Papers

AVS1996 Session EM-TuM: Growth and Processing of Wide Band Gap Semiconductors for Optoelectronics

Tuesday, October 15, 1996 8:20 AM in Room 204A

Tuesday Morning

Start	Invited?	Item
8:20 AM		EM-TuM-1 High Quality GaN Growth by MOVPE System with Three Flow Injection Reactor N. Akutsu (Nippon Sanso Corporation, Japan); K. Uchida (Nippon Sanso Corporation , Japan); H. Tokunaga, K. Uematsu, A. Yamaguchi (Nippon Sanso Corporation, Japan) We developed novel GaN-MOVPE system with a three layered laminar flow injection horizontal reactor. In this system, NH3, metal organic precursor(MO) with H2 carrier gas and subflow N2 are separately injected. This type of MOVPE system has a large potential especially in increasing utilization efficiency, because unfav orable deposition at the up stream area of the substrate is small. The reactor was de signed for high speed gas flow, therefore gas phase reaction will be suppressed at high temperature growth. We have grown GaN on AlN buffer layer deposited on 2" sapphire(0001) using this system with three laminar flow injection horizontal reactor at atmospheric pressure and clarified the effect of flow balance between the inje cted flows on a growth rate and film morphology. Using this system, effective five to thr ee ratio can be changed easily by changing MO carrier gas flow rate and subflow N2 flow rat e, without changing MO flow rate and NH3. The reduction of NH3 and MO carrier gas decreased the growth rate of GaN and the increasing of subflow increased growt h rate. By reducing NH3 flow rate from 10slm to 5slm and MO carrier gas flow rate from 10slm to 5slm, GaN growth rate at 1050C became about 2 times larger at a constant MO flow rate of 4.4E-5mol/min. On the other hand, morphology changed from mirror like flat surface to rough surface filled with pyramid shaped hillock. Carrier concentra tion and electric mobility at RT of the undoped GaN film was 1.1E17 and 400cm2/V/sec, respectively. PL measurement at RT of undoped GaN film was performed and a str ong band edge emission with very weak yellow luminescence was observed.
8:40 AM		EM-TuM-2 Blue-Light Semiconductor Lasers: Status and Prognosis for Practical Devices S. DenBaars (University of California, Santa Barbara) The recent announcement of the achievement of laser action in GaN-based semiconductor diodes has stirred considerable interest due to the potential impact of this technology on a variety of optoelectronic applications ranging from high brightness displays to dense optical storage. This talk will critically asses the material challenges and physical issues related to these devices and those of potentially competing material systems.
9:20 AM		EM-TuM-4 Rapid Thermal Processing of III-nitrides J. Hong, C. Vartuli, C. Abernathy, J. MacKenzie, S. Pearton (University of Florida, Gainesville); J. Zolper (Sandia National Laboratories) High temperature annealing is necessary in a number of applications for III-nitrides, including activation of Si/super +/ or Mg/super +/ implants for doping, maximization of implant-isolated regions and ohmic contact sintering. We have compared two methods for protection against surface dissociation of GaN, AlN, InN, In/sub X/Ga/sub 1-X/N and In/sub X/Al/sub 1-X/N during rapid thermal processing in N/sub 2/ ambients. In the first, InN powder is placed in the reservoirs of a SiC-coated graphite susceptor and provides a N/sub 2/ overpressure for the nitride samples within the susceptor. In the second method, the nitrides are placed face down on other III-V substrates during annealing. In both techniques N loss from the nitride surface is found to occur at \>=\ 1050\super o\C for GaN, \.>=\1100\super o\C for AlN and \>=\ 550\super o\ for InN, as measured by Auger Electron Spectroscopy. Surface roughening is significant only for the In-containing materials, with GaN and AlN retaining smooth morphologies even up to 1150\super o\ unless H\sub 2\ is present in the annealing ambient. The N\sub 2\-deficient surfaces of the binary nitrides become strongly n-type, while those of ternaries become less conducting. At temperature of 850-900/super o/C the In droplets on thermally degraded ternaries also begin to evaporate, leading to an apparent improvement in morphology. The presence of H\sub 2\ or O\sub 2\ in the annealing ambient lowers the dissociation temperature of each of the nitrides by 100-200/super o/C, due to an enhancement in N/sub 2/ removal.
