and lead to changes in the TD density by as much as two orders of magnitude. We present dislocation-based models to the TD generation and evolution, for which the essential features have been verified by TEM studies. The stacking disorder between the NL and HT GaN is accommodated by Frank and Shockley partial dislocations, which do not generate TDs, whereas regions of the NL that have coalesced and transformed to hexagonal GaN are a source of TDs in the HT GaN. The morphological evolution is consistent with models of diffusion-limited morphological evolution of epitaxial growth in which the film exhibits partial wetting or non-wetting behavior on the substrate. Furthermore, we will present data for both MOCVD and MBE growth that demonstrates that morphological instabilities in both bulk alloy layer growth (e.g., AlGaN or InGaN) or superlattice growth are nearly always associated with mixed character TDs.

2:00 PM D3.2 
TIME-RESOLVED X-RAY SCATTERING STUDIES OF GALLIUM-NITRIDE NUCLEATION AND GROWTH. R.L. Headrick1, J.D. Brock2, S. Kycia1, M.V.R. Murty3, and A.R. Woll2 1Cornell High Energy Synchrotron Source, Cornell University, Ithaca, NY; 2School of Applied and Engineering Physics and Materials Science Center, Cornell University, Ithaca, NY; 3Physics Department and Materials Science Center, Cornell University, Ithaca, NY.

In-situ x-ray scattering studies of the nucleation and growth of GaN onto sapphire(0001) have been performed. Growth was performed by metal organic MBE using TEG and NH3 as the precursors. Ga K-fluorescence was used to monitor the deposition, which followed a power law in time with an exponent of approximately three. Simultaneous time-resolved x ray reflectivity studies show that the growth mode is three-dimensional, producing an array of nano-scale clusters. AFM studies confirmed this growth mode. The nucleation is modeled by incomplete condensation of Ga onto the sapphire surface leading to a deposition rate that is proportional to the size of GaN clusters, and hence the power law growth. This is a new mechanism for three-dimensional growth, specific to gas-source heteroepitaxy with incomplete condensation. Low energy NHx+ ions were also used to influence the growth mode during the nucleation stage for some studies. The results were striking, showing layer-by-layer intensity oscillations at the GaN [0001] anti-Bragg reflection during growth from TEG and 30eV NHx+. This shows that low-energy ion assisted growth induces a two-dimensional growth mode. The mechanism of the change of growth mode is believed to be an increase of the nucleation density induced by the ion-irradiation. The deposition rate was constant, in contrast to growth from thermal precursors only. X-ray diffraction was also used to monitor the in-plane GaN lattice parameter during ion-assisted growth. An early relaxation, well before the expected critical thickness, was observed. These results show that the initial growth mode and lattice relaxation can be controlled. The three dimensional cluster growth mechanism also appears to be a promising method for producing a two-dimensional array of GaN quantum dots.

2:15 PM D3.3 
RADIATIVE AND NONRADIATIVE RELAXATION OF EXCITONS IN GAN. A. Hoffmann, A. Goldner, L. Eckey, I. Brose, Inst fur Festorperphysik, Technische Universitat Berlin, GERMANY; B. Gil, O. Briot, Groupe d'Etude des Semiconducteurs, Universite Montpellier II, FRANCE.

Nonradiative recombination processes are detrimental to the laser action in III-V-nitride devices. Since there is only little information on these nonradiative processes we performed low-temperature time-resolved photoluminescence and calorimetric experiments near the band gap on a series of GaN epilayers grown on (0001) sapphire by MOCVD. The excitonic luminescences dominating in this spectral range exhibit decay times between some 10 and 300 ps. Using the simultaneous highly sensitive detection of the calorimetric absorption, transmission, and reflection at 50mK we determine the quantum efficiency of the excitonic decay processes involved. They range between 10% and 50%. From these data a detailed picture of radiative and nonradiative relaxation and recombination processes in the spectral range near the band gap is obtained.

2:30 PM D3.4 
CRITICAL THICKNESSES AND DOPING EFFECTS, AND ROOM TEMPERATURE EMISSION MECHANISMS IN InGaN/GaN AND GaN/A1GaN MULTIPLE QUANTUM WELLS. H.X. Jiang, J.Y. Lin, M. Smith, and K.C. Zeng, Department of Physics, Kansas State University, Manhattan, KS; H. Morkoc, H. Tang, A. Salvador, and G. Popovici, Materials Laboratory and Coordinated Science Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL; M. Asif Khan, Department of Electrical and Computing Engineering, University of South Carolina, Columbia, SC.

Picosecond time-resolved photoluminescence (PL) emission spectroscopy have been employed to study the optical properties of InxGa1-xN/GaN (x 0.2, grown by MOCVD) and GaN/AlxGa1-xN (x 0.07, grown by MBE) with different well and barrier thicknesses. Our focuses are on the following three important issues (i) the critical thickness, (ii) doping effects on the electronic properties, and (iii) room temperature emission mechanisms in InGaN/GaN and GaN/AlGaN multiple quantum wells (MQWs). Our results have yielded that (i) the optical transitions in nominally undoped MQWs with narrow well thicknesses (Lw < 40 Å) were blue shifted with respect to the GaN epilayer due to quantum confinement, however, no such blue shift was evident for the MQWs with well thicknesses larger than 40 Å, (ii) the band-to-impurity transitions were the dominant emission lines in nominally undoped MQWs of large well thicknesses (Lw > 40 Å) at low temperatures, and (iii) Si-doping improved significantly the crystalline quality of MQWs of large well thicknesses (Lw > 40 Å). The room temperature (RT) emission mechanisms in high quality and purity GaN epilayers and MQWs have also been investigated. Our results showed that in MOCVD grown layers the band-to-band instead of excitonic transition is the dominant emission at RT. This conclusion is supported by the observation of the excitation intensity dependence of the photoluminescence (PL) emission peak position and by a model calculation. Our results thus suggest that the electron-hole plasma is most likely responsible for gain in GaN blue lasers, similar to the cases in other III-V semiconductor lasers.

2:45 PM D3.5 
OPTICAL PROPERTIES OF InGaN/GaN AND GaN/A1GaN MICRO-DISKS. H.X. Jiang, J.Y. Lin, R. Mair, and K.C. Zeng, Department of Physics, Kansas State University, Manhattan, KS; H. Morkoc, H. Tang, A. Salvador, and G. Popovici, Materials Laboratory and Coordinated Science Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL; M. Asif Khan, Department of Electrical and Computing Engineering, University of South Carolina, Columbia, SC.

Arrays of micro-disks with diameter of about 10 m and spacing of 50 m have been fabricated by dry etching from 45 Å/45 Å InxGa1-xN/GaN (x 0.2) and 50 Å/50 Å GaN/AlxGa1-xN (x 0.07) multiple quantum well (MQW) structures grown by MOCVD and MBE, respectively. Optical properties of these micro-disks have been studied by picosecond time-resolved photoluminescence (PL) emission spectroscopy. PL emission spectra and decay dynamics were measured at different temperatures, excitation intensities, polarizations, and delay times. With respect to the original MQW samples, we observed in MQW micro-disks (i) enhanced intrinsic transitions or quantum efficiencies, (ii) enhanced barrier transitions, and (iii) a modified rate of carrier transfer from the barrier to the well regions. Dynamic processes of the optical transitions in the micro-disks were found to depend strongly upon temperature and excitation intensity. Upon strong optical excitation, a red shift of the intrinsic transition (as well as broadening) was observed and is most likely due to enhanced carrier-carrier interaction. Enhanced carrier-photon interactions and spontaneous emission dynamics in the MQW micro-disks will also be discussed in terms of the radiative coupling between the optical and electronic quantum states. These results are expected to have important implications on the design of future micro-cavity and surface emitting vertical cavity blue lasers based on III-nitrides.

Chair: Tadeusz Suski 
Monday Afternoon, December 1, 1997 
Salon F (M)

3:30 PM *D4.1 
ELEMENTAL ANALYSES ON GROUP-III NITRIDES USING HEAVY ION ERD. G. Dollinger, S. Karsch*, A. Bergmaier*, C.M. Frey*, Physik-Department E12, Technische Universität München, GERMANY; O. Ambacher**, M. Stutzmann**, Walter Schottky Institute, Technische Universität München, GERMANY.

Elastic recoil detection (ERD) using energetic heavy ion beams (e.g. 170 MeV 127I) is a suitable method to measure depth profiles of light and medium heavy elements in thin films. The main advantages of ERD, which makes it favorable over many other techniques for elemental analysis, is the possibility to obtain reliable and quantitative results. Using modern detection systems a sensitivity in the ppm region can be obtained, even for all light elements. This technique is employed to the analyses of light elements in group-III nitrides to work on a wide spectrum of questions. First heavy ion ERD was used to obtain quantitative information about the aluminium content x in MBE grown AlxGa1-xN layers on Al2O3 substrates. Using this elemental information and the lattice constants obtained from high resolution X-ray diffraction, Vegard¥s law could be confirmed with high accuracy. Second the impurity content of group-III nitrides was investigated in dependence on deposition conditions for both, MBE and MOCVD, samples. In all samples investigated an oxygen concentration in the range of 100 to 400 ppm was detected which is much higher than the intrinsic charge carrier density of these samples. In addition it is shown that the efficiency of p-doping by Mg cannot be hindered by hydrogen only but also by carbon impurities. As a third, nitridation of heated Al2O3 substrates in NH3 atmosphere was followed using high resolution ERD. A substantial nitrogen content on the substrates surface was detected which means a nearly complete AlN layer grown on the Al2O3 surface by a heat treatment only. Such a nitridation layer can be the base for the growth of group III-nitrides on Al2O3 surfaces.

4:00 PM D4.2 
OBSERVATION OF NATIVE GALLIUM VACANCIES IN GALLIUM NITRIDE BY POSITRON ANNIHILATION. K. Saarinen, T. Laine, S. Kuisma, J. Nissila, P. Hautojarvi, Laboratory of Physics, Helsinki University of Technology, Espoo, FINLAND; L. Dobrzynski, Institute of Physics, Warsaw University Branch, Soltan Institute of Nuclear Studies, Otwock - Swierk, POLAND; J. M. Baranowski, K. Pakula, R. Stepniewski, M. Wojdak, A. Wysmolek, Institute of Experimental Physics, University of Warsaw, Warsaw, POLAND; T. Suski, M. Leszczynski, I. Grzegory, S. Porowski, UNIPRESS, High Pressure Research Center, Polish Academy of Sciences, Warsaw, POLAND.

The role of various defects in the properties of GaN still awaits a detailed description. For example, the parasitic optical transition leading to the yellow luminescence band at about 2.2 - 2.3 eV is observed both in GaN bulk crystals and layers, but the nature of the electronic levels participating this optical process are under discussion. There is an increasing amount of evidence that this transition takes place between a shallow donor and a deep acceptor. The Ga vacancy has been suggested as the defect responsible for the acceptor level. In this work we use positron annihilation spectroscopy to obtain structural information on native point defects in GaN materials. Bulk crystals grown under 1.5 GPa nitrogen pressure at 1500 C were studied by positron lifetime and Doppler broadening techniques. Epitaxial layers grown by MOCVD on sapphire substrates were characterized by a low-energy positron beam. The results show that negative vacancies are present at concentrations in both GaN bulk crystals and layers. The vacancies are in the Ga sublattice and their concentration correlates with the intensity of the yellow luminescence. We conclude that the Ga vacancies contribute to the electrical compensation of n-type GaN and their acceptor levels are involved in the yellow luminescence transition.

4:15 PM D4.3 
LOCAL ELECTRONIC STRUCTURE OF DEFECTS IN GaN FROM SPATIALLY RESOLVED ELECTRON ENERGY-LOSS SPECTROSCOPY. Michael Natusch, Gianluigi Botton, Ronald Broom, Paul Brown, David Tricker, Colin Humphreys, University of Cambridge, Dept of Materials Science and Metallurgy, Cambridge, UNITED KINGDOM.

The influence of the high defect density in wurtzite GaN on its luminescence properties is not well understood. We describe an attempt to measure the effect of defects on the local bandstructure and to correlate this with the luminescence properties, using spatially resolved electron energy-loss spectroscopy (EELS). Defects and surfaces in semiconductors can introduce defect states in the forbidden energy gap and so modify the macroscopic properties of these materials. EELS in a FEG-STEM extracts the imaginary part of the dielectric function in the interband region with high spatial resolution. For low losses (1-100 eV range) this information can be used to reconstruct the optical properties (e.g. reflectivity) of bulk GaN via Kramers-Kronig transformation and can hence provide a model for the joint density of states in a momentum-resolved measurement. At high losses, the Nitrogen 1s core excitation can be used to study the angular projected local density of states in greater detail at 0.4 eV energy resolution and 1 nm spatial resolution. We compare experimental data from wurtzite GaN with various models in the low loss regime. Detailed calculations for the near-edge fine structure of the core excitation agree well with the experimental data for the bulk crystal and enable us to associate the experimental peaks with features in the bandstructure. Measurements in the bulk can now be compared with measurements on isolated structural defects to reveal possible defect states in the band gap, and we have used spatially resolved line-scans across grain boundaries to achieve this.

4:30 PM D4.4 
SCANNING TUNNELING MICROSCOPY OBSERVATION OF SURFACE RECONSTRUCTIONS OF GAN ON SAPPHIRE AND 6H-SIC. A.R. Smith, V. Ramachandran, R.M. Feenstra, Department of Physics, and D.W. Greve, Department of Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA.

Observations of surface reconstructions can provide significant insight into the nucleation and growth mechanism of epitaxial layers. In this paper, we report STM observations of the surface structures of wurtzite GaN epitaxial layers grown on sapphire and 6H (0001) silicon carbide. Growth is performed by molecular beam epitaxy using an RF plasma source, followed by STM imaging without breaking vacuum. In this work, a GaN nucleation layer a few hundred ‰ in thickness was deposited at 685 C followed by growth of about 2000 A of GaN at 775 C. Large, atomically flat terraces are observed by STM separated by atomic height steps. On sapphire, the reconstructions have been explored in detail as a function of annealing and the amount of additional deposited gallium. Surfaces annealed at 800 C (above the Ga condensation temperature) exhibit a (11) reconstruction. Subsequent deposition of gallium onto this surface at 60 C results in the observation of (33), (66), and c(612) reconstructions (in order of increasing gallium coverage). Growth on 6H SiC shows some of the same reconstructions. The (22) reconstruction observed by previous workers using RHEED was notably absent. Based on theoretical and TEM studies [1,2], these results are attributed to dominance of the (000) or N-polar GaN surface under our growth conditions.

4:45 PM D4.5 
UNIAXIAL STRESS EFFECTS ON VALENCE BAND STRUCTURES OF GaN. A.Atsushi Yamaguchi, Yasunori Mochizuki, NEC Corporation, Fundamental Research Laboratories, Tsukuba, JAPAN; Chiaki Sasaoka, Akitaka Kimura, Masaaki Nido, Akira Usui, NEC Corporation, Opto-electronic Research Laboratories, Tsukuba, JAPAN.

Valence band modification by uniaxial stress in GaN is investigated experimentally. In GaN, three valence bands (A, B, and C) exist in a small energy region near the valence band maximum (VBM) and the density of states (DOS) at VBM is large. This feature results in a large threshold current density in GaN-based laser diodes. From such an aspect, it is expected that uniaxial strain in the c-plane leads to an increased energy splitting of the A and B valence bands and to a reduction of the DOS. In this paper, we investigate the effects of uniaxial stress in the c-plane on valence band structures of GaN by reflectance spectroscopy. The samples used were GaN films grown by MOCVD on sapphire (0001) substrates. The uniaxial stress in the c-plane was applied up to around 0.5 GPa. It is observed, in the reflectance spectra, that the energy separation between the A and B excitons increases with the applied uniaxial stress. This result reflects the valence band modification caused by the anisotropic strain in the c-plane. The wave functions of the valence bands are also modified by the strain. It is found from polarization characteristics of the exciton transitions, that the A valence band state becomes X>-like and the B band state becomes Y>-like under uniaxial stress along X direction. The experimental results are analyzed on the basis of kp theory, and deformation potential D5, which relates the anisotropic strain in the c-plane with the valence band structures, is experimentally determined as -3.3 eV, for the first time. Based on this D5 value, we also discuss improvement of GaN-based laser performance by the anisotropic strain.

Monday Evening, December 1, 1997 
8:00 P.M. 
Salons G-K (M)

EXCITON LUMINESCENCE OF STRAIN FREE GaN CRYSTALS GROWN BY AMMONO METHOD. R. Dwilinski, M. Kaminska, Institute of Experimental Physics, Warsaw Univ., Warsaw, POLAND; R. Doradzinski, Institute of Theoretical Physics, Warsaw Univ., Warsaw, Poland; J. Garczynski, L. Sierzputowski, Dept. of Chemistry, Warsaw Univ. of Technology, Warsaw, POLAND.

GaN in a form of microcrystals was grown using a new technique names AMMONO method. It consisted in gallium nitridization in supercritical ammonia. The growth was performed at relatively low temperature (up to 550 C) and pressure conditions (4 - 5 kbar) in comparison with other techniques used for GaN growth. The AMMONO crystals were of pure hexagonal phase. Depending on the kind of mineralizer added during growth, the crystals had either a form of regular grains of a few micrometer size or were of a needle shape up to 25 m length. Free electron concentration of AMMONO GaN was less than few times 1015cm3, as estimated from EPR signal of shallow donor. Luminescence of the crystals was very intensive, even at room temperature. Photoluminescence spectra at T = 4 K were characterized by fixed positions of very narrow exciton lines (FWHM down to 1 meV, comparable only with homoexpitaxial GaN). Free excitons A, B, C, resolved two donor bound excitons and acceptor bound exciton were visible. The fixed position of exciton lines was in contrast to small changes of line energies which have been always observe for epitaxial GaN layers due to strain present in them. This proved that AMMONO GaN crystals were strain-free.

SUBLIMATION SANDWICH GROWTH OF FREE STANDING GaN CRYSTALS. Yu.A. Vodakov, E.N. Mokhov, M.G. Ramm, A.D. Roenkov, A.G. Ostroumov, A.A. Wolfson, A.F. Ioffe Physical Technical Institute, St. Petersburg, RUSSIA; S.Yu. Karpov, Advanced Technology Center, St. Petersburg, RUSSIA; Yu. N. Makarov, Fluid Mechanics Institute, University of Erlangen-Nürnberg, Erlangen, GERMANY; H. Jürgensen, AIXTRON GmbH, Aachen, GERMANY.

It was shown in [1] that the sublimation sandwich method [2] can be used for growth of thick epitaxial layers of GaN with the growth rates up to 0.5 mm/hour. In the present paper a further development of this technique is presented. Thick monocrystalline GaN layers are grown on SiC and sapphire. In some growth experiments a specially prepared seed is used which consists of a few micrometer thick layer of GaN grown by MOVPE on the sapphire substrate. The sublimation growth is performed in a sandwich cell similar to that used in [1] at temperatures higher than 1000C in atmosphere of ammonia. Growth rates up to 1 mm/hour are demonstrated. The GaN layers of the best quality are grown at the growth rates 0.2-0.3 mm/hour. It is shown that the GaN growth rate during the sublimation sandwich growth is limited by supply of Ga, but control of transport and chemical decomposition of ammonia in the cell is of critical importance. In the paper the mechanisms of gallium and ammonia transport are discussed. A theoretical model of the sublimation growth is proposed which is used for optimization of the growth system design and the process. Using this method the free standing GaN crystals of more than 600 micrometer in thickness and of 10-15 mm in size are obtained. The crystals are transparent and non-colored. Rocking curve measurements give full width of half maximum (FWHM) in the range between 150 and 450 arc. sec.

GROWTH OF HEXAGONAL GALLIUM NITRIDE FILMS BY MOCVD USING NOVEL SINGLE PRECURSORS. C. G. Kim, K.-W. Lee, Y. Kim, Korea Research Institute of Chemical Technology, Advanced Materials Division, Taejon, KOREA.

Volatile single precursors were synthesized by the reaction of alkylgallium (R3Ga where R= -CH3, -CH2CH3) with alkylhydrazine (H2NNHR' where R'= -CH3, -C(CH3)3). Novel alkylgallium hydrazide precursors were obtained as liquids or volatile solids depending on alkyl groups of gallium metal. The physical properties of the precursors were investigated and showed they were reasonably suitable as precursors for CVD. These precursors with N/Ga ratio of 2 might overcome the deficiency of nitrogen observed commonly in GaN films produced by metal-organic chemical vapor deposition (MOCVD). They were applied for the deposition of GaN films on Si(111) substrates in the range of temperature between 500 and 800 oC by MOCVD. The produced h-GaN films were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM).


Numerous recent studies have demonstrated that dimethylethylamine alane (DMEAA) is a viable group III precursor for depositing high quality aluminum nitride thin films when operating in atomic layer growth mode with ammonia as the group V source. However, a practical consideration that is questioned but seldom investigated is whether one should initiate the growth with the group III or the group V source. Clearly DMEAA interacts with silicon very differently from ammonia, hence reversing deposition sequence may lead to different interfacial composition. Earlier studies involving TMAA and ammonia indicate that direct interaction of group III precursor may lead to higher carbon contamination. In this work, adsorption processes of DMEAA on Si(100) and on ammonia-covered Si(100) are characterized with X-ray Photoelectron Spectroscopy, Temperature-Programmed Desorption, and Secondary Ion Mass Spectrometry. Preliminary results indicate that DMEAA may adsorb molecularly on Si(100), but not necessarily so on ammonia-covered Si(100). Results from reversing the adsorption sequence, i.e. ammonia first then DMEAA, will be compared as a possibility for interface quality control.

