About the 8th Biennial Workshop on OMVPE: Technical Program, Tuesday Morning Sessions (April 15)

The following papers will be presented at the 8th Biennial Workshop on OMVPE, on Tuesday morning, April 15th, 1997. The calendar of events describes the entire technical program.

SESSION V: NITRIDES II

The Stability of InxGa1-xN/InyGa1-yN Structures Grown by MOVPE on Sapphire: J. Ramer, A. Hecht, and S.D. Hersee, Center for High Technology Materials, University of New Mexico, Albuquerque, NM 87131

The full development of IIIN device technology will require the growth of epitaxial structures in which InGaN layers can be combined with high quality GaN and AlGaN confinement layers. This paper addresses a potential compatibility problem between the MOCVD growth of InGaN and high quality GaN in the same epistructure. To incorporate useful amounts of In it has been found necessary to grow InGaN at temperatures between 750C and 850C, with more In being incorporated at lower growth temperature. In contrast, high quality GaN is typically grown at 1050C. X-ray diffraction (XRD) analysis of periodic InxGal-xN/InyGal-yN structures indicates that when a high temperature GaN growth follows the lower temperature growth of InGaN there is a degradation of the InxGalxN/InyGalyN heterojunction abruptness. The intensity of higher order satellite peaks in the X-ray diffraction spectra of periodic InxGal-xN/lnyGa1-yN structures is used to characterize heterojunction abruptness. It is shown that the intensity of these satellite peaks decreases as we increase the duration of subsequent GaN growth at 1050C. This paper will relate the changes in XRD spectra to heterojunction abruptness and will present evidence for the diffusion of In during MOCVD growth. A thin Al0.20Ga0.80N layer, grown after the InGaN layers and before the transition to the higher, GaN growth temperature, will be shown to improve the stability of the structure.

10:40 am

In situ Monitoring of GaN on Sapphire using Optical Reflectance: Jung Han, R.M. Biefeld, M.H. Crawford, and J.C. Zolper, Sandia National Laboratories, Albuquerque, NM 87111-0601

Interest in wide bandgap IIIV nitride compounds has increased rapidly with the recent demonstrations of promising optoelectronic and electronic applications. So far the majority of the nitride-based devices were grown by OMVPE where in-situ diagnostics has been traditionally lacking. Characterization using electron beams was prohibited by high ambient pressure, and optical probing was often hindered by coatings on the reactor chamber walls. In-situ monitoring is especially desirable considering the complicated nature of initial nucleation of nitrides on sapphire substrates with a large lattice mismatch (~14%) as well as the stacking sequence. The deposited nitride epilayers evolve from a heavily-disordered interfacial region with multiple phases and domains into essentially a single crystalline material through mechanisms (grain growth, surface recrystallization, and orientation selection, for example) not typically present in the epitaxy of conventional semiconductors. In this paper we report that. with the use of a vertical rotating disc reactor, the evolution of growth morphology can be monitored through use of an in-situ optical reflectometer. The correlation of the nucleation transients with GaN material parameters, as studied by PL, Hall-effect, and AFM measurements, will also be presented. *This work was supported by the United States Department Of Energy under Contract DE-AC04-94AL85000. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy.

11:00 am

MOVPE Growth of GaN at Low Temperature: O. Kryliouk(1), T. Dann(1), T.J. Anderson(1), and A. Gaskov(2), K.S. Jones(3), J.H. Li(3), and B. Chai(4), (1) Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, (2) Department of Chemistry, Moscow State University, Moscow, Russia, (3) Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, (4) CREOL, University of Central Florida, Orlando, FL 32826

The high growth temperature typically used for MOVPE growth of GaN on sapphire (~1025°C) limits the extent of In incorporation in InxGa1-xN alloys, and thus the range of accessible wavelengths for light emitting devices. The results of using alternative substrates to lower the growth temperature (< 850C) of GaN are reported. Our first approach was to use selective area growth of GaN on patterned sapphire and Si substrates, for which the mask material was a known NH3 decomposition catalyst (e.g., W, Ta, SiO2). Selective area growth was observed under certain growth conditions, but the catalysts were easily poisoned. The next approach was to investigate the new oxide-based substrates (LiGaO2 and LiAlO2) that are closely lattice-matched to GaN. Cross-sectional TEM micrographs show a superior structural quality of GaN grown on (001) LiGaO2, with an extended defect density approaching that reported for homoepitaxy of GaN. It was discovered that proper pretreatment of the LiGaO2 was critical to obtaining high structural quality material. For growth of GaN at 800°C, a rms surface roughness of 0.08 mn was measured using AFM. The results of optical (PL, FTIR), electrical (Hall), compositional (SIMS, SNMS) and structural (TEM, XRD) measurements will be discussed.

11:20 am

Characterization of MOVPE-Grown (AlGaIn)N Heterostructures by Quantitative Analytical Electron Microscopy: H. Lakner, C. Mendorf, G. Brockt, A. Redfeld, F. Scholz* , V. Harle*, and A. Sohmer*, Werkstoffe der Elecktrotechnik, Gerhard-Mercator-Universitat Duisberg, 47048 Duisberg, Germany, * 4, Physikalisches Institut, Universitat Stuttgart, 70550, Germany

Low pressure MOVPE growth of wurtzite (Al,Ga,In)N heterostructures on sapphire substrates was investigated by quantitative analytical scanning transmission electron microscopy (STEM) techniques like atomic number (Z-) contrast imaging, electron -loss spectroscopy (EELS) and convergent beam electron diffraction (CBED). Especially (Ga,In)N quantum wells of different thicknesses were analysed with respect to chemical composition variations, interface abruptness (grading) and strain (relaxation) effects. The interfaces in In0.12Ga0.88N/GaN quantum wells appear to be asymmetric. The lower interface in growth direction is more abrupt than the upper one, where a grading in the In-concentration is significant. Additionally, we found hints for composition variations within the InGaN quantum wells. The application of electron diffraction techniques (CBED) yields quantitative information on strain and relaxation effects. For the case of 17 nm thick InGaN quantum wells we observed relaxation effects, which are not present in the investigated thin quantum wells of 2 nm thickness. The experimentally obtained diffraction patterns were compared to simulations in order to get values for strain within the quantum wells.

11:40 am

The Effect of Substrate Surface Roughness on GaN Growth: Dongjin Byun and Dong-Wha Kum, Division of Metals, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea

Efficiency and lifetime of light emitting diodes (LEDs) and laser diodes (LDs) inversely depend on defect density of the crystal. Reduction of defect density is accomplished by proper choice of the substrate or deliberate modification of substrate surface. Roughness of substrate surface for GaN deposition can be controlled by buffer growth and/or nitridation. Buffer layers or nitrided layers promote lateral growth of films due to decrease in interfacial free energy between the film and substrate. Optimum conditions for GaN-buffer growth on Al2O3(0001) were determined by means of atomic force microscopy (AFM). AFM analysis of nitridated sapphire surfaces was also carried out to find the optimum condition for nitridation of sapphire substrate before GaN deposition. Nitridation of sapphires was performed only with nitrogen. Based on the fact that GaN deposited on more smooth surface exhibited the better crystal quality and optical property, use of AFM roughness as a reliable criterion is suggested for process optimization of GaN film growth by metallorganic chemical vapor deposition.