Xue-Chao Liu

Oxygen enhanced ferromagnetism in Cr-doped ZnO films

Cr-doped ZnO films have been prepared by inductively coupled plasma enhanced physical vapor deposition, and an in-depth study is performed on the chromium doping and oxygen partial pressure dependence of ferromagnetism. The x-ray diffraction and photoluminescence results indicate that the moderate oxygen can relax the lattice strain thus enhancing ferromagnetism which, as confirmed by soft x-ray absorption spectroscopy, is mainly attributed to the Cr valence state transition from Cr3+ to Cr6+. However, excessive oxygen suppresses the oxygen vacancies and the ferromagnetic exchange. Furthermore, the parabola-like dependence of ferromagnetism on oxygen partial pressure is consistent with the bound magnetic polaron scenario.

High Quality Strained Ge Epilayers on a Si0.2Ge0.8/Ge/Si(100) Global Strain-Tuning Platform

High structural quality, compressively strained Ge surface epilayers have been grown on Si(100) substrates of up to 200 mm diameter. The epitaxial growth by industrially compatible reduced pressure chemical vapor deposition proceeded uninterruptedly via an intermediate relaxed Si0.2Ge0.8/Ge buffer. In-depth characterization of the epilayers revealed a relatively smooth surface and a low threading dislocation density. The Ge layers were demonstrated to remain fully strained for thicknesses of more than 100 nm. The high quality of the material and flexibility to choose the Ge layer thickness over a wide range make these heterostructures very attractive for fabricating various electronic and photonic devices

Effect of donor localization on the magnetic properties of Zn–Co–O system

The effect of donor localization on the magnetic properties of Zn–Co–O system was investigated by resonant inelastic x-ray scattering and first principle calculations. It was found that the ferromagnetic coupling was induced through charge transfer between Co 3d and donor defect. The charge transfer was dependent on the electron localization of donor defect. The electron localization of oxygen vacancy defect was much stronger than that of donor defect formed by the substitution of Al3+ for Zn2+. In order to elucidate the magnetic mechanism, a series of Zn0.95Co0.05O and Zn0.94Co0.05Al0.01O films were prepared under different oxygen partial pressure (PO2). The magnetic properties of Zn0.95Co0.05O films were dependent on PO2, while Zn0.94Co0.05Al0.01O films showed stable ferromagnetism.

Effect of oxygen partial pressure on the local structure and magnetic properties of Co-doped ZnO films

Zn0.95Co0.05O films were prepared under different oxygen partial pressures (PO2) by inductively coupled plasma enhanced physical vapor deposition. The effect of PO2 on the local structure and magnetic properties was investigated. The x-ray absorption spectroscopy at the Co K-edge, Co L-edge, and O K-edge revealed that the main defects were oxygen vacancies when the films were deposited under very low PO2. The change from room-temperature ferromagnetism to paramagnetism was observed with increasing PO2. It was experimentally demonstrated that the oxygen vacancy defect is absolutely necessary to induce ferromagnetic couplings in Co-doped ZnO films.

Strong correlation between oxygen vacancy and ferromagnetism in Yb-doped ZnO thin films

Zn1−xYbxO (0 ≤ x ≤ 0.02) thin films have been prepared by inductively coupled plasma enhanced physical vapor deposition method. All the Yb-doped ZnO thin films show room-temperature ferromagnetism. The correlation between oxygen vacancy and magnetism in Yb-doped ZnO thin films is studied. It is found that Yb irons initially substitute for Zn sites when x ≤ 0.01 and then enter the interstitial sites of ZnO with increasing Yb concentration of x > 0.01. The ferromagnetism is induced by the coexistence of oxygen vacancy and Yb point defects. A strong correlation between oxygen vacancy and saturation magnetization is observed.

Ionized zinc vacancy mediated ferromagnetism in copper doped ZnO thin films

This paper reports the origin of ferromagnetism in Cu-doped ZnO thin films. Room-temperature ferromagnetism is obtained in all the thin films when deposited at different oxygen partial pressure. An obviously enhanced peak corresponding to zinc vacancy is observed in the photoluminescence spectra, while the electrical spin resonance measurement implies the zinc vacancy is negative charged. After excluding the possibility of direct exchange mechanisms (via free carriers), we tentatively propose a quasi-indirect exchange model (via ionized zinc vacancy) for Cu-doped ZnO system.

Silicon–germanium interdiffusion in strained Ge/SiGe multiple quantum well structures

The corresponding author, Xue-Chao Liu, would like to apologize for the omission of some of the contributing authors from this published paper. The full list of contributing authors should read: Xue-Chao Liu, R J H Morris, M Myronov, A Dobbie and D R Leadley Dr R J H Morris contributed the secondary ion mass spectrometry measurement and analysis, shown in figure 2. Drs M Myronov and A Dobbie contributed by growing the samples.

Magnetic Properties of Mn-Doped ZnO Nanostructures Synthesized by Chemical Vapor Transport

Mn-doped ZnO nanostructures have been fabricated by chemical vapor transport (CVT). Nanobelts, nanorods and nanowires have different growth directions because of their different growth environments. X-ray diffraction, electron paramagnetic resonance and Raman spectrum methods have been used to identify the substitution of a Zn site with Mn ions. Despite their different morphologies, these nanostructures possess the same magnetic properties. Magnetization was paramagnetic and antiferromagnetic. No ferromagnetism was observed even at T=5 K.

Zinc vacancy and erbium cluster jointly promote ferromagnetism in erbium-doped ZnO thin film

Zn1-xErxO (0.005 ≤ x ≤ 0.04) thin films have been prepared by inductively coupled plasma enhanced physical vapor deposition method. Ferromagnetism, crystal structure, microstructure and photoluminescence properties of the films were characterized. It is found that the chemical valence state of Er is trivalent, and the Er3+ cations play an important role in ferromagnetism. Both saturated magnetization (Ms) and zinc vacancy (VZn) are decreased with the increase of x from 0.005 to 0.03. However, further increasing x to 0.04, the Ms is quenched due to the generation of Er clusters. It reveals that the intensity of Ms is not only associated with the VZn concentration, but also related to the Er clusters. The VZn concentration and the Er clusters can jointly boost the ferromagnetism in the Zn1-xErxO thin films.

High-quality Ge/Si0.4Ge0.6multiple quantum wells for photonic applications : growth by reduced pressure chemical vapour deposition and structural characteristics

Strain-symmetrized Ge/SiGe multiple quantum wells have been grown on a thin (2.1 µm) relaxed Si0.2Ge0.8/Ge/Si(1 0 0) virtual substrate (VS) by reduced pressure chemical vapour deposition. Such structures are of interest in optoelectronic applications for which the structural integrity of the quantum well layers, after processing, is critical. The layer composition, thickness and interface quality have been studied for wafers both as-grown and after annealing between 550 and 700 °C. Transmission electron microscopy indicated precise thickness control of ±0.1 nm and sharp abruptness between the Ge QWs and SiGe barrier layers. A smooth surface was observed, with an average rms roughness of 1.5 ± 0.1 nm determined by atomic force microscopy. High-resolution x-ray diffraction (HR-XRD) indicated that both the QWs and barriers were fully strained compared with the relaxed VS. The thermal stability of the epilayers was investigated both by ultra low energy secondary ion mass spectroscopy of post-growth annealed layers and by in situ annealing in a high temperature HR-XRD stage. No obvious interdiffusion and strain relaxation was observed provided the annealing temperature was below 600 °C, but significant atomic rearrangement was evident for greater thermal budgets, thereby setting an upper processing temperature for this type of structure.