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Applied Surface Science (v.252, #16)

Oxidation process of SiGe on SOI substrates by Bo Jin; Xi Wang; Jing Chen; Feng Zhang; Xinli Cheng; Zhijun Chen (pp. 5627-5631).

The oxidation of SiGe film epitaxial grown on top of SOI wafers has been studied. These SiGe/SOI samples were oxidized at 700, 900, 1100°C. Germanium atoms were rejected from SiGe film to SOI layer. A new Si1−_xGe_x ( x is minimal) layer formed at SiGe/Si interface. As the germanium atoms diffused, the new Si1−_xGe_x ( x is minimal) layer moved to Si/SiO₂ interface. Propagation of threading dislocation in SiGe film to SOI substrate was hindered by the new SiGe/Si interface. Strain in SOI substrate transferred from SiGe film was released through dislocation nucleation and propagation inner. The relaxation of SiGe film could be described as: strain relaxed through strain equalization and transfer process between SiGe film and SOI substrates. Raman spectroscopy was used to characterize the strain of SiGe film. Microstructure of SiGe/SOI was observed by transmission electron microscope (TEM).

Keywords: Strain relaxation; SiGe; SOI

Work function distribution for W–Ir mixed metal matrix cathodes by K. Santhosh Kumar; P. Durga Devi; M. Ravi; K.S. Bhat (pp. 5632-5635).

Mixed metal matrix cathodes have inherent non-uniformity and patchiness of emission due to the presence of two-alloy phase structure on the surface. I– V characteristics of cathode studied in a close spaced diode configuration is one of the easy and cost effective methods to estimate the variation of work function on the cathode surface. Tungsten iridium mixed metal matrix dispenser cathodes of Ø1.4mm (80wt.% W–20wt.% Ir) have been fabricated in the laboratory and their I– V characteristics have been investigated in diode configuration. In this paper the model suggested by Tonnerre et al. has been used to find out the work function distribution of W–Ir cathodes from I– V characteristics. An attempt has been made to correlate the microstructure with the work function values.

Keywords: PACS; 79.40.+zMixed metal matrix cathode; Work function distribution

Coadsorption of CN and O on Cu (100) surface: A density functional study by Yi Li; Jian Ming Hu; Yong Fan Zhang; Jun Qian Li (pp. 5636-5644).

The adsorption of cyanide (CN) or oxygen atom, as well as the coadsorption of CN+O on Cu (100) surface is studied by using density functional theory (DFT) and the cluster model method. Cu₁₄ cluster is used to simulate the surface. Perpendicular and parallel bonding geometries of CN adsorbed on Cu (100) surface are considered, respectively. The present calculations show that the CN may be absorbed on top and bridge sites by carbon atom of cyanide (C-down), and C-down on top site is the most favorable. The adsorbed C–N stretch frequencies compared with that of the gaseous CN species are all red-shifted, except the C-down on top site. The charge transfer from the surface to the CN species leads to an increase in work function for the Cu surface. The oxygen atom adsorbed on the four-fold hollow site of Cu (100) is the most favorable, and is consistent with the experimental study. The coadsorption of O at a four-fold hollow site tends to block adsorption of CN at the nearby sites. If O coverage increases, the CN may be adsorbed on the top and bridges sites with the C-down model. The reaction CN+O→OCN on the Cu (100) is predicted to be exothermic, and formed OCN species may be stably absorbed on the Cu (100).

Keywords: Density functional theory; Coadsorption; Cyanide; Oxygen; Cu (1; 0; 0) surface

Scanning probe microscopy studies of PbS surfaces oxidized in air and etched in aqueous acid solutions by Yuri L. Mikhlin; Alexander S. Romanchenko; Alexander A. Shagaev (pp. 5645-5658).

Natural n-type PbS single crystals have been studied using AFM, STM and STS after long-term oxidation in air at ambient temperatures and extensive etching in aqueous acid solutions, in contrast to previous work devoted to initial corrosion of fresh surfaces. The exposure of PbS to atmosphere at high relative humidity for several days yields widespread loose oxidation products; the process is much slower at low humidity. Surface morphologies diverge after the treatment in 1M perchloric and hydrochloric acid solutions at room temperature and become widely different at elevated temperatures, displaying commonly etch pits up to several micrometers in size and depth along with rather uniformly distributed 20–100nm protrusions of PbS phase. The changes both in topography and semiconducting properties of PbS found by tunneling spectroscopy have been explained in terms of the non-uniform distribution of donor- and acceptor-type defects D⁺/D⁻ in the metal depleted surface layer, which are generated by chemical reactions and, in turn, determine the rates of the PbS corrosion. In particular, the D⁻ centers exhibit a self-catalyzing effect on the non-oxidative local dissolution of PbS in HCl media, resulting in the deep etch pits.

Keywords: Lead sulfide; Oxidation; Acidic dissolution; Scanning probe microscopy; Tunneling spectroscopy; Surface topography

Nonideality of Au/Si and Au/GaAs Schottky barriers due to process-induced defects by Keiji Maeda (pp. 5659-5675).

A mechanism of local lowering of the Schottky barrier height (SBH) is proposed, which causes nonideality in nearly ideal Au/ n-Si and Au/ n-GaAs Schottky barriers. Positively ionized defects generated by the process very close to the interface induce electrons in the metal-induced gap states (MIGS) and lower the SBH locally. The spatial density distribution of the ionized defects obtained from the SBH distribution is determined by the unique interaction with the MIGS. The defects are considered to have the negative-U property and are neutralized at very close positions to the MIGS. The potential distributions close to the interface have a considerable potential drop due to the large defect density. These inhomogeneous potentials are coincident with the energy level scheme of the defect identified as the defect causing the nonideality. This defect is Si self-interstitial in Au/Si SB, and As antisite in Au/ n-GaAs SB. This MIGS with process-induced defect model supersedes the previously proposed two major Fermi level pinning models. The mystery of the T₀ effect is solved. The thermionic-field emission current taking place in the strong electric field has influence on the I– V characteristics at low temperatures. Regarding the C– V characteristics of Au/Si SB, the observed extra capacitance under the forward bias is an experimental evidence in accordance with the proposed model.