LOW TEMPERATURE CHEMICAL VAPOR DEPOSITION OF GROUP-III-NITRIDES. Wolfram Rogge, Roland A. Fischer, Anorganisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, GERMANY.

The group-III nitrides AlN, GaN. and InN are interesting materials for light-emitting diodes and semiconductor lasers operating at short wavelengths as well as high frequency acoustic wave devices. Vapour phase heteroepitaxy using the trialkyls MR, (M = Al, Ga, In) and ammonia as reagents is the method of choice to fabricate device quality group-III nitride materials. High substrate temperatures of typically >900C are necessary to activate ammonia efficiently. N-deficient growth and hydrogen incorporation are severe problems. The use of single molecule precursors which contain nitrogen in a chemically activated form may allow stoichiometric growth even in the absence of ammonia and at much lower temperatures. An ideal single source precursor would be a non-pyrophoric, non-explosive, non-toxic, air-stable liquid compound with a high vapour pressure exhibiting a molecular structure with strong covalent M-N bonds and no or at least only few M-C, C-C, C-H, N-C or N-H bonds and no other chemical bonds. The precursor should be nitrogen rich and the thermal stability of the compound should be comparably low (dec. 250C). Nitride films were grown in the absence old additional N-sources in vacuo using (N3)2Ga[(CH2)3NMe2] (1), (N3)ln[(CH2)3NMe2]2 (2), (N3)Al[(CH2)3NMe2]2 (3) and (N3)AlMe2(H2NBu) (4). The obtained films were characterized by SEM-EDX, XRD and PL measurements. From precursor 1 highly textured GaN films on (0001)Al2O3 were grown at 700C (002 reflection: rocking curve FWHM 2) as well as AlN films (FWHM 0.16) using 4 at 900C. Very smooth, amorphous MN coatings with the correct band-gap (by photothermal deflection spectroscopy, PDS) were obtained at 400C.

CHARACTERIZATION OF GALLIUM NITRIDE THIN FILMS GROWN BY LOW TEMPERATURE CVD FROM GALLIUM TRIIODIDE AND AMMONIA ON Si(100) SUBSTRATES. Anna Topol, Denis Endisch, Alain Kaloyeros, Center for Advanced Thin Film Technology and Physics Department, The University at Albany-SUNY, Albany, NY.

Gallium nitride (GaN) allows fabrication of efficient light emitting diodes (LED's) even when deposited on substrates with large lattice mismatch (> 20 %) and resulting high defect densities exceeding 1010 cm-2. Growth of gallium nitride on silicon is therefore of great interest for integration of optoelectronics into existing silicon technology. However, for commercially viable solutions the process temperatures need to be reduced and less hazardous precursors have to be implemented. In this paper we report on the chemical vapor deposition (CVD) of GaN films on unbuffered and buffered Si(100) substrates using novel inorganic Ga precursor. Deposition was performed at temperatures ranging front 450C to 650C without plasma assistance. Results from optimized growth conditions with corresponding structural and chemical characterization of the GaN films, will be presented. Results obtained by Rutherford backscattering spectroscopy (RBS), x-ray diffraction (XRD), atomic force microscopy (AFM), and x-ray photoelectron spectroscopy (XPS) will be discussed.

BEHAVIOUR OF GaN BUFFER LAYER AND IT'S EFFECT ON THE EPILAYER DURING FILM GROWTH OF GaN BY USING LOW PRESSURE METALORGANIC VAPOR PHASE EPITAXY ON SAPPHIRE SUBSTRATES. Lisen Cheng, Ze Zhang, Beijing Laboratory of Electron Microscopy,Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing, CHINA; Guoyi Zhang, Dapeng Yu, Laboratory of Mesoscopic Physics, Dept of Physics, Peking Univ, Beijing, CHINA.

Behaviour of GaN buffer layer during film growth of GaN by low pressure metalorganic vapor phase epitaxy on sapphire substrates was investigated systematically by utilizing transmission electron microscopy. Buffer layers grown at 550 degree centigrade and 76 Torr are predominantly hexagonal and in polycrystalline form. Tilt angles of [0001] GaN crystallites with respect to [0001] sapphire substrate were measured by selected area electron diffraction to be in the range from 0 to 3.5 degree. In addition, selective distribution of a small portion of cubic GaN in grain boundary areas in the as-grown GaN buffer layer was also viewed, and it's formation is related to further nucleation of GaN at some specific sites along grain boundaries. After the subsequent growth of GaN epitaxial layer over the buffer layer at the temperature as high as 1080 degree centigrade, GaN in the buffer layer will transform from the predominantly hexagonal to the predominantly cubic phase. Due to this kind of phase transition, very high density of stacking faults and grain boundaries are present in the buffer layer. Grain boundaries in the buffer layer after the phase transition can be divided into two classes. One is the boundary between two adjacent hexagonal and cubic crystallites, and the other is the cubic twin boundary. It was also found that only the twin boundaries can extend into the epilayer deposited over the buffer layer to form domain boundaries. The as-formed domain boundaries in the epilayer were deduced to be inversion domain walls.

NITRIDATION OF SAPPHIRE: CHEMICAL AND STRUCTURAL ANALYSIS. Yonah Cho, Yihwan Kim, Eicke R. Weber, University of California, Berkeley, Dept of MSME, Berkeley, CA; Sergei Ruvimov, Zuzanna Liliental-Weber, Lawrence Berkeley National Laboratory, Berkeley, CA.

Despite the substantial lattice mismatch between sapphire and GaN, sapphire is far most favored substrate for the growth of blue LED material GaN. Structural quality of GaN layers has been shown to depend on substrate treatment prior the growth. Niridation of sapphire substrate combined with GaN buffer growth are currently used for the optimization of film quality. We investigated the chemical and structural effect of plasma source nitridation on the sapphire substrate. The substrates were nitridated for 5, 10, 15 and 30 minutes. Angle-resolved x-ray photoelectron spectroscopic (ARXPS) technique was used for chemical analysis up to 5nm while cross sectional high resolution electron microscopy (HREM) was employed to study the structure of the nitridated substrate. In addition, atomic force microscope (AFM) was used to monitor the nucleation of GaN buffer. Increasing nitrogen concentration was observed upon longer nitridation. However, no substantial nitrogen gradient was observed within the probing depth of ARXPS. That is consistent with HREM which indicates structural changes throughout 6-10 nm- thick surface layer. Formation of thin polycrystalline AlN layer on sapphire was detected for 30 min - nitridation. AFM measurements indicate that this nitridation regime provides a largest density of GaN islands during the deposition of GaN buffer on sapphire substrate.

EFFECT OF NITRIDATION AND BUFFER IN GaN FILMS GROWN ON A-PLANE (11-20) SAPPHIRE. D. Doppalapudi, E. Iliopoulos, S.N. Basu* and T.D. Moustakas, Center for Photonics Research, Dept. of Electrical Engineering, Boston, University, Boston, MA. *Dept. of Manufacturing Engineering, Boston University, Boston, MA.

The study of growth of III-V nitrides on the A-plane (ll-20) sapphire is important, since such a substrate is suitable for the formation of edge emitting lasers by cleaving. Since the in-plane symmetry of the substrate in this orientation is different from that of the epitaxial film, the growth mechanism is expected to be different from the growth on the c-plane (0001) sapphire. In this paper, we report on a systematic study of GaN growth on the A-plane sapphire by ECR-assisted MBE. Particular emphasis was placed on understanding the early stages of nucleation and growth. The structural quality of the samples was investigated by RHEED, SEM, AFM, XRD, TEM and their optoelectronic properties by Hall effect and photoluminescence measurements. The carrier concentration of the films doped n-type with Si was varied systematically from 1016 to 1019 cm-3. It was found that if the substrate is not nitrided prior to growth, the films are inferior in both structure and optoelectronic properties. Specifically, the TEM studies indicate that a considerable fraction of the film has zinc-blende structure and the diffraction studies show that GaN does not grow with the expected c orientation perpendicular to the substrate surface. On the other hand, the orientation of the GaN film was found to be independent of the existence of a low temperature GaN buffer. However, the low temperature buffer was found to affect the optoelectronic properties of the films, especially those with low carrier concentrations. Specifically, we observe that lightly doped films grown without low temperature buffer show strong luminescence features in the spectral region of 3.2 eV, which we attribute to stacking faults and zinc-blende domains in such films.

SUBSTRATE SURFACE TREATMENTS AND "CONTROLLED CONTAMINATION" IN GaN/SAPPHIRE MOCVD. Y. Golan, P. Fini, S.P. DenBaars and J.S. Speck, Materials Department, University of California, Santa Barbara, CA.

Although sapphire is considered to be a very stable oxide surfaces it has been shown that the treatment of sapphire by gaseous H2 at high temperature (HT) results in large, hexagonal etch pits on the {00.1} surface via the etching reaction [1]: 
A1203(s) + 2H2(g) Al20(v) + H2O(v) 
The product of this reaction is monovalent aluminum oxide, which is stable at temperatures >1000C. In addition to H2, trace amounts of metals could either catalyze the reaction with H2, or be engaged in direct redox reactions with the sapphire surface. Annealing the sapphire substrates in H2 at HT prior to growth has become a standard surface treatment for GaN MOCVO. In this work, we show using atomic force microscopy that the H2 exposures resulted in numerous pits on the sapphire surface. Interestingly, various surface morphologies were obtained upon repeating the experiments under ``identical'' conditions, suggesting that either the GaN precursors (in various possible forms) or other elements residing in the reactor (e.g. Si, Mg) might be acting as contaminants in the surface pretreatment process. In order to investigate the role of these contaminants, we have introduced the concept of ``controlled contamination'', namely, exposure of the sapphire surfaces to controlled amounts of potential contaminants and investigation of the resulting sapphire surface chemistry and morphology. In order to simulate the effect of Ga in the surface treatments, the substrates were exposed to controlled amounts of trimethyl gallium (TMGa). This resulted in Ga droplets on the sapphire surfaces. Consecutive annealing of these samples in H2 showed damage and roughening (pit depths of up to 40 non) of the substrate surface which is proportional to the TMGa exposure time. Similarly, deposition of HT GaN islands directly on the sapphire (without a nucleation layer) resulted in damage to the sapphire. This can be explained by decomposition of the GaN to its elements, leaving Ga metal to react with the substrate. The implications of sapphire reactivity in the GaN MOCVO environment on GaN epitaxy will be discussed.

SURFACE CHARACTERIZATION OF GaN FORMATION ON GaAs(100) USING AMMONIA. Chul Huh, Seong-Ju Park, Kwangju Institute of Science and Technology, Dept of Materials Science and Engineering, Kwangju, KOREA; Sook Ahn, Jeong Yeul Han, Keum Jae Cho, Jae Myung Seo, Jeonbuk Nat. Univ., Dept of Physics, Jeonju, KOREA.

The thermal nitridation and GaN formation on GaAs(100) using NH3 has been examined employing highly surface sensitive synchrotron radiation photoemission spectroscopy and atomic force microscope (AFM) to analyze the compositional change, the chemical states, and the surface morphological changes in the nitridated surface layer depending on the nitridation temperature. From the photoemission spectra analysis, it is deduced that ammonia can be decomposed to activated nitrogen atom above 700C. As the nitridation temperature increased, the amount of Ga-N species increased due to the efficient thermal decomposition of ammonia and the As desorption from the GaAs surface. Furthermore, the photoemission spectra of As3d indicated that the remaining As atoms are not bonded to N, suggesting that As-N species are not formed during nitridation. AFM images showed that the islands on surface have elliptical shape elongated along the [011] direction to relax the tensile strain in the [01] direction caused by the large lattice mismatch between GaN and GaAs substrate. It was also found that the size becomes larger with increasing temperature due to the increased surface migration of Ga atom. From the AFM analysis, it was also observed that the surface morphology in the nitridated layer is sensitively dependent on nitridation temperature, and that a smooth surface morphology can be achieved by nitridation at 700C. We will further discuss the mechanism of thermal nitridation and GaN formation on GaAs(100) using ammonia.

THE EFFECT OF LOW-TEMPERATURE NUCLEATION ON THE GROWTH OF GaN FILMS DURING HVPE PROCESS. Liang Junwu, Wen Ruimei, Guo Zhongguang, Deng Lisheng and Yang Hui, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, CHINA.

The growth of GaN thick films on sapphire using halid vapor phase epitaxy (HVPE) will be presented. We use a two-step growth technique, which involves a low temperaturenucleation step at about 750C prior to the high temperaturegrowth step. During the low temperature nucleation step thesapphire substrate is treated firstly by a mixed gas of GaCland hydrogen and then the GaN nuclei are deposited on sapphireby using GaCl+NH3 gas. The low temperature nucleation step accelerates the surface coverage and increases the epitaxialgrowth rate of GaN. Moreover, according to the double crystal X-ray diffraction and photoluminescence measurement, the crystallinity and optical quality of the GaN epilayers are improved by using this technique. The nucleation mechnism has been analysed by a surface kinetics model which considers the free energy for critical nucleus formation and the interface energy between precipitate and substrate. The kinetics calculation has shown that the formation of nuclei containing small number of GaN moleculeshas a maximum rate at about 750C.


Recently, the study of GaN has been proceeded above 1000C. But the sublimation and thermal decomposition of GaN are expected at this high temperature, making GaN film nonstoichiometric due to N vacancy. Carbon contamination and an insufficient incorporation of In into InGaN are also expected at high temperature. Thus we have investigated the low temperature growth of nucleation layer GaN layer at low temperature(450 - 650C) by RPE-UHVCVD system which can supply excited nitrogen sources. Atomic force microscopy(AFM) images of GaN showed two different surface morphologies. Many hexagonal facets appeared on the surface of GaN grown on nitridated sapphire but round shapes were observed on the GaN grown on sapphire without nitridation. This indicates that the nitridation provides the nucleation sites for facet formation. The FWHM of XRD rocking curve increased with increasing the thickness of nucleation layer at 500C. We believe that the nitridation evokes this phenomenon to the nucleation layer and consequently decreases the number of nucleation sites which induce the columnar growth. Root-mean-square(r.m.s) roughness measured by AFM were closely related to the columnar growth in the GaN film. And these were very sensitive to the growth temperature and independent of N2 flow rate in the plasma source. Columnar growth was observed in the GaN film grown at 500C. From these results, it was found that high crystalline GaN film can be obtained only when the indication of columnar growth appeared on the surface and this columnar growth disappeared with increasing nucleation layer thickness. In this study, we found that high crystalline GaN epitaxial film could be grown by two step growth method at low temperature by controlling the nitridation of substrate and growth of nucleation layer.


Al2O3 (0001) plane was nitridated at a low temperature range of 300-550C using an RPE-UHVCVD system which enabled us to grow III-V nitride film using the activated nitrogen species produced by inductively coupled plasma (ICP).

8:30 AM *D6.1 
CHARACTERIZATION AND GROWTH OF NITRIDE BASED QUANTUM STRUCTURES. Hiroshi Amano, Tetsuya Takeuchi, Shigeo Yamaguchi, Christian Wetzel and Isamu Akasaki, Department of Electrical and Electronic Eng., Meijo University, Nagoya , JAPAN.

High-quality GaN-based quantum well structures have been fabricated by MOVPE. It was found that both GaInN and AlGaN ternary alloys can be grown pseudomorphically on the underlying thick GaN layer. Since nitride has large piezoelectric constant, electric dipole caused by the strain and the corresponding electric field is intrinsically induced. Sub-band structures of the nitride-based strained quantum well have been investigated both theoretically and experimentally. Effect of intrinsic electric field on the performance of the nitride based lasers, and also new optical device based on quantum structure will be discussed.

9:00 AM D6.2 
THE FIRST NITRIDE LASER DIODE ON SILICON CARBIDE. J.D. Brown, J.T. Swindell and J.F. Schetzina, North Carolina State University, Raleigh, NC; G.E. Bulman, K. Doverspike, S.T. Sheppard, T.W. Weeks, M. Leonard, H.S. Kong, H. Dieringer, C. Carter and J.A. Edmond, Cree Research Inc., Durham, NC.

This paper reports the events at NCSU leading up to and including those of June 5, 1997 which produced the first demonstration of a nitride laser diode on silicon carbide - and the very first nitride laser demonstration outside of Japan. All of the laser diode samples tested at NCSU were designed, grown, and fabricated into cleaved cavity test structures at Cree Research. The laser structure is a multiquantum well (MQW), separate confinement heterostructure (SCH) grown on 6H-SiC and consists of an 8-well In0.14Ga0.86N/GaN MQW region, with Al0.06Ga0.94N waveguide regions and Al0.13Ga0.87N cladding layers. The devices were fabricated into index-guided structures and cleaved into bars corresponding to a laser cavity length of 500 m prior to delivery to NCSU. Laser testing at NCSU consisted of spectral emission measurements versus current, light output versus current (L-I) measurements, and polarization measurements using a rotatable linear polarizer placed in front of a diode array detector. Prior to June 15, a number of test structures of various design were measured at NCSU. All of these devices emitted bright but broad (10-20 nm) blue/violet light and failed to display laser action. However, at a drive current of 2.0 A, the first laser device displayed a very narrow emission peak (0.8 ) which provided the initial evidence of laser action. Over the next sixteen hours, a threshold current of Ith = 1.2 A (42 V) corresponding to a current density of 48 kA/cm2 was determined, L-I measurements were completed which showed a sharp increase in light output above Ith, and a high TE/TM polarization ratio of 135 above threshold was measured. The NCSU laser results were made known to Cree Research the next morning where they were immediately confirmed by G. Bulman - and the rest is history.

9:15 AM D6.3
INTERACTIONS OF LO PHONONS WITH BOUND EXCITONS IN HOMOEPITAXIAL GaN. K.P. Korona, A. Wysmolek, J.M. Baranowski, K. Pakula, Institute of Experimental Physics Warsaw University, Warsaw, POLAND; I. Grzegory and S. Porowski, High Pressure Research Centre Polish Academy of Sciences, Warsaw, POLAND.

Photoluminescence spectra of homoepitaxial GaN epilayers grown on GaN single crystal substrates are characterized at low temperature by very narrow emission lines connected with donor (DBE) an acceptor (ABE) bound excitons. Due to that, it is possible to analyze the first and second LO phonon replica of the DBE and ABE lines. In undoped homoepitaxial GaN layer, in spite that the no-phonon emission due to DBE is o ne order of magnitude stronger than the ABE one, the predominant feature of the first LO phonon replica of the excitonic spectrum is related to ABE. On the other hand, the second LO phonon replica of the excitonic spectrum is predominantly connected with DBE. These observations are explained in the following way. In the bound exciton recombination in which ne LO phonon is involved the conservation of k- vector has to be strictly obeyed. The ABE is more strongly localized in the real space, thus more delocalized in the k- space. Therefore, in the case of ABE recombination, phonons from larger fraction of the Brillouin zone near I point may participate in the first LO replica. That creates the higher effective phonon density of states for LO phonons coupled to the ABE than to the DBE. On the other hand, in the second LO phonon replica the DBE and ABE, the k conservation is not so strictly obeyed. Combination of two phonons from vicinity of the center of Brillouin zone easily may leads to k conservation. Therefore, in the second LO phonon replica the no-phonon excitonic structure, in which the DBE emission is the most intense is directly reproduces. In addition, the temperature dependence of LO phonon replica of excitonic structures is also reported. It is shown that with increase of temperature the coupling of LO phonon changes from bound to free excitons. The shape of the LO phonon replica of free exciton line has been describe as well.

9:30 AM D6.4 
BAND GAP PHOTOLUMINESCENCE BROADENING IN n-GaN FILMS. E. Iliopoulos, D. Doppalapudi, H.M. Ng, T.D. Moustakas, Center for Photonics Research and Electrical and Computer Engineering Dept., Boston University, Boston, MA.

This paper addresses the broadening mechanism of the near band gap photoluminescene in GaN films doped n-type with silicon. The films were produced by plasma assisted MBE and their carrier concentration was varied systematically from 1015 to 1020 cm^-3%%. The photoluminescence was excited with a 10mW He-Cd laser at 77K. At low carrier concentration (<1017 cm-3) the photoluminesoence peak has FWHM of about 18meV, while at higher carrier concentration (>1018 cm-3) the FWHM increase monotonically with the carrier concentration up to about 60meV. The broadening of the line at high carrier concentration is attributed to impurity band broadening and tailing of the density of states. The data were quantitatively analyzed, as a function of canier concentration and compensation ratio, using the impurity band broadening model of Morgan(1) and the agreement between model and experimental data supports the model's validity and suggest a potential method of determining the compensation in degenerate semiconductors.

9:45 AM D6.5 
MATERIALS CHARACTERIZATION ON OPTICALLY PUMPED InGaN/GaN LASERS BY FAR FIELD MEASUREMENTS AND BY FOURIER ANALYSIS OF THE EMISSION SPECTRUM. Daniel Hofstetter, Robert L. Thornton, Linda T. Romano, David P. Bour, and Noble M. Johnson, Xerox Palo Alto Research Center, Palo Alto, CA.

