JOURNAL OF ELECTRONIC MATERIALS
	ABSTRACTS Volume 24, Number 7, July 1995 This Month Featuring: Regular Issue Papers and 1995 Electronic Materials Conference Program. View July 1995 Table of Contents.

Preparation and Gas-Sensing Properties of -Fe₂O₃ Thin Films
C.C. CHAI, J. PENG, and B.P. YAN
Microelectronics Institute, Xidian University, Xi'an, Shaanxi 710071 P.R. China.

KEY WORDS -Fe2O3 thin films, atmospheric pressure chemical vapordeposition (APCVD), gas-sensing property, plasma-enhanced chemical vapor deposition (PECVD)

Haematite (-Fe₂O₃) thin films are prepared by two different chemical vapor deposition (CVD) processes: the atmospheric pressure CVD (APCVD) and the plasma enhanced CVD (PECVD). The films are analyzed by x-ray diffraction and scanning electron microscopy; their gas-sensing properties are also investigated. Experimental results show that APCVD -Fe₂O₃ films are highly sensitive and selective to smoke while PECVD films are highly sensitive and selective to alcohol. A certain amount of quadrivalent metal in the films has an effect on their sensitivity and selectivity to gases. It is found that the films will "break down" under certain conditions.

Process Parameter Dependence of Impurity-Free Interdiffusion in GaAs/Al_xGa_1-xAs and In_yGa_1-yAs/GaAs Multiple Quantum Wells
S. BüRKNER,¹ M. MAIER,¹ E.C. LARKINS,¹ W. ROTHEMUND,¹ E.P. O'REILLY,² and J.D. RALSTON¹
1--Fraunhofer-Institut für Angewandte Festkörperphysik, Tullastrasse 72, D-79108 Freiburg, Germany. 2--Permanent address: Department of Physics, University of Surrey, Guildford, Surrey GU2 5XH, England.

KEY WORDS GaAs/AlGaAs, InGaAs/GaAs, interdiffusion, multiple quantum well (MQW)

The dependence of the impurity-free interdiffusion process on the properties of the dielectric cap layer has been studied, for both unstrained GaAs/Al_xGa_1-xAs and pseudomorphic In_yGa_1-yAs/GaAs MQW structures grown by molecular beam epitaxy. The influence of the cap layer thickness, composition, and deposition technique on the degree of interdiffusion were all systematically investigated. Electron-beam evaporated SiO₂ films of varying thickness, chemical-vapor-deposited SiO_xN_y films of varying composition, and spin-on SiO₂ films were used as cap layers during rapid thermal annealing (850-950°C). Photoluminescence at 10K has been employed to determine the interdiffusion-induced bandgap shifts and to calculate the corresponding Al-Ga and In-Ga interdiffusion coefficients. The latter were found to increase with the cap layer thickness (e-beam SiO₂) up to a limit determined by saturation of the out-diffused Ga concentration in the SiO₂ caps. A maximum concentration of [Ga] = 4-7 x10¹⁹ cm^-3 in the SiO₂ caps was determined using secondary ion mass spectroscopy profiling. Larger band-edge shifts are also obtained when the oxygen content of SiO_xN_y cap layers is increased, although the differences are not sufficiently large for a laterally selective interdiffusion process based on variations in cap layer composition alone. Much larger differences are obtained by using different deposition techniques for the cap layers, indicating that the porosity of the cap layer is a much more important parameter than the film composition for the realization of a laterally selective interdiffusion process. For the calculated In_0.2Ga_0.8As/GaAs interdiffusion coefficients, activation energies E_A and prefactors D₀ were estimated to ranging from 3.04 to 4.74 eV and 5 x 10^-3 to 2 x 10⁵ cm²/s, respectively, dependent on the cap layer deposition technique and the depth of the MQW from the sample surface.

Investigations on Au, Ag, and Al Schottky Diodes on Liquid Encapsulated Czochralski Grown n-GaAs<100>
S. ARULKUMARAN,¹ J. AROKIARAJ,¹ M. UDHAYASANKAR,¹ P. SANTHANARAGHAVAN,¹ J. KUMAR,² and P. RAMASAMY¹
1--Crystal Growth Centre, Anna University, Madras 600 025,India. 2--FAX: 0091-44-2352774.