Keywords: PACS; 73.20.−r; 73.30.+ySchottky barrier; Silicon; Gallium arsenide; Defect; Potential distribution

Investigation of growth, coverage and effectiveness of plasma assisted nano-films of fluorocarbon by Pratik P. Joshi; Rajasekhar Pulikollu; Steven R. Higgins; Xiaoming Hu; S.M. Mukhopadhyay (pp. 5676-5686).

Plasma-assisted functional films have significant potential in various engineering applications. They can be tailored to impart desired properties by bonding specific molecular groups to the substrate surface. The aim of this investigation was to develop a fundamental understanding of the atomic level growth, coverage and functional effectiveness of plasma nano-films on flat surfaces and to explore their application-potential for complex and uneven shaped nano-materials. In this paper, results on plasma-assisted nano-scale fluorocarbon films, which are known for imparting inertness or hydrophobicity to the surface, will be discussed. The film deposition was studied as a function of time on flat single crystal surfaces of silicon, sapphire and graphite, using microwave plasma. X-ray photoelectron spectroscopy (XPS) was used for detailed study of composition and chemistry of the substrate and coating atoms, at all stages of deposition. Atomic force microscopy (AFM) was performed in parallel to study the coverage and growth morphology of these films at each stage. Combined XPS and AFM results indicated complete coverage of all the substrates at the nanometer scale. It was also shown that these films grew in a layer-by-layer fashion. The nano-films were also applied to complex and uneven shaped nano-structured and porous materials, such as microcellular porous foam and nano fibers. It was seen that these nano-films can be a viable approach for effective surface modification of complex or uneven shaped nano-materials.

Keywords: Growth; Plasma; Nano-film

Influence of the deposition pressure on the properties of transparent conducting zirconium-doped zinc oxide films prepared by RF magnetron sputtering by Maoshui Lv; Xianwu Xiu; Zhiyong Pang; Ying Dai; Shenghao Han (pp. 5687-5692).

Transparent and conducting zirconium-doped zinc oxide films with high transparency and relatively low resistivity have been successfully prepared by RF magnetron sputtering at room temperature. The deposition pressure was varied from 0.6 to 2.5Pa. A transformation from a relatively compact structure to individual grains was observed with the increase of deposition pressure. As the deposition pressure increases, the resistivity increases sharply due to both, the decrease of hall mobility and carrier concentration. The lowest resistivity achieved was 2.07×10⁻³Ωcm at a deposition pressure of 0.6Pa with a hall mobility of 16cm²V⁻¹s⁻¹ and a carrier concentration of 1.95×10²⁰cm⁻³. The films are polycrystalline with a hexagonal structure and a preferred orientation along the c-axis. All the films present a high transmittance of above 90% in the visible range. The optical band gap decreases from 3.35 to 3.20eV as the deposition pressure increases from 0.6 to 2.5Pa.

Keywords: Zirconium; Zinc oxide; Sputtering; Transparent conducting films

Diluting thiol-derivatized oligonucleotide monolayers on Au(111) by mercaptohexanol replacement reaction by Yonghai Song; Yaqing Liu; Menglong Yang; Bailin Zhang; Zhuang Li (pp. 5693-5699).

Surface replacement reaction of thiol-derivatized, single-stranded oligonucleotide (HS-ssDNA) by mercaptohexanol (MCH) is investigated in order to reduce surface density of the HS-ssDNA adsorbed to Au(111) surface. Cyclic voltammograms (CVs) and scanning tunneling microscopy (STM) are employed to assess the composition and state of these mixed monolayers. It is found that each CV of mixed self-assembled monolayers (SAMs) only shows a single reductive desorption peak, which suggests that the resulted, mixed SAMs do not form discernable phase-separated domains. The peak potential gradually shifts to negative direction and the peak area increases step by step over the whole replacement process. By analyzing these peak areas, it is concluded that two MCH molecules will replace one HS-ssDNA molecule and relative coverage can also be estimated as a function of exposing time. The possible mechanism of the replacement reaction is also proposed. The DNA surface density exponentially reduces with the exposing time increasing, in other words, the replacement reaction is very fast in the first several hours and then gradually slows down. Moreover, the morphological change in the process is also followed by STM.

Keywords: Electrochemistry; Scanning tunneling microscopy; DNA; Surface; Monolayers

On the growth strain origin and stress evolution prediction during oxidation of metals by B. Panicaud; J.L. Grosseau-Poussard; J.F. Dinhut (pp. 5700-5713).

High temperature oxidation of metals leads to residual stresses in the metal and in the oxide. In this work, we try to predict the evolution of the residual stresses in the growing oxides layers, during isothermal oxidation. The origin of these stresses is based on the microstructural model of Clarke, however, another justification is proposed, assuming a proportional dependence of the growth strain with the oxide layer thickness. Using the mechanics of thin layers, as well as the analysis proposed to describe the growth strain, a system of equations are deduced that predict the stresses evolution with oxidation time. Numerical analysis is performed, leading to a set of theoretical curves.

Keywords: High temperature oxidation; Growth strain; Residual stresses; Modelling; Asymptotic solution; Numerical approach

Modification of rubber surface by UV surface grafting by A.M. Shanmugharaj; Jin Kuk Kim; Sung Hun Ryu (pp. 5714-5722).

