Chemically Stable Semiconductor Surface Layers Using Low-Temperature Grown GaAs
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NSF MRSEC for Technology Enabling Heterostructure Materials and School of Electrical and Computer Engineering Purdue University West Lafayette, IN 47907, [email protected] *
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Department of Physics, Purdue University, West Lafayette, IN 47907
ABSTRACT The chemical stability of a GaAs layer structure consisting of a thin (10 nm) layer of lowtemperature-grown GaAs (LTG:GaAs) on a heavily n-doped GaAs layer, both grown by molecular beam epitaxy, is described. Scanning tunneling spectroscopy and X-ray photoelectron spectroscopy performed after atmospheric exposure indicate that the LTG:GaAs surface layer oxidizes much less rapidly than comparable layers of stoichiometric GaAs. There is also evidence that the terminal oxide thickness is smaller than that of stoichiometric GaAs. The spectroscopy results are used to confirm a model for conduction in low resistance, nonalloyed contacts employing comparable layer structures. The inhibited surface oxidation rate is attributed to the bulk Fermi level pinning and the low minority carrier lifetime in unannealed LTG:GaAs. Device applications including low-resistance cap layers for field-effect transistors are described. INTRODUCTION In stoichiometric n-type GaAs, a significant layer of oxide (approximately 25 A in thickness) forms rapidly upon exposure to atmosphere. In order to avoid this rapid oxidation, a number of passivation procedures have been employed, including As or S cap layers [1, 2]. Further evidence for the instability of most GaAs surfaces is given by the special preparation procedures employed in previous scanning tunneling microscope (STM) spectroscopy studies to avoid surface oxidation and the associated loss of STM resolution. Cleaved (110) surfaces have been prepared by either in-situ cleaving in an ultra high vacuum (UHV) STM system or ex-situ cleaving followed by sulfide passivation [2-5]. GaAs (001) surfaces passivated with As cap layers have been studied in UHV STM experiments following removal of the As layer by heating in the STM vacuum system [1]. STM spectroscopy has been performed on an unannealed layer of LTG:GaAs (225 C) capped with a layer of GaAs grown at 350 'C. Characterization of a (110) surface of the LTG:GaAs layer exposed by cleaving in UHV identified a band of midgap states associated with the excess arsenic [3-5]. For heavily doped n-type (n+) layers, this band of states was located above the valence band edge of the material. The rapid surface oxidation in stoichiometric GaAs plays a major role in ex-situ ohmic contact and Schottky barrier characteristics. This paper describes the surface oxidation characteristics and related device applications of low-temperature grown GaAs (LTG:GaAs), i.e. layers of GaAs grown by molecular beam epitaxy (MBE) at substrate temperatures of 250-300 'C. LTG:GaAs materials exhibit a number of interesting electronic properties associated with the excess arsenic incorporated during growth [3,6]. The studies reported in this work involve as-grown LTG:GaAs material, in which the excess arsenic results in
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