Applied Surface Science (v.172, #3-4)

Copper phthalocyanine and tetra-tert-butyl copper phthalocyanine were vacuum deposited onto gold electrode. The effects of substrate materials (glass and gold) and post-deposition annealing on the film morphology and crystalline structure were studied. The film characteristics were related to their sensing propertied to NO2. The results show that the growth behaviors of CuPc on glass and gold surfaces were different in the early growth stage, but their surface morphologies were similar at the later stage. The CuPc films have structures of amorphous and β form crystallite, respectively, on gold and glass surfaces. After heat annealing, the grains grow larger and a β form structure were obtained on both surfaces. The films without heat treatment show a short response time and higher sensitivity to NO2 due to the higher surface area of their loosely packed fine-grain structure. But this characteristic also causes in-diffusion of NO2 deeply into the bulk crystal and thus, the recovery process is slow and not completely reversible. Comparing with CuPc films, ttb–CuPc films have smoother morphology both on glass and gold and thus, the gas sensitivity is much smaller due to their smaller exposing area for the adsorption of NO2. The ttb–CuPc films have a diffraction peak at D=16.04 Å after heat annealing. The substitution of tetra-tert-butyl group into the phthalocyanine ring leads to a larger lattice spacing of crystal, which may be the reason for ttb–CuPc films to have faster recovery process. Because no apparent variation of the crystal morphology was found on the heat-treated ttb–CuPc films, the gas sensing properties vary little due to the heat treatment.
Keywords: Phthalocyanine; Vacuum deposition; Heat treatment; Gas sensor; Film morphology;

On the growth mechanism of UV laser deposited a-C:H from CH2I2 at room temperature by Mikael Lindstam; Mats Boman; Klaus Piglmayer (200-206).
Hydrogenated amorphous carbon films have been deposited on tungsten and quartz substrates at room temperature by photolytic dissociation of CH2I2. An Ar+ cw laser operating at 350 nm was used as the excitation source. The laser beam was focused parallel to the substrate surface. The deposition process was investigated as a function of laser power and total pressure. The carbon films were analysed by micro-Raman spectroscopy, IR spectroscopy, atomic force microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. The growth mechanism is discussed from results and analysis.
Keywords: Amorphous hydrogenated carbon; Laser CVD; Photolysis; Modeling;

We have developed a new method which enables us to atomically control the height and the periodicity of the step distribution on vicinal SrTiO3 (1 0 0) substrates. This method also enables us to obtain precipitate-free epitaxy of oxides. Practically thick YBa2Cu3O7−δ films grown on such terraced vicinal substrates exhibited a regular epitaxial growth, and smooth and precipitate-free surfaces of practical device sizes.
Keywords: YBa2Cu3O7−δ ; Precipitates; Vicinal substrate; Step-terrace structure;

Electroreflectance study of macroporous silicon surfaces by R.Yu Holiney; L.A Matveeva; E.F Venger; A.O Livinenko; L.A Karachevtseva (214-219).
The surfaces of a number of samples of macroporous silicon were characterized by electroreflectance spectroscopy. Specimens with pore diameters of 1–15 μm and pore depths from 15 to 80 μm were prepared by electrochemical etching with different values of current and bias. Etching enriches n-type silicon with majority carriers, and the existence of an intrinsic electric field is demonstrated. The intrinsic field develops due to the growth of an oxide layer on the pore walls. Correlation between the etching current and physical characteristics such as the phenomenological broadening parameter Γ, the magnitude of the intrinsic electric field, and the electro-optic energy is shown.
Keywords: Macroporous silicon; Electroreflectance spectroscopy; Electromagnetic waves;

KrF laser-induced photolysis of gaseous dimethyl selenium in excess of helium is controlled by cleavage of both Se–C bonds and affords ethane as a very dominant gaseous product together with elemental selenium. The photolysis mechanism being different from that of thermolysis shows potential for chemical vapour deposition of selenium films not contaminated with carbon impurities.
Keywords: Selenium films; Laser-induced photolysis; Dimethyl selenium;

