Quadruple-period ordering in MBE GaAsSb alloys
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Quadruple-period ordering in MBE GaAsSb alloys Iskander G. Batyrev, Andrew G. Norman, Shengbai Zhang, and Su-Huai Wei National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401, U.S.A. ABSTRACT Experimental and theoretical studies are reported for a quadruple period ordering found in GaAsSb alloy layers grown by molecular beam epitaxy at high growth temperatures. We propose a growth model that accounts for the observed three-dimensional (3D) ordered structure. It is shown that the already-ordered material in the previously grown layer affects the reconstruction of the growth front with respect to the underlying alloy template resulting in the correct stacking of the individual 2D ordered layers into the observed 3D ordered structure. INTRODUCTION Atomic ordering is frequently observed in epitaxially grown semiconductor AxB1-x alloys and is driven by surface thermodynamics and/or by growth kinetics, but not by bulk thermodynamics [1]. This is because the observed ordered phases often have a higher energy than either the disordered alloy or other as yet unobserved ordered structures [2]. Reconstruction at the growing surface, typically a (001) surface, frequently involves atomic dimerization that naturally provides an atomic-scale compressive/tensile strain field below the surface [3]. This creates a subsurface site preference for size-mismatched atoms and hence ordering [4,5]. This dimer-induced ordering mechanism is widely accepted but, strictly speaking, describes only a two-dimensional (2D) phenomenon. In order to obtain the observed three-dimensional (3D) ordered structures, the 2D ordered layers have to be correctly stacked. It has been proposed that surface steps are essential to complete the 3D ordering, as several experiments have shown that the degree of ordering correlates with the density and orientation of surface steps [6-8]. However, a microscopic model based on first-principles theory taking into account the stepinduced 3D stacking has so far not been reported. Recently, we reported a new quadruple-period (QP) ordering present in GaAs1-xSbx alloys (0.05 < x < 0.2) grown by molecular beam epitaxy (MBE) at 625˚C. This was revealed by transmission electron diffraction (TED) [9,10] and x-ray diffraction measurements [11]. The QP ordered structure has several extraordinary but unexplained physical features: (i) The QP ordered structure is only observed in layers grown at temperatures above 600°C. At such temperatures, reflection high energy electron diffraction (RHEED) measurements indicate the surface transforms into a high temperature form of (2x4) reconstruction differing from the β2(2x4) reconstruction present at lower growth temperatures. (ii) The QP ordering has a CuAu-like superlattice structure but contains a periodic array of antiphase boundaries along the [110] direction. This is the first report of such a structure in a semiconductor alloy, although similar antiphase superlattices have been observed previously in metal alloys [12]. (iii) The ordering direction is perpend
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