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Spectroscopy and magnetic properties of wide band gap diluted magnetic semiconductors

D. Ferrand, R. Giraud, D. Halley, H. Mariette, S. Marcet, W. Pacuski, E. Sarigiannidou, A. Titov
Overview and results
In II-VI and III-V diluted magnetic semiconductors (DMS), the spin carrier coupling between delocalized holes and localized spins is particularly efficient to induce ferromagnetism in highly p type doped epilayers. The highest Curie temperatures (about 170K in GaMnAs) still remain well below room temperature. According to mean field models, wide band gap diluted semiconductors may be good candidates to obtain higher transition temperatures, providing that a large concentration of magnetic ions can be incorporated and that a high p type doping or modulation doping can be achieved.
In collaboration with the Laboratoire Louis Néel, a detailed study of the magnetic properties of (Ga,Mn)N have been conducted using different techniques (X Ray, SQUID, magneto-optics). Magneto-optical studies have been done also on (Zn,Co)O, (Zn,Mn)O and (Ga,Fe)N wurtzite epilayers obtained through different collaborations. 3 PhD theses, S. Marcet (11/05), A Titov (7/06) W. Pacuski (defence planned in 10/07) have been conducted on these topics. Three fundamental issues have been mainly investigated : the valence and acceptor character of Mn in GaN, the spin carrier exchange coupling in GaN and ZnO and the intrinsic magnetic properties of (Ga,Mn)N. This work has been partly done in collaboration with the partners of the European project Feniks (2001-2004) and of the French action concertée Nanosciences “Decoress” (2002-2005).
In III-V DMS, the state of the hole trapped around a neutral manganese acceptor is often a matter of controversy. In (Ga,Mn)As, it is commonly assumed that the 3d states remain completely full with a hole trapped in an acceptor state. In (Ga,Mn)P, the holes are much more localized in the d shell with a Mn electronic configuration close to 3d4. Figure 1 summarizes our experimental results obtained on (Ga,Mn)N from the analysis of X ray near absorption spectra and of intraionic infrared d-d transition. Figure 1(a) shows the characteristic absorption peaks observed below the Mn K absorption edge [1]. A detailed analysis of these XANES spectra has been done on the basis of ab initio band structure calculations (PhD thesis of A. Titov in cotutelle with Moscow). The presence of two peaks in the spectra is attributed to a partial occupation of the 3d states of one spin, suggesting the presence of a strongly localized hole around the manganese. This result is also consistent with of the evolution in magnetic field of the internal d-d transition observed at 1.41 eV [2]. As shown in figure 1b, the evolution of the line can be well reproduced using the extreme assumption of a complete localization of the hole in the 3d shell (3d4 ionic configuration) (PhD thesis of S. Marcet).
Although the magnetic ion solubility in ZnO or GaN can be relatively high, the line broadening and luminescence quenching restrict unfortunately the magneto-optical study to lightly doped and paramagnetic samples. We used specifically these diluted samples to study the spin carrier exchange interactions between the magnetic ions and optically injected carriers. These studies have been done for (Zn,Co)O, (Zn,Mn)O, (Ga,Mn)N and (Ga,Fe)N (PhD thesis of W. Pacuski, in cotutelle with Warsaw University). We used the giant Zeeman splitting of the free A and B excitons observed in reflectivity and we developed a numerical model in order to calculate properly the excitonic Zeeman shift [3]. An estimation of the exchange integrals N0 and N0have been obtained for the first time for all of these materials. An example of reflectivity spectra under magnetic field is shown in figure 2a for (Ga,Mn)N. The energy position of A and B excitons and the calculated curves used to determine the exchange integrals are shown in figure 2b. For all materials, we found an exchange integral with holes (N0 always below 1eV, which is in strong disagreement with the 1/a3 empirical law (where a designs the lattice parameter) observed in II-VI DMS. Moreover, the exchange interaction between magnetic ions and holes is ferromagnetic (positive N0 for (Ga,Mn)N and (Ga,Fe)N. The valence band ordering uncertainty prevent the determination of the sign of N0 in (Zn,Co)O and (Zn,Mn)O. These results stimulate recently new theoretical calculations of the manganese acceptor state in wide band gap DMS.
In collaboration with the Laboratoire Louis Néel (R.M Galera, J. Cibert), magnetization measurements have been done on (Ga,Mn)N. The goal was to characterize the intrinsic magnetic properties and the measurements have been carried out on well characterized concentrated samples, where no secondary phases can be detected during the growth or by structural analyses (X Ray, EXAFS). For diluted samples, the SQUID measurements show a strong paramagnetic behaviour down to 2K with a strong anisotropy (easy axis perpendicular to c axis) in agreement with magnetic circular dichroism observed close to the band gap (See figure 3a). For highly concentrated samples (up to 6%) a ferromagnetic transition appear below a critical temperature comprised between 2K and 8K [4] (See figure 3b). The intrinsic character of the observed ferromagnetism is confirmed by XMCD measurements performed at Mn K edge [4]. Its origin is still not yet understood, but it could be related to a partial delocalisation of the holes when the Mn concentration is high enough.
In conclusion, (Ga,Mn)N can not be understood as an extrapolation of (Ga,Mn)As. It appears that in layers of good quality Mn is incorporated mainly in the “3d4 configuration” and do not behave as a shallow acceptor. As a result, the hypotheses leading to the prediction of high Curie temperature are not fulfilled and the experimental Curie temperatures are low. The general study of wide band gap DMS has led to many original observations on the magneto-optical properties. In particular those due to the strong excitonic character of the transitions, the low spin orbit coupling and the strength of the spin carrier coupling. All of these features would have to be taken into account for magneto-optical applications. Present trends in the domain focus now onto inhomogeneous distribution of the magnetic impurities in the semiconductor, which would ensure high critical temperatures while preserving strong magneto-transport and magneto-optical properties (spinodal decomposition effect).
[1] A. Titov et al, Phys. Rev B 72 115209 (2005) ; A. Titov, PhD thesis Grenoble, December 2006
[2] S. Marcet et al, Phys Rev B 74 125201 (2006) ; S. Marcet PhD thesis Grenoble, November 2005
[3] W. Pacuski et al, Phys. Rev B 73 035214 (2006) ; ibid, cond-mat 2007 ; W. Pacuski, PhD thesis, October 2007
[4] E. Sarigiannidou et al, Phys. Rev B 74 041306(R) (2006)
In collaboration with
J. Cibert1, R.M Galera1, J. Gaj2, P. Kossacki2, E. Kulatov3, S. Kuroda4
(1) Institut Néel, Grenoble (2) Instytut Fisyki Doswiadczalnej, Warsaw University, (3) A. M. Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow (4) Institute of Materials Science, Tsukuba University

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