Growth of fractal patterns in ZnO films doped with Fe

ZnO films both undoped and doped with Fe were deposited on Si substrates using rf-magnetron sputtering. The results showed that fractal features were clearly exhibited in the ZnO film doped with Fe. It is proposed that the fractal aggregates were the result of cluster diffusion-limited aggregation (CDLA) of magnetic particles on the surface of the film. The fractal dimension of a main branch (D=1.47) was smaller than that expected by the CDLA model (D=1.72). In this paper the growth mechanism of the observed fractal aggregates is discussed in terms of the magnetism of FeO, nanoparticle aggregation and surface tension changes.     

Structural and morphological properties of ZnO:Ga thin films

Zinc oxide (ZnO) is a promising semiconductor material with a great variety of applications. Compared to undoped ZnO, impurity-doped ZnO has a lower resistivity and better stability. With this aim, Ga has been proposed as a dopant. In this study, the structural characteristics and surface morphology of ZnO films produced by PEMOCVD at a substrate temperature of 250 °C on the c-plane (001) of sapphire were investigated. Doping was realized with 1, 3, 5 and 10 wt.% of Ga₂(AA)₃ in the precursor's mixture. At lower contents, Ga stimulates growth of (002) oriented textured films and the smallest FWHM was obtained as low as 0.17° for ZnO:Ga with 1 wt.%. A change in preferential orientation as well as surface smoothing and roughness decreasing of the films were observed with further increasing Ga content in precursor's mixture. We assume a key role of Ga and note that such a feature would be beneficial for the application of ZnO thin films for formation of abrupt junctions in p-n device structures.     

Preparation of Li and Er codoped ZnO thin films and their photoluminescence

Li-Er codoped ZnO thin films have been prepared on Si(100) substrates by pulsed laser deposition (PLD). Both the as-grown and post-annealed films exhibit good crystalline quality with preferred c-axis orientation. After post-annealing at 850 °C, the photoluminescence (PL) related to intra-4f shell of Er³⁺ can be clearly observed. The Li-Er codoped ZnO film shows higher intensity of PL around 1.54 μm than the Er monodoped ZnO film. The behavior is attributed to the lowering of the symmetry of the crystal field around Er³⁺ ions by introducing Li⁺ into ZnO lattice, which is also confirmed by Raman scattering spectra.     

阴极电沉积法同时制备TiO2薄膜和颗粒

采用阴极电沉积法电解钛酸四丁酯同时制备TiO2膜和颗粒,并讨论沉积条件对产物的影响。pH值是控制产物的主要参数。对比在紫外灯光照下不同产物不同时间下的光催化性能,比较其差异得出,酸性在pH值为1-2(加浓硫酸2-3mL)时的产物效果最好,光催化性能最好。并横向比较了加入硫酸体积相等的条件下,颗粒和薄膜的光催化效率。     

One- and two-phonon Raman scattering from hydrogenated nanocrystalline silicon films

One- and two-phonon Raman scattering from intrinsic and boron as well as phosphorus doped hydrogenated nanocrystalline silicon films prepared by plasma enhanced chemical vapor deposition technique were investigated. With regard to one-phonon Raman measurements of intrinsic films, redshifts attending by asymmetrical broadening of one-phonon transverse optical (TO) mode with diminishing mean dimension of Si nanocrystals can be ascribed to incorporating effects of phonon confinement and tensile strain. Photoluminescence behavior of these intrinsic specimens can be interpreted by a consistent way when recombination of quantum-confined excitons in Si nanocrystals is assumed. As to one-phonon Raman signals of doped nc-Si:H materials, besides joint effects of phonon confinement and tensile strain, additional redshifts accompanying with asymmetrical broadening of one-phonon TO band with increasing doping level can be assigned to carrier effect and disorder from doping. With diminishing average size of Si nanocrystals or increasing dopants, the decay of two-phonon Raman amplitudes of intrinsic and doped samples can be attributed to disorder. Raman experiments indicate that all the energies of the two-phonon TO branches are different from twice the energies of the one-phonon TO active bands, which reveal that the two-phonon TO modes do not come from the Raman active phonons at wavevector k=0. The peak shift of two-phonon transverse optical (2TO) modes relates to phonon confinement and disorder. Negligible peak shift in TO (2TO) modes of intrinsic and doped films on temperature indicates that the interface strain in nc-Si:H/c-Si can be ignored.     

