Applied Surface Science (v.214, #1-4)
Author Index (I-III).
Publisher's Note (vii).
Subject Index (IV-XIII).
Gallium nitride surface preparation optimised using in situ scanning tunnelling microscopy by R.A. Oliver; C. Nörenberg; M.G. Martin; A. Crossley; M.R. Castell; G.A.D. Briggs (1-10).
Effective preparation of clean GaN surfaces is vital for surface studies, and also has a key role to play in the development of nitride-based electronic devices, due to the importance of forming good metal–semiconductor contacts. We identify the effect of two common surface cleaning treatments (annealing in ammonia, and sputtering in nitrogen followed by in vacuo annealing) on surface reconstruction, morphology and stoichiometry, by employing a combination of in situ RHEED and scanning tunnelling microscopy (STM), and ex situ X-ray photoelectron spectroscopy (XPS). We show how the defect densities in as-treated GaN surfaces may be reduced by growing thin layers of GaN by low-pressure MBE, a technique compatible with most experimental surface science systems. By using a combination of in situ etching and low-pressure growth, we show that it is possible to obtain very similar surface structures to those obtained by full-scale MBE.
Keywords: Gallium nitride; Scanning tunnelling microscopy; Surface cleaning; Surface morphology; X-ray photoelectron spectroscopy;
Effects of post-deposition annealing on the mechanical and chemical properties of the Si3N4/NbN multilayer coatings by J.J. Jeong; C.M. Lee (11-19).
Multilayered thin films consisting of alternate layers of silicon nitride (Si3N4) and niobium nitride (NbN) have been prepared by a dc reactive sputtering technique in nitrogen and argon atmosphere using high purity Nb and Si targets for various flow ratios of N2/Ar. It has been found that the hardness of the multilayered system is higher than that of the constituent individual layers of equal thickness. Although a single layer of amorphous Si3N4 has higher hardness compared with a single layer of nanocrystalline NbN at all the deposition conditions used in this experiment, the hardness of the multilayer coatings consisting of consecutive Si3N4/NbN layers strongly follows the hardness variation of the polycrystalline NbN. When the multilayer coatings are subjected to post-deposition annealing at high temperatures, it has been found that both the hardness and the adhesion strength of the coating decrease with increasing annealing temperature. X-ray photoelectron spectroscopy (XPS) results reveal that oxidation of the coatings during annealing plays a crucial role behind such deterioration in mechanical properties. Further, it has been noted that NbN is a more oxidation resistant material than Si3N4. Therefore, it has been proposed that during preparation of multilayers with consecutive thin layers of NbN and Si3N4, the topmost layer should be made of NbN, instead of Si3N4, to prevent the oxygen diffusion from the top surface layer to the next layer underneath.
Keywords: Thin Films; Multilayers; Mechanical properties; Thermal effects;
Hydrogen uptake on film surfaces produced by a unique codeposition process by Kenneth V. Salazar; David W. Carroll; Mitchell Trkula; Cynthia W. Sandoval (20-26).
Hydrogen uptake on several different film surfaces has been achieved by deposition of a conventional hydrogen gettering system via a novel combination of physical vapor deposition (PVD) and chemical vapor deposition (CVD) processes. We decided to use a conventional hydrogen gettering system, developed by Smith and Schicker [J.R. Schicker, AS/KCD Project No. EPN-047620, May 1994], that uses an acetylenic organic compound mixed with carbon supported palladium metal. The organic material, 1,4-bis-(phenylethynyl) benzene (DEB), is mixed with palladium and carbon by employing conventional solid state ceramic preparative techniques. Our novel codeposition process combines PVD and CVD techniques for fabricating thin-film coatings of the palladium-catalyzed DEB hydrogen gettering system. Hydrogen uptake was confirmed by 1 H NMR and our novel process lends itself well to placing hydrogen getter onto complex shapes and substrates of various compositions.
Keywords: DEB; Codeposition; PVD; CVD; Hydrogen getter;
Spray pyrolytic deposition and characterization of lanthanum selenide (La2Se3) thin films by G.D. Bagde; S.D. Sartale; C.D. Lokhande (27-35).
The versatile spray pyrolysis technique was employed to prepare thin films of lanthanum selenide (La2Se3) on glass and fluorine doped tin oxide (FTO) coated glass substrates under optimized conditions. The deposition temperature was 250 °C. The X-ray studies reveal that the films are polycrystalline with single La2Se3 phase. The estimated optical band gap was found to be 2.6 eV. The dielectric properties such as dielectric constant and dielectric loss of the films deposited on FTO coated glass substrates were measured with FTO-La2Se3-Ag structure as a function of frequency and the results are reported. At room temperature dielectric constant and dielectric loss for 1 kHz frequency were found to be 6.2 and 0.048, respectively. The room temperature electrical resistivity was of the order of 105 Ω cm. The La2Se3 films are found to be n-type semiconductor.
Keywords: Spray pyrolysis; Lanthanum selenide (La2Se3) thin films; Crystal structure; Surface morphology; Electric and dielectric properties;
Oxygen chemisorption on an electrolytic silver catalyst: a combined TPD and Raman spectroscopic study by Geoffrey I.N. Waterhouse; Graham A. Bowmaker; James B. Metson (36-51).
