1997 EMC: Wednesday Morning Sessions

The following sessions will be held during the 39th Electronic Materials Conference (EMC) on Wednesday morning June 25, at Colorado State University, Fort Collins, Colorado. To view other programming planned for the meeting, go to the EMC Calendar of Events.

10:00 am, Invited

Boron Nitride Cold Cathodes: R.W. Pryor and L. Li, Institute for Manufacturing Research, Wayne State University, Detroit, MI 48201; H.H. Busta, David Sarnoff Research Center, Princeton, NJ 08543

Typical electron emitting materials used as cathodes required relatively high temperatures (from ~1000°K to ~2600°K) and/or relatively high applied fields (~10³ to 10⁴V/µm) to obtain adequate electron emission per unit area for use in modern high power device structures. This paper presents observations on the room temperature emission of electrons from the n-type boron nitride-based (BN) group of cold cathode materials and also some results from n-type polycrystalline diamond for comparison, both as synthesized and after post-synthesis annealing. Both the BN and the diamond films have been observed to show an increase in electron emission after annealing by one to several orders of magnitude, depending upon the type of emitter and the specific surface treatment. The nitride-based cold cathode materials were synthesized by two different processes; a reactive laser ablation process and a reactive magnetron sputtering process. The n-type polycrystalline diamond was synthesized in the Diamond Laboratory at Wayne State University using a microwave plasma enhanced chemical vapor deposition (MPECVD) technique. Observations on the emission current and the BN resistance as a function of applied field will be presented. The annealed BN cold cathodes have been observed to yield stable emission currents as high as 2 A cm^-2 (~4 mA total current) at fields as low as ~30 V µm^-1. This is believed to be the highest reported current density yet observed from a planar film cold cathode emitter.

10:40 am, Student Paper

Electron Emission Properties of Nitrogen Containing MPCVD Diamond Films: B.L.Ward, A.T. Sowers and R.J. Nemanich, Department of Physics, North Carolina State University, PO Box 8202, Raleigh, NC 27695-8202

Wide bandgap materials such as diamond are thought to be the next generation of materials for vacuum microelectronic devices. The search for a diamond cold cathode has been inhibited by the availability of a shallow donor in diamond. Nitrogen is one dopant that is relatively easy to incorporate into the diamond lattice and can result in a donor level 1.7 eV below the conduction band edge. Recently, Geis et. al. and Okano et. al have reported field emission from Nitrogen containing diamond samples. The mechanisms for field emission from Nitrogen containing diamond films has not been determined. We will discuss the field emission properties of CVD diamond films grown by microwave plasma CVD with different concentrations of methane and nitrogen in the process gas. The films were grown with bias enhanced nucleation which gives uniform coverage for thin films. The film thickness is monitored in situ with laser reflectance interferometry (LRI), and films as thin as 0.13 µm can be grown. Each film can be transferred in vacuo to the testing chamber where field emission can be completed with a position variable anode. The anode is controlled with a UHV stepper motor with a step size of 0.05 µm. Initial measurements indicate between 50-70 V/µm for films of various thickness and Nitrogen concentration. In many instances severe surface damage is observed in the diamond film. Furthermore the measurements often vary by factors of 2-3. The relationship of the field emission to the Nitrogen incorporation and the film quality as studied with optical techniques such as Raman spectroscopy and photoluminesence will be explored. We will also discuss the relationship of the field emission to the diamond film thickness.

11:00 am, Student Paper

Growth of Oriented Diamond Film on Singlecrystalline 6H-SiC Substrates: X. Li, Y. Hayashi, S.K. Lilov and S. Nishino, Department of Electronics and Information Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan

