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Heavily Mn-doped SiGe thin films were grown by radio frequency magnetron sputtering and then treated by post-growth thermal annealing. Structural characterizations reveal the coexistence of Mn-diluted SiGe crystals and Mn-rich nanoclusters in the annealed films. Magnetic measurements indicate the ferromagnetic ordering of the annealed samples above room temperature . The data suggest that the ferromagnetism is probably mainly contributed by the Ge-rich nanoclusters and partially contributed by the tensile-strained Mn-diluted SiGe crystals. The results may be useful for room temperature spintronic applications based on group IV semiconductors.
Diluted magnetic semiconductors (DMSs) have attracted lots of interest in the last two decades due to their potential applications in spintronic devices.[1–11] Substantial works have been carried out to dope transition-metals (TMs) in various semiconductors such as III–V,[1–4] II–VI,[5,6] group-IV,[7,8] and I–II–V[9–11] semiconductors. Among them, Mn doped group-IV semiconductors have drawn considerable attention for their compatibility with current silicon process technology. However, owing to the low solubility of Mn in group-IV semiconductors,[12] the ferromagnetic (FM) nanostructures of manganese silicide[13,14] and manganese germanide[15,16] compounds were usually formed in heavily Mn-doped Ge and Si samples. Such Mn-rich nanostructures embedded in the semiconductors have been obtained through self-organized aggregation[17] and could be an effective way to achieve high TC spintronic devices.[18] Although extensive studies have been performed on heavily Mn-doped Ge and Si samples,[13–19] no such studies have been reported on the Mn-doped SiGe compound yet. Considering that the SiGe alloy as an important semiconductor has been widely used in high-speed transistors in which the lattice strain and electronic properties are closely related to the alloy components,[20] it will be useful to explore the correlation between the microstructures and magnetic properties of heavily Mn-doped SiGe.
In this work, heavily (above 10 %) Mn-doped Si0.25Ge0.75 thin films were synthesized by radio frequency (r.f.) magnetron sputtering and then treated by post-growth rapid thermal annealing. The structural characterizations reveal that the Mn diluted SiGe crystals and Mn-rich nanoclusters coexisted in the annealed films, owing to the crystallization and phase separation during the treatment process. The magnetic measurements of the annealed samples show a TC around room temperature. Further analysis reveals the relationship between the FM ordering and the Mn-rich nanoclusters as well as Mn diluted SiGe crystals in the annealed samples.
Si0.25Ge0.75:Mnx thin films (x = 0.1 and 0.2) were grown by r.f. magnetron sputtering using 99.999% germanium (Ge) target, 99.999% silicon (Si) target, and 99.99% manganese (Mn) target. We used semi-insulating one-side polished Ge (100) wafers as the substrates. The Ge wafers were first ultrasonically cleaned by using ethanol, acetone, and diluted hydrofluoric acid solution (10%), then flushed by using deionized water, and finally loaded into the growth chamber. The base pressure prior to deposition was about 1.0 × 10–5 Pa. During the deposition, the substrate temperature was kept at 250°C. Argon was used as the working gas. A 150 nm Si0.25Ge0.75:Mnx film was deposited on the Ge substrate. After deposition, some samples were annealed at 800 °C for 30 s by using a rapid thermal annealing furnace (NBD-HR1200-110IT) in 95% Ar2/5% H2 atmosphere.
The films were deposited on Al2O3 substrate for detecting the composition by using a field-emission scanning electron microscope (EDAX, X-MaxN 80, Oxford). Grazing incidence x-ray diffraction (GIXRD, Bruker D8, Empyrean Panalytical diffractometer) using a Cu Kα radiation source with γ = 0.154 nm was applied to detect the structural properties of the samples. The vibration modes of the lattice were characterized by Raman spectroscopy using a custom-built confocal micro-Raman optical assembly with a 532 nm laser and an Andor EMCCD detector. Transmission electronic micrograph (TEM, Tecnai F30, FEI) was applied to detect the structural properties. The cross-section samples for the TEM measurement were prepared by a focus ion beam (FIB) milling procedure in a Helios (Tescan, LYRA 3 XMU). Also, the element distribution in our cross-section samples was characterized by using energy dispersive x-ray spectrometry in TEM measurement (EDS, Bruker Super-X, Bruker). The magnetic properties were measured by using a superconducting quantum interference device (SQUID) magnetometer (MPMS XL-7, Quantum Design).
The GIXRD measurements were first performed to study the crystalline structures of the samples. For simplicity, the three representative samples mainly discussed in this work are named as S1 (x = 0.1, as-grown), S2 (x = 0.1, annealed), and S3 (x = 0.2, annealed). As shown in Fig.
To study the lattice vibration modes, the Raman spectra of samples S1, S2 and S3 were taken. Since the penetration depth of a 532 nm laser in Si or Ge is less than 50 nm, the influence of the Ge substrate could be excluded. As shown in Fig.
To investigate the elementary distribution in the annealed sample S2, the high-angle annular-dark-field (HAADF) image and EDS-mappings for Si, Ge, and Mn were obtained. It shows that the Si and Ge elements were homogeneously distributed, but the Mn elements were not. Similar results were observed in the heavily (above 8%) Mn-doped GaAs grown by low temperature molecular epitaxy, owing to the high concentration and low solubility of the Mn element.[1–3] The inhomogeneous distribution of the Mn elements implies that the Mn-related secondary phases may be formed in the annealed samples, which is consistent with the finding of the GIXRD.
Further structural characterizations were performed by using high-resolution TEM (HRTREM). Figures
Considering the possible combinations of various kinds of manganese silicide and manganese germanide nanostructures in the alloy background, it is not easy to identify these nanoclusters. We found by using HRTEM that most of the nanoclusters were surrounded by amorphous phase and the diameters of these nanoclusters varied from 10 nm to 20 nm. Figure
The magnetic properties of the samples were measured by SQUID. Figure
In summary, heavily Mn-doped Si0.25Ge0.75 thin films were fabricated by using magnetron sputtering and post-growth annealing. Both Mn diluted SiGe crystals and Mn-rich nanoclusters were detected in the annealed samples owing to the high Mn concentration above 10%. The annealed samples showed a TC above room temperature which was probably caused by the Ge3Mn5 nanoclusters. The results could be potentially useful for spintronic applications based on group IV materials.
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