Free and bound excitons and the effect of alloy disorder in MBE grown AlxGa1-xAs

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Abstract
Photoluminescence and excitation spectroscopy are performed on the ternary random alloy AlxGa1-xAs grown by molecular beam epitaxy (MBE). The high-energy photoluminescence line is conclusively identified as a bound-exciton transition. Excitons are stable in the disordered potential of the alloy semiconductor, and they do not dissociate despite the random alloy electric fields which are caused by statistical composition fluctuations. The free-exciton transition is observed only in excitation spectroscopy but not in photoluminescence spectroscopy. The depth of the carbon acceptor above the valence band determined by excitation spectroscopy does not change with alloy composition x. This indicates that the heavy-hole effective mass does not depend on alloy composition for xxGa1-xAs are 1.8 meV and 2.4 meV, respectively, and are thus smaller than the corresponding energies in GaAs. The lowering of the binding energies is caused by internal electric fields via the Poole-Frenkel effect. Up-conversion of photoluminescence observed in AlxGa1-xAs at low temperatures is interpreted by a many-carrier model resulting from the bandgap fluctuations. The density of states of a random alloy semiconductor is calculated, and we show that pronounced alloy tail states which resemble impurity tail states are presented in AlxGa1-xAs.