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|Title:||Resonance Raman Scattering Studies of Gallium-Arsenide - Aluminum-Arsenide Superlattices|
|Author(s):||Gant, Thomas Andrew|
|Doctoral Committee Chair(s):||Klein, Miles V.|
|Department / Program:||Physics|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Subject(s):||Physics, Condensed Matter|
|Abstract:||We have made resonance Raman scattering studies of folded LA phonons and quantized LO phonons in several GaAs-AlAs superlattices. The motivation for this work was to study the electronic structure and the electron-phonon interaction in these structures. The samples were not intentionally doped. The Raman spectra of optic phonons were usually taken at a temperature of 10 K or less. The folded acoustic phonon work was taken at temperatures ranging from 200-300 K in order to enhance the scattering by the thermal factor.
Two samples in particular have received very close attention: sample 2292 (50 A GaAs- 20 A AlAs) and sample 3250 (20 A GaAs- 50 A AlAs). In sample 2292 we have made resonance studies of the folded LA phonons and the GaAs-like confined LO$\sb2$ mode near the second heavy hole exciton. The results on the folded acoustic phonons show a very strong resonance enhancement for the second order folded phonons, but very little for the first order. An interference between two different scattering channels (the n = 1 light hole and the n = 2 heavy hole subbands) seems to be responsible for this effect.
The resonance profile for the LO$\sb2$ confined optic phonon in sample 2292 shows 4 peaks in the region from 1.8 eV to 2.05 eV. We have studied the dependence of this resonance profile on the power density. A higher power density was achieved by using the same laser power with a tighter focus. At the higher power density the peak at 1.93 eV (formerly the strongest peak present) vanished. This "bleaching" effect is related to screening due to the higher carrier density.
In sample 3250 we have studied the polarization dependence of the resonance profiles of four peaks (LO$\sb2$, LO$\sb4$, LO$\sb6$, and an interface mode) near the lowest direct gap. The A$\sb1$ symmetry confined LO modes are seen in both polarized and depolarized geometries, in violation of the usual selection rule (polarized). A mechanism is proposed to explain this result, which has been previously observed by other workers.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1988.
|Date Available in IDEALS:||2015-05-13|