Rubber surface is subjected to ultraviolet radiation (UV) in the presence of allylamine and radiation sensitizer benzophenone (BP). Fourier transform infrared spectral studies reveal the presence of allylamine on the surface. The presence of irregular needle shapes on the surface as observed in scanning electron micrographs also confirms the polymerized allylamine on the surface. Allylamine coatings have been further confirmed from atomic force microscopy (AFM) analysis. Thermogravimetric analysis (TGA) reveals that allylamine coating on the rubber surface lowers the thermal degradation rate. The contact angle between the water and rubber surface decreases for the modified rubber surface confirming the surface modification due to UV surface grafting.

Keywords: PACS; 61.80.Ba; 33.20.Ea; 68.37.Hk; 68.37.Ps; 81.70.PgUV grafting; Allylamine; Atomic force microscopy; Contact angle

Influence of carrier gas pressure and flow rate on atomic layer deposition of HfO₂ and ZrO₂ thin films by Jaan Aarik; Aleks Aidla; Aarne Kasikov; Hugo Mändar; Raul Rammula; Väino Sammelselg (pp. 5723-5734).

Influence of the carrier gas on HfCl₄–H₂O and ZrCl₄–H₂O atomic layer processes was investigated. The growth rates of HfO₂ and ZrO₂ decreased with increasing flow rate and pressure of the N₂ carrier gas. Data of real-time quartz crystal microbalance measurements demonstrated that the effect observed was mainly due to influence of carrier gas on surface reactions and the role of overlapping the precursor pulses was negligible. At the same increase of the carrier gas mass flow, the increase of the linear flow rate led to more significant changes of thin-film properties than the increase of the carrier gas pressure did. Thin films with higher density, higher refractive index and, particularly, lower concentration of residual chlorine were obtained at higher carrier gas flow rates. Increase of the carrier gas flow rate also resulted in a higher concentration of a metastable phase in HfO₂ thin films deposited at 300°C.

Keywords: PACS; 68.55; 77.55; 78.65Hafnium dioxide; Zirconium dioxide; Atomic layer deposition; Surface reactions; Crystal structure; Optical properties

Modification of tribology and high-temperature behavior of Ti–48Al–2Cr–2Nb intermetallic alloy by laser cladding by Xiu-Bo Liu; Hua-Ming Wang (pp. 5735-5744).

In order to improve the tribology and high-temperature oxidation properties of the Ti–48Al–2Cr–2Nb intermetallic alloy simultaneously, mixed NiCr–Cr₃C₂ precursor powders had been investigated for laser cladding treatment to modify wear and high-temperature oxidation resistance of the material. The alloy samples were pre-placed with NiCr–80, 50 and 20%Cr₃C₂ (wt.%), respectively, and laser treated at the same parameters, i.e., laser output power 2.8kW, beam scanning speed 2.0mm/s, beam dimension 1mm×18mm. The treated samples underwent tests of microhardness, wear and high-temperature oxidation. The results showed that laser cladding with different constitution of mixed precursor NiCr–Cr₃C₂ powders improved surface hardness in all cases. Laser cladding with NiCr–50%Cr₃C₂ resulted in the best modification of tribology and high-temperature oxidation behavior. X-ray diffraction (XRD), optical microscope (OM), scanning electron microscopy (SEM) and energy-dispersive spectrometer (EDS) analyses indicated that the formation of reinforced Cr₇C₃, TiC and both continuous and dense Al₂O₃, Cr₂O₃ oxide scales were supposed to be responsible for the modification of the relevant properties. As a result, the present work had laid beneficial surface engineering foundation for TiAl alloy applied as future light weight and high-temperature structural candidate materials.

Keywords: TiAl intermetallic alloy; Microstructure; Wear resistance; High-temperature oxidation; Laser cladding

Ellipsometric investigation of optical constant and energy band gap of Zn1−_xMn_xSe/GaAs (100) epilayers by D.-J. Kim; Y.-M. Yu; Y.D. Choi; J.-W. Lee (pp. 5745-5751).

Zn1−_xMn_xSe/GaAs (100) epilayers were grown using a hot-wall epitaxy method. The spectroscopic ellipsometry was used to determine the optical dielectric constant. The obtained pseudodielectric function spectra revealed the distinct structures at energies of E₀, E₀+Δ₀, E₁, E₁+Δ₁, E₂ andE0′+Δ₀ critical points (CPs) at lower Mn composition range. These critical points were determined by analytical line-shapes fitted to numerically calculated derivatives of their pseudodielectric functions. The peak characteristics were changed with the change in Mn composition. The spectral dependence of pseudodielectric function 〈 ɛ〉 was used to obtain the fundamental energy gaps E₀ including a unique relation with Mn composition. Also, the shifting and broadening of the CPs were observed with increasing Mn composition.

Keywords: Hot-wall epitaxy; ZnMnSe; Spectroscopic ellipsometer; Pseudodielectric constant

Heterogeneity of activated carbons with different surface chemistry in adsorption of phenol from aqueous solutions by K. László; P. Podkościelny; A. Dąbrowski (pp. 5752-5762).

The heterogeneity of activated carbons is investigated on the basis of adsorption isotherms of phenol from dilute aqueous solutions at different values of pH in the solution. The original carbon studied was prepared from polyethyleneterephtalate (PET). Its various oxygen surface functionalities were systematically changed by additional nitric acid and heat treatments. The Dubinin–Astakhov adsorption-isotherm equation was used to evaluate the parameters characterizing the adsorption of phenol from dilute water solutions on activated carbon surfaces. Adsorption energy distribution functions were calculated by the INTEG algorithm, based on a regularization method. Analysis of distribution functions for activated carbons provides significant comparative information about their energetic heterogeneity. Moreover, a comparison of the resulting energies obtained from the distributions can be made with enthalpic data.

