1998 MRS Spring Meeting & Exhibit

Chairs

John Palmour
Cree Research Inc
Durham, NC 27703
919-361-4770

Michael Shur 
Dept of ECSE 
Rensselaer Polytechnic Inst 
JEC 7003 Rm 7009 
Troy, NY 12180-3590 
518-276-2201

Michael Spencer
Dept of Electrical Engr
Howard Univ
Washington, DC 20059
202-806-6618

* Invited paper

SESSION F1: GaN MATERIALS AND DEVICES 
Chairs: Michael S. Shur and Steven P. DenBaars 
Monday Morning, April 13, 1998 
Golden Gate C2

8:30 AM *F1.1 
GaN MATERIALS FOR HIGH POWER MICROWAVE AMPLIFIERS. Lester F. Eastman, Cornell University, Ithaca, NY. 

AlGaN/GaN structures on SiC substrates can be used for transistors capable of high power microwave amplifiers. 200 Å  Al₃Ga₇ N/GaN undoped structures are capable of > 1000 cm²/vs mobility for the 1 x 10¹³/cm² two-dimentional electron gas. A built-in piezoelectric charge, theoretically equivalent to 3.4 x 10¹³/cm², and experimentally foumd to be 5 x 10¹³/cm², occurs in the usual AlN layer adjacent to the GaN. Electric field strength of more than 2 MV/cm eesults under the Schottky gate, on the AlGaN, with the electron current pinched off. Means of minimizing the gate leakage will be required for drain-source voltage >100V. With gates less than .25 m long, unity current gain frequency of 50 GHz and unity power gain frequency of 100 GHz have been obtained. Optimum load resistance of 100  mm results, and average microwave output power > 15 W/mm is expected. The use of high quality channels in GaN that has been overgrown over a several-micron wide mask will further improve the device frequency performance. Vertical current flow parallel to the usual dislocations will have high carrier mobility, and the use of an AlGaN/GaN heterojunction will yield injection of ballistic electron into the gated channel regions. Current technological results on material growth and processing will be included. 

9:00 AM *F1.2 
GaN/AlGaN MATERIALS FOR MICROWAVE HEMTs. U. K. Mishra, Electrical & Computer Engineering, University of California, Santa Barbara, CA. 

Using two-flow horizontal MOCVD deposition system we have obtained high quality AlGaN/GaN heterostructure materials and devices. We have recently achieved 0.25 micron gate-length AlGaN/GaN MODFETs on sapphire substrates which exhibit CW output power densities 3.1 W/mm at 18 GHz, the highest reported to date for microwave FETs in the K band. This confirms the promise of the high Al-content AlGaN/GaN MODFET structure. The AlGaN/GaN epi-structure exhibited a sheet carrier density 8*1e+12/ cm-2/ and mobility of 1500 cm2//Vs. High channel saturation current and transconductance of 800 mA/mm and 240 mS/mm respectively were also achieved along with breakdown voltages. These excellent characteristics indicate that GaN materials systems is an attractive technology for high output power microwave devices. 

9:30 AM *F1.3 
RECENT PROGRESS IN DOPED CHANEL GaN-AlGaN HFETs FOR HIGH POWER APPLICATIONS. M. Asif Khan, Dept of ECE, University of South Carolina, Columbia, SC. 

Recent results on doped channel GaN-AlGaN HFETs show that these devices are superior for high power high temperature applications. This is related to their high current capabilities up to 1.8 A/mm [1], demonstated breakdown voltages as high as 300 V and thermal impendence as low as 2 oC mm/W demonstrated for devices grown over SiC substrates. [2] The reason for these and potentially even better performances is the reduction in the ionized impurity scattering in the doped channel. The high breakdown voltage is a direct outcome of the large band-gap of the nitride system. The novel offset gate design allows us to combine the high current capability with high breakdown voltages. [3] In this talk we will review the history of development and the physics of operation and the state-of-art and future prospects of nitride based doped channel HFET devices. These structures are rapidly emerging as the devices of choice for microwave high power operation. 

10:30 AM F1.4 
RF-ASSISTED MBE GROWTH OF GaN/AlGaN MODFETs. Chanh Nguyen, X. Nguyen, and David Grider, Hughes Research Laboratories, Malibu, CA. 

