Electrical and optical properties of Mn-doped Hg$_3$In$_2$Te$_6$ crystals

The effect of Mn impurities on the properties of Hg$_3$In$_2$Te$_6$ crystals is studied by electrical and optical measurements. It is shown that, despite the high dopant concentration (1 $\times$ 10$^{19}$ cm$^{-3}$), the electron concentration remains the same as that in an undoped crystal ($\sim$ 10$^{13}$ cm$^{-3}$ at 300 K). At the same time, narrowing of the band gap from 0.74 to 0.7 eV is observed. From an analysis of the absorption spectra, it is found that the absorption edge is formed by optical transitions involving density-of-states (DoS) tails and that two acceptor- and donor-type impurity bands are formed in the band gap. The two bands are described by a Gaussian distribution of the DoS, with an energy gap between the peaks of $E_0=E_d^0-E_a^0$ = 0.4 eV. The total donor and acceptor concentration $N_d+N_a$ and the degree of compensation $K=N_a/N_d\to1$ are determined. Such compensation is responsible for pinning of the Fermi level near the middle of the band gap and for quasi-intrinsic conductivity at temperatures $T\ge$ 300 K.