In situ Raman spectroscopy, temperature programmed desorption (TPD) and scanning electron microscopy (SEM) were used to study the interaction of an electrolytic silver catalyst with O2 at atmospheric pressure and temperatures between 300 and 923 K. Raman spectroscopy and TPD were applied to characterise the chemisorbed oxygen species formed during catalyst exposure to O2 under these conditions. SEM was used to examine changes in catalyst morphology caused by high temperature oxygen treatment. The silver catalyst chemisorbed oxygen in a variety of distinct states over the temperature range examined. Oxygen species, identified on the basis of their vibrational and thermal desorption spectra, were a molecular superoxo species, weakly chemisorbed surface atomic oxygen (Oα), strongly chemisorbed surface atomic oxygen (Oγ) and subsurface oxygen (Oβ). The thermal stability and formation conditions of each species were established through a combination of in situ Raman and TPD data. With heating in O2 at 923 K, the silver catalyst restructured to form well-defined surface facets of dimension 1–2 μm. The influence of the restructuring on the oxygen chemisorptive properties of the silver catalyst is discussed. The results of these studies were used to develop a scheme for the interaction of O2 with silver surfaces at 923 K.
Keywords: Oxidation; Catalysis; Raman; SEM; Oxygen; Silver; Formaldehyde;
Chemical composition of plasma treated polyimide microspheres by Barbara Gawdzik; Magdalena Sobiesiak (52-57).
Synthetic carbon microspheres for chromatography were obtained from porous copolymer of 4,4′-bis(maleimidodiphenyl)methane (BM) and divinylbenzene (DVB) using arc discharge argon plasma treatment. Chemical structure of the obtained material was determined by elemental analysis, scanning electron microscopy with X-ray detector, acid and base titrations, and atomic absorption spectroscopy. The results suggest that the carbon microspheres contain copper coming from the plasma reactor electrodes. To remove it two complexion agents were used: EDTA and 20% HNO3. Copper can be removed from the surface using these methods. The other amount was permanently built into internal structure of the microspheres.
Keywords: Synthetic microspheres; Porous copolymer; Plasma treatment;
Tungsten nitride films grown via pulsed laser deposition studied in situ by electron spectroscopies by G. Soto; W. de la Cruz; F.F. Castillón; J.A. Dı́az; R. Machorro; M.H. Farı́as (58-67).
Tungsten nitride (WN x ) films were grown on silicon and glass slide substrates by laser ablating a tungsten target in molecular nitrogen ambient. By in situ Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS), the films density, elemental composition and chemical state were determined. Ex situ, the films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Also, the transmittance and resistivity of the film on glass substrates were determined. The results show that the reaction of tungsten and nitrogen is effective; the nitrogen is integrated in the tungsten matrix changing gradually the electronic configuration, chemical states and film properties. Since with this preparation method the obtained films are of high quality, low resistivity and dense, this makes attractive to growth tungsten nitride films for technological applications.
Keywords: Tungsten nitride; WN x ; Pulsed laser deposition; EELS; XPS; AES; Thin films;
Effect of niobium on the surface properties of Nb2O5-SiO2-supported Mo catalysts by S. Damyanova; L. Dimitrov; L. Petrov; P. Grange (68-74).
Nb2O5–SiO2 oxides with different Nb content (0.8–4.4 wt.%) were prepared by impregnation of silica with ethanol solution of niobium ethoxide, Nb(OC2H5)5. Mo catalysts supported on the Nb2O5-SiO2 carriers were prepared by wetness impregnation method of the supports with an aqueous solution of ammonium heptamolybdate, (NH4)6Mo7O24. The samples were characterized by X-ray photoelectron spectroscopy (XPS) and diffuse reflectance spectroscopy (DRS). The chemical state and dispersion of metal components (Nb and Mo) were studied in the oxides and Mo-containing catalysts. It was shown a good correlation between the amount of Nb and XPS intensity Nb/Si ratios in oxide supports. Generation of new acid sites (Lewis and Brönsted) in Nb2O5-SiO2 samples was easily understood when considering the change in the binding energies of Nb 3d and O 1s photoelectrons, due to a strong interaction between Nb atoms and silica. It was found that addition of niobium improves the dispersion of molybdenum on silica.
Keywords: Nb2O5–SiO2 oxides; Molybdenum; Acidity; Dispersion; XPS;
Optical determination of thickness and composition of buried strained Si1−x Ge x HBT alloys by C.M. Scheirer; R.F. Jones; P. Nguyen; H. Pois; S. Zangooie (75-82).
We report on optical characterization of epitaxially grown SiGe heterojunction bipolar transistor (HBT) structures. Using a commercially available system with non-invasive optical techniques such as, spectroscopic ellipsometry and near normal angle of incidence reflectometry, we have established excellent repeatability and accuracy for product development and manufacturing crucial processes. Germanium content of the buried SiGe spacer layers was determined using the Bruggeman effective medium approximation, as well as an alternative method utilizing a linear interpolation of optical spectra. The optical techniques are also used to monitor responses of different deposition factors on structural properties of the material. For example, a kinetically induced island formation phenomenon is investigated and addressed by optimizing the deposition parameters. Comparison of the optically measured germanium depth profile with SIMS showed good agreement. Optical measurements agreed to within 3.7% of the values obtained by SIMS. These results demonstrate that optical measurements are possible for ex situ monitoring of the epitaxial growth production process for high performance SiGe HBT structures.
Keywords: Silicon germanium; Optical modeling; Heterojunction bipolar transistor;
Thermal decomposition of the rare earth sulfates of cerium(III), cerium(IV), lanthanum(III) and samarium(III) by James A. Poston; Ranjani V. Siriwardane; Edward P. Fisher; Angela L. Miltz (83-102).
