Physical properties of (La,Sr)Ti(O,N)3 thin films grown by pulsed laser deposition

Thin films of La_xSr_1−xTiO_3+x/2 (x = 0, 0.25, 0.5, 0.75, 1) were grown by laser ablation on two different kinds of substrates (SrTiO₃ (STO) and MgO) and were subsequently ammonolysed to yield the corresponding oxynitrides La_xSr_1−xTi(O,N)₃. For both substrates all films were found to grow epitaxially to the (1 0 0) direction of the cubic perovskite structure, except for x = 0.5 that grew parallel to the (1 1 0) direction. For some of the films TiN was detected as impurity phase. Scanning electron microscopy revealed that the films are dense and homogeneous with thicknesses around 350 nm. Atomic force microscopy showed that the surface roughness of the films varied between 4.2 and 14.1 nm. The employed substrate had a strong influence on the electrical properties. Films grown on STO exhibited a metallic behaviour, in contrast to the films grown on MgO, which were insulating.     

Fabrication of hollow VO2 microspheres by a facile template-free process

In the work, we report a facile template-free approach to prepare hollow VO₂ microspheres. Uniform hollow VO₂ assemblies composed of ordered microspheres with an average diameter of 3.0 µm and a hollow interior of 2.0 µm were prepared by calcining the precursor, which was synthesized via heat preservation process sealed in Teflon-lined stainless autoclave in the presence of urea. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scan electron microscopy (SEM) have been used to characterize VO₂ microspheres. The proposed mechanism for the formation of hollow VO₂ microspheres is discussed.     

Role of oxygen in enhancing N-type conductivity of CuInS2 thin films

Post-growth treatments in air atmosphere were performed on CuInS₂ films prepared by the single-source thermal evaporation method. Their effect on the structural, optical and electrical properties of the films was studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), optical reflection and transmission and resistance measurements. The films were annealed from 100 to 350 °C in air. The stability of the observed N-type conductivity after annealing depends strongly on the annealing temperature. Indeed it is shown that for annealing temperatures above 200 °C the N-type conductivity is stable. The resistance of the N-CuInS₂ thin films correlates well with the corresponding annealing temperature. The samples after annealing have direct bandgap energies of 1.45-1.50 eV.     

Synthesis of SrSnO3 thin films by pulsed laser deposition: Influence of substrate and deposition temperature

SrSnO₃ thin films were prepared by pulsed laser deposition on amorphous silica and single crystal substrates of R-sapphire, (100)LaAlO₃ and (100)SrTiO₃. High quality epitaxial (100) oriented films were obtained on LaAlO₃ and SrTiO₃ while a texture was revealed for films on sapphire deposited at the same deposition temperature of 700 °C. Amorphous films were obtained on silica but a post annealing at 800 °C induced crystallization with a random orientation. The screening of deposition temperature showed epitaxial features on SrTiO₃ from 650 °C while no crystallization was observed at 600 °C. The influence of substrate and deposition temperature was confirmed by Scanning Electron Microscopy and Atomic Force Microscopy observations.     

Rare-earth substituted HfO2 thin films grown by metalorganic chemical vapor deposition

Thin films of HfGdO_x and HfDyO_x were deposited by metalorganic chemical vapor deposition (MOCVD) utilizing guanidinate precursors for Hf, Gd and Dy. The close match in the thermal properties of the precursors enabled the MOCVD of rare-earth (RE) substituted HfO₂ over a wide temperature window. Film deposition was carried out in the temperature range 300-700 °C in the presence of oxygen on Si(100) substrates. HfGdO_x films were analyzed in detail for their structure, composition and morphology using X-ray diffraction, Rutherford backscattering spectrometry, proton induced X-ray emission, X-ray photoelectron spectroscopy and scanning electron microscopy. The electrical properties of HfGdO_x in terms of capacitance-voltage and current-voltage characteristics of metal-insulator-semiconductor device structures were evaluated.     