KEY WORDS Ag/GaAs, Al/GaAs, Au/GaAs, constant capacitance deep level transient spectroscopy (CC-DLTS), GaAs, ohmic contact, specific contact resistance

The Schottky barrier heights of metals Au, Ag, and Al fabricated by vacuum vapor deposition on liquid encapsulated Czochralski (LEC) grown undoped n-type GaAs (n = 2.35 x 10¹⁵ cm^-3) were measured with current-voltage (I-V) and capacitance-voltage (C-V) techniques. Good ohmic contacts were obtained through an after deposition anneal at 430°C for two minutes in an argon gas atmosphere. In the as-deposited state, Au, Ag, and Al gave very similar I-V characteristics for n-type substrates with the barrier height q_b= 0.81-1.16 eV and ideality factor n = 1.02-1.15. The C-V measurement also gives the same value of barrier height. The distribution of carrier concentration along the radial distance of the wafer is of 'M' shape. The Al/GaAs interfaces give the nonideal rectification behavior. The Au/GaAs interfaces give the near ideal rectification behavior. The barrier height of this interface is 0.89-0.92 eV and the ideality factor is about 1.10-1.19. Electron traps in the wafer have been found by constant capacitance deep level transient spectroscopy (CC-DLTS). Mainly the EL2, EL6, and EL3 (EI1) trap levels are prominent.

Microstructures of InAs_1-xSb_x (x = 0.07-0.14) Alloys and Strained-Layer Superlattices
D.M. FOLLSTAEDT, R.M. BIEFELD, S.R. KURTZ, and K.C. BAUCOM
Sandia National Laboratory, Albuquerque, NM 87185-1056.

KEY WORDS CuPt ordering, InAsSb alloys, strained-layer superlattices

Growth of InAs_1-xSb_x alloys by metalorganic chemical vapor deposition at 475°C results in CuPt ordering even at Sb concentrations as low as x = 0.07-0.14. The two {111}_B variants are present, but each exists separately in 1-2 µm regions. However, the ordering is incomplete: it occurs in platelet domains lying on habit planes tilted 30° from (001) within a disordered matrix and is not continuous at the atomic scale within the domains. This ordering apparently explains the reduction in infrared emission energies relative to the bandgaps of bulk alloys. Similar ordering is found in an InAs_0.91Sb_0.09/In_0.87Ga_0.13As strained-layer superlattice with lower-than-expected emission energy. High-resolution images indicate that the superlattice layers flat and regularly spaced. Infrared LEDs have been made from such superlattices.

Chemical Beam Epitaxy and Laser-Modified Chemical Beam Epitaxy of InGaAs Using Tris-dimethylaminoarsenic
H.K. DONG, N.Y. LI, and C.W. TU
Department of Electrical and Computer Engineering, University of California at San Diego, La Jolla, CA 92093-0407.

KEY WORDS Chemical beam epitaxy (CBE), InGaAs, laser irradiation, photoluminescence (PL), quantum wells (QW), tris-dimethylaminoarsenic, x-ray rocking curve

The growth of In_xGa_1-xAs (x = 0.13-0.25) on GaAs by chemical beam epitaxy (CBE) and laser-modified CBE using trimethylindium (TMIn), triethylgallium (TEGa), and tris-dimethylaminoarsenic (TDMAAs) has been studied. Reflection high-energy electron diffraction measurements were used to investigate the growth behavior of InGaAs at different conditions. X-ray rocking curve and low-temperature photoluminescence (PL) measurements were used to characterize the InGaAs/GaAs pseudomorphic strained quantum well structures. Good InGaAs/GaAs interface and optical property were obtained by optimizing the growth condition. As determined by the x-ray simulation, laser irradiation during the InGaAs quantum well growth was found to enhance the InGaAs growth rate and reduce the indium composition in the substrate temperature range studied, 440-500°C, where good interfaces can be achieved. These changes, which are believed to be caused by laser-enhanced decomposition of TEGa and laser-enhanced desorption of TDMAAs, were found to depend on the laser power density as well. With laser irradiation, lateral variation of PL exciton peaks was observed, and the PL peaks became narrower.

