Origin of robust nanoscale ferromagnetism in Fe-doped Ge revealed by angle-resolved photoemission spectroscopy and first-principles calculation

Abstract

Ge 1−x Fe x (Ge:Fe) shows ferromagnetic behavior up to a relatively high temperature of 210 K, and hence is a promising material for spintronic applications compatible with Si technology. We have studied its electronic structure by soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) measurements in order to elucidate the mechanism of the ferromagnetism. We observed finite Fe 3d components in the states at the Fermi level (E F ) in a wide region in momentum space and E F was located above the valenceband maximum (VBM). First-principles supercell calculation also suggested that the E F is located above the VBM, within the narrow spin-down d(e) band and within the spin-up impurity band of the deep acceptor-level origin derived from the strong p-d(t 2 ) hybridization. We conclude that the narrow d(e) band is responsible for the ferromagnetic coupling between Fe atoms while the acceptor-level-originated band is responsible for the transport properties of Ge:Fe. 1Ferromagnetic semiconductors (FMSs) such as (Ga,Mn)As [1, 2] have attracted much attention both from scientific and technological points of view [3][4][5][6][7][8]. Group-IV FMSs are particularly important because they are compatible with mature Si-based technology. Ge 1−x Fe x (Ge:Fe) is a promising material [9][10][11][12], and indeed can be grown epitaxially on Ge and Si substrates by the low-temperature molecular beam epitaxy (LT-MBE) method without the formation of intermetallic precipitates [13]. It shows p-type conduction, but the carrier concentration of ∼10 18 cm −3 [13] is orders of magnitude smaller than that of doped Fe atoms (∼10 21 cm −3 ). The Curie temperature (T C ) increases with the Fe content and with the inhomogeneity of Fe atom distribution [11,12], and reaches ∼210 K at highest by post-growth annealing [11], which is above the highest T C of (Ga,Mn)As, ∼200 K [14]. Unlike (Ga,Mn)As, the T C does not depend on carrier concentration [13]. Recent x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements [15] have revealed the valence of Fe substituting Ge to be 2+, which indicates that each Fe atoms would provide two holes. It was also found that nanoscale ferromagnetic domains exist even above the T C , the origin of which was attributed to the inhomogeneous distribution of Fe atoms on the nanoscale.In order to explain the origin of the ferromagnetism in (Ga,Mn)As and related FMSs, two models have been proposed so far [5,16,17], namely, the valence-band model [18,19] and the impurity-band model [20][21][22][23]. In the valence-band model, acceptor levels derived from the magnetic impurities are merged into the valence band and itinerant holes occupying states around the valence-band maximum (VBM) mediate ferromagnetism through Zener's p-d exchange mechanism.In the case of the impurity-band model, on the other hand, impurity levels are detached from the VBM and lies within the band gap of the host semiconductor and hence ferromagnetism is stabilized through a double-exchange-like mec...

Electronic structure of the high- TC ferromagnetic semiconductor (Ga,Fe)Sb: X-ray magnetic circular dichroism and resonance photoemission spectroscopy studies

“…Table I summarizes recombination of the excited electron with the core hole and is normally observed in metallic systems 51,52 . It is worth mentioning that such a strong Auger peak was also observed in the case of Fe-doped Ge 35 . In addition to the Auger peak, there are resonantly enhanced features around -1.7 eV and -10.3 eV, denoted by α and β, respectively.…”

“…Here, the T S was set to 200 • C for sample A and 250 • C for sample B. Lastly sub-nanometer-thick amorphous As cap layer was deposited to prevent surface oxidation. Note that sample A was paramagnetic down to 5 K, and sample B was ferromagnetic with T C = 170 K. In order to remove the oxidized surface, we etched the sample by hydrochloric acid (HCl) (2.4 mol/L) for 5 seconds and subsequently rinsed it by water just before loading the sample in the vacuum chamber of the spectrometer 35,38 .…”

“…Those local excitation peaks were almost absent in the case of Fedoped Ge (Ref. 35 ), suggesting that Fe 3d electrons in (Ga,Fe)Sb are more correlated or more For the purpose of examining the nature of resonance enhancement, it is useful to plot the intensity at a fixed binding energy as a function of incident photon energy, namely, a constantinitial-state (CIS) spectrum. In order to eliminate the effect of the overlapping Auger peak from the CIS spectra and to extract the resonance behavior of features α and β, we have employed curve fitting as shown in Fig.…”

Magnetization process of the insulating ferromagnetic semiconductor (Al,Fe)Sb

“…The origin of the nanoscale FM domains can be attributed to the nanoscale fluctuation of Fe distribution as discussed for the other Fe-doped semiconductors (In,Fe)As:Be [13,24,25] and Ge:Fe [14,26,27]. In addition, the previous theoretical studies where chemical pair interactions were calculated suggested that Fe atoms tend to segregate and form Fe-rich regions in the InAs [24] and Ge [26] matrices, while maintaining the zinc blende and diamond lattice structures.…”

Minority-spin impurity band in n -type (In,Fe)As: A materials perspective for ferromagnetic semiconductors

“…In this article, we have investigated the band structure of the prototypical n-type FMS (In,Fe)As:Be using soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) and spin-density functional theory (SDFT) calculation to understand the origin of the carrier-induced ferromagnetism. SX-ARPES is one of the most powerful experimental techniques to study the electronic structures of FMSs [17][18][19][20]. By using SX-ARPES we reveal the entire band structure of (In,Fe)As:Be, including the Fe-3d IB and electron occupation of the InAs-derived CB.…”