The increasing interest in semiconductor devices for use at high power, high temperatures and high frequencies results in growing research and development activities in wide bandgap materials. In principal, the ideal material for these applications is diamond. To achieve large area epitaxial diamond films at reasonable cost it is very important to grow them on non-diamond substrates. Singlecrystalline 6H-SiC substrates with large sizes and high quality are commercially available at present. The lattice mismatch between diamond and 6h-SiC (about 13%) is significantly less than for diamond and Si (about 52%) usually used now as substrate material. For these reasons it could be expected that 6H-SiC will have potential application prospects as a non-diamond substrate material for the epitaxial growth of diamond films in the near future. The bias-enhanced nucleation (BEN) technique and hot-filament chemical vapor deposition (HF-CVD) method has been applied to singlecrystalline 6H-SiC substrates for the deposition of oriented diamond films. The experimental results showed that on (0) face not only oriented diamond with the relationship of (111) Dia. // (0)SiC and <1-10>Dia.// <>6H-SiC but also high nucleation density (>108 cm^-2) have been achieved. The differences in oriented diamond nucleation and nucleation density on (0001) and (0) faces of 6H-SiC substrate could be attributed to the specific character of the chemical bonds on these two polar faces leading to the significant difference of their specific free surface energy. Scanning electron microscopy (SEM) micrographs of as-grown samples under our experimental conditions revealed many small triangular hills which have a certain angles with substrate surface and the same arrangement direction each other. Diamond nuclei occur on the top of the triangular hills. The investigation of the relationship between the oriented diamond particles and the triangular hills are in progress now. Detailed results will be presented.

11:20 am

Valence Band Splitting and Bank Offsets of AlN, GaN, and InN: S.-H. Wei and A. Zunger, National Renewable Energy Laboratory, Golden, CO 80401

The optical properties of III-V nitrides, their alloys and superlattices depend strongly on the crystal-field and spin-orbit splittings and on the band offsets. We have systematically calculated these quantities using first-principles local density approximation (LDA) as implemented by the full potential linear augmented plan wave (FLAPW) method. We have studied the electronic structures of both wurtzite and zinc-blende A1N and GaN, and InN. In the wurtzite structure we predict crystal-field splitting parameters _CF of -217, 42 and 41 meV, respectively, and spin-orbit splitting parameters ₀ if 19, 13 and 1 meV respectively. The crystal-filed splitting _CF is found to depend strongly on the internal structural parameter u, thus accurate calculations or measurements of _CF requires careful determination of u. In the zinc-blende structure _CF = 0 and ₀ are predicted to be 19, 15 and 6 meV, respectively. The trend that D0(A1N) > ₀(GaN)> ₀(InN) is explained in terms of the d-hybridization-induced reduction in ₀. The unstrained A1N/GaN, GaN/InN, and A1N/InN valence band offsets for the wurtzite (zinc-blende) materials are predicted to be 0.81 (0.84), 0.48 (0.26), and 1.25 (1.04) eV, respectively. The difference between the wurtzite band offsets and zinc-blende band offsets is caused by strain relaxation at the interface. Our results are compared with previously published theoretical and experimental data. This work is supported by DOE/EE under contract No. DE-AC02-83-CH10093.

11:40 am

Angle-Resolved Photoemission Study of Wurtzite- and Cubic-GaN: T. Maruyama, Y. Miyajima, K. Hata, S.H. Cho and K. Akimoto, Institute of Materials Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305, Japan; H. Okumura and S. Yoshida, Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan; H. Kato, Photon Factory, National Laboratory for High Energy Physics, Oho, Tsukuba, Ibaraki 305, Japan

GaN is a promising material for near-ultraviolet optoelectronic applications. So far, many theoretical works have been reported about the band structures of both wurtzite- (w-) and cubic- (c-) GaN. Optical gain calculations based on these theoretical results have also appeared. However, only a few experimental studies have been performed for verification of the band calculations. In this study, we investigated the valence band structures of both w- and c-GaN by means of angle-resolved photoemission spectroscopy (ARPES). Comparing our results with theory, the experimental bands obtained from normal emission spectra were, on the whole, consistent with previous band calculations for both w- and c-GaN. In w-GaN, it was found the bandwidth of the upper-valence band is 6.7 eV and Ga 3d state is positioned at 15.7 eV below the valence band maximum. These results show the larger splitting between the upper valence band and the Ga 3d state than theoretically predicted, which may affect the properties of w-GaN predicted by previous calculations. The experimental band structure of c-GaN is also presented and discussed.

