Direct band gap determination in AlxGa1-xAs epitaxial layers in the indirect gap region 0.4 < x < 0.9

GaAs and AlxGa1-xAs alloys are materials widely studied during the last thirty years; however, there are not enough data concerning the electrical and optical properties of the ternary AlxGa1−xAs alloys when the Al concentration in the layer is greater than 40%, that is when the AlxGa1−xAs band structure changes from a direct to an indirect one. Using the liquid phase epitaxy (LPE) growth technique, AlxGa1−xAs epitaxial layers with x-values in the range 0.35–0.90 were grown. The high crystalline quality of these layers was verified through the presence of an intense and sharp excitonic peak in the low temperature photoluminescence spectra, even for those layers with a high Al concentration in which there is a low probability for radiative indirect transitions. In order to measure the energy of the direct band gap transition at the Γ-point of the Brillouin zone, E0, we have used thermoreflectance spectroscopy to monitor the evolution of the E0 transition and its dependence on the alloy composition, x. From these results we propose a new E0(x) empirical expression, and we compare it with the commonly used expression due to Lee based on indirect measurements of the band gap energy.