The origin of the ferromagnetic behaviour persisting above room temperature (RT) and discovered in semiconductors doped with transition metals and rare earths may certainly be considered as one of the most controversial issues in today’s materials science. Many misleading assumptions and conclusions on the actual magnetic interactions in magnetically doped semiconductors and oxides have been, until recently, caused by lack of a proper correlation between fabrication parameters and structural characterisation at the nanoscale. Now, there is an increasing amount of evidence that, owing to specific features of magnetic impurities in semiconductors and oxides, the epitaxial growth of these systems can result in the self-organised aggregation of magnetically robust nanocrystals embedded in the host paramagnetic matrix. This finding holds enormous potential for the fabrication of a range of multifunctional nanosystems relevant to spintronics, nanoelectronics, photonics, and plasmonics. However, it has also been realised that difficulties in the experimental resolution and identification of the embedded nanostructures endure and hamper the progress in the control of the mechanisms accounting for associated and hitherto unexplored nanoassembly processes. Synchrotron radiation microprobe [1] and other nanoanalytical synchrotron tools, in combination with high-resolution transmission electron microscopy (HRTEM) [2], represent a suitable blend of techniques that, when complemented by magnetic measurements, can lead to a thorough characterisation of magnetically doped semiconductors.

The model system (Ga,Fe)N has been found from SQUID magnetometry data to be ferromagnetic at RT for a concentration of magnetic ions above 0.4% under the employed growth conditions. While laboratory high-resolution X-ray diffraction (XRD) does not evidence any phase separation in these samples, powder diffraction XRD measurements carried out at the beamline ID31 by using a photon energy of 15.5 keV reveal the presence of new diffraction peaks identified as the (002) and (111) of the phase -Fe3N with a Curie temperature TC ~ 575 K. Moreover, it has been possible to confirm a recent theory [3], according to which it is possible to modify the chemical valence of transition metal cations by doping with shallow acceptors (Mg in the case of GaN) or donors (Si for GaN), and, consequently, affect their aggregation in the semiconducting matrix. Figure 113 indicates that doping with Si does indeed hinder the formation of Fe-rich regions: panel (a) reproduces the full /2 scan and panel (b) a reduced scan with focus on the range with the diffraction peaks originating from the embedded nanocrystals. This is confirmed by the HRTEM images in the inset of Figure 113.

Fig. 113: Synchrotron radiation powder XRD spectra: effect of Si doping on the aggregation of Fe ions into the GaN matrix. a) full scan; b) reduced scan focussing on the region with the diffraction peaks from Fe-rich embedded nanocrystals. Inset: HRTEM images confirming the Fe aggregation hampered by Si doping.

The results on the effect of Si on the aggregation of the Fe ions in the GaN matrix are also corroborated by X-ray absorption near-edge structure (XANES) measurements at the Fe K-edge carried out at the GILDA beamline (BM08). As evidenced in Figure 114, XANES data point to the expected presence of Fe in the Fe3+ charge state in the nominally undoped (Ga,Fe)N samples, whereas they confirm the coexistence of Fe3+ and Fe2+ ions in the case of doping with Si, supporting the onset of a shift of the Fermi level and consequent modification of Fe charge state in the system. An analogous effect has been verified in the case of doping with Mg.

Fig. 114: XANES spectra with the region of interest expanded in the inset: in addition to the peak at 7114.3 ± 0.1 eV assigned to the Fe3+ charge state, a shoulder at 7112.7 ± 0.1 eV is visible in the Si-doped samples pointing to the reduction of a part of the Fe ions to the Fe2+ charge state by doping with donors.

By combining HRTEM with synchrotron XRD and XANES it has been possible to identify the distinct ways by which Fe incorporates into the GaN lattice giving an important contribution to the elucidation of the origin of the ferromagnetic features in magnetically doped semiconductors. Importantly, the doping with either acceptors or donors hampers the nanocrystal assembling and offers a way to functionally control the aggregation of magnetic elements in a non-magnetic host.

 

Principal publication and authors

A. Bonanni (a), A. Navarro-Quezada (a), T. Li (a), M. Wegscheider (a), Z. Matey (b), V. Hol’y (b), R.T. Lechner (a), G. Bauer (a), M. Rovezzi (c), F. D’Acapito (c), M. Kiecana (d), M. Sawicki (d), and T. Dietl (d), Phys. Rev. Lett. 101, 135502 (2008).
(a) University of Linz (Austria)
(b) Charles University Prague (Czech Republic)
(c) CNR-INFM-OGG c/o ESRF GILDA CRG Grenoble (France)
(d) Polish Academy of Sciences Warsaw (Poland)

References

[1] G. Martinez-Criado, A. Somogyi, S. Ramos, J. Campo, R. Tucoulou, M. Salome, J. Susini, M. Hermann, M. Eickhoff, and M. Stutzmann, Appl. Phys. Lett. 86, 131927 (2005).
[2] M. Jamet, A. Barski, T. Devillers, V. Poydenot, R. Dujardin, P. Bayle-Guillmaud, J. Rotheman, E. Bellet-Amalric, A. Marty, J. Cibert, R. Mattana, and S. Tatarenko, Nature Mater. 5, 653 (2006).
[3] T. Dietl, Nature Mater. 5, 673 (2006).