Correlation between structural and electrical properties of ZnO thin films

Thin ZnO films were deposited by radio frequency (r.f.) and direct current (d.c.) magnetron sputtering techniques onto glass substrates. Microstructural and electrical properties of ZnO films were studied using X-ray diffractometer (XRD), scanning electron microscope (SEM) and resistivity measurements. It was found that the size of the crystallites in the d.c. deposited films increased with increasing film thickness, while the crystallite size of r.f. deposited films remained unchanged. The d.c. deposited grains also had much stronger orientation related to the substrate than the r.f. films. XRD data indicated that the thin films with d<350 nm for r.f. and <750 nm for d.c. films have a very high degree of ZnO nonstoichiometry. This agreed well with the conductivity measurements and R(T) behaviour of the films with different resistance R. It was also found that the electrical resistivity of the samples increased exponentially with the thickness of films.     

Improved efficiency of the chemical bath deposition method during growth of ZnO thin films

Chemical bath deposition (CBD) is an inexpensive and low temperature method (25-90 °C) that allows to deposit large area semiconductor thin films. However, the extent of the desired heterogeneous reaction upon the substrate surface is limited first by the competing homogeneous reaction, which is responsible for colloidal particles formation in the bulk solution, and second, by the material deposition on the CBD reactor walls. Therefore, the CBD method exhibits low efficiency in terms of profiting the whole amount of starting materials. The present work describes a procedure to deposit ZnO thin films by CBD in an efficient way, since it offers the possibility to minimize both the undesirable homogeneous reaction in the bulk solution and the material deposition on the CBD reactor walls. In a first stage, zinc peroxide (ZnO₂) crystallizing with cubic structure is obtained. This compound shows a good average transparency (90%) and an optical bandgap of 4.2 eV. After an annealing process, the ZnO₂ suffers a transformation toward polycrystalline ZnO with hexagonal structure and 3.25 eV of optical bandgap. The surface morphology of the films, analyzed by atomic force microscope (AFM), reveals three-dimensional growth features as well as no colloidal particles upon the surface, therefore indicating the predominance of the heterogeneous reaction during the growth.     

X-ray reflectometry analyses of chromium thin films

A series of thin (less than 100 nm) chromium films on Si and SiO₂/Si substrates has been examined using X-ray reflectometry (XRR) and cross-section scanning electron microscopy. Comparisons of the measured film thicknesses from the two disparate methods were in good agreement. Because of this accord, it was possible to use these chromium thin films as model systems in order to probe the impact of both random and systematic errors on quantitative XRR measurements. In general, the errors associated with common XRR operations such as sample placement, system alignment, and choice of an instrumental broadening characteristic were much smaller than the statistical error obtained from a genetic algorithm fitting process to the experimental XRR curve.     

Cathodic electrodeposition of Ag-doped manganese dioxide films for electrodes of electrochemical supercapacitors

Cathodic electrodeposition method has been developed for the fabrication of Ag-doped MnO₂ films from the KMnO₄ aqueous solutions containing AgNO₃ for the application in electrodes of electrochemical supercapacitors (ES). The films were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), cyclic voltammetry (CV) and impedance spectroscopy. The Ag-doped MnO₂ films showed improved capacitive behaviour and lower electrical resistance compared to pure MnO₂ films. The highest specific capacitance (SC) of 770 F g^− 1 was obtained at a scan rate of 2 mV s^− 1 in the 0.5 M Na₂SO₄ electrolyte.     

