Oxygen vacancies effects on phase diagram of epitaxial La_(1-x)Sr_xMnO_3 thin films

We investigated the effects of oxygen vacancies on the structural,magnetic,and transport properties of La_(1-x)Sr_xMnO_3(x=0.1,0.2,0.33,0.4,and 0.5)grown around a critical point(without/with oxygen vacancies)under low oxygen pressure(10 Pa)and high oxygen pressure(40 Pa).We found that all films exhibit ferromagnetic behavior below the magnetic critical temperature,and that the films grown under low oxygen pressures have degraded magnetic properties with lower Curie temperatures and smaller magnetic moments.These results show that in epitaxial La_(1-x)Sr_xMnO_3 thin films,the magnetic and transport properties are very sensitive to doping concentration and oxygen vacancies.Phase diagrams of the films based on the doping concentration and oxygen vacancies were plotted and discussed.     

Strain relaxation and vacancy creation in thin platinum films

Synchrotron based combined in situ x-ray diffractometry and reflectometry is used to investigate the role of vacancies for the relaxation of residual stress in thin metallic Pt films. From the experimentally determined relative changes of the lattice parameter a and of the film thickness L the modification of vacancy concentration and residual strain was derived as a function of annealing time at 130 °C. The results indicate that relaxation of strain resulting from compressive stress is accompanied by the creation of vacancies at the free film surface. This proves experimentally the postulated dominant role of vacancies for stress relaxation in thin metal films close to room temperature.     

Ferromagnetism in carbon-doped In2O3 thin films

Carbon-doped In₂O₃ thin films exhibiting ferromagnetism at room temperature were prepared on Si (100) substrates by the rf-magnetron co-sputtering technique. The effects of carbon concentration as well as oxygen atmosphere on the ferromagnetic property of the thin films were investigated. The saturated magnetizations of thin films varied from 1.23 to 4.86 emu/cm³ with different carbon concentrations. The ferromagnetic signal was found stronger in samples with higher oxygen vacancy concentrations. In addition, deposition temperature and different types of substrates also affect the ferromagnetic properties of carbon-doped In₂O₃ thin films. This may be related to the oxygen vacancies in the thin film system. The experiment suggests that oxygen vacancies play an important role in introducing ferromagnetism in thin films.     

Strain and high temperature superconductivity: unexpected results from direct electronic structure measurements in thin films

Angle-resolved photoemission spectroscopy reveals very surprising strain-induced effects on the electronic band dispersion of epitaxial La(2-x)Sr(x)CuO(4-delta) thin films. In strained films we measure a band that crosses the Fermi level (E(F)) well before the Brillouin zone boundary. This is in contrast to the flat band reported in unstrained single crystals and in our unstrained films, as well as in contrast to the band flattening predicted by band structure calculations for in-plane compressive strain. In spite of the density of states reduction near E(F), the critical temperature increases in strained films with respect to unstrained samples. These results require a radical departure from commonly accepted notions about strain effects on high temperature superconductors, with possible general repercussions on superconductivity theory.     

显微拉曼研究外延生长的BaTiO_(3-x)薄膜的结构相变与结构动力学(英文)

我们用显微拉曼、卢瑟福背散射谱、X射线散射和非对称摇摆等实验手段研究了在1 0 - 2 到1 0 - 5帕氧气压下用激光分子束外延技术生长的BaTiO3-x薄膜的结构动力学特性。测量结果表明生长氧压越低,晶格常数c和c/a越大,晶格常数a稍微减小,晶胞体积变大。随着生长氧压的减小,薄膜中氧含量减小。在3 0×1 0 - 5帕氧气压下生长的薄膜中氧缺陷可达0 48,但是样品的四方相结构可以很好的维持。显微拉曼研究进一步确定了样品都是四方相结构。另外,在BaTiO2 52 薄膜的拉曼光谱中发现高频段有两个新峰,其可能是由于氧缺陷导致的二级拉曼散射引起的。随着生长氧压的减小,拉曼峰向低频移动,表明薄膜中的应力减小。同时,拉曼峰变宽,这可能是由于氧缺陷导致的结构畸变引起的。由于在薄膜中存在二维张应力,BaTiO3单晶样品中的结构相变特征在我们的样品中从78到5 5 0K的温度范围内不存在。     

Equilibrium configurations of partial misfit dislocations in thin-film heterosystems

