Applied Surface Science (v.222, #1-4)

Subject Index (V-XVII).

Author Index (I-IV).

Light-stimulated switching of azobenzene-containing self-assembled monolayers by F Hamelmann; U Heinzmann; U Siemeling; F Bretthauer; J Vor der Brüggen (1-5).
Azobenzene-containing self-assembled monolayers (SAM) have been prepared on silicon by chemisorption of 4-trichlorosilylazobenzene and mixtures of 4-trichlorosilylazobenzene (TSA) and phenyltrichlorosilane (PTS). The resulting films contain azobenzene units connected to the surface by means of a short, rigid spacer. The films have been analysed by single-wavelength ellipsometry and measurement of the water contact angle. Irradiation of the films with 360 nm ultraviolet light-induced a switching effect, which results in a decrease of the water contact angle and a change in the ellipsometric parameters. The switching could be made reversible by exposing the sample to visible light for 24 h.
Keywords: Azobenzene; Self-assembled monolayers; Photoisomerisation;

Nitridation of epitaxially grown 6.1 Å semiconductors studied by X-ray photoelectron spectroscopy by E.J Preisler; R.P Strittmatter; T.C McGill; C.J Hill (6-12).
We present a detailed analysis of the state of surfaces of the 6.1 Å family of semiconductors (InAs, GaSb and AlSb) under exposure to a nitrogen plasma. X-ray photoelectron spectroscopy (XPS) is used to determine the species created at the surface and their relative abundance. We find evidence that clean epitaxial surfaces exposed to the plasma form both VN (SbN and AsN) and IIIN (AlN, GaN and InN) compounds, but in different amounts and different proportions for each of the materials.
Keywords: Semiconductors; X-ray photoelectron spectroscopy; Epitaxial surfaces;

Noble gas ion effects on the XPS valence band spectra of silicon by Elaine Walker; Christopher P. Lund; Philip Jennings; John C.L. Cornish; Craig Klauber; Glenn Hefter (13-16).
X-ray photoelectron spectroscopy (XPS) has been used to study crystalline silicon (c-Si) (1 0 0) surfaces bombarded with argon, xenon and neon to examine the interaction of core peaks from these noble gases with the valence band region of silicon. XPS valence band spectra of xenon- and argon-bombarded silicon were found to have prominent peaks at binding energies of approximately 6 eV for the xenon (5p1/2, 5p3/2) and 9.3 eV for the argon (3p) core levels, respectively. These core level peaks are within the silicon valence band energy range.Attempts to compensate for the interfering peaks are reported but it is concluded that it is better to select a bombarding ion whose core levels do not overlap with the silicon valence band. Results for the ion bombardment are reported for neon, which has a peak at approximately 15.5 eV that does not significantly interfere with the photoelectron valence band spectrum of silicon.
Keywords: Amorphous silicon; Noble gases; Ion bombardment; XPS valence band; Lineshape analysis;

Using a neural network, a profile roughness of plasma etching is characterized. The etching was conducted in a CHF3/CF4 inductively coupled plasma. The etch process was characterized by a 23 full factorial experiment. The process parameters that were varied in the design include radio frequency source and bias powers, and gas ratio. Relationships between the parameters and profile roughness were captured by training neural network with eight experiments plus one center experiment. Model appropriateness was tested with six experiments not pertaining to the training data. Model prediction capability was optimized by means of a genetic algorithm (GA). Compared to a conventional model, GA-optimized model demonstrated a drastic improvement of about 54% in predicting profile roughness. From the optimized model, several plots were generated to examine parameter effects on the profile roughness. Increasing the source power (or bias power) under high bias power (or source power) increased the profile roughness. More significant effect of the bias power was revealed. The profile roughness decreased with increasing the gas ratio was strongly correlated to the dc bias. The little variation in the profile roughness was ascribed to chamber plasma condition maintained at relatively low dc bias.
Keywords: Plasma etching; Profile roughness; Neural network; Genetic algorithm;

A study of the surface structure and composition of annealed Ga0.96Mn0.04As(1 0 0) by A Mikkelsen; J Gustafson; J Sadowski; J.N Andersen; J Kanski; E Lundgren (23-32).
The surface structure and chemical composition of annealed Ga0.96Mn0.04As(1 0 0) have been studied by scanning tunneling microscopy (STM), auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The samples were As capped and subsequently transferred in-air from the MBE system to the STM chamber. After annealing to 600 K it is found that the Mn segregates to the surface and forms a compound, which is stable up to annealing temperatures of 790 K. For annealing temperatures above 825 K a well-ordered phase exists signified by a LEED pattern consisting of a superposition of a (1×6) and a (4×2) pattern. LEED and STM measurements demonstrate that the surface is dominated by (1×6) domains coexisting with small patches of (4×2) domains. By comparing the STM images of the high temperature phase found on Ga0.96Mn0.04As(1 0 0) with the high temperature phases found on ordinary GaAs(1 0 0), we demonstrate differences between annealed Ga0.96Mn0.04As(1 0 0) and GaAs(1 0 0) in both surface morphology and atomic structure. We argue that the Ga0.96Mn0.04As surface is more As rich than the GaAs surface prepared in a similar fashion. Reasons for these differences are discussed.
Keywords: STM; LEED; AES; GaAs; GaMnAs; Manganese; Surface; Structure;

On-line monitoring of laser modification of titanium dioxide using optical surface second harmonic by Guerric Vrillet; Soo Keun Lee; Daniel McStay; Peter K.J Robertson (33-42).
TiO2 photocatalysis is a promising technology for the destruction of organic pollutants in both waste and potable waters with the mineralisation of a wide range of compounds having been reported. TiO2 has many advantages over other semiconductors, it is highly photoreactive, cheap, non-toxic, chemically and biologically inert, and photostable. The photocatalytic activity of TiO2 has been shown to depend upon many criteria including the ratio of anatase/rutile crystal phase, particle size and oxidation state. This paper reports the use of optical surface second harmonic generation (SSHG) to monitor modifications in TiO2 powder induced following laser treatment. SSHG is a non-contact, non-destructive technique, which is highly sensitive to both surface chemical and physical changes. Results show that three different SSH intensities were observable as the TiO2 samples were irradiated with the laser light. These regions were related to changes in chemical characteristics and particle size of the TiO2 powder.
Keywords: Photocatalysis; TiO2; Surface second harmonic generation; Laser treatment; Colour change; Structure change;