Improved electrochromical properties of sol-gel WO3 thin films by doping gold nanocrystals

In this investigation, the effect of gold nanocrystals on the electrochromical properties of sol-gel Au doped WO₃ thin films has been studied. The Au-WO₃ thin films were dip-coated on both glass and indium tin oxide coated conducting glass substrates with various gold concentrations of 0, 3.2 and 6.4 mol%. Optical properties of the samples were studied by UV-visible spectrophotometry in a range of 300-1100 nm. The optical density spectra of the films showed the formation of gold nanoparticles in the films. The optical bandgap energy of Au-WO₃ films decreased with increasing the Au concentration. Crystalline structure of the doped films was investigated by X-ray diffractometry, which indicated formation of gold nanocrystals in amorphous WO₃ thin films. X-ray photoelectron spectroscopy (XPS) was used to study the surface chemical composition of the samples. XPS analysis indicated the presence of gold in metallic state and the formation of stoichiometric WO₃. The electrochromic properties of the Au-WO₃ samples were also characterized using lithium-based electrolyte. It was found that doping of Au nanocrystals in WO₃ thin films improved the coloration time of the layer. In addition, it was shown that variation of Au concentration led to color change in the colored state of the Au-WO₃ thin films.     

Effect of carbon additive on microstructure evolution and magnetic properties of epitaxial FePt (001) thin films

FePt:C thin films were deposited on CrRu underlayers by DC magnetron co-sputtering. The effects of C content, FePt:C film thickness and substrate temperature on the microstructural and magnetic properties of the epitaxial FePt (001) films were studied. Experimental results showed that even with 30 vol.% C doping, the FePt films could keep a (001) preferred orientation at 350 °C. When a FePt:C film was very thin (< 5 nm), the film had a continuous microstructure instead of a granual structure with C diffused onto the film surface. With further increased film thickness, the film started to nucleate and formed a column microstructure over continuous FePt films. A strong exchange coupling in the FePt:C films was believed to be due to the presence of a thin continuous FePt layer attributed to the carbon diffusion during the initial stage of the FePt:C film growth. Despite the presence of a strong exchange coupling in the FePt:C (20 vol.% C) film, the SNR ratio of the FePt:C media was about 10 dB better than that of the pure FePt media. The epitaxial growth of the FePt:C films on the Pt layers was observed from high resolution TEM cross sectional images even for the films grown at about 200 °C. The TEM images did not show an obvious change in the morphology of the FePt:C films deposited at different temperatures (from 200 °C to 350 °C), though the ordering degree and coercivity of the films increased with increased substrate temperature.     

Dielectric properties investigation of Cu2O/ZnO heterojunction thin films by electrodeposition

Structures and morphologies of the Cu₂O/ZnO heterojunction electrodeposited on indium tin oxide (ITO) flexible substrate (polyethylene terephthalate-PET) were investigated by X-ray diffraction (XRD), scanning electronic microscopy (SEM), high resolution transmission electron microscopy (HRTEM), respectively. The dielectric response of bottom-up self-assembly Cu₂O/ZnO heterojunction was investigated. The low frequency dielectric dispersion (LFDD) was observed. The universal dielectric response (UDR) was used to investigate the frequency dependence of dielectric response for Cu₂O/ZnO heterojunction, which was attributed to the long range and the short range hopping charge carriers at the low frequency and the high frequency region, respectively.     

Effect of Al concentrations on the electrodeposition and properties of transparent Al-doped ZnO thin films

Al-doped zinc oxide (AZO) thin films are prepared on polycrystalline fluorine-doped tin oxide-coated conducting glass substrates from nitrates baths by the electrodeposition process at 70 °C. The electrochemical, morphological, structural and optical properties of the AZO thin films were investigated in terms of different Al concentration in the starting solution. It was found that the carrier density of AZO thin films varied between ?3.11 and ?5.56 × 10²⁰ cm^?3 when the Al concentration was between 0 and 5 at.%. Atomic force microscopy images reveal that the concentration of Al has a very significant influence on the surface morphology and roughness of thin AZO. X-ray diffraction spectra demonstrate preferential (002) crystallographic orientation having c-axis perpendicular to the surface of the substrate and average crystallites size of the films was about 33–54 nm. With increasing Al doping, AZO films have a strong improved crystalline quality. As compared to pure ZnO, Al-doped ZnO exhibited lower crystallinity and there is a shift in the (002) diffraction peak to higher angles. Due to the doping of Al of any concentration, the films were found to be showing >80 % transparency. As Al concentration increased the optical band gap was also found to be increase from 3.22 to 3.47 eV. The room-temperature photoluminescence spectra indicated that the introduction of Al can improve the intensity of ultraviolet (UV) emission, thus suggesting its greater prospects in UV optoelectronic devices. A detailed comparison and apprehension of electrochemical, optical and structural properties of ZnO and ZnO:Al thin films is done for the determination of optimum concentration of Al doping.     