Porous silicon as a potentiometric transducer for ion detection: effect of the porosity on the sensor response by S. Zairi; C. Martelet; N. Jaffrezic-Renault; R. Lamartine; R. M’gaı̈eth; H. Maaref; M. Gamoudi; G. Guillaud (225-234).
This paper shows the possibility to use the oxidized porous silicon (PS) as a transducer material for ion sensor application. It aims to study the over Nernstian behavior of the porous material towards the concentration of sodium ions in contact. We have studied the dependence of the PS sensitivity on the porosity of the samples, which are prepared from a lightly doped silicon substrate. Then, we have presented a model to explain the mechanism of the ionic species adsorption at the electrolyte/SiO2 interface, and to interpret the observed large sensitivity against the different concentrations of the cations. The reproducibility of the sensor response and its lifetime were satisfactory for a frequent use.
Keywords: Transducer; Sensor; Over Nernstian; Porous silicon; Sensitivity;

Moderator gratings for the generation of epithermal positive muons by T. Prokscha; E. Morenzoni; C. David; A. Hofer; H. Glückler; L. Scandella (235-244).
We studied the effect of surface enlargement on the efficiency of a muon (μ+) moderator by comparing the flux of epithermal μ+ emerging from a solid Ar (s-Ar) layer deposited on a flat Al substrate with the fluxes from an Ar layer deposited on gratings manufactured in Si and Ag substrates. We developed a simple replication technique to emboss a Si grating into a thin Ag foil, where the gratings consist of V-shaped grooves with 20  μm depth and ridge distance of 30  μm on an area of 30×30  mm 2 . We obtain a gain in epithermal μ+ flux of 1.5 with respect to that of a flat moderator geometry. The 15% reduction with respect to the geometrical enlargement factor of 1.73 is due to μ+ absorption losses in the V-groove walls. The data imply a nearly isotropic angular distribution of epithermal μ+ inside the s-Ar moderator and a reflection coefficient of about 80% for epithermal μ+ impinging on s-Ar.
Keywords: Muon moderation; Surface patterning; Low-energy muon beam;

Preparation of indium tin oxide films and doped tin oxide films by an ultrasonic spray CVD process by Z.B. Zhou; R.Q. Cui; Q.J. Pang; Y.D. Wang; F.Y. Meng; T.T. Sun; Z.M. Ding; X.B. Yu (245-252).
We deposited high quality doped indium oxide and tin oxide thin films by an improved spray CVD process, which we characterize as ultrasonic spraying. The microstructure and electrical properties of these thin films are analyzed by XRD, AFM, and van der Pauw four-point-probe technique and the results discussed. Absorptance and transmittance spectra in the visible–near-ultraviolet spectral region are also presented. The optical band gaps are 3.90 eV for Sn-doped In2O3 and 4.05 eV for F-doped SnO2. The minima of electrical resistivity of Sn-doped In2O3 and F-doped SnO2 films are 1.5×10−4 and 4.0×10−4   Ω cm , respectively.
Keywords: Ultrasonic spraying CVD; Indium tin oxide thin films; F-doped SnO2 films;

High energy photoemission investigations of SiO2/SiC samples by C. Virojanadara; P.-A. Glans; L.I. Johansson; Th. Eickhoff; W. Drube (253-259).
Chemically etched and directly load-locked SiO2/SiC samples are investigated using a photon energy of 3.0 keV. Si 2p and C 1s spectra recorded at different electron emission angles each show two components originating from SiC, SiO2 and graphite like carbon, respectively. The relative intensity of these are extracted and compared to calculated intensity variations. For the samples investigated, best agreement between experimental and calculated intensity variations is obtained when assuming a graphite like layer on top of the oxide. No graphite like carbon at the SiO2/SiC interface was detected, even on a sample for which the graphite like carbon contribution at the surface corresponds to a layer thickness of only 0.05 Å. The energy separation between the oxide and carbide components in the Si 2p spectrum was monitored before and after Ar+ sputtering cycles and before and after in situ heating. The separation increased directly upon sputtering while only in situ heating does not affect it. We suggest that defects induced by the sputtering give rise to the increase, observed in the energy separation.
Keywords: SiC/SiO2 interfaces; Silicon carbide;

