JOURNAL OF ELECTRONIC MATERIALS
	ABSTRACTS Volume 24, Number 4, April 1995 This Month Featuring: Proceedings of the symposium on III-V Nitrides and Silicon Carbide from the 36th Electronic Materials Conference in Boulder, Colorado, June 22-24, 1994. View April 1995 Table of Contents.

SPECIAL ISSUE PAPERS

We would like to thank the authors, reviewers, and the Journal of Electronic Materials for their efforts, which resulted in this Special Issue.

Michael R. Melloch
Purdue University
West Lafayette, Indiana

Theodore D. Moustakas
Boston University
Boston, Massachusetts

Guest Editors

Epitaxial Growth of Cubic GaN on (111) GaAs by Metalorganic Chemical Vapor Deposition
C.H. HONG, K. WANG, and D. PAVLIDIS
Solid-State Electronics Laboratory, Department of Electrical Engineering and Computer Science, The University of Michigan, Ann Arbor, MI 48109-2122.

KEY WORDS (111) GaAs, cubic GaN, GaN, interface characteristics, low-pressure metalorganic chemical vapor deposition, stacking sequence

We are reporting the first comprehensive investigation of the structural properties of cubic GaN grown on (111) GaAs substrates by low-pressure metalorganic chemical vapor deposition. The minimum full width at half maximum (FWHM) of the x-ray diffraction (XRD) peak of (111) GaN was found to be ~12 min. The use of low temperature GaN buffers helps to reduce the FWHM of the XRD. Cross-sectional transmission electron microscopy (XTEM) revealed the presence of columnar structures in the GaN film with widths of the order of 600Å. Selected area electron diffraction (SAD) patterns at the interface confirmed that cubic (111) GaN was grown in-plane with the (111) GaAs substrate. High-resolution transmission electron microscopy (HRTEM) showed that the interface characteristics of GaN on (111)A GaAs substrate were better than those of the GaN on (111)B GaAs substrate.

Native Defects and Dopants in GaN Studied through Photoluminescence and Optically Detected Magnetic Resonance
T.A. KENNEDY,¹ E.R. GLASER,¹ J.A. FREITAS, JR.,^1,4 W.E. CARLOS,¹ M. ASIF KHAN,² and D.K. WICKENDEN³
1--Naval Research Laboratory, Washington, DC 20376. 2--APA Optics, Inc., 2950 N.E. 84th Lane, Blaine, MN 55434. 3--Johns Hopkins University, Applied Physics Laboratory, Laurel, MD 20723. 4--SFA, Landover, MD 20785.

KEY WORDS Acceptors, donors, energy levels, GaN, Mg doped, optically detected magnetic resonance (ODMR), photoluminescence (PL)

Native defects and dopants in GaN grown by organometallic chemical vapor deposition have been studied with photoluminescence and optically detected magnetic resonance. For undoped samples, the combined results indicate the presence of residual shallow donors and acceptors and deep donors. A model for the capture and recombination among these defects is developed. For Mg-doped samples, the experiments reveal shallow and perturbed acceptors and shallow and deep donors. Hence, shallow and deep states for the native donor or donors appear in all samples. The Mg-acceptor is perturbed from its effective-mass state by nearby point defects.

Valence-Band Discontinuity Between GaN and AlN Measured by X-Ray Photoemission Spectroscopy
G. MARTIN,¹ S. STRITE,² A. BOTCHKAREV,¹ A. AGARWAL,¹ A. ROCKETT,¹ W.R.L. LAMBRECHT,³ B. SEGALL,³ and H. MORKOǹ
1--University of Illinois at Urbana-Champaign, Materials Research Laboratory, 104 South Goodwin Avenue, Urbana, IL 61801. 2--IBM Zürich Research Laboratory, Rüschlikon, Switzerland. 3--Department of Physics, Case Western Reserve University, Cleveland, OH 44106-7079.

KEY WORDS AlN, GaN, valence band discontinuity, x-ray photoemission

The valence-band discontinuity at a wurtzite GaN/AlN (0001) heterojunction is measured by means of x-ray photoemission spectroscopy. The method first measures the core level binding energies with respect to the valence-band maximum in both GaN and AlN bulk films. The precise location of the valence-band maximum is determined by aligning prominent features in the valence-band spectrum with calculated densities of states. Subsequent measurements of separations between Ga and Al core levels for thin overlayers of GaN film grown on AlN and vice versa yield a valence-band discontinuity of E_v = 0.8 ± 0.3 eV in the standard Type I heterojunction alignment.

Chemically Assisted Ion Beam Etching of Gallium Nitride
A.T. PING,¹ C. YOUTSEY,¹ I. ADESIDA,¹ M. ASIF KHAN,² and J.N. KUZNIA²
1--Center for Compound Semiconductor Microelectronics, Material Research Laboratory, and Department of Electrical and Computer Engineering, University of Illinois, Urbana-Champaign, IL 61801. 2--APA Optics, Inc., Blaine, MN 55449.

