Symposium Organizers
Cammy R. Abernathy University of Florida
Hongxing Jiang Kansas State University
John M. Zavada U.S. Army Research Office
I1: Nitride Based Spintronics
Session Chairs
Monday PM, November 27, 2006
Room 311 (Hynes)
9:30 AM - **I1.1
Spinodal Decomposition and Super-Paramagnetism in Dilute Magnetic Nitride Semiconductors.
Kazunori Sato 1 , Tetsuya Fukushima 1 , Hiroshi Katayama-Yoshida 1
1 , ISIR, Osaka University , Ibaraki, Osaka, Japan
Show AbstractToday, it becomes possible to reproduce experimental Curie temperatures (TC) of dilute magnetic semiconductors (DMS) such as (Ga, Mn)As and (Zn, Cr)Te accurately from first-principles [1]. In contrast to this success, agreement between the theory and experiments is not satisfactory in wide band gap DMS such as nitride DMS, and more realistic description of wide band gap DMS is needed. For example, in general, DMS systems have solubility gap and in thermal equilibrium they show phase separation (spinodal decomposition), however in the previous theoretical approaches homogeneous impurity distribution is supposed [1]. In this paper we focus on the spinodal decomposition in nitride DMS, such as (Ga, Mn)N, (Ga, Cr)N and (Al, Cr)N, and study how inhomogeneous impurity distribution affects the ferromagnetism. We calculate electronic structure of nitride DMS from first-principles by using the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) method. Then, chemical pair interactions between magnetic impurities in the effective CPA medium are estimated by using the generalized perturbation method proposed by Ducastelle et al. Magnetic exchange interactions are calculated by the Liechtenstein’s method [1]. The spinodal decomposition of DMS is simulated by applying the Monte Carlo method for the Ising model with the calculated pair interactions. Curie temperatures of simulated spinodal decomposition phases are calculated within the random phase approximation with taking disorder into account [2]. It is shown that above the percolation threshold the system is ferromagnetic and TC goes up during the decomposition process in wide band gap DMS [3]. For low concentrations, the system is super-paramagnetic (TC =0) because small isolated clusters are formed in DMS due to the decomposition. However, the Monte Carlo simulation for the magnetization process of the decomposition phases indicates that super-paramagnetic blocking temperature could be higher because the activation energy to flip the magnetization becomes larger for the decomposition phases. Finally, we take into account a layer-by-layer crystal growth condition in our simulations and show that under this condition, quasi-one-dimensional nano-structures of impurities are formed in DMS even for low concentrations [4]. This simulation could explain the ferromagnetic behavior of wide gap DMS at high temperature. 1. L. Bergqvist et al, Phys. Rev. Lett. 93, 137202 (2004), K. Sato et al., Phys. Rev. B 70, 201202 (2004), T. Fukushima et al., Jpn. J. Appl. Phys. 43, L1416 (2004) 2. S. Hilbert et al., Phys. Rev. B 70, 165203 (2004)3. K. Sato et al., Jpn. J. Appl. Phys., 44, L948 (2005) 4. T. Fukushima et al., Jpn. J. Appl. Phys., 45, L416 (2006)
10:00 AM - I1.2
Spin-orbit Coupling and Zero-field Electron Spin Splitting in AlGaN/AlN/GaN Heterostructures with a Polarization Induced Two-dimensional Electron Gas.
Cagliyan Kurdak 1 2 , Necmi Biyikli 2 , Umit Ozgur 2 , Hadis Morkoç 2 , Vladimir Litvinov 3
1 , University of Michigan, Ann Arbor, Michigan, United States, 2 , Virginia Commonwealth University, Richmond, Virginia, United States, 3 , WaveBand/Sierra Nevada Corporation, Irvine, California, United States
Show Abstract10:15 AM - I1.3
First Growth of Cr-doped GaN Layers by MOVPE for Spintronics
Nicoleta Kaluza 1 , Yong Cho 1 , Nicolas Thillosen 1 , Martina von der Ahe 1 , Vitalij Guzenko 1 , Thomas Schaepers 1 , Hilde Hardtdegen 1 , Uwe Breuer 2 , Reza Ghadimi 3 , Marian Fecioru-Morianu 3 , Bernd Beschoten 3
1 Institute of Bio- and Nanosystems, Research Center Juelich, Juelich Germany, 2 Central Division of Analytical Chemistry, Research Center Jülich, Juelich Germany, 3 II. Physikalisches Institut, RWTH Aachen, Aachen Germany
Show AbstractChromium doped GaN is a very interesting candidate for the realization of spin-based electronics due its expected Curie-temperature above 300K [1]. Up to now Cr-doped GaN layers were grown exclusively by MBE [2]. MOVPE is however the preferred growth technique for the production of high quality group III nitride layers. Our group is the first to report on the MOVPE growth of chromium doping in GaN. The growth parameters are varied with the intention of achieving specular layers with ferromagnetic properties.The experiments were carried out in a conventional horizontal MOVPE reactor. A newly designed chromium precursor, (C5H5)2Cr (bis (cyclopentadienyl) chromium) was employed. The chosen substrate was sapphire, c-plane Al2O3 and the common precursors TMGa and NH3 were used for GaN growth. The precursors supply and partial pressure ratios, carrier gas type and growth temperature were varied. The influence of the parameters on layer quality and Cr incorporation efficiency in the GaN layers was investigated. The layers were characterized by secondary ion mass spectrometry (SIMS) scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and photoluminescence (PL) measurements. Superconducting quantum interference device (SQUID) measurements were performed at zero field heating from 5 – 350 K. They were cooled down at zero and 7 Tesla before measuring.The Cr Incorporation is linearly dependent on the Cp2Cr molar fraction in the gas phase. The major growth parameter for specular Cr-doped GaN surfaces is the growth temperature which needs to be lowered by 100 K compared to the temperature used for optimum GaN growth. For a Cr-concentration of 2 x 1019 cm-3 a remnant magnetization above room temperature for the Cr-doped GaN layers was observed. Complete results of the growth study will be presented and indicate the suitability of MOVPE for the achievement of high quality Cr-doped GaN layers with magnetic properties.[1]. T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, Science 287 (2000) 1019[2]. C. Liu, F. Yun and H. Morkoç, J. of Materials Science: Materials in Electronics 16 (2005) 555.
