Excitonic and Raman Properties of Zinc SELENIDE/ZINC(1-X) Cadmium(x) Selenium Strained-Layer Quantum Wells

This dissertation presents for the first time the detailed excitonic and Raman scattering investigations of strained-layer single quantum-well ZnSe/Zn_ {1-x}Cd_{ x}Se heterostructures. Recently, the existing group of molecular-beam epitaxially (MBE) grown II-VI materials has been enlarged by the successful growth of cubic (zinc-blende) CdSe and Zn_ {1-x}Cd_{ x}Se on GaAs substrate. The heteroepitaxy of Zn_{rm 1-x}Cd _{x}Se on (100) GaAs by MBE results in single-phase zinc-blende crystals over the entire composition range from CdSe to ZnSe. In wide-gap II-VI semiconductors the luminescence spectra are dominated by excitonic transitions involving the electron subbands and hole subbands. The photoluminescence under direct and indirect excitations are investigated in detail to study the carrier generation, transport and recombination mechanisms. The temperature dependence of photoluminescence and resonant Raman scattering are investigated to study the exciton -phonon interaction and luminescence quenching mechanisms. Very strong 2LO phonon Raman scattering has been observed with single Zn_{0.86}Cd _{0.14}Se quantum wells, where the scattered photon energy is in resonance with an exciton transition. The experimental confined exciton energies are compared with a finite-square potential-well model including band-nonparabolicity. The bandgap of Zn _{1-x}Cd_ {x}Se as a function of composition 'x' has been computed. Lattice mismatch as high as 1.011% exists in samples of ZnSe/Zn_{1-x}Cd_{x} Se quantum wells with x = 0.14. The strain effect shifts the band-edge energy in the quantum-well and removes the degeneracy present at the zone center. The strain -induced effects on band structures are calculated and observed in the photoluminescence spectra. The main emission peak is attributed to the free-exciton transition between the lowest electron subband and the ground heavy-hole band of the Zn_{0.86}Cd _{0.14}Se quantum-well. The photoluminescence emission spectra show clear shifts to lower energies as the well width is increased. The temperature dependence of peak position, their intensities, and the linewidth are studied. The temperature dependency of the linewidth is consistent with a broadening model in which free excitons are scattered by LO and TO phonons and ionized donor impurities. By temperature tuning the bandgap, the free-exciton transition is brought into resonance with the Raman scattered photon hbaromega_{S2LO }. This investigation reveals the potential for realization of narrow-band emitters as blue light-emitting diodes (LED's), lasers, and photodetectors.