KEY WORDS Chemically assisted ion beam etching, gallium nitride (GaN)

Chemically assisted ion beam etching of gallium nitride (GaN) grown by metalorganic chemical vapor deposition has been characterized using an Ar ion beam and Cl₂ gas. The etch rate of GaN was found to increase linearly with Ar ion beam current density, increase linearly then saturate with Ar ion beam energy, vary slightly with Cl₂ flow rate, and lastly, increase moderately with substrate temperature. Etch rates as high as 330 nm/min were obtained at high beam energies and 210 nm/min at a more nominal level of 500 eV. The anisotropy of etched profiles improved in the presence of Cl₂ in comparison to those etched by Ar ion milling only. Elevated substrate temperatures further enhanced the anisotropy to obtain near-vertical profiles for fairly deep-etched structures. Auger electron spectroscopy was used to investigate etch-induced surface changes. Oxygen contamination was observed on the as-etched surface but a dilute HCl treatment restored the stoichiometry of the material to its unetched state.

The Effects of N⁺ Dose in Implantation into 6H-SiC Epilayers
TSUNENOBU KIMOTO,¹ AKIRA ITOH,¹ HIROYUKI MATSUNAMI,¹ TOSHITAKE NAKATA,² and MASANORI WATANABE²
1--Department of Electrical Engineering, Faculty of Engineering, Kyoto University, Sakyo, Kyoto 606-01, Japan. 2--Ion Engineering Research Institute Corporation, Tsuda, Hirakata, Osaka 573-01, Japan.

KEY WORDS Electrical activation, implant dose, ion implantation, Raman scattering, Rutherford backscattering spectroscopy, silicon carbide

Ion implantation of nitrogen (N) into p-type 6H-SiC {0001} epilayers was investigated as a function of implant dose. Lattice damage induced by implantation was characterized by Rutherford backscattering spectroscopy and Raman scattering. The damage severely increased when the implant dose exceeds 1 x 10¹⁵ cm^-2, and amorphous layers were formed at doses higher than 4 x 10¹⁵ cm^-2. By high-temperature annealing at 1500°C, relatively high electrical activation ratios (~50%) can be obtained in the case of low-dose implantation (<1 x 10¹⁵cm^-2). However, the electrical activation showed sharp decrease with increasing implant dose, which may be caused by the residual damage in implanted layers.

A Microstructural Comparison of the Initial Growth of AlN and GaN Layers on Basal Plane Sapphire and SiC Substrates by Low Pressure Metalorganic Chemical Vapor Deposition
T. GEORGE,¹ W.T. PIKE,¹ M.A. KHAN,² J.N. KUZNIA,² and P. CHANG-CHIEN¹
1--Center for Space Microelectronics Technology, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109. 2--APA Optics Inc., 2950 N. E. 84th Lane, Blaine, MN 55434.

KEY WORDS AlN, GaN, high resolution transmission electron microscopy (HRTEM), interfacial bonding, low pressure metalorganic chemical vapor deposition (MOCVD), sapphire substrates

The initial growth by low pressure metalorganic chemical vapor deposition and subsequent thermal annealing of AlN and GaN epitaxial layers on SiC and sapphire substrates is examined using high resolution transmission electron microscopy and atomic force microscopy. Growth under low pressure conditions on sapphire substrates is significantly different from that reported for convenitional (atmospheric pressure) conditions. Smooth, single crystal AlN and GaN layers were deposited on sapphire in the initial low temperature (600°C) growth step. Interfacial bonding and not lattice mismatch was found to be the determining factor for obtaining good crystallinity for the epitaxial layers as indicated by the growth results on SiC substrates.

The Influence of Nitrogen Ion Energy on the Quality of GaN Films Grown with Molecular Beam Epitaxy
T.C. FU,¹ N. NEWMAN,^2,3 E. JONES,^2,3 J.S. CHAN,¹ X. LIU,² M.D. RUBIN,² N.W. CHEUNG,¹ and E.R. WEBER^2,3
1--Department of Electrical Engineering and Computer Science, University of California, Berkeley, CA 94720. 2--Lawrence Berkeley Laboratories, 3--Department of Material Science, University of California, Berkeley, CA 94720.

KEY WORDS Activated nitrogen, GaN, molecular beam epitaxy (MBE), nitrogen ion energy

Since the growth of GaN using molecular beam epitaxy (MBE) occurs under metastable growth conditions, activated nitrogen is required to drive the forward synthesis reaction. In the process of exciting the nitrogen using a plasma or ion-beam source, species with large kinetic energies are generated. Impingement on the growth surface by these species can result in subsurface damage to the growing film, as well as an enhancement of the reverse decomposition reaction rate. In this study, we investigate the effect of the kinetic energy of the impinging nitrogen ions during growth on the resulting optical and structural properties of GaN films. Strong band-edge photoluminescence and cathodoluminescence are found when a kinetic energy of ~10 eV are used, while luminescence is not detectable when the kinetic energies exceeds 18 eV. Also, we find that the use of conductive SiC substrates results in more homogeneous luminescence than the use of insulating sapphire substrates. This is attributed to sample surface charging in the case of sapphire substrates and subsequent variation in the incident ion flux and kinetic energy across the growth surface.This study clearly shows that the quality of GaN films grown by MBE are presently limited by damage from the impingement of high energy species on the growth surface.

High-Quality GaN Heteroepitaxial Films Grown By Metalorganic Chemical Vapor Deposition
K.G. FERTITTA,¹ A.L. HOLMES,¹ F.J. CIUBA,¹ R.D. DUPUIS,¹ and F.A. PONCE²
1--Microelectronics Research Center, The University of Texas at Austin, MER 1.606/R9900, Austin, TX 78712-1100. 2--Xerox Palo Alto Research Center, Palo Alto, CA 94304.

