Growth mechanism of ZnO nanostructures in wet-oxidation process

Zinc (Zn) thin films were prepared by direct current magnetron sputtering as precursors with different deposition times. Zinc oxide (ZnO) nanostructures such as nanowires, nanobelts and nanoblades were then synthesized from the Zn precursors by wet-oxidation process. The microstructures of the Zn precursor and ZnO nanostructures have been studied by scanning electronic microscopy and X-ray diffractometry. The optoelectronic properties were analyzed by photoluminescence measurement. It was found that the Zn precursor film with a porous top layer consisting of well-crystallized Zn grains is an essential for formation of ZnO nanowires. Along with time dependence study and temperature dependence studies, the ZnO nanostructure growth mechanisms during the wet-oxidation process are proposed: water vapor has a major influence on the initial stage, and the final dimensions of the nanostructure are controlled by the vapor-solid process.     

Anomalous behavior in ZnMgO thin films deposited by sol-gel method

The magnesium doped zinc oxide is a promising optical material to enhance the luminescence for possible application in solid state lighting. Magnesium doped zinc oxide thin films (Zn_0.85Mg_0.15O) were deposited by sol-gel route on p-type silicon and annealed at different temperatures in oxygen environment for an hour. The doping of magnesium in zinc oxide was confirmed by X-Ray diffraction and the samples were found to have wurtzite crystal structure with (002) preferred orientation. The films were characterized by Hall-effect, atomic force microscopy, UV-VIS spectroscopy, photoluminescence (PL) and work function measurements. The different studies exhibited an anomalous behavior for the film annealed at 900 °C. The Hall effect, work function measurements and UV-VIS spectroscopy indicated that the resistivity, work function and optical band gap increased as a function of annealing temperature (from 300 °C to 700 °C) however these parameters were found to decrease for the films annealed above 700 °C. The particle size increased with the annealing but for the samples annealed at 900 °C, the shape of the grains changed and became elongated like fibers as observed by the atomic force microscopy. The PL measurements displayed the existence of oxygen vacancies defects for the samples annealed at and above 600 °C. The possible mechanism for this anomaly has been discussed in this work.     

Properties of electrochemically deposited CdTe thin films: annealing effect

CdTe thin film have been deposited onto stainless steel and fluorine doped tin oxide coated glass substrates from aqueous acidic bath using electrodeposition technique. The different preparative parameters, such as deposition time, bath temperature, pH of the bath have been optimized by photoelectrochemical (PEC) technique get good quality photosensitive material. The deposited films are annealed at different temperature in presence of air. Annealing temperature is also optimized by PEC technique. The film annealed at 200 °C showed maximum photosensitivity. Different techniques have been used to characterize the as deposited and also annealed (at 200 °C) CdTe thin film. The X-ray diffraction (XRD) analysis showed the polycrystalline nature and a significant increase in the XRD peak intensities is observed for the CdTe films after annealing. Optical absorption shows the presence of direct transition with band gap energy 1.64 eV and after annealing it decreases to 1.50 eV. Energy dispersive analysis by X-ray study for the as-deposited and annealed films showed nearly stoichiometric compound formation. Scanning electron microscopy reveals that spherically shaped grains are more uniformly distributed over the surface of the substrate for the annealed CdTe film. Photovoltaic output characteristics and spectral response of the annealed film have been carried. The fill factor and power conversion efficiency (η) of the cell are found to be 71 and 3.89 %.     

Synthesis and luminescence properties of europium doped silica thin film

Mixture of europium oxide (Eu₂O₃) nanoparticles and spin-on glass (SOG) solution without annealing exhibited a strong room temperature photoluminescence (PL) at 610 nm. We developed a one-step synthesis to incorporate europium ions in silica thin film by mixing the Eu₂O₃ nanoparticles with the SOG solution and found that the weight ratio of the nanoparticles and the SOG solution was 1:5 for maximum PL. We also studied the temperature effect on the light emission of the europium doped thin film by time-of-flight secondary ion mass spectrometry (SIMS). The PL intensity of the thin film doubled after annealing. SIMS study confirmed the reduction of hydroxyl groups and explained the PL enhancement in the annealed europium doped silica thin film.     