10:30 AM - I1.4
Properties of Ferromagnetic GaGdN.
Jennifer Hite 1 , R. Frazier 1 , K. Allums 1 , R. Davies 1 , G. Thaler 1 , C. Abernathy 1 , S. Pearton 1 , J. Zavada 2
1 Materials Science & Engineering, University of Florida, Gainesville, Florida, United States, 2 , Army Research Office, Research Triangle Park, North Carolina, United States
Show AbstractOver the past decade, prospects of using dilute magnetic semiconductors in spintronic devices have spurred a great deal of interest in these materials. One of the latest to show room temperature ferromagnetism is GaGdN. The focus of this paper is on the effects of growth parameters, additional dopant species, and radiation on the magnetic properties of dilutely doped GaGdN. The films were grown via gas source molecular beam epitaxy with elemental sources for Ga and Gd and RF plasma for the nitrogen. Results from SQUID magnetometry indicated that the magnetization of the films depended on both crystalline quality and Gd cell temperatures. The films were also highly insulating. The addition of Si into GaGdN resulted in n-type material, where conductivity increased with the Si concentration. In addition, the co-doped films also exhibited room temperature ferromagnetism, reaching levels higher than the singly doped films. Irradiating GaGdN with high energy protons (10 MeV and 40 MeV) under a fluence of 5 x 10^9 cm^-2 showed a varying degree of degradation in both photoluminescence and magnetization of the material. Finally, the prospects of using this material in a spintronic device such as a spin valve will be discussed. This work was supported by the Army Research Office.
10:45 AM - I1.5
Gadolinium and Oxygen co-doping of Gallium Nitride: an LSDA+U study.
Walter Lambrecht 1 , Paul Larson 1
1 Department of Physics, Case Western Reserve University, Cleveland, Ohio, United States
Show AbstractDoping of Gd in GaN was recently shown to produce anomalously high magnetizations per Gd in the dilute limit and ferromagnetic coupling. The origin of this effect is not yet clear but there are experimental indications that oxygen impurities are involved in it. Here, we present first-principles calculations of Gd and O-impurities in GaN and their mutual interactions. The full-potential linearized muffin-tin orbital method is used in conjunction with the LSDA+U approach, in which orbital-dependent Coulomb interactions for the f-states are added to the local spin density approximation. Supercells of various sizes based on wurtzite and zincblende are used to study the effects of the Gd concentration and the distance between O and Gd dopants. Particular attention is paid to the size of the spin splitting induced in the conduction bands as function of Gd and O concentration. We show that the observed magnetizations of order 1000 Bohr magneton per Gd could be explained by a spin splitting of order 50 meV and ionization of the donor electrons into this spin-split conduction band. The challenge to first-principles calculations is now to explain how such a large splitting can come about in the dilute limit.
11:00 AM - I1:Spintronics
BREAK
I2: UV Materials and Devices
Session Chairs
Monday PM, November 27, 2006
Room 311 (Hynes)
11:30 AM - **I2.1
AlxGa1-xN Based Deep Ultraviolet Emitters and Detectors.
