Character of states near the Fermi level in (Ga,Mn)As: Impurity to valence band crossover

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 midinfrared peak at energy ប Ϸ 200 meV in the ac conductivity, and the hot photoluminescence spectra indicate the presence of an impurity band in low-doped ͑Ӷ1% Mn͒ 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 midinfrared peak move to lower energies, and the peak broadens with increasing Mn concentration. In metallic materials with Ͼ2% 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 ͑ប Ϸ 250 meV͒ midinfrared feature which appears in the metallic samples is associated with inter-valence-band transitions. Its redshift 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.

Hole -character and delocalization in (Ga,Mn)As revised with pSIC and MLWF approaches – Newly found spin-unpolarized gap states of s-type below 1 of Mn

“…Fermi level is pinned within the valence band for donorfree samples, or within the localized gap states of s-type for donor-rich samples and low Mn dopings -these samples are insulating and nonmagnetic. With the above findings, models for Curie temperature in dilute magnetic semiconductor (Ga,Mn)As, which assume an extended hole density function over the valence band 7,25,26 , are clearly justified.…”

“…As for the gap states at lower dopings, some publications imply that the origin of these states is from the valence band and their spin-polarization is of the d-type 26,28 . We have found that these states are spinunpolarized and of the s-type, and they are formed by a separation from the conduction band.…”

“…Experimentally, some of diluted samples are conducting and a few among all samples are insulating; the conducting samples have extended HDF and the insulating samples are characterized by the localized HDF 26 . This matches perfectly with our Figure 4 and donor scenario of Ref.…”

Microscopic mechanism of the noncrystalline anisotropic magnetoresistance in (Ga,Mn)As

“…7,23 The description is based on the canonical Schrieffer-Wolff transformation of the Anderson Hamiltonian 24 which for ͑Ga,Mn͒As replaces hybridization of Mn d orbitals with As and Ga sp orbitals by an effective spin-spin interaction of ͑L =0; S =5/ 2͒ local moments with host valence-band states. This step proves essential to effectively separate the different AMR mechanisms ͑a,b,c͒, symbolized in Fig.…”

Electronic- and band-structure evolution in low-doped (Ga,Mn)As

“…Mn Ga acceptors, with an impurity binding energy of 0.11 eV, 3,4 are more localized than shallower, hydrogeniclike acceptors in GaAs. This results in a higher critical carrier density for the metal-insulator transition (MIT) in Mn doped GaAs, of about 1×10 20 cm −3 , 5,6 as compared to the critical density for the shallow acceptors in GaAs, which is two orders of magnitude lower.…”