Impurity Band Conduction in a High Temperature Ferromagnetic Semiconductor

Abstract

The band structure of a prototypical dilute magnetic semiconductor (DMS), ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$, is studied across the phase diagram via infrared and optical spectroscopy. We prove that the Fermi energy (${E}_{F}$) resides in a Mn-induced impurity band (IB). Specifically the changes in the frequency dependent optical conductivity [${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$] with carrier density are only consistent with ${E}_{F}$ lying in an IB. Furthermore, the large effective mass (${m}^{*}$) of the carriers inferred from our analysis of ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ supports this conclusion. Our findings demonstrate that the metal to insulator transition in this DMS is qualitatively different from other III-V semiconductors doped with nonmagnetic impurities. We also provide insights into the anomalous transport properties of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$.