Photoluminescence and Photoconductivity in Undoped Epitaxial GaAs

We have measured the photoluminescence and the photoconductivity spectra of pure epitaxial GaAs. The photoluminescence spectra at 2°K show 11 sharp peaks (widths 0.1-1.0 meV) between 1.509 and 1.516 eV. The photoconductivity spectra show relatively sharp peaks (widths ≅2meV) which correspond in energy to some of the emission lines. From excitation-intensity dependence and comparison of different spectra, we classify the emission lines as follows: the free exciton at 1.5155±0.0003 eV; two lines due to "partially radiative" transitions involving this exciton and a neutral donor ($E_d\approx 6.4$ meV); a doublet due to an exciton-neutral-acceptor complex ($E_A\approx 40$ meV); a single line due to an exciton bound to the same neutral donor as above; five lines, shown to be a group, possibly due to excitons bound to a pair of impurities. The sharp photoconductivity structures are attributed to Auger recombination of the bound excitons or dissociation of the free exciton.