Group III nitride materials have emerged as a particularly promising materials system for high power and high temperature electronics and opto-electronics devices. However, because of the unavailability of bulk GaN substrate, device materials have to be grown on substrates with large lattice mismatches such as sapphire and SiC. Due to the difficulty of heteroepitaxy, growing good epitaxial films has remained a critical issue in device development efforts. Recently we have demonstrated electronic device-quality materials grown directly on sapphire by rf-assisted MBE. In this work, we report our study on the effects of growth parameters on the characteristics of the device. Optimizing the growth condition, we have obtained GaN/AlGaN MODFETs with f_r of 28 GHz and f_max of 40 GHz. High uniformity in terms of both material and device characteristics has been achieved; less than 5% variation is observed across two-inch wafers. This level of uniformity is suitable for the scaling of MODFETs to larger gate peripheries which are necessary for practical applications. GaN/AlGaN MODFETs with 1.0 mm gate perphery have been successfully fabricated in this work using MBE grown materials. The electrical behaviors of these devices scale very well with the total gate width. Our results indicate that rf-assisted MBE can offer a viable solution to the problem of scaling GaN-based electronic devices to the level of practical interest. 

10:45 AM F1.5 
DOUBLE CHANNEL AlGaN/GaN HETEROSTRUCTURE FIELD EFFECT TRANSISTORS. R. Gaska, J.W. Yang, APA Optics, Inc., Blaine, MN; M.S. Shur, T.A. Fjeldly, Center for Integrated Electronics and Electronics Manufacturing, Rensselaer Polytechnic Institute, Troy, NY. 

We report on the fabrication, characterization, and modeling of double channel AlGaN/GaN Heterostructure Field Effect Transistors. The epilayer structure was grown by low pressure Metal Organic Vapor Pressure Epitaxy on sapphire substrates. A 50 nm AlN layer growth on sapphire was followed by the deposition of a 0.8 m nominally undoped GaN layer, 25 nm of AlGaN, a second 0.1 m nominally undoped GaN layer, and finally capped with 30 nm AlGaN barrier layer. Both AlGaN barrier layers had an Al molefraction of 0.25, and were unintentionally doped with electron concentration of approximately 10¹⁸ cm^-3. Devices with the source-drain spacing of 5 m, the gate length of 2 m and the gate width of 50 m were fabricated. Ti/Al/Ti/Au of thickness 250/700/500/1000 angstroms was annealed at 900ºC for 30 sec to make ohmic contacts, Pt/Au was used for the offset gate fabrication. The maximum source-drain current at zero gate bias was 0.5 A/mm, the maximum transconductance 140 mS/mm was measured at the gate bias of -1.5 V. The threshold voltage of these devices was -4.5 V. The 25 nm AlGaN layer sandwiched between two GaN layers forms a Semiconductor-Insulator-Semiconductor structure. As shown in [1], strong piezoelectric effects cause the depletion at the top interface of this structure and accumulation at the bottom interface. This accumulation layer creates the second conducting channel in our devices. This conducting channel is separated from the ohmic source and drain contacts by the depleted region in GaN and by the thin AlGaN layer. Hence, this bottom conducting has very large series resistances at low drain biases. At high drain biases, the electron injection drastically decreases the series resistances for the bottom channel, and it starts contributing to the overall drain current. This contribution is responsible for a characteristic kink that we observe in the output characteristics at high drain biases. We developed an analytical device model for a double channel HFET based on the Unified Charge Control Model [2]. The model describes both subthreshold and above threshold regimes of operation and accounts for the drain-bias dependent series resistances for the bottom channel and accurately reproduces the measured current-voltage characteristics. The problem of a large, drain-bias dependent series resistance for the bottom channel can be solved using ion implantation or selective epitaxial growth. Based on the modeling results, we demonstrate that such improved multichannel AlGaN-GaN heterostructures will be very promising for applications in power GaN-based HFETs. 

11:00 AM F1.6 
TEMPERATURE DEPENDENCE OF BREAKDOWN FIELD IN P--N GaN DIODES. Andrei V. Osinsky APA Optics, Inc., Blaine, MN; Michael S. Shur, Dept of ECSE, Rensselaer Polytechnic Institute Troy, NY; Remigijus Gaska, APA Optics, Inc., Blaine, MN. 

p--n GaN diode CV and IV characteristics were measured from 298 K to 600 K at reverse bias voltages up to -90 Volts. The measured temperature coefficient of the breakdown voltage  is 1x10^-4 1/degree K. This coefficient, corrected for the experimental temperature dependence of the doping density is 8x10^-41/degree K. The increase of the breakdown voltage is consistent with observations by Dmitriev et al. [1] The estimated breakdown field is of the order of 1 to 2 MV/cm. Reverse current is an exponentially increasing function of the reverse bias. At the breakdown, the current first rises sharply and then depends linearly with the reverse bias (with the slope of approximately 4 KOhms). The breakdown is accompanied by the appearance of the microplasmas emitting blue light, clearly seen by naked eye through semitransparent top contact and the sapphire substrate. The estimated microplasmas density is 10⁵ microplasms/cm² @ -90V reverse bias. The intensity of the radiation is roughly proportional to the diode current. The emission spectrum peaks at 425 nm, suggesting Mg acceptor levels are involved in optical transitions. Hence, we conclude that nonuniform impact ionization is the breakdown mechanism in p--n GaN diodes. The breakdown field increases with temperature (which is a major advantage compared to SiC). We expect that the breakdown field (on the order of 1 to 2 MV/cm) can be substantially increased by optimizing the doping profile and device geometry. 

