Theroretical investigations of intersubband relaxation in quantum wells
In this thesis intersubband relaxation of electrons in quantum wells is theoretically investigated. Firstly, the in-plane kinetic energy, and also well width dependences of electron intra- or intersubband scattering rates (or times), associated by longitudinal optical (LO) phonon emission in a semiconductor single quantum well (SQW) structure are presented. Semi-analytic calculations are carried out for a GaAs/AlO. 3GaO. 7As SQW structure. The results show that the scattering rates (both for intra- and intersubband scattering) weakly depend on in-plane kinetic energy of the electron. Further- more, the resulting calculations of well width dependence show that intrasubband scattering times gradually increase with well width contrasting with the intersubband scattering times which display a monotonic decrease. A theoretical study of the condition to achieve inverted population in a semiconductor double quantum well (DQW) structure is also presented. The LO-phonon assisted tunneling rates, based on the Fr6hlich interaction and Fermi's golden rule, has been performed for a GaAs/Alo. 3Gao. 7As DQW structure. The calculated results show that the tunneling rates monotonically decrease with the energy difference El- El and strongly depend on the magnitude of the transfer integral M. This work has been extended to calculate the electron transport and its kinetics, due to various types of scattering and tunneling mechanisms in a triple barrier resonant tunneling structure (TBRTS). A system of coupled kinetic equations that describe the nonequilibrium electrons in the structure has been solved analytically to obtain sub- band distribution functions and gain spectra. Finally, the concept of sequential tunneling has been introduced to explain an in- plane magnetic field dependence of resonant tunneling in a TBRTS. Typical current- voltage characteristics and derivatives for the TBRTS with particular design parameters have been calculated. It is found in the second derivative of the current that the resonance between El and Ej is manifested as a visible feature in the background of a wide E2 resonance. This feature has a sharp local maximum in the absence of applied magnetic field, and becomes flattened with increasing magnetic field in agreement with experiments.