The deformed state, dimensions and mosaicity of subgrains, arrangement and density of excess dislocations in Cu/Al coating deposited by mechanochemical technique are investigated by X-ray diffraction method. For Cu coating the estimates 10 nm, 6.4×1011  cm−2 and 0.24 GPa are evaluated for average size of subgrains, excess dislocation density and shear strength, respectively. The strain-hardening mechanism of constraint on dislocation motion is proposed to be a dominant channel for the coating strengthening. A model is proposed for interdiffusion process of the Cu and the Al atoms at the Cu/Al interface. It is assumed that this interdiffusion is activated by the impact temperature oscillations due to repeated “ball–coating” collisions over the deposition process. On the basis of analysis of X-ray diffraction data and the obtained estimate for diffusive run of the Cu atoms into the Al substrate, formation of a Al1−x Cu x transition layer with compound parameter x≈0.02 and thickness 5.1 μm is predicted.
Keywords: Coating; Subgrain; Deformation; Dislocation; Vacancy; Diffusion; X-ray diffraction;

An increased attention is devoted to interfacial In2S3 thin films because of their potential application as a new generation of buffer layer in copper indium gallium diselenide (CIGS)-based solar cells. In this paper, thin films of In2S3 were deposited on soda lime glass and SnO2-coated glass using indium acetylacetonate (In(acac)3) and H2S precursors by atomic layer deposition (ALD), a sequential chemical vapour deposition technique allowing the formation of dense and homogeneous films. The effect of temperature on the film kinetics has been studied. In a temperature window between 130 and 260 °C, a maximum growth rate of about 0.7 Å per cycle is obtained at 180 °C. The structure and morphology of films were characterised by XRD, SEM and TEM. The ALD-In2S3 thin films are crystallised in a tetragonal form with band gap values of about 2.7 eV. Electrical properties have been addressed by using impedance measurements on semiconductor electrolyte junctions. The films are n-type semiconductors with a doping level around 1016–1017  cm−3 and possess a good blocking behaviour under reverse bias. The flat band potential is about −1.1 V versus MSE. These figures are close to those measured under similar conditions with CdS buffer layers and could explain the good cell performances obtained with ALD-In2S3.
Keywords: Atomic layer deposition; In2S3; Characterisation; Thin film deposition;

BCl3/Ne etching of III–V semiconductors in a planar inductively coupled plasma reactor by W.T. Lim; I.K. Baek; J.W. Lee; E.S. Lee; M.H. Jeon; G.S. Cho; S.J. Pearton (74-81).
A BCl3/Ne plasma chemistry was used to etch Ga-based (GaAs, AlGaAs, GaSb) and In-based (InGaP, InP, InAs and InGaAsP) compound semiconductors in a planar inductively coupled plasma (ICP) reactor. The addition of the Ne allows the plasma to maintain high ion density conditions over a broader range of operating pressures. All of the materials exhibit a maximum etch rate at BCl3-to-Ne ratios of 0.25–0.5. Under all conditions, the Ga-based materials etched at significantly higher rates than the In-based materials, due to the higher volatilities of their trichloride etch products relative to InCl3. The etched surfaces of both AlGaAs and GaAs have comparable root-mean-square (rms) roughness and similar stoichiometry to the unetched control samples, while the surfaces of In-based materials are degraded by the BCl3/Ne etching. Etch anisotropy of the GaAs and AlGaAs relies on some degree of sidewall passivation by redeposition of etch products and photoresist from the mask.
Keywords: High density plasma; Inductively coupled plasma reactor; Semiconductor;

Model study of initial adsorption of SO2 on calcite and dolomite by Katarina Malaga-Starzec; Itai Panas; Oliver Lindqvist (82-88).
The rate of calcareous stone degradation is to a significant extent controlled by their surface chemistry with SO2. Initial surface sulphite is converted to a harmful gypsum upon, e.g. NO2 catalysed oxidation. However, it has been observed by scanning electron microscopy that the lateral distributions of gypsum crystals differ between calcitic and dolomitic marbles. The first-principles density functional theory is employed to understand the origin of these fundamentally different morphologies. Here, the stability differences of surface sulphite at calcite CaCO3 (s) and dolomite Ca x Mg1−x CO3 (s) are determined. A qualitative difference in surface sulphite stability, favouring the former, is reported. This is taken to imply that calcitic micro-crystals embedded in a dolomitic matrix act as sinks in the surface sulphation process, controlled by SO2 diffusion. The subsequent formation of gypsum under such conditions will not require SO4 2− (aq) ion transport. This explains the homogeneous distribution of gypsum observed on the calcitic micro-crystals in dolomite. In contrast, sulphation on purely calcitic marbles never reaches such high SO2 coverage. Rather, upon oxidation, SO4 2− (aq) transport to nucleation centres, such as grain boundaries, is required for the growth of gypsum crystals.
Keywords: Calcite; Dolomite; Sulphation; Sulphite stability; Sulphate distribution;

The effects of anodic aging time and potential on the corrosion resistance, stability and constitution of the passive film formed on an Fe-24Mn-4Al-5Cr alloy in 50% HNO3 solution were studied by using combined electrochemical measurements and Auger electron spectroscopic (AES)/X-ray photoelectron spectroscopic (XPS) analysis. In the anodic passive region, prolonged anodic aging time or increased passivating potential can induce better protective and stable properties of the passive film and better resistance to corrosion. With increasing aging time from 15 min to 5 h, the time required for the potential decay from the passive to active state increases from about 300 up to above 12,000 s, and the corrosion resistance in 1 mol l−1 Na2SO4 solution of Fe-24Mn-4Al-5Cr alloy, characterized by polarization curves, is superior to that of Fe-13% Cr-0.1% C stainless steel. AES and XPS analyses of the aging passive film show that these improvements of properties are related to modifications of the passive layer with time. The increase of resistance to corrosion is attributed to Al2O3 and Cr2O3 enrichment and oxides of Fe and Mn depletion in the passive film and a thickening of the effective barrier layer of oxides.
Keywords: Passive film; Anodic passivation aging of passive film; Surface modification; Potential decline; XPS/AES analysis; Fe-Mn-Al-Cr alloys;

Copper chemical vapour deposition on organosilane-treated SiO2 surfaces by N.G Semaltianos; J.-L Pastol; P Doppelt (102-109).
Copper thin films were grown on SiO2 substrates by chemical vapour deposition using the precursor (2-methyl-1-hexene-3-yne)Cu(1,1,1,5,5,5-hexafluoroacetylacetonate) ((MHY)Cu(hfac)) and were examined by scanning electron and atomic force microscopy. The affinity for copper chemical vapour deposition of the substrate surface is higher after the formation of self-assembled monolayers of organosilanes onto the substrate surface. Furthermore, the affinity is greatly enhanced by a subsequent UV light irradiation of the organosilane monolayer, in air, prior to deposition. The dependence of film morphology and statistical surface parameters on substrate temperature and amount of vapour precursor introduced during deposition, provide information for the optimisation of external parameters towards obtaining a thin yet continuous film. Self-assembled monolayers of organosilanes can be used for a selective metallization of SiO2 substrates by copper chemical vapour deposition, in addition to acting as ultrathin barriers which prevent copper diffusion into the SiO2, thus opening a route for a technology useful in microelectronic industrial applications.
Keywords: Chemical vapour deposition; Organosilane; Self-assembled monolayers;

