Gas Phase and Surface Reactions in Mocvd of GaAs from Triethylgallium, Trimethylgallium, and Organometallic Arsenic Prec
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GAS PHASE AND SURFACE REACTIONS IN MOCVD OF GaAs FROM TRIETHYLGALLIUM, TRIMETHYLGALLIUM, AND ORGANOMETALLIC ARSENIC PRECURSORS Thomas R. Omstead, Penny M. Van Sickle and Klavs F. Jensen Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 ABSTRACT The growth of GaAs from triethylgallium (TEG) and trimethylgallium (TMG) with tertiarybutylarsine (tBAs), triethylarsenic (TEAs), and trimethylarsenic (TMAs), has been investigated by using a reactor equipped with a recording microbalance for in situ rate measurements. Rate data show that the growth with these precursors is dominated by the formation of adduct compounds in the gas lines, by adduct related parasitic gas phase reactions in the heated zone, and by the surface reactions. A model is proposed for the competition between deposition reactions and the parasitic gas phase reactions. Model predictions are in very good agreement with experimental data for all combinations of precursors except for TEG/TMAs where extensive gallium droplet formation is observed at low temperatures. Growth of reasonable quality GaAs with Hall mobilities of 7600 cm 2/Vs at 77 K using TEG and tBAs is reported for the first time. INTRODUCTION Although the use of arsine in organometallic chemical vapor deposition (MOCVD) of GaAs has been demonstrated to give excellent electrical properties [1 and references within], its use has a number of disadvantages. Since arsine is supplied in pressurized cylinders there is the possibility for an accidental large release of toxic gas, particularly during the changing of cylinders. Tertiarybutylarsine (tBAs), triethylarsenic (TEAs), and trimethylarsenic (TMAs) are alternative, liquid organometallic arsenic sources to arsine with the potential for growing good quality GaAs films [2,3] at relatively low V/III ratios. The gas phase chemistry associated with the use of TMAs, TEAs, and tBAs in conjunction with trimethylgallium (TMG) and triethylgallium (TEG) is complex, involving free radical reactions and possible adduct formation. A mass spectroscopy study of gas phase decomposition reactions and related possible adduct formations is described elsewhere [4,5]. Here we report on deposition rate measurements at 1 torr to demonstrate the relative influence of adduct formation, parasitic gas phase reactions and surface kinetics on the growth of GaAs by using organometallic precursors with TMG and TEG. The low pressure conditions were selected to minimize gas phase reactions and reduce mass transfer limitations that would otherwise complicate the data interpretation. For the same reasons, as well as the growth of abrupt interfaces and improvement in uniformity, there is a trend toward low pressure operation in device processing [1]. EXPERIMENTAL A radiantly heated, microbalance equipped, MOCVD reactor was employed as the primary means of investigating the growth rate of organometallic arsenic precursors with trimethylgallium (TMG) and triethylgallium (TEG). This reactor is shown in Figure 1 and it is des
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