We present two different characterization methods to evaluate the material quality of optically pumped InGaN/GaN-based MQW lasers. The first of these techniques is based on far field measurements and calculations of the investigated devices. These experiments have revealed that our lasers oscillate in the 7th order transverse mode; this observation could be confirmed by calculations of the confinement factors, the near field and the far field distributions. A good agreement between the measured and the calculated far field data was seen. The observation of such a higher order mode in the transverse direction allowed us to identify the entire epitaxial layer stack acting as a waveguide for this particular mode. However, the 7th order mode of this thick waveguide with air and sapphire as cladding layers happened to coincide with the zero order mode of the actual InGaN/GaN waveguide with the AlGaN cladding layers. The second method for the characterization of the material quality is based on Fourier analysis on spectra of optically pumped InGaN/GaN lasers. Doing Fourier transforms on the high resolution spectra of such devices at threshold enabled us to distinguish between several different oscillation periods occurring as peaks in the transform. Since one would expect only one peak corresponding to the cavity length, we tried to relate the additional peaks to crystal imperfections. AFM scans and TEM micro-graphs of material which was used for the above experiments showed 250 nm wide and 300 nm deep pits in the surface. Since laser action takes place in the form of narrow filaments, pits along these lines are a likely explanation for the features we have seen in the transformed spectra. In order to support this explanation, we fabricated a sample with a dry-etched, 5 micron wide and 300 nm deep groove almost parallel to the facets. The mode separations attributed to the two sub-cavities formed by the groove could be identified clearly as additional peaks in the Fourier transformed laser spectrum.

Chair: Christian Wetzel 
Tuesday Morning, December 2, 1997 
Salon F (M)

10:30 AM *D7.1 
STRUCTURAL AND OPTICAL PROPERTIES OF GROUP III-NITRIDE QUANTUM WELLS STUDIED BY (S)TEM AND CL. Hubert Lakner, Werkstoffe der Elektrotechnik, Gerhard-Mercator-Univ Duisburg, GERMANY; Ferdinand Scholz, 4. Phys Inst, Univ Stuttgart, GERMANY.

In order to investigate structural properties as defects, interface abruptness (grading), compositional fluctuations and effects of strain (relaxation) we applied quantitative analytical scanning transmission electron microscopy (STEM) to cross-sectional specimens of MOVPE-grown wurtzite (Al, Ga, In)N heterostructures. Additionally, cathodoluminescence (CL) and secondary electron (SE) imaging was used to analyze the electro-optical properties, as well as the topography of heterostructure layers. The interfaces in e.g. InGaN/GaN quantum wells appear to be asymmetric. The lower interface in the growth direction is more abrupt than the upper one, where a grading in the In-concentration is significant. Additionally, we found composition fluctuations in the nanometer range within the InGaN quantum wells, which are supposed to cause the localized exitonic behaviour of the observed emission. The thick InGaN quantum wells are relaxed which is not the case for the investigated thin quantum wells of 2 nm thickness. Misfit dislocations in the vicinity of the thick InGaN quantum wells have been observed. The quality of InGaN/GaN multiquantum wells or superlattices (SL) is degraded by threading dislocations emerging from the AlN nucleation layers. The SL periodicity is increasing in growth direction. 
The topography of samples containing InGaN layers is dominated by three types of defects: mesa-like hexagonal structures, hexagonal hillocks and micropipes. The growth process, the luminescence efficiency as well as the emission wavelength are influenced by these defects. CL images show intensity inhomogeneities in both InGaN/GaN and AlGaN/GaN heterostructures which are related to local variations of the interface quality. In AlGaN/GaN single quantum well structures a broad deep-level luminescence band was observed, which is generally absent in InGaN/GaN heterostructures. This deep-level emission is strongly enhanced at defect positions.

11:00 AM D7.2 
ROLE OF DOPANTS AND IMPURITIES ON PINHOLE FORMATION; DEFECTS FORMED AT InGaN/GaN AND AlGaN/GaN QUANTUM WELLS. Z. Liliental-Weber, S. Ruvimov. W. Swider, Y. Kim, and J. Washburn, Lawrence Berkeley National Laboratory, Berkeley, CA; S. Nakamura, Nichia Chemical Industries Ltd., Tokushima, JAPAN; R.S. Kern and Y. Chen, Hewlett Packard; and J.W. Yang, APA Optics, Inc.

11:15 AM D7.3 
ATOMIC STRUCTURE OF GRAIN BOUNDARIES AND INTERFACES IN III-NITRIDE EPITAXIAL SYSTEMS. S. Ruvimov, Z. Liliental-Weber, H. Amano*, and I. Akasaki*, Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California at Berkeley, Berkeley, CA; *Meijo University, Tempakuku-ku, Nagoya, JAPAN.

High resolution electron microscopy (HREM) was applied to study atomic structure of stacking faults, grain boundaries and interfaces in III-nitrides epitaxial layers grown by MOVPE on sapphire. Image analysis and contrast simulations were employed to verify the atomic structure of both basal and prismatic stacking faults as well as various boundaries in GaN and AIN. It has been shown that the formation of vertical boundaries m epitaxial layers is often associated with specific defects at the interface with a substrate. Deterioration of sapphire was also observed at those defects suggesting the oxygen outdiffusion along the boundaries. Oxygen outdiffusion into GaN layer might change its physical properties and cause a formation of other defects. This result was confirm by study of cleaved 60 wedge-shaped crystals in order to exclude the effect of ion milling on the interface structure. A cleavage at the (100) crystallographic planes allows the fast fabrication of wedge-shaped GaN crystals for HREM without further processing. This technique is also addressed to the problems such as phase transformations at the interfaces, nitridation of sapphire surface, and clustering in InGaN. Transmission electron microscopy has been applied to study defects in epitaxial doped and undoped GaN layers grown by MOCVD on sapphire substrates. Samples with InGaN/GaN and AlGaN/GaN QW's have also been investigated. The results of this study show that GaN does not like incorporation of any ''foreign'' atoms. This always leads to increased formation of nanotubes and pinholes. The highest density of these defects was formed close to the interface with sapphire where oxygen outdiffusion might be expected, or in the subsurface area in the samples where oxygen was added deliberately. Addition of In (or Al) at QW's leads to formation of prismatic dislocation loops at each interface and also to pinhole formation. Increased In fraction or a larger number of QWs leads to greater density of pinholes and larger surface roughness. Nonuniform thickness of QWs and localized growth in different crystallographic directions (c-direction and pinhole facet walls) will influence device properties (2D-gas, contacts). Many of the ''hollow'' nanotube defects are terminated during growth. Comparison with state of the art samples will be discussed.

11:30 AM D7.4 
IN-SITU TEM OBSERVATION OF AIN FORMATION DURING NITRIDATION OF (0001) SAPPHIRE. Mark Yeadon, Michael T. Marshall, J. Murray Gibson, Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL.

Using a novel ultrahigh vacuum transmission electron microscope (UHV TEM) with in-situ molecular beam epitaxy (MBE) we have studied the nitridation of (0001) sapphire upon exposure to ammonia. Atomically flat sapphire surfaces were obtained by high temperature annealing and surface steps were clearly imaged using both the TEM and atomic force microscope (AFM). Subsequent exposure to ammonia flow at 950C led to the successful synthesis of epitaxial AlN; the films were characterized in-situ using transmission and reflection electron microscopy (TEM and REM), transmission and reflection high energy electron diffraction (TED and RHEED), and Auger electron spectroscopy (AES). Our experiments show that AlN grows by a 3D island mechanism. Electron diffraction patterns suggest an abrupt AlN/sapphire interface with no evidence of the formation of Al-O-N compounds. The rate limiting step in the nitridation reaction appears to be the diffusion of nitrogen and oxygen species between the free surface of the growing AlN film and the reaction interface. It is inferred from kinetic measurements that diffusion of these species most likely occurs along the boundaries between coalescing AlN islands.

11:45 AM D7.5 
BUFFER LAYER ENGINEERING:CONTROL AND QUANTIFICATION OF RESIDUAL STRAIN IN GaN LAYERS. N.V. Edwardsa, M.D. Bremsera, T. Zhelevaa, W.G. Perrya, S.D. Yoob, R.F. Davisa, and D.E. Aspnesa; aDepartment of Materials Science and Engineering, NC State University; bDepartment of Physics, NC State University, Raleigh, NC.

We report three strategies for achieving relatively compressively strained GaN layers on 6H-SiC substrates, as quantified by XRD, TEM and low-temperature reflectance: (1)growth on (0001) vs. vicinal substrates; (2)growth of very thin (0.6m) GaN films, and (3)modification of the high temperature AlN buffer layer typically deposited on substrates prior to GaN growth. Further, we report for the first time the growth conditions under which material on 6H-SiC can be engineered to have optical properties and residual strains similar to material grown on Al2O3 substrates, achievable at this point only through strategy (3). For example, a 2m thick film yielded an A excitonic energy EA=3.4767 eV and excitonic splittings EBA=6.2meV, ECA=23.9meV and xx0kbar, comparable to values observed for a 250 m thick HVPE-grown sample on Al2O_3with no buffer layer measured in this laboratory. An investigation of the boundaries of this engineering capability will be presented, and will be given within the context of the least squares fit to the reflectance data expressed as E_Avs.E_BAandE_CA, basically the Hopfield quasi-cubic model calculation of the variation of GaN valence bands with biaxial strain. The excitonic splittings of these samples, plus those from GaN samples on LiAlO_2, LiGaO_2 and ScAM substrates, newly measured in this laboratory, should offer novel input into this fit, providing a critical test of the valence band parameters calculated in this laboratory and by other workers.

Chair: Olivier Briot 
Tuesday Afternoon, December 2, 1997 
Salon F (M)

2:00 PM *D8.1 
SELECTIVE AREA GROWTH OF GaN BY MOVPE AND HVPE. Kazumasa Hiramatsu, Mie University, Dept. of Electrical & Electronic Eng., Tsu, JAPAN; Hidetada Matsushima, Takumi Shibata, Nobuhiko Sawaki, Nagoya University, Dept. of Electronics, Nagoya, JAPAN.

The selective area growth (SAG) of III-V nitride semiconductors is important techniques not only for fabricating various kinds of device structures such as cold cathode emitter tips, field effect transistors and low dimensional quantum structures but also for realizing dislocation free layer using the epitaxially lateral overgrowth (ELO). In this paper, we report recent successful results on the SAG of GaN that has been done by MOVPE (metal organic vapor phase epitaxy) and HVPE (hydride vapor phase epitaxy). Both the growths were carried out on MOVPE-grown GaN (0001 ) / sapphire substrates with lined or dotted SiO2 masks. Sub-micron GaN dot and line structures were fabricated by the SAG in MOVPE and also buried structures of the periodically aligned SiO2 lines with a GaN layer was successfully realized using the ELO technique. The buried structures were confirmed to have smooth surfaces and no grain boundaries. Furthermore, high quality GaN bulk single crystals without any cracks were obtained by the SAG in HVPE. Crystalline and optical properties of the GaN bulk were much improved. The reduction in the thermal strain due to the growth on the limited area as well as the ELO on the mask area were found to be effective to reduce crystalline defects of the GaN bulk single crystals.

2:30 PM D8.2 
WIDE GaN STRIPES BY LATERAL GROWTH IN METALORGANIC VAPOR PHASE EPITAXY. Akitaka Kimura, Chiaki Sasaoka, Akira Usui, NEC Corp., Opto-electron Res. Labs., Ibaraki, JAPAN; Akira Sakai, NEC Corp., Fundamental Res. Labs., Ibaraki, JAPAN.

In our experiments, a metalorganic vapor phase epitaxy (MOVPE) reactor operating at 100 Torr using trimethylgallium (TMG) and NH3 as source gases was used. As the substrates for the selective growth, GaN epitaxial layers on sapphire (0001) substrates were used. Either 3000 -thick silicon dioxide (SiO2) or 2000--thick silicon nitride (SiNx) was used as the mask material. These masks were patterned to form 2-m-wide stripe windows in the GaN <1-100> and <11-20> directions. After the patterning, GaN layers were selectively grown in the MOVPE reactor at 1050C for I hour. The flow rate was 58 mol/min for TMG and 0.18 mol/min for NH3. The V/III ratio was 3100. Scanning electron microscope images of the selectively grown GaN stripes are shown in Fig. 1. Smooth (0001) surfaces and sidewall facets were obtained with SiO2 and SiNx masks for the <11-20> and <1 100> stripes. The growth rate of the <11-20> stripes with SiO2 and SiNx masks was 2.2 m/h in the vertical direction and less than 0.1 m/h in the lateral direction. The lateral/vertical growth rate ratio of the <11-20> stripes was less than 0.05. The growth rate of the <1-100> stripes with SiO2 and SiNx masks was 1.7 m/h in the vertical direction and 3.3 /h in the lateral direction. The lateral/vertical growth rate ratio of 1.9 was obtained for the <1-100> stripes. The lateral growth of the <11-20> stripes was terminated by the (1-101) facets, whose growth rate is extremely slow1). The large lateral/vertical growth rate ratio of the <1-100> stripes was achieved by the fact that the sidewall (11-22) and (11-20) facets of the <1-100> stripes have a faster growth rate than the (1 101) facets. The <11-20> stripes had a trapezoidal cross-sectional shape1) in the cases both of the SiO2 and the SiNx masks as shown in Figs. 1(a) and (b). The cross-sectional shape of the <1-100> stripes was trapezoidal in the case of the SiO2 masks as shown in Fig. 1(c), and rectangular in the case of the SiNx masks as shown in Fig 1(d). However, in the experiments with a reduced TMG flow rate of 29 mol/min, it was trapezoidal in the case both of the SiO2 and the SiNx masks. This result suggests that the effective V/III ratio on the growing surface depends on mask materials and this ratio influenced the sidewall facets of the <1-100> stripes. 
In our experiments, a metalorganic vapor phase epitaxy (MOVPE) reactor operating at 100 Torr using trimethylgallium (TMG) and NH3 as source gases was used. As the substrates for the selective growth, GaN epitaxial layers on sapphire (0001) substrates were used. Either 3000 -thick silicon dioxide (SiO2) or 2000--thick silicon nitride (SiNx) was used as the mask material. These masks were patterned to form 2-m-wide stripe windows in the GaN <1-100> and <11-20> directions. After the patterning, GaN layers were selectively grown in the MOVPE reactor at 1050C for I hour. The flow rate was 58 mol/min for TMG and 0.18 mol/min for NH3. The V/III ratio was 3100. Scanning electron microscope images of the selectively grown GaN stripes are shown in Fig. 1. Smooth (0001) surfaces and sidewall facets were obtained with SiO2 and SiNx masks for the <11-20> and <1 100> stripes. The growth rate of the <11-20> stripes with SiO2 and SiNx masks was 2.2 m/h in the vertical direction and less than 0.1 m/h in the lateral direction. The lateral/vertical growth rate ratio of the <11-20> stripes was less than 0.05. The growth rate of the <1-100> stripes with SiO2 and SiNx masks was 1.7 m/h in the vertical direction and 3.3 /h in the lateral direction. The lateral/vertical growth rate ratio of 1.9 was obtained for the <1-100> stripes. The lateral growth of the <11-20> stripes was terminated by the (1-101) facets, whose growth rate is extremely slow1). The large lateral/vertical growth rate ratio of the <1-100> stripes was achieved by the fact that the sidewall (11-22) and (11-20) facets of the <1-100> stripes have a faster growth rate than the (1 101) facets. The <11-20> stripes had a trapezoidal cross-sectional shape1) in the cases both of the SiO2 and the SiNx masks as shown in Figs. 1(a) and (b). The cross-sectional shape of the <1-100> stripes was trapezoidal in the case of the SiO2 masks as shown in Fig. 1(c), and rectangular in the case of the SiNx masks as shown in Fig 1(d). However, in the experiments with a reduced TMG flow rate of 29 mol/min, it was trapezoidal in the case both of the SiO2 and the SiNx masks. This result suggests that the effective V/III ratio on the growing surface depends on mask materials and this ratio influenced the sidewall facets of the <1-100> stripes.

2:45 PM D8.3 
LATERAL EPITAXY FORMATION MECHANISM AND MICROSTRUCTURE OF SELECTIVELY GROWN GaN STRUCTURES. Tsvetanka S. Zheleva, Ok-Hyun Nam, Jason D. Griffin, and Robert F. Davis, Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC.

The microstructure and the mechanism of formation homoepitaxially and selectively grown GaN structures within window Si02 masks have been investigated by transmission electron micros (TEM) and scanning electron microscopy (SEM). Three types of samples and their microstructures as a function of the geometry of the mask pattern and the growth parameters have been studied: GaN pyramids, triangular stripes and rectangular stripes. The structures were produced by organometallic vapor phase epitaxy (OMVPB) for field emission studies. A GaN layer underlying the SiO2 mask provided crystallographic template for the initial vertical growth of the structures. The SiO2 film provided an amorphous stage on which lateral growth of the GaN occurred and possibly very limited compliancy in terms of atomic arrangement during the lateral growth and in accommodation of the mismatch in the coefficients of thermal expansion during cooling. The primary materials source for both the vertical and lateral growth of the structures was the vapor phase. Essentially no GaN deposited on the SiO2 because of the substantial differences in sticking coefficients of Ga and N species on GaN (s=1) and SiO2 (s0). Observations with TEM show that large areas of the laterally grown GaN exhibit four orders of magnitude reduction in dislocation density compared to the GaN films grown vertically.

Chair: Theodore D. Moustakas 
Tuesday Afternoon, December 2, 1997 
Salon F (M)

3:30 PM *D9.1 
GaN DEVICE PROCESSING. S.J. Pearton, Department of Materials Science and Engineering, University of Florida, Gainesville, FL; F. Ren, Bell Laboratories, Lucent Technologies, Murray Hill, NJ; J.C. Zolper and R.J. Shul, Sandia National Laboratories, Albuquerque, NM.

Recent progress in development of dry and wet etching techniques, implant doping and isolation, thermal processing, gate insulator technology and high-reliability contacts will be reviewed. A comprehensive study of dry etch damage and its removal, and of selective etching of one nitride relative to another, has been carried out in both ICP and ECR high density tools for Cl2, CH4/H2, ICl and BCl3 plasma chemistries. The effects of gas additive (Ar, n2, SF6, H2), rf chuck and source power, and process pressure are discussed. Improved ohmic contact resistances in FET structures are obtained by selective implantation, followed by very high temperature (>1300C) anneals in which the thermal budget is minimized and AlN encapsulation prevents GaN surface decomposition. Implant isolation is effective in GaN, AlGaN and AlInN but marginal in InGaN. Candidate gate insulators include AlN, AlON and Ga(Gs)Ox, but interface state densities on nitrides are still too high to realize MIS devices.

4:00 PM D9.2 
RECENT PROGRESS IN IMPLANTATION AND ANNEALING OF GaN AND AlGaN. J.C. Zolper, J. Han, S.B. Van Deusen, R. Biefeld, M.H. Crawford, Sandia National Laboratories, Albuquerque, NM; J. Jun, T. Suski, J.M. Baranowski, High Pressure Reserach Center, Polish Academy of Sciences, Warsaw, POLAND; S.J. Pearton, Univ of Florida, Gainesville, FL.

Heterostructure modulation doped transistors (MODFETs) based on AlGaN/GaN structures have demonstrated impressive DC and microwave performance often despite high transistor access resistance. One approach to reducing the access resistance is to use selective area Si-implantation. While several reports exist on Si-implantation in GaN, little work has been done on implantation in AlGaN. In addition, more information on the annealing of implantation damage in GaN is needed to optimize its use in FETs and thyristors. We reported the electrical and structural properties of Si-implanted Al0.15Ga0.85N based on Hall and Rutherford Backscattcring (RBS) spectra, respectively. Al0.15Ga0.85N shows less damage accumulation than GaN for a room temperature Si-implant dose of 5x1015 cm-2 based on the minimum channeling yield (26% for AlGaN as compared to 34% for GaN), however, as with GaN, this damage is difficult to remove by thermal annealing at 1100 C. We also report on high pressure (up to 15 kbar) and high temperature (up to 1500 C) annealing for Si-implanted GaN. At 1250 C 50% electrical activation is achieved which increases to 90% at 1500C. The photoluminescence of these samples is also restored, or even enhanced, by the high temperature treatments.

4:15 PM D9.3 
LASER-PROCESSING FOR PATTERNED AND FREE-STANDING NITRIDE FILMS. M. K. Kelly, O. Ambacher, R. Dimitrov, H. Angerer, R. Handschuh, and M. Stutzmann, Walter Schottky Institut, TU-Munich, Garching, GERMANY.

Films of GaN and related materials can be processed by methods that invoke thermal decomposition, induced by intense illumination with a pulsed laser. At elevated temperatures, the nitride semiconductors undergo decomposition, with the effusion of nitrogen gas. Residual gallium can be easily removed by chemical means. The use of nanosecond-range pulses allows a deposition of heat that is highly localized, enabling high-resolution structuring. Up to 80 nm of GaN have been removed with single pulses of 355 nm light, and gratings with 250 nm period have been fabricated by illumination with an interference pattern. This offers a favorable alternative to chemical etching. Internal interfaces can also be illuminated and disintegrated, by the choice of a wavelength of light that is not absorbed before it reaches the interface. In this way, we have produced free-standing GaN films, removed from their sapphire growth substrate, as well as isolated mesa structures remaining on the substrate, again with a single laser pulse. We have also used this method to expose the back side of pn-diode structures for electrical contact. We will present details and results of the processing as well as characterization of the resulting films.