A novel technique for finely controlled near-infrared beam transmission of titania by H.Y Zheng; A Soutar; G.C Lim; J.W Chai; Bill Freeman; H.J Jiang (110-115).
The paper discusses a novel method of controlling near-infrared beam transmissions through a silica–titania dual layer. It was found that a KrF excimer laser was able to change the transmission characteristics of the dual layer coatings over a wide range from 0.1 to 12 dB at a small step of less than 1 dB for the transmission at both 850 and 1550 nm. The controlled beam transmission was attributed to the controllable laser-effected darkening of the titania layer.The developed method can be applied for the manufacture of adjustable filters widely used for optical communication systems.
Keywords: Laser-evoked coloration; Laser-induced darkening; Titania coating; Near-infrared transmission;

Ag-enhanced layered growth in Fe(1 0 0) homoepitaxy by M Kamiko; H Mizuno; H Chihaya; J.-H Xu; I Kojima; R Yamamoto (116-124).
We have investigated the effect of Ag on the homoepitaxial growth of Fe(1 0 0) by means of reflection high-energy electron diffraction (RHEED). It was clearly found that Ag enhances layer-by-layer growth of Fe on Fe(1 0 0)-c(2×2)O reconstruction surface. The result of the dependence of the growth behavior as a function of Ag layer thickness suggests that there exists the most suitable amount of Ag surfactant layer that enhances the smoother layer-by-layer growth.
Keywords: Surface structure; Epitaxy; Metallic films; RHEED; MBE; Iron; Silver;

Characterization of oxygen impurities in thermally evaporated LaF3 thin films suitable for oxygen sensor by M Vijayakumar; S Selvasekarapandian; T Gnanasekaran; Shinobu Fujihara; Shinnosuke Koji (125-130).
The lanthanum fluoride thin films has been prepared by means of thermal evaporation method. The XRD analysis shows the formation of polycrystalline hexagonal LaF3. The depth profile X-ray photoelectron spectroscopy (XPS) analysis shows the presence of oxide ions throughout the films. The formation of lanthanum oxyfluoride (LaOF) has been identified. The [O]/[F] ratio has been found to be 0.35 which is higher than the previously reported values of LaF3 film applied for the oxygen sensor.
Keywords: LaF3; Oxygen sensor; XRD analysis; XPS analysis;

Conductivity and distribution of charge on electroluminescent Si/SiO2 structures investigated by electrostatic force microscopy by T Suominen; P Paturi; H Huhtinen; L Heikkilä; H.-P Hedman; R Punkkinen; R Laiho (131-137).
Electroluminescent Si/SiO2/Au layer structures on a p-Si wafer are investigated with electrostatic force microscopy and atomic force microscopy. The samples comprise either four Si/SiO2 layer pairs prepared by chemical vapor deposition or a SiO2 thermal oxide layer grown at 950 °C on the wafer. A 9–13 nm thick Au-film electrode is sputtered on top of the samples. A correlation between the density of electroluminescent dots and distribution of charge on the surface of these structures is found. Measurements of the excitation current through the samples show that the four-layer Si/SiO2/Au structure has Poole–Frenkel type conductivity and the thermal oxide sample is excited through Fowler–Nordheim tunneling.
Keywords: Electrostatic force microscopy; Electroluminescence; Silicon/silicon dioxide; Conductivity processes;

Enhancement of material ablation and photoacoustic excitation by an artificially deposited liquid film in the process of pulsed-laser ablation (PLA) is investigated in this paper. Ablation threshold, ablation rate, surface topography, and acoustic-transient emission are also measured for dry and liquid film-coated surfaces. The physical mechanisms of enhanced ablation in the liquid-assisted process are analyzed at relatively low laser fluences with negligible effect of laser-produced plasma. Particularly, correlation between material ablation and acoustic-transient generation is examined. In the experiment, aluminum thin-films and bulk foils are ablated by Q-switched Nd:YAG laser pulses. The dependence of ablation rate and laser-induced topography on liquid film thickness and chemical composition is also examined. Photoacoustic emission is measured by the probe beam deflection method utilizing a CW HeNe laser and a microphone. In comparison with a dry ablation process, the liquid-assisted ablation process results in substantially augmented ablation efficiency and reduced ablation threshold. The results indicate that both increased laser-energy coupling, i.e., lowered reflectance, and amplified photoacoustic excitation in explosive vaporization of liquid are responsible for the enhanced material ablation.
Keywords: Pulsed-laser ablation; Liquid; Photoacoustic effect; Explosive vaporization;

Surface characterisation of electrografted random poly[carbazole-co-3-methylthiophene] copolymers on carbon fiber: XPS, AFM and Raman spectroscopy by A.Sezai Sarac; Syed A.M Tofail; Marina Serantoni; John Henry; Vincent J Cunnane; James B McMonagle (148-165).
Surface characterizations of thin film coatings of carbazole containing random poly(carbazole-co-3-methylthiophene) P[Cz-co-3-MeTh] copolymers on carbon fiber were performed. Coatings with different initial feed ratios of comonomers were electrochemically formed (grafted) onto carbon fibers by constant current electrolysis. The resulting copolymers were characterised with scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The characterization of the thin copolymer films was performed on the surface of the carbon fiber. This allows the relationship between the initial feed ratio of the comonomer, the resulting morphology and the surface structure of these thin film coatings to be investigated. The range of thickness increase (diameter) was from 200 nm to 6.74 μm depending on the polymer and copolymers used.
Keywords: Random conductive copolymers; Carbon fiber; AFM; XPS; Raman spectroscopy;

The layer-by-layer atomic composition of major elements (Cu, Ag, and Au) in the medieval Venetian coin has been measured by mass-resolved ion scattering spectrometry (MARISS) using 1 keV Ne+–He+ mixed ion beam for both controlled erosion of the sample (up to 0.3 μm in a depth) and simultaneous generation of analytical signals. Quantification was carried out by calibration using pure standard materials. It was shown that the surface of the coin is significantly enriched in silver (ca. 80 at.%), whose content tends to decrease inward the coin. Formation of the Ag-enriched layer (or surface depletion in copper) could be caused by the environment the coin was exposed to for many centuries.
Keywords: Archaeometry; Depth profiling; Low-energy ion scattering; Standard materials; Quantification;

