The Optical Properties of Narrow-Gap Low Dimensional Structures

Abstract

The paper reviews recent developments in the optical properties of narrow-gap semiconductors. The extension of optoelectronics to wavelengths beyond 1.5 μm has required the development of novel device concepts and, unlike the situation with GaAs based systems, new materials technologies. A case in question is the interest in replacing mercury cadmium telluride alloys for devices operating in the region of 10 μm wavelength, because of the severe structural and stability problems. The following examples are given of differing approaches currently being investigated.

Mismatch epitaxy is now widely employed either to modify the band structure by fabricating strained layer superlattices or to integrate grossly mismatched materials such as InSb with GaAs or GaAs with silicon. In the latter case the strain is relaxed by generation of large concentrations of misfit dislocations. Nevertheless it is shown that the carrier mobility need not be strongly degraded even close to the interface between the mismatched materials.

Strained layer superlattices based on the alloy system In(As, Sb) have been used to generate band edge photoluminescence at wavelengths beyond 10 μm. Examples are given of structural problems encountered with these III–V alloys. Remarkable photoluminescence results have also been obtained with single monolayers of InAs inserted into GaAs where intense sub band gap emission is observed with samples grown by flow-modulation MOCVD.

Other novel narrow-gap systems which are currently being investigated include alpha-tin and its alloys with germanium where the alpha phase is stabilized by built-in strain, and alloys of InSb and InAs with bismuth.

Alternative approaches to midinfrared photonics include i) the use of doping superlattices (n-i-p-i structures) to modify the band gap and to produce highly nonlinear optical effects, ii) the exploitation of intersubband absorption in quantum well structures.