Copper and iron based thin film nanocomposites prepared by radio-frequency sputtering. Part II: elaboration and characterization of oxide/oxide thin film nanocomposites using controlled ex-situ oxidation process

CuO/CuFe₂O₄ thin films were obtained on glass substrate by ex situ oxidation in air at 450 °C for 12 h from various starting metal/oxide nanocomposites by radio-frequency sputtering technique. The structure and microstructure of the films were examined using grazing incidence X-ray diffraction, Raman spectroscopy, scanning and transmission electron microscopies, X-ray photoelectron spectroscopy, and electron probe microanalysis. These studies reveal that a self-organized bi-layered microstructure with CuO (surface layer) and CuFe₂O₄ (heart layer) was systematically obtained. Due to the porosity of the upper layer formed during annealing, an increase in total thickness of the film was observed and is directly correlated to the oxidation of the metallic copper content initially present in the as-deposited sample. A self-organization in two stacked layers CuO/CuFe₂O₄ with various void fractions ranging from 0 to 41 % can be obtained by controlling the as-deposited elaboration step described in the part I of this paper. The highest porosities were observed for films deposited at low argon pressure and low target-to-substrate distance. Due to their specific self-organization in p- and n-type layers associated with their high porosity, such structured films exhibited the best electrical sensitivity to CO₂ gas sensing. The obtained results demonstrated the importance of microstructure control to improve the response of sensing layers.     

Elaboration and characterization of Fe1-xO thin films sputter deposited from magnetite target

Majority of the authors report elaboration of iron oxide thin films by reactive magnetron sputtering from an iron target with Ar-O₂ gas mixture. Instead of using the reactive sputtering of a metallic target we report here the preparation of Fe_1-xO thin films, directly sputtered from a magnetite target in a pure argon gas flow with a bias power applied. This oxide is generally obtained at very low partial oxygen pressure and high temperature. We showed that bias sputtering which can be controlled very easily can lead to reducing conditions during deposition of oxide thin film on simple glass substrates. The proportion of wustite was directly adjusted by modifying the power of the substrate polarization. Atomic force microscopy was used to observe these nanostructured layers. Mössbauer measurements and electrical properties versus bias polarization and annealing temperature are also reported.     

Optimization of a MoSe2 thin film deposition technique

MoSe₂ thin films deposited by DC diode sputtering have been investigated by scanning electron microscopy, electron microprobe analysis X ray analysis optical absorption and electron spectroscopy (XPS).It has been found that stoichiometric thin films are obtained after appropriate annealing for any kind of substrate. Thin films crystallize in the hexagonal structure The optical gaps and the chemical shifts of the XPS lines show that thin films are composed of good MoSe₂ crystallites     

Al-Mg-B thin films prepared by magnetron sputtering

Hard, nanocomposite aluminum magnesium boride thin films were prepared on Si (100) substrates with a three target magnetron sputtering system. The films were characterized by X-ray diffraction, atomic force microscope, electron micro-probe, Fourier transform infrared spectroscopy and nanoindentation. The results show that the maximum hardness of the as-deposited films is about 30.7 GPa and these films are all X-ray amorphous with smooth surfaces. The influences of substrate temperature and boron sputtering power on the quality of the films are discussed. From the results of this work, magnetron sputtering is a promising method to deposit Al-Mg-B thin films.     

Preparation of delafossite CuFeO2 thin films by rf-sputtering on conventional glass substrate

CuFeO₂ is a delafossite-type compound and is a well known p-type semiconductor. The growth of delafossite CuFeO₂ thin films on conventional glass substrate by radio-frequency sputtering is reported. The deposition, performed at room temperature leads to an amorphous phase with extremely low roughness and high density. The films consisted of a well crystallized delafossite CuFeO₂ after heat treatment at 450 °C in inert atmosphere. The electrical conductivity of the film was 1 mS/cm. The direct optical band gap was estimated to be 2 eV.     