Keywords: Liquid-phase adsorption; Activated carbon; Phenol; Heterogeneity; Porosity

Two-photon absorption study of the large-sized nanocrystallites by J. Ebothé; I.V. Kityk; I. Fuks-Janczarek (pp. 5763-5767).

The influence of size and shape for nanocrystallines (NC) of semiconducting materials (like GaN, ZnO, CdS, ZnS with sizes about 20–30nm) incorporated into olygoetheracrylate polymer matrices on photoinduced optical properties, particularly the two-photon absorption (TPA), was investigated. We have shown that maximal values of the TPA diagonal tensor component was observed at low temperatures for the GaN NC (about 4.2K), which corresponds to a NC concentration of 2.6–3.2wt.% and size of about 24nm. The shape and value of ellipticity play crucial roles in the observed non-linear optical phenomena. This contradicts the generally adopted model wherein the non-linear optical properties are mainly determined by averaged NC sizes. The observed temperature dependence is explained within a framework of anharmonic electron–phonon interaction in the large-sized nanocrystallites.

Keywords: Semiconducting nanocrystallines; Non-linear optics; Temperature dependence

Use of modified chemical route for ZnSe nanocrystalline thin films growth: Study on surface morphology and physical properties by R.B. Kale; C.D. Lokhande; R.S. Mane; Sung-Hwan Han (pp. 5768-5775).

The zinc selenide thin films have been deposited using modified chemical bath deposition (M-CBD) method. Zinc acetate and sodium selenosulphate were used as Zn²⁺ and Se²⁻ ion sources, respectively. The preparative parameters such as concentration, pH, number of deposition cycles have been optimized in order to deposit ZnSe thin films. The as-deposited ZnSe thin films are specularly reflective and faint yellowish in color. The as-deposited ZnSe films are annealed in an air atmosphere at 473K for 2h. The films are characterized using structural, morphological, compositional, optical and electrical properties.

Keywords: Thin Films; ZnSe; XRD; SEM; AFM

X-ray photoelectron spectroscopy characterization of oxidated Si particles formed by pulsed ion-beam ablation by X.P. Zhu; Tomiyuki Yukawa; Makoto Hirai; Hisayuki Suematsu; Weihua Jiang; Kiyoshi Yatsui; H. Nishiyama; Y. Inoue (pp. 5776-5782).

X-ray photoelectron spectroscopy (XPS) is used to probe oxidation states of Si species in particles deposited using a pulsed ion-beam evaporation method. The effects of He ambient gas, ion beam intensity and post-treatments on the oxides composition and oxygen content have been studied. It is found that presence of He ambient gas led to a profound oxidation of Si species as compared to that prepared in vacuum at the same ion-beam ablation energy, i.e. both increase of SiO₂ component and oxygen concentration in the oxides coverage. The deposition in He also resulted in an increase of oxygen concentration even under lower ablation intensity, but a higher Si suboxides concentration. It is revealed that the reaction between Si and O was controlled by the ion beam intensity (temperature of Si plasma) and the gas ambient (collision probability of Si and O species). The difference in structure of oxide layers for samples obtained under various conditions is discussed based on the results of XPS analyses.

Keywords: Silicon oxides; Pulsed ion-beam evaporation; Ablation plasma; X-ray photoelectron spectroscopy

Electrochemical and AFM characterization of the electropolimerization of pyrrole over a graphite–epoxy resin solid composite electrode, in the presence of different anions by G.A. �?lvarez-Romero; E. Garfias-García; M.T. Ramírez-Silva; C. Galán-Vidal; M. Romero-Romo; M. Palomar-Pardavé (pp. 5783-5792).

This work studied the process of electrochemical formation of polypyrrole, Ppy, over a graphite–epoxy resin composite electrode in the presence of different anions: DS⁻, NO₃⁻ and Cl⁻. It was shown by means of AFM that the morphological characteristics and the formation kinetics of the Ppy deposit, from analysis of experimental current density transients, are strongly dependent on the nature of the anion present. It was found that for all cases, the formation mechanism involves the simultaneous presence of adsorption and 3D nucleation, limited by diffusion of the different pyrrole oligomer species, processes.

Keywords: Conducting polymers; Polypyrrole; Electrodeposition

Effect of interface traps on Debye thickness semiconductor films by V. Sandomirsky; A.V. Butenko; I.G. Kolobov; A. Ronen; Y. Schlesinger (pp. 5793-5802).

The presence of the boundary interface trapping states and their role in determining the properties of Debye thickness thin semiconductor films, are demonstrated experimentally, using PbTe films deposited on mica. These charged states could not be observed earlier and be studied directly, because of the screening by the relatively high carrier density of the ordinary PbTe. Thin, Debye length thickness, PbTe films with a high concentration of interface trapping centers, possess an extraordinary high resistance. In this case the thermostimulated capacitor discharge method can be successfully applied to determine the energy of these levels, their carrier capture cross-sections and their donor- or acceptor-like character. The experimental results and theoretical calculations are discussed.

Keywords: PACS; 73.20.−r; 73.40 Qv; 73.50.−hInterface traps; PbTe; Debye length thickness film; Relaxation; Electric field effect

I– V curve oscillation observed by atomic force microscopy by L.X. Li; R.P. Liu; C.Z. Fan; M.Y. Lv; J. Li; W.K. Wang (pp. 5803-5807).

Oscillation on the current–voltage curve measured by atomic force microscopy is observed when the distance between the tip and sample is large enough and beyond a critical value. The appearance of the oscillation is attributed to the excitation of electron standing waves between the tip and sample. From the first peak position and the voltage difference between the first two peaks on the current–voltage curve, the value of the work function at the detected point on silver film surface and the distance between the tip and the detected point can be calculated.