KEY WORDS GaN, heteroepitaxial, metalorganic chemical vapor deposition (MOCVD), x-ray diffraction

In this paper, we describe the growth and characterization of high-quality GaN heteroepitaxial films grown on basal-plane sapphire substrates using metalorganic chemical vapor deposition. The quality of these films is analyzed by a variety of methods, including high-resolution x-ray diffraction, optical transmission spectroscopy, transmission electron microscopy (TEM), room-temperature photoluminescence, and room-temperature Hall measurements. The x-ray diffraction full width at half maximum value of ~37 arc s is the narrowest reported to date for any III-V nitride film on any substrate. The x-ray rocking curves for ~0.48 µm thick GaN/Al₂O₃ heteroepitaxial layers exhibit Pendellösung fringes, indicating that even relatively thin films can be of high quality. High-resolution TEM lattice images further attest to the excellent structural quality, showing the films to be completely free of stacking faults. Furthermore, no evidence of columnar growth is observed.

The Growth and Properties of Mixed Group V Nitrides
J.W. ORTON,¹ D.E. LACKLISON,¹ N. BABA-ALI,¹ C.T. FOXON,² T.S. CHENG,² S.V. NOVIKOV,² D.F.C. JOHNSTON,² S.E. HOOPER,² L.C. JENKINS,² L.J. CHALLIS,² and T.L. TANSLEY³
1--Department of Electrical and Electronic Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, England. 2--Department of Physics, University of Nottingham, University Park, Nottingham NG7 2RD, England. 3--School of Mathematics, Physics, Computing and Electronics, Maquarie University, NSW 2109, Australia.

KEY WORDS Cubic GaN, GaAsN, mixed group V nitrides

Bearing in mind the problems of finding a lattice-matched substrate for the growth of binary group III nitride films and the detrimental effect of the large activation energy associated with acceptors in GaN, we propose the study of the alloy system AlGaAsN. We predict that it may be possible to obtain a direct gap alloy, with a band gap as wide as 2.8eV, which is lattice-matched to silicon substrates. The paper reports our attempts to grow GaAsN alloy films by molecular beam epitaxy on either GaAs or GaP substrates, using a radio frequency plasma source to supply active nitrogen. Auger electron spectra demonstrate that it is possible to incorporate several tens of percent of nitrogen into GaAs films, though x-ray diffraction measurements show that such films contain mixed binary phases rather than true alloys. An interesting observation concerns the fact that it is possible to control the crystal structure of GaN films by the application of an As flux during growth. In films grown at 620°C a high As flux tends to increase the proportion of cubic GaN while also resulting in the incorporation of GaAs. Films grown at 700°C show no evidence for GaAs incorporation; at this temperature, it is possible to grow either purely cubic or purely hexagonal GaN depending on the presence or absence of the As beam.

The Effect of GaN and AlN Buffer Layers on GaN Film Properties Grown on Both C-Plane and A-Plane Sapphire
K. DOVERSPIKE,¹ L.B. ROWLAND,¹ D.K. GASKILL,¹ and J.A. FREITAS, JR,²
1--Laboratory for Advanced Materials Synthesis, Naval Research Laboratory, Washington, D.C. 2--Sachs Freeman Associates, Landover, MD.

KEY WORDS A-plane sapphire, GaN growth, organometallic vapor phase epitaxy (OMVPE)

This paper presents a comparative study of the properties of GaN grown by organometallic vapor phase epitaxy, using both a GaN and AlN buffer layer, as a function of sapphire orientation (c-plane vs a-plane). Results are presented for varying the thickness of the buffer layer, varying the growth temperature of the GaN film, and also varying the ammonia/trimethylgallium mass flow ratio. The electron Hall mobilities of GaN films grown on an AlN buffer layer were, in general, higher compared to films grown using a GaN buffer layer. In addition, growth on a-plane sapphire resulted in higher quality films (over a wider range of buffer thicknesses) than growth on c-plane sapphire. The room temperature electron mobilities were also found to be dependent on, not only the growth temperature, but also the ammonia/trimethylgallium mass flow ratio.

Operation of a Compact Electron Cyclotron Resonance Source for the Growth of Gallium Nitride by Molecular Beam Epitaxy (ECR-MBE)
R.J. MOLNAR,² R. SINGH,¹ and T.D. MOUSTAKAS¹
Molecular Beam Epitaxy Laboratory, Department of Electrical, Computer and Systems Engineering, Boston University, Boston, MA 02215. 2--Present address: MIT Lincoln Laboratory, Lexington, MA 02173.

KEY WORDS Electron cyclotron resonance-molecular beam epitaxy (ECR MBE), electron cyclotron resonance source, GaN

The operation of an ASTeX compact electron cyclotron resonance plasma source and its effect on the growth of GaN thin films by electron cyclotron resonance molecular beam epitaxy has been investigated. The role a flow limiting orifice plays in increasing plasma stability as well as reducing ion damage and impurities in resultant films has also been studied. Both optical emission spectroscopy as well as electrostatic (Langmuir) probe studies have been employed to elucidate the generation and transport of charged and neutral species. With the introduction of the flow orifice, a substantial decrease in ion induced damage as well as surface roughening in the films is observed. This can be accounted for in terms of a collisionally induced relaxation of the grad-B acceleration of charged species toward the substrate in plasma sources employ ing axial solenoidal fields.

