Chung Wo Ong

Reactive pulsed laser deposition of CNx films

Carbon nitride (CNx) films were prepared by reactive pulsed laser deposition at nitrogen partial pressure PN2varying from 0 to 300 mTorr. It is found that the atomic fraction of nitrogen f in the films first increases with increasing PN2, reaches a maximum of 0.32 at PN2=100 mTorr, and then decreases to a saturated value of 0.26 at PN2≳200 mTorr. Because of the absence of energetic particles in reactive pulsed laser deposition, the limited nitrogen content cannot be attributed to preferential sputtering of nitrogen that is generally observed in particle‐assisted deposition of CNx films. Infrared absorption experiments show the existence of C≡N bonds and graphitic sp2 bonds. The sp2 bonds become IR active because of symmetry breaking of graphitic rings as a consequence of nitrogen incorporation. CNx films deposited at low PN2 (e.g., 5 mTorr) are more graphitic than the diamondlike pure carbon sample deposited at PN2=0, so have a slightly narrower electron band gap Eopt and a significantly higher room‐tempe...

X-ray photoemission spectroscopy of nonmetallic materials: Electronic structures of boron and BxOy

Although an increasing volume of x-ray photoemission spectroscopic (XPS) data has been accumulated on boron and boron-rich compounds because of their unusual properties, including a unique three-center, two-electron bonding configuration, their common nonmetallic nature has been overlooked. Typically, the measured energy-state data are not clarified by surface Fermi level positions of these nonmetallic samples, which compromises the scientific contents of the data. In the present study, we revisited the XPS studies of sputter-cleaned β-rhombohedral boron (βr-B), the oxidized surface of βr-B, B6O pellet, and polished B2O3, to illustrate the impact and resolution of this scientific issue. These samples were chosen because βr-B is the most thermodynamically stable polytype of pure boron, B2O3 is its fully oxidized form, and B6O is the best known superhard family member of boron-rich compounds. From our XPS measurements, including those from a sputter-cleaned gold as a metal reference, we deduced that our βr-...

Effects of ion beam bombardment on electrochromic tungsten oxide films studied by X-ray photoelectron spectroscopy and Rutherford back-scattering

Abstract The effect of ion bombardment on thermally evaporated and magnetron sputtered tungsten oxide films were investigated using X-ray photoelectron spectroscopy (XPS). Results show that irrespective of the porosity and crystallinity of the film samples formed with different techniques and conditions, ion bombardment induced preferential sputtering of oxygen, resulting in a decrease of oxygen/tungsten (O/W) ratio with increasing sputtering time. Samples experienced electrochromic switching cycles also show the same effect, except that a higher O/W ratio is detected because the tungsten oxide film reacts with the LiClO 4 -propylene carbonate electrolyte. Angle-resolved XPS experiments further confirm preferential sputtering of oxygen, suggesting that ion beam sputtering used in XPS for pre-cleaning and depth profile analysis of tungsten oxide must be used with caution. Rutherford back-scattering gives more reliable composition data of tungsten oxide, since it does not involve any sputtering process.

Effects of substrate temperature on the structure and properties of reactive pulsed laser deposited CNx films

Abstract The effects of substrate temperature (25 ≤ Ts ≤ 437°C) on the structure and properties of reactive pulsed laser deposited (RPLD) carbon nitride (CNx) films prepared at a fixed nitrogen ambient pressure of 200 mTorr were investigated. The structure and composition of the films were characterized by X-ray diffraction, X-ray photoelectron spectroscopy and IR absorption, while the electrical and optical properties were studied by photothermal deflection spectroscopy, optical transmission and electrical conductivity measurements. The results show that the films contain graphitic rings, in which some of the carbon atoms are replaced by nitrogen atoms. The atomic fraction of nitrogen drops from 0.27 to 0.13 as Ts increases from 25 to 437°C, indicating that the incorporation of nitrogen is hindered by a rise in Ts. As a consequence, the optical band gap and the width of electron tail states become narrower, and the room temperature electrical conductivity increases greatly such that the properties of the films deposited at high temperature approach those of graphite.

Tensile strength of zinc oxide films measured by a microbridge method

Double-layered ZnO/silicon nitride microbridges were fabricated for microbridge tests. In a test, a load was applied to the center of the microbridge specimen by using a microwedge tip, where the displacement was recorded as a function of load until the specimen broke. The silicon nitride layer in the structure served to enhance the robustness of the specimen. By fitting the data to a theory, the elastic modulus, residual stress, and tensile strength of the ZnO film were found to be 137 ± 18 GPa, �0.041 ± 0.02 GPa, and 0.412 ± 0.05 GPa, respectively. The analysis required the elastic modulus, internal stress, and tensile strength of the silicon nitride layer. They were measured separately by microbridge tests on single-layered silicon nitride microbridges. The measured tensile strength of the ZnO films represents the maximum tolerable tensile stress that the films can sustain when they are used as the functional component in devices.