10:00 am, Student Paper

Organic Thin Film Transistors Using Polyphenyl Active Layers: D.J. Gundlach, Y.Y. Lin and T.N. Jackson, Department of Electrical Engineering, Penn State University, University Park, PA 16802; S.F. Nelson, Department of Physics, Colby College, Waterville, ME 04901; D.G. Schlom, Department of Materials Science and Engineering, Penn State University, University Park, PA 16802

Semiconducting organic thin films are of interest for use in displays and other broad-area electronics applications. Recently, pentacene thin film transistors with mobility and current on/off ratio comparable to hydrogenated amorphous silicon have been demonstrated. We report here that TFT's using active layers of the polyphenyls p-quaterphenyl (p-4P), p-quinquiphenyl (p-5P), and p-sexiphenyl (p-6P) also have relatively high mobility and current on/off ratio. Sexiphenyl has been considered previously as an active material for organic thin film transistors, but was not found to yield working devices [1]. We have fabricated TFT's using p-4P, p-5P, and p-6P as the active layer and find that the field effect mobility increases with increasing chain length. All devices were fabricated using vacuum evaporated films of as-purchased material with no additional purification. Heavily doped silicon was used as the device substrate and gate electrode, with a thermally grown SiO₂ layer as the gate dielectric. After active layer disposition, gold source and drain contacts were deposited through a shadow mask, and device field effect mobility was extracted from saturation region measurements. We find that p-4P devices have the lowest field-effect mobility, near 0.01 cm²/V-s, and p-5P devices have still higher field-effect mobility, near 0.04 cm²/V-s, and p-6P devices have still higher field-effect mobility near 0.1cm²/V-S. Devices fabricated using all three materials have current on/off ratios in 10⁵-10⁶ range. X-ray diffraction measurements show that these polyphenyl films deposit with the long axis of the molecule perpendicular to the SiO₂ substrate. Strong, sharp diffraction peaks indicate a high degree of molecular ordering. All three materials have dendritic grain structure when deposited on SiO₂ at room temperature, and grain size increases with increasing substrate temperature. TFT field effect mobility also increases with increasing substrate temperature, suggesting a correlation between molecular ordering and carrier transport. This research was supported by the Defense Advanced Research Project Agency (DARPA) #F33615-94-1-1464 and the National Science Foundation (NSF) #ECS-9409444.

10:20 am, Student Paper

Atomic Force Microscopy Study of High Mobility Pentacene Thin Films: Y.Y. Lin, D.J. Gundlach and T.N. Jackson, Department of Electrical Engineering, Penn State University, University Park, PA 16802; S.F. Nelson, Department of Physics, Colby College, Waterville, ME 04901; D.G. Schlom, Department of Materials Science and Engineering, Penn State University, University Park, PA 16802

Organic thin film transistors(TFTs) using pentacene as the active layer have recently shown both mobility and current on/off ratios comparable to amorphous silicon based devices. Thin films of pentacene deposited by vacuum sublimation on to smooth SiO₂ substrates held at a temperature in the 50-100°C range show a surprising degree of ordering. Using atomic force microscopy we find that film nucleation, growth, and ordering is strongly affected by substrate preparation. Pentacene deposited on smooth SiO₂ (rms roughness <3 Å) forms highly ordered layers even at room temperature. Grain size increases with substrate temperature during deposition, from less then a few tenths of a micron at room temperature to several microns at 90°C. For substrates held at about 40°C or higher, the grain structure is highly dendritic, and grains often show single molecular layer steps of pentacene. Thin film transistors fabricated using pentacene layers deposited onto smooth SiO₂ have shown field-effect mobility larger than 1 cm²/V-s. Films deposited onto SiO₂ films roughened by reactive ion etching have dramatically different characteristics. This SiO₂ has a rms roughness of about 30 Å and films deposited at all temperatures used here show only small grains and no molecular steps. Thin film transistors fabricated using pentacene layers deposited onto rough SiO₂ have significantly degraded characteristics and greatly reduced field-effect mobility. We find that treating the SiO₂ surface with a self-organizing molecule (SAM) can also greatly reduce pentacene film ordering. Treated SiO₂ surfaces remain relatively flat on a microscopic scale; AFM typically shows surfaces with rms roughness <3 Å. However, the atomic scale interactions of at least some SAMs lead to pentacene films with poor ordering, even for depositions held at elevated temperature. By using various SAMs, we are able to grow either poorly-organized, or well-organized layers. Interface control is likely to be significant problem for organic devices, and SAMs offer a powerful technique to vary the character of the interface. This research was supported by the Defense Advanced Research Project Agency (DARPA) #F33615-94-1-1464 and the National Science Foundation (NSF) #ECS-9409444.

