TEMPERATURE-DEPENDENCE OF THE DIELECTRIC FUNCTION AND THE INTERBAND CRITICAL-POINT PARAMETERS OF ALXGA1-XAS

The complex dielectric function epsilon(omega) of AlxGa1-xAs was studied in the 1.4-5.6-eV photon energy region and between 12 and 800 K. By performing a line-shape analysis of the observed structures, the temperature dependence of the interband critical-point energies, broadenings, strength, and excitonic phase angle have been determined. The compositional dependence of these parameters was also obtained, e.g., the temperature dependence shift of the E0 exciton and E1 transitions was found to be 5.5 + 3.35x (10(-4) eV/K). The decrease (increase) in energy (broadening) with increasing temperature was analyzed in terms of average phonon frequencies, giving rise to renormalization of the energies and a broadening of the band gaps. The statistical fluctuation of the alloy composition, which appears as an additional broadening, was found to affect the E0, E0 + DELTA-0, E1, and E1 + DELTA-1 structures. The excitonic character of the E1 and E1 + DELTA-1 transitions shows a systematic change from a localized Lorentzian to two-dimensional character with increasing both Al content and temperature. The E2 structure [E0', E2(X), E2(SIGMA), and E2(P) transitions] was found to change its strength and to increase in complexity by going from the Ga- to the Al-rich regime.