Xu Fa-Qiang

Electronic structure of ZnO and its defects

The electronic structure of ZnO and its native point defects has been calculated using full potential linear Muffin-tin orbital ( FP-LMTO) method for the first time. The results show that Zn3d electrons play an important role in the bonding of ZnO. Vacant Zn (Vzn) and interstitial O (Oi) produce the shallow acceptor levels at 0.3 eV and 0.4 eV above the top of the valence band (VB), while interstitial Zn (Zni) produces a shallow donor level at 0.5 eV bellow the bottom of the conduction band (CB). However, Vacant O (Vo) produces a deep donor level at 1.3 eV below the bottom of CB. On the basis of these results, we confirm that Zni is the main factor to induce the native n-type conductivity in ZnO

Native Point Defect States in ZnO

The native point defect states in ZnO have been calculated by using a full-potential linear muffin-tin orbital method. The results show that Zn vacancy and O interstitial produce the shallow acceptor levels above the valence band. The O vacancy produces a deep donor level, while Zn interstitial produces a shallow donor level, both below the conduction band. The Zn interstitial is the main factor which induces the native n-type conductivity in ZnO.

A FP–LMTO study on the native shallow donor in ZnO

Abstract First-principle calculations are performed on ZnO:V O , ZnO:V Zn and ZnO:Zn I using a FP–LMTO (full potential linear muffin-tin orbital) method. The results prove that the Zn I is the dominant donor in ZnO, and V Zn , a shallow acceptor, while V O is a deep donor.

Atomic and Electronic Structures of Cd0.96Zn0.04Te(110) Surface

X-Ray diffraction is used to analyse the lattice structure of Cd0.96Zn0.04Te (CZT), and the lattice constant is measured to be 0.647 nm. The atomic structure of the clean CZT(110) surface obtained by Ar+ etching in vacuum is observed by low-energy electron diffraction, where no surface reconstruction is discovered. Angle-resolved photoemission spectroscopy was used to characterize the surface state of the clean CZT (110) surface, by which we find a 1.5-eV-wide surface band with the peak at 0.9 eV below the Fermi energy containing about 6.9×1014 electrons/cm2, approximately one electron per surface atom.

Electronic structure of PCBM

We have studied the electronic structure of [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) using synchrotron radiation photoelectron spectroscopy (PES) measurements and first-principles calculations. The PES spectrum of the entire occupied valence band is reported, which exhibits abundant spectral features from the Fermi level to ~ 24 eV binding energy. All the spectral features are broadened as compared with the cases of C60. The reasons for the broadening are analysed by comparing the experimental data with the calculated energy levels and density of states. Special attention is paid to the analysis of the C60 highest occupied molecular orbital (HOMO)-1 derived states, which can play a crucial role in the bonding at the interfaces of PCBM/polymer blenders or PCBM/electrodes. Besides the well-known energy level splitting of the C60 backbone caused by the lowered symmetry, C 2p states from the side chain mix or hybridize with the molecular orbitals of parent C60. The contribution of the O 2p states can substantially modify the PES spectrum.

X-Ray Magnetic Circular Dichroism Measurement of Fe--Co Alloy Films Prepared by Electrodeposition

The macro- and micro-magnetic properties of Fe–Co alloy films eletrodeposited on GaAs(100) are studied by synchrotron radiation x-ray magnetic circular dichroism (XMCD) in combination with the magneto-optical Kerr effect (MOKE) measurements and magnetic force microscopy (MFM). The orbital and spin magnetic moments of each element in the Fe–Co alloy are determined by the sum rules of XMCD. Element-specific hysteresis loops (ESHL) are obtained by recording the L3 MCD signals as a function of applied magnetic field. MOKE results reveal that the amorphous films are magnetically isotropic in the surface plane. The MFM image shows that the dimension of the magnetic domains is about 1–2 μm, which is much larger than that of the grains, indicating that there are intergranular correlations among these grains. Both ESHL and MOKE hysteresis loops indicate the strong ferromagnetic coupling of Fe and Co in the alloy films.

Theoretical study on the band structure and optical properties of 4H-SiC

We have studied the band structure and optical properties of 4H-SiC by using a full potential linearized augmented plane waves (FPLAPW) method. The density of states (DOS) and band structure are presented. The imaginary part of the dielectric function has been obtained directly from the band structure calculation. With band gap correction, the real part of the dielectric function has been derived from the imaginary part by the Kramers–Kronig (KK) dispersion relationship. The values of reflectivity for normal incidence as a function of photon energy have also been calculated. We found the theoretical results are in good agreement with the experimental data.

A new method to enhance the magnetism of Fe overlayer on GaAs(100): sulfur passivation using CH3CSNH2

Ferromagnetic resonance (FMR) has been used to investigat the magnetism of Fe overlayer on S-passivated GaAs(100) pretreated by CH3CSNH2. Comparing with the magnetism of Fe overlayer on clean GaAs(100), we find that sulfur passivation can prevent As diffusion into Fe overlayer and weaken the interaction of As and Fe. It results in enhancing the magnetism of Fe overlayer on GaAs(100). We also investigate the effects of the pre-annealing of S- passivated GaAs(100) substrate on the magnetism of Fe overlayers. The results show that the maximum effective magnetization can be obtained at annealing temperature of 400 °C. According to the experimental results of synchrotron radiation photoemission, it can be explained by the change of chemical composition and surface structure of the passivation layer on GaAs(100) surface after the annealing.

Study of iron overlayer on sulphur passivated GaAs(100) by synchrotron radiation photoemission

We have studied the interface electronic structures and the chemical reaction of the Fe overlayer deposited on S-passivated GaAs(100). The chemical bond and electronic structure are different from Fe/GaAs, and the reaction between As and Fe is weakened by S atoms. This is beneficial to the magnetism in the interface. In the first stage of deposition, Fe clusters is form near S atoms due to the large electronegativity of S. The S atoms remain at the interface with Fe coverage. Magnetic ordering feature is found at a coverage higher than 0.6 nm. According to the large exchange splitting in valence band spectra, we suggest that Fe phase transition from bcc to fcc occurs with increasing coverage.