Depth dependences of the ion bombardment induced roughness and of the interdiffusion coefficient observed by Auger electron spectroscopical (AES) depth profiling of stationary as-deposited and annealed Si/Al multilayered specimens were evaluated by fitting calculated concentration–depth profiles to measured ones. The model used for calculation of the concentration–depth profile accounts for the instrumental smearing (interface broadening) upon AES depth profiling and, if relevant, interdiffusion. The instrumental smearing incorporates the effects of atomic mixing, roughness, escape depth of the Auger electrons, and preferential sputtering. The depth profile recorded from a stationary as-deposited Si/Al multilayered specimen was fitted by assuming, additionally, that the ion bombardment induced roughness increases with the sputter depth, keeping the other fitting parameters (including the inherent surface roughness) equal to the values determined for a rotating specimen. The roughness values determined by fitting to the measured depth profiles agree well with those determined directly by atomic force microscopy (AFM). Interdiffusion at the Si/Al interfaces was induced by annealing the specimens isothermally in an argon atmosphere at 120 °C for 240 min, 135 °C for 60 min, and 150 °C for 20 min. It was found that interdiffusion across interfaces near the surface of the multilayer is more pronounced than across interfaces in the deeper part of the layer. To account for this depth dependence of the extent of interdiffusion, calculated depth profiles for the annealed specimens were fitted to measured ones by assuming that diffusion annealing can be described as an additional “roughening” of the interfaces. As a result, values of the interdiffusion coefficient as a function of the depth beneath the surface were obtained. The depth dependence of the interdiffusion coefficient was discussed in terms of the microstructural development.
Keywords: AES depth profiling; Interdiffusion coefficient; Sputter induced roughness; Si/Al multilayer;

Photoluminescence and transmission spectrum characterization of Er-implanted Al2O3 films by H.B Xiao; C.S Zhang; X.L Jia; Y.J Wang; X.L Cheng; G.B Cao; F Zhang; S.C Zou (180-185).
Al2O3 films were deposited on thermally oxidized Si(1 0 0) substrates by ion beam enhanced deposition (IBED), and then implanted with 120 keV Er ions. A thermal anneal at 773–1273 K in Ar environment was performed for 1 h for each sample studied. Photoluminescence and optical transmission spectra of all samples were measured with optical spectrometer Nicolet 860. Annealed at 973 K, the pump absorption cross-section reaches its minimum value which lead to the appearance of the lowest PL intensity and 1140 nm spectrum. Optical transmission spectra indicate that the pump absorption cross-section relates to the minimum optical absorption of the Al2O3 materials.
Keywords: Photoluminescence; EDWA; Absorption cross-section;

The Cation Mass Spectrometer (CMS) is a new instrument allowing to optimize the advantageous quantification technique consisting in analyzing MCs x + clusters. To further enhance the potential of this instrument, we have developed a column that delivers a collimated and adjustable stream of neutral Cs atoms to be deposited on the surface of the sample while this one is being analyzed by Secondary Ion Mass Spectrometry (SIMS). Using this new column, it was possible to introduce an analysis technique consisting of a X y+ ion bombardment (where X stands for any element except Cs) accompanied by a simultaneous deposition of Cs0 at the surface of the sample. This experimental technique permits a successful decoupling of the sputtering and Cs introduction processes by avoiding the constraints imposed by an energetic Cs+ ion bombardment. As a consequence, it becomes possible to optimize simultaneously the sensitivity of the analysis, by carefully adjusting the Cs concentration to its optimum value, and the depth resolution of the analysis, by choosing adequate primary bombardment conditions. In this paper, we will first describe the new Cs0 column and outline its performances. In a second part, we will focus on the analytical aspect and study the ability to vary the Cs concentration over a complete and continuous range. Experiments using a Ga+ primary bombardment show in particular that the Cs concentration produced during analyses performed with the mentioned technique only depends on the characteristics of the analyzed material and the ratio between the erosion and deposition rates, but not on the individual values of these two rates.
Keywords: Secondary Ion Mass Spectrometry; Quantification; Cesium evaporator; Cesium deposition; Cesium concentration; Sputtering;

Photoinduced second harmonic generation in the In2O3 crystalline films doped by Al and Sn by H. Herschel; J. Ebothé; I.V. Kityk; U. Yamoda (198-207).
Photoinduced optical and second-order non-linear optical effects in nanolayers separating indium oxide (In2O3) crystalline films doped by Al, Sn, and glass substrates were found. Photoinduced optical second harmonic generation was applied as a particular non-linear optical method. The photoinduced second-order non-linear susceptibilities (at λ=1.76 μm) show good correlation with the photoinduced linear optical susceptibilities, particularly with the optical absorption. The performed experimental measurements show that the observed effects are caused by two factors. The first one is related with the potential gradient on the glass–In2O3 film border. The second one is caused by additional polarization due to doping by the Al and Sn atoms. Such near-the-border nanolayers can be considered as new type of second-order optical materials.
Keywords: Semiconducting crystaline films; Photoinduced effects; Second harmonic generation;

Application of hydrocarbon plasmas for modifying near-surface characteristics of bearing steel by K. Sridharan; E.H. Wilson; D.F. Lawrence; J.R. Jacobs (208-214).
The near-surface hardness and wear resistance of AISI 52100, a widely used bearing steel, have been enhanced by energetic ion bombardment using a plasma immersion process. Test flat samples of the steel were immersed in a plasma of methane or acetylene and pulse biased to a negative voltage so as to result in either ion implantation of carbon into the steel surface or deposition of diamond-like carbon film onto its surface. The nanometer size scale surface roughness of the initial sample flats was generally retained after the plasma surface treatments. The surface treatments were carried out with the samples at near-room temperature, which is an important consideration for this type of steel because of its relatively low softening temperature. However, by intentionally using the energy of the bombarding ions to marginally heat the surface of the samples, the depth of the carbon ion implanted layer could be increased by over a factor of two while maintaining the bulk hardness of the steel at acceptable levels.
Keywords: Plasmas; Ion implantation; Bearing steel; Wear;

