Microstructures induced in the bulk of SrTiO3 crystal by a femtosecond laser

Microstructures with the total length of hundreds of mu m were induced by fixing the focal point of the femtosecond laser at a certain depth in the bulk of SrTiO3 crystal. By different combination of the focusing conditions with the laser parameters, different morphologies have been observed, such as void array, necklace-shaped structures, continuous/segmental filaments and etc. The possible mechanism of the formation of those diversiform structures is discussed.

Orientation-controllable self-organized microgratings induced in the bulk SrTiO3 crystal by a single femtosecond laser beam

Self-organized microgratings were induced in the bulk SrTiO3 crystal by readily scanning the laser focus in the direction perpendicular to the laser propagation axis. The groove orientations of those gratings could be controlled by changing the irradiation pulse number per unit scanning length, which could be implemented either through adjusting the scanning velocity at a fixed pulse repetition rate or through varying the pulse repetition rate at a fixed scanning velocity. This high-speed method for fabrication of microgratings will have many potential applications in the integration of micro-optical elements. The possible formation mechanism of the self-organized microgratings is also discussed. (C) 2007 Optical Society of America.

Origin of insulating behavior of the p-type LaAlO3/SrTiO3 interface: Polarization-induced asymmetric distribution of oxygen vacancies

It is revealed from first-principles calculations that polarization-induced asymmetric distribution of oxygen vacancies plays an important role in the insulating behavior at p-type LaAlO3/SrTiO3 interface. The formation energy of the oxygen vacancy (V-O) is much smaller than that at the surface of the LaAlO3 overlayer, causing all the carriers to be compensated by the spontaneously formed V-O's at the interface. In contrast, at an n-type interface, the formation energy of V-O is much higher than that at the surface, and the V-O's formed at the surface enhance the carrier density at the interface. This explains the puzzling behavior of why the p-type interface is always insulating but the n-type interface can be conducting.

Structural and electrical properties of (110) ZnO epitaxial thin films on (001) SrTiO3 substrates

(110) oriented ZnO thin films were epitaxially prepared on (001) SrTiO3 single crystal substrates by a pulsed laser deposition method. The evolution of structure, surface morphology, and electrical conductivity of ZnO films was investigated on changing the growth temperature. Two domain configurations with 90 degrees rotation to each other in the film plane were found to exist to reduce the lattice mismatch between the films and substrates. In the measured temperature range between 80 K and 300 K, the electrical conductivity can be perfectly fitted by a formula of a (T) = sigma(0) + aT(b/2). implying that the electron-phonon scattering might have a significant contribution to the conductivity. (C) 2008 Elsevier Ltd. All rights reserved.

Fabrication and transport properties of ZnO/Nb-1 wt %-doped SrTiO3 epitaxial heterojunctions

(110) ZnO/(001) Nb-1 wt %-doped SrTiO3 n-n type heteroepitaxial junctions were fabricated using the pulse laser deposition method. A diodelike current behavior was observed. Different from conventional p-n junctions or Schottky diodes, the diffusion voltage was found to increase with temperature. At all temperatures, the forward current was perfectly fitted on the thermionic emission model. The band bending at the interface can qualitatively explain our results, and the extracted high ideality factor at low temperatures, as well as large saturation currents, is ascribed to the deep-level-assisted tunneling current through the junction. (C) 2008 American Institute of Physics.

Valence band offset of ZnO/SrTiO3 heterojunction measured by x-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy has been used to measure the valence band offset (VBO) of the ZnO/SrTiO3 heterojunction. It is found that a type-II band alignment forms at the interface. The VBO and conduction band offset (CBO) are determined to be 0.62 +/- 0.23 and 0.79 +/- 0.23 eV, respectively. The directly obtained VBO value is in good agreement with the result of theoretical calculations based on the interface-induced gap states and the chemical electronegativity theory. Furthermore, the CBO value is also consistent with the electrical transport investigations.

Structural and optical properties of ZnO films on SrTiO3 substrates by MOCVD

ZnO films have been fabricated on (0 0 1), (0 1 1) and (1 1 1) SrTiO3 (STO) substrates by metal-organic chemical vapour deposition (MOCVD). It is interesting that the ZnO films on (0 0 1) and (0 1 1) STO substrates show polar and semipolar orientations, which are different from previous reports, while the same growing direction of polar ZnO with previous results is found on (1 1 1) STO. For the atomic arrangements, two orthogonal domains and a single domain are observed on (0 0 1) and (1 1 1) STO, respectively. Photoluminescence spectra show that every sample has a sharp near-band-edge emission peak at about 3.28 eV without any deep-level emission band between 1.5 and 2.8 eV, implying a high optical quality. A violet emission around 3.0 eV is observed only in ZnO films on (0 0 1) and (0 1 1) STO substrates grown at 600 degrees C, which is discussed briefly. Additionally, the semipolar ZnO does not weaken the emission efficiency along with the reduction in the polarization effect compared with polar ZnO. These results show that high-quality polar and semipolar ZnO films can be grown on STO substrates by MOCVD.

