Direct and Indirect Energy Gap Dependence on Al Concentration in AlGaSb (x < 0.41)

The absorption spectra of AlxGa1-xSb alloys (x less than or equal to 0.41), measured at low temperatures, have been fitted including the contribution of direct and indirect transitions, taking into account the excitonic effects. This procedure yields accurate values of the energy of the direct Gamma-Gamma gap, E-g(Gamma)(x), as a function of the composition and allows the determination of the composition dependence of the energy of the indirect Gamma-L gap, E-g(L)(x). The results show that the dependence of the energies of the two gaps on the Al mole fraction for x less than or equal to 0.41 is linear instead of quadratic, as reported so far. The extrapolation of this result to the whole x range (0 less than or equal to x less than or equal to 1) is discussed. A value of the Gamma-L crossover concentration (x(c) = 0.22) significantly lower than the commonly accepted one (x(c) = 0.35) has been obtained