Progress in Silicon Carbide Semiconductor Electronics Technology
PHILIP G. NEUDECK
NASA Lewis Research Center, M.S. 77-1, 21000 Brookpark Road, Cleveland, OH 44135.

KEY WORDS Power electronics, silicon carbide

Silicon carbide's demonstrated ability to function under extreme high-temperature, high-power, and/or high-radiation conditions is expected to enable significant enhancements to a far-ranging variety of applications and systems. However, improvements in crystal growth and device fabrication processes are needed before SiC-based devices and circuits can be scaled-up and incorporated into electronic systems. This paper surveys the present status of SiC-based semiconductor electronics and identifies areas where technological maturation is needed. The prospects for resolving these obstacles are discussed. Recent achievements include the monolithic realization of SiC integrated circuit operational amplifiers and digital logic circuits, as well as significant improvements to epitaxial and bulk crystal growth processes that impact the viability of this rapidly emerging technology.

Hydrogen Incorporation in Boron-Doped 6H-SiC CVD Epilayers Produced Using Site-Competition Epitaxy
D.J. LARKIN,¹ S.G. SRIDHARA,² R.P. DEVATY,² and W.J. CHOYKE²
1--NASA Lewis Research Center, Cleveland, OH 44135. 2--Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260.

KEY WORDS Boron, hydrogen passivation, silicon carbide, site-competition epitaxy

We report on the initial investigations of using site-competition epitaxy to control boron incorporation in chemical vapor deposition (CVD) 6H-SiC epilayers. Also reported herein is the detection of hydrogen in boron-doped CVD SiC epilayers and hydrogen-passivation of the boron-acceptors. Results from low temperature photoluminescence (LTPL) spectroscopy indicate that the hydrogen content increased as the capacitance-voltage (C-V) measured net hole concentration increased. Secondary ion mass spectrometry (SIMS) analysis revealed that the boron and the hydrogen incorporation both increased as the Si/C ratio was sequentially decreased within the CVD reactor during epilayer growth. Epilayers that were annealed at 1700°C in argon no longer exhibited hydrogen-related LTPL lines, and subsequent SIMS analysis confirmed the outdiffusion of hydrogen from the boron-doped SiC epilayers. The C-V measured net hole concentration increased more than threefold as a result of the 1700°C anneal, which is consistent with hydrogen passivation of the boron-acceptors. However, boron related LTPL lines were not observed before or after the 1700°C anneal.

Surface Morphology of Silicon Carbide Epitaxial Films
J. ANTHONY POWELL, DAVID J. LARKIN, and PHILLIP B. ABEL
NASA Lewis Research Center, M.S. 77-1, 21000 Brookpark Road, Cleveland, OH 44135.

KEY WORDS Epitaxy, morphology, silicon carbide (SIC)

Silicon carbide (SiC) semiconductor technology has been advancing rapidly, but there are numerous crystal growth problems that need to be solved before SiC can reach its full potential. Among these problems is a need for an improvement in the surface morphology of epitaxial films that are grown to produce device structures. Because of advantageous electrical properties, SiC development is shifting from the 6H to the 4H polytype. In this study of both 6H and 4H-SiC epilayers, atomic force microscopy and other techniques were used to characterize SiC epilayer surface morphology. Observed features included isolated growth pits a few micrometers in size in both polytypes and triangles (in 4H only) approximately 50 µm in size for epilayers 3 µm in thickness. Also observed in some epilayers were large steps with heights greater than 20 nm. We found that there are significant differences between the morphology of 6H and 4H epilayers grown under identical conditions. We were able to improve surface morphology by avoiding conditions that lead to excess silicon during the initial startup of the growth process. However, the observed morphological defect density in both 6H and 4H epilayers was still the order of 10⁴ cm^-2 and varied widely from run to run. As expected, we found that morphological defects in the SiC substrates play a role in the formation of some epilayer surface features.

Characterization and Optimization of the SiO₂/SiC Metal-Oxide Semiconductor Interface
J.N. SHENOY,¹ G.L. CHINDALORE,¹ M.R. MELLOCH,¹ J.A. COOPER, JR,¹ J.W. PALMOUR,² and K.G. IRVINE²
1--School of Electrical Engineering, Purdue University, West Lafayette, IN 47907. 2--Cree Research, Inc., 2810 Meridian Pkwy., Ste.176, Durham, NC 27713.

KEY WORDS ac conductance technique, metal-oxide semiconductor interface, SiO2/SiC

The response time of deep-lying interface states in silicon carbide metal-oxide semiconductor (MOS) capacitors may be thousands of years at room temperature. To accurately measure interface state density beyond about 0.6 eV from the band edge, it is necessary either to raise the temperature well above 300K so that all states can follow changes in DC bias, or to utilize photoexcitation to modulate the interface state population at room temperature. In this paper, we use the hi-lo capacitance-voltage technique and the ac conductance technique at elevated temperatures to characterize the MOS interface of p-type 6H-SiC. We report on the effect of surface cleaning and push/pull rates, and give the first detailed comparison of the effect of aluminum vs boron as the p-type dopant on the MOS interface. Oxides grown on 6H-SiC at 1150°C in wet O₂ followed by a 30 min in situ argon anneal have fixed charge densities as low as 9 x 10¹¹ cm^-2 and interface state densities as low as 1.5 x 10¹¹ cm^-2 eV^-l.

