Huan-hua Wang

Sb-doped SrTiO3 transparent semiconductor thin films

Optically transparent Sb-doped SrTiO3 thin films with a transmittance higher than 95% in most of the visible region have been grown on SrTiO3 (001) substrate by pulsed laser deposition. The films behave as an n-type semiconductor between 10 K and room temperature. The carrier concentration and mobility of the films at room temperature are ∼5.8×1017 cm−3 and ∼6.4 cm2/V s, respectively. X-ray photoelectron spectroscopy measurement reveals that the delocalized electrons from the Sb dopants give rise to deep impurity levels within the band gap of the parent compound, which are responsible for the electrical conduction observed. The wide band gap and low density of states in the conduction band account for transparency of the films.

Nearly strain-free heteroepitaxial system for fundamental studies of pulsed laser deposition: EuTiO3 on SrTiO3

High-quality epitaxial thin films of EuTiO3 have been grown on the (001) surface of SrTiO3 using pulsed laser deposition. In situ x-ray reflectivity measurements reveal that the growth is two dimensional and enable real-time monitoring of the film thickness and roughness during growth. The film thickness, surface mosaic, surface roughness, and strain were characterized in detail by using ex situ x-ray diffraction. The thickness and composition were confirmed with Rutherford backscattering spectroscopy. The EuTiO3 thin films grow two dimensionally, epitaxially, and pseudomorphically, with no measurable in-plane lattice mismatch.

Optical and transport properties of Sb-doped SrTiO3 thin films

Optically transparent SrTi1−xSbxO3 (=0.05, 0.10, 0.15, and 0.20) thin films with transmittances higher than 85% in the visible region have been grown on SrTiO3 substrates by pulsed-laser deposition. Unless overdoped, the films possess a single-crystal phase and impurity conduction. The temperature dependence of the resistivities shows a metal–semiconductor transition for the film with x=0.05, and semiconducting behaviors for the films with x=0.10 and 0.15. The overdoped film with x=0.20 is an insulator. Sb concentration has a dominant effect on the electrical properties of the films, and the Anderson localization is probably the mechanism. X-ray photoelectron spectroscopy results indicate that the Sb impurity atoms provide donor electrons to form impurity states within the band gap, which is responsible for the electrical localized impurity. The wide band gap and the low density of states in the conduction band result in the transparency of the films. The disorder increases with Sb concentration, which is...

Hydrogen Impurity Defects in Rutile TiO2

Hydrogen-related defects play crucial roles in determining physical properties of their host oxides. In this work, we report our systematic experimental and theoretical (based on density functional theory) studies of the defect states formed in hydrogenated-rutile TiO2 in gaseous H2 and atomic H. In gas-hydrogenated TiO2, the incorporated hydrogen tends to occupy the oxygen vacancy site and negatively charged. The incorporated hydrogen takes the interstitial position in atom-hydrogenated TiO2, forming a weak O-H bond with the closest oxygen ion, and becomes positive. Both states of hydrogen affect the electronic structure of TiO2 mainly through changes of Ti 3d and O 2p states instead of the direct contributions of hydrogen. The resulted electronic structures of the hydrogenated TiO2 are manifested in modifications of the electrical and optical properties that will be useful for the design of new materials capable for green energy economy.

Growth and characterization of SrMoO3 thin films

Highly conductive SrMoO3 thin films with good crystallinity and smooth surface were grown on SrTiO3 (0 0 1) substrates by pulsed laser deposition. The effects of substrate temperature and oxygen pressure on the structure, surface morphology, and electrical properties were studied. In the range of substrate temperatures from 560 degreesC to 640 degreesC and oxygen pressure from 10 (-3) to 10 (-4) Pa studied in our experiments, high-quality SrMoO3 thin films were produced. Beyond the top temperature or oxygen pressure limit, SrMoO4 appears as an impurity phase. High-resolution transmission electron microscopy (HRTEM) study shows that the SrMoO3 film has high-quality crystallinity and an epitaxial nature. The root-mean-square surface roughness of the film deposited at 2.5 x 10 (-4) Pa is 3.9 Angstrom. The films exhibit metallic conduction, which results from the delocalized electrons from Mo. X-ray photoelectron spectroscopy (XPS) measurements characterized its core level spectra and the valence band

