Two-band electron transport in a double quantum well

Abstract

The carrier densities and mobilities have been measured for the first two populated subbands in a GaAs double quantum well (DQW) as a function of the top gate voltage ${V}_{g}$. The densities and quantum mobilities (${\ensuremath{\mu}}_{i}^{q}$, $i=1,2$) were obtained from the de Haas-Shubnikov oscillations. The transport mobilities $({\ensuremath{\mu}}_{i}^{t})$ were determined from the semiclassical low-field magnetoresistance with intersubband scattering taken into account. At $0.32\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the experimental data on both ${\ensuremath{\mu}}_{i}^{t}$ and ${\ensuremath{\mu}}_{i}^{q}$, as a function of ${V}_{g}$, lie on two curves which cross at the resonance point as expected from theoretical considerations. At $1.09\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the ${\ensuremath{\mu}}_{i}^{t}$ curves no longer cross at resonance, but show a gap. The reason for this is not known. The mobilities have been calculated in the low-temperature limit within the Boltzmann framework by assuming that they are limited by scattering due to ionized impurities located at the outside interfaces. The assumption of short-range scattering is justified by the relatively small value of the ratio ${\ensuremath{\mu}}_{i}^{t}∕{\ensuremath{\mu}}_{i}^{q}$ that is measured in the present system. The theoretical values obtained for ${\ensuremath{\mu}}_{i}^{t}$ and ${\ensuremath{\mu}}_{i}^{q}$ are in reasonable agreement with the experiment for all values of ${V}_{g}$ examined. We have also calculated the resistivity and intersubband scattering rates of the DQW as a function of ${V}_{g}$ and again find good agreement with measured values.