Anomalous Fermi level behavior in GaMnAs at the onset of ferromagnetism

Abstract

We present the systematic study of the resonant tunneling spectroscopy on a series of ferromagnetic-semiconductor Ga 1−x Mn x As with the Mn content x from ∼0.01 to 3.2%. The Fermi level of Ga 1−x Mn x As exists in the band gap in the whole x region. The Fermi level is closest to the valence band (VB) at x=1.0% corresponding to the onset of ferromagnetism near the metal-insulator transition (MIT), but it moves away from the VB as x increases or decreases from 1.0%. This anomalous behavior of the Fermi level indicates that the ferromagnetism and MIT emerge in the Mn-derived impurity band.The origin of the ferromagnetism and the metal-insulator transition (MIT) has been a long-debated issue in the prototype ferromagnetic semiconductor GaMnAs. 1-4 Previously, the valence band (VB) conduction picture has been widely accepted in this material, 5 where the MIT of GaMnAs was understood by the Fermi level crossing over the VB 2,6 similarly to p-type GaAs doped with non-magnetic acceptors such as Be or Zn. The ferromagnetism in GaMnAs has been thought to be induced by the VB holes interacting with the localized d electrons of the Mn atoms. 5,7 However, recently, many experiments have shown the strong evidence that the Fermi level exists in the impurity band (IB) in the band gap, 4,8-21 which requires reconsideration on the above scenario. Therefore, to clarify the origin of the ferromagnetism and MIT in GaMnAs, it is essential to precisely investigate the Fermi level position and the VB structure of GaMnAs in the low Mn content region including the onset of ferromagnetism and MIT.Resonant tunneling spectroscopy is a powerful method to investigate the VB structure and the Fermi level position in GaMnAs with a precision of several meV. The advantage of this method is that we can detect energy bands only with the same wave-function symmetry as that of the p-wave functions of the VB holes. Also, it is sensitive to the effective mass of the energy bands. Furthermore, by carefully analyzing the quantum-well (QW) thickness dependence of the resonant levels, we can avoid the unwanted effects induced at the surface, which prevent the precise determination of the VB or the Fermi level position. 3,22 Recently, from the resonant tunneling experiments in the double-barrier (DB) QW heterostructures 14,15 and the single-barrier structures with an ultra-thin surface GaMnAs layer, 16 it was shown that the Fermi level exists in the band gap in Ga 1−x Mn x As with the Mn content x higher than ∼5%. In this Letter, we carefully analyze the VB structure and the Fermi level position in a series of Ga 1−x Mn x As from the unexplored insulating region (x=∼0.01%) to the metallic region (x=3.2%). We find that the VB structure is not largely affected by the Mn doping and that the Fermi level never crosses over the VB near the MIT: The a) Electronic

Recent progress in III-V based ferromagnetic semiconductors: Band structure, Fermi level, and tunneling transport

“…To fabricate the RTDs with various GaMnAs-QW thickness d on a wafer, we linearly moved a shutter in front of the substrate while growing the Ga 1-x Mn x As QW, in which d was varied from 10 nm to 16 nm within the wafer of 15 Â 10 mm. 95 We observed clear resonant tunneling peaks in all the samples, which shifted and tended to converge at certain negative bias voltages as d increases.…”

“…95 Especially, metal-insulator transition (MIT), which occurs at x ¼ 1%-2%, has not been well understood in GaMnAs. Previously, the VB conduction picture has been widely used in GaMnAs, 47,56 where the MIT of GaMnAs was understood by the Fermi level crossing over the VB, 96,97 similarly to p-type GaAs doped with non-magnetic acceptors such as Be or Zn.…”

Intrinsic magneto-optical spectra of GaMnAs

“…4,14 This blue shift was attributed to the Moss-Burstein effect, in which the Fermi level moves toward lower energy in the valence band as the Mn content x increases; however, this interpretation is inconsistent with the impurity-band conduction picture, which is supported by the recent studies. 2,3,5,[15][16][17][18]21 In particular, this interpretation is inconsistent with the transmission MCD results, 2,3,5 although both of reflection and transmission MCD can be expressed by the same dielectric tensor which is intrinsic to materials. In order to correctly understand the observed MCD spectra, it is important to determine the dielectric tensor of GaMnAs and to derive its intrinsic MCD spectra that are free from the optical interference.…”

“…Our results indicate that the blue shifts of E 0 originate from optical interference and are consistent with the impurity-band conduction picture, in which the Fermi level exists in the band gap even in heavily Mn-doped GaMnAs. 2,3,5,[15][16][17][18]21 Table I summarizes the structural parameters, growth conditions, and properties of the samples examined in this study, which are composed of Ga 1Àx Mn x As (d nm)/GaAs (100 nm) grown on semi-insulating GaAs (001) substrates by low-temperature molecular-beam epitaxy, as illustrated in Fig. 1.…”

Spin-dependent transport properties of a GaMnAs-based vertical spin metal-oxide-semiconductor field-effect transistor structure

“…12 Note that holes in GaMnAs are in the impurity band (IB) in the band gap and the Fermi level E F is located in the IB. 13,14,15 Since the position of E F depends on the Mn content x in Ga 1-x Mn x As, 13,14 we estimate E V -E F to be ~ 40 meV (in terms of hole energy) in our device, where E V and E F are the valence band edge and Fermi level, respectively. When a gate-source voltage V GS is not applied ( Fig.…”