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

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

SESSION III: CHEMISTRY & MODELLING

Combined Reactor and Surface Morphology Simulations of OMVPE Growth: R. Venkataramani, K.F. Jensen, Massachusetts Institute of Technology, Cambridge, MA 02139, T.F. Kuech, University of Wisconsin, Department of Chemical Engineering and Materials Science Program, 1415 Engineering Dr., Madison, WI

Finite element simulations of OMCVD reactors and Monte Carlo (MC) simulations of the evolving surface during growth are coupled in order to understand how macroscopic process parameters affect the microscopic morphology of the surface. GaAs growth from arsine and triethylgallium or trimethylgallium precursors is used as a test case. MC simulations have been developed that correctly predict the transition between step-flow and layer-by-layer growth modes on the GaAs surface as the flux of precursors, the temperature, and the initial surface miscut is changed. The segregation of impurities, such as carbon. on the surface is also modeled using MC simulations. The MC simulations have been validated by comparison to AFM pictures of the GaAs surface, as well as dynamic x-ray scattering experiments. Finite element simulations of typical OMCVD reactors are coupled to the MC simulations through the flux of species to and from the surface in order to examine how the microscopic surface morphology affects the macroscopic concentration profiles and vice versa.

7:40 pm

Design of a High Pressure Metal Organic Chemical Vapor Deposition System: C. Hoepfner#, G. Kepler+, J. Scroggs+, S. LeSure+, K.J. Bachmann#, North Carolina State University, Materials Research Center# and Center for Scientific Computing+, Raleigh, NC 27695-7919

The use of high total pressures in a metal organic chemical vapor deposition (MOCVD) system should result in the suppression of thermal decomposition and therefore higher upper growth temperature limits which, in turn, may allow gains in throughput. However, one tradeoff of a high total pressure is increased buoyancy forces which cause convection, particularly in the presence of large temperature gradients. In order to evaluate the characteristics of MOCVD of GaxInlxP, GaN and related compounds under the conditions of high pressure we designed a MOCVD system which allows us to work with (i) total pressures up to 10 bar7 (ii) pulsed sequences of precursors and (iii) in-situ characterization of the growth process by p-polarized reflectance spectroscopy (PRS) as well as mass spectroscopy. In addition, we performed three-dimensional finite element simulations to study the details of flow patterns and temperature distributions of the rectangular cross section reactor. Thermal imaging experiments were used to specify the boundary condition for the simulations. Flow visualization experiments provide a means of validation of the computations. The results suggest, that buoyancy driven convection can be controlled by using high flow rates and providing for channel flow across substrate wafer. The validated computational model will in future be used as a means for evaluating the design of a MOCVD reactor for pressures >100 bar.

8:00 pm

Trisneopentylgallium as a Precursor for Atomic Layer Epitaxy of GaAs: P. Yu, R. Ayers, and S.P. Watkins, Department of Physics, Simon Fraser University, Burnaby, BC, V5A 1S6, Canada, G.A. Horley, P. Obrien, Department of Chemistry, Imperial College of Science, Medicine and Technology, London, SW7 2A2, UK, and A.C. Jones, Epichem Limited, Wirral, Merseyside, L35 7JW, UK

Trimethylgallium (TMG) has been the traditional precursor for atomic layer epitaxy (ALE) of GaAs but has been plagued by high levels of carbon incorporation due to the strong affinity of methyl radicals for the GaAs surface. Other precursors such as triethylgallium (TEG) and triisobutylgallium (UBG) do not produce self-limiting behavior. Both of the latter Ga sources undergo a beta elimination reaction which leads to the formation of Ga hydrides which then presumably form Ga droplets for exposures greater than one monolayer. This suggests that a higher molecular weight precursor could be used for ALE provided its preferred decomposition pathway was homolysis rather than beta elimination. In the present study we report the use of such a precursor, trisneopentylgallium (TNPG) as an alternative to TMG. In contrast to TEG and TIBG, we observe clear self-limiting behaviour at one monolayer per gas cycle over a temperature range of around 430C to 500C. By detailed comparison with TEG and TMG growth results, we propose a simple model for the ALE self-limiting mechanism in GaAs.