9:40 AM		EM-TuM-5 Low Energy Electron Enhanced Etching of III-N Semiconductors in a Hydrogen/Chlorine DC Plasma H. Gillis (University of California, Los Angeles); D. Choutov, K. Martin (Georgia Institute of Technology); S. Pearton, C. Abernathy (University of Florida, Gainesville) Special etching challenges arise in the fabrication of III-N based devices. RIE gives very slow rates, and requires unusually high ion energies, that can modify surface stoichiometry. High-density ECR microwave plasma etching for T < 200 C, produces acceptable etch rates; however, further increase of microwave or RF power again roughens the surface and alters the stoichiometry. We have developed a new, low-damage dry etching technique called Low Energy Electron Enhanced Etching (LE4) whereby electrons at energies 1-15 eV and reactive species at thermal velocities arrive at the sample. These electrons impart negligible momentum to the etching surface, and thereby avoid the ion bombardment damage intrinsic to RIE. LE4 of GaN films in a hydrogen/chlorine plasma shows good anisotropy, high selectivity over the Si substrate, and etch rates ranging from 7 to 100 nm/min (depending on temperature and reactive gas composition). Auger spectra indicated that the surface was essentially unchanged during LE4 in a pure hydrogen plasma, for T < 100C. Films of InN and AlN were also successfully etched by LE4 in a pure hydrogen plasma. Results of LE4 on III-N materials will be discussed in terms of anisotropy, stoichiometry, selectivity, rate, and surface roughness as a function of process parameters (temperature, gas composition, and pressure). This work has been supported in part by the NSF Division of Materials Research through grant DMR-9202879 and ONR grant N00014-92-J-1895.
10:00 AM		EM-TuM-6 Comparison of Ohmic Metallization Schemes for InGaAlN F. Ren (Bell Laboratories); C. Vartuli, S. Pearton (University of Florida, Gainesville); R. Shul, J. Zolper, M. Lovejoy (Sandia National Laboratories) W, WSi\sub 0.44\ and Ti/Al contacts were examined on n\super +\ In\sub 0.65\Ga\sub 0.35\N, InN and In\sub 0.75\Al\sub 0.25\N. W was found to produce low specific contact resistance (\rho\\sub c\ ~ 10\super -7\ \Ohm\(cm\super2\) ohmic contacts to InGaN, with significant reaction between metal and semiconductor at 900 \degrees\C mainly due to out diffusion of In and N. WSi\sub x\ showed an as-deposited \rho\\sub c\ of 4x 10\super-7\ \Ohm\(cm\super 2\ but this degraded significantly with subsequent annealing. Ti/Al contacts were stable to ~ 600 \degrees\C (\rho\\sub c\ ~ 4x10\super -7\ \Ohm\(cm\super2\ at \<=\ 600 \degrees\C). The surfaces of these contacts remain smooth to 800 \degrees\C for W and WSi\sub x\ and 650 \degrees\C for Ti/Al. InN contacted with W and Ti/Al produced ohmic contacts with \rho\\sub c\ ~ 10\super -7\ \Ohm\(cm\super 2\ and for WSi\sub x\ \rho\\sub c\ ~ 10\super -6\ \Ohm\(cm\super 2\. All remained smooth to ~ 600 \degrees\C, but exhibited significant interdiffusion of In, N, W and Ti respectively at higher temperatures. The contact resistances for all three metalization schemes were \>=\ 10\super -4\ \Ohm\(cm\super 2\ on InAlN, and degrades with subsequent annealing. The Ti/Al was found to react with the InAlN above 400 (C, causing the contact resistance to increase rapidly. W and WSi\sub x\ proved to be more stable with \rho\\sub c\ ~ 10\super -2\ and 10\super -3\ \Ohm\(cm\super 2\ up to 650 \degrees\C and 700 \degrees\C respectively. Graded contacts with InN were also examined. Measurements of the conduction mechanism in these contact structures further elucidated their properties.
10:20 AM		EM-TuM-7 Electronic Structure of Wurtzite GaN(0001) from Angle Resolved Photoemission C. Chen, M. Keeffe, G. Lapeyre (Montana State University) GaN as a promising wide-band-gap semiconductor has generated tremendous interest. Nevertheless, the fundamental electronic structural properties of the material needs to be investigated. The goal of this work is to use synchrotron-radiation photoemission to measure the electronic structure. The samples are heat cleaned by pressing them against a silicon wafer which is resistively heated until the oxygen and carbon 1s photoemission signal cannot be detected. The appearance of dispersing interband transitions and N 2s emission is used as evidence for a successful cleaning process. Normal emission measurements of the interband transitions were obtained as a function of photon energy. Data are collected from 14 to 36 eV with a near-normal-incidence monochromator and 38 to 90 eV with a grazing monochromator. Our beamline is unique in that one can select in situ either beam. An angle-integrating CMA is also used in the chamber. The width of the essentially p-derived valence bands (VB) is about 7.7 eV. The centroid of the Ga 3d and N 2s emission lines are separated by about 3.1 eV with a Ga 3d binding energy of about 17.9 eV referenced to the VBM. The latter value should help to differentiate between the various AlN-GaN band offsets determined by photoemission. The shape of the secondaries shows a minimum which is due to a minimum in the conduction-band density of states. The interband transitions are used for band mapping[1] and are compared to theoretical results. Evidence is found for a surface state about 0.6 eV below the VBM. *Supported by ONR/DEPSCoR 1. T.-C. Chiang et al., Phys. Rev. B 21, 3513 (1980) and G. Williams et al., Phys. Rev. B 34, 5548 (1986).
