Optical Studies of Strained-Layer and Diluted Magnetic Semiconductors
Optical properties of InGaAs/GaAs strained-layer -superlattices (SLSs) grown along (001) and (111) crystallographic axes were investigated. Significant differences in the spectra for the SLSs oriented in the two directions were observed. Magnetoreflectivity spectra showed features whose magnetic field dependence demonstrated their excitonic origin. For the (111) -grown samples, doublet structures were observed in the reflectivity spectra, whose energy separation are correlated with Landau-level-confinement -subband coupling experiments in a tilted magnetic field. The ground state exciton feature from the (111) -grown sample is significantly lower in energy than that from the sample with nominally identical parameters but grown along (001). PL spectra from the (111) sample showed dramatic pumping power dependent emission energies. Our results provide direct evidence for the theoretically predicted large built-in piezoelectric fields in the (111) -grown SLSs. Another part of this thesis consists of magneto -optical studies of diluted magnetic semiconductor (DMS) systems. The DMS system exhibits extraordinarily large spin splittings of the band edges arising from the strong exchange interactions between band carriers and the localized magnetic moments, as examined in CdFeSe and ZnFeSe materials. This property offers the possibility of magnetically tuning the quantum confinement potentials is a quantum-well structure consisting of DMS layers. Quantum-well structures of ZnFeSe/ZnSe were studied in this thesis, aiming to investigate such effects. Strongly asymmetric spin splitting of the excitonic components and dramatic intensity variations with magnetic field were observed. In a magnetic field, the small, zero -field valence band offset is overwhelmed by the spin-splitting of holes in the ZnFeSe layers. This results in a field -induced spin-dependent type-I/type-II band alignment and a consequent spatial separation of holes with different spin. The asymmetric behavior of the two components associated with the spin segregation of holes provides an accurate measure of the valence band offset in this unique system.
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- MAGNETICALLY TUNED;
- Physics: Condensed Matter; Physics: Optics; Engineering: Materials Science