Optical studies of strained pseudomorphic semiconductor heterostructures under external pressure

Epilayers and quantum well heterostructures of group III-V and II-VI binary and ternary compounds offer an exciting array of band gaps and physical properties. The band gaps range from the infrared to the near ultraviolet region (0.1 to 3 eV); continuous tuning of the band gap is achieved by means of alloying, strain and quantum confinement. Pseudomorphic heterostructures with large lattice mismatch and minimal dislocations and defects at the interface have been successfully grown by various techniques. This paper reviews the effects of strain on the optical and electronic properties, determination of strain, tuning of strain, and the structural stability of these metastable structures. Photoreflectance, photoluminescence and Raman scattering studies as a function of temperature and externally applied hydrostatic pressure are discussed. Owing to the different compressibilities and thermal expansion coefficients of the constituent members of a heterostructure, the strain can be continuously tuned by these perturbations. Depending on the relative signs of the lattice mismatch strain and the pressure (or temperature) induced strain, the net strain in the structure could increase or decrease. Examples of both cases are presented.