Surface and bulk chemical and elemental composition of the rare earth sulfates of cerium(III), cerium(IV), lanthanum(III) and samarium(III) were characterized during various stages of thermal decomposition. Decomposition was conducted under both vacuum and atmospheric conditions. In situ analysis was conducted on samples decomposed in vacuum. As identified by X-ray diffraction, the bulk decomposition of all the rare earth sulfate samples to their corresponding oxide, in atmosphere, proceeded via the formation of an oxysulfate. For the exception of cerium(III) sulfate, similar results were obtained in thermogravimetric analysis. The thermal decomposition profile, as determined by X-ray microanalysis was similar to that observed in thermogravimetric analysis and X-ray diffraction. Elemental maps revealed no observable concentration gradients of sulfur. Surface composition was not necessarily representative of the bulk composition. Thermal decomposition of sulfates to an oxide initiated at a lower temperature in vacuum than that observed at atmospheric pressure.
Keywords: Rare earth sulfates; Sulfate decomposition; Thermal decomposition; Oxysulfate; X-ray photoelectron spectroscopy;
The study of magnetism of Mn clusters on Si surfaces by S.M. Prokes; O.J. Glembocki; W.E. Carlos; T.A. Kennedy (103-108).
We have formed Mn clusters on Si(1 1 1) and Si(1 1 2) surfaces and we have examined their magnetic behavior using circularly polarized optical scattering. We show that Mn clusters of diameter 15–25 nm exhibit a temperature dependent ferromagnet-like behavior for temperatures below 10 K. As we use magnetic fields perpendicular to the plane of the surface, our results indicate that the magnetic moment of some of these clusters is out of the plane. In addition, our experiments verify previous theoretical suggestions that polarized optical scattering can be used to detect magnetism in small clusters.
Keywords: Clusters; Surfaces; Magnetism; Optical dichroism;
Self-assembly of micro- and nano-scale particles using bio-inspired events by H. McNally; M. Pingle; S.W. Lee; D. Guo; D.E. Bergstrom; R. Bashir (109-119).
High sensitivity chemical and biological detection techniques and the development of future electronic systems can greatly benefit from self-assembly processes and techniques. We have approached this challenge using biologically inspired events such as the hybridization of single (ss)- to double-stranded (ds) DNA and the strong affinity between the protein avidin and its associated Vitamin, biotin. Using these molecules, micro-scale polystyrene beads and nano-scale gold particles were assembled with high efficiency on gold patterns and the procedures used for these processes were optimized. The DNA and avidin–biotin complex was also used to demonstrate the attachment of micro-scale silicon islands to each other in a fluid. This work also provides insight into the techniques for the self-assembly of heterogeneous materials.
Keywords: Bio-inspired self-assembly; Nanoparticles; Heterogeneous integration;
A study of copper chemical mechanical polishing in urea–hydrogen peroxide slurry by electrochemical impedance spectroscopy by Tzu-Hsuan Tsai; Yung-Fu Wu; Shi-Chern Yen (120-135).
The electrochemical impedance spectroscopy (EIS) technique has been used to investigate the feasibility of urea–hydrogen peroxide (urea–H2O2) slurries in copper chemical mechanical polishing (Cu CMP). The performance of the inhibiting-type and the chelating-type additives, BTA and NH4OH, were also explored. In order to analyze the surface-reaction characteristics of Cu, the equivalent circuit of double capacitor mode was mainly used to simulate the corrosion behaviors of Cu CMP in various slurries. In addition, via measuring dc potentiodynamic curves and open circuit potential (OCP), the corrosion characteristics were obtained in various slurries. Both EIS and AFM experimental results indicate that the slurry composed of 5 wt.% urea–H2O2+0.1 wt.% BTA+1 wt.% NH4OH can achieve the better Cu CMP performance. Its rms-roughness (R q) after CMP and the removal rate (RR) attain to 2.636 nm and 552.49 nm/min, respectively.
Keywords: Copper; Chemical mechanical polishing; Urea–hydrogen peroxide slurry; Electrochemical impedance spectroscopy; Potentiodynamic curve;
Optical and dielectric properties of highly oriented (Zr0.8,Sn0.2)TiO4 thin films prepared by rf magnetron sputtering by W.X. Cheng; A.L. Ding; P.S. Qiu; X.Y. He; X.S.H. Zheng (136-142).
(Zr0.8,Sn0.2)TiO4 (ZST) thin films (∼150 nm) were grown on Pt/Ti/SiO2/Si(1 0 0) and fused quartz glass substrates by radio frequency (rf) magnetron sputtering. The microstructure and the surface morphology of ZST thin film have been studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). The optical properties of ZST thin film were obtained by spectroscopic ellipsometry and UV-Vis spectrometry for the first time. The optical band gap was found to be 3.30 eV of indirect-transition type. The low frequency (1 kHz–1 MHz) dielectric properties of ZST thin film were also discussed. The temperature coefficient of capacitance (TCC) of ZST thin film is about 80.2 ppm/°C at 1 MHz. The dielectric constant and dielectric loss at 100 kHz are 36.6 and 0.0069, respectively. The large dielectric loss compared with that of ZST ceramic is caused by the structure disorder in the thin film.
Keywords: (Zr0.2,Sn0.8)TiO4 (ZST) thin film; rf sputtering; Optical properties; Dielectric properties;
Magnetic nanocomposite thin films of NiFe2O4/SiO2 prepared by sol–gel process by Nelcy D.S. Mohallem; Luciana M. Seara (143-150).
