Comparison of AlGaAs Oxidation in MBE and MOCVD Grown Samples
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Comparison of AlGaAs Oxidation in MBE and MOCVD Grown Samples∗ Y. Chen, A. Roshko, K.A. Bertness, National Institute of Standards and Technology. D.W. Readey, Colorado School of Mines. A.A. Allerman, Sandia National Laboratories. M. Tan, A. Tandon, Agilent Technologies. ABSTRACT Simultaneous wet-thermal oxidation of MBE and MOCVD grown AlxGa1-xAs layers (x = 0.1 to 1.0) showed that the epitaxial growth method does not influence the oxidation rate. Nearly identical oxidation depths were measured for samples grown by both techniques. It was found, however, that the oxidation rate is very sensitive to non-uniformities in the Al concentration in the AlxGa1-xAs layers, and that maintaining consistent and uniform Al concentrations is critical to achieving reproducible oxidation rates. The study also showed that the oxidation rate was not affected by the V/III ratio during growth nor by impurities at concentrations less than or equal to 10 ppm. INTRODUCTION Native oxides formed by wet-thermal oxidation of AlxGa1-xAs are playing increasingly important roles in optoelectronic devices such as vertical cavity surface emitting lasers (VCSELs) [1, 2, 3]. They are also being considered for the gate material in field effect transistors (FET) [4]. In spite of the growing importance of these native oxides, there are several issues that complicate their use, including mechanical stability and fabrication reproducibility. The multi-layers for VCSEL devices are fabricated by molecular beam epitaxy (MBE) [5, 6] or metal organic chemical vapor deposition (MOCVD) [7, 8]. The MBE-grown AlGaAs layers are sometimes pseudo or digital alloys, rather than the random alloys grown by MOCVD. Specifically, the MBE layers are made up of very thin (0.5 to 6 nm) alternating layers of AlAs and AlGaAs or GaAs rather than a single true AlGaAs. Other differences between these growth techniques are that MBE uses solid sources for growth under ultra high vacuum, while MOCVD uses organic precursors at pressures close to ambient. The purpose of this study is to investigate how these two growth methods and different growth systems affect the wet-thermal oxidation of AlGaAs. The effects of V/III ratio during growth and impurity concentrations on the oxidation rate are also reported. EXPERIMENTAL PROCEDURES Three labs provided four specimens for the study. One wafer was fabricated by MBE and the others were grown by MOCVD. In addition to the already mentioned differences between these two growth techniques, the MBE sample was grown at a lower temperature, 600oC, and at a slower rate, GaAs-0.18/AlAs-0.3/AlGaAs-0.52 nm/sec. The MOCVD samples were grown at 720oC, with MOCVD1 grown at 1.5 nm/sec, MOCVD2 at 0.6 nm/sec, MOCVD3 at 1.0 and 1.5 nm/sec. Three of the wafers had nominally identical structures with different AlGaAs layers ranging in composition from 90 % Al (Al.90Ga.10As) to 100 % (AlAs), in increments of 2 %. The fourth sample, MOCVD3, had three sets of ∗
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layers grown with V/III rati
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