Gas Phase and Surface Reactions of Organometallic Arsenic Sources
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GAS PHASE AND SURFACE REACTIONS OF ORGANOMETALLIC ARSENIC SOURCES T.R. Omstead, S. Brandon, M. Hoveland, and K.F. Jensen Department of Chemical Engineering and Materials Science University of Minnesota Minneapolis, MN 55455 D.A. Bohling and G.T. Muhr Air Products and Chemicals P.O. Box 538 Allentown, PA 18105 ABSTRACT The chemical kinetics of two new organometallic arsenic substitutes, tris-trifluoromethylarsenic (As(CF3)3) and phenylarsine (PhAsH2), for use in the MOCVD of GaAs have been characterized through the use of microbalance gravimetry and molecular beam mass spectrometry. Both growth rate and gas-phase cracking studies demonstrate that phenylarsine interacts only slightly with the common gallium precursors, which may make it a useful alternative to arsine. Growth with tris-trifluoromethylarsenic is achieved only at low V/III ratios and with pressures above 250 Torr. The compound etches GaAs under most conditions. INTRODUCTION There is an increasing interest in replacing arsine with less hazardous and more easily handled arsenic sources for the metalorganic chemical vapor deposition (MOCVD) of III-V compounds.
The use of these compounds has been hindered, however, by a limited understanding of the fundamental chemical kinetics associated with growth from these precursors. To address this problem we have investigated the growth mechanisms of two of the newest organometallic arsenic precursors tris-trifluoromethylarsenic (As(CF 3 ) 3 , T"FMAs, or TRIS) and phenylarsine (PhAsH 2 , PhAs) and compared them to tertiarybutylarsine (tertBu-AsH 2 or tBAs), which is currently the most commonly used arsine alternative. We have employed a microbalance MOCVD system for the continuous monitoring of growth rates through weight change, and a molecular beam mass spectroscopy system for the direct sampling and analysis of gas-phase composition. Both of these systems have been described in detail elsewhere [1,2]. The most common approach to developing a liquid organometallic arsine substitute has
been to substitute one or more alkyl groups for hydrogen. Precursors such as trimethylarsenic, triethylarsenic, diethylarsine, and tertiarybutylarsine have met with varying degrees of success [3,4]. The substitution of alkyl ligands onto arsine, while lowering the toxicity and the vapor pressure, creates problems due to their electron donating nature. The presence of these alkyl ligands increases the basicity of the lone electron pair on the sp3 bonded arsenic atom, enhancing the formation of adduct compounds with the Lewis acid gallium precursors such as trimethylgallium and triethylgallium. These adduct complexes can lead to parasitic reactions, which cause source depletion and possibly increased impurity incorporation. Both of the compounds under
investigation here are designed to reduce adduct formation through the substitution of groups less electron donating than alkyls. The fluoromethyl ligand in tris-trifluoromethylarsenic is electron Mat. Res. Soc. Symp. Proc. Vol. 145. 01989 Materials Research Society
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