Dynamics of intrinsic and nitrogen-induced exciton emission in indirect-gapGa1−xAlxAs

We report measurements of low-temperature luminescence spectra, lifetime, and excitation spectra for excitons in ${\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x\mathrm{As}$ ($x_c<x<\mathrm{}0.55\mathrm{}$, where $x_c=0.435\mathrm{}$ is the direct-to-indirect crossover value) over a wide range of excitation levels. The no-phonon line, ∼6 meV wide, decays nonexponentially at low excitation levels. The decay rate depends strongly on excitation intensity and on temperature (for $2<T<\mathrm{}30\mathrm{}$ K) while the position and width remain unchanged. We show that in the low-temperature, low-excitation limit, the nonexponential decay, and its dependence on $x$, can be quantitatively explained in terms of emission from a small number of localized indirect excitons scattered by alloy fluctuations. Above 8 K these excitons become mobile and their decay is exponential. Most of the excitons are mobile even at 2 K. They dominate the emission when the excitation is sufficiently strong to neutralize the ionized impurities, which quench the luminescence at low intensities. The localized excitons show strong LO-phonon sidebands, while the mobile ones do not. The theory of the decay rate yields a mean value of the scattering strength $J\sim 0.2$ eV, in reasonable agreement with estimates from the Al-Ga electronegativity difference. The nitrogen-bound exciton with a wide range of binding energies, previously reported in ion-implanted samples, is found to be split, possibly by a disorder-induced axial field.