Thermoreflectance study of the direct optical gap in epitaxialAlxGa1−xSb(x<~0.5)

Thermoreflectance spectroscopy was used to precisely determine the direct optical gap ${E}_{g}^{\ensuremath{\Gamma}},$ as a function of composition and temperature of a series of ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{Sb}$ layers $(0.0l~xl~0.5)$ epitaxially grown on GaSb. The experimental line shapes were fitted with a critical-point functional form including excitonic effects, to derive the direct gap and broadening parameter values. The relation between ${E}_{g}^{\ensuremath{\Gamma}}$ and x shows a x-dependent bowing, which was compared with previous results and theoretical models, leading to the conclusion that ${E}_{g}^{\ensuremath{\Gamma}}(x)$ curves in ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{Sb}$ alloys have a cubic polynomial form.