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

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