10:40 am, Student Paper

Ink-Jet Printing of Highly Fluorescent Molecularly Doped Polymers: C.C. Wu, D. Marcy, M. Lu and J.C. Sturm, Department of Electrical Engineering, Princeton University, Princeton, NJ 08544

Patterning of organic thin films for individual devices in multicolor organic light emitting devices or displays has been problematic because of the difficulty associated with the wet processing of organic thin films and photolithography process after the organic deposition. In this paper, we demonstrate simultaneous deposition and patterning of fluorescent molecularly doped polymer (MPD) thin films directly by ink-jet printing. To avoid possible thermal damage to the active organic materials from thermal ink-jet printing head, we employ a ink-jet printing head with a piezolectric transducer, in which the polymer solution droplet is ejected from the nozzle by a pressure wave when a voltage is applied to the transducer surrounding the nozzle tube. The solution is applied from cartridge to the nozzle through a thin tube by capillarity. The polymer solution used is similar to that used for making devices by spin-coating, but with reduced concentration and therefore viscosity to assure the smooth passage of the polymer solution through the printing head. The MPD composition used in this work is poly(N-vinylcarbazole):coumarin 6 ~ 100:1 (wt), with the concentration ranging from 2 mg/ml to 10 mg/ml. We have used the ink-jet printer to print small dots on papers and ITO-coated plastic thin films. The polymer dot size is about 200 µm and the height of the dot could be a thin as several hundred angstroms, depending on the solution concentration. The dot size on ITO-coated plastic thin films will also depend on the wetability of the ITO by the solvent carrying the chemicals. The small dots of the green dyed coumarin 6-doped polymer shows green fluorescence under UV excitation, similar to the spin-coating films. Arbitrary patterns formed by these dye-doped polymer dots, such as letters and Arabic numbers, has been printed with a ~90 dpi printer containing such piezoelectric ink-jet printing heads. More work is in progress to fulfill full-color (R,G,B) printing of fluorescent polymers.

11:00 am

Operational Characteristics of Organic Crystalsf for NLO and EO Applications: N.B. Singh, T. Rajalakshmi, and I. Liberman, Science and Technology Center, Northrop Grumman Corporation, 1350 Beulah Road, Pittsburgh, PA 15235; N. Fernelius and D.E. Zelmon, Materials Directorate, Wright Laboratory, Wright-Patterson AFB, OH 45433; M.E. Glicksman, Materials Science and Engineering Department, R.P.I., 110 8th St., MRC-104, Troy, NY 12180

Organic crystals have shown promising properties for their applications in ultraviolet and near-infrared wavelength region. Very high values of the second harmonic conversion efficiency and the electo-optic coefficient have been observed. Thermal and environmental stability, growth of large optical quality crystals and fabricability are main concerns which prohibit the applications of these crystals in practical devices. Due to the lack of experimental data on physical properties and solidification of nonlinear optical organic materials, growth technology is not very developed. We have studied binary organic alloys based 3.nitoraniline (m.NA) and 2-chloro-4nitoraniline (CNA) for their solidification behavior, crystal growth and second harmonic conversion efficiency. Crystals were grown from the melt using the Bridgman method. The second harmonic efficiency was measures by using a beam of 1.5 mm diameter spot 10ns pulse and 1.06 µm wavelength. The measured efficiency of 7.7% with a 2mm fabricated sample indicated that a crystal of 1 cm length will produce very high efficiency. Crystal did not show any damage when exposed to 1.06 µm radiation at 1mJ per pulse (10 Hz repetition rate, 0.35 mm spot size and 10 ns pulse) corresponding to a 100MW/cm² power density. These results will be compared with the commercially available inorganic crystals.