Magnetic nanocomposite thin films formed by nickel ferrite particles dispersed in silica matrix have been prepared by sol–gel process. The starting solution was prepared using tetraethylorthosilicate (TEOS) as a precursor of silica, and metallic nitrates as precursors of the ferrite. NiFe2O4/SiO2 films were prepared with 5–30 wt.% ferrite contents and deposited on glass, quartz and silicon plates using the dip-coating process. The films obtained were adherent, transparent, homogeneous, and free of microcracks, with thickness between 70 and 100 nm. The nanocomposite films presented the formation of crystalline NiFe2O4 nanoparticles measuring between 6 and 10 nm dispersed in amorphous SiO2 matrix and a saturation magnetization above 8 emu/g.
Keywords: Magnetic thin films; Sol–gel process; Nickel ferrite nanocomposite;
Laser-controlled photoluminescence characteristics of silicon nanocrystallites produced by laser-induced etching by R.K Soni; G.R Bassam; S.C Abbi (151-160).
Various laser wavelengths have been employed to synthesize luminescent nanocrystallites of n-type silicon by laser-induced etching (LIE) process. These nanocrystallites exhibit light emission in the visible region. Both the photoluminescence (PL) emission and the surface morphology of the photosynthesized layer were significantly controlled by laser processing parameters. An estimate of the nanocrystallite sizes present in the photosynthesized layer was obtained from a modified quantum confinement model appropriate for spherical nanocrystallites.
Keywords: Nanostructures; Photoluminescence; Laser-induced etching;
The importance of carbon content beneath iron borides after boriding of chromium and nickel-based low-carbon steel by M. Kulka; A. Pertek (161-171).
The complex (B+C) diffusion layers have been formed on chromium- and nickel-based low-carbon steels. Gas boriding applied to these steels that have been previously carburized enables the production of wear-resistant borocarburized layers. After combined surface hardening with boron and carbon in the microstructure two zones have been observed: iron borides (FeB+Fe2B) and carburized zones. The iron borides in borocarburized layer show the tendencies towards lose of the needle-like nature. The depth of iron borides zone depend on pre-carburizing parameters. Although borocarburizing causes reducing of the depth of iron borides zone, it also reduces the microhardness gradient across the case. An increase of distance from the surface is accompanied by a gradual decrease of carbon content and microhardness in carburized zone. Probably, the brittleness of borocarburized layer is lower. In the consequence, the frictional wear resistance of borocarburized layers is essentially higher than that obtained in case of only borided layers. There is every indication that the carbon content beneath iron borides has an important effect on wear resistance, too. The highest wear resistance of borocarburized layer has been observed in case of about 1.2% C below iron borides.
Keywords: Diffusion; Carburizing; Boriding; Borocarburizing; Wear resistance;
Microstructure and structure of NiO–SnO2 and Fe2O3–SnO2 systems by Ricardo H.R. Castro; Pilar Hidalgo; R. Muccillo; Douglas Gouvêa (172-177).
The microstructure and the structure of NiO–SnO2 and Fe2O3–SnO2 were studied by X-ray diffraction (XRD) and adsorption analysis (BET). The segregation of Ni or Fe on the surface of SnO2 was observed even in specimens with 30 mol% addition. The segregation caused a decrease in the surface energy of the powders and, therefore, an increase in the surface area as expected by the Ostwald ripening model. After the saturation of the tin oxide surface, there is no increase in the surface area, but a decrease due to second phase nucleation. Adding more Ni to the system, similar segregation of Sn on the surface of NiO as well as the effect on the surface area were observed. In Fe2O3–SnO2, an iron phase other than hematite was detected at high concentrations of Fe (Fe2O3–20% SnO2), suggesting the formation of magnetite or a ferrite spinel. Gibbs energy diagrams that could be used to predict stable phases of certain compositions were drawn for both systems based on the observed data.
Keywords: Tin oxide; Microstructure; Additives; Surface;
Structural characteristics of a carbon adsorbent and influence of organic solvents on interfacial water by V.M. Gun’ko; V.V. Turov; J. Skubiszewska-Zi e ̨ ba; R. Leboda; M.D. Tsapko; D. Palijczuk (178-189).
Different treatments (suspending in water, drying, freezing by liquid nitrogen, de-ashing, oxidation by H2O2, reduction by H2, and some combinations of these methods) of activated carbon Norit R 0.8 Extra lead to changes in micropores to a greater extent than mesopores. Influence of polar (CD3CN, (CD3)2CO and (CD3)2SO) and nonpolar (CCl4, CDCl3 and C6D6) solvents on the structure of interfacial water pre-adsorbed on the carbon adsorbent was studied by 1 H NMR spectroscopy with freezing-out of the adsorbed water at 180 K<T<273 K and modelled by using quantum chemistry methods with consideration for the solvation effects. It was found that water interaction with the carbon surfaces depends strongly on the properties of solvents, which can remove a major (by nonpolar solvents insoluble in water) or minor (by polar solvents soluble in water) portion of water from narrow micropores to larger pores.
Keywords: Activated carbon; Pore size distribution; Surface heterogeneity; Organic solvents; 1 H NMR spectra; Interfacial water;
High-temperature sliding wear behaviors of laser clad Mo2Ni3Si/NiSi metal silicide composite coatings by X.D Lu; H.M Wang (190-195).
