Electron beam deposited VC and NbC thin films on titanium: Hardness and energy-dispersive X-ray diffraction study

Films of VC and NbC of about 200 nm thickness were electron beam deposited on the sandblasted surface of metallic Ti substrates, preheated at 350 and 500 °C, to improve the surface hardness of Ti implants intended for application in orthopaedics. According to both standard angular-dispersive X-ray diffraction measurements and rocking curve analysis performed by energy-dispersive X-ray diffraction, the films were found to be textured preferentially along the (200) crystallographic direction. The (200)-oriented crystallites are randomly rotated around their growth axes, with no correlation among adjacent domains. The measured intrinsic hardness of the films is 24-25 GPa for VC and 18-21 GPa for NbC.     

Electrochemical lithium storage performance of ternary Zn-Co-Ni alloy thin film as negative electrodes by electrodeposition method

Ternary Zn-Co-Ni alloy film electrode as an anode has been investigated, for the first time, for the purpose of electrochemical lithium storage in lithium-ion batteries. In this study, the ternary Zn-Co-Ni alloy film electrode is prepared by electroplating method. The electrodes were examined using X-ray diffraction (XRD), FE-SEM with EDX, and impedance studies. The electrochemical results demonstrate that the Zn-Co-Ni alloy film electrode delivers an initial discharge capacity of 281 mAh g⁻¹ and improves to 650 mAh g⁻¹ at the end of 30th cycling with no capacity fading at 0.1 C rate. The charge-discharge properties of the Zn-Co-Ni alloy film electrode are as follows: insertion capacity of 650 mAh g⁻¹ and delithiation capacity of 512 mAh g⁻¹ in the 30th cycling, coulombic efficiency of about 80.0% and good cycling behavior. The results suggest that the ternary Zn-Co-Ni alloy thin film electrode obtained via electroplating shows a good candidate anode material for lithium-ion batteries.     

Preparation and electrochemical study of cerium–silica sol–gel thin films

Design and development of suitable multilayered systems for delaying corrosion advance in metals requires that both the alteration mechanisms of the metal and the behaviour and properties of the protective coatings be known. Coatings prepared by the sol–gel method provide a good approach as protective layers on metallic surfaces. This kind of coatings can be prepared from pure chemical reagents at room temperature and atmospheric pressure, with compositions in a very wide range of environmentally non-aggressive precursors. Sol–gel coatings based on siloxane bonded units were prepared starting from an organic–inorganic hybrid system. The precursors were γ-methacryloxypropyltrimethoxysilane (MAP) and tetramethoxysilane (TMOS). Cerium nitrate hexahydrate in three different concentrations was added. Cerium salts may perform a similar protective effect to that carried out by the well-known lead oxides and chromium salts, even though in this case a negative environmental impact is not expected. Application of coatings upon pure zinc substrates and common glass slides were performed by spinning. Coated samples were heat treated at 40 °C for 6 days. Optical measurements (UV-Vis absorption and diffuse reflectance spectroscopies) pointed out that the coatings were colourless and transparent, reducing the diffuse reflectance of the metallic surface up to 60%. Optical and scanning electron microscopies (SEM) allowed observation of the texture and microstructure of the coated samples, both before and after the corrosion tests were carried out. Likewise, the remaining sols were kept to gelify at 60 °C for 4 days and then powdered to obtain suitable samples for analysing them by other characterisation techniques (Fourier transformed infrared, FTIR and differential thermal analysis, DTA). Electrochemical measurements were performed by impedance spectroscopy. This technique was used to clarify the anticorrosive protection role of cerium ions incorporated into the hybrid sol–gel network. The effect of cerium concentration on the impedance spectra was analysed, as well as the system behaviour against the corrosive medium (0.6 M NaCl aqueous solutions), as a function of exposure time. From the electrochemical point of view, the sol–gel films behave as a conversion coating on the metallic surface.     