Liquid Phase Epitaxy Growth of InGaAs with Rare-Earth Gettering: Characterization and Deep Level Transient Spectroscopy Studies
A. KUMAR, D. PAL, and D.N. BOSE
Semiconductor Divison, Materials Science Centre, I.I.T. Kharagpur 721302 India.

KEY WORDS InGaAs, liquid phase epitaxy (LPE), rare-earth gettering

The liquid phase epitaxial growth of high purity InGaAs layers lattice matched to InP has been studied using rare earth dysprosium (Dy) as an impurity getter. Using this getter, the electron concentration decreased from 1.2 x 10¹⁸ cm^-3 to 1 x 10¹⁵ cm^-3 while the mobility at 300K increased from 3920 to 10200 cm²/V-s. The epilayers were characterized by resistivity, Hall effect, electrochemical capacitance-voltage profiling, photoluminescence, secondary ion mass spectroscopy, double crystal x-ray diffractometry, and deep level transient spectroscopy (DLTS). Significant improvement was observed in both electrical and optical properties of the layers with an increasing amount of Dy in the melt. The amount of Dy was thus optimized (6 x 10^-4 atomic fraction) for the highest purity layer. The major background impurity was identified as silicon. The gettering of both acceptors, as well as donors, by Dy was established and gettering of oxygen was confirmed for the first time through DLTS studies.

Copper-Matrix Molybdenum Particle Composites Made from Copper Coated Molybdenum Powder
PAY YIH and D.D.L. CHUNG
Composite Materials Research Laboratory, State University of New York at Buffalo, Buffalo, NY 14260-4400.

KEY WORDS Cu, Mo, hot pressing, particulate composites, sintering

Copper-matrix molybdenum particle composites containing 33 ~ 73 wt.% Mo were fabricated by hot pressing copper coated molybdenum powder. For comparison, corresponding composites fabricated by hot pressing a mixture of copper and molybdenum powders were also made. The former method gave composites of lower porosity, higher hardness, higher compressive yield strength, lower coefficient of thermal expansion, lower electrical resistivity and higher thermal conductivity than the latter method. These differences are partly due to the separation of the molybdenum particles by the copper coating in the former case. In contrast, molybdenum particle clustering occurred in the latter case when the Mo content exceeded 53 wt.%.

Electrical Properties of Inhomogeneous SiC MIS Structures
M. KARLSTEEN, A. BARANZAHI, A. LLOYD SPETZ, M. WILLANDER, and I. LUNDSTRÖM
Department of Physics, Linköping University, S-581 83 Linköping, Sweden.

KEY WORDS 6H-SiC, gas sensor, inhomogeneous, MIS, Schottky

Current-voltage characteristics of metal contacts on 6H-SiC with a thin (5-20Å) oxide layer have been measured in the temperature range 300 to 1000K. The contacts were investigated in both H₂ and O₂-atmospheres. As the SiC surface was nonideal due to pin holes and other defects generated during the growth process, it was necessary to treat the Schottky contacts as inhomogeneous contacts. The inhomogeneity explains the nonideal current-voltage behavior of the contacts such as ideality factors much larger than unity and voltage dependent ideality factors. It was found that some metals gave Schottky contacts in the entire temperature range, while other metals were ohmic at higher temperatures. Several different contact metals were investigated: Al, Ti, TaSi_x, and Pd were found to be ohmic at high temperatures, while Pt, Pt+Cr, Ni, Cr and another TaSi_x contact were found to behave like Schottky contacts in the entire temperature range. This is a preliminary investigation of the electrical characteristics of different metals that could be useful for high temperature gas sensor purposes.

Low-Temperature Processing of Shallow Junctions Using Epitaxial and Polycrystalline CoSi₂
ERIN C. JONES,¹ NATHAN W. CHEUNG,¹ and DAVID B. FRASER²
1--Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA 94720-1772. 2--Components Research, Intel Corporation, 3065 Bowers Ave., Santa Clara, CA 95052.

