Electronic Structure and Optical Properties of Iii-V and II-Vi Semiconductor Superlattices.

This thesis presents a theoretical analysis of the electronic structure and optical properties of superlattices (SL) consisting of direct-gap bulk III-V and II-VI semiconductors. The philosophy underlying the present theoretical approach is to consider the superlattice as a periodic anisotropic solid to which a SL representation formalism for electronic and optical properties can be applied. The superlattice electronic properties at SL wavevector K = 0 are calculated within the envelope function approach using a modified bulk Kane k cdot p model as input. A detailed account of the envelope function approach is given. The SL band structure at finite K is generated using superlattice K cdot p perturbation theory. The electronic properties of three technologically important superlattices, namely GaAs/GaAlAs (Type I), InAs/GaSb (Type II) and HgTe/CdTe (Type III) are investigated for a wide range of layer widths. Superlattice effective masses are calculated analytically using the f-sum rule. Good agreement is obtained with recent experimental data for GaAs/GaAlAs. Only a few SL bands contribute to the f-sum rule in all three SL systems. The highly anisotropic oscillator strength connecting the lowest two SL conduction bands C1 and C2 is responsible for the difference between the values of the SL electron masses perpendicular and parallel to the planes. A resolution of the valence band offset controversy in HgTe/CdTe SL is proposed based on the discovery of a semiconductor to semimetal to semiconductor transition which occurs as the valence band offset is increased from zero. The calculated fundamental absorption coefficients for InAs/GaSb and HgTe/CdTe are in excellent agreement with experimental data. The intersubband C1 to C2 absorption is predicted to be appreciable, particularly in the thick barrier SL limit where the absorption linewidth is comparable to that of a laser. Large variations of the refractive index accompany the sharply peaked C1 to C2 absorption in thick barrier SLs, leading to the proposal of a novel class of Carrier Activated Light Modulators.