Asif Khan 1
1 Dept. of Electrical Engineering, University of South Carolina, Columbia, South Carolina, United States
Show AbstractAlGaN material system is ideally suited for the fabrication of deep ultraviolet devices with operation wavelengths in the solar-blind region (λ < 290 nm). Such optoelectronic devices are the key components for several commercial and military systems. To meet the transparency and absorption requirements, fabrication of solar-blind opto-devices requires epilayers with Al-alloy compositions well over 40%. Furthermore, at present, it also limits the choice of the substrate to sapphire. The lattice mismatched growth of AlxGa1-xN layers with x>0.4 leads to significant defect generation and doping issues that have a huge impact on device efficiencies. In this paper we will discuss the strategies that our group has followed to overcome these material growth issues. The key to our success is the use of a migration enhanced MOCVD (MEMOCVD) procedure for the deposition of device epilayers. This approach is ideal for avoiding the adduct formation which is a significant problem during the MOCVD growth of AlGaN. New pulsed lateral overgrowth approaches that have successfully yielded extremely low-defect AlGaN layers with Al-compositions well over 50% will also be described. These high quality AlGaN layers were then used to fabricate deep UV light-emitting-diodes (LEDs) and solar blind pin-detectors for avalanche mode operation. Characterization results for milliwatt-power deep UV LEDs will be presented. Some initial data for AlGaN pin photodetctors will also be discussed for high voltage reverse-bias operation and peak responsivity at 280 nm.
12:00 PM - I2.2
Extremely Large S/N Ratio of UV Detector Based on AlGaN/GaN JH-FET with p-GaN Gate Contact.
Shuichi Miura 1 2 , Takahiro Fujii 1 2 , Motoaki Iwaya 1 2 , Satoshi Kamiyama 1 2 , Hiroshi Amano 1 2 , Isamu Akasaki 1 2
1 Department of Materials Science and Engineering, Meijo University, Nagoya Japan, 2 21st-Century COE Program “Nano-Factory”, Meijo University, Nagoya Japan
Show Abstract12:15 PM - I2.3
MOCVD Growth of AlGaN UV LEDs (λ~300 nm) on Bulk AlN Substrates
Zaiyuan Ren 1 , Qian Sun 1 , Soon-Yong Kwon 1 , Jung Han 1 , Kristina Davitt 2 , Yoon-Kyu Song 2 , Arto Nurmikko 2 , Wayne Liu 3 , Joe Smart 3 , Leo Schowalter 3
1 Electrical Engineering, Yale University, New Haven, Connecticut, United States, 2 Division of engineering, Brown University, Providence, Rhode Island, United States, 3 , Crystal IS, Green Island, New York, United States
Show AbstractBulk AlN substrates are attractive for deep UV optoelectronic applications because of the close matches in lattice parameters and thermal expansion coefficients, as well as good thermal conductivity. Advances in bulk crystal growth over the past decade has resulted in a steady increase in wafer area and a decline in dislocation density to less than 104 cm-2. In this talk we will report the growth of AlGaN on low-dislocation bulk AlN substrates toward the realization of high-brightness UV light emitting devices.Bulk AlN substrates typically exhibit an XRD rocking curve linewidth from 0.01° to 0.03° for (0002) and 0.02° to 0.04° for (101bar2). AFM images after homoeptiaxy of 0.3 μm AlN indicate an overall smooth background with parallel and straight atomic steps. Study of nucleation evolution of AlGaN on AlN with AFM and XRD reveals a compression-driven surface roughening and strain relaxation. The rocking curve linewidths of (0002) and (101bar2) diffractions for a 1-μm n-Al0.50Ga0.50N are 0.03° and 0.05°, respectively; these numbers are among the lowest reported for MOCVD-grown AlGaN. The much-reduced dislocation density significantly impacts the device performance, to be described below, yet also appears to reduce the efficacy of plastic relaxation (through dislocations), resulting in elastic relaxation through surface roughening. A comparative study of LEDs grown on AlN with similar devices grown on conventional sapphire was conducted to benchmark the potential impact exerted by different substrates (AlN versus sapphire). Both of these devices were measured under identical conditions through their respective substrates without packaging or any additional heat sinking or light extraction enhancement schemes applied. The dc L-I-V characteristics measured directly off the chip for identical device geometries of 50 μm diameter UV LEDs grown on AlN and sapphire substrates will be presented. For UV LEDs on AlN, the electrical characteristics are markedly improved, showing a sharp turn-on and greater than 30% decrease in series resistance. TLM studies on the n-Al0.50Ga0.50N contact layer indicate a 5 times lower sheet resistance than device on sapphire. An on-chip cw light output power of 210 μW is achieved at 2.0 kA/cm2 (I=39.3 mA), corresponding to a light output power density of ~11 W/cm2. Furthermore, the light output power shows no thermal rollover, which is attributed to the 10 times increase in thermal conductivity of AlN over sapphire. A series of EL spectra at increasing current injection with emitting wavelength at around 300 nm for devices on AlN will be shown. An improvement of LED external quantum efficiency by more than 5 times points to a promising route of using AlN substrates in ultraviolet optoelectronics.
12:30 PM - I2.4
Growth and Optical Properties of Al rich AlN/AlGaN Quantum Wells.
