Q.Y. Zhang

Preparation, structure and properties of TaN and TaC films obtained by ion beam assisted deposition

Abstract Tantalum nitride and tantalum carbide films were prepared by the method of ion beam assisted deposition (IBAD). The results of tranmission electron microscopy (TEM) and X-ray diffraction showed that the films are all fcc structures and the grain sizes in the films are very small (about 10 to 20 nm). The composition depth profile and chemical binding character of films were determined by Auger electron spectroscopy (AES) and X-ray photo-electron spectroscopy (XPS). Measurements of film properties, such as microhardness, adhesion of film to substrate, and surface resistance, were made on a microhardness tester, a scratch tester and a bridge of resistance measurement with four-pole probe, respectively. The residual stress, dislocation density and dislocation distribution in the films were calculated by a method called X-ray diffraction line profile analysis developed by Wilkens and Wang et al. According to the elastic theory of dislocations, the microhardness of films was also calculated by using the parameters of dislocations obtained.

Optical and electrical properties of Zn1−xCdxO films grown on Si substrates by reactive radio-frequency magnetron sputtering

Zn1−xCdxO films (0⩽x⩽0.179) were grown on Si (001) substrates at 750°C with a radio-frequency reactive magnetron sputtering method. Difference between the photoluminescence (PL) spectra taken at room temperature (RT) and at 12K is reported and is deduced to be the result of PL emission from the ZnCdO phases with wurtzite and zinc blende structures. It is also found that the RT PL intensity is in inverse proportion to the carrier concentration in the films. Cd incorporation results in the transform of conductivity from p type to n type and a decrease of carrier mobility.

Magnetic and transport properties of epitaxial (LaBa)Co2O5.5+δ thin films on (001) SrTiO3

The (LaBa)Co2O5+δ thin films were grown on (001) SrTiO3 single crystal substrates by using pulsed laser deposition. Microstructure studies from x-ray diffraction and electron microscopy show that the films have good epitaxial quality with a-axis orientation and sharp atomic interface. Transport property and isothermal magnetoresistance measurements have been used to understand the physical properties of the films with anomalous magnetic phenomena and the largest reported magnetoresistance value of 19% at 40 K.

Improvement of the wear behavior of stircast Al-Si-Pb alloys by hot extrusion

Abstract The wear behavior of as-cast and hot extruded Al–Si–Pb alloys were investigated under dry conditions using a pin-on-disc type wear testing machine. The results show that the microstructure and mechanical properties can be greatly improved and porosity can be significantly decreased by hot extrusion. These factors contribute to great increase in wear resistance of hot extruded Al–Si–Pb alloys. Optical observation and X-ray photoelectron spectroscopy (XPS) analysis reveal the almost constant wear rate at mediate load levels. Better resistance to seizure for Al–Si–Pb alloys with more than 15 wt% lead are due to a film of lubricant covering almost the entire worn surface. This film is a mixture of different constituents containing Al, Fe, Si, O and Pb.

Photoluminescence and extinction enhancement from ZnO films embedded with Ag nanoparticles

ZnO films embedded with Ag nanoparticles are deposited at 750 °C with a reactive radio-frequency magnetron sputtering. The films are found to have a large enhancement in the intensity of photoluminescence emission and in the extinction of incident light. The enhancement is assigned to be from the interaction between the localized surface plasmons in the Ag nanoparticels and the light. The surface plasmons in the films can be excited in a wide range, from ultraviolet to near infrared wavelength of light.

PO2 dependant resistance switch effect in highly epitaxial (LaBa)Co2O5+δ thin films

This research was partially supported by Department of Energy under Grant No. DE-FG26-07NT43063, the National Science Foundation under Grant No. NSF-NIRT-0709293, the Texas ARP Program under Grant No. 003656-0103-2007, the State of Texas through the Texas Center for Superconductivity at the University of Houston, and the South Texas Technology Management Program.

Epitaxial relationship of ZnO film with Si (001) substrate and its effect on growth and morphology

Using reactive radio-frequency magnetron sputtering, epitaxial growth of ZnO film was observed on Si (001) substrate at different temperatures ranging from room temperature to 750°C. The epitaxial relationship was determined to be ZnO(001)‖Si(001) in the direction normal to the surface of the films with a deviated angle less than 3° and ZnO[100]‖Si[110] or ZnO[310]‖Si[110] in the plan view. Based on (2×1) reconstruction of Si (001), a heteroepitaxial model was suggested to discuss the influence of Si (001) substrate on the growth and morphology of ZnO films at different temperatures.

Absorption and photoluminescence properties of Er-doped and Er∕Yb codoped soda-silicate laser glasses

Er-doped and Er∕Yb codoped soda-silicate laser glasses with various concentrations of Er and Yb were fabricated. The absorption and the photoluminescence (PL) spectra were measured and analyzed. For the Er- doped soda-slilicate glasses, the optimum Er concentration for the PL intensity at 1536nm turns out to be 0.5at.%, and the full width at half maximum (FWHM) of PL spectrum increases from 18 to 26nm, with the increase of the concentration from 0.1 to 0.8at.%. The PL intensity of Er∕Yb codoped soda-silicate glasses with an Er concentration of 0.5at.% is enhanced approximately by four times, and the optimum Yb concentration for the PL intensity at 1536nm is analyzed to be 3.0at.%. The PL spectrum becomes broader with increasing the Yb concentration, up to a FWHM of 80nm at 6.0at.%. Yb. The relation between the absorption and PL spectra, together with the mechanism of PL broadening, has also been addressed.

Ultrafast atomic layer-by-layer oxygen vacancy-exchange diffusion in double-perovskite LnBaCo2O5.5+δ thin films.

Surface exchange and oxygen vacancy diffusion dynamics were studied in double-perovskites LnBaCo2O5.5+δ (LnBCO) single-crystalline thin films (Ln = Er, Pr; −0.5 < δ < 0.5) by carefully monitoring the resistance changes under a switching flow of oxidizing gas (O2) and reducing gas (H2) in the temperature range of 250 ~ 800°C. A giant resistance change ΔR by three to four orders of magnitude in less than 0.1 s was found with a fast oscillation behavior in the resistance change rates in the ΔR vs. t plots, suggesting that the oxygen vacancy exchange diffusion with oxygen/hydrogen atoms in the LnBCO thin films is taking the layer by layer oxygen-vacancy-exchange mechanism. The first principles density functional theory calculations indicate that hydrogen atoms are present in LnBCO as bound to oxygen forming O-H bonds. This unprecedented oscillation phenomenon provides the first direct experimental evidence of the layer by layer oxygen vacancy exchange diffusion mechanism.

The Origin of Local Strain in Highly Epitaxial Oxide Thin Films

The ability to control the microstructures and physical properties of hetero-epitaxial functional oxide thin films and artificial structures is a long-sought goal in functional materials research. Normally, only the lattice misfit between the film and the substrate is considered to govern the physical properties of the epitaxial films. In fact, the mismatch of film unit cell arrangement and the Surface-Step-Terrace (SST) dimension of the substrate, named as “SST residual matching”, is another key factor that significantly influence the properties of the epitaxial film. The nature of strong local strain induced from both lattice mismatch and the SST residual matching on ferroelectric (Ba,Sr)TiO3 and ferromagnetic (La,Ca)MnO3 thin films are systematically investigated and it is demonstrated that this combined effect has a dramatic impact on the physical properties of highly epitaxial oxide thin films. A giant anomalous magnetoresistance effect (~1010) was achieved from the as-designed vicinal surfaces.