Jian-Hua Yang

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.

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.

Friction Stir Processing: A Novel Approach for Microstructure Refinement of Magnesium Alloys

In this article, recent investigations on magnesium alloys by friction stir processing (FSP) are addressed. It indicates that remarkable grain refinement and breakup/dissolution of second-phase particles could be achieved simultaneously by FSP. High values of superplastic elongation were achieved in the FSP magnesium alloys at a wide range of strain rates and temperatures. The pinning of heat resistant particles on the grain boundaries in Mg-RE alloys stabilized the fine microstructure, leading to the occurrence of superplasticity at higher temperature and higher strain rate.

Influence of Ga doping on the Cr valence state and ferromagnetism in Cr: ZnO films

The Zn0.97−xCr0.03GaxO (0u2009≤u2009xu2009≤u20090.03) films have been prepared by inductively coupled plasma enhanced physical vapor deposition and investigated by soft x-ray absorption spectroscopy. The soft x-ray absorption spectroscopy results indicate Ga doping can substantially transform Cr3+ to Cr2+ and improve the average magnetic moment of Cr ions. The strong localization of carriers suggests the bound magnetic polarons scenario, wherein the reduced localization radius of variable range hopping by the enhancement of high valence state Cr ions can suppress the hopping probability of carriers and stabilize the bound magnetic polarons, leading to a monotonic increase in saturation magnetization.

Nano-Scale Native Oxide on 6H-SiC Surface and its Effect on the Ni/Native Oxide/SiC Interface Band Bending

Native oxide layer with thickness of about 1 nm was found easy to form on 6H-SiC surface during transporting from cleaning process to vacuum chambers, which was examined by x-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The interface band bending was studied by synchrotron radiation photoelectron spectroscopy (SRPES). For the native-oxide/SiC surface, after Ni deposition, the binding energy of Si 2p red-shifted about 0.34 eV, which suggested the upward bending of the interface energy band. Therefore, the native oxide layer should be considered on the study of SiC devices because it may affect the electron transport properties significantly.

Analysis of polytype stability in PVT grown silicon carbide single crystal using competitive lattice model Monte Carlo simulations

Polytype stability is very important for high quality SiC single crystal growth. However, the growth conditions for the 4H, 6H and 15R polytypes are similar, and the mechanism of polytype stability is not clear. The kinetics aspects, such as surface-step nucleation, are important. The kinetic Monte Carlo method is a common tool to study surface kinetics in crystal growth. However, the present lattice models for kinetic Monte Carlo simulations cannot solve the problem of the competitive growth of two or more lattice structures. In this study, a competitive lattice model was developed for kinetic Monte Carlo simulation of the competition growth of the 4H and 6H polytypes of SiC. The site positions are fixed at the perfect crystal lattice positions without any adjustment of the site positions. Surface steps on seeds and large ratios of diffusion/deposition have positive effects on the 4H polytype stability. The 3D polytype distribution in a physical vapor transport method grown SiC ingot showed that the facet preserved the 4H polytype even if the 6H polytype dominated the growth surface. The theoretical and experimental results of polytype growth in SiC suggest that retaining the step growth mode is an important factor to maintain a stable single 4H polytype during SiC growth.

The dual effects of Al-doping on the ferromagnetism of Zn0.98–yEr0.02AlyO thin films

Zn 0.98–yEr0.02AlyO (0 ≤ y ≤ 0.04) thin films have been prepared by inductively coupled plasma enhanced physical vapor deposition method. It is found that Er3+ substitutes Zn 2+ in ZnO lattice without forming any magnetic secondary phase. Al-doping has dual effects on the electron transport and magnetic properties of Er-doped ZnO films, wherein Al Zn and Ali play different roles. When 0 ≤ y ≤ 0.02, the dominant Al Zn increases and induces both carrier concentration and saturation magnetization (M s) increasing. When 0.02 < y ≤ 0.04, Ali becomes main defect and enhances the probability of electron scattering, thus reduces the M s.

Direct Observation of Nanoscale Native Oxide on 6H-SiC Surface and Its Effect on the Surface Band Bending

The RCA-cleaned 6H-SiC surface has a 1 nm native oxide layer, which was directly observed by high-resolution transmission electron microscopy and confirmed by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The surface band bending caused by the native oxide layer was studied by synchrotron radiation photoelectron spectroscopy. The binding energy of Si 2p core level for the Ni/oxygen-free SiC interface showed almost zero shift (<0.07 eV). However, it red-shifted about 0.34 eV for the Ni/native-oxide/SiC interface, it indicated that negative charged interface states induced in the Ni/native-oxide interface resulted in the upward bending of the interface energy band.

Effect of annealing temperature on the contact properties of Ni/V/4H-SiC structure

A sandwich structure of Ni/V/4H-SiC was prepared and annealed at different temperatures from 650u2009°C to 1050u2009°C. The electrical properties and microstructures were characterized by transmission line method, X-ray diffraction, Raman spectroscopy and transmission electron microscopy. A low specific contact resistance of 3.3 × 10-5 Ω·cm2 was obtained when the Ni/V contact was annealed at 1050u2009°C for 2 min. It was found that the silicide changed from Ni3Si to Ni2Si with increasing annealing temperature, while the vanadium compounds appeared at 950u2009°C and their concentration increased at higher annealing temperature. A schematic diagram was proposed to explain the ohmic contact mechanism of Ni/V/4H-SiC structure.

Effect of Interface Native Oxide Layer on the Properties of Annealed Ni/SiC Contacts

The near-SiC-interfaces of annealed Ni/SiC contacts were observed directly by high-resolution transmission electron microscopy (HRTEM). 1 nm native oxide layer was observed in the as-deposited contact interface. The native oxide layer cannot be removed at 650°C through rapid thermal annealing (RTA) and it was completely removed at 1000°C RTA. The residue of native oxide layer resulted in the Schottky characters. High temperature annealing (>950°C) not only removes the oxide layer in the near-SiC-interface, but also forms a well arranged flat Ni2Si/SiC interface, which contribute to the formation of ohmic behavior.