A Novel Method for Etching Trenches in Silicon Carbide
DEV ALOK and B.J. BALIGA
Power Semiconductor Research Center, North Carolina State University, Raleigh, NC 27695.

KEY WORDS Etching, ion implantation, silicon carbide (SiC)

In this paper, a novel trench etching technique for silicon carbide is described. In this technique, ion implantation is used to first create an amorphous silicon carbide region. The amorphous layer is then etched away by wet chemical etching. Trenches of 0.3 to 0.8 µm have been obtained using a single implantation/etching step. It has been demonstrated that deeper trenches can be obtained by repeating the implantation/etching step with platinum as a masking material. The etched surface was found to be smooth when compared with reactive ion etched surfaces reported for silicon carbide.

Low Resistivity As-Deposited Ohmic Contacts to 3C-SiC
A. MOKI,³ P. SHENOY,¹ D. ALOK,¹ B.J. BALIGA,¹ K. WONGCHOTIGUL,² and M.G. SPENCER²
1--Power Semiconductor Research Center, North Carolina State University, Raleigh, NC 27695-7924. 2--Materials Science Research Center of Excellence, School of Engineering, Howard University, Washington, DC 20059. 3--Permanent address: Sanken Electric Company Ltd., Niiza, Saitama 352, Japan.

KEY WORDS 3C-SiC, Al, contact resistivity, ohmic contact, Ti

The contact resistivities of Al and Ti ohmic contacts to n-type 3C-SiC were measured using the circular TLM method. The surface doping concentration under the contact was increased by ion-implantation of nitrogen into SiC. The contact resistivity was observed to decrease with increasing surface doping concentration for both Al and Ti contacts. The minimum value for the contact resistivities for Al and Ti contacts was 1.4 x 10^-5 and 1.5 x 10^-5 cm², respectively, at the surface doping concentration of 3 x 10²⁰ cm^-3 without any annealing of the contacts. These values are an order of magnitude lower than previously reported minimum values for as-deposited ohmic contacts on n-type 3C-SiC.

REGULAR ISSUE PAPERS

KEY WORDS Composition modulation, molecular beam epitaxy (MBE), transmission electron microscopy (TEM), ZnSeTe

Molecular beam epitaxial growth ofthe ZnSe_1-xTe_x (x = 0.44-0.47) alloy on vicinal (001) GaAs substrates tilted four, six, and nine degree-[111]A or B results in partial phase separation of the alloy with a vertical modulation between different compositions. Transmission electron microscopy images of samples grown on four degree-tilted substrates showed superlattice-like structures, with periods in the range 13.4-28.9Å. Lattice images reveal diffuse interfaces between light and dark bands. Period variations were detected in isolated regions of some samples. We present evidence that the modulation develops at the growth surface, and remains stable in the bulk at temperatures up to 450°C. Satellite spot pairs with approximate indices (h k 1 + ) were present near the zinc-blende spots in electron diffraction patterns and x-ray diffraction data, as expected from material with a sinusoidal composition profile. The orientation of the spots reveals that the modulation vector is parallel to the growth direction, rather than to [001]. The [111]A- and B-tilted samples showed significant modulation, while the five degree-[110] and on-axis material showed no detectable modulation. The modulation wavelength did not strongly depend on growth a temperature in the range examined (285-335°C). Samples showing composition modulation did not exhibit significantly altered low-temperature luminescence ispectra from material with no modulation.

The Effects of Laser Irradiation on InGaAs/GaAs Multiple Quantum Wells Grown by Metalorganic Molecular Beam Epitaxy
H.K DONG,¹ S.C.H. HUNG,² and C.W. TU¹
1--Department of Electrical and Computer Engineering, University of California at San Diego, La Jolla, CA 92093-0407. 2--Present address: Deptartment of Electrical Engineering, Stanford University, Stanford, CA 94305.

KEY WORDS Ar ion laser irradiation, InGaAs/GaAs, metalorganic molecular beam epitaxy, multiple quantum wells (MQW)

We studied the effects of Ar ion laser irradiation during the growth of InGaAs/GaAs multiple quantum wells (MQW) structures by metalorganic molecular beam epitaxy. Structural and optical properties were characterized by Nomarski microscopy, Dektak stylus profiler, and low-temperature photoluminescence (PL) measurements. For MQW structures grown at a relatively low substrate temperature (500°C), the laser irradiation influences greatly the growth process of the In_xGa_1-xAs well and results in a large blue shift of about 2000Å in the PL peak. Such a large blue shift suggests that laser modification during growth could have some novel applications in optoelectronics. On the other hand, the laser irradiation has relatively small effects on samples grown at a higher substrate temperature (550°C).

