P. S. Xu

The electronic structure and spectral properties of ZnO and its defects

The electronic structure of ZnO and its defects, which include intrinsic point defects and their complexes, have been calculated using full-potential linear Muffin-tin orbital method. According to our calculation data, the positions of the defect state levels have been determined in the energy band of ZnO. Based on the results above, we analysis the mechanism of the absorption and emission spectra of ZnO and discuss the effects of the electronic structure of complete ZnO and its defects on the spectral properties.

A sulfur passivation for GaAs surface by an organic molecular, CH3CSNH2 treatment

A sulfur passivation method for GaAs, CH3CSNH2 treatment has been developed. It is quite effective for removing the surface oxide layer and forming the sulfide passivation layer on GaAs surface. The enhancements of the PL intensity reveal the reduction of the surface recombination velocity and the reduction of density of defect states by this treatment. The synchrotron radiation photoemission spectroscopy measurements show that sulfur atoms bond both Ga and As atoms. After being annealed, a stable sulfur passivation layer is terminated on the surface due to the As2S3 component react with GaAs into the GaS component.

NEUTRALIZED (NH4)2S SOLUTION PASSIVATION OF III-V PHOSPHIDE SURFACES

Synchrotron radiation photoelectron spectroscopy has been used to investigate III–V phosphide GaP and InP (100) surfaces treated with a neutralized (NH4)2S solution. Compared to the conventional basic (NH4)2S solution treatment, a thick sulfide layer with P–S bond and strong Ga–S (In–S) bond of high thermal stability is formed on the neutralized (NH4)2S-treated GaP (InP) (100) surfaces. The possible passivation mechanisms of the two (NH4)2S solutions to III–V phosphide surfaces are also discussed.

Investigation of neutralized (NH4)2S solution passivation of GaAs (100) surfaces

Synchrotron radiation photoelectron spectroscopy combined with scanning electron microscopy (SEM) and gravimetry has been used to study GaAs (100) surfaces treated with a neutralized (NH4)2S solution. Compared to the conventional basic (NH4)2S solution treatment, a thick Ga sulfide layer and strong Ga–S bond were formed on the GaAs surface after dipping GaAs wafers in a neutralized (NH4)2S solution. Gravimetric data show that the etching rate of GaAs in the neutralized (NH4)2S solution is about 15% slower than that in the conventional (NH4)2S solution. From SEM observation, fewer etching pits with smaller sizes were found on the neutralized (NH4)2S-treated GaAs surface.

Few-layer graphene growth on 6H-SiC(0001) surface at low temperature via Ni-silicidation reactions

Few-layer graphene (FLG) has been prepared by thermal annealing of SiC crystal via the surface Ni-silicidation reactions. Results reveal that the temperature plays an important role for the final FLG quality and the optimized annealing temperature is about 800u2009°C. The investigation of surface morphology and microstructure for the FLG sample indicates that after the rapid cooling, the carbon atoms will segregate to form the FLG layer and the NiSix particles will congregate on the top surface. The mechanism of the FLG formation on SiC surface assisted by the Ni ultra-thin layer is briefly discussed based on the experimental results.

Structures and magnetic properties of (Fe, Li)-codoped NiO thin films

Rock-salt structured single-phase Ni0.98Fe0.02O and Ni0.93Fe0.02Li0.05O thin films with room temperature ferromagnetism were grown by pulsed laser deposition. It is found that the Li codoping into Ni0.98Fe0.02O significantly increases the saturation magnetic moment by a factor of 2, i.e., from 0.32μB∕Fe rising to 0.67μB∕Fe. The x-ray absorption fine structure spectral analyses at Fe and O K edge reveal that in both samples, the impurities are substitutionally incorporated into the NiO host. The first-principles calculations show that substitutionally doping Li ions into NiO leads to a strong hybridization between the Fe 3d states and the spin-split acceptor band at the Fermi level. As a result, the Fe 3d electron configuration is altered and the effective magnetic moment per Fe ion is enhanced.

The role of S passivation on magnetic properties of Fe overlayers grown on GaAs(100)

We have produced epitaxial Fe overlayers on sulfur-passivated GaAs(100) surfaces by CH3CSNH2 treatment, and investigated the correlation between magnetic properties of the overlayers and surface chemical structure of GaAs(100) surfaces by ferromagnetic resonance and synchrotron radiation photoemission. The surface chemical properties were modified by changing the annealing temperature of the surfaces prior to the growth. The results show that the magnetization of Fe overlayers is crucially determined by the presence of Ga–S chemical bonds and excess As after the anneals. A comparative investigation of the magnetization has been made on both S passivated and clean GaAs(100). It is confirmed that S passivation on the GaAs surface can effectively eliminate the magnetization deficiency previously attributed to interdiffusion of As into the Fe overlayer.

Studies of interface formation between Co with GaAs(100) and S-passivated GaAs(100)

Abstract Interface formation between Co with GaAs(100) and S-passivated GaAs(100) by CH 3 CSNH 2 treatment has been studied with synchrotron radiation photoemission. Strong interface disruption and reaction occurs between the overlayer with GaAs(100) even at low Co coverage (∼0.2 nm), while the reaction is much weaker on S/GaAs(100); a stable interface forms at a coverage of 1 nm and 0.8 nm, respectively. For S-passivated GaAs(100), Ga atoms bonded with S at the surface exchange with Co atoms and cause the formation of Co–S bonding, the amount of As bonded with Co is much less than that on GaAs(100), no segregated As appears at the surface of Co overlayer, in contrast with the case of Co/GaAs(100), indicating that S-passivation on GaAs(100) is an effective way of inhibiting the interdiffusion of As and Ga through the overlayer.

XRD and PES Studies of the Bi2Sr2Ca1−xPrxCu2O8+δ System

The Bi2Sr2CaCu2O8+δ system samples doped with Pr on Ca sites were synthesized. Resistivity measurements, x-ray powder diffraction, and photoemission experiments were performed. The experiment results show that Pr ions entered the lattice and chemical environment plays a major role in the shift of core-level spectra and valence-band spectra.

Carbonization process and SiC formation at C60/Si(111) interface studied by SRPES

Carbonization process and SiC formation upon annealing the Si(111) surface covered by C60 molecules with the thickness of 1.3 nm have been investigated by using synchrotron radiation photoelectron spectroscopy (SRPES), X-ray photoemission (XPS) and reflection high energy electron diffraction (RHEED) in NSRL. C60 molecules are chemisorbed on the Si(111) surface at room temperature, via Si-C60 hybridization to form covalent bonds, which can be explained by adsorption model including two adsorption configurations S3 and L With annealing the sample, the Si-C60 hybridization weakened C-C bonds internally in C60 molecules and enhanced the formation of SixC60, an intermediate species. Further annealing the sample to 650°C will lead to the decomposition of C60 molecules, the released carbon fragment will bond with external silicon atoms to form SiC. While annealing the sample to 850°C, decomposition of all C60 molecules was accomplished, and only a SiC film was left on the surface.