A new one-dimensional tungsten carbide nanostructured material

A new medium specific surface area one-dimensional tungsten carbide nanostructure was obtained by the Shape Memory Synthesis method, in which the macrostructural features of a carbonaceous 1D-preformed template were maintained during the carburization of tungsten oxide and determined the resulting carbide morphology.     

Beam-supported AlN thin film bulk acoustic resonators

A novel, suspended thin film bulk acoustic wave resonator (SFBAR) has been fabricated from an aluminum nitride film sputtered directly on a (100) silicon substrate. The suspended membrane design uses thin beams to support, as well as electrically connect, the resonator and has been fabricated using both thin film processing and bulk silicon micromachining. The quality factor and the effective electromechanical coupling coefficient were characterized as a function of the number and the length of the support beams. The length of the support beams was found to affect neither the quality factor at resonance nor the effective electromechanical coupling factor. However, longer support beams did facilitate better frequency pair response. Device performance varied with the number of support beams: 70% of the resonators tested showed a higher figure of merit with eight support beams than with four support beams.     

R.F. magnetron sputtered tungsten carbide thin films

Thin films of tungsten carbide have been deposited on stainless steel substrates held at 500°C by r.f. reactive magnetron sputtering in two different modes of introducing argon and acetylene gases called normal and high rate mode. A single phase fcc-WC is formed in the normal mode whereas a mixture of A-15-W₃C, hexagonal-WC and graphitic- and diamond-carbon is found in the high rate mode. A microhardness value as high as 3200 kgf/mm² (as compared to the bulk value of 1800 kgf/mm²) is obtained in the film deposited by normal mode.     

Investigation of electrochromic properties of nanocrystalline tungsten oxide thin film

Thin films of tungsten oxide were grown by organometallic chemical vapor deposition (OMCVD) using tetra(allyl)tungsten, W(η³-C₃H₅)₄. X-Ray diffraction (XRD) analyses showed amorphous films at substrate temperatures (T_s) <350°C and polycrystalline films at T_s>350°C. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) revealed grain sizes in the range 20–40 nm. In situ electrochemical reduction of WO_3.2/ITO (2.0 M HCl) produced a faint blue color in less than 1 s. The maximum coloration efficiency (CE) was found to be 22 cm²/mC at 630 nm. The density of the films decreases from 4.53 to 4.29 g/cm³ after annealing. An optical bandgap (E_g) of ∼3.2 eV was estimated for both as-deposited and annealed films.     

Structural investigation by the Rietveld method of sputtered tungsten carbide thin films

Thin films of tungsten carbides deposited by reactive radio-frequency sputtering were investigated by X-ray diffraction using the Rietveld method. Two films were selected for the structural refinement. One was biased, the other unbiased. The unbiased film was found to consist of a cubic phase WC_1 − x (C_0.9) in the space group Fm3m with a lattice parameter of 4.263 (5) Å. A negative substrate bias of − 40 V leads to a multiphasic film: a cubic phase WC_1 − x with a lattice parameter of 4.301 (6) Å and a hexagonal phase W₂C (P-3m1) with lattice parameters of a = b = 2.787 (1) and c = 4.549 (2) Å. The domain size was found to be of ~ 5 nm. The coexistence of nanocrystalline phases WC_1 − x and W₂C is in accordance with the decrease of the carbon content (WC_0.7) in the biased film.     

Growth of germanium–carbide thin film on crystal substrate

The formation of germanium-carbide in crystalline germanium substrate is studied using the perturbed γ–γ angular correlation (PAC) method. The growth of Ge–C micro-crystalline system in the host matrix was observed after annealing the sample above 450°C in vacuum. The Ge–C complexes have been detected at high dose carbon implantation in germanium (≥1 × 10¹⁵cm ⁻²). Information about the lattice locations of the carbon atoms in the host lattice can be obtained via the interaction between carbon atoms with unstable probe nucleus (¹¹¹In). Several carbon related complexes have been detected in this investigations which can be characterized by unique quadrupole interaction frequencies. The parameters of the hyperfine interactions drawn from the time spectra provide additional information about the formation of Ge–C system in germanium.     