Phase transformations after long-time annealing in metastable Fe–Cr alloy films prepared by pulsed laser deposition

Metastable Fe–Cr alloy films of various composition prepared by cross-beam pulsed laser deposition using two different procedures are investigated by wide-angle X-ray scattering. Depending on the Fe–Cr composition of the samples in an extended range, a body-centered cubic (bcc) phase or metastable phases with body-centered tetragonal (bct), face-centered orthorhombic (fco) or primitive orthorhombic (po) and primitive cubic (pc) lattices are formed in the films prepared by simultaneous co-deposition of Fe and Cr. In the films produced by layer-by-layer deposition of thin separate Fe and Cr layers (thickness of about 1 nm), only bcc and bct Fe–Cr phases were observed. A long-time annealing (50 h) at a temperature of 425 °C near the low-temperature existence limit of the σ-phase under equilibrium conditions followed by slow cooling (rate 0.5 °C/min) has been performed and various phase transformations were observed. In addition to known equilibrium and metastable Fe–Cr crystalline phases (mainly bcc and bct phases in the films prepared by layer-by-layer technique and bcc, bct and σ-FeCr phases in co-deposited films), a new metastable Fe–Cr superstructure characterised by a primitive tetragonal lattice with parameters a and c of about 0.57 and 0.63 nm, respectively, has been identified. It is shown that the formation of ″-crystallites with preferred orientation in the metastable Fe–Cr alloy films during dedicated long-time annealing gives rise to a spatially periodic modulation of chemical composition resulting in the formation of multilayers with periods of one or a few atomic monolayers of individual Fe and Cr components.     

Synthesis and structural characterization of undoped and Co doped zinc oxide thin films obtained by aerosol assisted chemical vapour deposition

Dilute magnetic oxides are transparent, wide-bandgap materials that behave ferromagnetically when doped with a few percent of a magnetic 3d cation. They have attracted a great deal of interest due to the integration of semiconducting and magnetic properties in a material, that is a prerequisite for successful fabrication of useful devices for the emerging technologies of spintronics. Here we report a study of growth characteristics and microstructural properties of undoped and Co doped ZnO films grown onto borosilicate glass substrates, using aerosol assisted chemical vapour deposition method. The obtained films are single phase, of Wurtzite type, some of them with a strong c-axis orientation, i.e. with the c-axis normal to the substrate surface.     

Nucleation and growth kinetics of co-deposited copper and selenium precursors to form metastable copper selenides

The nucleation and growth kinetics of binary copper-selenium compounds from co-deposited copper and selenium films as a function of annealing temperature and time was investigated. The thermally driven evolution of crystalline phases was followed using differential scanning calorimetry and X-ray diffraction. Below 60% selenium, hexagonal α-CuSe formed during the deposition and a reversible endothermic transition at ∼130 °C was observed for the phase transition into hexagonal γ-CuSe. Above 60% selenium the samples are amorphous as deposited and there is competition between the formation of γ-CuSe and cubic CuSe₂ as annealing temperature is increased. Slow rates of temperature increase favor the formation of CuSe₂ over γ-CuSe and near 66% selenium only cubic CuSe₂ forms during an exothermic event between 100 °C and 110 °C. It is surprising that the metastable cubic CuSe₂ initially nucleates and grows rather than the thermodynamically stable orthorhombic CuSe₂ polymorph. Kissinger analysis yields an activation energy for nucleation of 1.6 eV for cubic CuSe₂. CuSe nucleates throughout the composition region investigated. Hexagonal α-CuSe reacts with selenium to form the thermodynamically stable orthorhombic polymorph of CuSe₂ as the temperature approaches the melting point of selenium.     