Thin films preparation by rf-sputtering of copper/iron ceramic targets with Cu/Fe = 1: From nanocomposites to delafossite compounds

In the Cu-Fe-O phase diagram, delafossite CuFeO₂ is obtained for the Cu^I oxidation state and for the Cu/Fe = 1 ratio. By decreasing the oxygen content, copper/spinel oxide composite can be obtained because of the reduction and the disproponation of cuprous ions. Many physical properties as for instance, electrical, optical, catalytic properties can then be affected by the control of the oxygen stoichiometry.In rf-sputtering technique, the bombardment energies on the substrate can be controlled by the deposition conditions leading to different oxygen stoichiometry in the growing layers.By this technique, thin films have been prepared from two ceramic targets: CuFeO₂ and CuO + CuFe₂O₄. We thus synthesized either Cu⁰/Cu_xFe_1−xO₄ nanocomposites thin films with various Cu⁰ quantities or CuFeO₂-based thin films.Two-probes conductivity measurements were permitted to comparatively evaluate the Cu⁰ content, while optical microscopy evidenced a self-assembly phenomenon during thermal annealing.     

Effects of substrate temperature on the magnetic and electrical properties of cobalt ferrite/metal composite thin films

The magnetic and electrical properties of the cobalt ferrite/metal composite thin films, prepared by reactive sputtering, were studied as a function of substrate temperature. With increasing substrate temperature, the saturation magnetization of the thin films increased owing to precipitation of the Co_0.67Fe_0.33 phase. Also, the electrical resistivity of the thin films decreased. From Hall experiments, the decrease of electrical resistivity of the composite thin films was mainly attributed to the increase of electron concentration. The Seebeck coefficient measurement shows that the electrical conduction mechanism of the thin films containing 37.8 and 33.7 at % Co changes from p-type to n-type and that of the thin films containing 28.5 at % Co remains n-type with increasing substrate temperature. This might be attributed to the change in composition of the cobalt ferrite matrix to Fe-excess with precipitation of Co-rich Fe alloy. ©1999 Kluwer Academic Publishers     

RF magnetron sputtered TiNiCu shape memory alloy thin film

Shape memory alloys (SMAs) offer a unique combination of novel properties, such as shape memory effect, super-elasticity, biocompatibility and high damping capacity, and thin film SMAs have the potential to become a primary actuating mechanism for micro-actuators. In this study, TiNiCu films were successfully prepared by mix sputtering of a Ti₅₅Ni₄₅ target with a separated Cu target. Crystalline structure, residual stress and phase transformation properties of the TiNiCu films were investigated using X-ray diffraction (XRD), differential scanning calorimeter (DSC), and curvature measurement methods. Effects of the processing parameters on the film composition, phase transformation and shape-memory effects were analyzed. Results showed that films prepared at a high Ar gas pressure exhibited a columnar structure, while films deposited at a low Ar gas pressure showed smooth and featureless structure. Chemical composition of TiNiCu thin films was dependent on the DC power of copper target. DSC, XRD and curvature measurement revealed clearly the martensitic transformation of the deposited TiNiCu films. When the free-standing film was heated and cooled, a ‘two-way’ shape-memory effect can be clearly observed.     

CO<Subscript>2</Subscript> gas sensitivity of sputtered zinc oxide thin films

For the first time, sputtered zinc oxide (ZnO) thin films have been used as a CO₂ gas sensor. Zinc oxide thin films have been synthesized using reactive d.c. sputtering method for gas sensor applications, in the deposition temperature range from 130–153°C at a chamber pressure of 8·5 mbar for 18 h. Argon and oxygen gases were used as sputtering and reactive gases, respectively. ZnO phase could be crystallized using a pure metal target of zinc. The structure of the films determined by means of X-ray diffraction method indicates that the zinc oxide single phase can be fabricated in this substrate temperature range. The sensitivity of the film synthesized at substrate temperature of 130°C is 2·17 in the presence of CO₂ gas at a measuring temperature of 100°C.     