The energy characteristics of orthogonal rows of partial misfit dislocations with V-shaped stacking faults in thin-film heteroepitaxial systems are analyzed theoretically. It is shown that they should appear only in very thin epitaxial films of nanoscopic thickness and for high values of the mismatch exceeding a definite value. Under these conditions partial misfit dislocations associated with V-shaped stacking faults are typical elements of the defect structure of nanolayer heterosystems. For smaller mismatches and larger films thicknesses total misfit dislocations should form. Fiz. Tverd. Tela (St. Petersburg) 40, 2059–2064 (November 1998)     

A simple growth route towards ZnO thin films and nanorods

Highly orientated ZnO thin films and the self-organized ZnO nanorods can be easily prepared by a simple chemical vapor deposition method using zinc acetate as a source material at the growth temperature of 180 and 320 °C, respectively. The ZnO thin films deposited on Si (100) substrate have good crystallite quality with the thickness of 490 nm after annealing in oxygen at 800 °C. The ZnO nanorods grown along the [0001] direction have average diameter of 40 nm with length up to 700 nm. The growth mechanism for ZnO nanorods can be explained by a vapor-solid (VS) mechanism. Photoluminescence (PL) properties of ZnO thin films and self-organized nanorods were investigated. The luminescence mechanism for green band emission was attributed to oxygen vacancies and the surface states related to oxygen vacancy played a significant role in PL spectra of ZnO nanorods.     

Effects of different annealing atmospheres on the surface and microstructural properties of ZnO thin films grown on p-Si (1 0 0) substrates

Effects of different annealing atmospheres on the surface and microstructural properties of ZnO thin films grown on Si (1 0 0) substrates were investigated. X-ray diffraction results showed that the crystallinity of the ZnO thin film annealed in an oxygen atmosphere was better than that annealed in a nitrogen atmosphere. Atomic force microscopy and transmission electron microscopy (TEM) images showed that the surfaces of the ZnO thin films annealed in a nitrogen atmosphere became very rough in contrast to those annealed in an oxygen atmosphere. High-resolution TEM images showed that many stacking faults and tilted grains could be observed in the ZnO thin films annealed in a nitrogen atmosphere in contrast to those annealed in an oxygen atmosphere. Surface morphology and microstructural property variations due to different annealing atmospheres in ZnO thin films are also described on the basis of the experimental results.     

Structure and properties of ZnO films grown on Si substrates with low temperature buffer layers

Zinc oxide (ZnO) thin films were grown on Si (1 0 0) substrates by pulsed laser deposition (PLD) using two-step epitaxial growth method. Low temperature buffer layer (LTBL) was initially deposited in order to obtain high quality ZnO thin film; the as-deposited films were then annealed in air at 700 °C. The effects of LTBL and annealing treatment on the structural and luminescent properties of ZnO thin film were investigated. It was found that tensile strain was remarkably relaxed by employing LTBL and the band-gap redshifted, correspondingly. The shift value was larger than that calculated from band-gap theories. After annealing treatment, it was found that the annealing temperature with 700 °C has little influence on strains of ZnO films with LTBLs other than directly deposited film in our experiments. Interestingly, the different behaviors in terms of the shift of ultraviolet (UV) emission after annealing between films with and without buffer were observed, and a tentative explanation was given in this paper.     

Growth and evolution of epitaxial erbium disilicide nanowires on Si (001)

Sub-monolayer amounts of Er deposited onto Si (001)react with the substrate to form epitaxial nanowires of crystalline ErSi₂. The growth of uniaxial structures occurs because the different crystal structures of ErSi₂ and Si have a good lattice match along one Si<110> crystallographic axis but a significant mismatch along the perpendicular Si<110> axis. The nucleation, growth, and subsequent evolution of ErSi₂ nanowires were investigated as functions of erbium coverage on the Si (001) surface, annealing time, and annealing temperature. Low annealing temperatures (620 °C) and times (5 min) produced ErSi₂ nanowires with widths of a few nanometers, heights less than one nanometer, and lengths of several hundred nanometers. For longer annealing times at low temperature, the surface roughened without significant ripening of the wires. Annealing at intermediate temperatures (∼700 °C) caused stacking faults to form along the long axis of the nanowires and their lengths to ripen. At high temperature (800 °C), the wires broke apart into short segments with stacking faults. Received: 30 January 2002 / Accepted: 31 January 2002 / Published online: 3 May 2002     

