Raman scattering in mixed crystal semiconductors

"We have studied the effects of substitutional disorder on the inelastic light scattering by phonons in three semiconductor alloys; (GaSbh-x(Ge2)x, GaAsxSbl-x' and AlxGal-xAs. We measure the asymmetric broadening and shifting of the longitudinal optic (LO) phonon lines as a function of disorder and observe scattering from disorder-activated longitudinal acoustic (DALA) and transverse acoustic(DAT A) modes. Our observation of zone-center (k=O) selection rules for the dominant scattering, even in the most disordered alloys, demonstrates the importance of considering the k=O spectral projection of phonon-eigenstates in explaining the Raman lineshapes for LO phonons in mixed crystals. In a polarized resonance Raman study of three AlxGa l-xAs alloys - one having a direct gap (x~=0.30), one near crossover (x~=0.43), and another having an indirect band-gap (x~=0.50)- we utilize polarization selection rules to discriminate between the ""allowed"" deformation potential and ""forbidden"" intra-band Frohlich scattering by LO phonons. We observe strong outgoing resonances for both first-order and second-order LO phonon scattering. Of particular interest is the discovery of an incident-wavelength-dependent shift between the deformation potential and intra-band Frohlich scattering in the indirect samples (x~=0.43 and x~=0.50). The intermediate electronic states involved in the Raman scattering by LO phonons in the indirect alloys have an increased effective mass and thus an enhanced localized character. We attribute the ""forbidden""- ""allowed"" energy shift to a localization effect manifested through the correlation between local values of the energy gap and local phonon frequencies."

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Year dc:date

2011

Author dc:creator

• McGlinn, Thomas Carl

Contributors dc:contributor

• Klein, Miles V.

• Raman scattering
• mixed crystal semiconductors
• substitutional disorder
• inelastic light scattering
• longitudinal optic (LO) phonon lines
• disorder-activated longitudinal acoustic (DALA)
• disorder-activated transverse acoustic (DATA)

Statement dc:rights

• 1987 Thomas Carl McGlinn

Language dc:language

en

Identifier

1322700

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/25673