Electric characterization of HfO2 thin films prepared by chemical solution deposition

Hafnium dioxide (HfO₂) thin films were prepared on Si substrates using the chemical solution deposition (CSD) method. The Au/HfO₂/n-Si/Ag structures were characterized by X-ray diffraction (XRD), C–V curves and leakage current measurements. A relative dielectric constant of about 13.5 was obtained for the 65 nm HfO₂ film. Atomic force microscopy (AFM) measurements show uniform surfaces of the films. C–V hysteresis was found for the metal-oxide-semiconductor (MOS) structures with HfO₂ films of 52 and 65 nm thick. It is found that the width of C–V windows is related with the thickness of the HfO₂ films. Furthermore, the C–V hysteresis reveals the possibility of stress-effect, suggesting that it is possible to use HfO₂ to build an MOS structure with controllable C–V windows for memory devices. The leakage current decreases as the film thickness increases and a relatively low leakage current density has been achieved with the HfO₂ film of 65 nm.     

Electrical and optical characterization of Na: CuInS2 thin films grown by spray pyrolysis

The structural, electrical and optical properties of Na-doped CuInS₂ thin films grown by spray pyrolysis were studied. These films crystallized in the sphalerite structure of CuInS₂, and showed to contain traces of indium sulfide and CuIn₅S₈ as impurity phases. All films were In-rich and showed p-type conductivity. The film conductivity was strongly affected by Na-doping, which decreased from 10⁻² to 10⁻⁵ S/cm by increasing the [Na]/[Cu] ratio from 0.005 to 0.03 in the spray solution. The band gap energy was observed to increase, from 1.4 to 1.45 eV, with increasing the [Na]/[Cu] ratio. Our results suggested that Na could be an effective acceptor impurity in sprayed CuInS₂.     

Studying trivalent/bivalent metal ion doped TiO2 as p-TiO2 in bipolar heterojunction devices

Trivalent/bivalent metal ions doped TiO₂ thin films (M_xTi_1−xO₂, M = Cr³⁺, Fe³⁺, Ni²⁺, Co²⁺, Mn²⁺ and x = 0.01, 0.05, 0.1, 0.15, 0.2) were deposited on Indium–tin oxide (ITO) coated glass substrates by spin coating technique. X-ray photoelectron spectroscopy (XPS) showed Ti⁴⁺ oxidation state of the Ti2p band in the doped p-TiO₂. The homogenous M_xTi_1−xO₂ was used to support n-ZnO thin films with thickness ∼40–80 nm and vertically aligned n-ZnO nanorods (NR) with length ∼300 nm and 1.5 μm. Current (I)–voltage (V) characteristics for the Ag/n-ZnO/M_xTi_1−xO₂/ITO/glass assembly showed rectifying behavior with small turn-on voltages (V₀) < 1 V. The ideality factor (η) and the resistances in both forward and reverse bias were calculated. The temperature dependence performance of these bipolar devices was performed and variation of the parameters with temperature was studied.     

Characterization of ZnO-In2O3 transparent conducting films by pulsed laser deposition

Transparent conducting oxide (TCO) films in the ZnO-In₂O₃ system were prepared by a pulsed laser deposition method. A target that consists of the mixture of ZnO and In₂O₃ powders was used. Influences of the target composition x (x = [Zn]/([Zn] + [In])) and heater temperature on structural, electrical and optical properties of the TCO films were examined. Introduction of oxygen gas into the chamber during the deposition was necessary for improvement in the transparency of the deposited films. The amorphous phase was observed for a wide range of x = 0.20-0.60 at 110 °C. Minimum resistivity was 2.65 × 10⁻⁴ Ω cm at x = 0.20. The films that showed the minimum resistivity had an amorphous structure and the composition shifted toward larger x, as the substrate temperature increased. The films were enriched in indium compared to the target composition and the cationic In/Zn ratio increased as the substrate temperature was increased.     

SiO2/TiO2 multilayer films grown on cotton fibers surface at low temperature by a novel two-step process

A novel two-step process was developed to synthesize and deposit SiO₂/TiO₂ multilayer films onto the cotton fibers. In the first step, SiO₂ particles on cotton fiber surface were synthesized via tetraethoxysilane hydrolysis in the presence of cotton fibers, in order to protect the fibers against photo-catalytic decomposition by TiO₂ nanoparticles. In the second step, the growth of TiO₂ nanoparticles into the modified cotton fiber surface was carried out via a sol-gel method at the temperature as low as 100 °C. The as-obtained SiO₂/TiO₂ multilayer films coated on cotton fibers were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, atomic force microscopy and X-ray diffraction, respectively.     