11:20 am, Late News

11:40 am, Late News

10:00 am

Precipitation in Fe-doped GaAs or Ag-Implanted Al_0.3Ga_0.7As: J. Ye, J.C.P. Chang, D.T. McInturff, M.R. Melloch and J.M. Woodall, School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907; D.T. Crouse and D.D. Nolte, Physics Department, Purdue University, West Lafayette, IN 47907

The objective of this study is to search for an alternative metal/semiconductor composite to low-temperature-grown GaAs, i.e. the GaAs:As composite. GaAs:As containing a high density of As precipitates embedded in GaAs matrix has demonstrated many interesting optical and electrical features, such as high resistivity and sensitivity to subbandgap light. Calculations by Nolte¹ show that better metals can have stronger effects on the dielectric and optical properties of composites than the semi-metallic As. We have studied composites formed by ion implantation of various metals, including Cr, Cu, Fe, Ni, and Ag, into GaAs followed by subsequent anneals. Our results with the Fe-implanted GaAs,² further demonstrate the ability to fabricate a ferromagnetic/ semiconductor composite, which consists of ferromagnetic Fe₃GaAs precipitates in GaAs. Recently, we have synthesized the iron/GaAs composite by molecular beam epitaxy (MBE).³ In this paper, we report transmission electron microscopy (TEM) studies of phase identifications and precipitation behaviors in Fe-doped GaAs grown by MBE and Ag-implanted Al_0.3Ga_0.7As. Unlike most of the metals that tend to form compounds with Ga or As while precipitating, we have obtained pure Ag/AlGaAs composites by Ag implantation. The Fe-doped GaAs sample was grown at 600 or 400°C by co-depositing 1% Fe during MBE. Results indicate that different growth temperatures cause different phases of precipitates to be formed. Composites grown at 400°C have ellipsoidal precipitates with sizes ranging from 33 to 55 nm in diameter. TEM Analyses reveal that these precipitates are orthorhombic FeAs with an unfixed orientation relationship to the GaAs matrix system. This is compared to the hexagonal Fe₃GaAs precipitates observed in Fe-doped GaAs grown at 600°C and Fe-implanted GaAs. The Ag-implanted Al_0.3Ga_0.7As sample was implanted with 3x10¹⁶ ions/cm² of Ag at room temperature and then annealed at 650°C for 30 min. TEM analysis reveals a composite consisting of faceted precipitates with sizes ranging from 2 to 20 nm. These precipitates are identified to be elemental Ag (FCC structure with lattice constant a=0.409 nm) with orientation relationship to Al_0.3Ga_0.7As: (200)_Ag//(200)_AlGaAs, (02-2)_Ag//(02-2)_AlGaAs, and [011]_Ag//[011]_AlGaAs. Either Ag₉As or Ag₃Ga precipitates were observed in samples annealed at 900°C for 30 min. This work was partially supported by Materials Research Science and Engineering Center from the National Science Foundation grant No. DMR-9400415 and AFOSR Grant No. F49620-96-1-0234A.

10:20 am, Invited

Optical Investigation of the Properties of the Near Surface Electric Fields and Oxide Layers in LTG-GaAs (001): F.H. Pollak, Physics Department and NY State Center for Advanced Technology in Ultrafast Photonic Materials and Applications, Brooklyn College, Brooklyn, NY 11210

