The following papers will be presented at the 8th Biennial Workshop on OMVPE, on Wednesday morning, April 16th, 1997. The calendar of events describes the entire technical program.
Kevin Killeen, Hewlett Packard Laboratories, Palo Alto, CA
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GaAs Surface States during OMVPE: J.R. Creighton, H.K. Moffat, and K.C. Baucom, Chemical Processing Sciences Dept., Sandia National Laboratories, P.O. Box 5800, M.S. 0601, Albuquerque, NM 87185-0601
We have utilized reflectance difference spectroscopy (RDS) in our research rotating disk reactor (RDR) to investigate the state of the GaAs(100) surface during OMVPE. We have also used our UHV surface science machine to investigate the source of the Type II and Type III RDS lineshapes, which had not been adequately benchmarked. During OMVPE at 640C, the RDS lineshape exhibits a negative peak at 2.2-2.3 eV. This peak is intermediate to that found for Type I (at 2.5 eV), the c(4 X 4) surface, and Type II (at 2.1 eV) RDS lineshapes [Reinhardt and Richter, et al.], and suggests that the surface still maintains a "super" arsenic-rich coverage under typical OMVPE growth conditions. The magnitude of this peak increases with V/III ratio, as expected, indicating that it is a monitor of the arsenic coverage. We have used the behavior of this signal to test our site blocking model that explains the weak negative order V/III growth rate dependence observed in the RDR. At intermediate temperatures (500-550C) and low TMGa pressures, the surface normally exhibits a Type II RDS lineshape. Benchmarking work in our surface science machine indicates that the Type II surface is a metastable derivative of the c (4 X 4) super As-rich reconstruction. The RDS peak at 2.1 eV seen for this surface is not due to gallium dimers, as has been suggested. At higher TMGa pressures the surface exhibits a Type III RDS lineshape. Reinhardt and Richter et al. proposed that this surface is saturated by TMGa fragments. In our surface science machine we have found that a TMGa reaction with the c(4 X 4) surface creates a (1 X 2) adsorbate-induced reconstruction involving excess arsenic, gallium, and methyl groups. This (1 X 2) reconstruction exhibits a RDS lineshape similar to the Type III lineshape seen during OMVPE. Other RDS benchmarking experiments suggest that the ALE-type lineshape measured by Aspnes et al. originates from a methylene covered gallium rich surface.
In Situ Optical Monitoring of InAs/GaSb Superlattices: N.J. Mason and P.J. Walker, Clarendon Laboratory, Physics Dept., University of Oxford, OX1 3PU, UK
We have adapted Surface Photo Absotption (SPA), a p-polarised reflectance technique, to give information both about the bulk refractive index of the growing crystal, and its surface properties. We get information on the bulk growth rate and alloy composition [by measuring the Fabry-Perot oscillations in the same way as Reflectance Spectroscopy] and the surface dimers [similar to the results obtained from Reflectance Anisotropy Spectroscopy]. Many of the optical and electrical properties of InAs/GaSb superlattices are controlled by whether the interface approaches a monolayer of GaAs or InSb. These two different interfaces are achieved by biasing the alkyl switching at the interface. This biasing controls whether the growing crystal surface is either group III [In, Ga] or group V [As, Sb] rich at particular points in the switching sequence. We have been able to observe such monolayer changes on the growing surface using the above technique and correlate these changes with ex situ techniques such as atomic force microscopy, electron microscopy, tunnelling, magnetotransport, photoluminescence and Raman scattering.
Surface Photoabsorption Monitoring of the Growth of GaAs and InGaAs at 650°C by MOCVD: Y.D. Kim*, F. Nakamura**, D.V. Forbes, and J.J. Coleman***, *Department of Physics, Kyung Hee University, Seoul, 130-701, Korea, **SONY Corporation Research Center, Yokohama 240, Japan, ***Microelectronics Laboratory and Materials Research Laboratory, University of Illinois, Urbana, IL 61801
By monitoring the cyclic behavior of Surface Photoabsorption (SPA) reflectance changes during the growth of GaAs at 650°C and with sufficient H2 purging time between the supply of TMGa and AsH3, we have been able to achieve controlled growth of GaAs up to a monolayer (ML). To determine the ML growth rate of GaAs and its compatibility with the growth of other materials at this temperature, we grew AlGaAs single quantum well (SQW) samples by monitoring the SPA signal. The comparison of PL energies and linewidths with model calculations confirm that we have achieved ML epitaxy and that the roughness at the heterostructure was about 1 ML. We observed a continuous increase of the SPA signal with an initial temporary saturation during the TMGa exposure, which cannot be explained by existing models. We also discuss our PL measurements on the InGaAs SQWs grown by monitoring the SPA signal, which suggests that TMIn decomposes before reaching the surface, while TMGa constructs a methyl-terminated surface at this temperature.
On-line Growth Monitoring of InP-Based Device Structures by Reflectance Anisotropy Spectroscopy: P. Kurpas, M. Sato*, A. Knauer and M. Weyers, Ferdinand-Braun-Institut fur Hochfrequenztechnik Berlin, Rudower Chaussee 5, D-12489 Berlin, FR Germany, * NTT Basic Research Laboratories, Atsugi, Japan
Reflectance anisotropy spectroscopy (RAS) has been used to study different parts of the MOVPE growth process for GaInAsP/InP light emitting diodes. Depending on the substrate preparation of non-epi-ready InP:Fe substrates before growth a decrease of the reflectivity and strong increase of the surface anisotropy was observed during the growth of the first ~50 nm of the InP-buffer layer. With increasing layer thickness the RAS signals then recover to the initial values. Using AFM topograms the initial surface anisotropy can be correlated to the presence of huge islands (diameter 100 -500 nm, height 50 - 400 nm) on thin InP layers while thick layers are smooth. Based on the RAS results the pre-treatment procedure was optimized to minimize this initial surface roughening. From the RAS spectra also the dopine type of the InP layers can be determined. Variation of the GaInAsP composition leading to a different emission wavelength also show up in changes in the RAS spectra. These results indicate the possibility for composition control at growth temperature using RAS.
In Situ Monitoring of CdTe Atomic Layer Epitaxy using Spectroscopic Ellipsometry: Srikanteswara Dakshina Murthy and Ishwara Bhat, Electrical, Computer and Systems Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180-3590
Atomic layer epitaxy (ALE) is currently used for the growth of epitaxial layers of high uniformity, quality and well-controlled thickness. The use of in-situ diagnostics could further the understanding of the growth mechanism and aid the reproducibility of ALE within the usually narrow process "window". To this end, we have used in-situ spectroscopic ellipsometry (SE) as a diagnostic tool during the atomic layer epitaxy growth of CdTe on GaAs(100). The susceptor temperature, reactant partial pressures, as well as the flow and flush duration for each precursor are cmcia1 process variables for ALE growth. Growth was carried out for about 20 cycles under each set of process conditions and in-situ SE was used to monitor the presence of layer-by-layer growth. This enabled the quick determination of the necessary conditions within a few growth runs. The ALE growth of CdTe was also obtained at temperatures below 300°C, reconfirming the assertion that CdTe growth occurs via the surface catalyzed decomposition of the Te precursor.
10:00 am Break
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