Large third-order optical nonlinearity of SrBi2Nb2O9 thin films fabricated by pulsed laser deposition

SrBi₂Nb₂O₉ (SBN) thin films with a single phase of layered perovskite structure have been fabricated on fused quartz substrates at room temperature by pulsed laser deposition. The XRD and AFM analysis indicated that the films had better crystallinity, less rough surface morphology, and larger grain size with increasing oxygen pressure. The nonlinear optical properties of the samples were determined using a single beam Z-scan technique at a laser wavelength of 532 nm with laser duration of 25 ps. The real and imaginary parts of the third-order nonlinear optical susceptibility χ⁽³⁾ of the films were measured to be 3.18 × 10^− 8 esu and 5.94 × 10^− 9 esu, respectively.     

Phase transition behaviors and thermal conductivity measurements of nitrogen-doped Sb2Te3 thin films

Crystallization temperature of nitrogen-doped Sb₂Te₃ (ST) thin films increased with increasing nitrogen doping concentration, which indicates that the long-term stability of the metastable amorphous state can be improved by nitrogen doping. The root-mean-square (rms) roughness values of the films showed a significant decrease with nitrogen doping. Thermal conductivity of nitrogen-doped ST thin films was measured using a transient thermoreflectance (TTR) technique. It was found that the thermal conductivity decreased with increasing nitrogen doping concentration and increased with increasing annealing temperature. Nitrogen-doped ST thin films are suitable phase-change materials for low programming power consumption applications of phase-change random access memory (PCRAM).     

Sb2Te3-Ta2O5 nano-composite films for low-power phase-change memory application

Sb₂Te₃-Ta₂O₅ nano-composite films were deposited by the cosputtering of Sb₂Te₃ and Ta₂O₅ targets using radio frequency magnetron sputtering system at room temperature. A phase-change random access memory (PCRAM) device based on the Sb₂Te₃-Ta₂O₅ films was successfully fabricated. Compared to a pure Ge₂Sb₂Te₅ based PCRAM cell, the reset voltage of the Sb₂Te₃-Ta₂O₅ based cell was obviously reduced, which was attributed to the reduced thermal conductivity and lower melting point of the Sb₂Te₃-Ta₂O₅ films. In addition, the device with the Sb₂Te₃-Ta₂O₅ layer could work with much shorter pulse widths for both SET and RESET, suggesting that the Sb₂Te₃-Ta₂O₅ based compounds are promising candidates for low-power and fast-speed PCRAM application.     

Preparation of CuAlO2 and CuCrO2 thin films by sol-gel processing

CuAlO₂ and CuCrO₂ thin films were prepared by sol-gel processing and subsequent thermal treatment in air and inert gas atmosphere. Resistivities of 700 Ω cm and 60 Ω cm with optical transmissions of 65% and 32% were achieved respectively. The crystallization temperature of 700 °C allows the preparation of CuCrO₂ on borosilicate glass. P-type conductivity was verified by Seebeck measurements and a transparent heterostructure including p-CuCrO₂ showed rectifying behavior.     

Morphological and structural investigations of Co-MgF2 granular thin films grown by thermionic vacuum arc

Co-MgF₂ granular films were deposited in thermionic vacuum arc plasma with simultaneous ignition of plasma in Co and MgF₂ vapours. The samples were investigated by transmission electron microscopy which revealed Co grains of a few nm in diameter embedded in the MgF₂ matrix. The crystalline phase for Co and MgF₂ was studied by electron diffraction after selecting a typical area of the sample. Low-angle x-ray diffraction method was used to verify the bulk crystalline structure of the samples. The surface morphologies of the films were investigated by atomic force microscopy. The magneto-optical longitudinal Kerr rotation spectra of the films were also measured and compared.     