Low temperature grown (LTG)-GaAs, i.e., layers grown by MBE at substrate temperatures between 250-300°C, possess a number of interesting electronic properties associated with the excess arsenic concentration incorporated during growth. In as-grown LTG-GaAs material, the excess arsenic results in a large concentration of point defects (1x10²⁰ cm^-3), due primarily to arsenic antisite defects. The surface/near surface characteristics of this material are important from both fundamental and applied perspectives. For example, in order to evaluate the suitability of nonalloyed LT-GaAs contacts for device applications it is important to determine the stability and nature of the surface layer following air exposure. The steadiness of LTG-GaAs against oxidation in air has recently been demonstrated using STM. We have investigated the near surface electric fields and oxide layers in LT-GaAs (001) using the optical techniques of reflection anisotropy spectroscopy (RAS) and spectral ellipsometry (SE), respectively. The measurements were done in air at 300K on both not-intentionally (NID) and intentionally [n (5x10¹⁸ cm^-3)- and p(2x10¹⁹ cm^-3)- type] doped material. Using RAS (related to the electro-optic effect in the vicinity of the spin-orbit split E₁, E₁ + ₁ optical features) on the NID material we have (a) demonstrated that midgap surface pinning occurs in air and (b) evaluated an "effective depletion width" (W_E) of 45Å and the nature of the near surface electric field, i.e., n-type (upward band bending). The effects of n-/p-type doping is to increase/decrease the surface electric field with an associated decrease/increase in W_E. However, even at the highest p-doping level the band-bending is still n-type. Quantitative values of W_E have been obtained from a comparison of the experimental data with a self-consistent Poisson's calculation based on the properties of the defects, doping and midgap surface Fermi level pinning in this material. The SE investigation, particularly in the region of the E₂ structure (4.5 eV), revealed that the oxide layer on the LT-GaAs (8-10Å) is at least a factor of two smaller than that on conventional GaAs (20-25Å), in agreement with the STM results. In addition, the time evolution of the oxide formation will be reported.

11:00 am

Interaction of Native Point Defects Under Thermal Treatment in Low-Temperature, MBE-Grown GaAs Measured by PAC Spectroscopy: D.I. Lubyshev, D.L. Miller and G.L. Catchen, Pennsylvania State University, Electronics Material and Processing Research Laboratory, University Park, PA 16802

Low-temperature gallium arsenide (LT GaAs) has many applications in the production of integrated circuits and fast photodetectors, because high resistivity and short carrier lifetimes characterize this material. These properties depend on concentrations of native point defects such as gallium vacancies VGa and antisite defects AS_Ga. To investigate stability and kinetics of these defects, we use perturbed-angular-correlation (PAC) spectroscopy. In this application, we introduce a smaller number of ¹¹¹In radioactive probe atoms (~10¹¹) into the sample during MBE growth at 180°C, and we measure the hyperfine interactions of ¹¹¹Cd nuclei, which are produced by electron-capture decay of ¹¹¹In. The resulting PAC measurements show that ~50% of the probe atoms the undergo nuclear electric-quadrupole interactions with defects In in the as-grown LT GaAs samples. This value is higher than would be expected based on the density of point defects usually formed in this material (<10²¹ cm^-3) and the very low concentration of ¹¹¹In probe atoms. The percent of ¹¹¹In atoms observed to interact with point defects most likely depends on the details of the initial crystal growth and on the effects of the somewhat larger (as compared to Ga) covalent radius of the In atoms. That is, the energetics of the incorporation process may favor the location of point defects near the probe atoms. We performed a series of isochronal annealing treatments at successively higher temperatures from 250°C to 560°C to reveal the behavior of the point defects. After each annealing treatment, we performed a laboratory-temperature, PAC measurement. From the analysis, we derived the fraction of probes that, at each temperature, were associated with defects. The temperature dependence of this fraction indicates that two thermally-activated processes may take place, which have corresponding activation energies of 0.04 eV and 0.8 eV. The lower-temperature process could correspond to the V_Ga annihilation via coalescence and delivery of nonstoichiometric arsenic atoms as precipitates, and the higher-temperature process could correspond to arsenic diffusion to growing arsenic clusters. We are investigating these phenomena in more detail to provide a clearer picture of this important process.