11:15 AM F1.7 
LOW FREQUENCY NOISE IN GALLIUM NITRIDE OF n-TYPE. Nina Dyakonova, Michael Levinshtein, Ioffe Inst, Dept of Solid State Electr, St Petersburg, Russia; Sylvie Contreras, Wojtek Knap, Universite Montpellier II, FRANCE; Bernard Beaumont, P Gibart, CRHEA, Valbonne, FRANCE. 

Low frequency noise has been investigated in hexagonal polytype of gallium nitride GaN) of n-type with equilibriumn electron concentration at 300K n₀ 10¹⁷ cm^-3. The frequency and temperature dependences of spectral density of noise S were studied in the range of frequency of analysis f from 20 Hz to 20 kHz in the temperature range from 80K to 400K over the whole temperature range the slope of S versus f dependence is very close to the 1/f (flicker noise). The tempcrature dependence of the noise is rather weak. The value of Hooge constant  is very large:  5 7. These large values manifest rather low level of the stuctural perfectness of the material. The effects of the infrared and band-to-band illumination on the low frequency noise in GaN have been studied for the first time. The noise level is unaffected by illumination with photon energy E_ph < E_g even at the relatively high photoconductivity value /  50% (E_g is the forbidden gap energy). Band-to band illumination (E_ph  E_g) has influence on the low frequency noisc level over the whole temperature range investigated. At relatively high temperatures the influence of the illumination is qualitatively similar to the effect of band-to-band illumination on the low frequency noise in Si and GaAs. At relatively low temperatures the influence of the illumination on the noise in GaN differs qualitatively from the results obtained earlier for Si and GaAs. 

11:30 AM F1.8 
THERMAL STABILITY AND ELECTRICAL PROPERTIES OF OHMIC/SCHOTTKY CONTACTS TO N-GaN. J.H. Chern, Dept. of Material Science and Engineering, University of Utah, Salt Lake City, UT; L.P. Sadwick and R.J. Hwu, Dept. of Electrical Engineering, University of Utah, Salt Lake City, UT. 

The work described in this paper is part of a systematic study of contacts for n-type GaN. It is an objective of this study to evaluate the thermal stability and contact resistivity of ohmic contacts, such as Ti-Al, Pd-Al, and Cu₃Ge to GaN. Uniform ohmic contacts having low contact resistivity and a high degree of thermal stability constitutes a major obstacle for wide band gap materials. Schottky barriers with W, Au, and Pt have been reported; however, the Schottky barrier height determined by current-voltage analysis of 0.67eV, 0.65eV, and 1.0eV for W, Au, and Pt, respectively, did not follow ideal Schottky behavior where a definite increase in the barrier height with work function was expected. Low resistance ohmic contacts to n-GaN were prepared by electron beam evaporation followed by conventional thermal annealing. The resistivities of Ti-Al and Pd-Al contacts were found to be very sensitive to the annealing conditions, while good ohmic contacts using Cu-Ge were achievable by vacuum annealing. The specific contact resistivity of Cu₃Ge, Ti-Al, and Pd-Al on n-type -GaN with a doping level of 1.0x10¹⁸cm^-3 are 7.2x10^-4, 2.0x10^-6, and 1.5x10^-5cm², respectively. To prevent specific contact resistance degraded from oxidation and Ga outdiffusion and subsequent reactions with Al or Ge which rendered the surface metal discontinuous and of high resistivity, diffusion barrier and capping layers were appropriately chosen. It was found that TiW and Cr exhibited better protection among those materials used in this work. The specific contact resistivity of Ti-Al contact remained unchanged after 150hr annealing at 400ºC if TiW/Cr layers were deposited on top of the Al layer, whereas, Pd-Al contact became rectifying after annealing at 350ºC for 2hr. The specific contact resistivity of TiW(1500A)/Al(1200A)/Ti(300A) on n-GaN ranged from 2x10^-6 to 4x10^-6 cm², independent of aging time at 350ºC. 