4:30 PM D9.4 
ELECTRON MOBILITY IN n-GaN FILMS. H. M. Ng, D. Doppalapudi, R. Singh, T.D. Moustakas, Center for Photonics Research and Electrical and Computer Engineering Dept., Boston University, Boston, MA.

This paper addresses the dependence of the transverse electron mobility on the net carrier concentration in GaN films doped n-type with silicon. The films were produced by plasma-assisted MBE and their carrier concentration was varied systematically from 1015 to 1020 cm-3. At high carrier concentration the electron mobility increases monotonically with a decrease in carrier concentration, while at low carrier concentration the electron mobility decreases monotonically with the decrease in carrier concentration. Depending on the growth conditions, we find that the electron mobility vs (ND-NA) data fall into multiple bell-shaped curves with the same high carrier concentration branch. This odd functional behavior was not observed in other III-V compounds and is attributed to the high concentration of threading dislocations in GaN. Specifically the high carrier concentration branch is consistent with scattering by ionized impurities while the low carrier concentration branch is attributed to scattering by charged dislocation lines. The data are in qualitative agreement with a theory developed by Read1 and Podor2 regarding scattering by charged dislocations.

4:45 PM D9.5 
POSSIBLE BIEXCITONIC EMISSION FROM GaN QUANTUM DOTS. Satoru Tanaka, Shintaro Nomura, Yoshinobu Aoyagi, The Institute of Physical and Chemical Research (RIKEN), Saitama, JAPAN; Yukio Narukawa, Yoichi Kawakami, Shizuo Fujita, Shigeo Fujita, Department of Electronic Engineering, Kyoto University, Kyoto, JAPAN.

Excitons and biexcitons in quantum dots (QDs) are of great concern in optical physics and device applications. It is expected that excitons of biexcitons, whose binding energies are enhanced due to quantum size effect, can improve room temperature laser diode (LD) performance. However, the presence of biexcitons has only been observed in thick GaN films Thus, biexcitons with large binding energies in QDs may provide a new trend for future LDs and also contribute to understand low dimensional optical phenomena. In this study, may body effects of excitons in GaN QDs, possibly due to biexcitons, are reported. Detailed PL studies using time-resolved(TR) PL and excitation power dependence are shown. A typical sample structure consisted of an Al0.12Ga0.88N capping layer, GaN QDs (25 nm width, 9 nm height and density of 1011 cm-2), and Al0.12Ga0.88N/Al0.20Ga0.80N buffer layers grown on 6H-SiC. [1] The time integrated Pl spectra indicated a superlinear behavior of the peak (Exx which appeared at 35 meV lower energy of the QD excitonic peak as the excitation intensity was increased. The TRPL spectra clearly showed that the Exx peak appeared between 0 ps and 100 ps. The transient response curve of the Exx showed a double exponential characteristic, with life times =42 ps, =230 ps. The results of two photon absorption measurements together with PL excitation spectroscopy are also reported.

Tuesday Evening, December 2, 1997 
8:00 P.M. 
Salons E-G (M)

A COMPREHENSIVE X-RAY CHARACTERIZATION OF THE REAL STRUCTURE OF GAN EPITAXIAL THIN FILMS. Jurgen Blaesing, Jurgen Christen, Otto-von-Guericke-Univ, Inst fúr Experimentelle Physik, Magdeburg, GERMANY; Alois Krost, Andre Strittmatter, Technische Univ, Inst fúr Festkúrperphysik, Berlin, GERMANY; Jurgen Off, Ferdinand Scholz, Volker Hurle, Univ Stuttgart, Physikalisches Inst, Stuttgart, GERMANY; David Lockwood, Jim Webb, NRC Ottawa, Inst for Microstructural Science, Ottawa, CANADA.

We present a comprehensive and comparative study onto the structural properties of hexagonal epitaxial GaN layers. The layers were deposited by different methods (low pressure MOCVD and Magnetron Sputter Epitaxie) on different substrates (SiC,Si,sapphire) and buffer layers (AlAs,AlN,AlGaN). We used following X-Ray Fine Structure investigation techniques : - High Resolution X-Ray Diffraction (HRXRD) for symmetric and asymmetric reflections - X-Ray Powder Diffraction (XRD) in grazing incidence,constant resolution and Bragg-Brentano geometry - X-Ray Texture Analysis (XTA) on symmetric and asymmetric reflections - X-Ray Reflectometry (XRR) with specular and nonspecular measurements - Small Angle X-ray Scattering (SAXS) in the grazing incidence and transmission geometry. The influence of the divers substrats and deposition methods onto the epitaxial growth leads to distinct differences in the corresponding X-ray spectra.


In this work we study imperfections of monocrystals with help of the modified method small angle scattering of X-rays (S-AS). The upper view web of GaN domains looked like network of 60120 degrees and periodicity of 5000060000nm. The equal crystallography directions on the (0001) plane of the substrate 6H SiC continued in similar directions of GaN epitaxial film. GaN films deposited on -Al2O3 substrate had no regular upper view. We carried out a comprehensive X-ray diffraction study examining in-plane structure of GaN films grown on (000l) SiC and -Al2O3. We study domain allocation model in GaN crystals. We measured domain parameters of the hexagonal prisms (columns in transmission electron microscopy) in layers GaN, existed on substrates silicon carbide. We have offered cluster-crystal model of domain distribution in gallium nitride. The estimated shape of GaN/SiC domains is hexagonal prism 200-300nm in length (perpendicularly surface) and 50nm along the surface. While the layers GaN, brought up on sapphire had variations of density distributed in gas model. The size of these grains is about 200 nm. We looked for distorted interface between SiC and GaN too. There was triclinic, not hexagonal, distortion of hexagonal crystal lattice of epitaxial GaN thick films.

A COMBINED TEM/RHEED, SEM/CL STUDY OF EPITAXIAL GaN. Paul D. Brown, David M. Tricker, Colin J. Humphreys, Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, UNITED KINGDOM; T. S. Cheng, C.T. Foxon, Department of Physics, University of Nottingham, University Park, Nottingham, UNITED KINGDOM; David Evans, DTC, Maidenhead, UNITED KINGDOM; Simon Galloway, Judith Brock, Oxford Instruments, Eynsham, Witney, Oxon, UNITED KINGDOM.

Although TEM characterisation is immensely valuable during the development of a new epitaxial growth technique or the demonstration of a novel materials system, the time delays introduced by sample preparation can limit its general applicability if growth problems need to be solved quickly. Hence, we have modified a Philips 400ST electron microscope to accept a Reflection High Energy Electron Diffraction (RHEED) stage for the rapid assessment of the structural integrity of deposited thin films. Centimetre square sections from as-grown wafers mounted vertically approximately 10cm above the TEM phosphor screen provide diffraction information for the rapid assessment of the near-surface microstructure which can then be correlated with the growth conditions used. Complimentary information gained from conventional TEM may be obtained to examine the nature of the fine scale defect microstructure if then required. Examples shown include that of cubic inclusions within hexagonal GaN grown by MBE. RHEED observations confirm the nature of the phase of the deposit and provide a useful estimate of the volume fraction of the included cubic phase. Complimentary TEM images provide evidence for the form and distribution of such inclusions and nicely correlates with distributed cathodoluminescence emission from TEM foils examined in SEM.

CRYSTAL DEFECTS IN GaN ON (0001) SAPPHIRE. Matthew T. Johnson, Jeffrey Farrer, C.Barry Carter, Dept of Chemical Engineering and Materials Science, University of Minnesota, MN.

Defect structures in GaN thin films grown on (0001) sapphire have been studied using a combination of different transmission electron microscopy (TEM) techniques. Two fundamentally different types of defects are found in these films. Planar faults which lie on planes perpendicular to the growth surface are common. In some regions of the films, other planar defects are present which run parallel to the surface of the substrate. The terminology used to describe these different defects varies quite widely in the literature including combinations of antiphase (inversion) domain boundaries and stacking faults. The second type of defect is generally referred to as a threading dislocation since many such dislocations thread through the whole thickness of the film. Dislocations with different Burgers vectors have been identified in this work and in previous studies; these dislocations usually have a component of their Burgers vector lying normal to the (0001) plane. The overall defect structures in these films have been characterized using conventional bright-field and dark-field imaging. The detailed structures of the individual defects have been examined using weak-beam microscopy both in plan view and in cross section. This paper will illustrate the different types of defects, both planar and linear, compare them to defects which have been characterized more thoroughly in related materials, and discuss the nomenclature of the different defect configurations.

XTEM CHARACTERIZATION OF GaN/AlGaN QUANTUM WELL SEPARATE CONFINEMENT HETEROSTRUCTURES ON 4H-SiC. Wen Chen, Joyce E. Palmer, Neal G. Anderson, University of Massachusetts, Dept. of ECE, Amherst, MA; Joan Redwing, J.S. Flynn, G.M. Smith, Epitronics, Danbury, CT.

GaN/AlGaN Quantum Well Separate Confinement Heterostructures are grown by OMVPE on 4H-SiC 0001 substrates with an AlN buffer layer. Each sample has a 1 m thick Al0.17Ga0.83N lower cladding layer, a 75 nm active layer consisting of GaN/Al0.07Ga0.93N QW(s) with total well thickness of 15nm and a 200nm Al0.17Ga0.83N upper cladding layer. Three samples were grown with 1, 3 and 6 QW layers respectively. Selected Area Diffraction shows that all these samples' epi-layers are aligned very well, the C directions and [100] directions are parallel to one another. The QW morphology shows some nonuniformity, especially in MQW samples, in which the thickness of the very thin layers may vary by a factor of 2 or 3 near a crystalline defect. XTEM observed 3 kinds of behavior for threading dislocations near the active layer. 1) passing through without bending; 2) passing through with some bending in the horizontal direction; 3) terminating in the active layer. Short defects running parallel to the sample surface are also observed. The threading dislocations appear to interact with MQW active layer more than the SQW active layer.

COMPARATIVE TEM STUDY OF TYPICAL DEFECTS IN III-NITRIDE THIN FILMS AND THEIR ALLOYS. Katharine Dovidenko, S. Oktyabrsky, J. Narayan, Dept. of Mat. Sci. & Eng., North Carolina State University, Raleigh, NC; M. Razeghi, Center for Quantum Devices, Northwestern University, Evanston, IL.

Structural defects in thin layers of AlN, GaN and GaAlN grown on (0001) sapphire by MOCVD were studied using various transmission electron microscopy techniques: high resolution, diffraction contrast, multiple dark field imaging, convergent beam electron diffraction, and atomic image simulation. Comparison between the typical defects in AlN and GaN-GaAlN films was made. Predominant defects in AlN films were found to be inverted domain boundaries and threading dislocations mostly with 1/3[110] Burgers vector with the density of about 1010 cm-2. In GaN-GaAlN films the variety of defects were found: inverted domain boundaries, threading dislocations of both edge (b=l/3[110]) and screw/mixed character, tilt sub-grain boundaries, and stacking faults. The dependence of the type of predominant defects as a function of growth conditions was established. The optical and electrical properties of the films were also characterized. In the device-quality GaN/GaAlN films the main defects were found to be edge dislocations with 1/3[110] Burgers vector associated with tilt sub-grain boundaries. The density of these edge dislocations was very high: 1010 - 1011 cm-2, while the density of screw/mixed dislocations in these films was only 106 - 107 cm-2. We envisage that edge dislocations are relatively electrically inactive and screw/mixed dislocations as well as inverted domain boundaries are the defects which can serve as recombination centers for carriers thus deteriorating electrical properties of the devices.

DISLOCATION DISTRIBUTION AND CELLULAR SUBSTRUCTURE OF GAN FILMS DEPOSITED ON SAPPHIRE BY HVPE AND MOVPE. K.A. Dunn and S.E. Babcock, Materials Science Program, University of Wisconsin-Madison, Madison, WI; R. Vaudo, V. Phanse, and J. Redwing, Advanced Technology Materials, Inc., Danbury, CT.

Transmission electron microscopy (TEM) in cross-sectional and plan view was used to characterize the microstructure in GaN thin films. The 11m HVPE film was deposited directly on the sapphire substrate, whereas the 8m MOVPE film was deposited on a 15 nm buffer layer of AlN on sapphire. Both films have dislocation densities 10-9cm-2 in the top layer. In the HPVE film this is almost exclusively threading dislocations, but the MOVPE sample also contained an appreciable number of dislocations lying in the basal plane. The HVPE film, however, contained more dislocations with Burger's vectors lying in the basal plane in all regions of the sample. The microstructure of each film is dominated by a subgrain structure of slightly misoriented cells. In the MOVPE specimen, approximately 90% of the threading dislocations are associated with cells walls, whereas only approximately 75% of the dislocations in the HVPE specimen are associated with cell walls. Both the HVPE and MOVPE samples experience 40% coarsening of the cells through the thickness of the film, although the cells of the MOVPE sample were 75% smaller than those in the HVPE sample. The cells walls are better defined in the MOVPE sample, and the average dislocation spacing in the walls appears to be 50% smaller than in the HVPE sample.

TRANSMISSION ELECTRON MICROSCOPY STUDY OF ROOM TEMPERATURE LASING EPITAXIAL ZnO FILMS ON SAPPHIRE. N.Wang, K.K.Fung, P.Yu, Z.K.Tang, G.K.L.Wong, Hong Kong Univ of Science and Technology, Dept of Physics, HONG KONG; M.Kawasaki, A.Ohtomo, H.Koinuma, Tokyo Inst. of Technology, Research Lab of Engineering Materials, JAPAN; Y.Segawa, RIKEN, Photodynamics Research Center, Sendai, JAPAN.

Current interest in blue-ultraviolet semiconductor diode lasers has mainly been focused on ZnSe- and GaN-based heterostructures. Recently, ultraviolet laser emission at room temperature from epitaxial ZnO films on sapphire grown by laser molecular beam epitaxy has been observed in our laboratory. ZnO, a wide bandgap (3.37 eV) semiconductor with a wurtzite structure (a=3.25 A, c=5.21 A) similar to that of GaN, has been targeted as a buffer layer for the growth of GaN on sapphire. X-ray diffraction and atomic force microscopy have shown that high quality single crystalline epitaxial films are composed of hexagonal columnar grains along the c-axis. The optical gain has been shown to be of excitonic nature. Transmission electron microscopy (TEM) study has confirmed the epitaxial orientation relationship:(0001)ZnO//(0001)sapphire, [1120]ZnO//[1010]sapphire. It has also shown that there is a high density of dislocations in the ZnO films ( 10 to the power 10 per cm square). Most of the dislocations are threading dislocations perpendicular to the substrate. These threading dislocations form very low-angle grain boundaries separating neighbouring columnar grains. The microstructure and crystal defects of ZnO films are strikingly similar to those of GaN films on sapphire. We have also found that laser emission only takes place in films with very low-angle boundary grains. There is no laser emission when the angle of misorientation reaches a few degrees.

AMORPHOUS DOMAINS IN GaN LAYERS GROWN ON 6H-SiC BY MBE. Phillippe Vermaut, Valérie Potin, Pierre Ruterana, Alain Hairie and Gérard Nouet, Laboratoire d'Études et de Recherches sur le Matériaux, UPRESA CNRS 6004, Institut des Sciences de la Matiére et du Rayonnement, Caen, FRANCE; Arnels Salvador and Harris Morkoc, University of Illinois-Urbana, Coordinated Science Laboratory, Urbana, Il.

Due to a lake of substrates, GaN has to be heteroepitaxially grown. The deposited layers contain a high density of defects: threading dislocations, stacking faults. Some of them, grown on sapphire substrates, also contain inversion domains and voids which have been identified as nanopipes or open core screw dislocations. 
In epilayers grown on the (0001)si surface of 6H-SiC substrates by Electron Cyclotron Resonance assisted Molecular Beam Epitaxy, a high density of similar defects with 100 facets have been observed close to the deposit/substrate interfacial region. They have been identified by conventional transmission electron microscopy and high resolution scanning electron microscopy, as amorphous domains. In addition, they are not systematically linked to screw dislocations, meaning that they are not open core dislocations. 
From the observations of the interface area, the polarity and the three dimensional growth mode of the layers, the origin of these defects is discussed.

ATOMIC STRUCTURES OF LOW-ANGLE AND HIGH-ANGLE GRAIN BOUNDARIES IN AIN GROWN OVER (0001) SAPPHIRE. Valérie Potin, Pierre Ruterana, Alain Hairie, Gérard Nouet, Laboratoire d'Études et de Recherches sur les Matériaux, UPRESA CNRS 6004, Institut des Sciences de la Matiére et du Rayonnement, Caen, FRANCE.

AIN buffer layer grown on (0001) sapphire substrate by molecular beam epitaxy has been analyzed by high resolution electron microscopy. This layer may contain some islands which have been identified as AIN crystals with their [0001] axis parallel to the [0001] axis of the AIN layer. These grains are rotated around this axis and the rotation angle is in the range 0-20. According to the value of the rotation angle, these boundaries can be classified as low or high-angle grain boundaries. Some of these high angle grain boundaries correspond to special misorientation which can be described in terms of coincidence site lattice. For one of these grains, the rotation angle is equal to 18.94 that is very close to the coincidence orientation, sigma = 31 with a rotation angle equal to 17.89. 
Thanks to dichromatic complex, a CSL unit cell containing two atoms is defined and the atomic structure of the grain boundary plane {740} may be considered. It is based on different atomic rings: the common six atom ring of the hexagonal structure, a five-seven atom ring and an eight atom ring. This latter structural unit contains one dangling bond. 
Simulation of this array of structural units has been carried out by means of EMS software and a good agreement has been obtained between the experimental and calculated images.

RHEED-TRAXS AS AN IN-SITU MONITORING TOOL FOR GROWTH AND DECOMPOSITION OF ULTRA-THIN GaInN. Atsuya Ito, Hiromitsu Sakai, Takaki Yasuda, Tetsuya Takeuchi, Hiroshi Amano, Isamu Akasaki, Meijo Univ, Dept of Electrical and Electronic Engineering, Nagoya, JAPAN.

Ultra-thin GaInN-based quantum well structure is essential for the realizatino of high-performance devices. For the fabrication of well-controlled heterostructure based on GaInN, understanding the dynamics of growth and thermal decomposition of GaInN is important. In this study, we performed in-situ RHEED-TRAXS (Total-reflection angle X-ray spectroscopy) observation during growth and decomposition of ultra-thin GaInN layers by MBE. It is found that In-TRAXS signal increass linearly with GaInN thickness. It is also found that GaInN decomposes gradually with time at the growth temperature, for example, it took about 80min. for Ga0.75In0.25N about 3.0nm thick to decompose totally. We will also discuss the temperature and compositional dependence of the decomposition of ultra-thin GaInN layers.

STRAIN ANALYSIS OF MOCVD-GROWN GaN FILMS. Joachim Kruger, Christian Kisielowski, Dorina Corlatan, Yann Peyrot, Michael Rubin, and Eicke R. Weber, Lawrence Berkeley National Laboratory, Materials Science Division, Berkeley, CA; Brian T. McDermott, R. Pittman, and E.R. Gertner; Rockwell International Science Center, Thousand Oaks, CA.

The lattice mismatch, the difference in the thermal expansion coefficients as well as the incorporation of point defects into thin GaN layers grown on sapphire can cause large amounts of strain. Resulting stresses may reach values up to 1 GPa. Photoluminescence (PL) experiments at helium temperatures can be used to quantitatively assess the stress in the epilayer. In previous studies we established that an active utilization of hydrostatic and biaxial strain components allows to design MBE-grown GaN layers of desired optical, electrical and structural properties. In this report we prove by combination of PL and X-ray diffraction measurements that the application of the same methodologies controls the amount of stress and strain in MOCVD grown GaN:Si as they do in MBE-grown GaN. Therefore, we conclude that details of the applied growth method are less important than the common physical concept. Consequently, we expect that the strain induced by doping during a MOCVD growth process can be compensated in a similar manner than in the MBE growth process [1]. Thereby, film cracking at high doping levels could be avoided. The difference in growth temperature between MOCVD and MBE (1050C vs. 725C) can account for the different absolute stress values observed in MOCVD and in MBE grown samples.

COMPARATIVE STUDIES OF THE SURFACE OF GaN (0001) THIN FILMS WITH LEEM*. J.B. Maxson, B. Barnes, M.G. Lagally, Dept of Materials Science and Engineering, Univ of Wisconsin - Madison, Madison, WI.

Recent progress in the growth of Group III-nitride semiconductors has generated interest in methods to optimize film quality and innovative methods of characterizing the surface or growth front. Low-energy electron microscopy (LEEM) offers the possibility of both: investigation of the growth process with high spatial resolution in real time at actual growth temperatures. Because of the unknown surface properties of these wide-bandgap materials it is first necessary to establish that the surface can be cleaned and imaged. We report initial LEEM measurements of the surface morphology of GaN (0001) films grown on sapphire by halide vapor phase epitaxy (HVPE) and by metalorganic vapor phase epitaxy (MOVPE) (approximately 10 m and 1 m thick respectively). We compare differences in the two types of films and follow surface changes in-situ as we change the sample temperature. For example, on the as-prepared MOVPE surface, we observe a LEEM image with a ''cross-hatch'' pattern on the scale of 0.5 m, reminiscent of the stress-relief pattern seen in relaxed SiGe films. No LEED pattern is observed, however, indicating that the surface is contaminated. The cross-hatch pattern disappears with heating. We do not at present fully understand its causes or behavior. We monitor heating of the surfaces (up to 1100 C) with LEEM in efforts to clean them. After heating we observe a 1x1 LEED pattern, but the LEEM image indicates a rough stepped surface morphology. We compare our results with AFM and STM images of the surfaces of the films. *Research supported by ONR

MICROSTRUCTURE OF InGaN QUANTUM WELLS. F. A. Ponce, Xerox Palo Alto Research Center, Palo Alto, CA; D. Cherns, H. H. Wills Physics Laboratory, University of Bristol, Bristol, UNITED KINGDOM; W. Goetz and R. S. Kern, Hewlett Packard Optoelectronics Division, San Jose, CA.