ZrO2 buffer layers were successfully dip-coated on stainless steel (SS) foils via a sol–gel process, using 30 mol% SiO2 as a stabilizing agent and zirconyl chloride octahydrate (ZrOCl2·8H2O) as the precursor. The buffer layer was characterized by X-ray diffraction analysis, Fourier transform-infrared spectroscopy, Auger electron spectroscopy, and scanning electron microscopy. The introduction of SiO2 retarded ZrO2 phase transformation from tetragonal to monoclinic. The normal emittances of the Au/ZrO2/SS foils before and after heat-treatment in air at 850°C are 0.038 and 0.043, respectively. The ZrO2 buffer layer effectively suppresses interdiffusion between the gold film and the SS substrate at high temperature.
Keywords: ZrO2; Buffer layer; Sol–gel; Interdiffusion;

On the growth of Fe2As grains at the interface of the Fe/Al x Ga1−x As (x = 0.25) system by F Monteverde; A Michel; J.-P Eymery; J.-C Desoyer (265-275).
Iron thin films on ion-etched monocrystalline AlGaAs(0 0 1) substrates were prepared using ion-beam sputtering deposition. The interface reaction was characterised by planar and cross-sectional transmission electron microscopy after annealing in vacuum at 400°C for 1 h. Interdiffusion mainly results in the generation of Fe2As grains growing into the substrate, perpendicular to the Fe/AlGaAs interface. The iron-arsenide grains exhibit an either triangular or trapezoidal shape. Analyses on their morphology and size, as well as on the orientation relationships between Fe2As and AlGaAs phases were also carried out. The appearance of the Fe2As phase is discussed in terms of standard enthalpy for alloy formation. Finally a pyramidal growth model is proposed for Fe2As grains, which mainly involves the four close-packed {1 1 1}AlGaAs planes.
Keywords: Gallium arsenide; Iron; Interface; Epitaxy; Transmission electron microscopy (TEM);

Polypyrrole layers for steel protection by P. Herrasti; P. Ocón (276-284).
This report describes the behaviour of polypyrrole and polypyrrole/carboximethylcellulose (Ppy/CMC) prepared by different methods in order to prevent the corrosion of steel. Potentiodynamic polarization curves and open circuit potential were used to evaluate the capacity of these materials to protect the surface. The results obtained allowed to assume that Ppy/CMC is better material to prevent corrosion than polypyrrole (Ppy) and potentiostatic method better than galvanostatic to obtain the deposit.
Keywords: Polypyrrole; Polypyrrole/carboxymethylcellulose; Corrosion; Morphology; Stainless steel; Open circuit potential;

A chemical vapour deposition process enhanced by a flowing microwave ArBCl3 post-discharge has been carried out to synthesise at 733 K thin films containing zirconium diboride. In this process, zirconium atoms diffuse from the substrate (Zircaloy-4) to the gas/film interface and react with boron-containing molecules to synthesise ZrB2. Secondary neutral mass spectroscopy analyses show that these films contain oxygen also. Electron probe microanalysis results are not consistent with the possible defined compounds of the ZrBO system described in the literature. Thus, coatings obtained by post-discharge treatment are made of several phases, which are mixed: ZrB2, B2O3 and ZrO2. This point is confirmed by X-ray photoelectron spectroscopy. However, electron diffraction analyses show that only ZrB2 is crystallised. Thus, these films are composed of ZrB2 nanograins embedded in an amorphous solid solution of zirconium and boron oxides. These nanocomposite films contain Zr(Fe,Cr)2 precipitates also, i.e. the same than those present in zircaloy-4 substrates. However, the precipitates size in the film is about one order of magnitude higher than in substrates. According to the growth mechanism of the nanocomposite films, we present a possible explanation on the occurrence and on the size of the Zr(Fe,Cr)2 precipitates.
Keywords: Nanocomposite; ZrB2; Post-discharge; Structure;