Rh : BaTiO3 thin films with large nonlinear optical properties

We report the fabrication and the nonlinear optical properties of Rh-doped BaTiO3 thin films. The films were deposited on SrTiO3 (100) substrates by pulsed-laser deposition. The deposited Rh:BaTiO3 thin films were single phase and c-axis orientation investigated by x-ray diffraction. The films exhibited large nonlinear optical effects, which were determined using Z-scan technique at a wavelength of 532 nm with a laser duration of 10 ns. The real and imaginary parts of the third-order nonlinear susceptibility chi (3) were 5.71 x 10(-7) esu and 9.59 x 10(-8) esu, respectively. The value of Re chi (3) of Rh:BaTiO3 films is much larger than those of several representative nonlinear optical thin films. The results show that Rh:BaTiO3 thin films have great potential applications for nonlinear optical devices.

Nanostructure evolution of YBa2Cu3Ox thin films grown by pulsed-laser glancing-angle deposition

Nanostructured thin films of amorphous YBa2Cu3Ox were prepared using pulsed-laser glancing-angle deposition. Ambient oxygen pressure and laser fluence have a strong effect on the microstructure of the films. The films exhibit a structural evolution from isolated directional nanorods, through a network of vertical nanocolumns, to nanoparticles fractal with increasing ambient oxygen pressures from below 1 to above 180mtorr. The shadowing effect, surface diffusion, and flux scattering by ambient gas play main roles in determining the structural evolution.

Self-oxidized NiO on cube-textured Ni for YBCO coated superconductor

Abstract In this paper, we report a self-oxidation approach to obtain NiO on the surface of Ni substrates. Under optimal oxidizing conditions, high-textured NiO layers on the surface of Ni substrates were obtained. Detailed pole figure (PF) measurements show an improvement in the full width at half maximum of NiO layers compared with that of Ni substrate. Orientation distribution function calculated from two PFs shows that the best cubic texture of NiO can be formed at 680°C in 5 Pa oxygen. A grain size less than 1 μm is evident from scanning electron microscopy observation. Some possible ways were proposed to improve surface quality. The self-oxidation approach provides a potential to prepare high-Jc YBCO films on a large scale.

The point defect structure and its transformation in As-implanted ZnO crystals

The point defect structure of As-implanted ZnO crystals was investigated using diffuse x-ray scattering and spectroscopic methods. A dynamic picture regarding the defect structure transformation was revealed. The defect type, size and combination state experienced obvious changes during annealing. Before annealing the point defects distributed isolatedly and coexisted with dislocation loops. After annealing, As-O disappeared and other point defects either aggregated into large clusters or formed defect complexes including As-Zn-2V(Zn) whose ionization energy was measured and which was discerned as the main origin of p-type conductivity.

Insight into Metalized Interfaces in Nano Devices by Surface Analytical Techniques

Connections between metals and heterogeneous solid state materials form buried interfaces. These ubiquitous structures play an essential role in determining the performances of many nano- and microdevices. However, the information about the chemistry, structure, and properties of these real interfaces is intrinsically difficult to extract by traditional techniques. Therefore, approaches to efficiently discovering metalized interfaces are in high demand. Here, we demonstrate the transformation of nanoscale metal/oxide interface problems into surface problems through a novel metal-hydrogenation detaching method. We applied this technique to study the thickness dependence in Pb(Zr,Ti)O3 (PZT) ferroelectric thin films, a long-standing interface problem in a model metal/insulator device, and this allowed comprehensive surface analytical techniques to be adapted. A nonstoichiometric interfacial layer of 4.1 nm thick with low mass density, low permittivity, and weak ferroelectricity was quantified at the Pt/PZT interface and attributed to the preferential diffusions among the compositional elements. Targeted interface engineering by Pb rebalance led to a substantial recovery of ferroelectric properties. Our results therefore pave the way to a better understanding of metallized interface in ferroelectric and dielectric nanodevices. We hope that more useful information about metalized interfaces of other solid materials could, analogously, be accessed by surface analytical techniques.