The microstructure of InxGa1-xN single quantum wells has been studied by diffraction-contrast and high-resolution transmission electron microscopy. Lattice images and dark-field images show that the quantum wells (for x= 0.28 and 0.52) are inhomogeneous in character, exhibiting regions with interfacial misfit dislocations and clusters ranging from 3 to 10 nm in diameter, interspersed with other regions showing only misfit dislocations. Evidence is presented suggesting the extent of clustering depends on the exact orientation of the growth surface which is related to the columnar nature of the GaN epitaxy.

CORE STRUCTURE OF A THREADING DISLOCATIONS IN GAN/SIC LAYERS GROWN BY MBE. Pierre Ruterana, Philippe Vermaut, Valerie Potin, Gerard Nouet, Laboratoire d'Etudes et de Recherches sur les Materiaux, UPRESA 6004 CNRS, Institut de la Matiere et du Rayonnement, Caen, FRANCE.

The best layers of GaN grown on SiC or sapphire contain very high densities of threading dislocations, which do not seem to exhibit a critically important electrical activity. Actually, the diodes emitting in the blue range have demonstrated high stability. For the time being, the laser diodes are made on sapphire and their life time is of less than 50 hours. It seems that the electrical activity of these dislocations may change with time. In this work, the atomic structure of the a threading dislocation was analyzed. A study was done on the dislocation inside the bulk of the layer and inside sub grain boundaries which are mostly found either in bad quality epilayers, or in the buffer layers. In addition, a tentative analysis of the contrast was made in order to discriminate between a and a+c dislocations.

REDUCTION OF THREADING DISLOCATION DENSITY IN GaN FILMS BY SELECTIVE EPITAXIAL GROWTH. Akira Sakai, NEC Corporation, Fundamental Research Labs., Tsukuba, JAPAN; Haruo Sunakawa, Akitaka Kimura and Akira Usui, NEC Corporation, Optoelectronics Research Labs., Tsukuba, JAPAN.

Controlling the defect density in epitaxial GaN films is one of the crucial factors in successfully applying these films to electrical and optical devices. In particular, reducing the threading dislocation density is essential to improving the performance of GaN-based devices. In this work, we have successfully demonstrated the reduction of the threading dislocation density in GaN films grown by selective homo-epitaxy.

8:30 AM *D11.1 
GaN CRYSTALS: GROWTH AND DOPING UNDER PRESSURE. I. Grzegory, High Pressure Research Center, Polish Academy of Sciences, Warsaw, POLAND.

The recent progress in high pressure crystallization of GaN will be reported. Special attention will be paid on the mass transport and crystallization mechanisms occurring in large volume pressure reactors where crystals of stable morphology, with dimensions exceeding I cm are grown. The results of the growth from the solutions of N in pare Ga and in its alloys with II group metals (Mg, Ca, Zn) will be discussed. It will be shown that the growth mechanisms and the physical properties of the crystals depend on the composition of the growth solution In particular, high resistivity (104-10cm) GaN crystals of improved structural quality can be grown from solutions containing Mg. Physical properties of GaN crystals grown without and with the intentional doping will be compared. The preparation of surfaces of GaN substrates for homoepitaxy by mechanical and mechano-chemical polishing will be discussed. It will be shown that atomically flat thermally stable surfaces are possible to obtain by the applied procedures. Some most interesting results concerning homoepitaxial growth by MOCVD and MBE will be shortly reviewed. In particular, it will be shown that perfectly matched (strain free) GaN layers can be deposited on the highly resistive GaN:Mg substrates.

9:00 AM *D11.2 
GROWTH OF GAN FROM A LIQUID PHASE. V. Ivantsov, PhysTech WBG Research Group, Ioffe Institute, St. Petersburg, RUSSIA.

Main principles of bulk and epitaxial growth of GaN from a liquid phase will be considered on the base of equilibrium and non-equilibrium thermodynamics. GaN solution growth at a reduced nitrogen pressure is discussed in comparison to the high pressure growth. Novel results on the spontaneous and seeded GaN growth at a reduced pressure will be presented. Composition and impurity content of bulk GaN crystals and LPE grown layers were investigated using AES, SIMS and XPS methods. Structure of the grown crystals and layers was studied by x-ray diffraction and SEM. Physical properties of GaN crystals grown from melt-solution using different technological approaches will be compared.

9:30 AM D11.3 
PHYSICAL PROPERTIES OF GaN CRYSTALS GROWN AT HIGH N2 PRESSURE FROM THE SOLUTIONS IN Ga ALLOYED WITH Ca. S. Porowski, M. Bockowski, B. Lucznik, M. Wroblewski, I. Grzegory, T. Suski, M. Leszczynski, H. Teisseyre and E. Litwin-Staszerska, High Pressure Research Center, Polish Academy of Sciences, Warsaw, POLAND.

High quality GaN bulk single crystals have been obtained from the solutions in gallium alloyed with calcium, at N2 pressure of 12-20kbar. It is shown that calcium added to gallium at the amount of 0. 2-0.5 at.% influences optical and structural properties of GaN. In particular the character of the optical absorption and photoluminescence suggests substitutional incorporation of the impurity in the crystal lattice. However, in contrast to magnesium which added to the gallium at the same amount leads to high resistivity fully compensated crystals, calcium does not decrease significantly the concentration of free electrons in GaN. Therefore the amount of Ca in the solution was increased up to higher concentrations. The influence of Ca on the growth and properties of GaN crystals will be reported in this paper

9:45 AM D11.4 
PROPERTIES OF FREE-STANDING GaN BULK CRYSTALS GROWN BY HVPE. Yu. Melnik, A. Nikolaev, I. Nikitina, K. Vassilevski, and V. Dmitriev, PhysTech WBG Research Group, Ioffe Institute, St. Petersburg, RUSSIA.

Today, thick GaN layers grown by hydride vapor phase epitaxy (HVPE) are promising substrates for subsequent epitaxy of device structures. These layers have to be released from the substrate (SiC or sapphire) to realize an actually homoepitaxial process. We report on free-standing GaN bulk crystals grown by HVPE and their use as substrates for GaN homoepitaxy. Thick GaN layers were grown on SiC substrates at atmospheric pressure and the temperature about 1000ƒC. Free-standing GaN crystals ( 100 m thick) were obtained after removal of the substrate by reactive ion etching. The maximum size of these crystals (currently 7*6 was limited by cracks formed during cool down. The absence of residual strains in these crystals was proved by x-ray diffraction and Raman spectroscopy. The FWHM of omega scan (0002) x-ray rocking curve ranged from 170 to 350 arcsec. The lattice constants c= 5.18500 and a= 3.1890 were measured for both sides of a GaN crystal. Obtained GaN crystals were used as a substrates for growth of GaN layers. Homoepitaxy was conducted by the same growth method - HVPE. The properties of homoepitaxial layers will be presented. The mechanism of crack formation in thick GaN layers grown by HVPE on SiC substrates will be discussed. Possible ways to increase the size of free-standing GaN crystals will be proposed.

Chair: Hubert Lakner 
Wednesday Morning, December 3, 1997 
Salon F (M)

10:30 AM *D12.1 
LOCALIZED EXCITONS IN InGaN. Shigefusa Chichibu, Science University of Tokyo, Faculty of Science and Technology, Chiba, JAPAN; Takahiro Deguchi, Takayuki Sota, Waseda University, Department of Electrical, Electronics, and Computer Engineering, Tokyo, JAPAN; Shuji Nakamura, Nichia Chemical Industries Ltd., Department of Research and Development, Tokushima, JAPAN.

InxGa1-xN alloys are attracting special interest because of its potential for the fabrication of LEDs and LDs operating in the red to UV energy region. All of bulk 3D InGaN/AlGaN DH LEDs [1], SQW LEDs [1], purplish-blue MQW LDs [1,2], and SQW UV emitters [3] have InGaN active layers. The growth of InxGa1-xN having a large x has been difficult since InGaN essentially suffers from a kinetic compositional separation. However, it is necessary to investigate excitonic contribution on the EL from InGaN practical devices since the exciton Bohr radius of nitride semiconductors is very small compared to well-known III-V compounds. In this study, emission properties of light emitting devices consisting of 3D bulk or QW structure InGaN active layers were investigated using the devices themselves and their wafers. From the results of electroluminescence (EL), modulated-electro-photoabsorption (EA), photoluminescence (PL), photoluminescence excitation (PLE), photovoltage (PV), and time-resolved PL (TR-PL) measurements on DH and SQW LEDs and MQW LD structures, their static EL peak was assigned as being due to the recombination of excitons localized at certain potential minima in the QW[4] or InN-rich self-formed mesoscopic regions even in 3D epilayers [5]. TEM and AFM results will be presented to show their abrupt interfaces. Finally, the blueshift of the EL peak in QW devices caused by the increase of the driving current was explained as combined effects of the quantum-confinement Stark effect and band filling of the localized compositional tail states [4] by excitons.

11:00 AM D12.2 
CATHODOLUMINESCENCE STUDIES OF InxGa1-xN QUANTUM WELLS. F. A. Ponce, Xerox Palo Alto Research Center, Palo Alto, CA; S. A. Galloway, Oxford Instruments, Oxon, UNITED KINGDOM; W. Goetz and R. S. Kern, Hewlett Packard Optoelectronics Division, San Jose, CA.

Low temperature cathodoluminescence is used to investigate the spatial characteristics of light emission in InxGa1-xN single quantum wells and to establish its correlation with microstructure. High spatial resolution, narrow band pass imaging shows the luminescence to be strongly inhomogeneous in wavelength as well as in intensity on a sub-micron scale. Cathodoluminescence spectra correlates favorably with photoluminescence spectra. However, when spectra is recorded from different areas in spot mode, the quantum emission varies significantly in wavelength. The observed variations are consistent with composition inhomogeneities in the quantum well.

11:15 AM D12.3 
PHASE SEPARATION IN MOCVD GROWN InGaN FILMS. N. A. El-Masry, E. L. Piner, J. C. Roberts*, S. X. Liu, M. K. Behbehani, and S. M. Bedair*, Materials Science and Engineering Department, North Carolina State University, Raleigh, NC;* Electrical and Computer Engineering Department, North Carolina State University, Raleigh, NC.

InGaN ternary alloy plays an important role in optical devices in spite of the fact that there is still so much to understand in this ternary alloy system. For high concentration of indium in the InGaN films, the high lattice mismatch can lead to phase separation in film. Stringfellow et al. predicted the phase separation to occur in InxGa1-xN alloys for x>0.06. Moustakas et al. used x-ray diffraction and optical absorption to indicate the presence of a secondary phase in MBE grown InxGa1 xN for x>0.3. 
We report the first observation of phase separation in MOCVD bulk films using cross sectional transmission electron microscopy (TEM) and x-ray diffraction. Bulk InxGa1-xN films, 0.5 m thick, were grown in the temperature range 680-800C. The value of x in the films ranged from 0.05 to 0.43. The variation in film composition was achieved by varying the growth temperature. For example, samples grown at 680C yielded composition of x = 0.43. The phase separation of these InGaN bulk films were observed to occur for compositions above x = 0.20. The microstructure of the phase separated bulk films had a polycrystalline appearance However, the diffraction pattern and HRTEM of these films indicated a two-phase single crystalline film with compositions measured to be 0.43 and 0.24 in a film grown at 730C. The two-phase compositions were indicated by splitting in the diffraction spots of these films.

11:30 AM D12.4 
PHASE SEPARATION IN InGaN/GaN MULTIPLE QUANTUM WELLS. M.D. McCluskey, L.T. Romano, D.P. Bour, C.L. Chua, B.S. Krusor, N.M. Johnson, Xerox PARC, Palo Alto, CA; K.M. Yu, LBNL, Berkeley, CA.

Evidence is presented for phase separation in In0.27Ga0.73N multiple quantum wells (MQWs). The as-grown material has an optical absorption threshold at 2.95 eV (418 nm) which corresponds to an N=1 valence-to-conduction band transition of the InGaN quantum wells. After sputtering a 120 nm SiN cap onto the surface, the material was sequentially annealed in 1 min steps at a temperature of 1100C. After a total annealing time of 4 min, the absorption threshold at 2.95 eV is completely replaced by a broad peak at 2.65 eV (465 nm). We attribute this broad peak to In-rich regions in the InGaN quantum wells. The same results were obtained with a GaN proximity cap instead of a SiN cap. X-ray diffraction (XRD) measurements show a shift in the zeroth order diffraction peak toward GaN, consistent with the formation of an In-poor phase in the InGaN wells. For samples annealed at 950C, phase separation occurs only after 40 hr. Since the MQW active region of a laser diode heterostructure never experiences growth temperatures above 900C, our results suggest that negligible phase separation occurs as a result of annealing during growth.

11:45 AM D12.5 

It has been known for quite some time that strained GaN-based layers should be subject to strong piezoelectric fields. However, the consequences for the optical properties of such structures have not been clearly observed up to now. We have studied the static and time-resolved photoluminescence of GaInN/GaN double heterostructures and quantum wells with various layer thicknesses and compositions. In the static spectra of the thick layers we observe two emission lines, whereas there is only a single line for thin layers. In addition to the usual quantum confinement, we observe a red shift of the lower energy line with increasing layer thickness. This is accompanied by a dramatic increase in decay time by more than 4 orders of magnitude. Moreover, time-resolved spectra reveal a red-shift of the emission line for long delay times. Based on a quantitative comparison with a model calculation, we show that these effects are due to carrier separation due to the strong piezoelectric fields in coherently strained layers. The second higher energy line is attributed to strain-relaxed regions in the thick heterostructures. The consequences of the piezoelectric fields for devices like LED's and lasers will be discussed.

Chair: Stephen J. Pearton 
Wednesday Afternoon, December 3, 1997 
Salon F (M)

1:30 PM *D13.1 
MBE GROWTH OF III-V NITRIDES FOR DEVICE APPLICATIONS. M.A.L. Johnson, J.D. Brown, N.A. El-Masry, J.W. Cook Jr. and J.F. Schetzina, North Carolina State University, Raleigh, NC; M. Leonard, H.S. Kong, and J.A. Edmond, Cree Research Inc., Durham, NC.

Growth of III-V nitrides by molecular beam epitaxy (MBE) is being studied using rf nitrogen plasma sources from three different vendors. Film growth rates at 800C approaching 1 m/hr have been achieved using an EPI rf plasma source. Mg and Si have been used as dopants for p-type and n-type layers, respectively. The MBE-grown GaN films display excellent properties as determined from photoluminescence (PL), x-ray diffraction, and vertical-cross-section TEM micrographs. For planar electronic devices such as FETs, MBE may play a significant role since GaN:Si and AlGaN:Si films grown by MBE are comparable in quality to MOCVD-grown layers. Issues concerning p-type doping of GaN using Mg for p-n junction devices will also be discussed. InGaN is essential as the active recombination layer material in double-heterostructure light emitting devices. The growth of high quality InGaN is complicated by thermodynamic limitations: InN is unstable and tends to dissociate at typical MBE growth temperatures of 700-800C. To overcome these difficulties, we have developed a modulated beam MBE technique which employs alternating layers of InGaN and GaN. RHEED studies have revealed that a two-dimensional growth mode can be maintained during the entire modulated beam deposition process with (1x1) and (1x3) reconstructions observed during growth of each of the InGaN and GaN layers, respectively. GaN/InGaN quantum well structures consisting of twenty double-layers in which the individual layer thicknesses ranged from single monolayers to 30 each have been grown by MBE. The GaN/InGaN quantum well structures exhibit a single intense PL peak at room temperature in the 400-480 nm spectral region with FWHMs as narrow as 12 nm. Selected GaN/InGaN structures, grown and processed as diodes, exhibit similar EL spectra at room temperature. Supported by DARPA, ARO, and Cree Research.

2:00 PM *D13.2 
ATOMIC ORDERING AND PHASE SEPARATION IN AlGaInN ALLOYS. T.D. Moustakas, D. Korakakis, R. Singh, D. Doppalapudi, H.M. Ng, A. Sampath, E. Iliopoulos, M. Misra, Center for Photonics Research and Electrical and Computer Engineering Dept., Boston University, Boston, MA.

This paper reviews experimental results of atomic long-range order and phase separation in AlGalnN alloys, grown by plasma-assisted MBE. Atomic long-range ordering has been observed in AlxGa1-xN alloy over the entire alloy composition1. The phenomenon was investigated by studying the superlattice peaks (0001), (0003) and (0005) using X-ray diffraction (XRD). The relative intensity of these peaks was found to be largest for the Al content in the 40-50% range in qualitative agreement with expectations for an ordered structure of ideal Al)0.5Ga0.5N stoichiometry. Kinetic factors which promote ordering as well as the determination of the average size of the ordered domains will be discussed. Phase separation has been observed in InxGa1-xN alloys, using XRD and optical absorption studies2. Specifically we find that alloys with In content in excess of 30% show an extra diffraction peak corresponding to an alloy with high In concentration (close to pure InN). Correspondingly, the film becomes red in transmission and the optical absorption edge is consistent with a composite film. The optoelectronic properties of these films and the implications of the ordering and phase separation to devices will be discussed.

2:30 PM D13.3 
HOMOEPITAXIAL GROWTH OF GaN UNDER Ga-STABLE AND N-STABLE CONDITIONS BY rf-PLASMA MOLECULAR BEAM EPITAXY. E.J. Tarsa, B. Heying, X.H. Wu, P. Fini, S.P. DenBaars, and J.S. Speck, Materials Department, University of California, Santa Barbara, CA.

We have investigated the structural, morphological, and optical properties of homoepitaxial GaN layers grown by MBE as a function of the group III/ group V flux ratio during growth. The layers were deposited on MOCVD-GaN ``template'' layers to facilitate direct correlation of growth parameters and materials properties. GaN films grown with a high III/V ratio (Ga-stable conditions) displayed a smooth surface morphology with spiral growth features associated with the surface termination of mixed character threading dislocations and consistent with a step-flow growth mode. In contrast, films grown with a low III/V ratio (N-stable conditions) displayed a rough, granular morphology, the average rms roughness of which increased approximately as the square root of film thickness. Cross sectional and plan-view TEM analysis showed that films grown under Ga-stable conditions had a nearly identical defect structure to that of the template with no evidence of interfacial contamination. Films grown under N-stable conditions displayed a tilted columnar structure with a high density of stacking faults having displacement R=1/3[1010]. Photoluminescence measurements showed an increase in the ratio of deep-level (550 nm) to near-band edge emission for films grown under increasingly N-stable growth conditions. The abrupt transition from heavily defected N-stable growth to highly perfect Ga-stable growth represents a fundamental transition in growth mode. This transition will be qualitatively explained in terms of the relative surface adatom mobilities and the suppression of stacking fault formation on close-packed (0001) wurtzite planes under the step-flow growth mode.

2:45 PM D13.4 
CONTROLLING 2D/3D GROWTH OF GaN BY MOLECULAR BEAM EPITAXY: FROM SUPERLATTICES TO QUANTUM DOTS. B. Daudin, G. Feuillet, F. Widmann, Y. Samson, M. Arley, J.L. Rouvière, Département de Recherche Fondamentale sur la Matière Condensée, CEA-Grenoble, Grenoble, FRANCE; F. Fishman, V. Mandrillon, Laboratoire de Spectrométrie Physique, Université J. Fourier Grenoble, Martin d'Hères, FRANCE; P. Valiron, N. Maillard, Laboratoire d'Astrophysique Observatoire de Grenoble, Université J. Fourier, Grenoble, FRANCE.

The strain relaxation mechanisms of hexagonal AIN and GaN grown by Molecular Beam Epitaxy were investigated in detail by quantitative analysis of Reflection High Energy Electron Diffraction (RHEED) patterns. When AIN is deposited onto GaN, growth is 2-dimensional with gradual plastic strain relaxation. By contrast, GaN growth onto AIN is found to be either 2- or 3-dimensional, depending on growth temperature. Above 700C, GaN growth proceeds in a Stranki-Krastanov (SK) mode, with deposition of 2 bidimensional monolayers followed by 3D islanding. Then, either AIN/GaN superlattices or GaN dots were achieved. High Resolution Electron Microscopy (HREM) revealed that the superlattice interfaces were abrupt at the atomic scale, with no interdiffusion observed. In the SK mode, realization of GaN islands with heights ranging from 1.6 nm to 2.0 nm and lateral dimensions from 8 to 13 nm are clearly evidenced by AFM and HREM. The size and density of GaN islands were controlled by precise adjustment of growth temperature, deposition time and reorganization time under nitrogen plasma after GaN deposition. Cathodoluminescence experiments demonstrate confinement effects in close agreement with theoretical predictions.

