Papers

AVS1997 Session EM-MoA: Critical Issues in Wideband Nitrides

Monday, October 20, 1997 2:00 PM in Room C1/2

Monday Afternoon

Start	Invited?	Item
2:00 PM		EM-MoA-1 Doping of III-Nitrides K.H. Ploog (Paul-Drude-Institut fuer Festkoerperelektronik, Germany) The origin of the residual n-type doping and the rather low p-type conductivity of GaN and other III-Nitrides are major problem areas for device application of these materials. The low p-type conductivity, for example, results in extremely high threshold current densities for GaN-based laser diodes operating at room temperature. In this paper we show that the residual n-type doping mainly originates from oxygen donor impurities from the ambient incorporated on lattice sites during epitaxial growth. To improve the p-type conductivity to values as high as 50 (Ω cm)^-1 we have introduced the concept of reactive co-doping of GaN with Be acceptors and O as reactive donors. The formation of acceptor-donor pairs strongly reduces the ionized impurity scattering and thus enhances the hole mobility by more than an order of magnitude.
2:40 PM		EM-MoA-3 Characterization of Hexagonal GaN(0001) Thin Films by X-ray Photoelectron Diffraction R. Denecke (Lawrence Berkeley National Lab & Univ. of California, Davis); J. Morais (Lawrence Berkeley National Laboratory); R.X. Ynzunza (Lawrence Berkeley National Lab & Univ. of California, Davis); J. Liesegang (La Trobe University, Australia); C.S. Fadley (Lawrence Berkeley National Lab & Univ. of California, Davis) We report the first scanned-angle x-ray photoelectron diffraction measurements on GaN(0001) in the wurtzite structure, as grown on sapphire substrates using MOCVD. Using standard x-ray sources (Al K_alpha and Mg K_alpha) we measured full 2π diffraction patterns for Ga 3p and N 1s emission, as well as for O 1s and C 1s from the contaminant and/or oxide overlayer. The as- grown samples reveal forward scattering peaks for Ga and N in agreement with theoretical calculations using both single scattering and multiple scattering cluster formalisms. The diffraction from the surface contaminants does not exhibit any clear structure, and this is indicative of a more or less amorphous structure. From the combination of experimental and theoretical diffraction patterns and simple intensity- ratio arguments using the azimuthally-averaged Ga, N, O, and C intensities, the surface termination for these samples could be determined. The data also indicate that O is on average closer to the surface than C, but both contaminants are found preferentially on the surface, with no indication of significant adsorption between the hexagonal columns which are apparent in light microscope images of these surfaces. We have also studied the thermal desorption of these contaminants and the influence of heat treatment on the stoichiometry of the samples. It is found that the heating does not remove the contaminants completely and that it also influences the surface stoichiometry, in general agreement with previous results reported in the literature. Work has been supported by DOE, BES, Mat. Sci. Div. (Contract DE-AC03-76SF00098), DFG (Germany) and CNPq (Brazil).
3:00 PM		EM-MoA-4 A Novel Precursor for Aluminum Nitride Thin Film Growth: Quinuclidine Alane D.W. Robinson, J.W. Rogers, Jr. (University of Washington) Previous work in our group has shown that atomic layer growth of aluminum nitride films with very low carbon contamination can be grown by sequentially dosing ammonia and amine alanes ( trimethylamine alane (TMAA) and dimethylethylamine alane (DMEAA) ). The amine alanes are ideally suited for growth of aluminum nitride films. They contain an organic ligand ( the amine ) that limits adsorption of the precursor to one atomic layer when adsorbed molecularly, and the amine will easily be displaced by a stronger nucleophile such as ammonia thereby eliminating carbon contamination in the film. The amine alanes also contain free alane which will completely dehydrogenate adsorbed ND₂ species on Si(100) by ~ 600K leaving aluminum nitride. Unfortunately, to achieve atomic layer growth it is necessary to modulate the surface temperature to achieve both self-limiting adsorption of the amine alane as well as complete dehydrogenation of the ammonia to nitride. The temperature limit on self-limiting adsorption is set by the strength of the adduct bond between the alane and amine. DMEAA and TMAA will completely decompose at temperatures above ~ 400 K leading to aluminum film growth. Therefore, it was our goal to find a more stable amine alane precursor that would eliminate the necessity for temperature modulation by closing the temperature gap between self-limiting adsorption of the aluminum precursor and complete dehydrogenation of ammonia by the alane. We would like to avoid temperature modulation because it adds extra complexities to gas flow and temperature control in larger scale reactors. We find that quinuclidine alane possesses the properties we desire in an amine alane precursor. The greater donor strength of the quinuclidine compared to trimethylamine and dimethylethylamine adds extra stability to the precursor. We will show how this extra stability in the precursor increased the temperature for self-limiting adsorption thus eliminating the need for temperature modulation between dosing sequences of the aluminum and nitrogen precursors.
