Resonant Raman scattering by LO phonons in AlxGa1−xAs (0.2): Exciton broadening and alloying effects

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Abstract
The dipole-allowed resonant Raman scattering by LO phonons is studied around the E0 gap of Alx Ga1−xAs alloys at 100 K. Six high-purity n-type Alx Ga1−xAs samples with aluminum content in the range 0.2<x<0.7 were investigated. In the framework of the virtual-crystal approximation (VCA), a theoretical model of deformation-induced resonant Raman scattering by one LO phonon, including excitonic effects, has been used to interpret the experimental data. The measured absolute values of Raman polarizabilities for both GaAs and AlAs modes are in good agreement with the calculated ones for all alloy compositions x studied. These results explain the decisive role of excitons in the one-phonon Raman process and support the validity of the VCA for the Alx Ga1−xAs alloy. From the fit the dependence of the exciton lifetime broadening Γ0 on x is obtained: Γ0 varies slowly in the range x<0.45 and rapidly increases in the range x≳0.45. A qualitative explanation of this behavior is given in terms of intervalley exciton scattering from the Γ to X points and from the Γ to L points of the Brillouin zone.