INDIRECT-GAP ALXGA1-XAS AND ITS SIMILARITY TO GAP

We report on luminescence, transmission and luminescence excitation me asurements on indirect-gap AlxGa1-xAs in the composition range 0.42 le ss than or equal to x less than or equal to 0.86. The phonon coupling strength in transitions of bound excitons is investigated and found to be dependent on AlAs mole fraction and on the chemical species of the binding impurity. We find binding energy values of 35.5 +/- 5.5 meV f or x = 0.42 and 50.0 +/- 5.5 meV for x = 0.86. The recombination of in direct-gap free excitons is strongly influenced by a so-called 'camel' s back structure' (Delta = 349 meV, delta = 5.5 meV, xi = 0.092 for x = 0.42) within the lowest conduction band near the zone boundary X. Th e exciton binding energy amounts to 20 +/- 3 meV, much higher than rep orted recently in literature, and much higher than computable using a simple hydrogen model for the exciton. Temperature-dependent luminesce nce measurements-in the temperature range 4.2 K less than or equal to T less than or equal to 300 K-yielded a reliable relation for the temp erature dependence of the indirect excitonic gap at an AlAs mole fract ion of x = 0.42. From temperature-dependent measurements the binding e nergy of the sulphur donor was evaluated to be 95 +/- 9 meV in Al0.75G a0.25As, being representative of AlAs-rich material. This value, more than twice that computable from a simple effective-mass model, has to be seen as a further strong hint at a pronounced anisotropy within the lowest conduction band at the X point of the Brillouin zone. Because the band structure data of AlAs and indirect-gap AlxGa1-xAs are very s imilar to those of the well known binary compound Gap, we consequently use the detailed knowledge on near-gap electronic states in this mate rial to describe our experimental findings.