10:40 AM		EM-TuM-8 ZnO as a Substrate for GaN J. MacKenzie, C. Abernathy, S. Pearton, P. Holloway (University of Florida, Gainesville); B. Chai (University of Central Florida); R. Linares (Linares Management Associates) ZnO, LiGaO\sub 2\, and LiAlO\sub 2\ have been investigated as potential lattice-matched substrates for the growth of group III nitride compounds by metalorganic molecular beam epitaxy. AlN and InAlN films were deposited on single crystal ZnO substrates. Thermal stability of the ZnO was found to be a significant limitation on the growth conditions used for deposition and necessitated the use of low temperature capping layers. Even with the use of capping layers, films grown on ZnO exhibited poorer crystallinity as measured by x-ray diffraction than those grown simultaneously on 0001 Al\sub 2\O\sub 3\. Ion channeling analysis indicates significant defect formation at the substrate film interface along with poor crystallinity in the near surface region of the film. This suggests that surface damage during wafer processing or heat-up prior to growth is responsible for the poorer crystallinity. In addition to ZnO, LiGaO\sub 2\ and LiAlO\sub 2\ were also used as substrates for the deposition of GaN and AlN. These substrates were found to be more thermally stable, allowing growth temperatures up to 700 degrees C. The effects of growth temperature, V/III ration and oxide crystal orientation on structural quality will also be discussed.
11:00 AM		EM-TuM-9 Optical Spectroscopy of GaN and Related Materials F. Long, J. Burton, S. Lukacs, M. Pophristic, Y. Li, Y. Lu (Rutgers University) We have used Raman spectroscopy to study native, p-type and n-type doped GaN. Raman spectroscopy of InGaN has also been performed. Observed wavelength dependent changes in the Raman spectra will be discussed. We will attempt to correlate these measurements with other measurements such as photoluminescence measurements on the same sample.
11:20 AM		EM-TuM-10 STM Investigation of Zn Pre-Exposure of the GaAs(001)-(2x4) Surface for ZnSe Growth M. Pashley (Philips Research); S. Ahsan, A. Kahn (Princeton University) Progress on the development of blue-green lasers from ZnSe-based II-VI semiconductors grown on GaAs(001) has led to laser lifetimes of up to 100 hours [1]. This has been achieved by the reduction in stacking faults which nucleate at the ZnSe/GaAs interface. The key to the reduction in stacking faults is the nucleation of ZnSe on the GaAs surface. One of the steps used in the ZnSe nucleation is exposure of the GaAs surface to Zn [1,2], however the role that Zn plays is not understood. We have made the first Scanning Tunneling Microscopy (STM) study of the initial stages of ZnSe deposition on a GaAs(001)-(2x4) surface pre-exposed to Zn.When Zn is deposited onto the GaAs surface below 300C, diffraction shows that the 2x periodicity on the surface disappears, leaving a (1x4) surface. Zn clearly sticks to the surface. However, our STM images show that the As dimers remain unchanged, and no indication of Zn on the surface is seen. This implies that the Zn is located within the dimer vacancy row of the (2x4) reconstruction where it cannot be seen by STM. It has been established that the reaction of Se with the GaAs surface results in a high stacking fault density [3]. Comparison of STM images of submonolayer quantities of ZnSe on GaAs with and without a Zn pre-treatment suggests that the Zn reduces the interaction of Se with the GaAs surface.[1] Taniguchi et al. Electronics Lett. 32, (1996) 552. [2] Miller et al. - to be published. [3] M. D. Pashley and D. Li, Mat. Sci. Eng. B30, (1995) 73.
11:40 AM		EM-TuM-11 Computational Simulation of Epitaxial Growth: Wide Bandgap Materials R. Buggeln, M. Meyyappan (Scientific Research Associates) Wide bandgap materials such as III-V nitrides and silicon carbide are receiving much attention for a number of optoelectronics and photonics applications. These include red, yellow, green, and blue LEDs for full color displays, blue lasers for high density information storage, and high temperature electronics. Impressive results on device performance have been reported. Based on performance, the need for large scale manufacturing of devices and circuits is anticipated. In the present work, we have performed computational simulations of vapor phase epitaxial growth processes since CVD is an attractive method for large area growth and scaleup. The governing mass, momentum, and energy equations for a multicomponent system are solved numerically. Comprehensive gas phase and surface chemistry for the growth of SiC and nitrides are included in the model. For SiC, two precursor systems propane/silane/hydrogen and methyltrichlrosilane/hydrogen are studied and compared in terms of efficiency, growth rate and stoichiometry. The simulation results for growth rate and stoichiometry compare favorably with published growth data. Hence, the predictive capablity of the model can be exploited in equipment and process design in wide bandgap device processing.