CEMS of nitride coatings in agressive environments

The corrosion properties of single layered TiN and CrN films have been compared to bi-layered and multi-layered Ti/TiN films. XPS has showed that in humid SO₂ atmosphere the best corrosion properties have been achieved by a multi-layered Ti/TiN coating. Cyclic voltammetry in acetate buffer has been applied to measure the porousity and corrosion resistance of coatings. The best results have been achieved by multi-layered Ti/TiN and CrN films. Conversion electron Mössbauer spectroscopy has been used to study the changes in the interface Fe/TiN during thermal treatment in UHV. It has been shown that the amount of iron nitrides in the interface increases with increasing temperature.     

Optical properties of Fe-doped silica films on Si

Optical properties of Fe-doped silica films on Si were investigated by ellipsometric technique in the region 1-5 eV. Samples were produced by sol-gel method. Precursors were prepared by mixing tetraethoxysilane (TEOS) solution in ethanol and water with aqueous solution of Fe-chloride or Fe-acetate. The coating solution was deposited on Si substrates by spin on technique. The size of Fe-containing nanometric-sized particles depended on technology and varied from 20 to 100 nm. Optical response of complex hybrid samples SiO₂:Fe/Si was interpreted in a multi-layer model. In the inverse problem, the Maxwell equations were solved by transfer matrix technique. Dielectric function of Fe-doped silica layers was calculated in the model of effective media. Analysis of optical data has shown that various Fe-oxides formed. Experimental data for films obtained from precursors with Fe-acetate and annealed in hydrogen were well described by the model calculations taking into account a small contribution 1-5% of metal Fe imbedded in silica. The Fe/Fe-O contribution to optical response increased for samples grown from FeCl₃-precursor. Ellipsometric data for Fe-doped silica films on Si were interpreted taking into account the structural AFM studies as well as the results of magnetic measurements.     

Silicide formation in bilayer ultrathin iron and cobalt films on silicon

The processes that occur in ultrathin (up to 1 nm) Fe and Co layers during deposition onto the Si(100)2 × 1 surface in various sequences and during annealing of the formed structures to a temperature of 400°C are studied. The elemental and chemical compositions of the films are analyzed by in situ high-resolution X-ray photoelectron spectroscopy using synchrotron radiation, and their magnetic properties are determined using the magnetic linear dichroism effect in the angular distribution of Fe 3p and Co 3p electrons. It is shown that, when iron is first deposited, the formed structure consists of the layers of FeSi, Fe₃Si, Co-Si solid solution, and metallic cobalt with segregated silicon. The structure formed in the alternative case consists of the layers of CoSi, Co-Si solid solution, Co, Fe-Si solid solution, and Fe partly covered by silicon. All layers (apart from FeSi, CoSi) form general magnetic systems characterized by ferromagnetic ordering. Annealing of the structures at temperatures above 130dgC (for the Co/Fe/Si system) and ～200°C (for Fe/Co/Si) leads to the formation of nonmagnetic binary and ternary silicides (Fe _x Co_{1 ? x} Si, Fe _x Co_{2 ? x} Si).     

Magnetic Properties of Fe/Ni and Fe/Co Multilayer Thin Films

In this work, the magnetic and transport properties of Fe/SiO₂/Ni and Fe/SiO₂/Co multilayers grown on Si/SiO₂ substrates have been studied. The samples have been prepared by two-stage deposition process. In the first stage, Fe layer and SiO₂ interlayer of both samples are grown by ion beam deposition technique at room temperature. Then the samples are taken out to ambient atmosphere and loaded into a pulse laser deposition (PLD) chamber. Prior to the deposition of top layer, the samples are cleaned by annealing at 150 °C. In the second stage, Ni (or Co) layer is prepared by PLD technique at room temperature. The thickness of deposited layers has been measured by Rutherford back scattering (RBS). Magnetic properties of ferromagnetic bilayers have been investigated by room-temperature ferromagnetic resonance (FMR) and vibrating sample magnetometer (VSM) techniques. Standard four-point magneto-transport measurements at various temperatures have been performed. Two-step switching in the in-plane hysteresis loops of Fe/SiO₂/Ni and Fe/SiO₂/Co samples is observed. A crossing in the middle of hysteresis loops of both samples points to a weak antiferromagnetic interaction between the magnetic layers of the stacks. Saturation magnetization values have been obtained from the VSM measurements of samples with DC magnetic field perpendicular to the films surface. Magneto-transport measurements have shown the predominant contribution of anisotropic magnetic resistance both at room and low temperatures. FMR studies of Fe/SiO₂/Ni and Fe/SiO₂/Co samples have revealed additional non-uniform (surface and bulk SWR) modes, which behavior has been explained in the framework of the surface inhomogeneity model. An origin of the antiferromagnetic interaction has been discussed.     

Investigation on preparation and physical properties of nanocrystalline Si/SiO2 superlattices for Si-based light-emitting devices

Structures containing silicon nanocrystals (nc-Si) are very promising for Si-based light-emitting devices. Using a technology compatible with that of silicon, a broader wavelength range of the emitted photoluminescence (PL) was obtained with nc-Si/SiO₂ multilayer structures. The main characteristic of these structures is that both layers are light emitters. In this study we report results on a series of nc-Si/SiO₂ multilayer periods deposited on 200 nm thermal oxide SiO₂/Si substrate. Each period contains around 10 nm silicon thin films obtained by low-pressure chemical vapour deposition at T=625°C and 100 nmSiO₂ obtained by atmospheric pressure chemical vapour deposition T=400°C. Optical and microstructural properties of the multilayer structures have been studied by spectroscopic ellipsometry (using the Bruggemann effective medium approximation model for multilayer and multicomponent films), FTIR and UV–visible reflectance spectroscopy. IR spectroscopy revealed the presence of SiO_x structural entities in each nc-Si/SiO₂ interface. Investigation of the PL spectra (using continuous wave-CW 325 nm and pulsed 266 nm laser excitation) has shown several peaks at 1.7, 2, 2.3, 2.7, 3.2 and 3.7 eV, associated with the PL centres in SiO₂, nc-Si and Si–SiO₂ interface. Their contribution to the PL spectra depends on the number of layers in the stack.     