8:20 pm

Kinetics of Pyrolytic and Photoassisted MOVPE Growth of ZnSe for p-Type Doping to Produce Blue-Green LEDs: M.U. Ahmed, S.J.C. Irvine, A. Stafford, P. Prete, Optoelectronic Materials Research Laboratory, North East Wales Institute, Plas Coch, Mold Rd., Wrexham, LL11 2AW, UK, L.M. Smith, A.C. Jones and S.A. Rushworth, Epichem Ltd., Power Rd., Bromborough, Wirral, Merseyside, L62 3QF, UK

In this work a comparison is made of the quality and growth rates of MOVPE grown ZnSe using DMZn, TEN as the Zn precursor and DESe, DIPSe and DTBSe as the Se sources. The results shows that at the desired growth temperatures (300°C-400°C), the growth rates are similar for the various Se precursors and are very low even for DTBSe. By using photoassisted growth using above band-gap energy from an argon ion laser of wavelength 458nm the growth rates at these temperatures were increased by a factor of 10 for growth with DIPSe and DESe, but no significant photo-enhancement was observed in the case of DTBSe. SIMS analysis of hydrogen concentration in the undoped ZnSe layer shows high concentrations, up to 5.9El9 atoms cm3 for DTBSe grown ZnSe which will have significant detrimental implication for subsequent p-doping with nitrogen. Through the study of the growth kinetics, the mechanism of hydrogen incorporation is being investigated and possible routes for the reduction/elimination of hydrogen incorporation will be presented.

8:40 pm

Growth Study of AlGaAs using Dimethylethylaminealane: Hong Q.Hou, W.G. Breiland, and B.E. Hammons, Sandia National Laboratories, MS 0603, Albuquerque, NM 87185

Dimethylethylaminealane (DMEAA) is an attractive precursor compared to trimethylaluminum (TMA) for growth of high-purity AlGaAs with low C and O impurities. We present a growth rate and surface mophorlogy study of AlGaAs grown with arsine. DMEAA, trimethylgallium (TMG) and triethylgallium (TEG) in an EMCORE-made rotating disk reactor The growth rate, measured by an in situ reflectometer, was examined as a function of the growth temperature, growth pressure, substrate rotation speed, and arsine flow rate. Strong growth rate dependence on these parameters suggests that pre-reaction occurs for DMEAA itself and between DMEAA and TEG precursors. On the other hand, no measurable pre-reaction between reactants TMG and DMEAA is observed. The surface morphology is generally better for films grown at high temperatures. The steep dependence of the growth rate on the growth parameters make the growth rate reproducibility control much more difficult. The AlGaAs films grown with DMEAA were examined by SIMS, Hall and photoluminescence measurements. We will present a comparison of the material quality for films grown with DMEAA and TMA. This work is supported by the US DOE under contact No. DE-AC04-94AL85000.

9:00 pm

A Kinetic Model for Tris-(dimethylamino) Arsenic Decomposition on GaAs (100) Surfaces: BINQIANG SHI a) and Charles W. Tu b) , a) Department of AMES/Chemical Engineering, b) Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, CA 92093

In this paper, we outline the detailed arguments involved in developing a kinetic model describing surface pyrolysis of tris-(dimethylamino) arsenic (TDMAAs) on GaAs(100) surfaces. Two decomposition pathways of TDMAAs on GaAs(100) are assumed: simple scission of the arsenic and nitrogen bond and surface b-hydrogen transfer reactions. The pre-exponential factor for the scission of the parent is assumed to be the largest among the species going through the scission pathway. Removal of the last dimethylamino ligand is assumed to be essentially the rate-limiting step of the overall decomposition of TDMAAs. The reaction steps and constraints so constructed enable us to derive kinetic parameters of reasonable values from temperature-programmed desorption data in literature. Computer simulations based on the model reproduce well the behavior of data in literature of desorption products from a heated GaAs(100) surface inside a vacuum chamber filled with l0-5 Torr of TDMAAs. This model is being used to investigate mechanisms for laser-induced enhancement of the growth rate of GaAs with TDMAAs and triethylgallium.

9:20 pm

Design of Alternative Sources for OMVPE of Si:Er: William S. Rees, Jr., School of Chemistry and Biochemistry and School of Materials Science and Engineering and Molecular Design Institute, Georgia Institute of Technology, Atlanta, GA 30332-0400

As speed demands for silicon-based technologies impact on intrinsic transport rates and practical feature dimensions, attention progressively is turning toward optical improvements. One approach to surmount the inherent indirect gap of this material is to rely on rare earth dopants, which possess ionic states which are in-gap. Earlier work has demonstrated a relationship between device quality and oxygen content for Si:Er. To address this key issue, alternative sources have been designed which can deposit oxygen free films. Following a brief description of the preparation and purification of Er{N[Si(CH3)3]2}3, this lecture will focus on the significant improvements observed for layers deposited from this new precursor. Comparisons of deposition mechanisms will be discussed in perspective of substantial characterization data on the formed structures.