Large-scale electrodeposition of ZnO thin films with novel petal-like architectures

Large-scale and dense ZnO thin films with novel petal-like architectures were directly electrodeposited on the Au/ITO glass substrates from aqueous solution of Zn(NO₃)₂ and HMTA at a low temperature of 70 °C for the first time. Scanning electron microscopic (SEM) investigation revealed that the Au/ITO glass substrates are fully covered with densely distributed ZnO petal-like architectures. X-ray powder diffraction (XRD) pattern indicated that as-prepared unique films are highly crystalline and wurtzite hexagonal phase with extremely preferred orientation along [0001] direction. The optical band gap energy was found to be 3.85 eV for ZnO film with petal-like architectures on the Au/ITO substrate. Moreover, a strong and sharp ultraviolet (UV) emission at 386 nm but very weak defect-related deep level emission (DLE) in the room temperature photoluminescence (PL) spectrum also indicated that as-grown films are of good crystal quality. The possible growth mechanism for the novel petal-like architectures suggested that both the inherent highly anisotropic structure of ZnO and large lattice mismatch between ZnO and Au played crucial roles in determining final surface microstructures of the products.     

Optical and structural properties of Mg doped ZnO thin films by chemical bath deposition method

The chemical bath deposition method has often been employed to successfully deposit pure and Mg doped ZnO thin films on a glass substrate. The impact of Mg creates a strained stress in ZnO films affecting its structural and optical properties. XRD patterns revealed that all thin films possess a polycrystalline hexagonal wurtzite structure and Mg doped ZnO thin films (002) plane peak position is shifted towards a lower angle due to Mg doping. From the SEM image, it is understood that the Mg doped ZnO thin films are uniformly coated and are seen as dense rods like pillers deposited over the film. The energy dispersive X-ray analysis confirmed the presence of Mg in doped ZnO thin films. The transmittance spectra exhibit that it is possible for Mg doping to enhance ZnO thin films. The optical energy gap of the films was assessed by applying Tauc’s law and it is observed to show an increasing tendency with an improvement in Mg doping concentrations. The optical constants such as reflectance, index of refraction, extinction coefficient and optical conductivity are determined by using transmission at normal incidence of light by using wavelength range of 200–800 nm. In PL spectra, the band edge emission shifted to the blue with increasing amount of Mg doping.     

Effective optical properties of non-absorbing nanoporous thin films

Numerous effective medium models have been proposed for the effective optical properties of nanoporous media. However, validations of these models against experimental data are often contradictory and inconclusive. This issue was numerically investigated by solving the two-dimensional Maxwell's equations in non-absorbing nanoporous thin films with various morphologies. It was found that below a certain critical film thickness, the effective index of refraction depends on the porosity and on the pore size, shape and spatial distribution. For thick enough films the effective index of refraction depends solely on porosity and on the indices of refraction of the two constitutive phases. The numerical results agree very well with a recent model obtained by applying the Volume Averaging Theory to the Maxwell's equations. However, commonly used models systematically and sometimes significantly underpredict the numerical results.     

氧化锌薄膜的p型掺杂及光学和电学性质研究

为了实现氧化锌薄膜的p型掺杂,从而制备氧化锌同质p-n结,采用化学气相沉积法,以二水合醋酸锌为前驱,醋酸铵为氮源,在氧气氛下制备了氮掺杂的p型氧化锌薄膜.利用X射线光电子能谱和X射线衍射分析,表征了氧化锌薄膜的化学成分.通过霍尔效应测量证实了薄膜的p型导电特性.探讨了制备过程中温度、压强、源物质等条件对反应产物的影响,研究了薄膜的光学和电学特性,得到了p型氧化锌薄膜的吸收光谱和光致发光光谱,以及氧化锌p-n结的伏安特性曲线.