3:20 PM		EM-MoA-5 UHV Arcjet Nitrogen Source for Selected Energy Epitaxy of Group III Nitrides by MBE F.J. Grunthaner, R. Bicknell-Tassius, P. Deelman, P.J. Grunthaner (Jet Propulsion Laboratory); C.E. Bryson, E.J. Snyder (Surface/Interface, Inc.) A fundamental understanding of Group-III nitride epitaxy requires a nitrogen source capable of varying the type of nitrogen species delivered to the growth surface (neutral versus ionic, atomic versus molecular, ground versus excited state). Control of the translational energy is also desireable. Theoretical studies by Goddard, et al., have predicted that ground state atomic nitrogen will successfully incorporate into the growing GaN surface, while atomic nitrogen in either of the excited doublet states will lead to etching of the growth surface. Control and characterization of the nitrogen species is critical to unfolding the effects of the various species on the morphological, optical, and electronic properties of the film. In a departure from traditional RF and ECR sources, we report on the development of a MBE arcjet nitrogen source in which an electron arc is used to thermally dissociate nitrogen gas. The arcjet approach offers the possibility of selected species. We have adapted traditional arcjet thruster concepts to the UHV requirements for MBE growth by (a) adding differentially pumped skimming stages, (b) all rhenium construction in the areas where reactive nitrogen is present, and (c) extremely small diameter-to-length ratios for the gas constrictor. We have demonstrated operation of the arcjet at power levels frnaging from less than 10 to more than 300 W. To date, the arcjet has been in operation more than 300 hours without erosion of the cathode. Optical emission data show no metal impurities in the beam. In initial experiments utilizing a single skimmer stage, strong features due to atomic nitrogen were observed by optical emission and no charge species were detected by a Langmuir probe. Analysis of the mass spectrum of the beam with and without arc excitation showed an atomic nitrogen flux of about 5 monolayers per second at the MBE sample growth distance. Beam properties varied as a function of arcjet power and nitrogen flow rate.
3:40 PM		EM-MoA-6 Optimization of InN for Ohmic Contact Formation S.M. Donovan, C.R. Abernathy (University of Florida, Gainesville); F. Ren (Bell Laboratories, Lucent Technologies); S.J. Pearton, J.D. MacKenzie (University of Florida, Gainesville); J.M. Zavada (U.S. Army Research Office); B.H. Chai (University of Central Florida) InN is being considered for use as an Ohmic contact formation layer. In this paper we will discuss the effect of substrate and growth conditions on the structural, electrical and contact properties of InN grown by MOMBE. Material quality was determined by Hall measurement, x-ray diffraction (XRD), XTEM, SEM and AFM. Interfacial reactions between the substrates and the RF nitrogen plasma were studied using ESCA, AES and AFM to correlate the substrate surface characteristics with InN material quality. Substrates investigated include GaAs, sapphire and LiAlO₂. For all of the oxide substrates, exposure to a nitrogen plasma did not produce nitridation. As a result low temperature nucleation layers were necessary to produce 2D growth. Substrate choice had little or no effect on electrical behavior. By contrast, structural quality as measured by AFM and XRD was found to be a determining factor in the contact resistance obtained using WSi_x contacts on InN. Growth conditions resulting in poorer structural quality, such as the use of GaAs rather than oxide substrates, low nitrogen flows and low growth temperatures, produced contact resistances of 10^-4-10^-3 Ohms-cm². Using optimized conditions contact resistances of 3.5 x 10^-6 Ohms-cm² were obtained. Thermal stability of the contacts was investigated by annealing for five minutes at various temperatures. While structures with thin (50 nm) InN layers showed an initial reduction in resistance, annealing up to 500°C of structures with thick (200 nm) InN layers did not appreciably change the contact resistance. Annealing had an adverse effect on the stuctural integrity, which worsened as the InN thickness increased. Investigation of the metal/InN interface by Auger electron spectroscopy (AES) depth profiling will also be presented as a function of annealing temperature and InN thickness. Using an optimized process and thickness, structural integrity could be maintained even at temperatures up to 700°C.
