About the 8th Biennial Workshop on OMVPE: Technical Program, Monday Morning Sessions (April 14)

The following papers will be presented at the 8th Biennial Workshop on OMVPE, on Monday morning, April 14th, 1997. The calendar of events describes the entire technical program.

SESSION I: NITRIDES I

Welcome: R. Bhat, General Meeting Chairman, Bellcore, Red Bank, NJ 07701

8:40 am

Flow Modulation Epitaxy of Indium Gallium Nitride: S. Keller, U.K. Mishra, and S.P. DenBaars, Electrical & Computer Engineering and Materials Departments, University of California, Santa Barbara, CA 93106

InGaN layers were grown on GaN films by Flow Modulation Epitaxy (FME) at temperatures between 550 and 750°C. The precursors were trimethylgallium, trimethylindium and ammonia. The groupIII precursors had been injected for 5 s and the ammonia flow was modulated between 0.5 and 6 1/min, simultaneously. The indium composition in the FME grown layers was generally lower than in films grown under the same conditions in the continuos growth mode. Furthermore, the indium content of the layers increased with decreasing ammonia exposure time between groupIII precursor injection. The effect of the ammonia flow during groupIII injection will be discussed along with the kinetics of indium incorporation / desorption in the FME growth mode. Films grown under optimized conditions showed intense band edge related luminescence at room temperature (excitation density 220 mW/cm2). In addition, we will present results on the influence of the growth mode on the surface properties of the InGaN layers evaluated by atomic force microscopy.

9:00 am

Reaction and Transport Processes in InGaN Growth: T.G. Mihopoulos, H. Simka, and K.F. Jensen, Massachusetts Institute of Technology, Department of Chemical Engineering, Cambridge, MA 02139

OMVPE of InGaN involves complex chemistry and flow phenomena, which determine the quality of the deposited layers. The high growth temperatures used lead to significant gas-phase decomposition of precursors and large temperature gradients between cooled walls and the high temperature substrate. These gradients create complex mixed convection flow fields and further influence the precursor distribution in the reactor through thermal diffusion. Metallorganic precursors (e.g. TMGa) form Lewis acid-base type adducts with ammonia that are known to play a significant role in nitride epitaxial growth. We describe a modelling approach that helps identify the relative importance of different transport and reaction processes to the overall growth performance in different reactor geometries. Simulations demonstrate the effects of reactor geometry (horizontal and vertical reactors of different sizes, as well as two-flow reactors in horizontal and vertical orientation) and operating conditions (flow rates, pressures, carrier gas choices). A kinetic model for InGaN growth is presented that builds upon previously reported data for the gas-phase decomposition of TMGa, TMIn and ammonia. Very good agreement with experimental results for GaN growth rates is demonstrated over a wide range of temperature and pressure in several reactor configurations. The different flow and temperature distributions in the reactors result in varying degrees of wall depositions and parasitic reactions. The indium content appears to be controlled by competition between desorption kinetics and incorporation, the latter being influenced by the GaN growth rate.

9:20 am

Annealing Effects in AlN/Sapphire Buffer Layers on OMVPE GaN Growth: N.R. Perkins* and T.F. Kuech*,**, Department of Chemical Engineering** and Materials Science Program*, University of Wisconsin, 1415 Engineering Drive, Madison, WI 53706

The application of a low temperature nitride buffer layer is a recognized method to control the properties of OMVPE GaN on sapphire. This paper provides a detailed study of the nucleation and recrystallization of low temperature AlN on sapphire as a function of thickness. annealing time, and temperature. Recrystallization of the buffer layer, resulting from surface mass transport, leads to the development of regular surface features as observed by AFM, including change in buffer layer RMS roughness from 0.73nm to 1.21nm as well as systematic development of the physical structure as noted by RHEED and Read xray measurements. Treatment of the sapphire in ammonia prior to buffer layer deposition in ammonia is shown to significantly alter the buffer layer morphology (RMS roughness 0.54nm for NH3- annealed samples vs 1.2lnm for H2-annealed substrates). NH3-pretreatment of the substrate also results in modification of the epitaxial GaN properties, with a 20% decrease the (002) DCXRD FWHM values and decreased dependence of FWHM values with buffer layer thickness for 2 mm epitaxial GaN films. Assessment of optimized epitaxial GaN on these buffers demonstrates FWHM values of less than 300 arcsec, low yellow luminescence, and high electrical mobilities (n=2.5xl017/cm3, m=300 cm2/Vsec) for 2.5 mm thick epilayers.

9:40 am

Electrical and Optical Properties of Oxygen Doped GaN Grown by Mocvd Using N2O: R. Niebuhr, K.H. Bachem, U. Kaufmann, M. Maier, C. Merz, B. Santic, P. Schlotter, Fraunhofer-Institute for Applied Solid State Physics, Tullastr. 72, 79108, Freiburg, Germany, H. J-F. Crgensen, Aixtron GmbH, Kackertstr. 15-17, 52072 Aachen, Germany

We have grown oxygen doped GaN using N2O as oxygen precursor. The GaN layers were grown on 2" c-plane sapphire from Trimethylgallium (TMG) and especially dryed ammonia (NH3) using nitrogen as carrier gas. The GaN layers were grown at 1085°C and 50 mbar on an AlN buffer layer and have a thickness of about 1 micron. Undoped layers show high resistivities. The N2O concentration was varied between 25 ppm and 400 ppm. The doped layers were characterised by temperature dependent Photoluminescence (PL), Hall-measurements and Secondary Ion Mass Spectroscopie (SIMS). PL and Hall effect indicate that oxygen behaves as a shallow donor. Lightly doped samples are dominated by 2 PL peaks attributed to the A1 free exciton and 8 donor bound D° X exciton. With increasing N2O partial pressure the intensity of the free exciton decreases and the bound exciton finally dominates. Deeper PL bands are weak and do not show a correlation with the N2O flux. From 25 ppm to 400 ppm N2O the electron concentration measured by Hall effect increases from 4*1016 to 4*1018 cm3. The Hall mobility is low and around 40 cm2/V-s. Si doped samples of comparable concentration show an order of magnitude higher mobility. The observed carrier concentration compares well with the oxygen concentration determined by SIMS.

10:00 am

Nucleation and Growth Behaviour for GaN Grown on (0001) Sapphire Using a Close Space MOCVD Reactor: Junko T. Kobayashi, Nobuhiko P. Kobayashi, and P. Daniel Dapkus, Compound Semiconductor Laboratory, Department of Materials Science and Electrical Engineering/Electrophysics, University of Southern California, Los Angeles, CA 90089-0483

We report the characteristics of nucleation and growth of GaN grown on (0001) sapphire substrates by atmospheric pressure metalorganic chemical vapor deposition using a closed space shower head reactor. This reactor design utilizes a water cooled multi-inlet gas distribution showerhead in which the III and V sources are separately inlet directly (~1 cm) above the substrate. Growth behavior is observed that is related to the reduction of gas phase reactions in this geometry. A multi-step buffer layer approach consisting of layers of GaN grown at different temperatures to suppress thermal desorption and control mass transport of the low-temperature grown buffer layers results in predominantly lateral growth of truncated 3D islands with uniform height. This paper will report the evolution of the substrate nuclei and describe the growth parameters, including the temperature and gas phase composition, relevant to growing smooth, featureless, and colorless films using this reactor. These materials exhibit x-ray rocking curves for GaN (0002) with FWHM values of ~240 arc sec., carrier concentrations in the range 1-2 x 1017 cm-3 and mobilities in the range ~500 cm2/V-sec. This work supported in part by the JSEP program at USC and the Office of Naval Research.

10:20 am Break