Abstract
Normal-incidence reflectance spectra have been used to study the thermal evolution of the direct energy gap in a superlattice composed of 120-times-repeated pattern of nominally 9 monolayers (ML) AlAs and 15 ML GaAs. Very low noise level in the spectra allows the structure of the heavy- and light-hole transitions to be enhanced by numerical differentiations. Excellent agreement of the observed temperature shifts in the 15--300-K range with the results of Kronig-Penney calculations is found when a temperature-dependent electron effective mass is taken into account. Two components of the lowest direct energy gap are resolved at low temperatures; we interpret this fact as due to the presence of two superlattice regions differing in the average composition by 1 ML of GaAs.
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