Applications of Strained Layer Superlattices

ABSTRACTBecause of different band-edge lineups, strain conditions, and growth orientations, various strained-layer superlattice (SLS) materials can exhibit qualitatively new physical behavior in their optical properties. We describe two examples of new physical behavior in SLS: strain-generated electric fields in polar growth axis superlattices and strained type II superlattices. In SLS, large electric fields can be generated by the piezoelectric effect. The fields are largest for SLS with a [111] growth axis; they vanish for SLS with a [100] growth axis. The strain-generated electric fields strongly modify the optical properties of the superlattice. Photogenerated electron-hole pairs screen the fields leading to a large nonlinear optical response. Application of an external electric field leads to a large linear electrooptical response. The absorption edge can be either red or blue shifted. Optical studies of [100], [111], and [211] oriented GaInAs/GaAs superlattices confirm the existence of the strain-generated electric fields. Small band-gap semiconductors are useful for making intrinsic long wavelength infrared detectors. Arbitrarily small band gaps can be reached in the type II superlattice InAs/GaSb. However, for band gaps less than 0.1 eV, the layer thicknesses are large and the overlap of electron and hole wavefunctions are small. Thus, the absorption coefficient is too small for useful infrared (IR) detection. Including In in the GaSb introduces strain in the InAs/GaInSb superlattice, which shifts the band edges so that small band gaps can be reached in thin-layer superlattices. Good absorption at long IR wavelengths is thus achieved.