Keywords: Oscillation on current–voltage curve; Atomic force microscopy; Electron standing wave; Work function

Excitation mechanisms and localization sites of erbium-doped porous silicon by A. Najar; H. Elhouichet; N. Lorrain; M. Oueslati (pp. 5808-5813).

Porous silicon (PS) is doped with erbium by electrochemical anodisation. The penetration of erbium into the PS layer is confirmed by Rutherford backscattering spectroscopy (RBS) and energy dispersive X-ray (EDX) measurements. Efficient green and infrared emissions were observed at room temperature. The investigations are focused on the evolutions versus temperature and pump intensity of the green photoluminescence (PL) corresponding to the⁴S_3/2→⁴I_15/2 transition. It was found that an erbium related level defect can be involved on the excitation and emission processes of erbium. Pump intensity dependent PL studies revealed that for the electrochemical incorporation, most of the Er³⁺ ions are localized inside the Si nanocrystallites and not in stoichiometric SiO₂. The optical cross-section is close to that of erbium in Si nanocrystallites.

Keywords: Porous silicon; Erbium; Excitation mechanisms; Cross-section

Annealing and measurement temperature dependence of W₂B₅-based rectifying contacts to n-GaN by Rohit Khanna; S.J. Pearton; F. Ren; I. Kravchenko (pp. 5814-5819).

The thermal stability and measurement temperature dependence of Schottky contact characteristics on n-GaN using a W₂B₅/Ti/Au metallization scheme was studied using current–voltage ( I– V), scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) measurements. The elemental profile obtained from samples annealed at 350°C showed some titanium diffusion into the gold layer but little other difference from the as-deposited wafer. Annealing at 700°C produced significant diffusion of titanium. The Schottky barrier height increased with anneal temperature up to 200°C, reaching a maximum value of 0.65eV, but decreased at higher annealing temperatures. The reverse breakdown voltage from diodes fabricated using the W₂B₅-based contacts showed a similar dependence. The reverse current magnitude was larger than predicted by thermionic emission alone. The barrier height showed only minor changes with measurement temperature up to 150°C.

Keywords: Thermal stability; Temperature dependence; n; -GaN

Influence of transition metals (Cr, Mn, Fe, Co and Ni) on the methane combustion over Pd/Ce–Zr/Al₂O₃ catalyst by Baohua Yue; Renxian Zhou; Yuejuan Wang; Xiaoming Zheng (pp. 5820-5828).

The effects of transition metals (Cr, Mn, Fe, Co and Ni) on the catalytic properties of Pd/Ce–Zr/Al₂O₃ catalyst for methane combustion have been investigated. The supported Pd catalysts are characterized by BET, XRD, TEM, TPR, TPO and TPSR measurements. Activity tests in methane combustion show that Pd/Ce–Zr–Ni/Al₂O₃ has the highest catalytic activity and thermal stability among all catalysts. The results of TEM show that the addition of Ni to Pd/Ce–Zr/Al₂O₃ increases the dispersion of Pd component and inhibits the site growth. The results of TPO and TPSR show that the addition of Ni inhibits the decomposition of PdO particles and improves the reduction–reoxidation properties of the active PdO species, which increases the catalytic activity and thermal stability of the Pd/Ce–Zr/Al₂O₃ catalyst.

Keywords: PACS; 82.65.+rPalladium; Ceria–zirconia–alumina; Transition metals; Thermal stabilization; Methane combustion

Deposition of graded TiO₂ films featured both hydrophobic and photo-induced hydrophilic properties by Kee-Rong Wu; Jin-Jen Wang; Wu-Chien Liu; Zhi-Sheng Chen; Jiing-Kae Wu (pp. 5829-5838).

Graded TiO₂ films were deposited on unheated glass substrates by using a twin dc magnetron sputtering system. The graded TiO₂ films showed a highly polycrystalline structure of anatase with a little rutile phases revealed by X-ray diffraction spectra. The surface energy of the fresh and UV irradiated films were evaluated by water contact angle measurement. The results indicated that the water contact angle of the fresh graded TiO₂ films was found within 100–112°. The films then became a highly hydrophilic surface with the water contact angle of almost zero under 60min UV irradiation. The XPS spectrum of Ti 2p revealed that the graded TiO₂ films became a stoichiometric titanium oxide layer near the surface, proving that titanium was fully oxidized. It was found that the surface OH group density depended on the substrates employed for given sputtering conditions. In addition, AFM images revealed a considerably rough surface of the graded films with RMS roughness of 12.6–14.5nm. One can conclude that the unique properties of highly hydrophobicity and photo-induced hydrophilicity can be attributed to fully oxidized chemical composition and higher roughness on the film surface.

Keywords: Graded TiO; ₂; films; Water contact angle; Hydrophobic; Hydrophilic

Oxidation of Zircaloy-4 by H₂O followed by molecular desorption by N. Stojilovic; R.D. Ramsier (pp. 5839-5845).

The interaction of H₂O with Zircaloy-4 (Zry-4) is investigated using Auger electron spectroscopy (AES) and temperature programmed desorption (TPD) methods. Following adsorption of H₂O at 150K the Zr(MNV) and Zr(MNN) Auger features shift by ∼6.5 and 4.5eV, respectively, indicating surface oxidation. Heating H₂O/Zry-4 results in molecular desorption of water at both low and high temperatures. The low-temperature desorption is attributed to ice multilayers, whereas, three overlapping high-temperature features are presumably due to recombinative desorption. This high-temperature desorption begins before the surface oxide is dissolved, continues upon its removal, and is atypical for water/metal systems. Unexpectedly, no significant desorption of hydrogen is observed near 400K, as is typically observed following O₂ adsorption on Zr-based materials. However, we do observe that H₂O adsorption on Zry-4 surfaces roughened by argon ion sputtering results in H₂ desorption.