11:45 AM F1.9 
NITRIDE BASED HIGH POWER DEVICES: TRANSPORT PROPERTIES, LINEAR DEFECTS AND GOALS. Z.Z. Bandic, P.M. Bridger, E.C. Piquette, T.F. Kuech, T.C.McGill, Thomas J. Watson, Sr. Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA; Department of Chemical Engineering, University of Wisconsin, Madison, WI. 

The wide bandgap semiconductors GaN and AlGaN show promise for high voltage standoff layers in high power devices. Some of the candidates for nitride based high power devices include GaN based Schottky rectifiers and GaN/AlGaN thyristor-like switches. 
The material properties which significantly influence the device design and performance are electron and hole diffusion lengths, recombination lifetimes and the critical field for electric breakdown. Diffusion lengths and recombination lifetimes were measured by electron beam induced currents on unintentionally doped, n and p-type GaN, and undoped AlGaN samples grown by various epitaxial techniques. To establish the possible effects of linear dislocations and other defects on the transport and breakdown properties , sample surfaces and cross-sections were analyzed by AFM and SEM. On some of the samples, our measurements indicate that the dislocations appear to be electrically active and that recombination at dislocations occupying grain boundaries limit the minority carrier lifetime to the nanosecond range. Attempts have been made to process devices of various geometries to characterize electric breakdown and measure critical fields. 
Our estimates, based on the measurements of transport properties, critical fields and the modeling of the devices proposed indicate that DARPA/EPRI goals for megawatt electronics set at  standoff voltage and  on-state current might be achieved with  thick layers grown by HVPE, at approximately  doping levels, and  device active area. Preliminary results on nitride high power device design, material properties and fabrication are reported. 

SESSION F2: CRYSTAL GROWTH 
Monday Afternoon, April 13, 1998 
Golden Gate C2

1:30 PM *F2.1 
GROWTH OF ALUMINUM NITRIDE SINGLE CRYSTALS. Glen A. Slack, Rensselaer Polytechnic Institute, Dept of Physics, Applied Physics and Astronomy, Troy, NY. 

Aluminum nitride AlN has the same crystal structure as GaN, and has very nearly the same lattice parameters and thermal expansion coefficients. Thus it is a useful substrate for growing expitaxial layers of GaN and Al_xGa_1-xN. Single crystals as large as 7mm in diameter and 12mm long have been grown by a sublimation-recondensation technique in tungsten crucibles at temperatures near 2300ºC. Property measurements on these bulk crystals show that AlN possesses a large band-gap, a high Debye temperature, a high sound velocity, a high thermal conductivity, weak impurity optical absorption bands, and high electrical resistivity. Epitaxial layers of GaN grown on these crystals exhibit promising behavior. 

2:00 PM F2.2 
LARGE AREA GaN SUBSTRATES. Olga Kryliouk, Mike Reed, Todd Dann and Tim Anderson, Chemical Engineering Department, University of Florida, Gainesville, FL. 

There is interest in producing large area bulk GaN substrates for group III nitride optoelectronic and electronic device applications. Success with traditional bulk crystal growth processes has been limited because of high decomposition pressure and the high melting temperature. We report on the successful growth of large area bulk GaN single crystals using the rapid growth rates obtainable with hydride vapor phase epitaxy (HVPE). Seed crystals were grown by MOCVD in a low pressure horizontal cold-wall reactor on LiGaO₂ substrates. This film serves to protect the LiGaO₂2 from attack by HCl. The key to obtaining high quality MOCVD GaN on LiGaO₂2 is the initial surface nitridation step. It is believed that a surface reaction product is formed during nitridation that promotes recrystallization of the underlying LiGaO₂2 and shows a lattice parameter very close to that of GaN. Furthermore, this reaction product serves as an efficient barrier for Li transport into the GaN. The surface of the MOCVD grown GaN was atomically flat (surface roughness R_g=0.036 nm) and the bulk microstructure was excellent as judged by TEM and HRXRD analysis. The LiGaO₂2 substrate with the MOCVD grown GaN capping layer was transferred to the HVPE system. The deposition was conducted in the temperature range 850 to 950ºC and at atmospheric pressure with a growth rate 50 to 70 mm/hr. The oxide substrate was subsequently removed by wet chemical etching, leaving large area free-standing wafers of GaN. LRXRD spectra revealed the GaN (0002) and (0004) diffraction peaks. Growth studies and film characterization results of thick GaN films are presented. The surface morphology was determined by AFM, the structural quality was analyzed by TEM and XRD, while the composition was investigated by AES, SNMS, SIMS and ESCA. 