Single step deposition method for nearly stoichiometric CuInSe2 thin films

This paper reports the production of high quality copper indium diselenide thin films using pulsed DC magnetron sputtering from a powder target. As-grown thin films consisted of pin-hole free, densely packed grains. X-ray diffraction showed that films were highly orientated in the (112) and/or (204)/(220) direction with no secondary phases present. The most surprising and exciting outcome of this study was that the as-grown films were of near stoichiometric composition, almost independent of the composition of the starting material. No additional steps or substrate heating were necessary to incorporate selenium and create single phase CuInSe₂. Electrical properties obtained by hot point probe and four point probe gave values of low resistivity and showed that the films were all p-type. The physical and structural properties of these films were analyzed using X-ray diffraction, scanning electron microscopy and atomic force microscopy. Resistivity measurements were carried out using the four point probe and hot probe methods. The single step deposition process can cut down the cost of the complex multi step processes involved in the traditional vacuum based deposition techniques.     

Phase structure characteristics of r.f. reactively sputtered zirconia thin film

Zirconia thin films were deposited by an r.f. magnetron reactive sputtering method. Three kinds of target (pure Zr metal and compounds of Y-ZrO₂ and Mg-ZrO₂) were employed. The phase characteristics of the three series of films deposited with various oxygen partial pressures and substrate temperatures were investigated. It is found that only monoclonic phase was formed in the sputtered thin films when using Zr target, and both tetragonal and monoclinic phases were formed when using the compound targets.     

Fabrication of Ag/PVA nanocomposites and their potential applicability as dielectric layer in thin film capacitor

Over the past decade, polymer–metal nanocomposites have drawn a great deal of attention due to their extensive application in organic flexible devices. Here, we describe the synthesis and characterisation of silver–polyvinyl alcohol nanocomposite thin films and investigates the possibility of application of these composite films as dielectric layer in efficient capacitors. The material characterisation is done through UV–vis absorption, scanning electron microscopy and atomic force microscope. The dielectric property of the nanocomposite material is investigated with a LCR meter. The different fabricating parameters for generating homogeneous thin films are optimised in this investigation. The composite material thin films exhibit high capacitance density and low dielectric loss and hence they may be suitable ingredients for high capacitance capacitors.     

Investigation of Pb-Mg-Nb-O pyrochlore thin films for tunable material

PMN thin films have been investigated as a feasible material for tunable microwave applications. PMN thin films were deposited by RF magnetron sputtering from Pb₆MgNb₆O₂₂ ceramic target on platinized Si substrates. The crystallinity, thickness, surface morphology, dielectric property, and voltage tunable properties of thin films were investigated by means of X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), and an impedance analyzer. The influences of sputtering substrate temperature and post-annealing on the tunable dielectric properties of thin films were investigated. Increasing the sputtering substrate temperature and annealing temperature can significantly increase tunability; however, these relations had the limitation that overly annealing temperature degraded the tunability of the thin film. A tunability of 38% at 1100 kV/cm of dc bias field under 1 MHz was achieved for a sample sputtered at 550 °C and annealed at 700 °C.     

Epitaxial growth mechanism of La2Zr2O7 thin film on metal substrate

We have studied the growth behavior of pyrochlore structure La₂Zr₂O₇ (LZO) thin films with different thickness on biaxial texture NiW substrates prepared by metal?Corganic deposition (MOD) method. The structure and morphology of LZO films were investigated by x-ray diffraction (XRD) and scanning electron microscopy (SEM). Our results show that the surface morphology of LZO films varies with changes in the surface coverage ratio of the metal substrates after crystallization in argon?Chydrogen atmosphere. The degree of texture of the LZO films reaches a maximum when the film thickness increases to 37 nm, where the NiW substrate surface is just fully covered. The complete coverage of the substrate surface together with the enhanced texture suggests that there exists a possible connection between the surface morphology and the texture development of the films prepared by MOD. Information on the nucleation and growth of oxide thin films on metal substrates with biaxial texture is important for understanding the mechanism of buffer layer formation and also for optimizing the preparation conditions for high-temperature superconducting coated conductors.     