4:00 PM		EM-MoA-7 Status of High-Brightness III-Nitride Light Emitting Diodes (LEDs). D.A. Steigerwald (Hewlett-Packard Company) Group III-nitrides, GaN, AlN, and InN, and their alloys have recently gained considerable interest with the development of high-brightness blue and green light-emitting diodes (LEDs) and laser diodes (LDs). Blue and green LEDs with luminous efficiencies of approximately 5 and 30 lumens/watt, respectively, have recently been measured. This performance compares well with AlInGaP yellow and red-orange LEDs, where efficiencies as high as 50 lumens/watt have been reported. This level of performance permits these solid-state light sources to directly compete with conventional incandescent bulbs for a variety of applications. This paper will review the status of high-brightness III-nitride LEDs, their applications and future directions for research in visible LED technology.
4:40 PM		EM-MoA-9 Selective ICP Etching of Group-III Nitrides in Chlorine and Boron Trichloride Based Plasmas R.J. Shul, M.M. Bridges, C.L. Willison (Sandia National Laboratories); J.W. Lee, S.J. Pearton, C.R. Abernathy, J.D. MacKenzie, S.M. Donovan (University of Florida, Gainesville); J. Han (Sandia National Laboratories) Fabrication of group-III nitride electronic and photonic devices relies heavily on the ability to pattern features with anisotropic profiles, smooth surface morphologies, etch rates often exceeding 1 um/min, and a low degree of plasma-induced damage. Patterning these materials has been especially difficult due to their high bond energies and their relatively inert chemical nature as compared to other compound semiconductors. The use of high-density plasmas has resulted in improved GaN etch results possibly due to a two step process directly related to the plasma flux. Initially the high-density plasmas increase the bond breaking mechanism allowing the etch products to form and then produce efficient sputter desorption of the etch products. However, high-density plasma etching is often dominated by ion bombardment mechanisms which minimize etch selectivity. Development of etch processes with high selectivity, as well as equi-rate etching, will be necessary for specific applications as the growth of heterostructure devices becomes more substantial. In this study, we will report high-density inductively coupled plasma (ICP) etch rates and selectivities for GaN, AlN, and InN as a function of plasma chemistry. For example, GaN over AlN has a selectivity greater than 5 in chlorine/argon. Surface roughness and near-surface stoichiometry evaluated from atomic force microscopy (AFM) and Auger emission spectroscopy (AES), respectively, will also be reported. Optical emission spectroscopy (OES) will be used to identify plasma species and possible etch mechanisms. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-ACO4-94AL85000.
5:00 PM		EM-MoA-10 Composition and Doping Effects on Plasma Etching of Group III-Nitride Photonic Device Materials J.E. Spencer, C. Doughty, D.C. Knick (PlasmaQuest, Inc.); D. Schmitz (AIXTRON); B. Schineller (RWTH) Compound semiconductor photonic devices using Group III nitrides have seen rapid development in recent years. Reports of record holding devices including high brightness blue and green light emitting diodes (with emission intensity of up to 12cd for the green diodes) and blue laser diodes are quickly expanding the commercial applications of these materials. Commercial applications require well-controlled growth and patterning processes. Dry etch processes are desired for patterning, and a large body of literature exists, but etching is quite sensitive to both etch conditions and to material properties such as composition and microstructure. Because of this interaction it is important to investigate these aspects simultaneously in a controlled manner. We report results on the dry etching characteristics of various compositions of high-quality Group III-Nitride compounds such as GaN, AlGaN and InGaN. Etch results are correlated to growth conditions and material properties (such as composition, microstructure and morphology). Samples were etched in an a high-density ECR reactor using a variety of Cl based chemistries. The effect on the etch of plasma chemistry and process variables and their interaction with the material properties and structure are explored.