The study of the volume expansion of aluminum during porous oxide formation at galvanostatic regime by I. Vrublevsky; V. Parkoun; V. Sokol; J. Schreckenbach; G. Marx (215-225).
The volume expansion factor of porous alumina, formed by through anodizing of an Al foil of thickness 11.5 μm in the range of current densities of 4–35 mA cm−2 in oxalic and sulfuric acid at 18–24 °C has been studied. The microstructure of anodizing samples has been observed using scanning electron microscopy. The thickness of obtained porous alumina films was measured by a mechanical profilometer with a computer signal-processing. The volume expansion factor of porous alumina varied from 1.35 to 1.65. Linear dependences were obtained for the volume expansion factor of porous alumina versus the anodizing voltage and the ionic current-density logarithm versus the inverse volume expansion factor. Unlike oxide formation in sulfuric acid, these dependences have two subsequential rectilinear regions in oxalic acid. This peculiarity of the dependences in oxalic acid was explained by formation of a region of the immobile negative space charge in the barrier Al oxide layer and its influence on the ionic transport.
Keywords: Aluminum; Anodizing; Porous alumina; Volume expansion factor; Electron traps; Immobile negative space charge;

Femtosecond laser ablation of diamond-like carbon films by Gabriel Dumitru; Valerio Romano; Heinz P Weber; Sergei Pimenov; Taras Kononenko; Marc Sentis; Jörg Hermann; Sébastien Bruneau (226-233).
Diamond-like carbon (DLC) coatings were deposited on Si substrates using a hot filament diode discharge and they were irradiated with ultrashort laser pulses (800 nm, 150 fs, <4 J/cm2). The laser-treated films were examined using optical microscopy, Raman spectroscopy, SEM, AFM and white-light interferometery. Damage threshold of 0.16 J/cm2 and ablation rates below 110 nm/pulse were determined. Changes in the structure of the laser-irradiated films were showed by means of Raman investigations. The laser-treated samples were etched and the depths of modified material layers were determined. Ablation experiments with longer laser pulses (1064 nm, 100 ns, <3 J/cm2) were also performed and the irradiated DLC films were afterwards analyzed using the same procedures. Dissimilarities in the structure changes induced by fs-and ns-laser irradiation were observed and comments are given.
Keywords: Femtosecond pulses; Diamond-like carbon; Ablation properties; Structure changes;

Thermal decomposition of dimethylaluminum isopropoxide on Si(1 0 0) by Sung Yong Lee; Bing Luo; Yangming Sun; J.M White; Yunsoo Kim (234-242).
The thermal decomposition of dimethylaluminum isopropoxide dimer, [(CH 3)2 Al(O i C 3 H 7)]2 , on Si(1 0 0) is investigated by line-of-sight temperature programmed desorption (LOS-TPD), line-of-sight isothermal reaction spectroscopy (LOS-IRS), and Auger electron spectroscopy (AES). [(CH 3)2 Al(O i C 3 H 7)]2 does not decompose upon adsorption on Si(1 0 0) held at 100 K or during subsequent LOS-TPD. AES indicates that film growth starts when the precursor is dosed with the substrate at ∼650 K. LOS-IRS shows that the monomer, [(CH 3)2 Al(O i C 3 H 7)]2 , is an intermediate in the process of the dimer decomposition to aluminum-containing films, and that further decompositions occur via breaking the CAl and OC bonds.
Keywords: Dimethylaluminum isopropoxide; Aluminum oxide; Si(1 0 0) substrate; TPD; Isothermal reactions;

Electronic resonances in the photofield emission spectra from Ta(1 1 1) by P. H a ̨ dzel; L. Jurczyszyn; T. Radoń (243-252).
The energy distributions of photoelectrons obtained from field emission spectroscopy performed for Ta(1 1 1) indicate the presence of the fine structure of peaks separated by a few hundredths of an electron volt. New experimental data show also that these peaks appear practically for the same energies in the spectra of both the surface region and the bulk of the field emitter. Our theoretical study performed with the help of the simple one-dimensional model shows that this effect might be a result of the partial reflection of the electronic waves between the Schottky barrier and a border separating the topmost part of the field emitter from the rest of the tip. This mechanism may lead to the formation of the electronic resonance states in the apex part of field emitter, which is manifested in the photofield emission spectra by the fine structure of peaks. We have studied the influence of the changes of the morphology of the tip, caused by the annealing process, on the energy separation between the nearest peaks: experimental results and numerical computations give an additional support for the supposition that the observed dense structure of peaks represents electronic resonances existing in the topmost part of the field emitter.
Keywords: Field emission; Photoelectron emission; Surface electronic phenomena (work function, surface potential, surface states, etc.); Tantalum;

Adhesion behavior of electroless deposited Cu on Pt/Ta silicate and Pt/SiO2 by M Garza; J Liu; N.P Magtoto; J.A Kelber (253-262).
X-ray photoelectron spectroscopy (XPS) was used to characterize the growth mode of sputter deposited Pt at ∼300 K on Ta silicate and SiO2 thin films formed on a Si(1 0 0) substrate. The ability of Pt thin films to catalyze electroless Cu deposition on Ta silicate and SiO2 was also studied. The XPS data show that Pt deposition results in conformal growth or “wetting” on Ta silicate and 2-D cluster growth on SiO2. Electroless Cu deposition on ∼11 monolayers (MLs) Pt/Ta silicate film results in an adherent Cu film which passed the Scotch tape test. In contrast, electroless Cu deposition on ∼11 ML Pt/SiO2 results in a non-adherent Cu film due to weak Pt/SiO2 interaction.
Keywords: Adhesion; Copper; Growth mechanism; XPS;

Annealing effects on the microstructure and photoluminescence properties of Ni-doped ZnO films by D.J Qiu; H.Z Wu; A.M Feng; Y.F Lao; N.B Chen; T.N Xu (263-268).
Ni-doped ZnO (ZnO:Ni) films were fabricated on Si(0 0 1) substrates by reactive electron beam evaporation at low substrate temperature. The as-grown films were then annealed in oxygen ambient at higher temperatures. X-ray diffraction (XRD) results indicated that 5 at.% Ni-doped samples are still of single phase with the ZnO-like wurtzite structure. Photoluminescence (PL) measurements of Ni-doped samples illustrated the UV-PL emission centered at about 384 nm, which is ascribed to the near-band-edge (NBE) emissions of ZnO-like band structures. The UV-PL intensity becomes stronger along with the increase of annealing temperatures and reaches a maximum magnitude after annealed at 450 °C. However, along with the further increase of annealing temperatures, UV-PL intensity diminishes again. The UV-PL intensity of 450 °C-annealed samples is 213 times stronger than that of as-grown (doped) samples, which may render potential applications in optoelectronic devices, such as UV luminescent devices.
Keywords: Ni-doped ZnO films; Electron beam reactive deposition; Microstructure; Photoluminescence;

