Infrared conductivity of hole accumulation and depletion layers in (Ga,Mn)As- and (Ga,Be)As-based electric field-effect devices

摘要

We have fabricated electric double-layer field-effect devices toelectrostatically dope our active materials, either $x$=0.015Ga$_{1-x}$Mn$_x$As or $x$=3.2$\times10^{-4}$ Ga$_{1-x}$Be$_x$As. The devicesare tailored for interrogation of electric field induced changes to thefrequency dependent conductivity in the accumulation or depletions layers ofthe active material via infrared (IR) spectroscopy. The spectra of the(Ga,Be)As-based device reveal electric field induced changes to the IRconductivity consistent with an enhancement or reduction of the Drude responsein the accumulation and depletion polarities, respectively. The spectroscopicfeatures of this device are all indicative of metallic conduction within theGaAs host valence band (VB). For the (Ga,Mn)As-based device, the spectra showenhancement of the far-IR itinerant carrier response and broad mid-IR resonanceupon hole accumulation, with a decrease of these features in the depletionpolarity. These later spectral features demonstrate that conduction inferromagnetic (FM) Ga$_{1-x}$Mn$_x$As is distinct from genuine metallicbehavior due to extended states in the host VB. Furthermore, these data supportthe notion that a Mn-induced impurity band plays a vital role in the electrondynamics of FM Ga$_{1-x}$Mn$_x$As. We add, a sum-rule analysis of the spectraof our devices suggests that the Mn or Be doping does not lead to a substantialrenormalization of the GaAs host VB.