Changes of tungsten oxide nanostructures in flame vapor deposition process depending on process variables

Tungsten oxide thin film in 1-dimensional (1-D) nanostructures shows high photocatalytic activity and the flame vapor deposition (FVD) process is fast and economical method to prepare 1-D nanostructured tungsten oxide thin films of high purity and crystallinity. We investigated the morphology changes of tungsten oxide thin film prepared by FVD process for various process variables such as total gas flow rate, flame temperature and substrate temperature. For the experimental conditions in this study, we confirmed that the selection of suitable total flow rate is a key factor for 1-D nanostructure growth in fuel-rich condition. As we increase the flame and substrate temperatures, the longer and thinner 1-D nanotubes were obtained, which have the advantages of high surface area and shorter diffusion length of proton for the application to photoelectrochemical water splitting. This study would provide the basic information for the design of FVD process to prepare 1-D nanostructures in future.     

Influence of the growth conditions on the stoichiometry and on the optical properties of titanium oxide thin films prepared by reactive sputtering

Titanium oxide thin films are deposited at room temperature by reactive DC sputtering onto glass and Si (100) substrates. Different conditions of deposition were varied such as sputtering power, deposition time and oxygen partial pressure to study their influence on the titanium oxide thin films growth. The absolute amount of oxygen and the relative O/Ti composition of films have been determined by Nuclear Reaction Analysis and Rutherford Backscattering Spectroscopy, respectively. Additionally, the band-gap was determined by measuring the optical absorption and its behavior correlated with the oxygen film content. From the present study, it is possible to establish that the optical band-gap energy depends mainly on the sputtering oxygen partial pressure used at the preparation and that films prepared with a partial oxygen pressure of 4 × 10^− 2 Pa allows titanium oxide with near stoichiometric composition. Additionally, from the optical point of view, band-gap energies of 3.4 eV are obtained for near stoichiometric films and a decrease is observed for samples prepared with higher oxygen concentrations.     

Preparation and characterisation of tin-doped indium oxide films

Tin-doped In₂O₃ (ITO) thin films were prepared by reactive evaporation from new pulverulent mixture of indium oxide, tin oxide and metallic indium at different partial pressures of oxygen. The films were annealed in a secondary vacuum just after deposition. Under optimal conditions of evaporation, these films are stoichiometric, show a good crystallinity and feature high transmission in visible region (T>90%) and high reflection in near IR region versus oxygen pressure.     

Investigation of the effect of different oxygen partial pressure to LaAlO3 thin film properties and resistive switching characteristics

The pulsed laser deposition and growth of a high-k dielectric lanthanum aluminate LaAlO₃ (LAO) thin film on indium tin oxide/glass substrate at different oxygen partial pressure was studied. Based on the pulsed laser deposition growth mechanism, we explain how a difference in the oxygen partial pressure influences the surface roughness, formation of an interfacial layer, and the transparent resistive switching characteristics of LAO thin films. The micro-structure and oxygen concentration difference inside LAO thin films may be the main reason for the difference in electrical and resistive switching properties. Films grown at higher oxygen partial pressure displayed more reliable resistive switching performance, due to the formation of the interfacial layer and a lower concentration of oxygen vacancies. The interfacial layer serves as a good oxygen reservoir and the involvement of more oxygen ions ensures the switching reliability. The migration of oxygen ions between the interfacial layer and the LAO film under applied bias may be the switching mechanism.     

in situ Preparation of superconducting HgBa2CuO4 thin films

Superconducting thin films of HgBa₂CuO₄ have been grown in situ by using a sputtering method for the first time. (100) SrTiO₃ was used as substrates and heated between 500 ° C to 600 ° C during the film deposition. By setting the deposition conditions properly, c axis oriented HgBa₂CuO₄ films were grown perpendicularly to the substrate surface. It was found that Hg composition in the deposited films had close relation to the sputtering gas, namely, the oxygen partial pressure, Hg could remain in the film when the partial pressure of oxygen was lower than in the case of the other oxide superconductors such as Bi cuprates. The optimum oxygen partial pressure for the crystallized thin film ranged from 0.1 Pa to 0.01 Pa with total gas pressure of 0.6 Pa. The superconducting transition was observed at around 75K.     

