Mingshan Xue

Surface Structural Evolution in Iron Oxide Thin Films

Ordered iron oxide ultrathin films were fabricated on a single-crystal Mo(110) substrate under ultrahigh vacuum conditions by either depositing Fe in ambient oxygen or oxidizing preprepared Fe(110) films. The surface structure and electronic structure of the iron oxide films were investigated by various surface analytical techniques. The results indicate surface structural transformations from metastable FeO(111) and O-terminated Fe(2)O(3)(0001) to Fe(3)O(4)(111) films, respectively. The former depends strongly on the oxygen pressure and substrate temperature, and the latter relies mostly upon the annealing temperature. Our experimental observations are helpful in understanding the mechanisms of surface structural evolution in iron oxides. The model surfaces of Fe-oxide films, particularly O-terminated surfaces, can be used for further investigation in chemical reactions (e.g., in catalysis).

Layer-by-layer growth of polar MgO(111) ultrathin films

By alternate deposition of Mg and exposure of O2, layer-by-layer growth, polar MgO(111) ultrathin films with Mg-terminated or O-terminated surfaces have been successfully fabricated on Mo(110) substrate. The surface geometric structure and electronic structures of the polar MgO(111) films were investigated using surface analysis techniques including low-energy electron diffraction and photoelectron emission and electron energy loss spectroscopies. The results indicate that the O-terminated surface is of an insulating character, while for Mg-terminated surface, a prominent new surface state at 2-3 eV and appreciable density of states near Fermi level have been observed. The polar oxide films provide ideal model surfaces for further investigation of support-particle system.

Initial Oxidation and Interfacial Diffusion of Zn on Faceted MgO(111) Films

The interaction of zinc and faceted MgO(111) thin films prepared on a Mo(110) substrate was investigated in situ by using various surface analysis techniques, including X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Auger electron spectroscopy, high-resolution electron energy loss spectroscopy, and low-energy electron diffraction. The results revealed that three-dimensional Zn islands exist on the faceted MgO(111) films and that no chemical interaction takes place at the interface at room temperature. Initially, deposited Zn is stable at temperatures below 400 K and diffuses into MgO at temperatures above 425 K. A portion of Zn is oxidized at approximately 10 (-6) mbar O 2 at room temperature. An interfacial phase of Zn x Mg 1- x O was formed after Zn was exposed to approximately 10 (-6) mbar O 2 at temperatures >or=500 K. The faceted structure on the MgO(111) surface is of a disadvantage for the epitaxial growth of ZnO films.

Tunable surface band gap in MgxZn1−xO thin films

Mg(x)Zn(1-x)O thin films epitaxially grown on Mo(110) substrate under ultrahigh vacuum condition were studied in situ by various surface analysis techniques including x-ray photoelectron spectroscopy, Auger electron spectroscopy, low-energy electron diffraction, and high resolution electron energy loss spectroscopy. The results indicate that as-grown Mg(x)Zn(1-x)O films are soluble phase, and a phase transition from wurtzite to cubic structure occurs in the region of x = 0.55-0.67. The surface band gap can be tuned continuously with altering the content of Mg in Mg(x)Zn(1-x)O films, and its tunable window width is about 1.9 eV. Based on heterojunction and quantum well structure, this kind of materials can be applied in wide-band-gap semiconductor devices, such as short-wavelength light-emitting devices.

Unsteady Thermal Convection of Melts in a 2-D Horizontal Boat in a Centrifugal Field with Consideration of the Coriolis Effect

A rotating centrifuge introduces the centrifugal acceleration and the Coriolis force acting on melts while melt growth is being carried out in the centrifuge. These two forces influence melt convection and, in turn, modify the transport of dopant and impurities. In this paper the effects of varying the centrifugal acceleration and the Coriolis force were studied numerically. We paid attention to unsteady thermal convection of melts in a two-dimensional rectangular boat with relevance to crystal growth in a centrifuge by horizontal Bridgman technique. The mathematical model was constructed by the continuity, Navier-Stokes and energy equations with the Boussinesq approximation, which was solved by the finite control volume method with fully implicit, steady, time-marching, central-difference discretization. The calculations based on the simplified model reveal that the centrifugal acceleration enhances buoyancy force, which may dominate the convection and induce oscillation, and the Coriolis force may stabilize or destabilize the flow depending on the rotation sense of the centrifuge. This numerical results as well as the experiments of temperature measurement give a satisfactory explanation of the results described previously.12 13

Preliminary results of GaAs single crystal growth under high gravity conditions

GaAs single crystals have been grown under high gravity conditions, up to 9g0, by a recrystallization method with decreasing temperature. The impurity striations in GaAs grown under high gravity become weak and indistinct with smaller striation spacings. The dislocation density of surcharge-grown GaAs increases with increase of centrifugal force. The cathodoluminescence results also show worse perfection in the GaAs grown at high gravity than at normal earth gravity.

Growth and electronic structure of Ag on polar MgO(111) films

The growth and electronic structure of Ag on polar MgO(111) thin films with {100} facets were in situ investigated by low energy electron diffraction and photoemission spectroscopies. The Ag epitaxially grows on faceted MgO(111) surface as Ag(111) films due to the large surface energy of the MgO(111) face in spite of the existence of {100} facets, compared with a three-dimensional growth of Ag on MgO(100) films. Based on the photoemission spectroscopies studies, the shifts of the Ag 3d core level line at the initial coverage are mainly associated with size effects rather than chemical interaction. The results are helpful to the understanding of the growth mechanisms and electronic structure of metals on polar oxide surfaces.

RESPONSE OF TEMPERATURE OSCILLATIONS IN A TIN MELT TO CENTRIFUGAL EFFECTS

An experimental study was conducted on the flow and temperature oscillations of molten tin in a horizontal boat under centrifugation. A longitudinal temperature gradient was applied to an open boat containing molten tin so that convection was generated with a known direction of the basic flow. The experimental results demonstrated that both effects of centrifugal and Coriolis forces exist due to the rotation of the centrifuge. The former enhances the convection, decreasing the temperature gradient in the melt. The latter shows different influences on the flow stability depending on the rotation sense of the centrifuge. Temperature fluctuations in the melt are considerably retarded, provided the centrifuge rotation is in the same sense as the convection roll in the melt.

Effect of polar surface on the growth of Au

The growth of Au on faceted MgO(111) and MgO(100) films was investigated by Auger electron spectroscopy, low energy electron diffraction, scanning electron microscopy, atomic force microscopy, and Raman spectroscopy. On the polar MgO(111) surface, a continuous Au film is easily formed because of the large surface energy compared with MgO(100). Heating the Au films grown on MgO(111) results in the formation of nanoparticles of Au. A relatively high Raman activity of the Au nanoparticles is observed. The size of Au nanoparticles might be adjusted by controlling the density of facets on (111) through changing the thickness of the polar oxide films. Our results provide a new insight into the preparation of nanoparticles of metals by choosing a polar surface as a patterned substrate, which may also be applied to other metal/oxide systems.