Talal Al tahtamouni 1 , Neeraj Nepal 1 , Mim Nakarmi 1 , Jingyu Lin 1 , Hongxing Jiang 1
1 Physics, Kansas State University, Manhattan, Kansas, United States
Show AbstractAl rich AlGaN alloys are promising materials for the applications in the optoelectronic devices such as deep ultraviolet (UV) emitters and detectors in the spectral range down to 200 nm. AlGaN based UV emitters (λ<340nm) has applications in bio-chemical agent detection and medical research/ health care. To realize deep UV emission (λ< 280 nm), Al rich AlGaN based quantum wells (QWs) are required. We report here the growth ofAlN/AlxGa1-xN QWs (x ~ 0.65) on AlN/sapphire templates by metalorganic chemical vapor deposition. Deep UV photoluminescence (PL) was employed to study the optical properties of the QWs. Well width dependence was studied by varying the QW thickness (Lw) from 1 to 3 nm with fixed x ~ 0.65. Optical properties of these QWs such as the effects of alloy fluctuation, strain, spontaneous and piezoelectric fields, carrier and exciton localizations on the quantum efficiency have been studied. A systematic dependence of the PL emission peak position on Lw was observed. The PL emission line width was found to increase linearly with Lw. Furthermore, our results have shown that highest QE was obtained in AlN/Al0.65Ga0.35N QWs with well width between 2 and 2.5 nm. Polarity effect on surface morphology, structural, and electrical properties of Al rich AlGaN epilayers will be discussed. Implications of our findings on the applications of Al rich AlN/AlGaN QWs for UV emitters and detectors will also be discussed.
12:45 PM - I2.5
Fabrication of Deep Ultraviolet Light-Emitting Diodes with a Large Emission Area Using a Laser Lift-Off Technique
Koji Kawasaki 1 , Tomohiro Maegawa 1 , Misaichi Takeuchi 1 2 , Yoshinobu Aoyagi 1 2
1 , Tokyo Institute of Technology, Yokohama Japan, 2 , RIKEN, Wako Japan
Show AbstractCompact deep ultraviolet (UV) light source emitting at 200 to 300 nm with high efficiency is strongly required for numerous applications such as the efficient white light, sterilization, decomposition of pollutant, medical treatment. It is necessary for realization of a deep UV light emitting diode (LED) that not only high quality AlGaN active layer but also high extraction efficiency and reduction of operation voltage. AlGaN devices on sapphire substrate have several problems such as current crowding and self-heating due to high resistivity of thin n-type AlGaN layers. In this work, the laser lift-off technique of high Al content (>60%) AlGaN layers grown on sapphire substrate by MOCVD was studied in order to make a deep UV-LED working at low operation voltage with a large emission area.At first, a thin GaN light absorption layer was deposited after 800nm thick AlN buffer layer growth on the double-side polished (0001) sapphire substrate. Then an n-type Al0.6Ga0.3N layer was deposited and multiple-quantum wells consisting of 4 nm Al0.15Ga0.85N layers were deposited. Then, it was followed by a p-type Al0.3Ga0.7N layer, and capped by a thin p+-GaN contact layer. After Ni/Au p-type electrode was deposited on the p+-GaN surface, it was annealed at 350oC for 20min. Then the electrode surface and GaAs substrate was bonded each other by the AuGe solder. The laser-assisted lift-off process was carried out using a Q-switched Nd:YVO laser (266nm) defocused at the AlGaN active layer. An LED film on conductive substrate with the size as large as 5mm x 5mm was obtained. The n-type surface on remained LED structure was etched to 1um thickness by reactive ion etching technique using Ar and Cl2 gases and finally the n-type thin semi-transparent electrode of Al/Au was depositied It was found that the insertion of GaN laser absorption layers less than 6 nm was suitable for laser lift-off process for the AlGaN film with Al content of 60 % without any cracks on the surface. The peak wavelength of the LED was 325 nm. The vertical deep UV LED with a large area was fabricated using the AlGaN film using high Al content buffer structure. This device fabrication technique is prospective for shorter wavelength deep UV LED. The detail of a vertical deep UVLED emitting shorter wavelength than 325nm will be presented ay the conference.
