Diluted Magnetic Semiconductors InFeSb Prepared by Laser Ablation: Spectroscopic and Microscopic Investigations

We report optical and magneto-optical results as well as atomic force microscopy (AFM) and magnetic force microscopy (MFM) results for InFeSb samples prepared by laser ablation. AFM and MFM studies have revealed the presence of magnetic particles on the samples surface, whose sizes depend on the Fe content and substrate temperature. It has found that both optical and magneto-optical spectra are superposition of spectra from the doped InFeSb layers and particles on their surface.


Introduction
Spintronic devices suggested in theoretical studies may possess characteristics that are unreachable for traditional electronics. However, the implementation of such devices is restrained by the lack of ferromagnetic semiconducting (FMS) thin films, whose Curie temperature (Tc) is above room one. Recently, sets of FMS GaFeSb [1] and InFeSb [2] thin films obtained using molecular beam epitaxy and laser ablation with TC higher than 300 K have been reported. Magneto-optical spectroscopy is a powerful tool for probing the electronic structures of diluted magnetic semiconductors (DMS). It can provide unique information about such systems because MOKE spectra are sensitive to electronic and magnetic properties. We can distinguish features in the MO spectra associated with transitions in a doped semiconductor matrix (intrinsic ferromagnetism) and features associated with the presence of magnetic inclusions, since they are observed at different energies. In this work we study InFeSb thin films, which have been prepared by laser ablation [2], using optical (ellipsometry) and magneto-optical (transversal Kerr effect, TKE) spectroscopy as well as atomic and magnetic force microscopy (AFM and MFM, respectively).

Samples
InFeSb samples are ≈ 40 nm-thick films, which were fabricated on i-GaAs substrates by pulsed laser deposition. Solid InSb-and Fe-targets were periodically sputtered by Nd:YAG laser in a vacuum chamber. The ratio of the sputtering times, tFe/(tInSb+tFe) (YFe), was varied in the range of 0.08 -0.17. The substrate temperature, TS, was 20, 150, 200, 250°C and 300°C. The samples characteristics are displayed in Table 1.

Results
The AFM and MFM images (Fig. 1, left and right sides, respectively) obtained at room temperature show complex surface topography of the samples. There are particles, whose sizes depend on the Fe quantity and substrate temperature, on the samples surface.

AFM images
Sample number MFM images

N2
Magnetic structure was not detected (as well as for sample N1).   Fig. 3(a,b). For the samples, whose the growth parameter is YFe  0.12, we observe large magnetooptical response even at room temperature. In the TKE spectra, two bands around 1.  All the samples exhibit TKE signal increase as the temperature decreases. In the spectrum of sample N1 with the lowest Fe content even at the low temperature T=22 K only one band is present with the maximum around 1.2 eV. Similar band is observed in the spectra of other samples. There is a distinct peculiarity (minimum) around the Е1+1 transition region in the low temperature TKE spectra and its location depends on the Fe content and growth temperature. We do not observe any peculiarities near the E1 transition, however, a strong band is present around 1.7 eV. Most likely, the TKE spectra represent superpositions of contributions from InFeSb ferromagnetic matrix and magnetic surface particles containing Fe oxides.

N3
Studies of the temperature and field TKE dependences (Fig. 4) have shown that the InFeSb layers with YFe above 0.08 and TS ≥200°C exhibit ferromagnetic behavior even at room temperature. At the same time, saturation in the fields used (3 kOe) was observed only for sample N7 with the maximum Fe content. The TKE temperature dependences (Fig. 4a) display that the samples with the lower YFe values give more TKE contribution generated by a low-temperature ferromagnetic phase. It leads to the presence of well pronounced peculiarities in the TKE spectra related to this phase at temperatures lower than 100 K. In the spectra of the samples prepared at bigger YFe values and TS higher than 200°C, these peculiarities become less noticeable, since other magnetic phases make larger contribution to the effect.  For sample N1, we observe approximately linear dependence on the magnetic field in the all temperature range. At the same time, a superposition of ferromagnetic and paramagnetic contributions is observed at low temperatures for sample N2.

Conclusions
Samples with YFe >0.08 and TS ≥200°C not only exhibit ferromagnetic behavior, but also have high Curie temperature. Even at T=194°C the TKE signal is also relatively strong. However, one should check, whether these properties are related to the bulk material or the surface particles, since the experimental data leads to a conclusion that these samples are multiphase. Most likely, the surface particles are compounds such as FeO, Fe3O4, Fe2O3 (since the small particles being on the surface would inevitably oxidize), which are mixed with FeSb variations. Despite the fact that at room temperature a distinguishable signal is not detected from sample N2, it shows a low-temperature ferromagnetic response. However, without the substrate heating the overall effect is lower that may be related to less iron oxides content. Matrix doping in the sample with the Fe lowest content (YFe =0.08, N1) is apparently insufficient to create ferromagnetism. Further improvement of the growth technology aimed at the fabrication of singlephase InFeSb layers or those with negligibly small content of undesirable phases, as well as the studies continuation are required to confirm the intrinsic hightemperature ferromagnetism of the InFeSb layers obtained by laser ablation.