Humidity behavior of thermally carbonized porous silicon by Mikko Björkqvist; Jarno Salonen; Ensio Laine (269-274).
We have studied the humidity behavior of thermally carbonized (TC) p+-type porous silicon (PS) samples as a function of carbonization temperature. The Fourier transform infrared (FTIR) spectroscopy and gas adsorption measurements indicate that the bond configuration and structure of thermally carbonized porous silicon surface are highly dependent on carbonization temperature. Moreover, the surface affects on the stability of porous silicon but also significantly on its humidity behavior. The porous silicon samples carbonized at or below 650 °C remain hydrophobic, while the temperature of 730 °C was high enough to produce a hydrophilic surface. The water sorption rate in thermally carbonized porous silicon samples was quite slow, however, this was interpreted to be mostly a consequence of a thick sample. Hygroscopicity was studied by measuring the weight of the sample under various relative humidity values, and also by using isothermal microcalorimeter (IMC). For humidity sensor purposes, we also studied the effect of water adsorption on electrical parameters of thermally carbonized porous silicon layer. The changes of resistance and capacitance were observed both for hydrophobic and hydrophilic surface.
Keywords: Porous silicon; Thermal carbonization; Hygroscopicity;

The two-dimensional modelling of the etching of silicon oxide in CF4+H2 plasma is considered. The profiles of etched trenches at real dimensions are calculated as a function of mask dimensions, concentrations of chemically active and non-active plasma components and ion bombardment parameters. The chemical composition of CF4+H2 plasma is calculated, the plasmochemical etching (PCE) and reactive ion etching (RIE) of a SiO2 in the plasma are investigated to achieve the goal. It was found that the aspect ratio and etching anisotropy decrease with increase of H2 content in the feed during SiO2 etching in CF4+H2 plasma. The concentration of CF radical is insufficient to passivate the sidewalls completely. Meanwhile, the formation and subsequent growth of fluorocarbon film stop etching during one-dimensional PCE of SiO2 at critical H2 content in the feed.
Keywords: CF4 + H2 plasma; SiO2; Reactive ion etching;

Thin layers of cubic GaN were grown on GaAs(1 0 0) by atmospheric pressure metalorganic vapor phase epitaxy using dimethylhydrazine (DMHy) as the nitrogen precursor. Samples fabricated at various growth conditions were characterized using atomic force microscopy. The surface morphology of the samples was clearly affected by both the growth temperature and the V/III molar ratio. Two distinct growth modes of GaN, two-dimensional (2D) and island growth (3D), were observed. A critical temperature was discovered, below which the growth was 2D, and above which 3D growth took place. This critical temperature was found to increase with increasing V/III ratio. In this work, a smooth 8 nm thick GaN layer was grown at 600 °C with a V/III ratio of 100. Such layers have potential applications in GaAs heterostructure electronics.
Keywords: Cubic GaN; GaAs(1 0 0); Strained; DMHy; Growth mode;

A simple theoretical model involving heat conduction equation that includes the temperature and phase dependence of material properties is presented. It describes the heating, vaporization, and physics of laser created plasma for aluminum targets in ambient gas. One-dimensional (1D) heat conduction equation is numerically solved to calculate ablation threshold, ablation depth and ablation onset time and to investigate the effect of energy, nature of surface and laser pulse shape. Results show a marked difference in ablation characteristics between with and without the temperature and phase dependence. The temperature profiles inside the target and ablated vapors at various irradiance and pulse shapes are generated. Depending on the pulse shape and irradiance a certain region inside the plume becomes critically dense and reflects the incoming laser. The distance at which this shielding occurs varies with laser intensity, with the increase in intensity the time of occurrence of shielding decreases.
Keywords: Heat conduction equation; Aluminum plasma; Ablation threshold; Ablation onset;

The statistical rate theory (SRT) has frequently been used for interpretation of dynamics of many interfacial processes. In the case of adsorption kinetics it has successfully been applied to well-defined systems as well as to the systems where the energetic heterogeneity of solid surface plays significant role.Because the SRT approach involves the knowledge about the chemical potential of the adsorbed phase, different approximate expressions have been applied for estimating that chemical potential. Today, the most useful tool, for calculation of the chemical potential of the adsorbed phase, are the computer simulations. However, there are no algorithms of computer simulations of adsorption kinetics based on the SRT approach available in the literature. So, the main goal of this publication is design and investigation of such algorithm.The elaborated algorithm gives the results which are in agreement with slightly modified standard SRT equation for adsorption kinetics. This modification has its source in some dynamic limitations which the SRT equation alone does not account for.
Keywords: Adsorption/desorption kinetics; Monte Carlo simulations; Statistical rate theory;

Room-temperature ferromagnetism of Mg and Mn co-doped GaN films grown by PEMBE by Min-Chang Jeong; Moon-Ho Ham; Jae-Min Myoung; Sam Kyu Noh (322-326).
The magnetic properties of the epitaxial Mg and Mn co-doped GaN films with low Mg and Mn concentrations grown by plasma-enhanced molecular beam epitay (PEMBE) are reported. Surface morphology was smoother as Mn effusion cell temperature increased. Room-temperature ferromagnetic ordering was observed. The saturation magnetization (M s) of the films increased as Mn effusion cell temperature (T Mn) increased. Saturation magnetization change was not observed with the variation of Mg effusion cell temperature, and this supports that Mg incorporation in the films is not related to ferromagnetic ordering in the films. The samples exhibit n-type conductivity with the room-temperature ferromagnetism, and the films’ characteristics are determined by Mn flux at fixed growth temperature.
Keywords: PEMBE; DMSs; Room-temperature ferromagnetism; Ga(Mn,Mg)N;

Amorphous carbon nitride films have been deposited by vacuum cathodic arc method at various N2/Ar flow ratios and substrate temperatures. The properties of the films were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (IR) spectroscopy. At 200 and 400 °C, the N content could arrive at 16–18 at.% and showed the increasing tendency and then came to the saturation values with the N2/Ar ratio. While at room temperature it could be up to 31 at.% and the falling stage was even observed after the saturation. At 200 and 400 °C, from 0/50 to 50/50 sccm, the development of the aromatic cluster component was observed. In addition, there was probably some interlinked carbon backbone component in the films and the N atoms were mainly incorporated into the aromatic clusters. In the range of 150/50–400/50 sccm, it seems that the temperature played the similar role to N2/Ar ratio in affecting the microstructure. The films developed from polymeric-like type to the type similar to amorphous carbon with the temperature at 150/50 sccm and with N2/Ar ratio at room temperature, respectively. In addition, in the case of 400 °C with increasing N2/Ar flow ratio from 150/50 to 400/50 sccm or in another case of 250/50 sccm with increasing the substrate temperature from room temperature (RT) to 400 °C, the decreasing of polymeric component and then the development of aromatic component was observed. Furthermore in both cases of 400 °C and 400/50 sccm, respectively, the aromatic cluster component developed gradually. As for the film at 400/50 sccm: 400 °C, there was probably some interlinked N-containing backbone component. The chemical sputtering, which increased with the N2/Ar ratio and substrate temperature, might play an important role in deciding the N content of the films and the transferring of the microstructure.
Keywords: Carbon nitride; Raman spectroscopy; X-ray photoelectron spectroscopy; Fourier transform infrared spectroscopy;

