Valence band energy spectrum of HgTe quantum wells with an inverted band structure

Abstract

The energy spectrum of the valence band in $\mathrm{HgTe}/{\mathrm{Cd}}_{x}{\mathrm{Hg}}_{1\ensuremath{-}x}\mathrm{Te}$ quantum wells of a width ($8--20$) nm has been studied experimentally by magnetotransport effects and theoretically in the framework of a four-band $kP$ method. Comparison of the Hall density with the density found from a period of the Shubnikov--de Haas (SdH) oscillations clearly shows that the degeneracy of states of the top of the valence band is equal to 2 at the hole density $p<5.5\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. Such degeneracy does not agree with the calculations of the spectrum performed within the framework of the four-band $kP$ method for symmetric quantum wells. These calculations show that the top of the valence band consists of four spin-degenerate extremes located at $k\ensuremath{\ne}0$ (valleys) which gives the total degeneracy $K=8$. It is shown that taking into account the ``mixing of states'' at the interfaces leads to the removal of the spin degeneracy that reduces the degeneracy to $K=4$. Accounting for any additional asymmetry, for example, due to the difference in the mixing parameters at the interfaces, the different broadening of the boundaries of the well, etc., leads to reduction of the valleys degeneracy, making $K=2$. It is noteworthy that for our case twofold degeneracy occurs due to degeneracy of two single-spin valleys. The hole effective mass (${m}_{h}$) determined from analysis of the temperature dependence of the amplitude of the SdH oscillations shows that ${m}_{h}$ is equal to $(0.25\ifmmode\pm\else\textpm\fi{}0.02){m}_{0}$ and weakly increases with the hole density. Such a value of ${m}_{h}$ and its dependence on the hole density are in a good agreement with the calculated effective mass.