A novel wear- and oxidation-resistant Mo2Ni3Si/NiSi metal silicide composite coatings were fabricated on substrate of an austenitic stainless steel by laser cladding using Ni–Mo–Si elemental powder blends. The microstructure of the coating was characterized by optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). The high-temperature wear resistance of the coating was evaluated under sliding wear test conditions. Results show that microstructure of the coating consists of the Mo2Ni3Si primary dendrite and interdendritic Mo2Ni3Si/NiSi eutectic. Due to the presence of a large amount of hard and wear-resistant Mo2Ni3Si phase, the laser clad Mo2Ni3Si/NiSi metal silicide composite coating has excellent wear resistance under high-temperature sliding wear test conditions.
Keywords: Laser cladding; Metal silicide; Mo2Ni3Si; Wear; Coatings;
Initial stages of Yb/Si(1 0 0) interface growth: 2 × 3 and 2 × 6 reconstructions by M. Kuzmin; R.E. Perälä; P. Laukkanen; R.-L. Vaara; M.A. Mittsev; I.J. Väyrynen (196-207).
The initial stages of Yb/Si(1 0 0) interface growth have been studied by scanning tunneling microscopy, low-energy electron diffraction, Auger electron spectroscopy (AES), thermal desorption spectroscopy, and work-function change measurements. It is shown that a two-dimensional (2D) adsorbed layer of Yb forms at low coverage (<0.5–0.6 monolayers (ML)), followed by three-dimensional (3D) growth of Yb silicide phase at higher coverages. The two surface reconstructions of Yb/Si(1 0 0), 2×3 and 2×6 phases, are identified in submonolayer regime. The submonolayer adsorbed phases are found to exhibit higher thermal stability compared to the silicide one. The formation of adsorbed phases results in a large decrease of work-function (by up to 1.8 eV), which suggests that the reconstructions are terminated by Yb and a charge transfer occurs from top Yb atoms to underlying Si substrate. Moreover, the structural model of the 2×3 reconstruction is proposed and the features of the 2×6 phase are interpreted.
Keywords: Silicon; Ytterbium; Scanning tunneling microscopy; Low-energy electron diffraction; Thermal desorption spectroscopy; Work-function measurements;
Comparison of the electrical and optical properties for SnO2:Sb films deposited on polyimide and glass substrates by Jin Ma; Xiaotao Hao; Shulai Huang; Jie Huang; Yingge Yang; Honglei Ma (208-213).
Transparent conducting antimony-doped tin oxide (SnO2:Sb) films have been prepared on polyimide and Corning 7059 substrates by radio frequency magnetron-sputtering technique at low substrate temperature (30–220 °C). Polycrystalline SnO2:Sb films having the rutile structure were deposited with resistivity as low as 3.7×10−3 Ω cm on polyimide substrate and 2×10−3 Ω cm on glass substrate. The average transmittance exceeded 70.6 and 85.5% in the visible spectrum for 285 and 315 nm thick films deposited on polyimide and glass substrate, respectively. A comparison of the properties for the films deposited on glass and organic substrates was performed.
Keywords: Organic substrate; Antimony-doped tin oxide film; Transparent conducting film;
The pH-sensing and light-induced drift properties of titanium dioxide thin films deposited by MOCVD by Paik-Kyun Shin (214-221).
Titanium dioxide thin films were deposited on thermally oxidized silicon wafers by metal organic CVD (MOCVD). Then the resulting TiO2/SiO2/Si test structures were annealed in dry oxygen for 60 min at temperatures 700, 800, 900, and 1000 °C, respectively. Refractive index and relative permittivity of the as-deposited and annealed titanium dioxide layers were investigated by spectral ellipsometry and high frequency capacitance–voltage (HF-CV) measurements. The pH-sensing properties of the titanium dioxide layers were investigated by capacitance–voltage measurements on electrolyte/insulator/silicon (EIS) structures. In addition, light-induced drift of the test structures was investigated by illumination using 12 V light bulb for 60 min in a measurement chamber. The titanium dioxide layers revealed refractive index of 2.38–2.58 (λ=550 nm) and relative permittivity of 31.28–36.27. Integral pH-sensitivities (pH ranging from 3 to 11) of the titanium dioxide layers were 57.4–62.3 mV/pH (T=32 °C). All the titanium dioxide layers showed no light-induced drift. After long-term measurement, the titanium dioxide layer annealed at 900 °C revealed a better stability of the pH-sensing properties than other titanium dioxide layers investigated in this study.
Keywords: TiO2; MOCVD; pH-sensitivity; Light-induced drift; ISFET;
Pyridine-TiO2 surface interaction as a probe for surface active centers analysis by T. Bezrodna; G. Puchkovska; V. Shimanovska; I. Chashechnikova; T. Khalyavka; J. Baran (222-231).
Molecular interactions between pyridine molecules and titanium dioxide particles of different crystalline modifications (anatase and rutile) have been investigated by IR spectroscopy. Analysis of spectral data allowed to suggest possible mechanisms of these interactions and make some conclusions about TiO2 surface state and active centers. Anatase surface has active centers of both Lewis and Brønsted types (LAC and BAC), with which pyridine interacts via coordination bonding or by the creation of pyridinium-ion (PyH+). Rutile samples have only LAC on their surface, and this does not depend on thermal treatment. Also, pyridine can form hydrogen bonds with hydroxyl groups of TiO2 surface and adsorbed water molecules in these heterogeneous systems. There are at least two types of non-equivalent OH-groups with different bonding enthalpies. Their amount and enthalpy depend on the crystalline modification of TiO2, thermal treatment, and change after loading TiO2 particles into pyridine.