I3: Material Growth
Session Chairs
Vladimir Dmitriev
Zlatko Sitar
Monday PM, November 27, 2006
Room 311 (Hynes)
2:30 PM - I3.1
The Nucleation and Suppression of Threading Dislocations at Isolated Interfacial Steps for III-Nitride Films on 4H-SiC Mesa Substrates
Mark Twigg 1 , Nabil Bassim 1 , Michael Mastro 1 , Charles Eddy 1 , Thomas Zega 1 , Richard Henry 1 , James. Culbertson 1 , Ronald Holm 1 , Philip Neudeck 2 , J. Powell 3 , Andrew Trunek 4
1 , Naval Research Laboratory, Washington, District of Columbia, United States, 2 , NASA Glenn Research Center, Cleveland, Ohio, United States, 3 , OAI, Cleveland, Ohio, United States, 4 , Sest, Inc., Cleveland, Ohio, United States
Show AbstractThe nucleation and propagation of threading dislocations in heteroepitaxial films is an all too familiar problem plaguing device layers, especially for III-nitride films. The advent of step-free 4H-SiC mesa substrates [1], however, provides a superior environment for MOCVD III-nitride growth [2]. Recent observations using cross-sectional transmission electron microscopy (XTEM) show that such step-free substrates provide a template that encourages the nucleation and growth of dislocation half-loops that propagate laterally on the c-plane, thereby providing strain relief without the deleterious effects of threading dislocations compromising the device region near the film surface [3]. For the case where threading screw dislocations are present in the substrate, unfortunately, the mesa surface is not completely step free. XTEM observations of samples prepared by focused ion beam milling (FIB) reveal that 0.5 nm bilayer steps on the surface of a 4H-SiC mesa substrate serve as nucleation sites for threading dislocations in a 2 micron GaN film grown on a 100 nm AlN nucleation layer. It is also apparent that the nucleation of threading dislocations in these films is incompatible with nucleation and propagation of lateral half-loops, with c-plane half-loops completely absent in the presence of threading dislocations.In addition to observing the configuration of c-plane half-loops and step-nucleated threading dislocations, we have also considered the excess stresses driving strain relief by these defects. Using the strain profile as a function of the distance from the mesa edge [4] and the line tension of the c-plane threading arms, we have calculated the excess stress driving the half-loop from the mesa edge into the mesa interior. We have also compared the half-loop excess stress with the excess stress driving the tilt of threading edge dislocations, which has been proposed as one of the principal strain relief mechanisms in III-nitride films [5]. The excess stress driving c-plane half-loops ranges from a few 1000 MPa at the mesa edge to few 100 MPa towards the mesa interior, while the excess stress driving the tilt of threading edge dislocations is generally in excess of 10,000 MPa. We would then expect for threading dislocations to play the dominant role in strain relief for SiC substrates where a significant number of surface steps are present. Therefore, in order to allow strain relief while preventing the formation of the perennially undesirable threading dislocations, either substrate steps must be eliminated, or the nucleation of threading dislocations at these steps must be suppressed, possibly through the control of the population of point defects by adjustment of the MOCVD growth parameters.[1] J. A. Powell et al. APL 77, 1449 (2000).[2] N. D. Bassim et al. APL 84, 021902 (2005).[3] H. Du et al. Mat. Sci. Forum, in press.[4] A. Fischer et al. Phys. Stat. Sol. (a) 171, 475 (1999).[5] P. Cantu et al. JAP 97, 103534 (2005).
2:45 PM - I3.2
Optimizing Growth Conditions of Bulk Gallium Nitride under Acidic Ammonothermal Conditions
Dirk Ehrentraut 1 , Yuji Kagamitani 1 , Naruhiro Hoshino 1 , Akira Yoshikawa 1 , Tsuguo Fukuda 1 , Hirohisa Itoh 2 , Shinichiro Kawabata 2
1 , Tohoku University, Sendai Japan, 2 , Mitsubishi Chemical Corp., Ibaraki Japan
Show AbstractThe growth of bulk GaN crystals is currently approached by several technologies like HVPE, high-pressure melt growth, sodium flux, and ammonothermal growth. The latter is much inspired by the hydrothermal growth of quartz and ZnO and describes the growth of GaN from supercritical ammonia (NH3) by employment of mineralizer and precursor. If growth conditions are optimized, both high crystal quality and large throughput can be achieved. However, current research has not yet shown true bulk GaN of satisfying crystal quality. Beside the short time period ammonothermal growth of GaN crystal is subjected to research activities, many parameters do have an impact on the physico-chemical conditions inside the ammonothermal reactor. We will show how the temperature effect of ammonium halogenides NH4X (X = Cl, Br, I) on the phase stability of GaN synthesized over a wide temperature range up to 550°C affects the phase stability of GaN. Self-nucleated hexagonal GaN (h-GaN), cubic GaN (c-GaN), and gallium oxide has been crystallized. The latter was formed from mineralizers containing a relatively high amount of oxygen impurities. The tendency to form c-GaN increases from X = Cl to I. Also, decrasing the temperature supports the formation of c-GaN. Single-phase h-GaN can be grown from X = Cl, Br at 550°C. The use of h-GaN seed crystal has a phase-stabilizing effect and lowers the stability range for overgrown h-GaN crystal. We show how the choice of precursor will have an impact on the a and c lattice parameter of self-nucleated h-GaN. The choice of precursor influences the growth rate. Best results were achieved for the growth at a supersaturation from a combined Ga/GaN precursor. The surface morphology of the grown film is not affected by the nature of the precursor. Seed etching prior growth improves the nucleation behavior.
3:00 PM - I3.3
Ammonothermal Growth and Characterization of AlxGa1-xN Crystals.
