On the character of states near the Fermi level in (Ga,Mn)As: impurity to valence band crossover

TL;DR

This study investigates the electronic states near the Fermi level in Mn-doped GaAs, revealing a transition from impurity band to merged valence band with increasing Mn concentration, supported by transport and optical measurements.

Contribution

It provides comprehensive experimental evidence supporting the impurity band to valence band crossover in GaAs:Mn as doping increases, clarifying the nature of states near the Fermi level.

Findings

Impurity band exists in low-doped GaAs:Mn and merges with the valence band at higher doping levels.

In metallic samples, no impurity band features are observed in transport measurements.

Mid-infrared features shift with doping, indicating changes in electronic structure.

Abstract

We discuss the character of states near the Fermi level in Mn doped GaAs, as revealed by a survey of dc transport and optical studies over a wide range of Mn concentrations. A thermally activated valence band contribution to dc transport, a mid-infrared peak at energy hbar omega approx 200 meV in the ac- conductivity, and the hot photoluminescence spectra indicate the presence of an impurity band in low doped (<<1% Mn) insulating GaAs:Mn materials. Consistent with the implications of this picture, both the impurity band ionization energy inferred from the dc transport and the position of the mid-infrared peak move to lower energies and the peak broadens with increasing Mn concentration. In metallic materials with > 2% doping, no traces of Mn-related activated contribution can be identified in dc-transport, suggesting that the impurity band has merged with the valence band. No discrepancies with this perception are found when analyzing optical measurements in the high-doped GaAs:Mn. A higher energy (hbar omega approx 250 meV) mid-infrared feature which appears in the metallic samples is associated with inter-valence band transitions. Its red-shift with increased doping can be interpreted as a consequence of increased screening which narrows the localized-state valence-band tails and weakens higher energy transition amplitudes. Our examination of the dc and ac transport characteristics of GaAs:Mn is accompanied by comparisons with its shallow acceptor counterparts, confirming the disordered valence band picture of high-doped metallic GaAs:Mn material.