Xue Qi-Kun

Thermal stability of Mg2Si epitaxial film formed on Si（111） substrate by solid phase reaction

A single crystalline Mg2Si film was formed by solid phase reaction (SPR) of a Si(111) substrate with an Mg overlayer capped with an oxide layer(s), which was enhanced by post annealing from room temperature to 100 degrees C in a molecular beam epitaxy (MBE) system. The thermal stability of the Mg2Si film was then systematically investigated by post annealing in an oxygen-radical ambient at 300 degrees C, 450 degrees C and 650 degrees C, respectively. The Mg2Si film stayed stable until the annealing temperature reached 450 degrees C then it transformed into amorphous MgOx attributed to the decomposition of Mg2Si and the oxidization of dissociated Mg.

Electroluminescence of an n-ZnO/p-GaN heterojunction under forward and reverse biases

We investigate the carrier envelope phase (CEP) effects on high-order harmonic generation (HHG) in ultrashort pulses with the pulse duration 2.5 fs when the laser intensity is high enough so that the initial state is ionized effectively during the laser pulse but remains about 20% population at the end of the laser pulse. We End that the ionization process of the initial state is very sensitive to the CEP during the laser pulse. The ionization process of the initial state determines the continuum state population and hence influences dramatically the weights of the classical trajectories that contribute to HHG. In such a case we can not predict the cutoff and the structure of the harmonic spectrum only by the number and the kinetic energy of the classical trajectories. The harmonic spectrum exhibits abundant characters for different CEP cases. As a result, we can control the cutoff frequency and the plateau structure of the harmonic spectrum with CEP by controlling the time behaviour of the ionization of the initial state.

Influence of external voltage on electronic transport properties of molecular junctions： the nonlinear transport behaviour

The influence of external voltage on the charge transport properties of benzene-1,4-dithiolate molecular junction has been investigated. Variations of the geometric and electronic structures for the molecule caused by the applied voltage are calculated at ab initio level. Based on the numerical results, we find that the energy shift and expansion coefficients in the end-site of the molecule for the frontier molecular orbitals show nearly linear dependence on voltage in the interested voltage interval. The charge transport properties of the molecular junction are then studied by employing the elastic scattering Greens function method. It is shown that the voltage has an obvious effect on the I - V characteristic of the molecular junction, particularly the shape of the conductance curves. The I - V curves are consistent with the experimental measurement quite well.

STUDY OF THE Al/GRAPHITE INTERFACE

Thin Al films with a thickness of 20-30nm were prepared by ultra-high vacuum deposition of Al onto a graphite surface parallel to a (0001) basal plane. The samples were annealed up to 1070K. X-ray photoelectron spectroscopy analysis has shown that for temperatures just higher than 770K, a little carbide occurs in the Al film and only an Al-C phase is present at the Al/graphite interface. After annealing at 970K, the Al4C3 phase can be observed and the binding energy of the Al2p electrons increases continuously from 72.7 to 74.2eV with increasing temperature up to 1070K. Auger electron spectroscopy depth profiles are measured to investigate the phases existing in the Al film as well as at the Al/graphite interface. It is found that the Al4C3 phase at the interface is the final product of a series of Al carbides from the interfacial reaction between Al and graphite.

A Z-scan study of LiNbO3 thin films

We have investigated the morphology and microstructure of carbon nanotubes and nanoparticles in cathode deposits prepared by self-sustained arc discharge. Scanning electron microscopy images indicate that there are two regions exhibiting different morphologies on the top surface of the cathode deposits. In the central region, there is a triangular pattern of spots with a diameter up to 100mum, which consists of carbon nanotubes and nanoparticles. In the fringe region, carbon nanotubes and nanoparticles are distributed randomly. In addition, carbon nanotubes in the central region have a larger inner diameter, compared with those in the fringe region. The outer diameter distribution of tubes in the central region is narrower than that of tubes in the fringe region, while the former has a smaller peak value than the latter. For the nanoparticles, they exhibit a different behaviour from the tubes existing in the same region. The difference between the microstructure of tubes or particles in the two regions is attributed to the different temperatures and temperature gradients during their formation.

Two-step growth of MgO films on sapphire(0001) substrates by radio frequency plasma-assisted molecular beam epitaxy

We report on the growth of rock salt MgO films on sapphire (0001) substrates by rf plasma-assisted molecular beam epitaxy. A two-step method, i.e. high temperature epilayer growth after low-temperature buffer layer growth, was adopted to obtain the single crystal MgO film. The epitaxial orientation between the MgO epilayer and the sapphire (0001) substrate was studied by using in situ reflection high energy electron diffraction and ex situ x-ray diffraction, and it is found that the MgO film grows with [111] orientation. The role of the low temperature buffer layer in the improvement of crystal quality of the MgO epilayer is discussed based on the cross-sectional scanning electron microscopy.

Oscillatory electron phonon coupling in Pb/Si(111) deduced by temperature-dependent quantum well states

Photoemission study of atomically flat Pb films with a thickness from 15 to 24 monolayers (ML) have been performed within a temperature range 75–270K. Well-defined quantum well states (QWSs) are observed, which exhibit interesting temperature-dependent behaviours. The peak position of the QWSs shifts towards higher binding energy with increasing substrate temperature, whereas the peak width broadens linearly due to enhanced electron–phonon coupling strength (λ). An oscillatory λ with a period of 2ML is deduced. Preliminary analysis shows that the oscillation can be explained in terms of the interface induced phase variations and is thus a manifestation of the quantum size effects.

Room-Temperature Growth of Al Films on Si(111)-7×7 Surface

Reflection high energy electron diffraction and scanning tunnelling microscopy (STM) are used to investigate the structure and morphology of Al films deposited on Si(111)-7×7 surface at room temperature. The films are polycrystalline, made up of (100) and (111) oriented islands, which primarily result from the interface elastic effect and free surface energies of the Al (100) and (111) surfaces.

Atomic and Electronic Structures of Submonolayer In on Cu(111)

We have studied the atomic and electronic structures of submonolayer In on Cu(111) using synchrotron radiation photoemission and scanning tunnelling microscopy. At very low coverage, In atoms were favourable to occupy substitutional sites and to be distributed evenly on the Cu(111) surface. At a higher coverage of 0.33 and 0.50 ML, two ordered reconstructed phases of ((3)1/2×(3)1/2)R30o and (2×1) have been observed for the first time. The two-dimensional states of In-induced valence bands in the ((3)1/2×(3)1/2)R30o phase were measured by angle-resolved photoemission. The result is in good agreement with theoretical calculations.

Bi- and Au-Induced Reconstructions on GaAs(001)-2 ×4 Surface

Submonolayer Bi and Au adsorptions on the GaAs(001)-2 × 4 surface are investigated by scanning tunnelling microscopy, low energy electron diffraction and first-principles calculations. The 1 × 4 and 3 × 4 reconstructed surface induced by Bi and Au, respectively, are revealed and their structural models are proposed based on experiments and first-principles calculations. Moreover, the validity of the recently proposed generalized electron counting (GEC) model [Phys. Rev. Lett. 97 (2006) 126103] is examined in detail by using the two surfaces. The GEC model perfectly explains the structural features, such as the characteristic short double-line structure in the Bi-1 × 4 surface and the 3× arrangement of four-atom Au clusters.