Buguo Wang 1 , Michael Callahan 2 , Michael Suscavage 2 , David Bliss 2
1 , Solid State Scientific Corporation, Nashua, New Hampshire, United States, 2 Sensor Directorate, Air Force Research Laboratory, Hanscom AFB, Massachusetts, United States
Show AbstractDevelopment of optoelectronic devices based on the III-nitride family has shown considerable progress recently. AlxGa1-xN alloys are used in fabricating optoelectronic devices in the ultraviolet region for wide applications such as water purification and solar blind detection. These devices suffer from low efficiency due in part to the high number of defects in the active regions of these devices. A low defect AlGaN substrate is therefore needed to improve the performance of these devices. Ammonothermal growth is a promising technique for producing low-defect nitride substrates. Growth of GaN and AlN crystals by the ammonothermal technique has previously been investigated by our group. GaN single crystals up to 10 mm2 × 1 mm were obtained in high-strength autoclaves using HVPE templates as seeds (Crystal Growth & Design 2006, 6, 1227-1246). However, few works on the growth of AlxGa1-xN crystals by the ammonothermal technique have been reported. Here we present the ammonothermal synthesis AlxGa1-xN crystals from alkaline ammonia solutions, and characterization of the grown crystals by photoluminescence, X-ray, and elemental analysis. Several issues affecting synthesis and growth of AlGaN crystals will be discussed.
3:15 PM - I3.4
Strain-free Low-defect-density Bulk GaN with Nonpolar Orientations.
Tanya Paskova 1 , Plamen P. Paskov 2 , Vanya Darakchieva 2 , Roland Kroeger 1 , Detlef Hommel 1 , Bo Monemar 2 , Edward Preble 3 , Andrew Hanser 3 , Mark N. Williams 3 , Michael Tutor 3
1 , Universiy of Bremen, Bremen Germany, 2 , Linkoping University, Linkoping Sweden, 3 , Kyma Technologies Inc., Raleigh, North Carolina, United States
Show AbstractThe built-in electric fields, typical for the nitrides grown in the conventional [0001] direction are among the factors hampering the performance of the nitride-based devices. Aiming to rule out this problem, growth in nonpolar directions has been intensively investigated during last years, by employing different growth techniques, substrate materials, substrate and layer orientations. However, despite all the growth optimizations the heteroepitaxially grown material on sapphire, SiC and/or LiAlO3 possesses high structural defect density, including dislocations and stacking faults. The latter, being not typical for the polar c-plane nitrides, are found to have pronounced deterioration effect on the optical properties of the nonpolar material. In addition, strain-related effects play an important role, being anisotropically distributed both in growth and in-plane directions. All this makes the growth in nonpolar directions a real challenge with severe problems remaining to be solved. Alternative approach is a growth of boules in the conventional [0001] direction up to a significant thickness by hydride vapor phase epitaxy (HVPE), that could then be sliced into bars and/or wafers with desired nonpolar orientations.The material under investigation was produced at Kyma Technologies Inc. Boules with 2” diameter and thickness up to 7-8 mm were grown in the [0001] direction on sapphire. Rectangular bars with sizes of 10x6 mm were then sliced along the nonpolar (11-20) a-plane and (1-100) m-plane to produce bulk substrates. The lattice parameters were determined by high-resolution x-ray diffraction using symmetrical plan and edge geometries, and the values (c=5.1850(7) Å, a=3.1891(4) Å) imply the material as fully relaxed. The dislocation distribution was determined by both transmission electron microscopy and cathodoluminescence to be homogeneously distributed with density in the low range of 105 cm-2. Raman scattering (RS) and infrared spectroscopic ellipsometry (IRSE) were employed to estimate the free carrier concentration, which appears below their detection limits. The results demonstrate a high structural quality of the material, implying the approach as the most promising for nonpolar substrate applications. In addition, the material as being strain-free with low density of structural and impurity defects allows a study of fine fundamental properties. In particular, the low temperature photoluminescence spectra reveal narrow lines of the free and donor-bound exciton (DBE) as well as two electron satellite transitions, which occur when a DBE recombines leaving the donor in excited states. This provides a possibility for a thorough study of the complex recombination mechanisms. Also, the nonpolar GaN substrates provide access to the complete set of GaN phonons with A1, E1 and E2 symmetry and allows a detailed study of the vibrational properties of bulk high-quality GaN and determination of the strain-free phonon frequencies by RS and IRSE.
3:30 PM - I3:Growth
BREAK
4:30 PM - I3.5
Electrical Properties of Free Standing AlN Grown by Halide Chemical Vapor Deposition.
