Ultrahigh electron and hole mobilities in zero-gap Hg-based superlattices

Transport measurements on five HgTe/CdTe and ${\mathrm{Hg}}_{1-x}{\mathrm{Zn}}_x\frac{\mathrm{Te}}{\mathrm{Cd}}\mathrm{Te}$ superlattices have yielded the highest $p$-type mobilities ever reported for a II-VI semiconductor (> ${10}^5$ ${\mathrm{cm}}^2$/Vs). The temperature dependence of the intrinsic carrier density indicates near-zero energy gaps in all of the samples. Theoretical band-structure calculations by the tight-binding method are consistent with the experimental results in that they predict not only zero band gap but also electron and hole effective masses which are both considerably less than $0.01m_0$. Further features of the calculated band structure are mirrored in the data, such as a near equality of the electron and hole masses, "mass broadening" of the holes, and an extreme nonparabolicity of the highest valence band.