2:15 PM F2.3 
A NEW APPROACH TO GROWTH OF BULK ZnO CRYSTALS FOR WIDE BANDGAP APPLICATIONS. Gaurav Agarwal and Jeff Nause, Cermet, Inc. Atlanta, GA. 

ZnO has the same crystal structure as GaN and reasonably close lattice match. This makes ZnO a promising substrate for GaN growth. Studies on growth on ZnO have been limited by difficulty in growing substrate quality crystals. Other important potential applications of single crystal ZnO include a wide range of piezoelectric devices. The conventional methods of single crystal growth of ZnO are slow and prohibitively expensive. This paper discusses a new method to grow bulk crystals from ZnO melt by controlled solidification. In the preliminary experiments, we have demonstrated that cm size ZnO crystals can be produced using this technique. Besides being very cost efficient, this method provides a controlled atmosphere to yield near-stoichiometric ZnO crystals. The quality of these crystals were evaluated using XRD, rocking curve analysis, SEM, and energy dispersive spectroscopy, inter actively. The defect density was calculated using chemical etching. These tests have indicated that good quality ZnO crystals can be grown using the melt solidification technique. 

2:30 PM F2.4 
GaN NUCLEATION MECHANISM ON A SURFACE TEMPLATE OF OXIDIZED AlAs. Nobuhiko P. Kobayashi, Junko T. Kobayashi, Won-Jin Choi, P. Daniel Dapkus, Univ. of Southern California, Departments of Materials Science and Engineering, and Electrical Engineering and Electrophysics, Los Angeles, CA. 

Single crystal GaN is grown by metalorganic chemical vapor deposition (MOCVD) on Si(111) substrate using an oxidized AlAs (AlOx) as an intermediate layer. First an AlAs layer followed by a GaAs cap layer is grown on Si(111) substrate. Subsequently the AlAs layer is selectively oxidized and converted to an AlOx layer. After an annealing at a high temperature GaN is grown on the AlOx layer formed on Si(111) substrate. X-ray diffraction measurement indicates that a single crystal hexagonal GaN is grown on the AlOx layer with the (0001) plane parallel to the Si(111) plane. The AlOx layer on which a single crystal GaN can be grown is found to contain amorphous/fine-grain phase as a majority part of its volume. The study here is focused on the mechanism by which single crystal GaN is grown on AlOx that appears to be amorphous/fine-grain phase. Contact mode atomic force microscope (C-AFM) and reflection high energy electron diffraction (RHEED) are used to examine the AlOx surface on which single crystal GaN is grown. Microscopic morphology observed by C-AFM on the AlOx surface is found to consist of fairly periodic densely packed domain-like features. The height variation of the domain-like feature decreases upon high temperature annealing in forming gas ambient. RHEED patterns taken on on the AlOx surface clearly indicates that a crystallographic surface symmetry exists on the AlOx surface. Comparison among several AlOx surfaces prepared by different procedures suggests that a possible mechanism that leads to a single crystal GaN grown on AlOx is the surface template originally created at the interface between the GaAs cap layer and the AlAs layer that is oxidized. Based on this model, GaN nucleation and growth on AlOx layers formed on different types of substrates are studied 

2:45 PM F2.5 
MOCVD GROWTH AND CHARACTERIZATION OF GAN ON SI(111). Jung Han, J.G. Fleming, T. -B. Ng, R.M. Biefeld, and D.M. Follstaedt, Sandia National Laboratories*, Albuquerque, NM. 

Growth of GaN on (111) silicon wafers has received relatively little attention thus far. The apparent insensitivity of GaN active properties to the presence of structural defects suggested the possibility of photonic device demonstration on a silicon substrate. We will report the nucleation, growth, and characterization of GaN on (111) Si monitored by an in-situ optical reflectometer. An AlN buffer layer was employed to define the epitaxial template and avoid interfacial reaction between GaN and Si at elevated temperatures. In consistency with a previous report by Watanabe et al. (J. Cryst. Growth 128, 391 (1993)), the deposition temperature of AlN buffer is deterministic to the subsequent GaN growth. Pronounced islanding and surface roughening occurred, as revealed by both in-situ reflectance and SEM imaging, when the deposition temperature (of AlN) is less than 1100 C. On the other hand, optically smooth GaN epilayers can be grown on Si (111) when AlN is grown above 1100 C. Cracking associated with the mismatch of thermal expansion coefficients (between Si and GaN) was still observed, however, which led to a broadened x-ray rocking curve linewidth ( 800 arcsec) due to crack-induced tilting and twisting.