Influences of deposition rate and oxygen partial pressure on residual stress and microstructure of YSZ thin films

Yttria-Stabilized Zirconia (YSZ) thin films were deposited on borosilicate crown glass substrates using electron beam evaporation technique and controlling technological parameters: deposition rate and oxygen partial pressure. Spectrophotometry, optical interferometry and X-ray diffraction were used to investigate how the thin film optical properties, residual stresses, and structure depend on these parameters. The results showed that the deposition rate had a significant influence on the increase of the refractive index of YSZ thin films while the oxygen partial pressure had less influence on it. In all samples, the tensile stress increased with the increasing of deposition rate and the decreasing of oxygen partial pressure. Meanwhile, all deposited films were poly-crystallizations, while crystallite size and preferential orientation of YSZ thin films changed as a function of deposition rate and oxygen partial pressure. The variations of the optical spectra and residual stress corresponded to the evolution of the film structures induced by the deposition parameters.     

Characteristics of tin oxide-based thin film transistors prepared by DC magnetron sputtering

Here we demonstrate the fabrication of SnO(x) thin-film transistors (TFTs), where SnO(x) thin films are deposited as an active channel layer by DC magnetron sputtering. We analyzed the effects of the oxygen partial pressure ratio and post-deposition heat treatment (PDHT) on the characteristics of the SnO(x) thin films. We found improved performance of the TFTs obtained by using interface modification with the optimized deposition condition of SnO(x) thin films. These results are helpful for fabricating oxide-TFTs, including simple binary oxide semiconductors, as an active channel layer.     

阴极电沉积高c轴取向ZnO薄膜及其光学性能的研究

以硝酸锌水溶液为电沉积液,采用两步法电沉积技术在ITO导电玻璃上电沉积出透明致密的ZnO薄膜.利用电势-pH计算和循环伏安实验得到适宜的溶液pH值和电沉积电势,采用X射线衍射、热重-差热、光致发光谱和透射光谱对ZnO薄膜的结构、组成和光学性能进行了表征.结果表明,所制备的ZnO薄膜在c轴方向上具有高度的择优取向性质,平均粒径为纳米级,性能稳定、单一,发光性能良好,可见光区透射率较高.此类产品在太阳能电池和光电子器件等光学材料方面具有很高的应用价值.     

Magnesium metallic interlayer as an oxygen-diffusion-barrier between high-κ dielectric thin films and silicon substrate

The deposition of a thin magnesium metallic interlayer on an Si substrate prior to the deposition of an oxide thin film using rf-sputtering was investigated. The deposition of high-κ HfO₂ thin film was more particularly studied and it was demonstrated that the metallic interlayer acts as an oxygen barrier, preventing the formation of a low-κ layer at the high-κ/Si interface during the deposition. A post-deposition annealing treatment performed on the films induced the diffusion of the metal barrier into the HfO₂ film and allowed obtaining a sharp interface. However, the degree of diffusion depends not only on the interlayer thickness, but also on the thickness of the high-κ film. X-ray photoelectron spectroscopy was used to study the degree of oxidation of the Mg interlayer. High resolution transmission electron microscopy and energy filtered transmission electron microscopy were used to characterize the films and the diffusion of the Mg interlayer into the high-κ film after annealing. In this work we will stress on the engineering of the interface via the diffusion of the Mg interlayer during the growth process and on annealing.     

Phase formation and morphology control of niobium oxide nanopillars

Nanopillar metal oxide thin films offer versatility as ultra high surface area supports and conductors. Metal oxide properties (e.g. stability, conductivity) can be tuned via phase and composition control to achieve desired application-specific functionality. Here we demonstrate phase control of high surface area thin films grown by glancing angle deposition and transformed to desired phases through high temperature annealing in a reducing environment. The post-annealed properties such as stoichiometry, phase, and morphology are shown to be largely dependent on initial film structure and hydrogen forming gas flow rate. Initially amorphous films of approximate stoichiometry Nb₂O₅ are transformed to NbO₂ or NbN_xO_1−x through annealing. Transformation to oxygen-deficient phases is more easily achieved for films of higher initial porosity. Higher forming gas flow rates result in both increased oxygen removal and significantly less physical degradation of nanostructures. A phase map is included as a guide to phase formation and morphology control in annealed nanopillar niobium oxide films.     

Correlation between film quality and photoluminescence in sputtered ZnO thin films

A red photoluminescence (PL) is observed in as-deposited ZnO thin films which are prepared by a conventional sputtering method. The PL intensity strongly depends on sputtering deposition conditions. In as-deposited and annealed films, it is found that there exists a correlation between the red PL intensity and the film quality. The origin of the red PL is attributed to the native defect induced during the sputtering deposition process. It is shown that the red PL measurement is effective as an evaluation method of the film quality.     