Keywords: TiO2; Lewis and Brønsted centers; Pyridine;
Improving the wettability of 2024 aluminium alloy by means of cold plasma treatment by W Polini; L Sorrentino (232-242).
Aluminium alloys are heavily used to manufacture structural parts in the aeronautic industry because of its lightness and its corrosion resistance. These alloys are successfully used in other industrial fields too, such as railway, automotive and naval industries. The need to contrast the severe use conditions and the heavy stresses developing in aeronautic field implies to protect the surfaces of the structures in aluminium alloy by any deterioration. To preserve by deterioration, it is necessary to make aluminium more suitable to be coated by protective paint. In the aeronautic industry, a complex and critical process is used in order to enhance both wettability and adhesive properties of aluminium alloy surfaces. Cold plasma treatment represents an efficient, clean and economic alternative to activate aluminium surfaces.The present work deals with air cold plasma treatment of 2024 aluminium alloy surfaces. The influence of dc electrical discharge cold plasma parameters on wettability of 2024 aluminium alloy surfaces has been studied. A set of process variables (voltage, time and air flow rate) has been identified and used to conduct some experimental tests on the basis of design of experiment (DOE) techniques. The experimental results show that the proposed plasma process may considerably increase aluminium alloy wettability. These results represent the first step in trying to optimise the aluminium adhesion by means of this non-conventional manufacturing process.
Keywords: Cold plasma; Glow-discharge plasma; Aluminium alloy; Wettability;
Contact angle of 63Sn–37Pb and Pb-free solder on Cu plating by Chien-Tai Lin; Kwang-Lung Lin (243-258).
This study investigated the wetting behavior of 63Sn–37Pb and Pb-free solder on different surface roughness Cu plating. The Cu plating roughness was controlled by current density and deposition time. The contact angles were measured to investigate the wetting behavior. The contact angles of DI (de-ionized) water obey Wenzel’s equation and decrease with the increase of surface roughness. Daubing the flux on solder balls as well as on the Cu plating lowers the magnitude of contact angle by 2–23° and the deviation of measured values by 4–6°. The contact angles decrease with the increase of Cu substrate surface roughness in the temperature range of 250–280 °C. The cleaning effect of flux was analyzed by Auger electron spectroscopy. The oxide on the Cu substrate surface was well removed by flux. The carbon residue left behind on the substrate surface after reflow raises both the magnitude and the scattering of contact angle.
Keywords: Solder; Cu plating; Contact angle; Surface roughness; Wetting behavior;
Surface roughness and interface diffusion studies on thin Mo and W films and Mo/Si and W/Si interfaces by D. Bhattacharyya; A.K. Poswal; M. Senthilkumar; P.V. Satyam; A.K. Balamurugan; A.K. Tyagi; N.C. Das (259-271).
Single layers of Mo and W thin films and Mo/Si/Mo and W/Si/W tri-layers have been deposited by r.f. sputtering on c-Si substrates as precursor to the fabrication of Mo/Si and W/Si multilayer X-ray mirrors. The single layer thin films have been characterized primarily by phase modulated spectroscopic ellipsometry (SE) and information have been derived regarding the thickness and volume fraction of voids present in the layers. The results obtained by fitting the SE data have been verified by complementary techniques, viz. grazing-incidence X-ray reflectivity, Rutherford back-scattering and atomic force microscopy. With the optimized values of the sputtering parameters, Mo/Si/Mo and W/Si/W tri-layer structures have been deposited on c-Si substrates and have been characterized by secondary ion mass spectrometry (SIMS) technique using sputtering by a Cs+ ion source. The depth profile data obtained from the SIMS measurements have been analyzed to get the inter-diffusion at the metal–Si interfaces. Efforts have been given to correlate the interface diffusion at the interfaces to the thickness of the surface layers of the metal films.
Keywords: Surface roughness; Interface diffusion; Spectroscopic ellipsometry; SIMS;
High resolution transmission electron microscope observation of α-TiCl3 by Tomohiro Higuchi; Boping Liu; Hisayuki Nakatani; Nobuo Otsuka; Minoru Terano (272-277).
High resolution transmission electron microscopy (HRTEM) was used to observe the structure of α-TiCl3 in order to investigate the nature of active sites in Ziegler–Natta catalyst. The crystalline structures of the α-TiCl3 surface were directly observed in the HRTEM image and the electron diffraction pattern. The ordered arrangement of the crystal lattices in α-TiCl3 particles was clearly revealed. The rapid deterioration of the crystalline structure of the α-TiCl3 due to the exposure in ambient conditions was also observed on the atomic scale, reflecting its ultra-high sensitivity to the impurities in the air.
Keywords: Ziegler–Natta catalyst; HRTEM; α-TiCl3;
Microstructure and properties of borided 41Cr4 steel after laser surface modification with re-melting by M. Kulka; A. Pertek (278-288).