Room-temperature ferromagnetism in CuO sol-gel powders and films

Both CuO nanopowders and thin films prepared by sol-gel method exhibit room-temperature ferromagnetism (FM), even though CuO bulk presents a paramagnetism (PM) at room temperature above its antiferromagnetic Néel temperature. For CuO nanopowders at room temperature, FM likely occurs at surfaces of nanograins, while PM may remain in the inner of grains. The vacuum annealing greatly enhances room-temperature FM, and the air reheating after vacuum annealing reduces FM again, indicating that the observed FM in CuO sol-gel powders and thin films are connected with oxygen vacancies. The room-temperature FM of CuO nanograins decreases with an increase of grain size, possibly due to the decrease of oxygen vacancy concentrations at surfaces of nanograins.     

Structural Variation and Its Influence on the 1/f Noise of a-Si_(1-x)Ru_x Thin Films Embedded with Nanocrystals

The structural variation and its influence on the 1/f noise of a-Si_(1-x)Ru_x thin films are investigated by Raman spectroscopy,transmission electron microscopy, and low frequency noise measurement. The Ru atoms are introduced into the amorphous silicon thin films by rf magnetron co-sputtering. Ru2 Si nanocrystals are found in the as-deposited samples. It is shown that the 1/f noise of the films can be reduced by a slight doping with Ru atoms. Moreover, both the microstructure and the 1/f noise performance of a-Si_(1-x)Ru_x thin films could be improved through a high-temperature annealing treatment.     

Influence of impurities on the surface structures and fault generation in homoepitaxial Si (111) films

In situ LEED studies of the homoepitaxial growth of Si(111) films by uhv sublimation, indicate a strong correlation between the type of surface structure generated and the metallic impurity content of the silicon substrates as estimated from minority carrier lifetimes. The development of the familiar Si (111)−7 × 7 structure is favored by the presence of lifetime-killing impurities in the substrate. Experiments where Fe is introduced on high lifetime substrates prior to annealing and film growth, suggest that this impurity species plays a role in the generation of the 7 × 7 surface structure. Electron microscopy reveals that homoepitaxial Si(111) layers are generally faulted, the number density of which increases progressively as the growth temperature is lowered and the deposition rate increased. Films deposited on high lifetime silicon contain substantially fewer stacking faults than those grown on low lifetime substrates. These results suggest that the faults originate at the substrate surface at microprecipitates consisting of fast diffusing, low solubility impurity species.     

纳米ZnO薄膜可见发射机制研究

利用溶胶－凝胶法 （Sol-Gel）制备了纳米ZnO薄膜,获得了高强的近紫外发射室温下测量了样品的光致发光谱(PL )、吸收谱(ABS)、X射线衍射谱(XRD).X射线衍射(XRD)的结果表明：纳米ZnO薄膜呈多晶态,具有六角纤锌矿结构和良好的C轴取向;发现随退火温度升高,（002）衍射峰强度显著增强,衍射峰的半高宽（FWHM）减小、纳米颗粒的粒径增大.由吸收谱(ABS)给出了样品室温下带隙宽度为3.30 eV.在PL谱中观察到二个荧光发射带,一个是中心波长位于392 nm附近强而尖的紫带,另一个是519 nm附近弱而宽的绿带研究了不同退火温度样品的光致发光峰值强度的变化关系,发现随退火温度升高,紫带峰值强度增强、绿带峰值强度减弱,均近似呈线性变化.证实了纳米ZnO薄膜绿光发射主要来自氧空位(Vo)形成的浅施主能级与锌空位(VZn)形成的浅受主能级之间的复合,或氧空位(Vo)形成的深施主能级上的电子至价带顶的跃迁;紫带来自于导带中的电子与价带中的空位形成的激子复合.     