11:20 am

Dominant Donor Traps in Non-Stoichiometric GaAs: J.P. Ibbetson and U.K. Mishra, Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106

While there has long been controversy over whether the Fermi level pinning in annealed LT-GaAs is due to the isolated arsenic antisite, AsGa, or arsenic precipitates, the former is widely believed to be responsible for the electrical properties of unannealed material. However, we will present evidence that the Fermi level in as-grown LT-GaAs is in fact pinned in a broad band of unidentified donor states whose energies lie between 0.3-0.5 eV below the conduction band edge, and that As_Ga becomes the preeminent donor trap only following an anneal at ~500-600°C, depending on the growth temperature. These results are important for understanding carrier trapping and recombination behavior in LT-GaAs. In the main experiment, we measured the temperature-dependent conductivity of LT-GaAs layers grown at various temperatures (225-350°C) using n+GaAs/LT/metal structures. Following growth, pieces of each sample were annealed at various temperatures (up to 700°C) prior to device fabrication. In all LT-GaAs layers annealed at 600°C or higher, the band conductivity exhibits the familiar 0.7 eV activation energy associated with the As_Ga deep donor. However, band conductivity characterized by a different activation energy of 0.4 EV (and therefore a different donor) is observed in LT-GaAs grown at 350°C and annealed up to 550°C. We are not able to observe the 0.4 eV activation energy directly in films grown at lower temperature because their hopping conductivity is too high. However, by measuring the charge transfer from an adjacent doped channel, the Fermi level in LT-GaAs grown at 250°C and below is also shown to be pinned close to 0.4 EV below the conduction band edge. A detailed analysis of the hopping conductivity and characteristic hope energy in our films suggest that the observed 0.4 eV activation energy is not the ionization energy of a discrete donor level. Instead, this value arises from a 200-250 meV wide band of donor states "centered" near 0.4 eV below the band edge. There is some evidence that the width of this donor band is due to spatially-correlated donors and acceptors, complexes whose thermally-induced breakup could explain why the hopping conductivity in the film grown at 350°C increases by two orders of magnitude followed an anneal at 550°C. Efforts to independently measure the trap density of states in thin layers of LT-GaAs to confirm this, and to see if the isolated As_Ga donor coexists with the unidentified 0.4 eV donor band, will also be discussed.

11:40 am

Antisite Arsenic Incorporation in the Low Temperature MBE of Gallium Arsenide: Physics and Modeling: S. Muthuvenkatraman, S. Gorantla, R. Venkat, Department of Electrical and Computer Engineering, University of Nevada, Las Vegas, NV 89154-4026; D.L. Dorsey, Wright Laboratory, Materials Directorate, Wright-Patterson AFB, OH 45433-7707

Low temperature molecular beam epitaxially grown non-stoichiometric GaAs (LT GaAs) possesses technologically important electrical properties such as high resistivity, short carrier lifetimes and high dielectric strength. In spite of many detailed experimental research, theoretical understanding of the growth processes which control the properties is incomplete. A stochastic model of growth for simulating the growth processes and investigating the growth issues of LT GaAs, is developed. Three different kinetic models including the surface dynamics of physisorbed arsenic (PA) such as chemisorption, evaporation and creation of arsenic antisites (As_Ga) are considered. The model which allows the temperature dependent incorporation and evaporation of AsGa reproduces the experimental data fairly well. An activation energy of 1.16 eV for the evaporation of As_Ga obtained from simulations is in excellent agreement with theoretical value. At a constant substrate temperature and growth rate, the AsGa concentration increases with arsenic flux for low fluxes and saturates at high fluxes. The critical arsenic flux at which the As_Ga concentration saturates increases with decreasing temperature. From the analysis of our simulation results, it is observed that as the arsenic flux increases, the coverage of PA state increases and at a critical flux dictated by the temperature and growth rate, the coverage saturates at its maximum value of unity. Thus, the variation in concentration of As_Ga for different temperatures appears to be dictated by the variation in the availability of gallium sites with temperature. Influences of growth rate and temperature on As_Ga incorporation are also presented and physics of these dependencies are clarified. *This work is supported by the Air force Office of Scientific Research under DEPSCOR with grant #F49620-96-0275.