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arXiv · 0707.0665

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

T. Jungwirth·Jairo Sinova·A. H. MacDonald·B. L. Gallagher·V. Novak·K. W. Edmonds·A. W. Rushforth·R. P. Campion·C. T. Foxon·L. Eaves·K. Olejnik·J. Masek·S. -R. Eric Yang·J. Wunderlich·C. Gould·L. W. Molenkamp·T. Dietl·H. Ohno
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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.

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T. Jungwirth, Jairo Sinova, A. H. MacDonald, B. L. Gallagher, V. Novak, K. W. Edmonds, A. W. Rushforth, R. P. Campion, C. T. Foxon, L. Eaves, K. Olejnik, J. Masek, S. -R. Eric Yang, J. Wunderlich, C. Gould, L. W. Molenkamp, T. Dietl, H. Ohno. 2007-07-05. On the character of states near the Fermi level in (Ga,Mn)As: impurity to valence band crossover. https://doi.org/10.1103/physrevb.76.125206

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