Preparation of nanocrystalline Cu<Subscript>2</Subscript>O thin film by pulsed laser deposition

In this paper, the synthesis of nanocrystalline copper oxides Cu₂O and CuO thin films on glass substrates using a pulsed 532 nm Nd:YAG laser is presented. Deposition of films is achieved at two different substrate temperatures. The influence of substrate temperature on the structural and optical properties of copper oxide films are discussed and analyzed. The X-ray diffraction (XRD) results show that the deposited films are crystalline in nature. Films prepared at 300 °C substrate temperature were Cu₂O and has (111) and (200) diffracted peaks, while films grown at 500 °C were CuO and has (111) and (020) planes. The morphology of deposited films were characterized by scanning electron microscope (SEM) and atomic force microscope (AFM). The optical energy gap of Cu₂O and CuO films have been determined and found to be 2.04 and 1.35 eV respectively.     

Metal properties and thin film oxidation

The dependence of thin film oxidation rates on the metal properties is discussed in terms of a surface state charge at the metal-oxide interface and a space charge layer in the growing oxide. The properties considered are the magnetic change at the Curie temperature, allotropic transformation and crystal orientation of the metal substrate. Experimental data on the direct logarithmic oxidation of iron, nickel, cobalt and copper forming p-type semiconducting oxides are analysed.     

Fabrication and characterization of Fe3O4 thin films deposited by reactive magnetron sputtering

Fe-O thin films with different atomic ratio of iron to oxygen were deposited on glass and thermally oxidized silicon substrates at temperatures of 300, 473 and 593 K, by reactive magnetron sputtering in Ar+O₂ atmosphere. The composition and structure of the thin films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electrical resistivity. It was found from XRD that with increasing the oxygen partial pressure in the working gas, the crystalline structure of the Fe-O films deposited at the substrate temperature of 473 K gradually changed from α-Fe, amorphous Fe-O, Fe₃O₄, γ-Fe₂O₃ to Fe_21.34O₃₂. The structure and chemical valence of the Fe₃O₄ films were analyzed by electron microscopy and XPS, respectively.     

Property variations of direct-current reactive magnetron sputtered copper oxide thin films deposited at different oxygen partial pressures

Cuprous oxide (Cu₂O) and cupric oxide (CuO) thin films were deposited on glass substrates at different oxygen partial pressures by direct-current reactive magnetron sputtering of pure copper target in a mixture of argon and oxygen gases. Oxygen partial pressure was found to be a crucial parameter in controlling the phases and, thus, the physical properties of the deposited copper oxide thin films. Single-phase Cu₂O thin films with cubic structure were obtained at low oxygen partial pressure between 0.147 Pa and 0.200 Pa while higher oxygen partial pressure promoted the formation of CuO thin films with base-centered monoclinic structure. Polycrystalline Cu₂O thin films deposited with oxygen partial pressure at 0.147 Pa possessed the lowest p-type resistivity of 1.76 Ω cm as well as an optical band gap of 2.01 eV. On the other hand, polycrystalline CuO thin films deposited with oxygen partial pressure at 0.320 Pa were also single phase but showed a n-type resistivity of 0.19 Ω cm along with an optical band gap of 1.58 eV.     

In-situ fabrication and performance optimization of CIGS thin film deposited by ion-beam sputtering without post-selenization

Cu(In, Ga)Se₂ thin films were fabricated by an in situ fabrication process of ion beam sputtering deposition without post-selenization. X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and four-point probe technique were used to study the microstructures, surface morphology, composition and electrical properties, respectively. The results show that the films grown above 400 °C are of chalcopyrite structure and Cu (In_0.7Ga_0.3) Se₂ thin film was obtained at annealing temperature of 550 °C. With the increase in annealing temperature, the resistivity of the films decreases while the component is nearly steady. The degree of crystallization of the thin films increased at first with increasing of the annealing time and then decreased with the annealing time increasing further. It was demonstrated that the structural and electrical properties of the films improved significantly with adding selenium amount by sputtering selenium.     

Correlation between optical, electrical and structural properties of vanadium dioxide thin films

VO₂ films have been prepared on normal microscope glass slides by reactive rf magnetron sputtering of vanadium target in a mixture of argon and oxygen. Optical properties of the films were investigated by the UV/Vis/NIR Perkin–Elmer Lamda 9. Transmission electron microscope and atomic force microscope were used to investigate the structure of the films. Correlation between structural and optical properties of VO₂ thin films is investigated with respect to the dependence of both to substrate temperature.