Effects of nonuniform Mn distribution in (Ga,Mn)As

Abstract

Resonant in situ photoemission from Mn 3d states in Ga 1−x Mn x As is reported for Mn concentrations down to the very dilute level of 0.1%. Concentration-dependent spectral features are analyzed on the basis of first-principles calculations for systems with selected impurity positions as well as for random alloys. Effects of direct Mn-Mn interaction are found for concentrations as low as 2.5%, and are ascribed to statistical (nonuniform) distribution of Mn atoms. In the quest for magnetic semiconductors with potential use in spintronics, (Ga,Mn)As has emerged as the prototype system with documented spin-polarization of the electron states. Even though the Curie temperature of (Ga,Mn)As is still too low for practical implementations, the transport properties can be controlled to the extent that device structures based on the unusual magnetic characteristics can be explored [1]. In this perspective it is remarkable that the electronic structure of (Ga,Mn)As remains poorly characterized. It is generally agreed that the ferromagnetic state is due to Mn atoms in substitutional Ga sites [2]. From electron spin resonance (ESR) and optical data it is known that individual Mn atoms occur in either an ionized 3d 5 or a neutral (3d 5 + hole) state, the latter being the reason for the p-type behavior with the acceptor level at 113 meV above the valence band maximum (VBM) [3,4]. In systems with higher Mn concentrations the ESR data are found to be strongly broadened [4] and the situation is less clear. Interestingly, the broadening is observed in epitaxial layers with relatively low Mn concentration (0.5%), where one would expect to have a system of well-separated impurities. The broadening has been tentatively ascribed to effects of demagnetizing field [4].Photoelectron spectroscopy is the most direct method for probing the electron structure. An issue of concern in this context is the intrinsic surface sensitivity of this technique. This, in combination with the metastable character of (Ga,Mn)As at concentrations of interest (in the range of 1%), prohibits standard surface preparation involving ion beam surface cleaning and annealing at sufficiently high temperatures (above 300• C) to restore the surface order. Although this is obvious, it is remarkable that only a few in situ studies are available so far (see, e.g., [5,6]). Even more surprising, some of the "generally accepted" details about the electronic structure are derived from studies involving such treatment even though they are in conflict with results from in situ studies. Specifically it has been demonstrated that the energy of the main Mn 3d induced valence band structure is shifted by about 1 eV towards higher binding energy by such * krister.karlsson@his.se treatment [5], yet the most quoted value is that derived from ex situ treated samples [7].A particularly important issue in the context of magnetism in (Ga,Mn)As is the nature of electron states mediating the ferromagnetic coupling. The so far most successful description of ferromagnetism in (Ga,Mn)As has ...

Mn-induced modifications of Ga 3d photoemission from (Ga, Mn)As: evidence for long range effects

“…In other words, an individual Mn impurity affects all Ga atoms within a sphere of 1.4 ± 0.2 nm diameter (using the mentioned uncertainty in Mn concentration), and all atoms within this volume are affected in the same way. Regarding the small component at -0.39 eV, we note that according to a Monte Carlo simulation [14] the probability of finding two Mn atoms within the distance of interaction is around 6 % for a Mn concentration of 1 %. This corresponds quite well to the relative intensity of component (D).…”

“…If we assume that only the electrically uncompensated Mn atoms are active in this context, and take into account that up to 20% of the acceptors can be compensated (mainly by Mn in interstitial sites) [14], the diameter will be slightly larger (around 1.5 nm). Regarding the small component at −0.39 eV, we note that according to a Monte Carlo simulation [15] the probability of finding two Mn atoms within the distance of interaction is around 6% for a Mn concentration of 1%. This corresponds quite well to the relative intensity of component (D).…”

Electron correlations in MnxGa1–xAs as seen by resonant electron spectroscopy and dynamical mean field theory

“…non-uniform) Mn distribution. For Mn atoms occupying Ga sites in the zincblende structure the probability for finding two nearest neighbor sites occupied is 30% [43] at 3% Mn concentration. Note that we in Fig.…”