The paper presents the results of laser surface modification of the borided layers produced on the 41Cr4 medium carbon steel (0.43% C, 1.02% Cr). Laser tracks were arranged by CO2 laser beam as a single track and as multiple tracks formed in the shape of helical line. The microstructure in both cases consists of three following zones: re-melted zone (MZ) (eutectic mixture of borides and martensite), heat-affected zone (HAZ) (martensite) and the substrate (ferrite and pearlite). XRD scans of the laser-modified borided specimen confirmed the presence of the same two types of iron borides (FeB and Fe2B), like these indicated in the as-borided layer. After re-solidification in some places iron richer Fe3B phase was additionally formed. The layer after boriding and laser heat treatment (LHT) has a lower microhardness, than the only borided layer, reducing the hardness gradient between the diffusion layer and the substrate. Probably, the brittleness of this layer is lower. The improved wear resistance of this layer has been found in comparison with borided layers after conventional heat treatment.
Keywords: Boriding; Laser heat treatment; Microstructure; Microhardness; Wear resistance;
UV-light enhanced oxidation of carbon nanotubes by M. Grujicic; G. Cao; A.M. Rao; T.M. Tritt; S. Nayak (289-303).
Ab initio density functional theory (DFT) calculations of the interactions between selected semiconducting and metallic single-walled carbon nanotubes (SWCNTs) (as well between single and double graphene sheets) and single oxygen molecules are carried out in order to provide a rationale for the recent experimental observations of UV-light accelerated oxidation of carbon nanotubes and the accompanying changes in the thermoelectric power. The computational results obtained show that these experimental findings can be related to UV-light excitation of oxygen molecules from their ground spin-triplet state into a higher-energy spin-singlet state. Such excitation lowers the activation energy for molecular-oxygen chemisorption to a nanotube, increases the adsorption energy and promotes charge transfer from the nanotube to the oxygen molecule. Lattice defects such as 7-5-5-7 and Stone–Wales defects are found to play a critical role in enhancing oxygen molecule/nanotube bonding and in affecting the extent of charge transfer. Contrary to this, the effects of nanotube diameter and chirality and the number of walls appear to be less significant.
Keywords: Carbon nanotubes; Adsorption; Density functional theory (DFT) calculations;
A liquid aluminum corrosion resistance surface on steel substrate by Wang Deqing; Shi Ziyuan; Zou Longjiang (304-311).
The process of hot dipping pure aluminum on a steel substrate followed by oxidation was studied to form a surface layer of aluminum oxide resistant to the corrosion of aluminum melt. The thickness of the pure aluminum layer on the steel substrate is reduced with the increase in temperature and time in initial aluminizing, and the thickness of the aluminum layer does not increase with time at given temperature when identical temperature and complete wetting occur between liquid aluminum and the substrate surface. The thickness of the Fe–Al intermetallic layer on the steel base is increased with increasing bath temperature and time. Based on the experimental data and the mathematics model developed by the study, a maximum exists in the thickness of the Fe–Al intermetallic at certain dipping temperature. X-ray diffraction (XRD) and energy dispersive X-ray (EDX) analysis reveals that the top portion of the steel substrate is composed of a thin layer of α-Al2O3, followed by a thinner layer of FeAl3, and then a much thicker one of Fe2Al5 on the steel base side. In addition, there is a carbon enrichment zone in diffusion front. The aluminum oxide surface formed on the steel substrate is in perfect condition after corrosion test in liquid aluminum at 750 °C for 240 h, showing extremely good resistance to aluminum melt corrosion.
Keywords: Aluminizing; Diffusion; Intermetallic compounds; Corrosion protection;
Numerical simulation of vertical temperature distribution in heterogeneous multilayer structure with Mathematica by Hee-Joong Yun (312-318).
Temperature distributions produced by a scanning cw Nd:YAG (1.06 μm) laser beam in a heterogeneous multilayer structure (poly-Si/SiO2/Si) are calculated using Mathematica. A solution to the nonlinear heat equations including temperature-dependent thermal parameters is solved numerically for the temperature distributions of the heterogeneous multilayer structure. We present the linear temperature distributions of multilayer along the full depth of structure as a function of normalized thickness of the structure. This calculation scheme could cover a wide range of calculation of the temperature distribution produced by a scanning heat source in the heterogeneous multilayer structure.
Keywords: Numerical calculation; Temperature distribution; Multilayer structure; Mathematica;
Characterization of PdAu thin films on oxidized silicon wafers: interdiffusion and reaction by Senay Yalcin; Recep Avci (319-337).
Plasma-deposited thin films prepared at room temperature, ranging from 46 to 250 Å of PdAu on ∼45–50 Å Si-oxide and Si-oxynitride films grown on Si wafers were studied. Grazing incidence X-ray diffraction, X-ray reflectivity, and XPS depth profile techniques were used to characterize the thin films. A reactive interface involving Pd- and Au-silicides is formed, linking the thin film to the Si-oxide and Si-oxynitride films: a small fraction of Pd and Au atoms from PdAu migrate into the Si substrate, first penetrating the oxide layer, and the small fraction of Si atoms from the oxide layer migrate into the PdAu film and form a silicide interlayer consisting of a reactive interface made up of mixtures of Au- and Pd-silicides interspersed within the matrix of PdAu and substrate. The concentration profiles of these silicides have a maximum at the interface with decay on both sides. The density and the face-centered cubic (fcc) lattice parameter of the film are determined to be ∼13±1 g/cm3 and ∼4.004±0.014 Å, respectively. The ideal film density is expected to be ∼15.5 g/cm3, which suggests substantial defect density and light material mixture, causing more than 13% reduction in the mass density of the film.
Keywords: PdAu thin films; PdAu/SiO2/Si; Silicide; Interdiffusion and reaction; XPS; Reflectivity;
Low-energy ion-beam induced effects in Al(1 0 0) surface studied using Rutherford backscattering and channeling by C.V Ramana; B.S Choi; R.J Smith (338-350).