Growth and structure characterization of epitaxial Bi2Sr2Co2Oy thermoelectric thin films on LaAlO3 (001)

Epitaxial Bi₂Sr₂Co₂O_y thin films with excellent c-axis and ab-plane alignments have been grown on (001) LaAlO₃ substrates by chemical solution deposition using metal acetates as starting materials. Microstructure studies show that the resulting Bi₂Sr₂Co₂O_y films have a well-ordered layer structure with a flat and clear interface with the substrate. Scanning electron microscopy of the films reveals a step-terrace surface structure without any microcracks and pores. At room temperature, the epitaxial Bi₂Sr₂Co₂O_y films exhibit a resistivity of about 2 mΩ cm and a seebeck coefficient of about 115 μV/K comparable to those of single crystals.     

Pyrite (FeS2) thin films deposited by sol-gel method

Nanocrystalline pyrite (FeS₂) films were achieved by the sol-gel dip-coating process and sulfurization treatment. The microstructural, optical and electrical characteristics were investigated and the effect of sulfurization time on film properties was discussed. The XRD spectra show that FeS₂ film can be obtained for 1 h sulfurization and no other phase appears. The morphology of the precursor Fe₂O₃ films shows a porous and loose structure. However, with the sulfurization time increasing, the precursor films completely transformed into the pyrite films which have a compact and smooth structure. The pyrite films with a different sulfurization time have the optical absorption edges changed in the range of 0.90-0.99 eV. With the increase of sulfurization time, the carrier concentration increases and the carrier mobility decreases. It is speculated that crystallographic defects in the films could play an important role in film properties.     

Structure and optical properties of nanocrystalline BiFeO3 films prepared by chemical solution deposition

Bismuth ferric oxide (BiFeO₃) films grown on quartz substrate were prepared by a simple chemical solution deposition. Phase constitution characterization of the films were obtained by X-ray diffraction, surface morphology and transmittance of the films were studied by atomic force microscopy and ultraviolet-visible-infrared light spectrophotometer. The optical constants (refractive index n, extinction coefficient k) were calculated by straightforward method proposed by Swanepoel [Swanepoel R, J. Phys., E J. Sci. Instrum. 1983; 16: 1214-1222. [1]]. The influence of the processing parameters on microstructure and optical properties, especially band gap, were studied.     

A study of textured non-stoichiometric MoTe2 thin films used as substrates for textured stoichiometric MoS2 thin films

Textured MoTe₂ films have been prepared by sequential evaporation of the constituents followed by annealing under a tellurium pressure. The films are systematically textured with the c-axis of the crystallites perpendicular to the plane of substrate, however, the film composition is difficult to control and even after process optimization the films are tellurium deficient. This is thought to be caused by the electro negativity difference of the constituents.The textured MoTe₂ films have been used as substrates on which to grow MoS₂ films by annealing under a pressure of sulfur that allows textured MoS₂ films to be grown with good crystalline properties. The presence of sulfur at the surface and annealing under dynamic vacuum is important for this process and moreover, suppresses the superficial oxidation of the Mo and Te constituents.     

Structural, thermal and optical characterization of TiO2:ZrO2 thin films prepared by sol-gel method

We have studied the structural and optical properties of thin films of TiO₂, doped with 5% ZrO₂ and deposited on glass substrate (by the sol-gel method). The dip-coated thin films have been examined at different annealing temperatures (350 to 450 °C) and for various layer thicknesses (63-286 nm). Refractive index and porosity were calculated from the measured transmittance spectrum. The values of the index of refraction are in the range of 1.62-2.29 and the porosity is in the range of 0.21-0.70. The coefficient of transmission varies from 50 to 90%. In the case of the powder of TiO₂, doped with 5% ZrO₂, and aged for 3 months in ambient temperature, we have noticed the formation of the anatase phase (tetragonal structure with 14.8 nm grains). However, the undoped TiO₂ exhibits an amorphous phase. After heat treatments of thin films, titanium oxide starts to crystallize at the annealing temperature 350 °C. The obtained structures are anatase and brookite. The calculated grain size, depending on the annealing temperature and the layer thickness, is in the range (8.58-20.56 nm).