EXCITONS IN INDIRECT-GAP ALXGA1-XAS

We have studied unintentionally and deliberately Si- or Mg-doped indirect-gap AlxGa1-xAs (0.38 less-than-or-equal-to x less-than-or-equal-to 0.835) prepared by a variety of growth techniques, by means of photoluminescence in the temperature range 4.2 K less-than-or-equal-to T less-than-or-equal-to 300 K. At 4.2 K two high-energy transitions were determined to be due to the radiative decay of donor and acceptor bound excitons. The zero-phonon bound exciton transitions are accompanied by phonon replicas due to the emission of one or two AlAs- or GaAs-like phonons. The peak energy of the indirect donor bound exciton increases in the range 0 less-than-or-equal-to x less-than-or-equal-to 1 corresponding to the relation (T less-than-or-equal-to 30 K) hv(c)(D0X, x) = 1.9690 + 0.1975x + 0.0517x7.4 (in eV). The Al content was determined by electron beam microanalysis using different standards. The donor bound exciton binding energy was estimated using the intensity quenching versus the temperature. Including these values the indirect gap-composition relation is given. The direct-indirect crossover point was determined in excellent agreement with earlier estimations. Besides the radiative decay of bound excitons free exciton recombination was observed at elevated temperatures.