Resonant Raman scattering by LO phonons in AlxGa1−xAs (x < 0.1): Alloying and interference effects

Abstract

The resonant Raman scattering by LO and two LO phonons and LO-phonon interference effects are studied near the $E_0$+${\Delta }_0$ gap of liquid-phase-epitaxy ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As (0≤x≤0.07) samples at 100 K. The $E_0$+${\Delta }_0$ gap is shifted and broadened due to the alloying, whereas the interference between dipole-allowed and dipole-forbidden Raman scattering persists. Within the composition range studied the $E_0$+${\Delta }_0$ gap as well as its Lorentzian broadening η depends linearly on the alloy composition x [$E_0$+${\Delta }_0$(x)=1.847+1.56x eV, η(x)=3.8+120x meV]. Estimates for the η-versus-x behavior based on a simple tight-binding model and on binominal statistics for a random distribution of Al atoms on Ga sites do not explain the alloy broadening observed for the $E_0$+${\Delta }_0$ gap. Inhomogeneous broadening due to clustering fluctuations in x or other crystal defects cannot be discarded. The measured absolute values of the corresponding Raman efficiencies agree well with calculated ones. The comparison between the different ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As samples shows that aluminum in GaAs does not significantly enhance the impurity-induced dipole-forbidden Raman scattering by LO phonons.