Electronic structure of (001) and (111) growth axis InAs–Ga1−xInxSb strained-layer superlattices

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The electronic structure and optical properties of InAs–Ga1−xInxSb strained-layer superlattices grown along the (001) and (111) axis are investigated with the use of a realistic k⋅p theory. We assume that this superlattice is of the type II, i.e., the conduction band minimum of InAs lies at an energy below the valence band maximum of the alloy Ga1−xInxSb. We consider alloy compositions from x=0.1 to x=0.4, corresponding to lattice mismatches ranging approximately from 1% to 3%. Deformation potentials effects cause a substantial reduction of the band gap and allow the possibility to reach the technologically important 12 μm infrared region for very thin superlattices. This result is significant because type II superlattices exhibit optical matrix elements which decrease rapidly with increasing superlattice layer thickness. Reasonably large optical matrix elements can be obtained in the 12 μm region by properly tailoring the strain in the superlattice layers. In fact, optical matrix elements in InAs–Ga1−xInxSb strained-layer superlattices are shown to be comparable to those of HgTe–CdTe superlattices in the 12 μm region. Strained-layer superlattices grown along the (111) axis exhibit smaller quantum confinement energies and consequently appear to be slightly more advantageous than their (001) counterparts for optical applications. Furthermore, the presence of internal strain-induced piezoelectric fields in (111) growth-axis strained-layer superlattices produces a further reduction of the superlattice band gap. These results are significant and reveal that properly strained type II superlattices constitute a novel class of systems with useful materials properties for long-wavelength detection applications.