KEY WORDS B, CoSi2, epitaxial silicide, shallow junction, Si doping

Cobalt disilicide is grown epitaxially on (100) Si from a 15 nm Co/2 nm Ti bilayer by rapid thermal annealing (RTA) at 900°C. Polycrystalline CoSi₂ is grown on (100) Si using a 15 nm Co layer and the same annealing condition. Silicide/p⁺-Si/n-Si diodes are made using the silicide as dopant source: ¹¹B⁺ ions are implanted at 3.5-7.5 kV and activated by RTA at 600-900°C. Shallow junctions with total junction depth (silicide plus p⁺ region) measured by high-resolution secondary ion mass spectroscopy of 100 nm are fabricated. Areal leakage current densities of 13 nA/cm² and 2 nA/cm² at a reverse bias of -5V are obtained for the epitaxial silicide and polycrystalline silicide junctions, respectively, after 700°C post-implant annealing.

Low Temperature Heteroepitaxial Growth of Si_1-xGe_x-on-Si by Photo-Enhanced Ultra High VacuumChemical Vapor Deposition Using Si₂H₆ and Ge₂H₆
C. LI, S. JOHN, and S. BANERJEE
Microelectronics Research Center, University of Texas, Austin, TX 78758.

KEY WORDS Chemical vapor deposition (CVD), digermane, disilane, photo enhanced Si1-xGex

Strained-layer Si_1-xGe_x-on-Si heteroepitaxy has been achieved by photolytic decomposition of disilane (Si₂H₆) and digermane (G e₂H₆) in an ultra high vacuum (UHV) chamber at substrate temperatures as low as 275°C. An ArF excimer laser (193 nm) shining parallel to the Si substrate was used as the UV light source to avoid surface damage and substrate heating. The partial pressures of the source gases in the reactor were chosen to vary the Ge mole fraction x from 0.06 to 0.5 in the alloy. The Si₂H₆ partial pressure was kept at 10 mTorr and the Ge₂H₆ partial pressure was varied from 0.13 to 2 mTorr with the laser intensity fixed at 2.75 x 10¹⁵ photons/cm².pulse. To fit the Si_1-xGe_x growth rate and Ge mole fraction data, the absorption cross section of Ge₂H₆ at 193 nm was set to 1 x 10^-16 cm², which is 30 times larger than that of Si₂H₆ (3.4 x 10^-18 cm²). For Si_1-xGe_x alloy growth, the deposition rate of Si increases with Ge mole fraction, resulting in increased Si_1-xGe_x alloy growth rates for higher Ge content. The increase of the Si growth rate was attributed to the enhanced adsorption rate of Si₂H₆ pyrolytically in the presence of Ge, rather than due to photolytic decomposition reaction. The Ge mole fraction in Si_1-xGe_x alloys can be predicted by a new model for Si and Ge pyrolytic and photolytic growth. The model describes the increased growth rate of Si_1-xGe_x alloys due to a Ge₂H₆ catalytic effect during photo-enhanced chemical vapor deposition.

Microstructure and Domain Configurations in Ferroelectric PbTiO₃ and Pb(Zr,Ti)O₃ Thin Fims
JANE G. ZHU,¹ M.M. Al-JASSIM,² and MARIA HUFFMAN³
1--National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401. Present address: Oak Ridge National Laboratory, Solid State Division, P.O. Box 2008, Oak Ridge, TN 37831-6057. 2--National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401. 3--Symetrix Corporation, 5055 Mark Dabling Blvd, Colorado Springs, CO 80918.

KEY WORDS Domain configurations, ferroelectric PbTiO3, Pb(Zr,Ti)O3, thin films

Ferroelectric domain configurations in PbTiO₃ and Pb(Zr_xTi_1-x)O₃ (PZT, x = 0.3 or 0.5) thin films have been studied by transmission electron microscopy. The PbTiO₃ and PZT thin films have been deposited by the ionized cluster beam technique and radio frequency sputtering, respectively. The grain size in these thin films is typically less than 0.5 µm. Lamellar 90°-domain features have been observed in both PbTiO₃ and PZT (30/70) samples. The domain walls correspond to the {011} twin boundaries. La-doping and Ca-modification are shown to affect the microstructure of the PZT films. No clear domain feature occurs in the PZT thin film that has composition near the morphotropic phase boundary. The effects of grain sizes are briefly discussed.

Electronic Characterization of Heterojunctions by Surface Potential Monitoring
L. KRONIK, M. LEIBOVITCH, E. FEFER, V. KOROBOV, and YORAM SHAPIRA
Department of Electrical Engineering-Physical Electronics, Faculty of Engineering, Tel-Aviv University, Ramal-Aviv 69978, Israel.

