Formation of a Ba-Te Surface on GaAs
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Formation of a Ba-Te Surface on GaAs Kevin A. Boulais, Francisco Santiago, Karen J. Long and Victor H. Gehman Electromagnetic and Solid State Technologies Division, Naval Surface Warfare Center Dahlgren Division, Dahlgren, VA 22448, U.S.A. ABSTRACT The formation of a Ba-Te surface on GaAs has been investigated. The surface was created using molecular beam epitaxy (MRS). A GaAs (100) surface was first exposed to Te and characterized using x-ray photoelectron spectroscopy (XPS), reflective high energy electron diffraction (RHEED) and low energy electron diffraction (LEED). The Te-reacted surface was then exposed to BaF2 flux producing a second reaction. In this reaction, the BaF2 dissociated leaving barium on the surface but no fluorine. This is in contrast to the clean (no tellurium) GaAs (100) surface in which BaF2 has been shown to grow single crystal. Although high order exists during early stages of the Ba-Te growth, further exposure gives way to a polycrystalline form. This paper discusses the formation and analysis of the Ba-Te surface. INTRODUCTION There has been much interest in the chalcogen elements as applied to GaAs in recent years. One area is the heteroepitaxial growth of lattice mismatched materials by aid of a third species acting as a surfactant [1]. Tellurium as a surfactant promotes 2-dimensional growth of InAs [2,3] and ZnSe [4] on GaAs (001). For InAs, tellurium extends layer-by-layer growth from 1.5 monolayers to 6 monolayers. The lattice mismatch between InAs and GaAs is 7.2%. It has been proposed that the 2-dimensional growth occurs by an exchange process with the surfactant [5,6]. Thus, the tellurium floats on the surface and limits the kinetics by burying the growth material in underlying layers. Another driving force has been the promise of improved GaAs devices. It has been shown that some of the chalcogen elements, when applied to the surface of GaAs, result in an enhancement in the photoluminescence, a reduction in the surface recombination rate and a shift in the Fermi-level towards improved device performance. This is particularly true for selenium and sulfur [7] although changes have also been observed using tellurium [8,9]. However, Zahn, et. al. [10] attributes the enhancements to a redistribution of interface states within the bandgap rather than a true electrical passivation of the surface. There has been concern about the stability of chalcogenated GaAs in the presence of oxygen. An aging process has been observed with a corresponding reduction in photoluminescence [7] and increased Fermi-level pinning [11] over a period of hours. However, the reduction in the passivated-like surface appears to be somewhat reversible either by heating [12] or by deposition of an appropriate metal [11] which is believed to remove the oxygen from the subsurface layers. It has also been shown that some metals will stabilize a chalcogenated GaAs surface by creating a low-soluble chalcogenide [13] that minimizes oxidation. In this paper, we discuss the formation of a Ba-Te layer formed on (00
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