Keywords: Zircaloy-4; Water; TPD; AES; Zero-order desorption kinetics

Nano-deformation of a Ni–P coating surface after nanoparticle impacts by J. Xu; J.B. Luo; C.H. Zhang; W. Zhang; G.S. Pan (pp. 5846-5854).

Chemical mechanical polishing (CMP) has become a primary planarization technique required for the manufacture of a computer hard disk substrate. In CMP, erosive wear, which is regarded as one of the wear mechanisms underlying the interaction between the abrasive particles and polished surfaces, can occur when materials are removed by the surface collision of particles which are carried by a fluid medium. A fundamental understanding of the process in which nanoparticles impact on the surface of the nickel–phosphorous (Ni–P) coating plated on the computer hard disk substrate is important to the control and preventing of surface defects during CMP. In this study, a cylindrical liquid jet containing de-ionized water and SiO₂ nanoparticles impacts obliquely on the surface of Ni–P coating at a speed of 10m/s. Microscopic examinations of the impacted surface are performed using a high resolution transmission electron microscope, an atomic force microscope, etc. Experimental results indicate that craters and scratches in the surface have taken place after nanoparticle impacts, and crystal grains in nano-scale and an element phosphorus concentration can be found in the sub-surface layer of the impacted surface.

Keywords: Hard disk driver; Ni–P coating; Nanoparticle impact; Nano-deformation

The dissociative adsorption of borane on the Ge(100)-2×1 surface: A density functional theory study by Yi-Jun Xu; Jun-Qian Li (pp. 5855-5860).

By means of cluster models coupled with density functional theory, we have studied the hydroboration of the Ge(100)-2×1 surface with BH₃. It was found that the Ge(100) surface exhibits rather different surface reactivity toward the dissociative adsorption of BH₃ compared to the C(100) and Si(100) surfaces. The strong interaction still exists between the as-formed BH₂ and H adspeices although the dissociative adsorption of BH₃ on the Ge(100) surface occurs readily, which is in distinct contrast to that on the C(100) and Si(100) surfaces. This can be understood by the electrophilic nature of the down Ge atom, which makes it unfavourable to form a GeH bond with the dissociating proton-like hydrogen. Alternatively, it can be attributed to the weak proton affinity of the Ge(100) surface. Nevertheless, the overall dissociative adsorption of BH₃ on group IV semiconductor surfaces is favourable both thermodynamically and kinetically, suggesting the interesting analogy and similar diversity chemistry of solid surface in the same group.

Keywords: Adsorption; Borane; Ge(1; 0; 0) surface; Density functional theory

Studies on behaviors of dipalmitoylposphatidylcholine and bilirubin in mixed monolayer at the air/water interface by Yuhua Shen; Yufeng Tang; Anjian Xie; Jinmiao Zhu; Shikuo Li; Yong Zhang (pp. 5861-5867).

Mixed monolayers of dipalmitoylposphatidylcholine (DPPC) and bilirubin (BR) were prepared on different subphases. The properties of DPPC/BR monolayer, such as collapse pressure ( π_coll), limiting area per molecule ( A_lim), surface compressibility modulus, free energy (Δ G_mix) and excess free energy (Δ G_ex), were investigated based on the analysis of the surface pressure–area isotherms on pure water. The results showed that DPPC and BR were miscible and formed non-ideal mixed monolayers at the air/water interface. With the molar fraction of BR ( X_BR) increasing, the LE–LC coexistence region of DPPC monolayer was eliminated gradually. The DPPC/BR complex (M_D–B) of 1:2 stoichiometry formed as a result of the strong hydrogen bonds between the polar groups of DPPC and BR. The studies of effects of pH values and calcium ions in subphase on the DPPC/BR monolayers showed that the mixed monolayer became expanded on alkali aqueous solution and on 1mmol/L CaCl₂ aqueous solution. The orientation of DPPC and BR at air/water interface was also discussed.

Keywords: PACS; 34.50.Dy; 68.18.+p; 82.65.DpAir/water interface; Mixed monolayer; Surface pressure–area isotherm; Dipalmitoylphosphatidylcholine/bilirubin

Calculation of subband breadth of GaAs/AlGaAs superlattice by Li-jun He; Xing-kui Cheng; Hua Li; Jian Zhang; Jun-ming Zhou; Qi Huang (pp. 5868-5872).

From the view of electron waving, taking account of the electron wave reflections at the interface between the well and the potential barrier layer we discuss the electronic states above the barriers in a GaAs/AlGaAs superlattice. We present a new method on calculating the breadth of the subband and the calculated breadth of GaAs/AlGaAs superlattice is in good agreement with experimental results.

Keywords: Superlattice; Electron states; Subband breadth

The tungsten powder study of the dispenser cathode by Bao Ji-xiu; Wan Bao-fei (pp. 5873-5876).

The intercorrelation of tungsten powder properties, such as grain size, distribution and morphology, and porous matrix parameters with electron emission capability and longevity of Ba dispenser cathodes has been investigated for the different grain morphologies. It is shown that a fully cleaning step of the tungsten powder is so necessary that the tungsten powder will be reduction of oxide in hydrogen atmosphere above 700°C. The porosity of the tungsten matrix distributes more even and the closed pore is fewer, the average granule size of the tungsten powder distributes more convergent. The porosity of the tungsten matrix and the evaporation of the activator are bigger and the pulse of the cathode is smaller when the granularity is bigger by the analysis of the electronic microscope and diode experiment.

Keywords: Dispenser cathodes; Tungsten powder; Porosity; Granules; Size of porous; Cathode life

Chemical reaction of sputtered Cu film with PI modified by low energy reactive atomic beam by Jong-Yong Park; Yeon-Sik Jung; J. Cho; Won-Kook Choi (pp. 5877-5891).

