The origin of ferromagnetism in⁵⁷Fe ion-implanted semiconducting 6H-polytype silicon carbide

Abstract

Semiconducting (mostly p-doped) single crystals of the 6H-polytype of α-SiC(0001) were implanted with 57Fe ions with a nominal dose of 1.0 × 1016, 2.0 × 1016, 3.0 × 1016 or 2.0 × 1017 cm−2 (high-dose sample p-hd) at 100 or 200 keV ion energy in order to produce diluted magnetic semiconductors (DMSs). After implantation all samples (except p-hd) were subject to rapid thermal annealing at 1000 °C for 2 min. The structure was investigated by x-ray diffraction, high-resolution cross-sectional transmission electron microscopy and sputter-Auger depth profiling. The magnetic properties were obtained from superconducting quantum interference device (SQUID) magnetometry and 57Fe conversion electron Mössbauer spectroscopy (CEMS) at room temperature (RT) and 4.2 K. Our combined results obtained by several techniques prove unambiguously that ferromagnetism in 57Fe-implanted SiC for Fe concentrations above 3% originates mostly from epitaxial superparamagnetic Fe3Si (and possibly a small fraction of Fe nanoparticles) in the SiC matrix. We find a wide range of blocking temperatures, TB, which start from 400 K for a dose of 2.0 × 1016 cm−2, and shift downwards to ∼220 K for 3.0 × 1016 cm−2. For the lowest dose of 1.0 × 1016 cm−2 at 200 keV, we find evidence of ferromagnetism below 20 K via weak magnetic hyperfine interaction. Our measurements suggest that for a maximum Fe concentration in the range of 1–3%, which corresponds to this lowest Fe dose, the possibility exists to obtain a DMS in Fe-implanted SiC, prepared at lower or equal implantation doses.

Magnetic Properties of Transition-Metal-Doped Silicon Carbide Diluted Magnetic Semiconductors

“…Various impurity complexes can coexist in a statistical distribution, further complicating the physical picture. Contribution of secondary phases to the magnetic signal can also not be ruled out completely (Stromberg et al, 2006). SiC doped with Mn received more attention from the experimentalists compared to the other SiC DMSs and the reported promise of this impurity for obtaining high Curie temperatures is in-line with our conclusions.…”

“…Nevertheless, the existence of free holes may have led to an additional, hole-mediated exchange interaction playing the role in stabilizing ferromagnetism at larger distances between the impurities. At least some of the experimental studies of Fe-doped SiC reported ferromagnetic DMS behaviour in Fe-implanted 6H-SiC (Theodoropoulou et al, 2002;Stromberg et al, 2006). Our calculations reveal nonmagnetic ground state and antiferromagnetic ordering in the magnetic state of (Si, Fe) C. Such discrepancy may be explained by several factors.…”

“…This, for example, may lead to the magnetic state (ferromagnetic or antiferromagnetic) becoming energetically favourable, compared to the nonmagnetic state, in the case of Fe in SiC, for which the ground state solution is nonmagnetic. This possibility of changing the state preference may explain the magnetic response reported in several experimental studies of Fe-doped SiC (Theodoropoulou et al, 2002;Stromberg et al, 2006). Furthermore, the fact that the magnetic and nonmagnetic solutions are separated by an energy gap comparable to room temperature thermal energy, allows us to suggest that a new equilibrium mixed state with a certain distribution of TM atoms between the magnetic and nonmagnetic states, and an average over this distribution magnetic moment, may be created at a nonzero temperature.…”

Fe implantation effect in the 6H-SiC semiconductor investigated by Mössbauer spectrometry

“…Theodoropoulou et al showed that 5% of Fe atoms implanted in the 6H-SiC polytype lead to a ferromagnetic material at a temperature of about 250 K. 11 No secondary phase was detected in their samples. Similar findings have been reported by Pearton et al 12 and Stromberg et al 13 Indeed, these authors carried [14][15][16] The formation of nano-sized particles of nano-metric size is partly responsible for the magnetic properties observed on their samples after annealing.…”

Si-Based Magnetic Semiconductors

“…The origin of the magnetism in their samples was not clear, and the authors point out that further experiments are needed to identify the origin of the magnetism [179]. Later work on Fe-doped 6H-SiC by Stromberg et al [180] using Fe ion implantation, and Song et al [181] using polycrystalline powders, identified the formation of Fe 3 Si precipitates in their samples and suggested that the ferromagnetic magnetization appearing near room temperature was due to this phase.…”