Electron Intersubband Transitions in N(+) Indium Gallium Arsenide Quantum Wells: Optical Selection Rules and Strain Effects

This thesis presents an experimental and theoretical study of electron intersubband transitions in n ^+ InGaAs single quantum wells (SQWs). Using polarization-resolved techniques, we found electron intersubband transitions in n^+ InGaAs/AlAs SQWs exhibit two interesting features of (i) equally strong TE and TM absorption strength and (ii) strain dependent polarization splitting. A detailed account of 14 times 14 kcdotp perturbation theory and QW D_{2d} group symmetry analysis is given to explain these observations. In 14 times 14 kcdotp theory, interband mixing of QW subbands at k_ | = 0, is strongly enhanced by the quantization effects. High-lying QW conduction subbands, therefore, have appreciated amount of P-like character due to band mixing effects. Interband coupling of Gamma_sp {15}{c} - Gamma_sp{15}{v} (Q), Gamma_sp{1} {c} - Gamma_sp {15}{v} (P _0), and Gamma_sp{1 }{c} - Gamma _sp{15}{c} (P _1) are found important in determining the intersubband selection rules and excitation strength. In particular, for crystals possessing spherical symmetry, Gamma_sp{15}{c} - Gamma_sp{15 }{v} coupling vanishes so that TE-active intersubband IR absorption is diminished and intersubband Raman scattering is abolished. Selection rules of intersubband IR absorption and Raman scattering can also be derived from QW D _{2d} point group symmetry analysis. A compressive QW biaxial strain causes a polarization splitting of intersubband transitions with TM-active transition occurred at the higher energy side of TE-active transition.