Photoluminescence of shallow acceptors in Al0.28Ga0.72As

Low-temperature (5K) photoluminescence (PL) of lightly silicon-doped Al0.28Ga0.72As grown by molecular beam epitaxy is presented. Samples with different free electron concentrations are examined over five decades of laser excitation intensity. The PL data and Hall effect measurements verify that the samples are not nearly as compensated (n300K =2.2×1015 cm−3, μ300K =2470 cm2/V s) as samples studied previously by others. Narrow transition linewidth (3 meV) bound exciton recombination, ‘‘defect-induced’’ bound exciton recombination, and two prominent neutral donor-neutral acceptor (DA) peaks are observed. Free-to-bound recombination is distinguished from the DA recombination in the low-excitation intensity spectra. The acceptor ionization energies derived from the data are 33 meV for carbon and 53 meV for silicon. The difference in ionization energy for the carbon acceptor in GaAs (26 meV) and Al0.28Ga0.72As (33 meV) is accounted for by the effective mass theory of Baldereschi and Lipari. Differences among the ionization energy assignments reported in the literature and their probable causes are discusssed.