KEY WORDS GaAs, In2O3/InP, surface photovoltage, surface potential

An original approach for studying the formation of semiconductor heterojunctions and their electronic properties is discussed and illustrated. Monitoring the changes in the surface potential during the heterojunction formation lends itself to direct measurement of the band discontinuities, Debye length, and the width of the space-charge region at heterojunction interfaces. The contribution of an interface dipole is considered. The technique is demonstrated by a technologically significant experimental example.

T(x) Phase Diagram of the Cu₂Se-Al₂Se₃ System
B.V. KORZOUN,¹ L.A. MAKOVETSKAYA,² V.A. SAVCHUK,² V.A. RUBTSOV,² G.P. POPELNYUK,² and A.P. CHERNYAKOVA²
1--Institute of Physics of Solids and Semiconductors, P. Brovki 17, Minsk 220072, Belarus. Previous articles by this author were presented with the spelling of his name as B.V. Korzun. 2--Institute of Physics of Solids and Semiconductors, P. Brovki 17, Minsk 220072, Belarus.

KEY WORDS Differential thermal analysis, group I-III-YI2 compounds, microstructure investigation, phase diagram, semiconductor CuAlSe2, x-ray diffraction

Twenty alloys of various compositions in the Cu₂Se-Al₂Se₃ system were prepared and investigated. The T(x) phase diagram of the Cu₂Se-Al₂Se₃ system was obtained from x-ray diffraction, differential thermal analysis, and microstructure investigation for the first time. The homogeneity region of the CuAlSe₂ semiconducting compound was established.

DX-Center Energy Level Dependence on Silicon Doping Concentration in Al_0.3Ga_0.7As
G. MEDEIROS-RIBEIRO,¹ A.G. de OLIVEIRA,² G.M. RIBEIRO,² and D.A.W. SOARES³
1--University of California, Materials Department, Santa Barbara, CA 93106. 2--Departamento de Física, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, Caixa Postal 702 30161 Belo Horizonte, Brazil. 3--Departamento de Física e Química, Escola Federal de Engenharia de Itajubá, Caixa Postal 50, 37500-000, Itajubá, Brazil.

KEY WORDS Compension,DX centers, persistent photoconductivity

The Hall free carrier concentration of silicon-doped Al_0.3Ga_0.7As alloys is studied as a function of both temperature and illumination. Different thermal activation energies were observed for the DX center as a function of silicon concentration. Excitation of the samples using an infra-red source also provided data for better insight into the DX center population. We correlated the incremental free electron photo persistent population with the activation energies obtained from the temperature dependence of the measured Hall concentration. We concluded that large free carrier concentrations can yield perturbations in the lattice potential that may alter the configuration coordinate energy barriers for capture and emission of electrons by and from the DX centers.

LT-GaAs with High Breakdown Strength at Low Temperature for Power MISFET Applications
K.-M. LIPKA,¹ B. SPLINGART,¹ D. THERON,² J.K. LUO,³ G. SALMER,² H. THOMAS,³ D.V. MORGAN,³ and E. KOHN¹
1--University of Ulm, Department of Electron Devices and Circuits, Oberer Eselsberg, D-89069 Ulm, Germany. 2--IEMN, Ave. Poincare, BP 69, F-59652 Villeneuve d'Ascq, France. 3--University of Wales, School of EES Eng. Cardiff, CF1 3YH, Great Britain.

KEY WORDS Breakdown strength, LT-GaAs, power metal-insulator-semiconductor field-effect transistor (MISFET)

Low temperature grown GaAs has been fabricated containing a limited amount of excess arsenic. The material has a low conductivity in the order of 100K cm, due to hopping in a deep donor band. This -LT-GaAs was grown reproducibly by using the lattice mismatch as the primary parameter for substrate temperature calibration. Breakdown fields, in the order of 100kV/cm, are observed for planar structures and increased at low measurement emperatures. Low hopping conductivity and high breakdown field are also observed in the lossy dielectric metal-insulator-semiconductor field-effect transistor device using -LT-GaAs as a surface layer. The record radio frequency power density of 4.0W/mm at 77K is extracted from the dc output characteristics.