Timothy Bogart 1 , Mark Fanton 1 , Ed Oslosky 1 , Brian Weiland 1 , Rodney Ray 1 , Adam Dilts 1 , David Snyder 1
1 , Penn State Electro-Optics Center, Freeport, Pennsylvania, United States
Show AbstractAluminum nitride films greater than 300 μm thick were grown on (0001) 6H-SiC and (0001) sapphire substrates by a novel halide chemical vapor deposition (HCVD) process. The substrate was then removed to produce free standing single crystal AlN. Growth took place in a vertical hot wall impinging jet reactor by flowing a mixture of AlCl3 and NH3 diluted in N2 over the substrate at temperatures between 1100°C and 1500°C and pressures between 25 Torr and 400 Torr. Growth temperatures are significantly higher than those used in typical HVPE growth of AlN, yet well below the temperatures used for sublimation growth. The increased growth temperature allows CVD growth rates to be significantly increased. Growth rates as high as 120 μm/hr have been obtained for epitaxial growth of AlN on both sapphire and SiC substrates compared to 3 to 60 μm/hr for typical HVPE growth rates suggesting that this process may provide an alternate route for bulk growth of AlN single crystals. In addition to increasing growth rate, the use of very high purity Al and N source materials in HCVD growth has the potential to significantly reduce the oxygen concentration in the resulting AlN compared to sublimation grown crystals using powdered or solid AlN source materials. The electrical and optical properties of free standing AlN grown by HCVD will be discussed. The electrical properties were characterized by high temperature contactless resistivity mapping (COREMA) and thermally stimulated current spectroscopy (TSC), while the optical properties were characterized by photoluminescence (PL). The concentrations of key contaminants and dopants including oxygen, carbon, and silicon were characterized by secondary ion mass spectrometry (SIMS).
4:45 PM - I3.6
GaN-ready Native AlN Based Substrates for Device Applications
Joseph Smart 1 , Wayne Liu 1 , Robert Bondokov 1 , Kenneth Morgan 1 , Timothy Bettles 1 , Sandra Schujman 1 , Leo Schowalter 1
1 , Crystal IS, Inc. , Green Island, New York, United States
Show AbstractThere is great interest in the optoelectronics industry for a low dislocation substrate suitable for the growth of efficient, long lifetime, blue laser diodes (LDs) and high brightness light emitting diodes (LEDs) based on III-nitride semiconductors. Although very high quality bulk GaN small plates were demonstrated, they are not commercially available because of difficulty in scaling up the process in order to provide sufficient area for device applications. Quasi-bulk approaches using HVPE technology involve foreign substrates that result in unacceptably high defect densities or require complex growth techniques to bend or getter defects away from the active region of the device. Growth directly on foreign substrates such as sapphire is plagued with defects, poor thermal properties and lack of natural cleavage planes for laser facets. The large lattice mismatch between sapphire and GaN, and the low thermal conductivity of the substrate contribute to poor device performance and short lifetime. Silicon carbide substrates have less lattice mismatch (for the a-b plane) with GaN and AlGaN alloys than sapphire while the thermal conductivity is ten times larger. However, the SiC crystal structure is different from that of GaN, the thermal expansion mismatch over typical growth temperature is large and the band gap is small enough that UV wavelengths of interest from AlxGa1-xN devices will be absorbed in it and so it cannot be used in flip-chip package configurations. In addition, nucleation schemes used for epitaxial growth on both sapphire and SiC substrates generate high densities of dislocation defects that propagate into subsequent epitaxial layers grown on top, affecting the performance and lifetimes of devices. The recent availability of 2” diameter bulk AlN substrates with very low dislocation densities (below 104 cm-2) which can be used to fabricate low defect density GaN buffers is an attractive substrate alternative for nitride laser diodes and high brightness LEDs. A thick GaN layer synthesized by organometallic vapor phase epitaxy (OMVPE) is placed on top of a graded transitional structure on c-plane native AlN substrates. Atomic force microscopy (AFM) images of the resulting 1-µm-thick GaN buffer yield bi-layer atomic stepped terminated surfaces. The dark circular features commonly observed at step terminations used to estimate threading dislocation densities are not present on the GaN surface. Etch pit density (EPD) counts on the GaN layers are around 106 cm-2. Re-growth on the GaN-ready AlN based substrates will produce high quality epitaxial material with lower threading dislocation densities that will contribute to increased device performance and device reliability for laser diodes and high brightness LEDs. Details of the OMVPE growth techniques, AFM, EPD and X-ray diffraction analysis will be discussed.
5:00 PM - I3.7
Bulk AlN Crystal Growth on SiC Seeds and Defects Study
Peng Lu 1 , James Edgar 1 , Raoul Schlesser 2 , Rafael Dalmau 2 , Zlatko Sitar 2
1 Chemical Engineering, Kansas State University, Manhattan, Kansas, United States, 2 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show Abstract Seeded sublimation growth of AlN was performed on Si-face, 8 ° off-axis 4H-SiC (0001) and on-axis 6H-SiC (0001) substrates. AlN layers 400 μm thick and 20 mm in diameter were grown at 1830 °C ~ 1850 °C. The c-axis growth rate was approximately 5 to 8 µm/hr. On the 8 ° off-axis SiC substrate, the AlN had step flow growth mode; i.e. lateral growth along the micro-steps of the substrate was dominant, resulting in a pronounced step and terrace structure. The step heights and terrace widths increased as the AlN film grew thicker. On the on-axis SiC substrates, the AlN nucleated as top-flat hexagonal faceted islands and then grew vertically and laterally until they coalesced together to generate a continuous film. For AlN films on SiC, the c-lattice constant was larger than pure AlN, indicating that the film had substantial SiC incorporation. Polarity sensitive etching suggested only Al-polar layers were formed regardless of the substrate misorientation. Defect selective etching revealed that there were threading dislocations and basal plane dislocations on the AlN films grown on the 8 ° off-axis substrate but only threading dislocations on the AlN film grown on the on-axis substrate. This difference was caused by the different growth mode associated with the substrate’s misorientation.
