Effects Due to and Derived from Spontaneous Ordering in III-V Semiconductor Alloys
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Effects Due to and Derived from Spontaneous Ordering in III-V Semiconductor Alloys Yong Zhang*and A. Mascarenhas National Renewable Energy Laboratory, 1617 Cole Boulevard Golden, CO 80401, USA *[email protected] ABSTRACT Two interesting and important aspects of spontaneous CuPt ordering in III-V semiconductor alloys, which have only been investigated recently, are reviewed in this paper. The first aspect addresses the statistical effects that should be considered as the most unique consequence of the phenomenon of ordering, more specifically, how ordering affects the alloy fluctuations and hence the physical properties of the alloy. The second aspect tackles some intriguing properties of the domain twins of two CuPt ordered variants, specifically, considering the transmission of a ballistic electron beam through such a domain twin and its analogy to a highly interesting phenomenon, negative refraction, for light.
INTRODUCTION Spontaneous ordering in III-V semiconductor alloys has been studied for close to two decades. Despite the fact that the underlying mechanism for initiating the ordering during the epitaxial growth still remains controversial,1-3 a great deal of understanding toward the consequences of ordering has been achieved, especially for the CuPt ordering in the prototype system GaxIn1-xP (x ~ 0.5).4, 5 The so-called CuPt ordered structure is a mono-layer superlattice along one of the [111] directions of the cubic lattice with alternating Ga-rich and In-rich layers: Ga0.5+η/2In0.5-η/2P and Ga0.5-η/2In0.5+η/2P, with η being the order parameter varying from 0 (full random) to 1 (full ordered). Experimentally, by empirically adjusting the growth parameters, the order parameter is found to be tunable from η ≈ 0 to η ~ 0.6.6 Note that even though the maximum order parameter achieved so far is still not close to η = 1, it is actually higher than that reported for artificially ordered structures, e.g., GaAs/AlAs-[001] mono-layer superlattices.7 Beside the effort in attempting to reveal the mechanism for the formation of ordering, a vast amount of studies have been devoted to explore the effects of ordering. The most extensively investigated area is the modifications in the band structure properties due to or related to the symmetry change as a result of ordering. 4, 5 These properties are usually obtained by averaging the corresponding properties over a large volume probed by a given experimental technique. These macroscopic effects of ordering may include, for instance, band gap reduction, valence band splitting, changes in both conduction and valence band effective masses, optical anisotropy, band offsets with respect to other semiconductors, phonon modes, etc. Although these effects are critically important for understanding the ordering process and device applications, their underlying physics in fact often shares some similarity with that of other types of perturbations, say, uniaxial stress and quantum confinement. The unique feature and the kernel of the ordering phenomenon should be the statistic
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