Applications of Strained Layer Superlattices
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APPLICATIONS OF STRAINED LAYER SUPERLA'ITICES D. L. SMITH*, B. K. LAURICH* AND C. MAILHIOT** *LOS ALAMOS NATIONAL LABORATORY, Los Alamos, NM 87545 "**LAWRENCE LIVERMORE NATIONAL LABORATORY, Livermore, CA 94550 ABSTRACT Because 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 [2111 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. INTRODUCTION Pseudomorphic growth of thin semiconductor layers, whose lattice constant differs from that of the substrate upon which it is grown, has been clearly demonstrated [1-3]. For material layers thinner than a critical thickness, which depends on the difference in lattice constants, the lattice mismatch is accommodated by internal strain in the film rather than by the formation of dislocations. The internal strain can produce novel physical properties in SLS [4]. Here we consider two examples of strain-induced properties: electric fields generated by the piezoelectric effect and strained type II superlattices. Large electric fields occur in SLS grown along a polar axis, such as [111] or [211] (see Ref. 5). These fields strongly modify the optical properties of the SLS [6] by the Stark effect. Because the electric fields are intrinsic to the superlattice, we refer to them as intrinsic Stark effect superlattices (ISES). The fields can be modulated by screening from photogenerated electron-hole pairs [7] and by application of an external field [8] leading to nonlinear optical and electrooptical response, respectively. Strain can be used to tailor the optical respo
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