Polyimide (PMDA-ODA) surface was irradiated by low energy reactive atomic beam with energy 160–180eV to enhance the adhesion with metal Cu film. O₂⁺ and N₂⁺ ions were irradiated at the fluence from 5×10¹⁵ to 1×10¹⁸cm⁻². Wetting angle 78° of distilled deionized (DI) water for bare PI was greatly reduced down to 2–4° after critical ion flounce, and the surface energy was increased from 37 to 81.2erg/cm. From the analysis of O 1s core-level XPS spectra, such improvement seemed to result from the increment of hydrophilic carbonyl oxygen content on modified PI surface. To see more carefully correlation of the peel strength with interfacial reaction between Cu and PI, flexible copper clad laminate with Cu (9μm)/Cu (200nm) on modified PI substrate (25μm) was fabricated by successive sputtering and electroplating. Firstly, peel strength was measured by using t-test and it was largely increased from 0.2 to 0.5kgf/cm for Ar⁺ only irradiated PI to 0.72–0.8kgf/cm for O₂⁺ or N₂O⁺ irradiated PI. Chemical reaction at the interface was reasoned by analyzing C 1s, O 1s, N 1s, and Cu 2p core-level X-ray photoelectron spectroscopy over the as-cleaved Cu-side and PI side surface through depth profiling. From the C 1s spectra of cleaved Cu-side, by the electron transfer from Cu to carbonyl oxygen, carbonyl carbon atom became less positive and as a result shifted to lower binding energy not reaching the binding energy of C₂ and C₃. The binding energy shift of the peak C₄ as small as 1.7eV indicates that carbonyl oxygen atoms were not completely broken. From the analysis of the O 1s spectra, it was found that new peak at 530.5eV (O₃) was occurred and the increased area of the peak O₃ was almost the same with reduced area of the peak carbonyl oxygen peak O₁. Since there was no change in the relative intensity of ether oxygen (O₂) to carbonyl oxygen (O₁), and thus O₃ was believed to result from Cu oxide formation via a local bonding of Cu with carbonyl oxygen atoms. Moreover, from X-ray induced Auger emission spectra Cu LMM which was very sensitive to chemical bonding, Cu oxide or CuOC complex formation instead of CuNO complex was clearly identified by the observation of the peak at 570eV at higher 2eV than that of metal Cu. In conclusion, when Cu atoms were sputtered on modified PI by low energy ion beam irradiation, it can be suggested that two Cu atoms locally reacted with carbonyl oxygen in PMDA units and formed Cu⁺O⁻C complex linkage without being broken from carbon atoms and thus the chemically bound Cu was in the form of Cu₂O.

Keywords: PACS; 68.35.Np; 82.30.Fi; 82.35.Gh; 81.65.Mq; 87.64.GbLow energy atomic beam; Surface energy; Cu adhesion; Polyimide; Peel strength; X-ray induced Auger emission spectra

Investigation of chemical decomposition of CCl₄ on TiO₂ near room temperature by Yi-Shiue Lin; Meng-Tso Chen; Yu-Feng Lin; Shang-Ju Yang; Jong-Liang Lin (pp. 5892-5899).

Dissociative adsorption of CCl₄ on TiO₂ at 35°C has been studied by Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and electron spin resonance. CCl₄ decompose to form CO, CO₂, and CO₃ on the surface, at such a low temperature, in which CO₂ formation is not from CO oxidation on TiO₂, but CO₃ can be produced by CO and CO₂ adsorption. The Cl generated from CCl₄ decomposition is left on the surface and bonded to titanium ions. Mineralization of CCl₄ on TiO₂ involves the lattice oxygens. Thermodynamical driving force and possible reaction routes for CO and CO₂ formation in the CCl₄ decomposition on TiO₂ are discussed.

Keywords: Fourier-transform infrared spectroscopy; X-ray photoelectron spectroscopy; CCl; ₄; TiO; ₂

Integrated description for random adsorption and 2D-island growth kinetics in thin film growth: Autocatalytic-reaction model and kinetic Monte Carlo simulation by Hideaki Togashi; Yoshiharu Enta; Maki Suemitsu (pp. 5900-5906).

Because of the interplay among various surface processes, an integrated description of 2D film growth is usually a very difficult task. As far as adsorption–migration–desorption of precursors and the resultant nucleation, growth, and coalescence of 2D islands up to 1 monolayer are concerned, however, the autocatalytic-reaction (ACR) model [Phys. Rev. Lett. 82 (1999) 2334] can be a rare exception, which satisfactorily models the oxide film uptake curve during Si dry oxidation from a Langmuir to sigmoidal behaviors. A kinetic Monte Carlo (KMC) simulation, conducted to mimic the 2D growth of thin films, shows a similar shift from a Langmuir to sigmoidal uptake behavior by varying the parameters corresponding to adsorption, migration, and desorption. A comparison between ACR and KMC clarifies that the success of the ACR model lies in its effective inclusion of adsorption of species as well as of nucleation, growth, and coalescence of islands. A recipe is presented to translate the ACR parameters to KMC ones, which allows one to discuss the surface morphology based on the ACR analysis on the film uptake curve.

Keywords: PACS; 68.45.Da; 82.65.PaThin film growth; Autocatalytic reaction; Island growth; Nucleation; Coalescence; Si oxidation; Kinetic Monte Carlo simulation

Effect of Al on the electrochemical corrosion behaviour of Pb free Sn–8.5 Zn–0.5 Ag–XAl–0.5 Ga solder in 3.5% NaCl solution by Udit Surya Mohanty; Kwang-Lung Lin (pp. 5907-5916).