Well-controlled nanostructure of polymer brushes has been fabricated by combining atomic force microscopy (AFM), lithography and surface-initiated polymerization techniques. Self-assembled monolayers (SAMs) of octadecyltrichlorosilane (OTS) were made on silicon surface followed by introduction of nanostructures on these SAMs surface using a conductive AFM tip, where OTS SAMs are oxidized. The oxidized areas are then covered with hydroxyl-terminated silane SAMs, reacted sequentially with 2-bromoisobutyryl bromide to connect initiator groups. Finally, the poly(methyl methacrylate) (PMMA) brushes arrays are formed on this functionalized silicon surface. This technique provides a simple and efficient approach to create various nanostructures of polymer brushes potentially used for designing nanodevices.
Keywords: Nanobrushes; Surface-initiated polymerization; PMMA; AFM;

Suppression of interfacial diffusion by a predeposited Hf metal layer on SiO2/Si by Ruiqin Tan; Yasushi Azuma; Isao Kojima (346-350).
The reaction between predeposited ultrathin Hf and native SiO2 on Si substrate was investigated using in situ X-ray photoelectron spectroscopy (XPS). During Hf metal sputtering, the initially arriving highly reactive atoms consume the oxygen of the native SiO2 to form HfO2. This preformed interlayer is helpful in suppressing interfacial diffusion and reaction during the subsequent thermal oxidation process, by which stoichiometric HfO2 films can be successfully obtained. Compared with HfO2/SiO2/Si samples deposited directly by reactive sputtering, the interface thickness is greatly reduced from 3.3 to 1.4 nm.
Keywords: Hafnium dioxide; Sputtering deposition; Auger electron spectroscopy; X-ray photoelectron spectroscopy;

Dual-source chemical vapour deposition of strontium and zirconium β-diketonates for strontium zirconate perovskite films by Michel Andrieux; Virginie Viallet; Malwenn Le Stum; Laetitia Rapenne; Michaele Ghysel; C. Haut; Marc Condat (351-356).
Strontium zirconate perovskite thin films were deposited on Si(1 0 0) substrates by metal organic chemical vapour deposition (MOCVD) in a cold wall reactor using direct injection process of two β-diketonates precursors. Oxide growth was observed over a temperature ranging from 700 to 1000 °C and various metal Sr/Zr ratios in the solution.Using peculiar experimental conditions, an orthorhombic strontium zirconate (SrZrO3 (o)) phase is obtained. The results on the films are discussed in relation with the bulk ZrO2–SrO pseudo binary equilibrium phase diagram existing on this compound.
Keywords: Strontium zirconate; MOCVD; Perovskite oxide; Phase diagram;

Temperature effects on the growth of oxide islands on Cu(1 1 0) by Guangwen Zhou; Judith C Yang (357-364).
We examined the Cu2O island formation on Cu(1 1 0) as a function of oxidation temperature in the range of 450–650 °C and oxygen pressure of 0.1 Torr. Epitaxial three-dimensional trapezoid island formation was observed for oxidation at the all temperatures and it was found that increasing oxidation temperature increased the thickening rate of the oxide islands. The oxidation at 0.1 Torr was noted to have a smaller nucleation activation energy for the oxide formation as compared to lower pressures.
Keywords: Oxidation; Cu(1 1 0); Cu2O; Morphology; In situ ultra-high vacuum transmission electron microscope (UHV-TEM);

The early stages of the surface microstructural modification of silicon, induced by single pulses of CO2 laser irradiation (λ=10.6 μm), have been studied, in both vacuum and air, by contact mode AFM. The laser pulse was found to be absorbed at the front surface of the sample, facing the laser; this was shown to be due to the presence of native oxide, which absorbs at this wavelength. We found that this absorption of energy caused the stress-induced formation of vertically oriented, square-shaped fragments, 400–700 nm in length, often with short branches, that form a wall around the impact site; they oriented toward the plane of the sample with distance from the impact site, aligning more in the electric field direction of the pulse. In addition, electrically charged, branched fragments were redeposited at the outer extremities of the pulse site.
Keywords: CO2 laser; Silicon; Surface structure; Surface morphology;

We have investigated the tailoring of graphite layers using a scanning tunneling microscope (STM). Water-adsorbed graphite surfaces were tailored at ambient temperature and atmospheric pressure by oxidizing the surface carbon atoms under a high bias voltage between an STM tip and the surface. We thereby tailored the graphite surface from one to several layers in depth. We observed that graphite surface fabrication using an STM occurred above a threshold bias voltage, and found that the threshold voltage depended on the STM tip scanning speed. By optimizing the bias voltage and the tip scanning speed, we obtained graphite nanostructures within a square of about 30 nm minimum length. The STM tailoring could also be successfully applied to carbon nanotubes, scratching and slicing them. These results indicate that the STM can be used to tailor graphene sheets producing well-defined graphite nanostructures.
Keywords: Scanning tunneling microscopy; Oxidation; Surface chemical reaction; Graphite;

We report on the laser ablation of Si in vacuum, and in the presence of helium ambient at 1 and 10 Torr, respectively. The silicon nanoparticles were deposited on silicon substrate at room temperature by ablating silicon wafer in ambient atmosphere of helium at 1 Torr. The mean cluster size ranging from 1.8 to 4.4 nm is observed depending on the laser intensity. Optical emission spectroscopy and images of the plume are used to study the spatial and temporal variation of the silicon plasma. The electron density, measured by the Stark-broadening of Si I transition 3p 21 S–4s   1 P 0 at 390.55 nm and temperature, assuming thermal equilibrium, were found to be 1.2×1018  cm−3 and 2 eV, respectively. The temporal variation of Si I transition 3p 21 S–4s   1 P 0 at 390.55 nm showed a shift in peak position attributed to collisions at an early stage of plasma formation. The relative concentration of Si II/Si I estimated by using the Saha–Boltzmann relation showed abundance of Si I. Time resolved images of the plume were used to investigate the dynamics of the expanding plasma plume, estimating the vapor pressure, vapor temperature, velocity, and stopping distance of the plume. The photoluminescent spectra of the Si thin films showed three distinct emission bands at 2.7, 2.2 and 1.69 eV, the origin of these bands is attributed to defects and quantum confinement.