Annealing effects on microstructural and optical properties of Nanostructured-TiO2 thin films prepared by sol-gel technique

In this paper we report on the effect of annealing on the microsctructural and optoelectronic properties of titanium dioxide (TiO₂) thin films prepared using sol-gel method onto silicon (Si) (100) and quartz substrates. The annealing temperatures range from 200 to 1000 °C. The Microstructural properties of annealed thin films were investigated by Thermal gravimetric analyses (TGA), X-ray diffraction (XRD) and Raman Spectroscopy. The surface morphology of the film was examined using Atomic Force Microscopy (AFM) method. The optical properties of TiO₂ thin films were characterized using UV-VIS and Spectroscopic ellipsometry. The results have shown that the TiO₂ thin films persist in the anatase phase even after annealing at 800 °C. The phase transformation from anatase to rutile occurred only when the films were annealed at 1000 °C. AFM studies revealed nanocrystalline structure where their shape and density depend strongly on the annealing temperatures. The elaborated nanostructured-TiO₂ thin films present a high transparency in the visible range. Spectroscopic ellipsometry (SE) study was used to determine the effect of annealing temperature on the thickness and on the optical constant of TiO₂ thin films. Spectroscopic ellipsometry and UV-VIS shows that the band gap of TiO₂ thin films was found to decrease when the annealing temperature increases. The Anatase phase was find to show higher photocatalytic activity than the rutile one.     

Dependency of photoluminescence from SiO_2 thin films containing Si_(1-x)Ge_x quantum dots on Ge/Si doping ratio

SiO2 thin films containing Si1-xGex quantum dots （QDs） are prepared by ion implantation and annealing treatment. The photoluminescence （PL） and microstructural properties of thin films are investigated. The samples exhibit strong PL in the wavelength range of 400-470 nm and relatively weak PL peaks at 730 and 780 nm at room temperature. Blue shift is found for the 400-nm PL peak, and the intensity increases initially and then decreases with the increase of Ge-doping dose. We propose that the 400-470 nm PL band originates from multiple luminescence centers, and the 730- and 780-nm PL peaks are ascribed to the Si=O and GeO luminescence centers.     

Epitaxial growth of Xe on the Si(111) 7×7 surface observed by LEED

The epitaxial growth up to 20 monolayers of Xe on the Si(111) 7×7 surface at several temperatures between 25 and 50 K has been studied by LEED. A variety of different LEED patterns demonstrate that there is a significant influence of the silicon structure on the Xe growth mode. The 7×7 superstructure has a very peculiar influence depending on temperature, so that a periodic sequence of stacking faults is observed in the xenon even in thicker layers.     

Influence of oxygen vacancies on optical properties of anatase TiO2 thin films

This paper presents an investigation into the relaxation process of photo-excited carriers in anatase TiO₂ thin films, in which the concentration of oxygen vacancy is controlled by annealing at various temperatures in an oxygen atmosphere. The influence of oxygen vacancies on absorption spectra, photoluminescence (PL) spectra, and PL decay dynamics are discussed.     

电子束蒸发制备HfO_2高k薄膜的结构特性

使用高真空电子束蒸发在p型Si(1 0 0 )衬底上制备了高kHfO2 薄膜 .俄歇电子能谱证实薄膜组分符合化学配比 ;x射线衍射测量表明刚沉积的薄膜是近非晶的 ,高温退火后发生部分晶化 ;原子力显微镜和扫描电子显微镜检测显示在高温退火前后薄膜均具有相当平整的表面 ,表明薄膜具有优良的热稳定性 ;椭偏测得在 6 0 0nm处薄膜折射率为 2 0 9;电容 电压测试得到的薄膜介电常数为 1 9.这些特性表明高真空电子束蒸发是一种很有希望的制备作为栅介质的HfO2 薄膜的方法     

Luminescence mechanism of ZnO thin film investigated by XPS measurement

The effects of annealing environment on the luminescence characteristics of ZnO thin films that were deposited on SiO₂/Si substrates by reactive RF magnetron sputtering were investigated by X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL). An analysis of the O 1s peak of ZnO film revealed that the concentration of oxygen vacancies increased with the annealing temperature from 600 °C to 900 °C under an ambient atmosphere. The PL results demonstrated that the intensity of green light emission at 523 nm also increased with temperature. Under various annealing atmospheres, the analyses of PL indicated that only one emission peak (523 nm) was obtained, indicating that only one class of defect was responsible for the green luminescence. The green light emission was strongest and the concentration of oxygen vacancies was highest when the ZnO film was annealed in ambient atmosphere at 900 °C. The results in this investigation show that the luminescence mechanism of the emission of green light from a ZnO thin film is associated primarily with oxygen vacancies. PACS 81.15.Cd; 81.40.Ef; 78.55.-m; 78.55.Et