A thin film electrochromic display based on the tungsten bronzes

The main features of a passive thin film display cell based on the electrochemically reversible formation of a tungsten bronze according to the reaction

$\text{(colourless) WO}{}_3\text{+ x}\text{M}{}^{\mathrm{+}}\text{+ x}\text{e}{}^{\mathrm{?}}\text{?}\text{M}{}_{\mathrm{x}}\text{WO}{}_3\text{(blue)}$

where 0 < x < 1 are considered. Chemical analysis of an electrochemically coloured WO₃ film has confirmed the presence of M. It is shown that a critical requirement of these cells is that D_τ(qC_m/Q)² ≈ 1, where the symbols are, in order, the M⁺ diffusion coefficient, the required device response time, the electronic charge, the maximum practical volume concentration of M in the WO₃ film and lastly the area colouring charge. Typical energy requirements might be about 10 mJ cm^?2 per complete cycle in a favourable case.Ionic injection overpotentials and ionic diffusion both appear to play a significant role in determining cell currents. Preliminary diffusion coefficient results for Li⁺ in r.f. sputtered WO₃ films are reported, and their predicted dependence of film structure is discussed. The optical absorption of coloured WO₃ films is presented, and it is interpreted as being predominantly due to free-electron intraband transitions.     

Carbon self-diffusion in tungsten carbide

Dense, hot pressed tungsten carbide specimens were used to study the self-diffusion of ¹⁴C into WC in the temperature range 2238 to 2643 ° K. The necessity for extended diffusion anneal times was eliminated by using a submicron sectioning technique, and the diffusion penetration depth was determined by spectrophotometric W^V thiocyanate analysis of the sample sections. The existence of two clearly delineated diffusion mechanisms was demonstrated from the shape of the activity versus penetration curves. The first obeyed a bulk diffusion law, originated at the specimen surface, demonstrated anomalously low diffusion coefficients, dominated to a depth of about 0.5 m, and could be represented by the expression: D _vol=1.90×10^–6 exp–(88,000/RT).Autoradiography demonstrated that the second mechanism was grain-boundary diffusion which dominated at depths greater than 1 m. The Fisher grain-boundary diffusion analysis and the Suzuoka analysis gave apparent grain-boundary diffusion activation energies of 74 Kcal/mole and 71 Kcal/mole respectively. By using an estimated value of the bulk diffusion coefficient, the Suzuoka analysis permitted direct calculation of the grainboundary diffusion coefficient, and can be represented by: D _g.b.=4.57×10² exp–(71,000/RT).     

Deposition and optical studies of silicon carbide nitride thin films

Thin films of silicon carbide nitride (SiCN) have been prepared by reactive radioactive frequency (r.f.) sputtering using SiC target and nitrogen as the reactant gas. Deposition rates are studied as a function of deposition pressures and argon-nitrogen flow ratios. The optical absorption studies indicated the band edge shifting of the films when the nitrogen ratios are increased during deposition. Fourier transform infrared spectroscopy (FTIR) analysis on the films indicated several stretching modes corresponding to SiC, SiN and CN compositions.     

Probing the bulk ionic conductivity by thin film hetero-epitaxial engineering

Abstract

Highly textured thin films with small grain boundary regions can be used as model systems to directly measure the bulk conductivity of oxygen ion conducting oxides. Ionic conducting thin films and epitaxial heterostructures are also widely used to probe the effect of strain on the oxygen ion migration in oxide materials. For the purpose of these investigations a good lattice matching between the film and the substrate is required to promote the ordered film growth. Moreover, the substrate should be a good electrical insulator at high temperature to allow a reliable electrical characterization of the deposited film. Here we report the fabrication of an epitaxial heterostructure made with a double buffer layer of BaZrO₃ and SrTiO₃ grown on MgO substrates that fulfills both requirements. Based on such template platform, highly ordered (001) epitaxially oriented thin films of 15% Sm-doped CeO₂ and 8 mol% Y₂O₃ stabilized ZrO₂ are grown. Bulk conductivities as well as activation energies are measured for both materials, confirming the success of the approach. The reported insulating template platform promises potential application also for the electrical characterization of other novel electrolyte materials that still need a thorough understanding of their ionic conductivity.     