In_xGa_1-xAs Ohmic Contacts to n-Type GaAs Prepared by Sputter Deposition
MASAYUKI OKUNISHI,¹ CHIHIRO J. UCHIBORI,¹ T. OKU,¹ A. OTSUKI,¹ NAOKI ONO,² and MASANORI MURAKAMI³
1--Division of Materials Science and Engineering, Department of Engineering Science, Kyoto University, Sakyo-ku, Kyoto 606, Japan. 2--Mitsui Mining & Smelting Co, LTD, 2081 Karafune, Oaza, Omuta, Fukuoka 836, Japan. 3--Division of Materials Science and Engineering, Department of Engineering Science, Kyoto University, Sakyo-ku, Kyoto 606, Japan.

KEY WORDS InxGa1-xAs, n-type GaAs, ohmic contact, sputtering

Thermally stable, low contact resistance InAs/Ni/W contacts were previously prepared by sputter depositing InAs, Ni, and W targets in our laboratory. However, the optimum annealing temperature to provide low contact resistance (R_c) was high, resulting in rough contact surface. In the present experiment, the effects of the In concentrations of In_xGa_1-xAs targets on the optimum annealing temperature to prepare low R_c and the surface morphology of the In_xGa_1-xAs/Ni/W contacts were studied. In addition, the electrical properties and the interfacial microstructure were correlated to search the optimum In concentration to provide the minimum R_c, where the interfacial microstructure was analyzed by x-ray diffraction and transmission electron microscopy and the contact resistances (R_c) were measured by the transmission line method. The optimum annealing temperature to provide minimum R_c was reduced by 150°C by using the In_0.7Ga_0.3As targets instead of the previous targets. The contact resistance of 0.4 -mm was obtained for the In_0.7Ga_0.3As/Ni/W contacts after annealing at temperatures of around 600°C. The R_c values did not deteriorate after annealing at 400°C for 2 h. Also, the surface of this contact was smooth and no evidence of In outdiffusion on the contact surface was seen. Finally, the effect of the In concentrations at the metal/GaAs interfaces on the electrical properties will be discussed.

Nature and Evolution of Interfaces in Si/Si_1-xGe_x Superlattices
J.-M. BARIBEAU,¹ D.J. LOCKWOOD,¹ and R.L. HEADRICK²
1--Institute for Microstructural Sciences, National Research Council Canada, Ottawa, K1A OR6, Canada. 2--Cornell High Energy Synchrotron Source and Department of Applied and Engineering Physics, Cornell University, Ithaca NY 14853-8001.

KEY WORDS Correlation, interfaces, Si1-xGex/Si heterostructures, Raman scattering, x-ray reflectivity

The interfacial properties of a series of Si_1-xGe_x/Si superlattices with the same unit period (~10 nm Si and 5 nm Si_0.65Ge_0.35) but with different number of repetitions (5, 10, 20) or prepared at different temperatures in the range 250-750°C are studied by x-ray and Raman scattering techniques. For growth at 250°C, the interfaces are chemically abrupt, but exhibit a pronounced vertically correlated physical roughness that increases from ~0.3 nm near the substrate to ~2 nm at the surface. Growth at intermediate temperatures (400-550°C) resulted in structures with physically smooth interfaces, independent of the, number of periods, but intermixed over at least two monolayers, and asymmetrically broadened with an alloy/Si interface width about twice (0.25 vs 0.5 nm) that of the Si/alloy interface. In that temperature range, a roughness component aligned with the substrate miscut angle (~0.14° toward [001]) and with an in plane length scale of ~1.2 µm is observed. Higher growth temperatures (620-750°C) further enhanced the intermixing and caused undulation (100 nm length scale) of the interfaces.

The Formation of Large-Grain CuInSe₂ Films by Selenization by High-Rate Se Transport Under Moderate Vacuum Conditions
D.S. ALBIN, J.R. TUTTLE, and R. NOUFI
National Renewable Energy Laboratory, Golden, CO 80401.

KEY WORDS CuInSe2, large grain, selenization

A novel method for forming large-grain CuInSe₂ thin films by the reaction of In and Cu with Se inside a sealed ampoule is presented. A Se source and a selenization precursor consisting of a co-evaporated In+Cu film deposited onto an alumina substrate were placed at opposite ends of a closed quartz crucible sealed at 100 Torr. By establishing a small thermal gradient between the ends of the ampoule, Se was transported to the precursor where selenization of the In-Cu layer occurred. The maximum substrate temperature used was 475°C. Resulting film microstructures were extremely well formed, consisting of 3 to 5 µm grains of densely packed CuInSe₂ crystallites. It is suggested that such enhanced grain growth may be the result of the preferential formation of the Se- rich liquid, L3, under conditions of high Se flux. Such a mechanism may be exploited for fabricating large-grain CuInSe₂ films at temperatures compatible with substrates-of-choice in solar cell manufacturing.

Mobility of Modulation Doped AlGaAs/Low-Temperature MBE-Grown GaAs Heterostructures
D. SCHULTE, S. SUBRAMANIAN,* L. UNGIER, K BHATTACHARYYA, and J.R. ARTHUR
Department of Electrical and Computer Engineering, Center for Advanced Materials Research, Oregon State University, Corvallis, OR 97331. *On leave of absence from Tata Institute of Fundamental Research, Bombay, India.

