Cheng-Feng Yan

Investigation of room temperature ferromagnetism of 3C-SiC by vanadium carbide doping

Undoped and vanadium carbide (VC) doped 3C-SiC powders have been prepared, and an in-depth study is performed on the VC-doping dependence of room temperature ferromagnetism (FM). It is demonstrated that the FM originates in vacancy defects. The saturation magnetization (Ms) of VC is about 800 times than that of undoped 3C-SiC, while the Ms of VC-doped 3C-SiC is even smaller than that of the undoped one. The increase of doping concentration would result in the decrease of vacancy concentration and the increase of carrier concentration, suggesting that the FM of 3C-SiC is related to both vacancy and carrier concentrations.

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.

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.

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.

Strong correlation between B-Al-N doping concentration fluctuation and photoluminescence effects of f-SiC

This paper report the relationship between B-Al-N doping concentration fluctuation and photoluminescence effects of fluorescent 4H-SiC single crystals. The photoluminescence emission properties, dopant concentration, and internal quantum efficiency of B, Al and N co-doped 4H-SiC are characterized. It is found that the emission spectra exhibit a wide band that covers from about 450 nm to 750 nm. The peak intensity of the emission spectra is strongly affected by B, Al and N concentrations. By further analyses the roles of B-Al-N dopants, a hypothetical formula is proposed, which can help to profile the strong correlation between photoluminescence effects and B-Al-N doping concentrations.This paper report the relationship between B-Al-N doping concentration fluctuation and photoluminescence effects of fluorescent 4H-SiC single crystals. The photoluminescence emission properties, dopant concentration, and internal quantum efficiency of B, Al and N co-doped 4H-SiC are characterized. It is found that the emission spectra exhibit a wide band that covers from about 450 nm to 750 nm. The peak intensity of the emission spectra is strongly affected by B, Al and N concentrations. By further analyses the roles of B-Al-N dopants, a hypothetical formula is proposed, which can help to profile the strong correlation between photoluminescence effects and B-Al-N doping concentrations.

Study of Nitrogen Concentration in Silicon Carbide

This work focused on studying the nitrogen concentration (CN) in SiC. The variations of CN in the synthesis of SiC powder as well as the transport during SiC crystal growth have been investigated for broad ranges of temperature and Ar pressure. Before SiC crystal growth, SiC powders were synthesized from high-purity silicon and carbon powders. The concentrations of nitrogen, free C, and free Si in the as-prepared powders were all measured. CN in the SiC source powder decreased with increasing temperature and decreasing Ar pressure, whereas it did not show a remarkable trend with the molar ratio of free Si to free C. SiC crystal was then grown by the physical vapor transport (PVT) technique using the as-prepared powder. The distribution of CN in the remaining material indirectly indicated the temperature field of crystal growth. In addition, compared with introducing N2 during SiC crystal growth, doping with nitrogen during synthesis of the SiC source powder might be a better method to control CN in SiC crystals.

Effect of vanadium on the room temperature ferromagnetism of V-doped 6H—SiC powder

This study focuses on the effect of V-doping on the ferromagnetism (FM) of 6H—SiC powder. The X-ray diffraction results indicate that V is inserted into the 6H—SiC lattice. The Raman spectra reveal that with a V concentration of 25 ppm, the crystalline quality and carrier concentration of 6H—SiC are hardly varied. It is found that after the V-doping process, the saturation magnetization (Ms) and the vacancy concentration of 6H—SiC are both increased. From these results, it is deduced that the effect of V might contribute mainly to the increase of vacancy concentration, thus resulting in the increase of Ms of V-doped 6H—SiC.

Influence of surface bow on reconstruction on 2-inch SiC (0001) wafer

The step morphologies of a gas-etched 2-in. 6H-SiC (0001) wafer are investigated by the atomic force microscope. Due to the concave surface induced by bow, undulation surface morphologies were observed in the edge region. In the upside and downside region of the center along the 〈112¯0〉 miscut direction, ripples and bimodal steps are observed, respectively. In the other edge regions, a complex pattern of waves enveloped in microsteps was formed. The complex reorganized surface morphology is discussed by the viewpoint of the two or three-dimensional phase separation. Compared to the regular unit cell high steps, the formalism of waved surface is interpreted by competing free energy curve through changes in surface structure faceting, where the step separated at a specific orientation and favor to match to surface reconstruction with unit cell high steps.

Synthesis of Source Powder for SiC Crystal Growth Using High Purity Silicon and Carbon Powder

This work focused on the synthesis of source powder for SiC crystal growth. SiC powder was prepared using high purity silicon and carbon powder. Broad ranges of temperature and Ar pressure were studied on the property of the as-prepared powder. X-ray diffraction (XRD) results show that SiC polytypes were determined by synthesis temperature, while not related to the variation of Ar pressure. The lattice constant of SiC would expand when Ar pressure decreased. Raman results revealed that the variation of Ar pressure would influence SiC crystallization. It was found that the concentrations of free C, free Si and nitrogen all varied with the variation of temperature or Ar pressure.