“Exciton” photoconductivity in GaAs crystals

The spectral dependence of the photoconductivity in epitaxial GaAs of high purity and crystalline perfection grown on a semiinsulating GaAs substrate are measured at 1.7 and 77 K and analyzed theoretically. The developed theory of photoconductivity spectra takes into consideration the analytical shape of the exciton absorption edge in semiconductor crystals, the transport of photogenerated charge carriers, the finite thickness of the crystal, and the presence of surface recombination centers. It is shown that, when the excitation photon energy is lower than the band gap, the photoconductivity spectrum is determined by elementary excitations of the exciton type, while at higher excitation energies the spectrum is determined by surface states as well as by processes associated with optical-phonon emission. It is found that, in the samples under study, the probability of the direct formation of excitons upon the absorption of light can be comparable to the probability of their formation from a thermalized electron-hole gas.