KEY WORDS LT-GaAs, mobility, molecular beam epitaxy (MBE), two-dimensional electron gas (2DEG)

A study of the mobility of a novel modulation doped heterostructure in which the channel region is made of low-temperature molecular beam epitaxially grown GaAs (LT-GaAs) and all other layers are grown at normal temperatures is presented for the first time. The resistivity of the as-grown samples (in-situ annealed) is very high, as is that of single layers of bulk LT-GaAs. However, in the presence of light, the resistivity of the LT-GaAs modulation-doped field effect transistor (MODFET) is significantly lower, facilitating reliable Hall measurements. We speculate that the observed decrease in resistivity of the LT-GaAs MODFET is due to the formation of a two-dimensional electron gas (2DEG) at the heterointerface under illumination. A number of samples grown under different growth conditions were investigated. Mobilities for these samples were found to be in the range of 250 to 750 cm²/Vs at 300K and ~3000 to 5500 cm²/Vs at 77K. A first-order computer simulation was implemented to calculate the mobility of the 2DEG using the relaxation-time approximation to solve the Boltzmann equation, taking into account different scattering mechanisms. Scattering by the arsenic clusters and by ionized impurities in the LT-GaAs MODFET channel are found to be the two dominant mechanisms limiting the mobility of the LT-GaAs MODFET samples. Experimental values are in good agreement with theoretical results.

Reactivity of No-Clean Pastes and Fluxes for the Surface Mount Technology Process--Part I: Corrosion Behavior of Cu, Sn, and Pb
P.L. CAVALLOTTI,¹ G. ZANGARI,¹ and V. SIRTORI²
Dipartimento di Chimica Fisica Applicata, Politecnico di Milano, via Mancinelli 7, 20131 Milano, Italy. 2--IBM SEMEA - ECAT, Vimercate (MI), Italy.

KEY WORDS Corrosion, Cu, Sn, Pb, solder, solder pastes

No-clean fluxes allow simplifications in the surface mount technology process, but introduce reliability problems for electronic assemblies during their service life. An electrochemical method is devised to study the anodic reactivity of Sn, Pb, eutectic Sn-Pb solder alloy and Cu in aqueous solutions containing no-clean paste residues obtained during a reflow process. The potential corrosion risk of residues from two different pastes is evaluated, and the corrosion behavior of the different metals assessed. Cu corrodes faster than Sn, Pb, and the solder alloy in presence of the residues, but corrosion starts at higher overvoltages. Pb corrodes at low overvoltage; Sn is prone to oxidation and passivation. The solder alloy has an intermediate behavior between those of the pure metals.

Reactivity of No-Clean Pastes and Fluxes for the Surface Mount Technology Process--Part II: Corrosion Risk Measurements for Printed Circuit Boards and Solder Joints
P.L. CAVALLOTTI,¹ G. ZANGARI,¹ V. SIRTORI,² and A. MANARA³
Dipartimento di Chimica Fisica Applicata, Politecnico di Milano, via Mancinelli 7, 20131 Milano, Italy. 2--IBM SEMEA - ECAT, Vimercate (MI), Italy. 3--IAM - JRC Ispra, 21020 (VA), Italy.

KEY WORDS Corrosion, Cu, Pb, solder, solder pastes, Sn

The introduction of no-clean pastes and fluxes for the surface mount technology process needs testing methods of high sensitivity and reliability to evaluate the activity of the residues after the reflow process. An electrochemical method is proposed, suitable to ascertain the reactivity and the corrosion risk of these residues. Products of normal use are tested, and their behavior described and compared. Surface morphologies and compositions are examined after reflow and after testing. Local surface oxides are investigated with x-ray photoelectron spectroscopy. Surface analytical results are interpreted and related to the electrochemical behavior of solder joints. Tin is shown to have the most important role toward the onset of possible localized corrosion.

Quantitative Surface Photovoltage Spectroscopy of Semiconductor Interfaces
L. KRONIK, M. LEIBOVITCH, E. FEFER, L. BURSTEIN, and YORAM SHAPIRA
Department of Electrical Engineering, Physical Electronics, Faculty of Engineering, Tel-Aviv University, Ramat-Aviv 69978, Israel.

KEY WORDS Interface states, surface photovoltage spectroscopy

A comprehensive and quantitative method for extracting the important parameters of interface states is presented. The method is based on wavelength-, intensity-, and time-resolved surface photovoltage spectroscopy, as well as on measurements as a function of the thickness of an overlayer. Data analysis provides detailed information about interface state properties, including their energy position and distribution, density, and the transition probabilities, i.e. their thermal and optical cross sections. It is also possible to distinguish between surface and bulk states, and determine the spatial site of the states in the case of a heterostructure. Experimental examples for various III-V and II-VI compound semiconductors are given.

Molecular Beam Epitaxial Regrowth of InAs/AlSb/GaSb Heterostructures on Patterned Substrates
M. WALTHER,¹ G. KRAMER,¹ R. TSUI,¹ H. GORONKIN,¹ M. ADAM,¹ S. TEHRANI,¹ S. ROGERS,¹ and N. CAVE²
1--Motorola Inc., Phoenix Corporate Research Laboratories, 2100 E. Elliot Road, Tempe, AZ 85284. 2--Motorola Inc., Semiconductor Product Sector, 2200 W. Broadway Road, Mesa, AZ 85201.