Methodology for the evaluation of yield strength and hardening behavior of metallic materials by indentation with spherical tip

This article presents a methodology for evaluating the yield strength and hardening behavior of metallic materials by spherical indentation. Two types of assumed material behaviors with a pure elastic-Hollomon’s power law hardening and a pure elastic-linear hardening were considered separately in the models of spherical indentation. The numerical relationships between the material properties and indentation responses were established on the basis of dimensional and finite element analysis. As the first approximation to the real plastic flow properties, the yield strengths and hardening behaviors determined from the spherical indentation loading curve and the numerical relationships were used to derive the intersecting points between Hollomon’s power law hardening curve and linear hardening line. Through proceeding the three parameter’s regression analysis with Swift’s power law function for the intersecting points determined at different maximum indentation depths, the final yield strength and hardening b...

Physical properties of dual ion beam deposited (B0.5−xSix)N0.5 films

(B0.5−xSix)N0.5 films (0⩽x⩽0.5) were prepared by dual ion beam deposition. Buffer layers were added to improve the film adhesion. The film structure was characterized by x-ray photoelectron spectroscopy, infrared absorption, and x-ray diffraction. The hardness and elastic modulus were measured by a nanoindenter. The I–V curves of the Ti/(B0.5−xSix)N0.5/buffer/p-Si/Ti diodes were investigated. The films are composites of cubic-boron nitride (c-BN), h-BN, and Si–N. When x=0, the film contains 70–75 vol % c-BN and has a hardness ≈38 GPa, but peels off quickly from the substrate after exposure to air. When x increases to 0.013, a small amount of Si–N phase is formed, which serves to release part of the internal stress without affecting the volume fraction of c-BN or the mechanical strength, and good adhesion is achieved. For higher Si content (0.013<x⩽0.067), the c-BN phase is disrupted with simultaneous replacement by h-BN. Rapid drops in the hardness and elastic modulus follow. When the Si content continues...

Structural studies of reactive pulsed laser-deposited CNx films by X-ray photoelectron spectroscopy and infrared absorption

The changes in the structure of reactive pulsed laser-deposited (RPLD) CNx films with nitrogen content from 3.6–22 at% have been investigated by X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and Fourier transform–infrared (FT–IR) absorption. The films were found to be amorphous, and to consist of a network of rings. The rings that were composed solely of carbon atoms gave rise to an XPS peak located between 284.3 and 284.8 eV (C1 component). The rings containing nitrogen led to another peak located between 285.5 and 286.4 eV (C2 component). When the nitrogen content increased, the relative intensity of the C1 component fell, while that of the C2 component rose, indicating that some carbon atoms in the rings were replaced by nitrogen atoms. C≡N bonds also contributed to the C2 component. The FT–IR data were consistent with this interpretation. No evidence for the existence of a β-C3N4 phase was found in RPLD CNx films.

Room-temperature resistive H2 sensing response of Pd/WO3 nanocluster-based highly porous film

Hydrogen- (H₂-) induced resistive response of palladium (Pd) coated tungsten oxide (WO₃) films prepared by using supersonic cluster beam deposition (SCBD) was investigated. An SCBD WO₃ film is found to be constructed of WO₃ nanoclusters of diameters of 3-5 nm. The nanoclusters are loosely connected to form a structure of high porosity around 66%. With this structure, the film exhibits many excellent room-temperature H₂ sensing properties, including high sensitivity, broad detectable range of H₂ concentration, low detection limit, fast response rate, excellent cyclic stability (>2400 cycles), high selectivity against vapor of many organic compounds, mild ambient pressure dependence and many other advantages such as low power consumption, miniaturizability and high batch-to-batch reproducibility. These findings are useful for making new high-quality H₂ sensors for monitoring the leakage of H₂ and ensuring safe use of this gas.

Thermal stability of pulsed laser deposited diamond-like carbon films

Abstract Pulsed laser deposited diamond-like cabron (PLD DLC) films were prepared with the visible 532 nm Nd:YAG pulsed laser beam of power density Φ from 2.7 × 109 to 2.0 × 1010W cm−2, below the Nagel criterion (5 × 1010W cm−2 for 1064 nm Nd:YAG laser). The temperature dependence of the electrical conductivity, σ(T), the imaginary part of the refractive index, k, and the surface morphology of the films were studied as functions of Φ and post-annealing temperature Ta. Particulates appear on the surface of all samples, with their size increasing with increasing φ. For Φ between 2.7 × 109 and 5.9 × 109W cm−2, the room temperature electrical conductivity σR and k decrease from 1.28 × 10−4 to 3.37 × 10−5 Ω−1 cm−1, and from 0.2 to 0.18, respectively. These results indicate that higher Φ increases the diamond-like content in the films. For Φ from 5.9 × 109 to 2.0 × 1010W cm−2, σR and k increase to reach 3.69 × 10−4 Ω−1 cm−1 and 0.22, respectively. In this power density range the surface graphitic particulates dominate the film properties. Higher Φ generates larger graphite particulates and degrades the film quality, thus leading to increases in σR and k. The as-deposited samples were annealed at temperatures Ta between 100 and 900 °C. In this temperature range, ΦR increases by a factor of 106, while k rises from about 0.2 to 0.5, indicating that a diamond-like-to-graphitic transition has occurred. The degradation process was observable at Ta as low as 100 °C, and became significant for higher Ta. Finally, the films reach a graphitic state when Ta ⩾ 750 °C. Since the surface density and size of the particulates are not influenced significantly by annealing, we suggest that the variation of the films properties arises from changes in the matrix in which the graphitic particulates are embedded.