Sensitivity to humidity of TiO2 thin films obtained by reactive magnetron sputtering

This study presents results on the humidity-sensing properties of titanium dioxide thin films measured by a quartz microbalance. A novel two-layer structure, consisting of a polymer sub-layer and a sensing titanium dioxide layer, was fabricated on a quartz resonator. The polymer sub-layer was synthesized by a plasma process from hexamethyldisiloxane to protect the resonator's surface during the deposition of the titanium dioxide film by magnetron sputtering. The TiO₂ films were characterized by X-ray diffraction and Auger Electron Spectroscopy. The film composition was determined to be close to that of stoichiometric TiO₂. The sensitivity to humidity varied from 5 Hz/%RH to 7 Hz/%RH for TiO₂ film thickness lying in the range of 18-70 nm. An increase of film thickness in this interval led to a slight decrease in sensitivity, which is explained by water sorption occurring principally at the surface of the titanium dioxide film and a change of the morphology to a higher surface smoothness for thicker films. It was found that 30-60 min of sorption time is necessary to completely eliminate hysteresis, which suggests that the process is reversible.These results are promising for the development of sensor devices for measuring the relative humidity of air.     

Converting polycrystals into single crystals – Selective grain growth by high-energy ion bombardment

Common failure mechanisms in microelectronics such as electromigration, creep and fatigue can be positively influenced by microstructure optimization. In this paper a new mechanism of microstructure optimization in thin metal films is proposed. Post-deposition ion bombardment can produce an in-plane texture in originally highly fiber textured thin metal films by a selective grain growth process. In extreme cases the in-plane texture becomes as sharp as the out-of-plane fiber texture. A subset of grains oriented for ion channeling was found to grow significantly at the expense of the remaining grain fraction. We studied the selective grain growth as a function of ion species (N⁺, Ne⁺, Ar⁺), ion energy (1–3.5 MeV) and target temperature (liquid nitrogen to 400 °C). In a textured thin film the degree of preferred in-plane orientation can be strongly influenced by ion bombardment, and therefore this technique has the potential to become a powerful tool for the enhancement of reliability in micro- and nanosystems.     

Post annealing effect on transport properties of La0.67Ca0.33MnO3 films grown on vicinal cut substrates

La_0.67Ca_0.33MnO₃ thin films have been grown on 10°, 15°, and 20° vicinal cut SrTiO₃ (1 0 0) substrates by pulse laser deposition. The single phase and the least textured growth have been studied by X-ray diffraction analysis. The post annealing effect with high temperature and high oxygen pressure on the transport properties of films has been investigated by resistance versus temperature measurements. Films with post annealing show large enhancement of metal-insulator transition temperature T_p about 20-30 K towards higher temperature and obvious decrease of resistance, which is attributed to the refilling of oxygen, the change of Mn-O-Mn angle and the improvement of crystallinity by the post annealing effect. Specially, film on 20° vicinal cut substrate exhibits the biggest range gap of peak resistance drop, which may originate from more defects caused by steps at this tilt angle and many of these defects are removed after post annealing.     

Stress and preferred orientation in nitride-based PVD coatings

Nitride-based coatings are nowadays widely studied both from fundamental and technological point of views due to their unique physical and mechanical properties. Among the binary nitrides, TiN is the most stable thermodynamically and has been widely used due to the combination of its covalent and metal-like characteristics. Coatings produced by Physical Vapor Deposition (PVD) techniques generally exhibit a crystallographic texture, which in turn may strongly affect their properties, such as hardness, wear resistance, or diffusion barrier properties in microelectronic devices. Therefore great efforts have been made in recent years to understand the underlying mechanisms governing texture development in nitride thin films. In particular, the issue of stress build-up during PVD growth and its possible interplay with film preferred orientation is essential to address.We present a brief overview of stress and preferred orientation in nitride-based thin films, either in the form of single-, multi-layered or nanocomposite coatings. X-ray Diffraction (XRD) was used in the standard θ-2θ configuration to study the texture development with film thickness, while the sin²ψ method combined with linear elasticity theory was employed to determine the complete strain/stress state. XRD measurements were made in the framework of the crystallite group method, which is of prime importance in thin films exhibiting a mixed texture, as it enables to selectively measure the elastic strain in a given subset of grains. For PVD films grown with energetic particles, the appropriate modeling requires the use of a triaxial stress tensor, including a hydrostatic stress component to take into account the local distortions induced by growth-defects. This approach enables us to determine the ‘stress-free and defect-free lattice parameter’, a₀, solely linked to chemical effect.Illustrations will be given for fiber-textured TiN and ZrN films deposited on Si substrates, epitaxial TiN layers as well as epitaxial TiN sub-layers in TiN/Cu multilayers grown on (001) MgO single crystal substrates. Ternary TiN-based coatings, either in the form of solid solutions or nanocomposites will be also investigated.     

