W. Peng

One-step growth of ZnO from film to vertically well-aligned nanorods and the morphology-dependent Raman scattering

We observed a transition from film to vertically well-aligned nanorods for ZnO grown on sapphire (0001) substrates by metalorganic chemical vapor deposition. A growth mechanism was proposed to explain such a transition. Vertically well-aligned homogeneous nanorods with average diameters of similar to 30, 45, 60, and 70 nm were grown with the c-axis orientation. Raman scattering showed that the E-2 (high) mode shifted to high frequency with the decrease of nanorod diameters, which revealed the dependence of nanorod diameters on the stress state. This dependence suggests a stress-driven diameter-controlled mechanism for ZnO nanorod arrays grown on sapphire (0001) substrates. (c) 2005 American Institute of Physics.

Comparison of valence band x-ray photoelectron spectrum between Al-N-codoped and N-doped ZnO films

The valence band structures of Al-N-codoped [ZnO:(Al, N)] and N-doped (ZnO:N) ZnO films were studied by normal and soft x-ray photoelectron spectroscopy. The valence-band maximum of ZnO:(Al, N) shifts up to Fermi energy level by about 300 meV compared with that of ZnO:N. Such a shift can be attributed to the existence of a kind of Al-N in ZnO:(Al, N), as supported by core level XPS spectra and comparison of modified Auger parameters. Al-N increased the relative quantity of Zn-N in ZnO:(Al, N), while N-N decreased that of Zn-N in ZnO:N. (c) 2006 American Institute of Physics.

Synthesis and temperature-dependent near-band-edge emission of chain-like Mg-doped ZnO nanoparticles

Chain-like Mg-doped ZnO nanoparticles were prepared using a wet chemical method combined with subsequent heat treatment. The blueshifted near-band-edge emission of the doped ZnO sample with respect to the undoped one was investigated by temperature-dependent photoluminescence. Based on the energy shift of the free-exciton transition, a band gap enlargement of similar to 83 meV was estimated, which seems to result in the equivalent shift of the bound-exciton transition. At 50 K, the transformation from the donor-acceptor-pair to free-to-acceptor emissions was observed for both the undoped and doped samples. The results show that Mg doping leads to the decrease of the acceptor binding energy. (c) 2006 American Institute of Physics.

The origin of the weak ferroelectric-like hysteresis effect in paraelectric Ba0.5Sr0.5TiO3 thin films grown epitaxially on LaAlO3

Perovskite Ba0.5Sr0.5TiO3 (BST) thin films, with thickness of about 350 nm, have been epitaxially grown on (001) LaAlO3 substrates by pulsed-laser deposition. The good crystallography and epitaxy characteristics were confirmed using x-ray diffraction and transmission electron microscopy (TEM). The dielectric properties of the BST thin films were measured with a planar capacitor configuration in the temperature range of 77-300 K. The capacitance temperature characteristics, measured with no and several different DC biases, reveal that the BST films are in the paraelectric state at room temperature, with a very good dielectric tunability. However, a butterfly-shaped curve, typical for a ferroelectric material, was obtained from room temperature capacitance-voltage (C-V) measurements, suggesting a faint ferroelectric-like effect for the BST thin films. Careful dielectric property characterizations showed significant temperature and frequency dependence, probably indicating a behaviour of Maxwell-Wagner-type dielectric relaxation. As a result, the weak C-V hysteresis effect in our paraelectric BST thin films is believed to be ascribable both to oxygen vacancies and to the presence of other space charges, trapped at the grain boundaries and/or at the substrate/dielectric film interface, which give rise to local polar regions in the thin-film samples. Furthermore, this explanation is supported by cross-sectional TEM and off-axis electron holographic observations.

Structural and optical investigation of Mn-doped ZnS nanocrystals

Using a solution-based chemical method, we have prepared ZnS nanocrystals doped with high concentration of Mn2+. The X-ray diffraction analysis confirmed a zinc blende structure. The average size was about 3 nm. Photoluminescence spectrum showed room temperature emission in the visible spectrum, which consisted of the defect-related emission and the 4 T1-6 A1 emission of Mn2+ ions. Compared with the undoped sample, the luminescence of the ZnS:Mn sample is enhanced by more than an order of magnitude, which indicated that the Mn2+ ions can efficiently boost the luminescence of ZnS nanocrystals.

Investigation of YBa2Cu3O7−δ growth mode effect on the dielectric properties of BaxSr1−xTiO3 for BaxSr1−xTiO3/YBa2Cu3O7−δ bilayered thin films epitaxially grown on vicinal LaAlO3 substrates

Epitaxial bilayered thin films of BaxSr1−xTiO3 (BSTO, x = 0.1 and 0.5)/YBa2Cu3O7−δ (YBCO), with different YBCO film thicknesses, have been grown in situ by pulsed-laser deposition on 1.2° vicinal LaAlO3 substrates. The critical thickness for the transformation of the growth mode in YBCO between two-dimensional (2D) step flow and three-dimensional island is experimentally confirmed to be around 180 nm. As the YBCO-film thickness increases from 145 to 250 nm, the tremendous degradation of the film quality, due to the growth-mode change in YBCO, leads to drastic deterioration in the dielectric properties of BSTO thin films, with severe decrease in the figure of merit factor from 148 to 10 in Ba0.1Sr0.9TiO3/YBCO (at 77 K and 100 kHz) and from 56 to 6 in Ba0.5Sr0.5TiO3/YBCO (at 300 K and 100 kHz). As a result, the dielectric properties of BSTO thin films could be significantly enhanced by strictly tailoring the growth mode in YBCO as a 2D step flow for this kind of BSTO/YBCO bilayers.

Influence of tilting of SrTiO3 substrates on the morphology and superconductivity of YBa2Cu3O7−δ thin films

Morphology and superconductivity of YBa2Cu3-(7-delta) thin films grown on vicinal SrTiO3 substrates by pulsed laser deposition are studied. Ex situ atomic force microscopy and scanning tunneling microscopy are used to observe the film surface morphology, Novel anisotropic step-mediated island growth is found for the first time. The anisotropic evolution of the surface morphology is discussed in terms of the anisotropic surface diffusion. Relatively low deposition temperature (750 degreesC is found to be the optimum condition for ultra-smooth step-mediated growth of YBa2Cu3O7-delta thin films. Tilting of the substrates is found favorable for the superconductivity of YBa2Cu3O7-delta thin films grown an them, The critical temperature at zero resistance (T-c0) of YBCO films is found to be a function of the vicinal angle of the SrTiO3 substrates

Formation and orientation control of Y2O3 inclusions in pulsed laser deposited YBa2Cu3O7−δ films by using a melt-textured target

The formation and orientation Of Y2O3 inclusions in pulsed laser deposited YBa2Cu3O7-delta films by using Y2BaCuO5-containing melt-textured target have been studied by X-ray diffractometry and transmission electron microscopy. The results show that Y2O3 inclusions, rather than the Y2BaCuO5 phase contained in the target, are formed in the films. [1 1 1]- and [10 0]-oriented Y2O3 inclusions have been found in the YBa2Cu3O7-delta films. The orientation Of Y2O3 inclusions is dependent on the deposition temperature. If the deposition temperature is high enough, [I I 1]-oriented Y2O3 can be inhibited. The reasons for the formation Of Y2O3 inclusions and their orientation are discussed