Persistent photoconductivity and two-band effects in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterojunctions

We have measured the mobility and individual subband electron densities of GaAs/${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterojunctions (with x≃0.33) exhibiting two subband occupancy as a function of total two-dimensional electron-gas density. The density is varied by means of persistent photoconductivity using either red or infrared radiation. The latter is filtered to prevent electron-hole–pair excitations in the GaAs. Different behavior is observed in the two cases which can be attributed to differing excitation mechanisms. Infrared radiation leads to ionization of DX centers in the ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As while red radiation preferentially leads to the excitation of electron-hole pairs in the GaAs. This latter process continues until the acceptor depletion layer is exhausted, at which point DX excitations take over. Detailed calculations (with no adjustable parameters) of the electronic structure provide a good account of the observed electron areal densities in the two subbands as a function of total density for both types of illumination. The mobility is calculated with use of an estimate of the ionized-impurity charge distribution based on a model in which the Si donors in the ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As have both a shallow and a deep level. The general trends of the theoretical results are in good agreement with those of the experimental data, but the calculated mobility tends to be too low in absolute magnitude. The calculations indicate that complete ionization of all donors in the ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As at saturation is incompatible with the observed density dependence of the mobility. The theoretical results also show that the relative magnitudes of the transport and quantum lifetimes for electrons in the two subbands depend sensitively on the illumination conditions. For example, the second-subband transport lifetime can be roughly a factor of 2 greater (red) or smaller (ir) than that of the first subband.