Probing the bulk ionic conductivity by thin film hetero-epitaxial engineering

Highly textured thin films with small grain boundary regions can be used as model systems to directly measure the bulk conductivity of oxygen ion conducting oxides. Ionic conducting thin films and epitaxial heterostructures are also widely used to probe the effect of strain on the oxygen ion migration in oxide materials. For the purpose of these investigations a good lattice matching between the film and the substrate is required to promote the ordered film growth. Moreover, the substrate should be a good electrical insulator at high temperature to allow a reliable electrical characterization of the deposited film. Here we report the fabrication of an epitaxial heterostructure made with a double buffer layer of BaZrO₃ and SrTiO₃ grown on MgO substrates that fulfills both requirements. Based on such template platform, highly ordered (001) epitaxially oriented thin films of 15% Sm-doped CeO₂ and 8 mol% Y₂O₃ stabilized ZrO₂ are grown. Bulk conductivities as well as activation energies are measured for both materials, confirming the success of the approach. The reported insulating template platform promises potential application also for the electrical characterization of other novel electrolyte materials that still need a thorough understanding of their ionic conductivity.     

Simultaneous carbon and tungsten thin film deposition using two thermionic vacuum arcs

Carbon and tungsten films were deposited sequentially and simultaneously from two separate plasmas produced by the thermionic vacuum arc (TVA) method. Total film thickness and atomic concentration of C and W were measured for different substrate positions and arc parameters and a comparison between sequential and simultaneous deposition of carbon and tungsten is presented. For the simultaneous case, while keeping the same discharge parameters as in the sequential case, the carbon composition percentage is enhanced despite the presence of the tungsten plasma. When only the carbon deposition rate is increased 10 times, the tungsten atomic concentration drops to a lower level, while the content of carbon becomes dominant for the substrates positioned near the carbon anode.     

Nanoindentation studies of thin film coated systems

In this paper we report the results of ultralow load indentation (“nanoindentation”) tests carried out on a range of thin film coated systems. In addition to characterizing the properties of each system at high resolutions, a further principal aim has been to explore the extent to which it is possible to generalize the nanoindentation response of the various systems in terms of the relative mechanical properties of the coating and the substrate. In this case we classify the substrate and coating in terms of the relative elastic and plastic responses (i.e. the ratio of yield stress to Young's modulus). Some of the different possible responses are shown together with the transitions in response (elasticity-dominated to plasticity-dominated and vice versa) which can occur. We also report on the various steps which can occur in loading curves and our attempts to relate them, via high resolution scanning electron microscopy, to the fracture patterns (through-thickness cracks and interfacial cracks) within and around the indentations.     

Structural investigation of bulk and thin film PLZT using X-ray absorption spectroscopy

Thin film electro-optic and non-linear optical materials are of interest for applications in high-speed integrated optical devices. Materials of the system Pb_1−x/100La_x/100(Zr_y/100 Ti_1−y/100)_1−x/400O₃ or PLZT x/y/(100−y) are attractive since they can be integrated into Si and GaAs substrates using suitable deposition techniques. In this investigation we examine the structural properties of r.f. magnetron sputter-deposited PLZT using X-ray absorption near-edge spectroscopy (XANES). For XANES analysis, four samples were selected: (1) a highly oriented PLZT 28/0/100 film of ≈ 4500 Å deposited on SiO₂; (2) a highly oriented PLZT 28/0/100 film of ≈ 4500 Å deposited on a 2 ωm SiO₂ buffer layer over a Si(100) substrate; (3) a highly oriented PLZT 28/0/100 film of ≈4500 Å deposited on Al₂O₃ (1 02); and (4) a commercial ceramic wafer of PLZT 9/65/35. The XANES experiments were performed at the Stanford Synchrotron Radiation Laboratory (SSRL) using electron yield and fluorescence techniques. Data was taken at the Ti K-edge (4966.4eV) and compared to reference spectra. Of the reference spectra, the Ti K-edge spectra of the PLZT most closely resemble perovskite (SrTiO₃). The surface and bulk thin film are similar and all the 28/0/100 spectra resemble the spectra of 9/65/35, indicating similar cubic perovskite structures for these materials.     