Formation of ytterbium silicide nanowires on Si(0 0 1) by M Kuzmin; P Laukkanen; R.E Perälä; R.-L Vaara; I.J Väyrynen (394-398).
We demonstrate ability to form self-assembled nanowires of Yb silicide on Si(0 0 1) by means of solid phase epitaxy. Such nanowires, in contrast to those of other rare earth silicides, are shown to exhibit an unexpectedly wide variety of orientations with respect to the Si substrate. Their spontaneous formation is explained in terms of the hexagonal AlB2 structure of silicon-rich Yb disilicide.
Keywords: Ytterbium; Silicon; Silicides; Self-assembly; Surface structure, morphology, roughness, and topography; Scanning tunneling microscopy;

The chemical states and electronic properties of the interface between thermally evaporated aluminium and a photoluminescent conjugated copolymer containing 9,9′-dihexylfluorene and europium complex-chelated benzoate units in the main chain (PF6-Eu(dbm)2phen) were studied in situ by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The changes in C 1s, Eu 3d, N 1s, and Al 2p core-level lineshapes with progressive deposition of aluminium atoms were carefully monitored. Aluminium was found to interact with the conjugated backbone of the copolymer to form the Al carbide, Al–O–C complex, and Al(III)–N chelate at the interface. In addition, the europium ions were reduced to the metallic state by the deposited aluminium atoms, which were oxidized and chelated by the 1,10-phenanthroline ligands (phen). The changes in chemical states at the interface suggest that the intramolecular energy transfer process in this copolymer had been affected. Moreover, aluminium also interacted with the bulk-adsorbed oxygen, which migrates to the surface in response to the deposition of aluminium atoms, to form a layer of metal oxides. On the other hand, the evolution of the UPS spectra suggests that the π-states of the conjugated system were affected and an unfavorable dipole layer was induced by the deposited aluminium atoms.
Keywords: Aluminium; Photoluminescent; Conjugated copolymer; Europium; XPS; UPS; Intramolecular energy transfer;

Local barrier height of Au nanoparticles on a TiO2(1 1 0)-(1×2) surface by Y Maeda; M Okumura; S Tsubota; M Kohyama; M Haruta (409-414).
The local barrier height (LBH) was measured on TiO2(1 1 0) single crystal surfaces with and without Au deposition using scanning tunneling microscopy (STM). On the clean TiO2(1 1 0) surface, the LBH values measured at (1×1) and (1×2) rows were almost the same, while the value at defect sites was much smaller. This indicates that the LBH of TiO2 surfaces is dependent on the presence of topmost oxygen atoms. On the Au deposited surfaces, the LBH was dependent on the height of Au particles: the LBH of large particles (>0.4 nm in height) was about 0.3 eV larger than that of the substrate, whereas the LBH of small particles (<0.4 nm in height) was comparable to that of the substrate. The critical height of 0.4 nm for the LBH could be correlated with the metal–nonmetal transition of Au particles.
Keywords: STM; Au; TiO2; Local barrier height; Work function; Catalysis;

The specific dome-shaped structures were grown by multi-pulse Nd:YAG (λ=1.064 μm, τ=∼300 ns, and ν=30 kHz) laser irradiation of titanium targets in air at atmospheric pressure. The laser intensity values were chosen below the single-laser-pulse melting threshold of titanium. The chemical composition of the structures was studied as a function of laser pulse number as well as laser intensity, both at the outer surface layer and in depth. Micro-Raman spectroscopy, Auger electron spectroscopy (AES), and wavelength dispersive X-ray spectroscopy (WDX) were used as diagnostic techniques. Morphological investigations were performed by scanning electron microscopy. The obtained results revealed a lower oxygen concentration in the centre of the structures as compared to the borders and a lower concentration on the surface than in the depth. Moreover, it was found that the stoichiometry of the formed TiO2−x oxides increases from the structures centre towards the border and from the surface towards the depth.
Keywords: Pulsed laser irradiation; Surface structuring; Titanium; Oxidation;

Effect of the growth sequence on the properties of InGaP/GaAs/InGaP quantum wells grown by LP-MOVPE from group-V metalorganic sources by M Begotti; M Longo; R Magnanini; A Parisini; L Tarricone; C Bocchi; F Germini; L Lazzarini; L Nasi; M Geddo (423-431).
Lattice-matched, single and multiple InGaP/GaAs/InGaP quantum wells (QWs) were grown at 600 °C by low-pressure metalorganic vapour phase epitaxy (LP-MOVPE), with the use of the tertiarybuthylarsine (TBAs) and tertiarybuthylphosphine (TBP) group-V sources. In order to enhance the interface abruptness, different gas switching sequences were exploited during the growth of the interface, and the best results were obtained by inserting a few monolayer-thick GaAsP interlayers (IL), at the direct GaAs-on-InGaP interface. Low-temperature photoluminescence (PL), high resolution X-ray diffraction, transmission electron microscopy and photoreflectance spectroscopy analysis were performed on the grown heterostructures, to correlate the adopted growth sequence with the interface properties and the QW optical transitions.Promising results were obtained, among which: (a) the suppression of the anomalous PL emission at low energy, (b) optical emission from the InGaP/GaAs/InGaP QWs, exhibiting a good correlation with theoretical expectations, (c) direct interface fluctuations within 1 nm.
Keywords: GaAs-on-InGaP interface; MOVPE; Sequence dependence; Structural and optical properties;

Pd layers of thickness from a fraction to several monolayers (ML), deposited at room temperature (RT) on Nb(0 0 1) surface, are annealed from 400 to 2150 K. Changes of composition, atomic and electronic structure, caused by annealing, have been studied using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and contact potential measurement (CPD). Up to 2.8 ML the growth of Pd films is isomorphous with the substrate. Above this coverage, hcp palladium domains are formed with the orientation {1 1  2 ̄  0}Pd||{0 0 1}Nb and 〈0 0 0 1〉Pd||〈0 1 1〉Nb , and the stacking sequence AB|AB|, etc. along the [0 0 0 1] direction. The work function (WF) of the (1 1  2 ̄  0)Pd film amounts to 5.2 eV. For the annealing temperature from 600 to 900 K, beginning from coverage 6.8 ML, the Pd adlayer undergoes phase transition to another hcp structure with the stacking sequence ABAC|ABAC|, etc. For coverage higher than 1 ML, intermixing becomes noticeable above 700 K. After annealing at higher temperatures, surface alloy formation is observed in the entire coverage range studied, 0.5–9.5 ML. The alloy has a (1×1) lateral structure with reference to Nb(0 0 1). The topmost layer of the alloy consists of Pd atoms and the work function amounts to 4.8±0.1 eV.
Keywords: Surface alloys; Epitaxial growth; LEED; AES; CPD;