InN layers grown by MOCVD on SrTiO3 substrates

Epitaxial wurtzite InN thin films have been grown by metal-organic chemical vapor deposition on (1 1 1) SrTiO3 (STO) substrates. Interestingly, twin domain epitaxy induced by the surface reconstruction of STO is observed with the in-plane orientation relationships of [(1) over bar 1 0 0]InN parallel to [<(1)over bar > 1 0]STO and [2 <(1 1)over bar > 0]InN parallel to[<(1)over bar > 1 0]STO, which is helpful to release the strain. The InN films on STO substrates exhibit a strong photoluminescence emission around 0.78 eV. Particularly, using STO substrates opens up a possibility to integrate InN with the functional oxides. (C) 2009 Elsevier B.V. All rights reserved

SrTiO3衬底上MOCVD生长ZnO和InN薄膜

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溶胶-凝胶工艺制备SrTiO3纳米薄膜的研究

采用溶胶-凝胶方法,以醋酸锶、钛酸四丁酯为前驱体,乙酰丙酮为螯合剂,乙二醇甲醚为溶剂,乙酸为助溶剂和催化剂制备溶胶,用旋涂法在Si(100)衬底上制备出了具有典型钙钛矿型结构的SrTiO3纳米薄膜.用XRD、AFM、SEM和椭圆偏光仪等手段分析了薄膜的性能,结果显示薄膜的表面均匀、无裂纹、厚度20～30nm,折射率1.75～1.85,晶粒度35～60nm,表面平均粗糙度RMS为3.4～3.8 nm,晶粒形貌呈类圆锥形态.

在钛酸锶衬底上外延生长ZnO薄膜及其性能研究

用脉冲激光淀积法（PLD）在（111）面SrTiO3衬底上外延生长ZnO单晶薄膜。样品分别在衬底温度为350℃、500℃、600℃下外延生长。X射线衍射（XRD）的结果表明,所得的ZnO单晶薄膜结晶性能好,只出现（002）和（004）两个衍射峰,（002）峰的半高宽度（FWHM）为0.23°。在荧光光谱中我们只观察到来源于带边激子跃迁的强UV发射,并且随着生长温度的升高,紫外峰的强度逐渐增强。样品的SEM图像表明所得ZnO薄膜表面平整,晶粒均匀。衬底温度为600℃时,所得到的ZnO薄膜结构完整,晶粒尺寸最

Growth of c-oriented ZnO films on (001) SMO3 substrates by MOCVD

The growth direction of ZnO thin films grown by metal-organic chemical vapor deposition (MOCVD) is modulated by pretreatment of (001) SMO3 (STO) substrates. ZnO films show a-oriented smooth surface with epitaxial relationship of < 001 > ZnO//< 110 > STO on as-received SfO, and c-axis columnar growth with < 010 > ZnO//< 110 > STO on etched STO, respectively. The orientation alteration of ZnO films is supposed to be caused by the change of STO surface polarity. In addition, the c-ZnO films exhibit an enhanced photoluminescence (PL) intensity due to the improved crystal quality, while the blueshift of PL peak is attributed to the smaller tensile strain. These results show that high quality c-ZnO, which is essential for electronic and optoelectronic device applications, can be grown on (001) SfO by MOCVD. (C) 2008 Elsevier B.V. All rights reserved.

Valence band offset of ZnO/BaTiO3 heterojunction measured by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy has been used to measure the valence band offset of the ZnO/BaTiO3 heterojunction grown by metal-organic chemical vapor deposition. The valence band offset (VBO) is determined to be 0.48 +/- 0.09 eV, and the conduction band offset (CBO) is deduced to be about 0.75 eV using the band gap of 3.1 eV for bulk BaTiO3. It indicates that a type-II band alignment forms at the interface, in which the valence and conduction bands of ZnO are concomitantly higher than those of BaTiO3. The accurate determination of VBO and CBO is important for use of semiconductor/ferroelectric heterojunction multifunctional devices.