Self-Organized Superlattices in GaInAsSb Grown on Vicinal Substrates
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Self-Organized Superlattices in GaInAsSb Grown on Vicinal Substrates* C.A. Wang, C.J. Vineis†, and D.R. Calawa Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA 02420-9108 † now at AmberWave Systems Corporation, Salem, NH 03079 ABSTRACT Self-organized superlattices are observed in GaInAsSb epilayers grown lattice matched to vicinal GaSb substrates. The natural superlattice (NSL) is oriented at a slight angle of about 4° with respect to the vicinal (001) GaSb substrate. This vertical composition modulation is detected at the onset of growth. Layers in the NSL are continuous over the lateral extent of the substrate. Furthermore, the NSL persists throughout several microns of deposition. The NSLs have a period ranging from 10 to 30 nm, which is dependent on deposition temperature and GaInAsSb alloy composition. While the principle driving force for this type of phase separation is chemical, the mechanism for the self-organized microstructure is related to local strains associated with surface undulations. By using a substrate with surface undulations, the tilted NSL can be induced in layers with alloy compositions that normally do not exhibit this selforganized microstructure under typical growth conditions. These results underscore the complex interactions between compositional modulation and morphological perturbations. INTRODUCTION Phase separation in multi-component compound semiconductors has been widely reported [1]. Of particular interest are materials systems that spontaneously self-organize with a significant degree of regularity, since this periodicity can impact the electronic band structure and consequently, materials properties and device performance. The length scale of these ordered phases ranges from the atomic scale, e.g. CuPt ordering as observed in GaInP [2], GaAsP [3], GaAsSb, [4], and InAsSb [5], to microscopic dimensions on the order of ~50 nm, e.g. composition modulation. Composition modulation can persist either parallel (lateral) to the growth direction, or perpendicular (vertical) to the growth direction. Lateral composition modulation (LCM) has been reported in strained alloy systems such as bulk AlInAs and GaInP epilayers [6-8], as well as in intentionally grown short period superlattices such as GaP/InP, AlAs/InAs, GaAs/InAs, and InAs/GaSb [9-12]. Vertical composition modulation (VCM) and self-organized natural superlattices (NSLs) in alloy layers that are homogenously grown have also been observed, but these studies are less prevalent and the mechanism for their formation are less understood. NSLs have been reported in ZnSeTe grown on vicinal GaAs substrates [13] and SiGe grown on (001) Si [14]. The NSL period was 2 to 3 nm for both of these materials systems, and a model based on step-flow growth and local strain fields that are modulated during growth were developed to explain the phenomena [14]. In addition, InAsSb and GaAsSb [5,15] were reported to spontaneously form a periodic structure that consisted of platelets of alternating composition and periodici
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