Second quantum state transition in GaAs/AlGaAs resonant Bragg structure probed by modulation reflectance spectroscopy

Modulation spectroscopy, ever since its introduction by B.O. Seraphin in 1964, has been considered and widely used as a sensitive experiment technique for studying and characterizing the properties of varieties of semiconductor materials. Compared to general optical reflectance spectrum which measures the absolute reflection, the modulation spectroscopy evaluates the interpretation of the changes in the optical response from the sample caused by a periodic physical perturbation applied to the sample, such as temperature, electric fields, hydrostatic pressure, uniaxial stress, etc. Those modulation spectroscopies with an external electric field perturbation are known as electroreflectance spectroscopy, which provides sharp and derivative-like spectral features in the energy region of excitonic transitions in the semiconductors while suppressing uninteresting background effects that are not affected by the electro-modulation. One interesting phenomenon is that when the excitonic transition energy in a periodic dielectric structure, for example multiple quantum well (MQW) structure, meets the Bragg resonance condition, the reflectance spectrum shows an enlarged responding effect with enhanced reflectivity and broadened transition features. This kind of a structure is known as a resonant Bragg structure (RBS) and the coincidence of the exciton and Bragg resonances is called the double resonance condition.;In this thesis, we employed both electroreflectance and optical reflectance spectroscopies to probe excitonic transitions in a GaAs/AlGaAs RBS MQW structure. The sample structure was specially designed and fabricated to tune the double resonance condition around the second state of the heavy-hole exciton x(e2-hh2) transitions by variation of the incident angle and temperature. The sample used in this experiment consists of 60 periods of quantum well structures with GaAs well layer (13 nm) and AlGaAs barrier layer (94 nm), grown by solid source molecular beam expitaxy on a semi-insulating GaAs substrate. We observed a significant enhancement of excitonic features at the x(e2-hh2) exciton transitions around incident angle of 23° in both techniques, revealing the double resonance condition at low temperature. Additionally, heavy-hole and light-hole ground state exciton transitions x(e1-hh1) and x(e1-lh1) were also evaluated. In the temperature dependence of optical reflectance and electroreflectance from the double resonance condition, we observed redshift of the excitonic features. The electric field dependence measurement of electroreflectance exhibited a broadening effect for the x(e2-hh2) exciton transition.