Impurity Band Conduction in a High Temperature Ferromagnetic Semiconductor

The band structure of a prototypical dilute magnetic semiconductor (DMS), ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$, 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 [${\sigma }_1(\omega )$] 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 ${\sigma }_1(\omega )$ 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 ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$.