The improvement of electrophysical parameters of an SiO2/InAs interface were observed during a DC (direct current) sputter deposition of an ITO (indium tin oxide) film. We observed a reduced interface-state density for the ITO/SiO2/InAs structure from 2.5×1012 to 5.7×1011   cm −2   eV −1 with an electron-stimulated modification of a semiconductor–dielectric interface. The built-in positive charge density also decreased from 2.7×1012 to 6.7×1011   cm −2 . Experiments were conducted to verify that the main cause of the improvement was due to electron penetration through the SiO2 film into the bulk of the semiconductor crystal lattice. By using a DC sputter deposition for the ITO film, we have presented a new method for the modification of a semiconductor–dielectric interface of a ITO/SiO2/InAs structure.
Keywords: Electron–solid interactions; Indium arsenide; Indium oxide; Interface states; Semiconductor–insulator interfaces; Silicon oxides; Sputter deposition;

Simulated nc-AFM images of Si(0 0 1) surface with nanotube tip by Katsunori Tagami; Naruo Sasaki; Masaru Tsukada (301-306).
We predicted the non-contact atomic force microscopy (nc-AFM) images of Si(0 0 1) surface using the nanotube tip from the theoretical calculations based on the tight-binding model. The images are found to depend highly on the tip shape and orientation, and the ghost atoms are frequently observed. These abnormal images are due to the effect of the multi-atom apex, which is analogous to the STM.
Keywords: Nanotube; nc-AFM; Si(0 0 1); Simulated image; Tight-binding;

FT-IR-ATR study of depth profile of SiO2 ultra-thin films by N. Nagai; H. Hashimoto (307-311).
It is very important to characterize the depth change in chemical bonding structures of ultra-thin SiO2 films. The LO mode and the lower frequency shoulder peak of SiO2 on Si was detected by the FT-IR-ATR method, and this band was simulated by a gradient layered and effective medium model. Interface roughness estimated from the shoulder bands of ATR spectra was in good agreement with the GIXR results, and the shoulder band also reflects the change of Si–O–Si bonding angle. In our calculation, the shoulder band around 1150–1050 cm−1 reflects the interface roughness and the shoulder band around 1250–1150 cm−1 includes information about the change of Si–O–Si bonding angle and/or Si–O force constant. The FT-IR-ATR method is a useful technique with which to characterize SiO2 ultra-thin films.
Keywords: SiO2 film; Gate oxide film; LO mode; FT-IR; ATR; GIXR; Interface roughness;

Cerium-based conversion layers on aluminum alloys by Manuele Dabalà; Lidia Armelao; Alberto Buchberger; Irene Calliari (312-322).
Mixed layers of Al and Ce oxides have been obtained in conversion coatings on two aluminum alloys (AA6061 and AA2618). The microstructure and the chemical composition of the protective films have been examined by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). The corrosion resistance in NaCl solution was investigated and a comparison with a traditional chromate conversion coating on the same alloys is given. The coatings obtained by immersion in boiling solution of Ce(III) salts cover uniformly the alloys surface with a protective layer, whereas the H2O2–cerium conversion coating process produces a thick, but cracked layer with “dry-mud” morphology, which preferentially covers the intermetallic particles on the samples surface. The boiled cerium conversion coating treatments are as effective in corrosion resistance as the traditional chromating process, but the interest for industrial purposes is limited because of long application times. The H2O2–cerium conversion coating process produces effective corrosion resistance layers, thinner than the ones obtained by traditional processing, but very interesting for industrial applications due to the limited environmental impact.
Keywords: Chemical conversion coating; Aluminum alloys; Cerium chloride; SEM; XPS; SIMS; Corrosion resistance; EIS; Hydrogen peroxide;

Photoelectric conversion of a novel photoresponsive dendritic dye in LB monolayer films by Fu-You Li; Lin-Yong Zhu; Chun-Hui Huang; Lin-Pei Jin; Jie Zheng; Mei Shi; Jian-Quan Guo; Er-Jian Wang (323-330).
A novel D-π-A-σ-A′ dendritic dye (D3) was synthesized. The dye molecule was successfully transferred onto an ITO electrode by the Langmuir–Blodgett (LB) technique and its photoelectric conversion property in LB monolayer films was investigated. The results show that this dye exhibits good photocurrent generation that is dependent on the deposited pressure. Its photocurrent generation quantum yield at 25 mN m−1 is 0.32%, while that at 6 mN m−1 is 0.21%.
Keywords: Dendritic dye; LB monolayers; Photocurrent generation;