退火气氛对SnO2薄膜结构与成分的影响

研究了低氧分压反应溅射法生长的SnO2薄膜于600℃ O2气氛和Ar气氛退火处理后的表面形貌、晶体结构以及表面成分,发现经氧化性气氛退火处理的SnO2为具有金红石结构的表面多孔薄膜.相比较而言,Ar气氛退火处理后的SnO2薄膜表面较为致密,相结构包含朱氧化完全的β-Sn.XPS分析进一步表明,氧化性气氛退火形成单一组分的SnO2,而惰性气氛退火后薄膜表面含有Sn、SnO和SnO2.另外,研究还发现薄膜表面吸附氧形态与退火气氛有关.     

Synthetics of ZnO nanostructures by thermal oxidation in water vapor containing environments

Metallic Zn films deposited on glass were wet or dry oxidized at 390 °C in pure N₂ or O₂ to understand the effects of water vapor in different oxygen partial pressure on growth of ZnO nanostructure during thermal oxidation. As-prepared ZnO oxides were characterized by a scanning electron microscope (SEM), an X-ray diffractometer (XRD), and a transmission electron microscope (TEM). Optical and electric properties of these ZnO films were characterized by photoluminescence (PL) and resistance measurements, respectively. It was found that the oxygen partial pressure and water vapor of environment significantly affect the morphologies of ZnO nanostructures. Decreasing oxygen partial pressure in dry oxidation can enhance a green light peak at 500 nm on PL spectra arising from defect-related emission and reduce the resistivity of the oxide films. High H₂O^(g)/O₂ ratio in wet oxidation will significantly increase the intensity of a green light peak and reduce the resistivity of the oxide films. The effect of oxygen partial pressure and H₂O^(g)/O₂ ratio on the PL spectra and resistivity of ZnO films are explained by the theory of defects equilibrium during oxidation.     

Luminescent properties of SrZnO2:Tb3+ hybrid thin film phosphors grown by pulsed laser deposition

Novel green-light-emitting Tb(3+)-doped SrZnO2 phosphor thin films were grown via the pulsed-laser-deposition technique. The films were grown at various substrate temperatures and oxygen pressures. The crystallinity and surface morphology of the films were investigated via X-ray diffraction (XRD) and atomic-force microscopy (AFM), respectively. The luminescence properties were analyzed by measuring the excitation and photoluminescence spectra. The thin films showed a green emission radiated by the transitions from the 5D4 excited states to the 7FJ (J = 3-6) ground states of the Tb3+ ions found under ultraviolet excitation with a 272 nm wavelength. The crystallinity, surface morphology, and photoluminescence spectra of thin-film phosphors were found to be highly dependent on the deposition conditions, particularly the substrate temperature and oxygen pressure. The surface roughness and photoluminescence intensity of the films showed similar behaviors as a function of the substrate temperature and oxygen pressure.     

Influence of deposition temperature on structure and morphology of nanostructured SnO2 films synthesized by pulsed laser deposition

Nanostructured tin oxide thin films were deposited on the Si (100) substrate using the pulsed laser deposition technique at different substrate temperatures (300, 450 and 600 °C) in an oxygen atmosphere. The structure and morphology of the as-deposited films indicate that the film crystallinity and surface topography are influenced by the deposition temperature by changing from an almost amorphous to crystalline microstructure and smoother topography at a higher substrate temperature. The photoluminescence measurement of the SnO₂ films shows three stable emission peaks centered at respective wavelengths of 591, 554 and 560 nm with increasing deposition temperature, contributed by the oxygen vacancies.     

Epitaxial EuO thin films by pulsed laser deposition monitored by in situ x-ray photoelectron spectroscopy

We have grown EuO thin films on silicon [001] and yttrium aluminate [110] from a europium metal target using pulsed laser deposition. In situ x-ray photoelectron spectroscopy has been used to determine the parameter window for stoichiometric EuO deposition. EuO is observed to grow in the relatively high pressure regime of 10⁻⁶-10⁻⁵ mbar, due to the large Eu flux during ablation. EuO is proven to grow epitaxially on yttrium aluminate [110]. Magnetization measurements confirm the stoichiometry of the film.