KEY WORDS InAs/AlSb/GaSb, MBE regrowth, resonant interband tunneling diode

The molecular beam epitaxial growth of InAs/AlSb/GaSb heterostructures on GaSb epilayers patterned by dry etching is investigated. Faceted growth occurs at pattern edges, depending on the adatom species, crystallographic planes and growth parameters. The morphology of the overgrown structure is determined by (111), (110), and (100) planes near the edges of the patterned stripes, oriented in the [011], [001], or [] direction. During the regrowth of InAs, additional (311)A planes are formed at the edges for stripe orientations in the [] direction. Utilizing the faceted growth behavior at pattern edges, resonant interband tunneling diodes with a room temperature peak-to-valley current ratio of 13 have been fabricated on patterned substrates. The results indicate that this approach has the potential of realizing advanced devices with higher complexity.

Real-Time Monitoring of III-V Molecular Beam Epitaxial Growth Using Spectroscopic Ellipsometry
F.G. CELII,* W.M. DUNCAN, and Y.-C. KAO
Corporate Research & Development/Technology, MtS 147, Texas Instruments, P.O. Box 655936, Dallas, TX 75265. *Electronic mail address:celii@resbld,csc.ti-com

KEY WORDS AlAs, GaAs, in situ monitoring, reflection high energy electron diffraction (RHEED), spectroscopic ellipsometry

We report the real-time monitoring of monolayer thickness changes in AlAs and GaAs layer growth on rotating GaAs substrates using spectroscopic ellipsometry (SE). A phase-modulated spectroscopic ellipsometer was integrated with a III-V MBE system by triggering spectral acquisition synchronously with substrate rotation. Absolute thickness accuracy was verified using ex situSE measurement. Reasonable agreement was also obtained between in situ growth rate measurements by SE and reflection high energy electron diffraction. The precision and speed of this method appears suitable for real-time control of quantum devices, such as resonant-tunneling diodes.

Metal Removal from Silicon Surfaces in Wet Chemical Systems
GERD J. NORGA and LIONEL C. KIMERLING
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139.

KEY WORDS Metal contaminants, Si, wafer cleaning

The steady decrease in the minimum feature size of silicon ICs has tightened the wafer cleanliness standards needed to achieve satisfactory device yields. While wet cleaning cycles continue to be used because of their excellent ability to remove particles and native oxides, the deposition of trace amounts of metals from contaminated solutions is a growing concern. A quantitative description of metal removal and deposition phenomena is possible using E-pH diagrams. These diagrams define a thermodynamic driving force for deposition, which is the difference between the chemical potential of the metal ion in the cleaning mixture and the metal or metal oxide on the silicon surface. A mechanistic model, which combines the thermodynamic driving force with etching and mass transport effects, is presented for metal removal and deposition in oxidizing cleaning solutions.

The Temperature-Composition Phase Diagram and the Miscibility Gap of Hg_1-xCd_xTe Solid Solutions by Dynamic Mass-Loss Measurements
KUO-TONG CHEN and HERIBERT WIEDEMEIER
Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180-3590.

KEY WORDS Dynamic mass-loss measurements, Hg1-xCdxTe, Hg partial pressure, miscibility gap

The dynamic mass-loss technique has been employed to measure Hg partial pressures over Te-saturated Hg_1-xCd_xTe solid solutions with x = 0.40, 0.54, and 0.70 in the 10^-1 to 10^-4 atm range. The relative chemical potentials of HgTe in Hg_1-xCd_xTe solid solutions have been calculated using the measured Hg partial pressures at temperatures below 413°C, and fitted into an analytical expression. A Gibbs-Duhem integration yielded the relative chemical potentials of CdTe. By combining the relative chemical potentials of the binary components HgTe and CdTe, an expression for the Gibbs free energy of mixing was derived. The binodal (miscibility gap) and spinodal curves of the Hg_1-xCd_xTe solid solutions have been established with the critical temperature and composition of 221°C and Hg_0.40Cd_0.60Te.

Photoluminescence Study of ZnO Varistor Stability
M.S. RAMANACHALAM,¹ A. ROHATGI,¹ W.B. CARTER,¹ J.P. SCHAFFER,² and T.K. GUPTA³
1--Georgia Institute of Technology, Atlanta GA 30332-0245. 2--Chemical Engineering Department, Lafayatte College, Easton, PA 18042. 3--Alcoa Technical Center, ALCOA, PA 15609.

KEY WORDS Photoluminescence (PL), varistor, ZnO

Photoluminescence (PL) measurements were carried out on commercial ZnO varistor samples that were electrically stressed and/or annealed at different temperatures. Changes in the intensity of green and yellow luminescence centers were studied as a function of annealing treatment. It was found that the ZnO luminescence (green and yellow) decrease with increase in annealing temperature, reach a minimum at 700°C, and increase again beyond 800°C. Furthermore, these green and yellow luminescence bands observed in the PL spectra are quenched in the ZnO varistor samples, compared to pure ZnO. In an electrically stressed ZnO varistor sample, the luminescence intensity was found to be higher compared to the as-sintered varistor sample. Annealing of the stressed varistor sample resulted in a decrease of the luminescence intensity. These PL observations are consistent with previous deep level transient spectroscopy and doppler positron annihilation spectroscopy results. All of the experimental results are consistent with the ion migration model of degradation and can be explained using a grain boundary defect model.