The effect of Al on the electrochemical corrosion behaviour of Pb-free Sn–8.5 Zn–0.5 Ag–XAl–0.5 Ga solder in 3.5% NaCl solution was investigated by using potentiodynamic polarization techniques. The X content in the solder varied from 0.1 to 3wt.%. Polarization studies revealed that an increase in Al content upto 1.5wt.% decreased the corrosion current density ( I_corr), corrosion rate of the solder and shifted the corrosion potential ( E_corr) towards more noble values. However, higher content of Al, i.e. 3 (wt.%) in the five-element solder enhanced the corrosion rate and resulted in a significant increase in the E_corr towards more negative values. Passivation behaviour was noticed in all the solders having varying Al content, but the passive film formed at 1.5wt.% Al was most stable due to its low passivation current density ( i_p) and low critical current density ( i_cc) value in comparison to the other solders. XPS and Auger depth profile results revealed that the passive film consisted of oxides/hydroxides of Al and Zn formed on the surface of the solder with Sn being formed in the subsequent layer. Considerable aluminium segregation occurred towards the surface principally as Al₂O₃/Al(OH)₃ with increase in Al content to 1.5wt.% in the five element solder. The formation of Al₂O₃ seemed to prevent the oxidation of zinc on the surface of the solder.

Keywords: Aluminium; Anodic polarization; AES; Corrosion; Pb free solders; XPS

Two-dimensional recrystallisation processes of nanometric vanadium oxide thin films grown by atomic layer chemical vapor deposition (ALCVD) evidenced by AFM by H. Groult; E. Balnois; A. Mantoux; K. Le Van; D. Lincot (pp. 5917-5925).

The influence of thermal annealing on the morphology and structure of nanometer range thickness vanadium oxide films deposited by ALCVD on silicon substrate was investigated by AFM. The appearance of crystalline centres with typical rectangular V₂O₅ plates was clearly observed from 400°C. Furthermore, spectacular 2D-reorganisation phenomenon with increasing temperature was pointed out since, initial circular particles change to elongated ones with a rectangular shape with increasing temperature. This reorganisation process results from an increase in the high surface atomic mobilities with increasing temperature. The growth of V₂O₅ particles in the ab-plane occurs preferentially along the b-direction for which the atoms density is higher, in good agreement with results previously deduced from XRD analyses. The latter show limitation of the coherence domains values along the a-axis for temperatures higher than 450°C.

Keywords: PACS; 61.16; 61.46; 68.55; 68.60; 81.15; 81.20Vanadium oxide; Atomic layer chemical vapor deposition; AFM; Film growth

Low-temperature MOVPE growth of ZnO thin films by using a buffer layer by W.Z. Xu; Z.Z. Ye; L. Jiang; Y.J. Zeng; L.P. Zhu; B.H. Zhao (pp. 5926-5929).

ZnO thin films have been grown on a-plane (1,1,−2,0) sapphire substrates by metalorganic vapor phase epitaxy (MOVPE) at low substrate temperature of 350°C. It is showed that the crystal and electrical quality of the thin films was improved by using a ZnO buffer layer. The photoluminescence (PL) measurements indicate that the ZnO thin films grown at such a low substrate temperature have a strong UV emission.

Keywords: PACS; 61.10.N; 81.15.Kk; 68.55; 73.61.G; 78.66.HCrystal structure; Characterization; Metalorganic chemical vapor deposition; Zinc compounds

Schottky barrier properties of various metal (Zr, Ti, Cr, Pt) contact on p-GaN revealed from I– V– T measurement by C.K. Tan; A. Abdul Aziz; F.K. Yam (pp. 5930-5935).

Schottky barrier contact using three different metal (Zr, Ti, Cr and Pt) and Ohmic contact using Ni were made on same epitaxial growth layer of p-GaN. Measurements were carried out using current–voltage–temperature ( I– V– T) in the range of 27–100°C. Under forward bias and room-temperature (RT), the ideality factors ( η) were determined to be 2.38, 1.82, 1.51 and 2.63, respectively, for Zr, Ti, Cr and Pt. The Schottky barrier height (SBH) and effective Richardson coefficient A^** were measured through modified Norde plot as one of the analysis tools. Barrier heights of 0.84, 0.82, 0.77 and 0.41eV for Zr, Ti, Cr and Pt, respectively, were obtained from the modified Norde plot. Schottky barrier heights of Zr, Ti, or Cr/p-GaN were also measured through activation energy plot, and determined to be in the same range (∼0.87eV) and Pt at 0.49eV. These results indicate that the Fermi level seems to be pinned due to the value of slope parameter ( S) was very low ( S=−0.25).

Keywords: PACS; 73.20.−r; 73.30.+y; 73.61.Ey I; –; V; –; T; Pinning Fermi level; p-GaN; Schottky contact

Study of glasses with grisailles from historic stained glass windows of the cathedral of León (Spain) by N. Carmona; M.A. Villegas; J.M. Fernández Navarro (pp. 5936-5945).

This work concerns the study of grisailles of historic glass samples from stained glass windows of the Cathedral of León, which were removed during the restoration carried out in 19th century. Both the glass samples and their coloured grisailles showed very different chemical composition and macroscopic heterogeneity. As a general rule their deterioration degree is rather moderate, maybe due to the pieces removal that preserve them from the high atmospheric pollution occurred in the last century. The present research pointed out the physical characteristics, chemical compositions and deterioration degree of the samples selected from the most important Spanish ensemble of Medieval and Renaissance stained glass windows. Moreover, this work offers sufficient results to be compared with those formerly obtained for other stained glass windows from European cathedrals and churches.

Keywords: PACS; 81.05; 82.80; 61.16.BGrisailles; Stained glass windows; Medieval; Renaissance; Analytical study

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Applied Surface Science (v.252, #16)