Shi-Yi Zhuo

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.

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.

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.

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

The Zn0.97−xCr0.03GaxO (0 ≤ x ≤ 0.03) 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.

Ferromagnetism in proton irradiated 4H-SiC single crystal

Room-temperature ferromagnetism is observed in proton irradiated 4H-SiC single crystal. An initial increase in proton dose leads to pronounced ferromagnetism, accompanying with obvious increase in vacancy concentration. Further increase in irradiation dose lowers the saturation magnetization with the decrease in total vacancy defects due to the defects recombination. It is found that divacancies are the mainly defects in proton irradiated 4H-SiC and responsible for the observed ferromagnetism.

Effects of annealing temperature on the electrical property and microstructure of aluminum contact on n-type 3C–SiC

The 3C—SiC thin films used herein are grown on Si substrates by chemical vapor deposition. Al contacts with different thickness values are deposited on the 3C—SiC/Si (100) structure by the magnetron sputtering method and are annealed at different temperatures. We focus on the effects of the annealing temperature on the ohmic contact properties and microstructure of Al/3C—SiC structure. The electrical properties of Al contacts to n-type 3C—SiC are characterized by the transmission line method. The crystal structures and chemical phases of Al contacts are examined by X-ray diffraction, Raman spectra, and transmission electron microscopy, respectively. It is found that the Al contacts exhibit ohmic contact behaviors when the annealing temperature is below 550°C, and they become Schottky contacts when the annealing temperature is above 650°C. A minimum specific contact resistance of 1.8 × 10−4 Ωcm2 is obtained when the Al contact is annealed at 250°C.

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.

Donor-acceptor-pair emission in fluorescent 4H-SiC grown by PVT method

Fluorescent SiC, which contains donor and acceptor impurities with optimum concentrations, can work as a phosphor for visible light emission by donor-acceptor-pair (DAP) recombination. In this work, 3 inch N-B-Al co-doped fluorescent 4H-SiC crystals are prepared by PVT method. The p-type fluorescent 4H-SiC with low aluminum doping concentration can show intensive yellow-green fluorescence at room temperature. N-B DAP peak wavelength shifts from 578nm to 525nm and weak N-Al DAP emission occurred 403/420 nm quenches, when the temperature increases from 4K to 298K. The aluminum doping induces higher defect concentration in the fluorescent crystal and decreases optical transmissivity of the crystal in the visible light range. It triggers more non-radiative recombination and light absorption losses in the crystal.