Chair: Takeshi Uenoyama 
Wednesday Afternoon, December 3, 1997 
Salon F (M)

3:30 PM *D14.1 
THEORETICAL INVESTIGATION OF EXTENDED DEFECTS IN GROUP-III NITRIDES. A.F. Wright, Sandia National Laboratories, Semiconductor Material and Device Sciences Dept., Albuquerque, NM.

Experimental studies indicate that heteroepitaxially grown GaN and its alloys contain extended defects - stacking faults, dislocations and grain boundaries - at densities that would severely limit the performance of optical devices in other material systems. In spite of this, robust light- emitting diodes have been successfully fabricated from the group-III nitrides, and rapid progress is being made in the development of laser diodes. This suggests that defects in nitride compounds either have unusual properties when compared with other material systems, or perhaps that point defects are efficient at passivating them. To explore these suggestions, we will utilize density-functional techniques to determine the properties of extended defects and also their interactions with point defects. To begin, we discuss the energetics and electronic structure of stacking faults and the implications for polymorphism.

4:00 PM D14.2 
NITROGEN VACANCIES IN AlxGa1-xN ALLOYS. P. Boguslawski1,2 and J. Bernholc2; 1 Institute of Physics PAS, Warsaw, POLAND; 2 North Carolina State University, Raleigh, NC.

Nitrogen vacancies are important native donors in GaN, which compensate acceptors and may cause undoped samples to become n-type [1]. In this work, we investigate N vacancies in wurtzite AlxGa1-xN alloys by quantum molecular dynamics and show that their level structure and energetics are strongly dependent on the number of Ga nearest neighbors. These findings have important consequences for the electronic and structural properties of the alloys. In AlxGa1-xN alloys there are 5 possible variants of a nitrogen vacancy, with 0, 1, ..., 4 Ga nearest neighbors (or, equivalently, 4, ..., 0 Al neighbors), respectively. Each variant is characterized by a corresponding number of Ga and Al dangling bonds. The calculated properties of the various variants differ substantially. The variant with 4 Ga neighbors is lower in energy by as much as 2.8 eV than the variant with 4 Al neighbors. In general, the increase of the number of Ga neighbors lowers the total energy by about 0.7 eV. This implies that the energy of the dangling bond of Ga is lower than that of Al by 0.7 eV. Because of this strong tendency to form Ga-terminated vacancies, the distribution of vacancies should be highly non-statistical, i.e., most of the vacancies should exist in the Ga-terminated variant even in very Al-rich AlxGa1-xN alloys. Furthermore, the mobility of VN in AlxGa1-xN should be strongly reduced. Considering the electronic structure, we find a large spread of about 0.7 eV in the vacancy-induced donor level. The calculated energy difference of Ga and Al dangling bonds provides a driving force for surface segregation, i.e., for the tendency of the free surface of AlxGa1-xN alloys to be Ga-terminated. This effect is discussed in context of the abruptness of interfaces in the nitride-based structures.

4:15 PM D14.3 
NATIVE POINT DEFECTS IN AlN AND InN. C. Stampfl and C. G. Van de Walle, Xerox Palo Alto Research Center, Palo Alto, CA.

We performed density-functional pseudopotential calculations using the local density approximation (LDA) as well as the generalized gradient approximation (GGA) for the exchange-correlation functional to investigate the electronic and atomic structure, and formation energies, of native point defects in zinc-blende AlN and InN. For InN, most of the calculations were performed using the nonlinear core correction for In, but for the lowest energy defects, we repeated the calculations treating the In-4d state as a valence state. In both materials we find that nitrogen vacancies have the lowest formation energies in p-type material, and cation vacancies have the lowest energy in n-type material. The defect-induced state for the nitrogen vacancy lies above the conduction band; caution should therefore be exercised in performing calculations for the neutral or negative charge states of the defect to ensure correct occupation of the levels. For n-type conditions the formation energies of the nitrogen vacancy in both AlN and InN are suffiently high as to indicate that these defects will not occur in high concentrations. For the systems investigated in the present study, we find qualitatively the same results when using the GGA and LDA; there are, however, quantitative differences. We discuss the origin of some of these deviations.

4:30 PM D14.4 
POLAR GaN SURFACES: Ga-STABILIZED STRUCTURES AND CONSEQUENCES FOR GROWTH. Jørg Neugebauer, Tosja Zywietz, Matthias Scheffler, Fritz-Haber-Institut der MPG, Berlin, GERMANY; John E. Northrup, Xerox PARC, Palo Alto, CA.

Despite progress in material quality and device fabrication, the knowledge about the structure and properties of GaN surfaces and of adatom diffusion on these surfaces is still in its infancy. We therefore employed density-functional theory total-energy calculations to study the atomic geometry, energetics, and electronic states of GaN surfaces. We will focus on the technologically most important polar GaN (0001) and (001) surfaces. Our results reveal features not observed for other semiconductors: For instance, surfaces are mainly Ga-stabilized (with up to two Ga-layers on the surface), and for cubic GaN (001) a Ga-terminated surface is energetically most stable even under N-rich conditions. Even though GaN is a wide-band gap semiconductor, the surfaces exhibit metallic-like character under Ga-rich growth conditions. Further, we find unusual surface reconstructions that have never been observed for semiconductor surfaces but which closely resemble structures found at metal surfaces. We have also studied the diffusion of Ga and N adatoms on these surfaces. Our results show a very high diffusivity for Ga adatoms but a rather low diffusivity for N adatoms. We discuss how the unusual surface structures and the very different mobility of Ga and N adatoms affect the growth of GaN and the incorporation of impurities.

4:45 PM D14.5 
THEORY OF INTERFACES, SURFACES AND ADSORBATES IN WIDE GAP NITRIDES. Krzysztof Rapcewicz, Marco Buongiorno Nardelli, Claudia Bungaro and J. Bernholc, Dept. of Physics, North Carolina State Univ., Raleigh, NC.

We present the results of an extensive ab initio theoretical investigation of the nitrides. The phenomena that we have studied include band offsets at interfaces as a function of strain, pyro- and piezo-electric effects in multiquantum wells, surface reconstruction energetics and adsorbate-substrate interactions. 
Band offsets between the different epilayers in a heterojunction have a major impact on the carrier transport properties both along and across the interface. As heterojunctions of the nitrides are lattice-mismatched, the band offset can be tuned by changing the substrate material, which introduces strain. The strain-induced variation of the band offsets in GaN/AlN/InN multiquantum wells can alter the offset by as much as 40%. 
During growth, the surface morphology of the substrate is important. We have studied the influence of growth conditions and surface polarity upon the morphology of (0001) GaN surfaces, which are the primary growth faces. On the Ga-terminated surface under Ga-rich (N-rich) conditions, a Ga-adatom (N-adatom) reconstruction is the most stable 22 reconstruction. A N atom near the N-terminated surface spontaneously forms N2 molecule, which is weakly bound to a vacancy on the substrate. At the high temperatures under which growth takes place, this will result in N2 desorption and subsequent reconstructions. We have also investigated H and Mg adsorption on the (0001) surface. H stabilizes the ideally cleaved surface irrespective of polarity, and the surface states associated with its adsoption have been identified. Although Mg is the most important p-type dopant of GaN, its incorporation during MBE growth has been difficult. We investigated the behavior of monolayer and submonolayer coverages of Mg at the Ga-terminated (0001) surface and found that the most stable adsorption site changes as a function of coverage, and that surface adsorption is favored over subsurface incorporation

Wednesday Evening, December 3, 1997 
8:00 P.M. 
Salons G-K (M)

BAND-OFFSET TRENDS IN NITRIDE HETEROJUNCTIONS. N. Binggeli, P. Ferrara, and A. Baldereschi, Institute of Applied Physics, EPFL, Lausanne, SWITZERLAND.

We have performed extensive first-principles studies of the structural and chemical trends of the band offsets in nitride heterojunctions, including the offset dependence on interface orientation, heterovalency, and polytype. Selected prototypic zincblende (3C) and wurtzite (2H) systems were examined, including isovalent GaN/AlN and heterovalent GaN/SiC and AlN/SiC heterojunctions. In the isovalent GaN/AlN (100), (110) and (111) heterojunctions, the band offsets are relatively insensitive (within 0.1 eV) to the orientation and structural details of the interface, consistently with the trend established from linear-response theory for the conventional, smaller-gap, semiconductor heterojunctions. Bulk strain effects, however, produce modifications as large as 0.4 eV in the offsets of coherently strained AlN/GaN and GaN/AlN (100) heterojunctions. In the heterovalent polar GaN/SiC and AlN/SiC (111) systems, the band alignment critically depends on the interface composition, and differences in the offset as large as 1 eV are found between neutral interfaces including mixed anion (N/C) or mixed cation (Ga/Si or Al/Si) planes. We also find that atomic relaxation plays a major role in determining the offsets of the heterovalent polar nitride junctions. This is in contrast to the case of conventional semiconductor heterojunctions, and follows from the high ionicity of the heterovalent nitride systems. We will show, however, that even for such highly ionic systems, provided atomic relaxation is taken into account, the offset dependence on interface orientation and heterovalency can still be qualitatively explained using the linear-response approach. Finally, the change from zincblende to wurtzite crystal structure in GaN/AlN, GaN/SiC, AlN/SiC (111) and (0001) heterojunctions selectively affects the conduction band offset, and has only a minor influence on the valence discontinuity. This feature will be discussed in connection with the effect of stacking faults on the band alignment in 3C(111)/2H(0001) homojunctions and heterojunctions.

EFFECTS OF SUBSTRATE ORIENTATION ON THE VALENCE BAND SPLITTINGS AND VALENCE BAND OFFSETS IN GaN AND AlN FILMS. J. A. Majewski, M. Städele, and P. Vogl, Walter Schottky Institute, Technical University of Munich, Garching, GERMANY.

We present first-principles studies of the effect of epitaxial strain on the electronic band structure and structural properties of the wurtzite and cubic GaN and AlN films and their heterostroctures. In the present study we focus on two problems. First, we demonstrate how the valence band splittings in GaN and AlN depend on the growth direction of the film. Secondly, we investigate the dependence of the valence band offsets in wurtzite/wurtzite and cubic/cubic GaN/AlN heterostructures on the substrate orientation. In our calculations we consider main crystallographic directions, namely [0001], [11-20], [01-10], and [1-101] for wurtzite, and [001], [110], and [111] for cubic phase. Our calculations are based on the first-principles total-energy pseudopotential method and include full relaxation of the atomic positions. The band structure has been calculated including the spin-orbit interactions nonperturbatively. From our ab-initio calculations, we determine the full set of deformation potentials for both wurtzite and cubic phases, and show that the numerically computed band energies can be excellently represented in terms of 6-band model. In the case of biaxial strain in the basal (0001) plane, the theoretical results are in accord with available experimental data. Therefore, we believe that the predictions of valence band splittings may facilitate the interpretation of experimentally measured free exciton lines in nitride films grown on differently oriented substrates.

SIMULATIONS OF VACANCY PAIRS IN GaN USING TIGHT-BINDING MOLECULAR DYNAMICS. Derrick Boucher, Zoltan Gal, King's College Dept. of Chemistry and Physics, Wilkes-Barre, PA; Gary G. DeLeo, W. Beall Fowler, Dept. of Physics, Lehigh University, Bethlehem, PA.

The electronic structure, geometry and energetics of Ga vacancy pairs and N vacancy pairs in both wurtzite and zincblende GaN are investigated via molecular dynamics (MD) simulations using an empirical tight-binding (TB) model with total energy capabilities and supercells containing up to 216 atoms. Our calculations suggest that, by pairing, N vacancies, which in isolation act as shallow donors, can lower their collective formation energy by about 3 eV. In doing so, however, these N vacancies lose their shallow-donor character as the lattice relaxes in response to this aggregation. Contrasting with the N vacancies, the Ga vacancies are found to retain their isolated shallow acceptor behavior and do not gain significant energy upon aggregation. The possible implications for larger aggregate defects are discussed.

MONTE CARLO CALCULATION OF HIGH- AND LOW-FIELD AlxGa1-xN ELECTRON TRANSPORT CHARACTERISTICS. J.D. Albrecht, R.P. Wang, P.P. Ruden, University of Minnesota, Department of Electrical and Computer Engineering, Minneapolis, MN; M. Farahmand, E. Bellotti, and K.F. Brennan, Georgia Institute of Technology, School of Electrical and Computer Engineering, Atlanta, GA.

The Monte Carlo technique is used to simulate electron transport in wurtzite phase, bulk AlxGa1-xN. A three valley analytical band model consisting of spherical, non-parabolic conduction band valleys at the , K, and U symmetry points is matched to first-principles bands of GaN and AlN, with parameters of the AlxGa1-xN alloy obtained by interpolation. To the extent available, experimental data is used to correct the band parameters. The Monte Carlo simulations are performed for ambient temperatures in the range of 300K to 600K. Scattering mechanisms taken into account include ionized impurity and alloy scattering, in addition to deformation potential scattering (intra- and inter-valley) and polar optical phonon scattering. We present results for the electron steady-state drift velocity and the valley occupancy for electric fields up to 600kV/cm. Low-field drift mobilities are extracted from the Monte Carlo calculations as functions of the electron concentration, of the compensation ratio, and of the alloy composition. The sensitivity of the results to changes in the band parameters is examined, and the effect of possible material inhomogeneity is explored in the framework of simple models. We compare the calculated low-field drift mobilities to results of variational calculations and to experimental data for AlxGa1-xN with x<0.4. Work supported in part by NSF under contracts ECS-9408479 (University of Minnesota) and ECS-9313635 (Georgia Tech).

SPONTANEOUS POLARIZATION, PIEZOELECTRICITY, DIELECTRIC PROPERTIES, AND LO-TO-PHONON SPLITTINGS IN III-V NITRIDES. Fabio Bernardini and Vincenzo Fiorentini, Istituto Nazionale di Fisica della Materia and Dipartimento di Scienze Fisiche, Università di Cagliari, ITALY; David Vanderbilt, Department of Physics, Rutgers University.

We present an ab-initio density functional calculation of the macro scopic spontaneous polarizations piezoelectric constants, and effective Born charges for AIN, GaN and InN, of the static and high-frequency dielectric constants for AIN and GaN. The calculations are based on the Berry phase method for the polarization properties and on a novel scheme for the dielectric properties: the high-frequency dielectric constants are calculated as the ratio of zero-field and longitudinal polarization, while the static one is obtained connecting the zero field polarization with the elastic properties of the material. III-V nitrides turn out to have the largest piezoelectric constants among the tetrahedrally bonded semiconductors (up to 10 times larger than in conventional III-V and II-VI semiconductors, and slightly larger than those of ZnO). The nitrides are therefore extremely polarizable and may develop large piezoelectric fields in epilayers at comparatively small strains. The calculated dielectric constants compare well with available experimental results, also for other test cases (SiC, BeO ZnO). Finally, the LO-TO phonon splittings at zone center, extracted from the calculated Born charges and dielectric constants, match nicely recent experiments.

AB INITIO PREDICTION OF IMPURITY LEVELS AND FORMATION ENERGIES OF DOPANTS IN ALUMINUM NITRIDE. Antonella Fara, Fabio Bernardini, and Vincenzo Fiorentini, Istituto Nazionale di Fisica della Materia and Dipartimento di Scienze Fisiche, Università di Cagliari, ITALY.

While n-doping is relatively easy in GaN, in AlN the chances of the occurrence of deep donors is much higher given the larger gap (6.2eV, direct). p-doping on its part should not be easier. By means of ab-initio density-functional-theory calculations, we predicted thermal and optical impurity levels, formation energies, and equilibrium geometries for some acceptors (BeAl, MgAl) and donors (SiAl, VN) in wurtzite AlN. The thermal acceptor levels, extracted as differences of density-functional total energies and referred to the valence band top, are 0.25eV for BeAl and 0.45eV for MgAl. Unlike GaN, impurity states are mostly localized on N neighbors along the c-axis, As formation energies are somewhat larger, p-doping seems marginally harder than in GaN. As in GaN, antisite and heteroantisite behavior is strongly disfavored. As to donors, both the N vacancy and SiAl have the first donor level (+/0) at about 4.5-5 eV above the valence band. If the experimental gap is used for comparison, both would be deep donors, but such a procedure is not technically consistent. To give a more accurate estimate of the donor level depth, we are currently attempting to calculate the gap in the same way as for extrinsic levels as a total energy difference. Preliminary results are encouraging.

VELOCITY OVERSHOOT AND BALLISTIC ELECTRON TRANSPORT IN INDIUM NITRIDE. B.E. Foutz, L.F. Eastman, School of Electrical Engineering, Cornell University, Ithaca, NY; S.K. O'Leary, M.S. Shur, Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, NY; and U.V. Bhapkar, Naval Surface Warfare Center, Dahlgren, VA.

Using an ensemble Monte Carlo approach, ballistic transport and velocity overshoot effects are examined in InN and compared with those in GaN and GaAs. It is found that the peak overshoot velocity in InN is greater than cm/s which is larger than both GaN and GaAs. Furthermore, the velocity overshoot in InN occurs over distances in excess of 0.4 microns which is comparable to GaAs, but is larger than the 0.2 microns for the case of GaN. These strong overshoot effects combined with a high peak drift velocity, large low-field mobility, and large saturation drift velocity, should allow InN based field effect transistors to outperform GaN and even GaAs based devices.

Abstract Withdrawn.

PARALLEL TRANSPORT IN HETEROSTRUCTURES OF GaN/AIGaN. Mahesh Krishnan, A. Dimoulas and A. Christou, Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD.

Transport properties of the 2D electron gas as GaN/AlGaN have been calculated utilizing kinetic Monte Carlo (MC) simulations based on stochastic processes to take into account interfacial roughness and molecular clusters. The approach developed takes into account scattering degeneracies. The transport properties of GaN and their (110), (100) and (111) surfaces were calculated which take into account the full degeneracy at the GaN/AlGaN interface. Without this degeneracy, false velocity-field relationships are attained. The numerical calculations are in agreement with reported I-V curves for GaN/AlN HFETs.

AN INVESTIGATION OF THE ELECTRON ESCAPE TIME WITHIN A BIASED AlGaN/GaN QUANTUM WELL. Kevin R. Lefebvre and A. F. M. Anwar, Electrical and Systems Engineering, University of Connecticut, Storrs, CT .

A large number of semiconductor devices utilize an electric field applied perpendicular to the quantum well, modifying the electron escape rate. Several theorectical models have been introduced to theorectically explain experimental results of the escape time of electrons from a biased AlGaAs/GaAs quantum well. These models incorporating both tunneling and thermionic emission, have proven to be inadequate in verifying the experimental results. However, it has been demonstrated by the present authors that incorporating the redistribution of the density of states as a result of the applied electric field obtains excellent theorectical agreement with previously reported experimental results. In this presentation, the electron escape time from an Al.25Ga.75N/GaN/Al.25Ga.75N quantum well system is compared with Al.4Ga.6As/GaAs/Al.4Ga.6As systems. This study will investigate the escape time as a function of electric field as well as quantum well width. This calculation addresses the continuous DOS within a quantum well as a result of the electric field, the change in the group velocity and the proper partitioning of between the tunneling and thermionic emission currents by solving Schroedinger equation through the logarithmic derivative of the wavefunction and the retarded Greenís function. For a 100 Åquantum well and for electric fields between 10kV/cm to 100kV/cm, the escape time for Al.25Ga.75N/GaN/Al.25Ga.75N (10-8-10-9 sec) is an order of magnitude greater than the equivalent Al.4Ga.6As/GaAs/Al.4Ga.6As (10-9-10-10 sec) system. Furthermore, the electric field at which the current changes from thermionic emission dominance to tunneling dominance is higher for Al.25Ga.75N/GaN/Al.25Ga.75N system (70 kV/cm) than the equivalent Al.4Ga.6As/GaAs/Al.4Ga.6As system (50 kV/cm). Electron escape times for varying Al mole fraction in AlxGa1-xN/GaN/AlxGa1-xN will also be reported.

STRUCTURAL AND ELECTRONIC PROPERTIES OF THE GaN/Al INTERFACE. S. Massidda INFM - Dip. Scienze Fisiche, Univ. Cagliari , Cagliari, ITALY; S. Picozzi and A. Continenza, INFM - Dip. Fisica, Univ. L'Aquila, L'Aquila, ITALY; A. J. Freeman, Dept. of Physics and Astronomy and Materials Research Center* Northwestern University, Evanston, IL.

The structural and electronic properties of several possible GaN/Al interface configurations are determined from first principles local density full potential linearized augmented plane wave (FLAPW) calculations. Several important electronic properties (such as density of states, charge redistribution, etc.) as a function of the distance from the interface are presented; the gap states induced into the semiconductor by the presence of Al are found to be strongly localized in the interface region. We also study two auxiliary systems, i.e. the Al/AlN interface and the GaN/AlN interfaces, both grown on a GaN substrate. We show that Al does not provide good ohmic contacts on the clean nitrides considered, in contrast with experimental results on chemically treated GaN, but in agreement with recent measurements on the clean surface. Finally, we investigated the structural stability of the different interface morphologies and the relative changes in the Schottky barrier heights. Our theoretical results show that while the exchange between Ga and Al atoms at the unannealed interface is energetically favorable, AlN formation seems not to be favored - in agreement with recent experimental studies .* Supported by the N.S.F. V. M. Bermudez et al., J. Appl. Phys. 79, 110 (1996).

