Transport properties of Hg1-xZnxSe and Hg1-xMnxSe doped with Fe resonant donors.

Abstract

We present here a study of the influence of iron doping on transport and magnetotransport properties of HgSe-based narrow-gap semiconducting mixed crystals, namely ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$Se (x\ensuremath{\le}0.072) and ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se (x\ensuremath{\le}0.13). The crystals were grown by the Bridgman method. Measurements were performed in the temperature range 1.7--300 K in magnetic fields up to 6 T. For both systems, the electron concentration as a function of increasing concentration of Fe dopants changes in a similar way: it rises proportionally below a certain Fe donor concentration ${\mathit{n}}_{\mathrm{Fe}}^{\mathrm{*}}$ (which depends on the mole fraction x of mixed crystal) and then reaches a constant value (which also depends on x). The electron mobility in samples with ${\mathit{n}}_{\mathrm{Fe}}$>${\mathit{n}}_{\mathrm{Fe}}^{\mathrm{*}}$ shows anomalously high values, similarly to the case of HgSe:Fe studied earlier. We interpret the observed phenomena by assuming that iron dopants create, in the compounds investigated, a resonant donor state with an energy level degenerate with the conduction band continuum. For ${\mathit{n}}_{\mathrm{Fe}}$>${\mathit{n}}_{\mathrm{Fe}}^{\mathrm{*}}$ the system of Fe donors is only partially occupied by electrons, i.e., ${\mathrm{Fe}}^{2+}$ neutral donors and ${\mathrm{Fe}}^{3+}$ singly ionized donors coexist, resulting in a mixed valence regime.The Coulomb repulsion between the electrons localized on donors leads to a correlation of their positions and results in a dramatic reduction of the scattering rate by ionized impurity potentials. The analysis of the mobility (in the case of ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se also of the Dingle temperature) in terms of the scattering from ionized centers (with the possible spatial correlation of impurity charges taken into account) and the alloy scattering (in the case of ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se also spin-dependent scattering) leads to a fair agreement between the measured data and the theoretical description. The lack of any dependence of the electron concentration upon the level of iron doping in the mixed valence regime (when the Fermi level is pinned to the iron state) enabled us to determine the position of the Fe level as a function of the Zn or Mn mole fraction x. This, in turn, made it possible to estimate the ${\mathrm{\ensuremath{\Gamma}}}_{8}$ and ${\mathrm{\ensuremath{\Gamma}}}_{6}$ band offsets (denoted W and V, respectively) between HgSe and MnSe and between HgSe and ZnSe. The values determined by us are, in the ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$Se system W=-0.5 eV and V=2.6 eV, and in the case of the ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se system W=-1.2 eV and V=3 eV. Moreover, it is noted that even weak Fe doping (corresponding to ${\mathit{n}}_{\mathrm{Fe}}$${\mathit{n}}_{\mathrm{Fe}}^{\mathrm{*}}$) can lead to a substantial improvement of electrical properties of the compounds studied by us. This is particularly clearly seen for ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$Se.