Determination of bulk and interface density of states in polycrystalline silicon thin film transistors

The aim of our investigation is to determine the bulk and interface density of states in excimer laser annealed polycrystalline silicon thin film transistors (polysilicon TFTs). The exponential energy distribution of the band tail states in the bulk of the polysilicon layer is obtained from analysis of the space charge limited current in n⁺-i-n⁺ structures. The density of traps at the gate oxide/polysilicon interface and the slope of the exponential band tail states in a thin layer adjacent to the channel/gate oxide interface are extracted from low-frequency noise measurements. The experimental results indicate that the degree of disorder is improved in the upper part of the polysilicon layer due to its columnar growth.     

Analysis of surface preparation treatments for coating tungsten carbide substrates with diamond thin films

The effect of various substrate surface treatments on (i) the pre-growth topography and composition of the treated substrate, and (ii) the quality of diamond thin films produced by hot-filament chemical vapour deposition on tungsten carbide substrates, is reported. Two different substrate grain sizes were subjected to various surface treatments. They were then examined for surface material composition and topography using scanning electron microscopy and X-ray diffraction (XRD). Subsequently, diamond films were deposited on the samples and their quality analysed by Raman spectroscopy and XRD techniques. These analyses do not indicate a strong dependence between the substrate grain size and film quality. However, the surface-treatment method affects the resulting substrate surface topography and film quality. This revised version was published online in November 2006 with corrections to the Cover Date.     

Characteristics of MgxZn1-xO thin film bulk acoustic wave devices

Piezoelectric thin film zinc oxide (ZnO) and its ternary alloy magnesium zinc oxide (Mg/sub x/Zn/sub 1-x/O) have broad applications in transducers, resonators, and filters. In this work, we present a new bulk acoustic wave (BAW) structure consisting of Al/Mg/sub x/Zn/sub 1-x/O/n/sup +/-ZnO/r-sapphire, where Al and n/sup +/ type ZnO serve as the top and bottom electrode, respectively. The epitaxial Mg/sub x/Zn/sub 1-x/O films have the same epitaxial relationships with the substrate as ZnO on r-Al/sub 2/O/sub 3/, resulting in the c-axis of the Mg/sub x/Zn/sub 1-x/O being in the growth plane. This relationship promotes shear bulk wave propagation that affords sensing in liquid phase media without the dampening effects found in longitudinal wave mode BAW devices. The BAW velocity and electromechanical coupling coefficient of Mg/sub x/Zn/sub 1-x/O can be tailored by varying the Mg composition, which provides an alternative and complementary method to adjust the BAW characteristics by changing the piezoelectric film thickness. This provides flexibility to design the operating frequencies of thin film bulk acoustic wave devices. Frequency responses of devices with two acoustic wave modes propagating in the specified structure are analyzed using a transmission line model. Measured results show good agreement with simulation.     

Plastic deformation mechanisms in tungsten carbide

Transmission electron microscopy studies of dislocations formed by plastic deformation in tungsten carbide have confirmed a slip deformation mechanism involving the {1 0 ¯1 0} 0 0 0 1 system. Direct visual evidence also confirmed the existence of extended dislocations formed by a suggested dissociation 1/31 1 ¯2 3=1/62 0 ¯2 3+1/60 2 ¯2 3 which can further interact with other extended dislocations on pairs of intersecting {1 1 ¯2 2} pyramidal planes. The proposed interaction is suggested as a means of reducing both dislocation strain energy and atomic misfit strains due to antiphase boundary formation and leads to a form of sessile dislocation arrangement similar to the Lomer Cottrell lock.     

硫化温度对硫化钨薄膜摩擦性能的影响

采用硫化法以3Cr13马氏体不锈钢为基片制备硫化钨薄膜,研究了硫化温度对薄膜性能的影响.结果表明,硫化温度对硫化钨薄膜的表面形貌和结晶率有明显影响,但对膜层的化学成分影响不大,薄膜能够有效改善不锈钢基体的摩擦学性能.随着硫化温度的升高,摩擦系数降低,测试环境和测试条件对薄膜的摩擦系数也有一定影响.