The results of low-energy ion spectroscopy can be compromised by the damage that the ions cause in surface layers of target single crystals, so it is important to characterize the impact of the probe ion on the target. In this investigation Al(1 0 0) single crystal surfaces have been irradiated at room temperature with 1 keV He+ and Ar+ ions typical of low-energy ion scattering spectroscopy (LEIS) and sample sputter cleaning, respectively, and the effects have been studied as a function of ion dose and annealing. Rutherford backscattering and channeling measurements were used to probe the depth distribution and annealing characteristics of the dechanneling yield induced by low-energy ion bombardment. Analysis of the Al surface peak area for channeled MeV He ions, as well as the dechanneling yield from well below the surface, indicated an unusual increase in the depth distribution of the backscattered ion yield from Al with increasing low-energy He+ ion dose. Thermal annealing of the crystal following irradiation with the highest He+ dose of 4×1017 ions/cm2 restored the dechanneled ion yield to its lower starting value, with the relatively low activation energy of 0.05 eV. The increase in backscattered ions is attributed to implanted He atoms located in the [1 0 0] channels. The Ar+ ion induced effects in the channeling spectrum are not as significant as those for He+ ions, even at the highest Ar+ dose of ∼1017 ions/cm2.
Keywords: LEIS; Sputtering; Aluminum; Ion channeling; Metal surfaces; He implantation;
Field emission from modified nanocomposite carbon films prepared by filtered cathodic vacuum arc at high negative pulsed bias by J.X. Guo; Z. Sun; B.K. Tay; X.W. Sun (351-358).
Cobalt-containing amorphous carbon (a-C:Co) composite films have been prepared by filtered cathodic vacuum arc at high negative pulsed bias voltage. After heat treatment at 580 °C in a mixture of acetylene and hydrogen gases, field emission properties of the a-C:Co films were significantly improved and the films deposited at higher negative bias voltage possessed better field emission properties. A relatively low threshold field of 2.1 V/μm (for emission current 1 μA/cm2) and a high emission current density of 2 mA/cm2 at the electric field of 6 V/μm has been achieved for the films deposited at the negative bias voltage of 7 kV. The morphologies and structures of the a-C:Co films were also studied in detail. It is suggested that the employed negative bias voltage increases the grain size and the sp2 content in the films, and further improves the field emission properties.
Keywords: Field emission; Nanocomposite film; Amorphous carbon; Filtered cathodic vacuum arc; Annealing;
A two steps CVD process for the growth of silicon nano-crystals by F. Mazen; T. Baron; A.M. Papon; R. Truche; J.M. Hartmann (359-363).
We have developed a two steps chemical vapor deposition (CVD) process which permits to dissociate the nucleation and the growth of silicon nano-crystals. In the first step, silicon “clusters” of a diameter below 1 nm are nucleated by exposure of the SiO2 substrate to SiH4. In the second step, these clusters are grown selectively using SiH2Cl2 as silicon precursor. TEM analysis shows that the silicon quantum dots (Si-QDs) obtained with this process are mono-crystalline. The main advantage of this two steps process is that the size dispersion is sharpened compared to a standard one step process because of the dissociation of the nucleation and the growth of the Si-QDs.
Keywords: CVD; Silicon; Nano-crystals;
The preparation and characterization of CN x film with high nitrogen content by cathode electrodeposition by Jia-Tao Zhang; Chuan-Bao Cao; Qiang Lv; Chao Li; He-Sun Zhu (364-369).
CN x thin film with high nitrogen content was prepared on ITO conductive glass substrates by cathode electrodeposition, using dicyandiamide (C2H4N4) in acetone as precursors. The surface morphologies, atomic bonding state, and chemical composition were analyzed by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy. The CN x particles got nanometer level with the average size of 80 nm. The maximum value of the N/C atomic ratio was more than 1. Carbon and nitrogen existed mainly in the form of tetrahedral CN bonds, with a few CN bonds. From UV-Vis absorption spectrum, we found that during near-ultraviolet area the deposited CN x films appeared nonlinear optical absorption phenomena, and the ultraviolet light (200–280 nm) could be transmitted. The electrical resistivities of the films were in the range of 1012–1016 Ω cm.
Keywords: CN x ; Electrodeposition; Cathode;
Cation-deficient nano-dimensional particle size cobalt–manganese spinel mixed oxides by Dimitar G. Klissurski; Ellie L. Uzunova (370-374).
Cobalt–manganese binary spinel oxides of high-dispersity are synthesized by thermal decomposition of hydroxycarbonate precursors in the limits 0.33≤Mn/Co≤2. The mixed oxides are cation-deficient and contain a high surface concentration of non-stoichiometric oxygen. The oxygen-to-metal surface ratios, determined by ESCA are higher than the stoichiometric value for dioxides. The spinel lattice is tetragonally distorted for atomic ratios Mn/Co>1. The samples synthesized below 400 °C are composed from spherical-shaped particles with diameters in the nano-scale range 6–12 nm. Reduction of the binary spinel oxides with hydrogen proceeds in multiple steps that correspond to the reduction steps Co(III)⇒Co(II); Co(II)⇒Co reduction and Mn(IV), Mn(III)⇒Mn(II). In the temperature range 100–1200 °C reduction to metallic manganese has not been observed.
Keywords: Spinel; Cobalt; Manganese; Hydroxycarbonates; Cation-deficient;