ELECTRON-PHONON SCATTERING IN GaN/AlN AND GaAs/AlAs QUANTUM WELLS. C. R. McIntyre, C. R. McIntyre, George Mason Univ, Dept of Physics and Astronomy, Fairfax, VA; T. F. Forbang, George Mason Univ, Center for Computational Sciences and Informatics, Fairfax, VA.

We have studied the effects on the phonon spectrum and on the electron-longitudinal optical phonon scattering in GaN/AlN and GaAs/AlAs quantum wells. Phonon modes and electron-phonon scattering rates have been calculated for both systems. We will compare the relaxation times and the effect on electron mobility due to LO-phonon scattering in both systems.

BINDING ENERGIES FOR SUBSTITUTIONAL ACCEPTORS IN GaN AND AlN. Francisco Mireles*, Sergio E. Ulloa, Ohio Univ, Dept of Physics and Astronomy, Condensed Matter and Surface Sciences Program, Athens, OH.

We report the first binding energy calculations for Mg, Zn, Ca, C and Si substitutional shallow acceptors in bulk GaN and AlN for both, wurtzite (WZ) and zincblende (ZB) crystal phases. The calculations are performed within the effective mass theory through the Rashba-Sheka-Pikus and the Luttinger-Kohn 6 6 matrix Hamiltonians, respectively. An analytic representation for the pseudopotential is used to introduce the appropriate nature of the impurity atom. The parametrizations for GaN and AlN hole bands were taken from experiments and first principles band structure calculations. The energy shift due to polaron effects is also considered in this approach. The ionization energy estimates obtained are in excellent agreement with those reported experimentally. We find that the binding energies for ZB GaN acceptors are even shallower than the corresponding impurities in the WZ crystalline phase. We obtain that the Ca substitutional impurity in GaN shows relatively weak binding energy, suggesting Ca as a possible p-dopant in GaN. Ionization levels in AlN are found to have similar values as in GaN but with important differences. The light and heavy hole band mixing as a function of the impurity binding energy and the dependence of the acceptor ground state splitting with the strain in this nitrides are also discussed.

STUDY OF INTERFACE STRUCTURE OF GaN SEMICONDUCTOR WITH MOLECULAR DYNAMICS SIMULATION. Ryuji Miura, Isao Gunji, Tomonori Kanougi, Kazuomi Shiota, Akira Endou, Kazuya Tsujimichi, Momoji Kubo, Abhijit Chetterjee, Akira Miyamoto, Tohoku Univ., Graduate School of Eng., Sendai, JAPAN; Yasushi Iyechika, Takayoshi Maeda, Sumitomo Chem. Co., Ltd., Tsukuba Res. Lab., Ibaraki, JAPAN.

Recently, high power blue-LEDs and short wavelength LDs were realized using GaN based semicondutors[1]. However, properties of hetero-junction inteface in these devices, such as GaInN/GaN or GaN/ a-AI2O3, are not yet to be understood. Therefore, we have performed computer modeling to determine simulatlon parameters for GaN, InN, GalnN and -Al2O3(0001) surfaces, and analyzed their respective interface structure using Molecular Dynamics (MD). We used MXDORTO and RYUDO for MD calculation, and RYUKI for constructing structure and RYUGA for analysis and visualization. The two body central force interaction potential which was used for all our calculations include Coulomb potential, exchange repulsion interaction potential and Morse potential. We determined the potential parameters for bulk structures of GaN, InN and -Al2O3 in order to fit the lattice constant at 300K and melting point to the experimental data. Then we constructed the GaN(0001) and -Al2O3 (0001) surface structures and performed MD calculation of these structures with deposited InN and GaN molecules at 300K, 6OOK and 900K. We observed from the MD results, that the deposited N atoms were strongly attracted by surface Al atoms, and that the growth of GaN structure was affected by atomic arrangement of Al atom at -Al2O3 surface.

VELOCITY-FIELD CHARACTERISTICS OF INDIUM NITRIDE. S.K. O'Leary, M.S.Shur, Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, NY; B.E. Foutz, L.F. Eastman, School of Electrical Engineering, Cornell University, Ithaca, NY; U.V. Bhapkar, Naval Surface Warfare Center, Dahlgren, VA.

We determine the velocity-field characteristics of indium nitride, using an ensemble Monte Carlo approach. It is found that indium nitride exhibits an extremely high room temperature peak drift velocity, cm/s, at a doping concentration of cm-3. This exceeds that of gallium nitride, cm/s, by approximately 40 %, and is believed to be one of the highest peak drift velocities ever reported for a III-V semiconductor. The saturation drift velocity of indium nitride is found to be comparable to that of gallium nitride, roughly equal to cm/s. These results are shown to be valid over a wide range of temperatures, from 77 to 500 K, and doping concentrations, up to cm-3, suggesting that indium nitride has considerable potential for device applications. Some of these possible applications are explored.

Important characteristics of materials used in luminescence devices are rates of different electron-hole recombination processes. We calculated for the first time the interband radiative recombination rate in the wide-gap semiconductors GaN, InN and AlN crystallizing in the hexagonal wurtzite structure and in ternary In1-x-yGaxAlyN alloys including also binary solid solutions GaxAl1-xN , InxAl1-xN and GaxIn1-xN. All our calculations were based on experimental data on energy band structures and optical absorption spectra of the nitride materials. We estimated the matrix elements for the direct optical transitions between conductivity and valence bands of these semiconductors using the experimental photon energy dependence of the fundamental absorption. In our calculations we assumed that basic parameters of the solid solution (the matrix element, effective masses of carriers and so on) could been obtained by linear interpolation between their values in the alloy components. Taking the temperature dependence of the energy gap in the form proposed by Varshni, we performed the calculations of the radiative recombination rate R in the wide range of temperature and carrier concentrations and in the full range of the alloy composition. The radiative recombination coefficient B defined according to the equation R=Bnp, n and p being the carrier densities, is higher in InN and lower in GaN, taking intermediate values in AlN. For example, B=(2.7, 0.4 and 0.15) 10-10 cm3/s for InN, AlN and GaN, correspondingly. The carrier lifetime in GaN equals 60ns at 300K and n=1 1017 cm-3. Indium concentration increase in the ternary alloy leads to a rise of the radiative recombination coefficient.

PERFORMANCE OF WURTZITE AND ZINCBLENDE GaN QUANTUM WELL LASERS. R.J. Radtke, U. Waghmare, H. Ehrenreich, Harvard University, Div of Engineering and Applied Sciences, Cambridge, MA; C.H. Grein, University of Illinois at Chicago, Dept of Physics, Chicago, IL.

The radiative recombination rates, gain, and threshold current densities of ideal wurtzite and zincblende 20A InGaN / 70A GaN multiple quantum well laser diodes are calculated and compared. The formalism is based on a superlattice K.p theory which has been previously applied to other zincblende III-V compounds and has yielded excellent agreement between theory and experiment in both the energetic position and the magnitude of the absorption features.

8:30 AM *D16.1 
HYDRIDE VAPOR PHASE EPITAXY OF GALLIUM NITRIDE. R.J. Molnar, Massachusetts Institute of Technology, Lincoln Laboratory, Lexington, MA.

The growth of gallium nitride (GaN) by hydride vapor phase epitaxy (HVPE) has recently regained considerable interest, primarily as a growth technique for growing thick layers which can be used as substrates for subsequent device overgrowths. This technique has the advantage of a high growth rate as well as low operational and equipment costs. It also appears to be well suited for producing films with low structural defect densities (108 cm-2). As device layers deposited on these HVPE-grown GaN buffer layers by OMVPE and MBE tend to replicate the defect structure of the buffer layer, a net reduction of defects in nitride-based devices is achieved over conventional two-step techniques. A number of critical issues need to be addressed, however, in order for this technique to fulfill the role of a homoepitaxial substrate. An improved understanding and control of residual thermal strain in the HVPE layers is needed. Techniques have to be developed to avoid relief of the thermal strain through cracking. Ultimately, removal of the HVPE layer from the host substrate would not only eliminate the strain, but also allow vertical current transport and/or improved heatsinking of device structures. The observed reduced rate of dislocation annihilation with increased thickness tends to impose an apparent bottleneck on the minimum dislocation density that is readily obtainable, although it is yet unclear what dislocation density level is required for the various types of devices. Control of conductivity to address the different needs of different device technologies, such as lasers and FET's, needs to be developed. The current status of our efforts to address these issues, as well as materials characterization, will be presented.

9:00 AM *D16.2 
BULK GaN CRYSTAL WITH LOW DEFECT DENSITY GROWN BY HYDRIDE VAPOR PHASE EPITAXY. Akira Usui, Opto-electronics Research Laboratories, NEC Corporation, Ibaraki, JAPAN.

GaN and InGaN-based device structures have been grown mainly on sapphire. Due to the large difference of lattice parameter and thermal expansion coefficient between the sapphire and GaN or InGaN, many threading dislocations have been observed in the grown layers. The best way to avoid defect generation at the interface is to use a GaN bulk substrate. By hydride vapor phase epitaxy (HVPE), several attempts to grow thick GaN layers for this purpose have been so far reported. The advantage of HVPE is its high growth rate. In addition, optical properties of HVPE-grown layers are among the best ever reported. However, the occurrence of cracks was observed in HVPE-grown thicker GaN layers. In this paper, we grew a thick GaN layer with low defect density, on a 2-inch-diameter sapphire substrate, without cracks by using HVPE. The HVPE growth was carried out on a MOVPE-grown thin GaN layer on the sapphire substrate. Selective growth was carried out at the beginning of the growth. The coalescence of the selectively grown regions made it possible to achieve a flat surface over the entire substrate. As a result, crack-free GaN films with a mirror-like surface were successfully grown even in the thickness of a few hundred microns. The extended defect density was as low as 6x107 cm-2. Using low-pressure MOVPE, InGaN MQW LED structures were grown on the bulk GaN. From TEM observation, the broken area of the MQW(caused by threading dislocations) was found to be greatly reduced as compared with that on the sapphire. Moreover, the light-output efficiency from the LED on the bulk GaN was three times greater than that on the sapphire substrate. 
These results indicate that HVPE-grown bulk GaN will be useful for achieving high-performance light-emitting devices.

9:30 AM D16.3 
Si-DOPING OF GaN GROWN ON SAPPHIRE BY HVPE. Robert P. Vaudo and Vivek M. Phanse, Epitronics, Danbury, CT; J. Jayapalan, D. Wang and B.J. Skromme, Dept of Electrical Engineering and Center for Solid State Electronics, Arizona State University, Tempe, AZ.

Thick layers of Si-doped GaN have been grown on (0001) sapphire by hydride vapor phase epitaxy (HVPE). These GaN layers provide a high quality, epi-ready, strain-relaxed base layer for subsequent nitride epitaxial growth. In addition, thick HVPE GaN layers can be used to enhance conduction in the bottom layer of lateral devices. The undoped GaN layers are conductive as grown. Capacitance-voltage (C-V) and Hall effect analysis of these undoped layers revealed that the conductivity was mainly due to a thin region of high carrier concentration at the GaN/sapphire interface and that the bulk of the undoped GaN has an electron concentration of approximately 2E16 cm-3. In this work, silane was introduced into the HVPE environment to intentionally Si-dope and further lower the resistance of the bulk region of the HVPE GaN. To our knowledge, this is the first report of Si-doping of HVPE-grown GaN. The donor concentration in the HVPE GaN:Si films was controllably varied with silane flow from 2E16 to 5E18 cm-3. Si-doping was found to significantly enhance the electron mobility a all doping levels. Sheet resistances as low as 5/sq. were measured for heavily Si-doped 10 m thick GaN films. The optical properties of the HVPE GaN:Si layers were determined using photoluminescence (PL). Broadening of the near band edge feature at 363 nm was observed with increased Si doping. At 2K, a shallow donor peak (binding energy of 21.3 meV) was observed which was not present in undoped GaN films. A description of the electrical, optical and structural characteristics of the GaN:Si films grown by HVPE will be presented.

9:45 AM D16.4 
AlN/GaN AND AlGaN/GaN HETEROSTRUCTURES GROWN BY HVPE ON SiC SUBSTRATES. Yu. Melnik, A. Nikolaev, S. Stepanov, I. Nikitina, K. Vassilevski, A. Zubrilov, A. Babanin, V. Davydov and V. Dmitriev, PhysTech WBG Research Group, Ioffe Institute, St. Petersburg, RUSSIA.

AlxxGa1-xN (x <0.6) and AlN layers were successfully grown by hydride vapour phase epitaxy (HVPE) on SiC substrates using GaN intermediate layer. Layer thickness ranged from 0.5 to 5 m. Growth rate was controlled from 0.5 to 1.5 m/min. Composition of AlGaN alloy was determined using Auger Electron Spectroscopy, Electron Probe Micro-analysis, X-Ray Diffraction and cathodoluminescence. Complex analysis of Al content based on data obtained by different charachterization techniques was performed. Crystal structure of the grown samples was investigated by x-ray diffraction and transmission electron microscopy. The AlGaN films were also characterized by Raman spectroscopy. Electrical measurements were performed on AlGaN/GaN/SiC samples. Ti/Ni and Al metallization were used to form ohmic contacts to AlGaN layers. Mesa structures were fabricated by RIE technique. Measurements of I-V characteristics in vertical current flow geometry proved that AlxxGa1-xN layers with Al contents up to 60 were conductive. Physical properties of AlGaN/GaN heterostructures grown by HVPE will be compared with properties of AlGaN/GaN heterostructures grown by MOCVD.

Chair: Isamu Akasaki 
Thursday Morning, December 4, 1997 
Salon F (M)

10:30 AM *D17.1 
DOPING, ACTIVATION OF IMPURITIES, AND DEFECT ANNIHILATION IN GaN BY HIGH PRESSURE ANNEALING. Tadek Suski, Jan Jun, and Michal Leszczynski, Unipress, Polish Academy of Sciences, Warszawa, POLAND.

GaN semiconductor is characterized by strong bonding and high vapor pressure of nitrogen. The strong bonds limit an efficiency of annealing procedures required for variety of semiconductor technologies, e.g., post implantation annealing, or doping by diffussion. Maximum temperatures employed up to now in GaN annealing have not exceeded about 1100C. A desired increase of annealing temperatures causes a decomposition of GaN unless an elevated pressure of N2 is supplied. In this work, we report on application or high pressure annealing procedures (temperatures up to 1600C and pressures up to l.8 GPa) which enabled us to study the three important technological aspects: i) optical and electrical activation of ion-implanted impurities, ii) doping by diffusion and iii) improvement of GaN properties by annihilation of the growth-related defects.

11:00 AM D17.2 
InGaN DOUBLE-HETEROSTRUCTURES AND DH-LEDS ON HVPE GaN-ON-SAPPHIRE SUBSTRATES. K.S. Boutros, J.S. Flynn, V. Phanse, R.R. Vaudo, G. Smith, and J.M. Redwing, Epitronics, Danbury, CT; T.R. Tolliver and N.G. Anderson, Dept. of Electrical and Com. Eng., Univ. of Mass. at Amherst, Amherst, MA.

Growth of III-nitrides on sapphire requires the use of a low-temperature buffer layer to promote nucleation and achieve single crystal films. Alternatively, thick GaN deposited by HVPE on sapphire can be used as a template for MOVPE growth. In addition to significantly reducing the MOVPE growth complexity, HVPE GaN templates offer an improvement in the properties of subsequently deposited layers. We will report on the properties of InGaN films deposited by MOVPE on HVPE GaN templates. We have achieved InGaN films with band-to-band photoluminescence (PL) from 370 to 540 nm. InGaN/GaN double heterostructures (DH) and multiple-quantum-wells (MQW) have also been grown on the GaN HVPE templates with crystalline and optical properties comparable to the best reported in the literature. Typical In0.09Ga0.91N active layers have double-crystal XRD FWHM 300 arcsec, and 400 nm peak PL emission with FWHM 100 meV. DH-LEDs were fabricated with InGaN layers with various compositions, and produced strong EL emission in the blue and blue/green regions. We will present the results on InGaN DHs and MQWs, along with the properties of these films and general trends in the growth of InGaN. We will also report on the DH-LED characteristics, their electroluminescence (EL) spectra, and output power.

11:15 AM D17.3 
AGING OF InGaN/AlGaN/GaN LIGHT-EMITTING DIODES. A.N. Kovalev, F.I. Manyakhin, Moscow Institute of Steel and Alloys, Moscow, RUSSIA; V.E. Kudryashov, A.N. Turkin, A.E. Yunovich, Moscow State Lomonosov University, Department of Physics, Moscow, RUSSIA.

Changes in luminescent spectra, current- and capacitance-voltage characteristics of blue and green LEDs based on InGaN/AlGaN/GaN heterostructures were studied versus time of working at 30-80 mA. The effective temperature of the active layer at these currents, determined from the luminescent spectra, was 100-200 C. During the period of 50-100 hours for blue and 500-600 hours for green LEDs the light intensity is growing up to 10-40. The concentration of charged acceptors on the p-side of the junction is growing on 15-20. These changes are explained by additional activation of Mg because of residual H atoms are going out of complexes with Mg. During the second period, up to 104 hours, the light intensity decreased on 5-10. The spectral band is broadening to the long wavelength side. The tunnel component of the current at low voltages is growing. The concentration of charged acceptors on the p-side of the junction decreased 6-8 fold. The width of p-n junctions is growing only on 30. This can be explained by compensation of acceptors by donor defects created during a long injection and a possible migration of acceptors to charged walls on the InGaN/AlGaN heterointerface. The main changes of properties were detected at low current measurements (0.15 mA). The relative intensity of tunnel radiative recombination band, studied previously in [1], grew more than threefold; the overall intensity of luminescence decreased almost an order of magnitude. The observed changes can be explained by growing of ''weak'' spots in the structures were the electric field is high and a creation of point defects is probable. These point defects can create complexes with deep levels. Deep levels and potential fluctuations of various origin take part in the tunnel recombination. Breakdown luminescent spectra confirm this model.

11:30 AM D17.4 
PHOTOCURRENT RESPONSE IN Mg-DOPED GaN. C.H. Qiu and J.I. Pankove, Astralux Inc., Boulder, CO; W. Melton, University of Colorado, Dept of Electrical Engineering, Boulder, CO; I. Akasaki and H. Amano, Meiko University, Nagoya, JAPAN.

III-nitride semiconductors are promising materials for UV and x-ray radiation detection. In such applications, the photocurrent response is of primary interest. In the past, we reported the photo-conductivity spectra and product for both n-type and p-type GaN, as well as the photocurrent decay in n-type GaN[1,2] . We recently reported the photocurrent decay in the long time domain (the persistent photoconductivity) in Mg-doped GaN[3]. Here we present our study of the photocurrent response of Mg-doped GaN in the short time domain. The photocurrent is excited by a pulsed nitrogen laser and is recorded as a function of time by a computer. The photocurrent response exhibits several interesting features. The time when the photocurrent attains the maximum is after the laser pulse excitation. At 100K, for example, the photocurrent reaches a maximum at a time of 4 ms, much longer than the width of laser pulse (10 ns). At 390 K, the photocurrent reaches maximum at about 100 ns. The photocurrent decay is initially exponential and followed by a quasi power-law. The decay time constant as determined from the exponential decay region is temperature dependent, with an activation energy of 46 meV. The implications of the results will be discussed.

11:45 AM D17.5 
THERMOPOWER INVESTIGATION OF P-TYPE DOPING IN GaN. M.S. Brandt, P. Herbst, O. Ambacher, H. Angerer, M. Stutzmann, Walter Schottky Institut, Technische Universität München, Garching, GERMANY.

A detailed comparison of the activation energies of the carrier concentration Ea,n and thermopower Ea,t can provide valuable information on potential fluctuations in semiconductors. In particular, such potential fluctuations would be expected in p-type GaN if the hydrogen-induced reduction of carrier concentration is caused by compensation rather than by passivation. We have therefore conducted a detailed comparison of MBE-grown GaN using thermopower, conductivity and Hall experiments The Seebeck coefficient observed in the limit of high temperatures in unintentionally n-type GaN is S=3.9(k/e), indicating ionized impurity scattering as the dominant scattering mechanism. In p type material, slightly larger values for S>5(k/e) are found. Upon exposure to hydrogen, Ea,n and Ea,t increase. However, when the hole concentration has been reduced to below 1016 cm-3 by hydrogenation, Ea,t is 150-200 meV smaller than Ea,n, providing an estimate of the size of the valence band fluctuations. Possible implications on the properties of H in p-type GaN will be discussed.

12:05 PM *X3.1 - Room Salon E (M) 
III-NITRIDE LASERS AND OPTOELECTRONIC DEVICES. Shuji Nakamura, Nichia Chemical Industries, Ltd.,
Dept of Research & Development, Tokushima, JAPAN.

Chair: Alan F. Wright 
Thursday Afternoon, December 4, 1997 
Salon F (M)

2:00 PM *D18.1 
LOCALIZED DONORS IN GaN: SPECTROSCOPY USING LARGE PRESSURES. Christian Wetzel, H. Amano, I. Akasaki, High Tech Research Center, Meijo University, Nagoya, JAPAN; T. Suski, High Pressure Research Center, Warsaw, POLAND; J.W. Ager, E.R. Weber, E.E. Haller, Materials Science Division, Berkeley National Laboratory, Berkeley, CA; B.K. Meyer, 1. Physics Institute, Justus-Liebig-University Giessen, Giessen, GERMANY.