5:15 PM - I3.8
Fabrication and Characterization of 50mm diameter AlN Single-Crystal Wafers cut From Bulk Crystals
Robert Bondokov 1 , Kenneth Morgan 1 , Glen Slack 1 , Leo Schowalter 1
1 , Crystal IS, Inc. , Green Island, New York, United States
Show AbstractThe III-Nitride semiconductors are expected to play a significant role in next generation electronic and photonic devices. However, the industry needs further improvement of material quality to improve device performance and reliability. Most studies suggest that dislocation density and epitaxial layer crystallinity should be significantly improved. One of the reasons for high dislocation densities in GaN and AlGaN epitaxial layers is the use of foreign substrates such as sapphire or silicon carbide. The lattice and thermal expansion mismatch between these two substrates and GaN is far from ideal. High quality, native-nitride substrates have been shown to improve the crystallinity and reduce the dislocation densities in device layers and are greatly desired by the nitride device community. “Quasi-bulk” substrates of GaN and AlN are being developed where the initial nucleation is on a foreign substrate such as sapphire, GaAs or Si. However, those substrates still have relatively high defect densities (on average, greater than 10^6 cm-2) which seriously impact the yield and reliability of LED and RF power amplifiers. Native AlN substrates offer excellent lattice and thermal expansion match with AlGaN compounds and dislocation densities in the order of 10^3 cm-2. These substrates have been available for the past several years in limited amounts, generally in pieces of 25 mm diameter or smaller, with irregular shapes due to cracking. In this study we present the characterization of the first, crack-free 50 mm diameter, AlN substrates cut from boules grown by the sublimation-recondensation technique.Aluminum nitride boules larger than 50 mm in diameter were grown by the sublimation-recondensation technique. X-ray Laue diffraction was used to characterize the crystallinity and orientation of the boules, and 50 mm dia. substrates were sliced with typical thickness of ~500 μm. The wafers were then polished in order to meet the common standards for wafer thickness and flatness. The Al-terminated surface was finished with a proprietary chemical-mechanical process and showed RMS roughness of 0.5 nm or less as measured by atomic force microscopy (5x5 μm area). Currently, the substrates have some polycrystalline regions that are highly textured but about 50% of the total area is monocrystalline. The dislocation density in the crystalline regions of the substrate was measured by preferential chemical etching and then determining the resulting etch pit density (EPD). The etching technique involves potassium hydroxide and has been qualified through correlation with x-ray topography measurements of the dislocations. Measured EPD varied from 250 cm-2 to 3x10^4 cm-2. Other structural defects such as low angle grain boundaries, prismatic slip bands, inversion domains, have also been observed. The rare appearance of these defects will be discussed even though their role in the epitaxial growth of GaN and AlGaN is yet to be clarified.
5:30 PM - I3.9
Advances in AlN Substrate Fabrication.
Ziad Herro 1 , Dejin Zhuang 1 , Raoul Schlesser 1 , Rafael Dalmau 1 , Zlatko Sitar 1
1 Materials Science and Engineering, NC State University, Raleigh, North Carolina, United States
Show AbstractAluminum nitride (AlN) possesses a number of excellent properties that make it highly desirable for use as a III-nitride substrate material. While major crystal growth problems including lack of seeds, impurity contamination and subsequent parasitic nucleation were recently solved in our group, further increase in crystal diameter and quality is required for device preparation. In this study we present the latest advances in AlN substrate fabrication; well faceted, single crystalline AlN boules of 15 mm in diameter were grown using the seeded physical vapor transport method. High resolution XRD measurements were performed on different wafers cut from these boules. Crystal quality increased during growth as manifested in a drop in FWHM of x-ray rocking curves from 80 arcsec near the seed to 16 arcsec at growth surface. Longitudinal cuts containing the c-axis were prepared and polished to track defect generation and their origin during the growth. The main defects originated from voids existing between the seed and the seed holder prior to growth. Interestingly, longitudinal cuts have two areas with different coloration. The darker area above the seed expands during growth to cover the whole surface at the growth end. SIMS measurements were performed to track the change in impurity concentration radially and axially. AFM measurements were performed on the as-grown surfaces and showed a step-flow assisted growth mechanism.In the presentation, a detailed description of planar and point defects as well as their origin will be given. A model relating the color change to growth mode will be presented. A route to further diameter increase while maintaining high crystal quality will be discussed.
5:45 PM - I3.10
New Results on HVPE Growth of AlN, GaN, InN and Their Alloys
Alexander Usikov 1 , Vitali Sukhoveev 1 , Lisa Shapovalova