Mechanical spectroscopy and structural evolution of CuMo thin films

CuMo thin films with a typical thickness of 200 nm have been prepared by Ion Beam Sputtering (IBS) on oxidised silicon (100) substrates. Two samples with symmetric (atomic) composition Cu30Mo70 and Cu70Mo30 where studied. ‘Direct’ observations of the microstructure were performed by X-ray diffraction. The samples have been characterised in their as-sputtered state, then after annealing at increasing temperature. On the other hand, a vibrating reed device specially adapted for thin adherent films has been used to determine the mechanical properties over a temperature range between 20°C and 500°C. In this paper, it is shown that the structural evolution in temperature highly depends on the sample composition. Particularly, in the Mo-rich specimen, two independent stages have been shown. Indeed, the segregation process is preceded by a structural relaxation phenomenon. We have also shown that internal friction experiments are quite useful in the study of these structural modifications occurring in thin films.     

X-ray study of tin oxide films obtained by reactive DC sputtering from a metallic tin target in pure oxygen plasma

Tin oxide films deposited on glass substrates were prepared using a metallic tin target in a DC-magnetron sputtering system. Pure oxygen (99.9%) was used as oxidant and plasma gas. The experiments were carried out at a fixed current mode of the power supply and the cathode voltage was measured during 15 min, the first 5 min of pre-sputtering and the last 10 were used for film growth. The deposition was carried out under different conditions of current, oxygen pressure and substrate temperature. The crystalline phases present in the films were identified with an X-ray diffractometer, equipped with a grazing incidence attachment. The angle of incidence was 3°. The preferred orientation degree in the tin oxide films and the relative cassiterite (SnO₂) proportion for each reflection were calculated from the deconvoluted X-ray peaks. It was found that the synthesis conditions have a strong influence over the growth texture and the relative cassiterite proportion over romarchite formation (SnO). To obtain tin oxide films with a high SnO₂ proportion and a high texture in reactive oxygen sputtering, were necessary, relatively high values of substrate temperature, current and oxygen pressure.     

Crystal structure, thermal expansion and electrical properties of the new Al0.32ErGe2 compound

A new ternary compound Al_0.32ErGe₂ has been synthesized and studied from 298 K to 773 K using X-ray powder diffraction technique. Its structure has been determined at room temperature by Rietveld refinement of X-ray powder diffraction data. The ternary compound Al_0.32ErGe₂ crystallizes in the orthorhombic with the defect CeNiSi₂ structure type (space group Cmcm, a = 0.40701(2) nm, b = 1.60401(9) nm, c = 0.39240(2) nm, Z = 4 and D_calc = 8.326 g/cm³). The average thermal expansion coefficients , and of Al_0.32ErGe₂ are 1.72 × 10⁻⁵ K⁻¹, 1.11 × 10⁻⁵ K⁻¹ and 1.52 × 10⁻⁵ K⁻¹, respectively. The bulk thermal expansion coefficient is 4.35 × 10⁻⁵ K⁻¹. Electrical resistivity of Al_0.32ErGe₂ was measured between 5 K and 300 K.     

Brush plated ZnS films and their properties

Zinc sulphide thin films were deposited by the brush plating technique using AR grade zinc sulphate and sodium thiosulphate on titanium and conducting glass substrates at a current density of 80 mA cm⁻² and at different deposition temperatures in the range 30–80 °C. The films exhibited cubic structure. Band gap of the films were in the range of 3.79–3.93 eV. Auger spectra of the ZnS films deposited at different current densities indicated that the Zn/S ratio varies in the range of 1.02–1.04. Room temperature PL spectrum of the films deposited at 80 °C indicated two emission peaks at 420 and 480 nm for an excitation of 325 nm. Resistivity of the film varied from 200–769 Ω cm as the deposition temperature increased.     

