Band-gap-dependent electron and hole transport in<i>p</i>-type HgTe-CdTe superlattices

Abstract

Extensive magnetotransport and phototransport experiments have been performed on p-type molecular-beam-epitaxy-grown HgTe-CdTe superlattices. A mixed conduction analysis was performed in order to obtain accurate electron and hole densities and mobilities from the magnetic-field-dependent Hall and conductivity data. Band gaps determined from the temperature dependence of the intrinsic carrier density are found to systematically increase with decreasing well thickness, spanning the range 0--200 meV in the series of ten samples. Low-temperature electron and hole mobilities are in turn found to increase with decreasing gap, with both mobilities exceeding ${10}^{5}$ ${\mathrm{cm}}^{2}$/V s in zero-gap samples. The data display a number of other distinctive features, including an electron-to-hole mobility ratio near unity, an abrupt decrease in the hole (but not electron) mobility at temperatures in the range 30--50 K, and the presence of more than one species of high-mobility holes. Comparison with the results of a theoretical tight-binding calculation of the superlattice band structure indicates that all of these observations can be explained if one takes a large value for the valence-band offset (e.g., 350 meV). However, if the offset is assumed to be small (e.g., 40 meV) the band gaps for small-gap samples significantly exceed the experimental values. Furthermore, none of the qualitative features cited above are reproduced. The investigation demonstrates that, due to unusual aspects of the band structure, narrow-gap superlattices display a number of unique properties which differ considerably from those encountered in either narrow-gap alloys or III-V superlattices.