Band-gap-dependent electron and hole transport inp-type HgTe-CdTe superlattices

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.