Pengshou Xu

Experimental and theoretical investigations on ferromagnetic nature of Mn-doped dilute magnetic semiconductors

X-ray absorption fine structure (XAFS) and first-principles calculations are effectively combined to establish correlations of fabrications, atomic and electronic structures as well as ferromagnetism for the Mn-doped dilute magnetic semiconductors (DMS), and to shed light on the magnetism origin for a variety of ZnO-, GaN-, and Si-based DMSs. The results of Mn:ZnO and (Mn,N):ZnO thin films reveal that either the existence of Zn vacancy or N substitution of O sites can stabilize the ferromagnetic interactions between neighboring Mn-Mn pairs, and enhance the magnetic moment per Mn. In a 2.5at.% Mn-doped GaN film, a part of substantial Mn ions is found to locate at the interstitial sites near the substitutional Mn ions, forming Mn-Mn dimers that possess unique electronic and magnetic properties. Similar phenomena have also been found in the Mn-doped Si system in which the interstitial Mn atoms intend to assemble together via an intervening substitutional Mn ion. We have proposed a pathway to understand the microscopic origin of ferromagnetism in the DMS materials from the viewpoint of experimental determination and theoretical calculations.

First-principles studies of the electronic and optical properties of 6H–SiC

Abstract We study the electronic and optical properties of hexagonal 6H–SiC crystal, using ab initio full potential augmented plane wave method. The density of states (DOS) and band structure are presented based on local density function theory. From the electronic structure calculation, the imaginary part of the dielectric function has been obtained directly using the joint DOS and the optical matrix elements. With band gap correction, the real part of dielectric function can be derived from the imaginary part by the Kramers–Kronig relationship. The reflectivity for normal incidence is also calculated. The resulting spectrum is in good agreement with available experimental data in a wide energy range.

Stable ultraviolet photoluminescence from sol–gel silica containing nano-sized SiC/C powder

Abstract PL band of 340 nm has been observed from sol–gel silica with and without the addition of nano-sized SiC/C powder. With the increasing of annealing temperature, the intensity of the 340-nm PL band in pure silica decreases remarkably, while that in SiC/C powder-added silica is enhanced. Based on the Fourier-transform infrared (FTIR) absorption and X-ray photoelectron spectroscopy (XPS) spectra, the authors suggest that the addition of nano-sized SiC/C powder into porous silica (PSO) will strengthen the silica network and prevent it from shrinking during heat treatment. By this mechanism, the 340-nm PL band is stabilized by the presence of SiC/C powder.

Synchrotron radiation photoelectron spectroscopy study of ITO surface

Abstract Synchrotron radiation photoelectron spectroscopy (SRPES) has been applied to surface analysis of indium tin oxide (ITO) thin films. Several different components of In and Sn were observed at the clean ITO surface. By comparing the chemical compositions of the film before and after vacuum annealing, the contents of In 2 O 3− x and Sn 3 O 4 were found to be the major factors influencing the electrical conductivity and optical transparency of the film.

Fabrication and Luminescent Properties of 6H-SiC/3C-SiC/6H-SiC Quantum Well Structure

Abstract Quantum well structure film of 6H-SiC/3C-SiC/6H-SiC was fabricated on 6H-SiC (0001) with the substrate temperature of 1350 K by solid source molecular beam epitaxy (SSMBE) through the variation of Si flux rate. The crystal polytypes and luminescent properties of the film were characterized by reflection high energy electron diffraction (RHEED) and photoluminescence (PL), respectively. The results of RHEED indicated that the film was 6H-SiC/3C-SiC/6H-SiC with the quantum well structure. The results of PL excited by He-Gd laser at room temperature showed that there were intense emissions in the range of 480–600 nm, which could not be observed from the substrate. The fitting peaks were consistent with the results calculated from the model of quantum well structure, which showed that such intense emissions were probably from the quantum wells with different widths.

Studies of Mg overlayer on GaAs(100) surface treated by CH3CSNH2

Abstract A new sulfur passivation method for GaAs, CH 3 CSNH 2 treatment, has been developed. By Synchrotron Radiation Photoemission Spectroscopy(SRPES), the chemical states and electronic aspects of the passivated surfaces are investigated. It is found that the oxide layer of the GaAs is effectively removed, and Sulfur atoms bond both to Ga and As atom at room temperature. In addition, the behavior of Mg deposited on the passivated surfaces with and without annealing has been investigated. It is found that Ga atoms can be exchanged from GaS bond, and diffuse into Mg overlayer, but the sulfur atoms remain at the interfaces. At higher Mg coverage, a MgGa alloy may be formed due to excess Mg atoms reacting with Ga atoms at the overlayer.

Study of the Na - Si(111) 3 1 interface using core-level photoemission spectroscopy

The Na - Si(111) and Na - Si(111) interfaces with various coverages of Na have been investigated using low-energy electron diffraction Auger electron spectroscopy and photoemission spectroscopy (PES) with a synchrotron radiation source. In decomposing the PES peak from the Si 2p core level into the surface and bulk components, the relative shifts and intensities of the surface components as well as the band bending for the interface are consistent with the structure model proposed by Monch. The evolution of the band bending from about 1 monolayer to thick Na coverage corresponds to the theory of the metal-induced gap states. The final Schottky barrier height is determined to be eV and does not depend on whether a thicker Na layer is deposited on an ordered Na - Si(111) surface or a disordered thin Na layer on Si(111).

Commissioning and operation of the beamline for photoelectron spectroscopy in NSRL

A grazing incidence spherical grating monochromator (SGM) for photoelectron spectroscopy has been set up in NSRL. It covers the energy range 10–1000 eV. The primary results of commissioning and operation of the beamline have been described. The resolution power could be obtained from 500 to 1000 (E/ΔE) with 50 μm of slit opening in a wide range of photon energy. The improvement of the beamline performance is continuing.

The Re‐design of a Soft X‐ray Beamline for Photoemission Spectroscopy at NSRL

The monochromator of the old soft x‐ray beamline for photoemission spectroscopy covering spectral range from 60 –1000 eV was designed according to Rowland Circle Monochromator for energy resolution (E/ΔE) over 2000 with slit widths of 40 μm at NSRL. But the beamline suffers of low flux with high energy over 500 eV cut off and energy resolution bellow 1000 at practical flux. We reconstruct the beamline by replacement of the Kirkpatrick‐Baez pre‐focusing optics and the monochromator, energy resolution beyond 1000 and flux over 5×109 photons/s can be obtained with 50 μm slit widths.

Photoemission study on the growth and stability of Gd cluster films over Ni(110) surface

Abstract The growth of Gd films on a Ni(110) surface was studied by synchrotron radiation photoemission spectroscopy (SRPES) and XPS techniques. The results showed that at a thickness below 0.20 nm Gd4f exhibited a single-peak feature indicating that the Gd film grows in layer-by-layer mode. When the Gd coverage is larger than 0.20 nm, however, the Gd4f peak evolves into a double-peak feature with 2.3 eV separation at 2.00 nm. A similar phenomenon was observed in the Gd4d XPS spectra. It is suggested that the double-peak structure of Gd4f is attributed to Gd clusters which might exhibit different electronic states from Gd metal owing to their special structures. Anneal treatment induced the collapse of Gd clusters and uniform spread of Gd over the Ni(110) surface, implying that Gd clusters are unstable thermodynamically.