Studies on structural and thermal expansion properties of Ho2−xLnxMo4O15 (Ln = Er, Sm and Ce) solid solutions

Three new series of Ho_2−xEr_xMo₄O₁₅ (x = 0.0–2.0), Ho_2−xSm_xMo₄O₁₅ (x = 0.0–0.6) and Ho_2−xCe_xMo₄O₁₅ (x = 0.0–0.25) solid solutions have been prepared successfully by solid-state reaction and studied by powder X-ray diffraction. All the XRD patterns of these molybdates can be indexed in monoclinic space group P2₁/c. Lattice parameters a, b and c of Ho_2−xLn_xMo₄O₁₅ decrease linearly with increasing erbium content and increase with increasing samarium or cerium content. Thermal expansion behaviors of Ho_2−xLn_xMo₄O₁₅ have been investigated in the 25–500 °C temperature range with high-temperature X-ray diffraction. The temperature dependence of Mo(2)–O14 interaction looks like to be responsible for their thermal expansion behaviors.     

Determination of the composition of InxGa1−xN from strain measurements

The use of incorrect GaN and InN anisotropic elastic constants and unstrained lattice constants, or the erroneous interpolation of InGaN elastic coefficients, have led to several errors in the calculations of compositions of ternary alloys based on strain measurements. To avoid this, statistically estimated elastic constants are used to calculate the correct coefficients at any composition assuming they follow Vegard’s law as happens for relaxed lattice constants. In consequence, a general equation to extract x from experimentally determined a and c cell parameters in biaxial strained wurtzite In_xGa_1−xN is proposed. The validity of this equation is confirmed: inputting structural parameters deduced from fine electron diffraction of non-phase-segregated epilayers (0.4 < x < 0.8) gives outputs that are in agreement with compositions directly measured by energy-dispersive X-ray analysis in a scanning transmission electron microscope; high-resolution X-ray diffraction analysis also supports these findings. The proposed elastic behavior of InGaN correlates well with other experiments in the literature.     

Effects of processing parameters on the properties of tantalum nitride thin films deposited by reactive sputtering

The effects of processing parameters on the properties of tantalum nitride thin films deposited by radio frequency reactive sputtering have been investigated. The influence of the N₂ partial and (Ar + N₂) total gas pressures as well as the sputtering power on the microstructure and electrical properties is reported. Rising the N₂ partial pressure, from 2 to 10.7%, induces a change in the composition of the δ-TaN phase, from TaN to TaN_1.13. This composition change is associated with a drastic increase of the electrical resistivity over a 7.3% N₂ partial pressure. The total gas pressure is revealed to strongly affect the film microstructure since a variation in both composition and grain size is observed when the gas pressure rises from 6.8 to 24.6 Pa. When the sputtering power varied between 50 and 110 W, an increase of the grain size related to a decrease of the electrical resistivity is observed.     

Physical properties of tin oxide thin films improved by vapour chopping

The vapour chopping technique has been successfully used to lower the ambient air ageing effect on the tin oxide thin films. The films were prepared by thermal oxidation (in air) of vacuum evaporated vapour chopped and nonchopped tin thin films. The films showed SnO and SnO₂ phases with tetragonal and orthorhombic structure. All the films showed increase in optical transmittance with increase in oxidation temperature and duration. The vapour chopped films showed higher refractive index and band gap than those of nonchopped films. The refractive index was found to increase with the thickness. Due to air ageing, the refractive index of both the films was found to increase. The ageing effect was found lower on the vapour chopped (0.008) than those on nonchopped (0.02) tin oxide thin films. These films can have potential use in optical waveguides.