Adsorption and Decomposition of Trimethylarsenic on GaAs(100)
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ADSORPTION AND DECOMPOSITION OF TRIMETHYLARSENIC ON GaAs(100) J. R. CREIGHTON Sandia National Laboratories, NM87185
Division 1126, P.O. Box 5800, Albuquerque,
ABSTRACT Alkylated arsenic compounds have shown some promise as alternatives to arsine as the group-V source gas for GaAs MOCVD. However, little is known about the fundamental chemical interactions of these compounds with the GaAs surface. We have investigated the adsorption and reactivity of trimethylarsenic (TMAs) on GaAs(100) using temperature programmed desorption (TPD), Auger electron spectroscopy, and LEED. For the exposures and temperatures studied, TMAs did not pyrolytically decompose on the GaAs(100). TPD results indicate that TMAs chemisorbs, apparently non-dissociatively, and desorbs - 330 K. Multilayers of TMAs desorb - 140-160 K. Exposure of adsorbed TMAs to 70 eV electrons results in irreversible decomposition of the molecule. After electron irradiation, TPD shows that methyl radicals desorb at 660 K, which corresponds to a desorption activation energy of - 40 kcal/mol. At higher temperatures, As 2 , H2, C2 H2 , and a smaller amount of methyl radicals desorb, and a small coverage of carbon remains on the surface.
INTRODUCTION The hazard posed by the group-V hydride source gases (i.e. AsH3 and PH3 ) commonly used for III-V MOCVD has spawned interest in the search for safer alternatives [1,2]. Most of work to date has focused on alkylated derivatives of arsine such as trimethyl, triethyl, or tertiarybutylarsine. High levels of carbon incorporation has been a major problem in the development of growth processes utilizing the alkylated source gases. It is generally believed that carbon incorporation is the result of some intrinsic chemical reaction involving the alkyl groups of the source gas. As part of our overall effort to investigate the heterogeneous chemistry relevant to MOCVD, we have chosen to examine the surface chemistry (i.e. reaction pathways, carbon incorporation mechanisms) of the simple alklylated arsenic compounds. In this paper we report some of our initial results on the chemical interactions of trimethylarsenic (TMAs) with the GaAs(lO0) surface. The true pyrolytic behavior of TMAs will be compared to that observed when adsorbed TMAs is irradiated with 70 eV electrons.
EXPERIMENTAL Experiments were performed in a multilevel vacuum system equipped with a cylindrical mirror analyzer for Auger Spectroscopy, a quadrupole mass spectrometer for temperature programmed desorption (TPD), and rear-view LEED optics (see Fig. 1). The chamber was pumped by turbomolecular pumps ay liquid-nitrogen cold traps to a typical working base pressure of 5 X 10 Torr. A doser, with a liquid-nitrogen-cooled shutter, was used to dose TMAs in the upper chamber level. The absolute flux of TMAs impinging on the sample was not known, but it was normally adjusted to produce about 1 ML of adsorbed TMAs every 5-30 seconds. The sample was a 1.8-cm x 0.8-cm rectangular piece of semi-insulating GaAs(100) on which a 2-pm-thick conductive epilayer (Si doped, n-type)
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