Dopants and defects strongly affect light emission and structural properties of GaN and its ternary alloys. In order to simulate alloying and aspects of heteroepitaxial growth conditions, we perform defect spectroscopy of GaN under perturbation of large hydrostatic pressure ( GPa). As grown samples with unintenional doping and GaN doped with common donor type dopants O and Si are studied by photoluminescence and Raman spectroscopy. Analyzing the phonon-plasmon interaction and the photocarrier relaxation processes we distinguish localized traps from ideal bandstructure behavior. We find Si to behave like a hydrogenic donor while in O doped GaN pressure reversibly induces an electron capture to a deep gap state [1]. The observed transition pressure of GPa can be compared to in the AlxGa1-xN alloy. Resembling properties of DX centers in GaAs, we discuss the consequences for AlGaN alloys, properties of buffer layers and device performance. Work in part supported by US DOE Contract DE-AC03-76SF00098.

2:30 PM D18.2 
CAPACITANCE TRANSIENT SPECTROSCOPY AND PHOTOLUMINESCENCE CHARACTERIZATION OF THE DEFECT-RELATED YELLOW AND THE Zn- RELATED VIOLET LUMINESCENCE. Peter Hacke, Hideyo Okushi, ETL, Tsukuba, JAPAN; Satoru Tanaka, RIKEN, Wako, JAPAN; Tomoyuki Kuroda, Theeradetch Detchprohm, Kazumasa Hiramatsu, Nobuhiko Sawaki, Nagoya University, Nagoya, JAPAN.

Capacitance transient measurements of n-type GaN consistently indicate the existence of a deep level from which trapped carriers are photoionized to the conduction band in a broad range from below 1.6 eV to over 2.3 eV. The position, ubiquity, broad nature, high concentration and large optical cross section of the level as characterized by optical-isothermal capacitance transient spectroscopy (O-ICTS) suggest it to be defect related and responsible for the frequently observed yellow luminescence. Mg and Zn have been observed to form color centers in GaN causing blue-violet luminescence and suppression of the yellow emission. HVPE-grown GaN deposited on sapphire with a ZnO buffer layer that was examined displays blue-violet luminescence (a result of Zn in the growth environment) and no yellow luminescence. The O-ICTS results, however, indicate a non-negligible concentration of the broad deep level associated with the yellow luminescence. Thus, if the current D-A pair model for the yellow emission is correct, the present results show that photoluminescence cannot detect the dominant mid gap state believed responsible for the yellow luminescence under certain conditions, such as in the presence of Zn impurity.

2:45 PM D18.3 
MAGNETOLUMINESCENCE AND RESONANT ELECTRONIC RAMAN SCATTERING INVESTIGATION OF DONORS IN HYDRIDE VPE AND MOCVD GaN. B.J. Skromme, H. Zhao, J. Jayapalan, and D. Wang, Dept of Electrical Engineering and Center for Solid State Electronics Research, Arizona State Univ, Tempe, AZ; R.P. Vaudo and V.M. Phanse, Epitronics, Danbury, CT; R.J. Molnar, MIT Lincoln Laboratory, Lexington, MA; B. Goldenberg, Honeywell Technology Center, Plymouth, MN; O.F. Sankey, Dept of Physics & Astronomy, Arizona State University, Tempe, AZ.

We describe a detailed spectroscopic study of the properties of the donor excited states (n=2 and above) in epitaxial GaN layers grown by hydride vapor phase epitaxy (HVPE) or metalorganic chemical vapor deposition (MOCVD), using photoluminescence and resonant electronic Raman scattering measurements in high magnetic fields up to 13 T. We observe two-electron replicas of the neutral donor-bound exciton, which exhibit Zeeman splittings into 2s, 2p(0,+,‚) states in the magnetic field, allowing accurate determination of the donor binding energy and other relevant parameters. The results are compared to a theoretical calculation that includes the anisotropy in effective mass and dielectric constant as well as a nonperturbative treatment of the magnetic field. Even better spectral resolution is obtained from electronic Raman scattering measurements on some of the thicker layers, using the 364 nm line of an Ar laser. This measurement clearly shows the anisotropy splitting of the donor state, even at zero field. The residual donors are compared among the various growth techniques, and we also study the binding energy of intentionally doped Si donors in some of the HVPE material. We find that different residual donors can be present in HVPE material grown in different laboratories, having binding energies of about 35 meV in one case and about 30.3 meV in another. The residual donor in the MOCVD material has a binding energy of about 29 meV for comparison. The Si donors are shallower than either of the residual species, showing a binding energy of only 21 meV, which suggests that neither of the residual donors in HVPE material (or that in MOCVD material) is Si.

Chairs: Isamu Akasaki and Steven P. DenBaars 
Thursday Afternoon, December 4, 1997 
Salon F (M)

3:30 PM *D19.1 
EXCITONIC ENHANCED OPTICAL GAIN OF GaN/AlGaN QW WITH LOCALIZED STATES. Takeshi Uenoyama, Central Research Laboratory, Matsushita Electric Industrial Co. Ltd., Kyoto, JAPAN.

We review the optical gain of GaN/AlGaN quantum well structures within a single particle picture and show the problem such that the large density of states for the conduction and valence bands due to the strong electronegativity and the weak spin-orbit coupling of the N atom caused the high threshold carrier density. Then, we evaluate the optical gain of GaN/AlGaN quantum wells using a many-body approach to include the screening effect, band-gap renormalization, Coulomb enhancement and so on. Especially, the Coulomb interaction is a very important factor for wide-gap semiconductors, and the many-body approach makes it clear how the exciton, which is the electron-hole bound state, is related to the optical gain. In the two-band model where only the conduction and valence bands are adopted, the excitonic absorption peak is gone when the optical gain appears by increasing the carrier density. So, we can understand that the electron-hole bound state, so-called exciton, cannot produce the optical gain. Then, we introduce the localized level above the valence band and evaluate the optical gain for the three-band model. This situation corresponds to the case where InGaN well layers of GaN/InGaN QWs in experiments have the localized states by the alloy-fluctuations of the In content. It is found that the optical gain by the transition between the conduction band and the localized state is extremely enhanced by the electron-hole interaction with the small carrier density.

4:00 PM D19.2 
HRTEM INVESTIGATIONS OF A GaN/AlxGa1-xN/GaN MULTI-QUANTUM WELL STRUCTURE WITH 0.1 < x <1. C. Kisielowski; Lawrence Berkeley National Laboratory, and Department of Materials Science, UC Berkeley, Berkeley, CA; J. Yang, APA Optics, Blaine, MN.

High Resolution Transmission Electron Microscopy was applied to study the incorporation of Al into GaN. A quantum well structure with five stacks of AlxGa1-xN/GaN was grown by MOCVD . The thickness ratio of each stack was aimed to be 1/4. The Al concentration was step by step increased from well to well by choosing x to be 0.1, 0.25, 0.5, 0.75 and 1. Unexpectedly, HRTEM reveals that in the designed structures the thickness ratio decreases with increasing x. A dependence of growth rates on the incorporated Al concentration can be quantified by the performed experiments. A transition from a two dimensional (2D) growth on (0001) planes to a three dimensional (3D) growth on (0001) and (01-10) planes occurs in the AlxGa1-xN layers for x > 0.5. Overgrowth of a GaN cap layer restores the 2D growth mode. The lattice images reveal an unusual large pattern change across AlxGa1-xN/GaN interfaces. Compositional changes across the interfaces and the wells can be mapped by the application of Quantitative HRTEM if the pattern changes are solely attributed to a local Al fluctuations. In this case Al must accumulate at the interfaces. Other possible mechanisms such as the presence of piezoelectric fields that may induce the unusual the large pattern change across AlxGa1-xN/GaN interfaces are discussed.

4:15 PM D19.3 
OPTICAL PROPERTIES OF InGaN/GaN MULTIQUANTUM WELL STRUCTURES. J.P. Bergman, N. Saksul, B. Monemar, Department of Physics and Measurement Technology, Linköping University, Linköping, SWEDEN; H. Amano, I. Akasaki, Department of Electronic and Electrical Engineering, Meijo University, Nagoya, JAPAN.

A set of GaN/InGaN multiple quantum wells (QW's) with well thickness 30Å and barrier thickness 60 Å were grown by MOCVD on sapphire substrates. The n-type Si doping of the GaN barriers was varied, in order to produce a different electron concentration in the QW's for the different samples. Optical spectra was obtained by time resolved photoluminescence spectroscopy. The data shows excitonic spectra from the QW's as well as a deeper emission presumable from the quantum dot like structure inside the QW's. The spectral shape becomes narrower and the energy position shifts towards higher energies, due to screening of the piezoelectric field, with increasing barrier doping. The two different emissions are not easily separated in a CW or time integrated spectral measurement, but are clearly observed in a time resolved spectral measurement due to their different recombination rates. The deeper emission has a long and non exponential decay, with a decay time in the order of several hundreds of nanoseconds. The higher energy excition emission has a considerable faster decay of about 5 ns.

4:30 PM D19.4 

The excellent electron transport property of the two dimensional electron gas at the AlGaN-GaN interface formed on sapphire substrates has been exploited in recent years for heterostructure field effect transistors (HFETs). The low thermal conductivity of the substrate is expected to become the limiting factor for a fully implemented power device. Recently, AlGaN/GaN high electron mobility transistors (HEMTs) have been reported using SiC as the substrates with a transconductance (Gm) of 70 mS/mm and ft and fmax of 6 GHz and 11 GHz, respectively. In this talk we shall present significantly improved results of AlGaN-GaN HFETs grown on SiC substrates. 
The layer structures used in this work consisted of a 0.15 m AIN buffer on an n-and p-type SiC. This is followed by a thick semi-insulating GaN, 50 nm of n-GaN (n=5-7x1017 cm-3), 3 nm undoped Al0.2Ga0.8 N spacer, and 30 nm of doped Al0.2Ga0.8 N. The doping level in the AlGaN has resulted in a measured sheet carrier density-times-mobility product (ns) of 2.7x1016/V-s. The HFETs had nominal source-drain separation of 2 and 3 m, gate length of 0.25 m, and gate width of 50 m. Ti/AI metal scheme was used for the Ohmic contacts. The gates of the HFETs were formed by e-beam evaporated Ni/Au. 
Gm and maximum drain current (Imax) as high as 222 mS/mm and 1.71 A/mm have been obtained from HFETs grown on n-SiC. The HFETs on p-SiC have also exhibited Gm and Imax of 230 mS/mm and 1.43 A/mm. A more interesting feature to note is the nearly absence of apparent negative differential resistance in the drain characteristics often observed from high current density HFETs fabricated on sapphire substrates. We have obtained ft and fmax of 55 GHz and 56 GHz for HFETs on p-SiC.

4:45 PM D19.5 
GAN AND ALGAN PHOTODETECTORS FOR HIGH TEMPERATURE SENSING APPLICATIONS. J. M. Van Hove, P. P. Chow, R. Hickman, II, J. J. Klaassen, A. M. Wowchak, and C. J. Polley, SVT Associates, Inc., Eden Prairie, MN.

GaN and its ternary alloys with aluminum, AlGaN, are extremely robust and chemically inert materials. The wide bandgap of this material system lends itself to the fabrication of efficient ultraviolet (UV) photodetectors which are capable of sensitive UV detection even in a high background of visible and infrared radiation. These qualities make the AlGaN material system ideally suited for sensing applications in high temperature, hostile environments such as encountered in combustion processes. The bandgap of AlGaN can be adjusted between 365 nm (3.4 eV) and 200 nm (6.2 eV). This allows photodetectors to be fabricated which have peak sensitivities tuned for UV emission bands from important combustion species such as hydroxyl radical (OH). We report on AlGaN photodiode detectors grown on (0001) sapphire by RF atomic nitrogen plasma molecular beam epitaxy. Detectors are fabricated in 1x10 element arrays with individual elements ranging in size from 250x250 microns to 450x450 microns using a chlorine-based reactive ion etch (RIE) and refractory metal ohmic contacts. At room temperature, GaN p-i-n photovoltaic detectors show peak responsivity at 360 nm as high as 0.198 A/W, corresponding to an internal quantum efficiency of 85%. These detectors also exhibit five orders of magnitude of rejection for radiation longer than 500 nm. The electrical and spectral characteristics of these detectors at temperatures up to 400 C will be presented. The short wavelength UV responsivity remains fairly constant at elevated temperatures, while the peak responsivity actually increases with increasing temperature. Results will also be presented for n-i-p epitaxial layers. The smooth surface morphology of heavily doped p-type material grown by MBE makes possible diode structures with a p-type bottom layer. The effect of moving the spectrally broader p-type material deeper into the diode structure will be discussed.

Chair: Fernando A. Ponce 
Friday Morning, December 5, 1997 
Salon F (M)

8:30 AM *D20.1 
NITRIDE LASER DIODES WITH InGaN BASED MQW STRUCTURES. Masayuki Ishikawa, Johji Nishio, Lisa Sugiura, Masaaki Onomura, and Kazuhiko Itaya, Toshiba Advanced Semiconductor Devices Laboratories, Kawasaki, JAPAN.

We demonstrate the room temperature pulsed operation of InGaN based MQW laser diodes grown using a MOCVD technique that allows the fabrication of ultrathin and precisely controlled InGaN layers. A threshold current of 530 mA, corresponding to a current density of 10.6 kA/cm2, and an oscillation wavelength of 413 nm was obtained.

9:00 AM *D20.2 
InGaN/GaN MQW SCH LASER GROWN ON SiC SUBSTRATES. K. Doverspike, G.E. Bulman, S.T. Sheppard, T.W. Weeks, M. Leonard, H.S. Kong, H. Dieringer, C. Carter and J. Edmond, Cree Research, Inc., Durham, NC;J.D.Brown, J.T. Seindle and J.F. Schetzina, Department of Physics, North Carolina State University, Raleigh, NC; Y-K Song, M. Kuball and A. Nurmikko, Brown University, Providence, RI.

The material system composed of AIN-GaN-InN alloys are an important class of semiconductors for optoelectronic applications, such as LEDs and laser diodes. Blue GaN laser development has been dominated by the successful demonstration of pulsed and cw InGaN-based laser grown on sapphire and pulsed laser grown on MgAlO by Nakamura(1). SiC exhibits a much higher thermal conductivity than sapphire and is easily cleaved to produce smooth facets. In the present research, results will be presented of pulsed room temperature laser operation in an InGaN -GaN MQW laser fabricated on 6H-SiC and consists of an 8-well InGaN/GaN MQW region with Al0.06Ga0.94N waveguide regions and Al0.13Ga0.87N cladding layers. The devices were fabricated into index-guided structures. High quality bars were fabricated by facet cleaving with a cavity length of 500m. Below threshold a broad spontaneous emission peak is observed. As the current is increased, this peak shifts to shorter wavelengths and at threshold, narrow lasing modes abruptly appear. Laser operation is confirmed by the observation of a sharp increase in light output vs. current. Above threshold, the TE/TM polarization ratio is greater than 135.

9:30 AM *D20.3 
InGaN LASER DIODE GROWN ON SiC SUBSTRATE USING LOW-PRESSURE METAL ORGANIC VAPOR PHASE EPITAXY. Akito Kuramata, Kay Domen, Reiko Soejima, Kazuhiko Horino, Shin-ichi Kubota and Toshiyuki Tanahashi, Fujitsu Laboratories Ltd., Atsugi, JAPAN.

Room-temperature pulsed operation and continuous-wave (CW) operation of InGaN quantum well laser diodes (LDs) fabricated on a sapphire substrate have been reported. Some problems are inherent, however, in the use of sapphire substrate, in terms of cavity mirror fabrication and electrode processing. The use of a SiC substrate may solve these problems because of its cleavage characteristics and electrical conductivity. Furthermore, high thermal conductivity and small lattice mismatch are expected to become the additional advantage of SiC substrate. 
In addition to the issue of substrate selection, the growth pressure is another important subject to be studied in the field of GaN based material growth. Low-pressure metal organic vapor phase epitaxy (LP-MOVPE) is attractive for mass-production because of the ease in controlling gas flow patterns. However, InGaN LDs were conventionally grown using atmospheric-pressure metal organic vapor phase epitaxy (AP-MOVPE). 
In this paper, we report on the room-temperature pulsed operation of InGaN multiple quantum well (MQW) LDs fabricated on a SiC substrate using a LP-MOVPE. The laser oscillation was observed above the threshold current of 800 mA at a peak wavelength of 414.3 nm under pulsed current injection at room-temperature. The pulse duration was 300 ns and the repetition frequency was 1 kHz. The threshold current density and differential efficiency were estimated to be 16 kA/cm2 and 0.03 W/A, respectively. The full width at half maximum (FWHM) of the lasing emission lines was between 0.03 nm and 0.21 nm. Streak-shaped far field patterns were clearly observable. The lifetime of the laser diode was more than 5 hours. 
We obtained quite promising data for future development of InGaN LDs grown on SiC substrate. The feasibility of applying LP-MOVPE to the fabrication of GaN-based LDs has also been proven.

Chair: Toby Strite 
Friday Morning, December 5, 1997 
Salon F (M)

10:30 AM *D21.1 
APPLICATION OF BLUE LASERS TO PRINTING. Ross Bringans, Xerox Palo Alto Research Center, Palo Alto, CA.

The availability of blue diode lasers would have a significant impact on several printing applications. The laser printing market is moving towards higher resolutions and higher print speeds. The optical systems for polygon scanner laser printers are diffraction limited and therefore to improve the optical resolution it is very advantageous to reduce the wavelength of the scanning laser beam from that of the IR or red lasers currently being used. The other printing application of blue lasers is exposure of film, an area where argon ion lasers are currently being used. Replacement of these by diode lasers would potentially lower system costs. In this talk, the applications of blue lasers to printing will be outlined and the technical requirements that the lasers will need to satisfy will be discussed.

11:00 AM *D21.2 

DVD disk basic specifications being fixed, electronic appliance manufactures have rushed to develop DVD-video players: Toshiba and Matsushita, both of Japanese manufacture, marketed in November/*96 their DVD-video players. Other maker follows without interval. As the number/variety of DVD disk increases, the DVD player is increasing its number; over several hundreds of thousand players have installed in American homes since this March. Besides this well acceptance by the consumer market, the "next generation" DVD-video, HD-DVD, which can deal with theater quality images, has already shown its shape in this March by Toshiba. This HD-DVD disk can reproduce high definition images which conventional TV images can not share and will be usable as package media for images of the next generation digital TV, which is now planning to start at around the beginning of the 21st century. The experimental HD-DVD disk of 12cm in diameter, which Toshiba publicized this spring, can store up to 15GB per disk, which is sufficient to store over 130 minutes high-definition images. The demonstrated system was using as a green light source an SHG laser. Toshiba is planning, as a second step, to realize 15GB storing capacity on a single side and then, with a purple-blue semiconductor laser, to put the player into the market at around 2000. To realize this plan, a blue laser of 400nm - 430nm wavelength is indispensable. As the candidate for this blue laser we are thinking a GaN laser is most promising, which was recently reported to be lasing at about 410nm for several ten hours in a room temperature. System designers are expecting as the output power for the laser is around several mW for read-only application and several tens of mW for read/write application to realize enough SNR for the electronic signal and low-cost optical head. Besides this output power level, specifications for the laser are to include life-time, temperature dependence, aspect-ratio, and consuming power level. We are expecting to have a blue laser with reasonable cost within our requested time.

11:30 AM *D21.3 
LASER BASED DISPLAYS: OPPORTUNITIES AND REQUIREMENTS. R.L. Melcher, IBM Thomas J. Watson Research Center, Yorktown Heights, NY.

A new class of large screen, high information content displays may be made possible by the combination of laser illumination with emerging microdisplay technology. Exploitation of the synergy between these two technologies can enable displays with an unprecedented breadth of application and a unique quantitative capability. Today, the critical path to these displays is limited by the laser performance and cost. Two basic types of laser displays can be considered. In the first the laser is raster scanned, amplitude modulated and projected directly onto a screen for viewing much like the electron beam in a CRT. In the second the laser is used as an illumination source to replace an arc lamp in a projection display utilizing some sort of spatial light modulating element (i.e., a microdisplay). This talk will review the status of microdisplay technology and describe the benefits provided by laser illumination. It will also summarize the requirements placed on the laser by this application. The two main features that the laser provides to display technology are its extreme brightness and its stable and narrow optical bandwidth. Microdisplay technologies are emerging which achieve very high performance, can be very small and still have extremely high spatial resolution. Conventional arc lamps are unable to efficiently illuminate these microdisplays because of their relatively large arc size. Consequently, the cost, size and weight advantage enabled by reducing the microdisplay and optical component size is not fully realized in a practical system. The brightness of a laser makes possible the efficient illumination of very small microdisplays with all the attendant advantages. The laser wavelength stability means that highly quantitative and saturated colors can be projected, enabling displays of unprecedented color gamut and reproducibility. The opportunity for lasers to impact information display technology is great but the demands on the laser device is also great. Laser illuminators must provide all three primary colors at suitable wavelengths, optical power, size and efficiency. Speckle must be eliminated and the cost must be commensurate with the benefits. The development of diode (or other) lasers to satisfy these requirements will have a major impact on how data, graphics and video information is displayed.