Modified surface morphology in surface ablation of cobalt-cemented tungsten carbide with pulsed UV laser radiation by Tiejun Li; Qihong Lou; Jingxing Dong; Yunrong Wei; Jingru Liu (331-344).
Surface ablation of cobalt-cemented tungsten carbide hardmetal has been carried out in this work using a 308 nm, 20 ns XeCl excimer laser. The influence of ablation rate, surface roughness, surface micromorphology as well as surface phase structure on laser conditions including laser irradiance and pulse number have been investigated. The experimental results showed that the ablation rate and surface roughness were controlled by varying the number of pulses and laser irradiance. The microstructure and crystalline structure of irradiated surface layer varied greatly with different laser conditions. After 300 shots of laser irradiation at irradiance of 125 MW/cm2, the surface micromorphology characterizing a uniform framework pattern of “hill–valleys”. With the increment of laser shots at laser irradiance of 125 MW/cm2, the microstructure of cemented tungsten carbide transformed from original polygon grains with the size of 3 μm to interlaced large and long grains after 300 shots of laser irradiation, and finally to gross grains with the size of 10 μm with clear grain boundaries after 700 shots. The crystalline structure of irradiated area has partly transformed from original WC to β-WC1−x , then to α-W2C and CW3, and finally to W crystal. At proper laser irradiance and pulse number, cobalt binder has been selectively removed from the surface layer of hardmetal. It has been demonstrated that surface ablation with pulsed UV laser should be a feasible way to selectively remove cobalt binder from surface layer of cemented tungsten carbide hardmetal.
Keywords: Surface ablation; Tungsten carbide; Cobalt; Excimer laser;

Commonly used electrolytes (NH4F·HF and NaF/H2SO4) for doping profile analysis in Si are employed to clarify their capabilities and limitations using a boron-doped staircase test structure. The carrier density profiles at various measurement voltages for these electrolytes are compared with a SIMS profile of the structure. A variety of parameters affecting a carrier density profile such as static current–voltage behaviour, dissipation factor, flat band potential and series resistance at various doping levels are explored to optimise the measurement conditions. It is found that both electrolytes can be employed for doping profiling in Si for a broad range of doping levels. However, electrolyte NaF/H2SO4 allows wider measurement voltage range and suits better for profiling highly doped structures as indicated by lower series resistance and hence lower dissipation factor.
Keywords: Si; eCV; Electrolyte-semiconductor junction; Doping profiling;

Vacancies and adatoms complexes on silicon surface by Hansheng Guo; R. Souda (351-355).
By intentional introduction of Ni on Si(1 0 0) and Si(1 1 1) we have observed the reconstruction of the both surfaces. Scanning tunneling microscopy studies reveal morphologies of the Si(1 0 0)–2×n structure, which are partially different from those of the reported up to now. Heterogeneous reconstruction has been also observed on Si(1 1 1) surface: the 7×7 terraces and terraces being composed of disordered adatoms distribute alternatively on the surface. We present the results and give our primary exploration to their formation mechanism.
Keywords: Scanning tunneling microscopy; Silicon surface; Ni-induced reconstruction;

Ablation of cobalt with pulsed UV laser radiation by Tiejun Li; Qihong Lou; Jingxing Dong; Yunrong Wei; Jingru Liu (356-365).
The ablation of cobalt with 20 ns pulses of 308 nm XeCl excimer laser radiation is investigated. The dependence of the ablation rate on laser fluence, spot area, and pulse number is determined. The experimental results are compared with a numerical calculation based on an improved thermal model. The model, which considers proper vapor attenuation, is shown to be in good agreement with experiment.
Keywords: Cobalt; Surface ablation; Excimer laser;

AUTHOR INDEX (367-370).

SUBJECT INDEX (371-381).