Study of ultrathin iron silicide films grown by solid phase epitaxy on the Si(001) surface

Ultrathin films of iron silicide have been grown by high-temperature annealing of 0.14-to O.5O-nm-thick Fe films deposited on the Si(001) surface at room temperature. It has been found that annealing leads to the formation of nanoislands of iron silicide on the surface, so that their type depends on the thickness of the Fe film. High-energy electron diffraction and atomic force microscopy measurements have revealed that the deposition of Fe films less than 0.32 nm thick on the Si(001) surface stimulates epitaxial growth of both three-dimensional β-FeSi₂ and two-dimensional γ-FeSi₂ islands. It has been found that, for Fe coverages of more than 0.32 nm thick, a complete transition to solide phase epitaxy is observed only for two-dimensional β-FeSi₂ islands. The effect of prolonged annealing at 850°C on the morphology of the surface of the iron silicide film has been investigated.     

Ion tracks in ultrathin polymer films: The role of the substrate

Surface damage produced by single MeV-GeV heavy ions impacting ultrathin polymer films has been shown to be weaker than those observed under bulk (thick film) conditions. The decrease in damage efficiency has been attributed to the suppression of long-range effects arising from excited atoms lying deeply in the solid. This raises the possibility that the substrate of the films itself is relevant to the radiation effects seen at the top surface. Here, the role of the substrate on cratering induced by individual 1.1 GeV Au ions in ultrathin poly(methyl methacrylate) (PMMA) layers is investigated. Materials of different thermal and electrical properties (Si, SiO₂, and Au) are used as substrates to deposit PMMA thin films of various thicknesses from ∼1 to ∼300 nm. We show that in films thinner than ∼40 nm craters are modulated by the underlying substrate to a degree that depends on the transport properties of the medium. Crater size in ultrathin films deposited on the insulating SiO₂ is larger than in similar films deposited on the conducting Au layer. This is consistent with an inefficient coupling of the electronic excitation energy to the atomic cores in metals. On the other hand, the damage on films deposited on SiO₂ is not very different from the Si substrate with a native oxide layer, suggesting, in addition, poor energy transmission across the film/substrate interface. The experimental observations are also compared to calculations from an analytical model based on energy addition and transport from the excited ion track, which describe only partially the results.     

Microstructure and mechanical characteristics of hydroxyapatite coatings on Ti/TiN/Si substrates synthesized by pulsed laser deposition

We synthesized by pulsed laser deposition a bilayer of Ti/TiN on Si(100) wafers which was coated in a next step with hydroxyapatite (samples labelled HA-1). Some of the structures were further thermally treated in a water vapour jet (samples labelled HA-2). In SEM, the HA surface looked rough, with micronic droplets. TEM and SAED investigations revealed a compact organization of HA crystals in the case of the HA-1 sample, while two regions (one compact and one porous) were identified for the HA-2 sample, with triclinic HA crystals within the 500 nm range. In XTEM, at the Si/TiN border, a 2–3 nm SiO₂ layer was visible, whereas at the TiN/Ti border there was a smooth transition from fcc (111) TiN to hcp (100) Ti. The HA crystals were elongated normal to the surface. According to Berkovich indentation qualitative analyses, the sample HA-1 was more homogeneous and harder but brittle. Scratch tests confirmed quantitatively that HA-1 was more resistant and adherent than HA-2 films. In the first case, the big droplets only were removed by the indenter, while the HA-2 films were delaminated on large areas as a result of wedge spallation failure.     

Low cost fabrication of SiO2 optical waveguides by laser direct writing on Ti-doped Sol-Gel films

A commercial direct laser writing (DLW) system operating at 1070 nm was used to fabricate SiO₂ optical waveguides on silicon wafers. A Ti-doped SiO₂ Sol-Gel film was deposited on the SiO₂/Si substrate by the dip-coating technique, based on which SiO₂ optical waveguides were patterned by DLW using a Ytterbium fiber laser and followed by chemical etching. The effects of laser parameters and the preheated temperature of Sol-Gel films on the dimensions of optical waveguides were studied systematically. The differences of etching rate between laser irradiated and non-irradiated areas in Sol-Gel films preheated at various temperatures are characterized by measuring the thickness of the films. Results demonstrate that the available laser power density range for laser densification and the width of the patterned optical waveguides are influenced strongly by the preheated temperature of the Sol-Gel films. The width of the optimized optical waveguide in this work is 25 μm. The minimum propagation loss of the fabricated optical waveguides is 1.7 dB cm⁻¹ at the wavelength of 1550 nm.     

Synthesis of chromium silicide with laser pulses

Thin chromium films, 60 nm thick, were deposited onto single-crystal silicon wafers. The samples were irradiated with 30 ns single pulses from a Nd: glass laser at fluences ranging from 0.4 to 2.25 J/cm². Rutherford backscattering spectrometry, transmission electron microscopy and electron diffraction measurements evidence the formation of CrSi₂ layers at the Cr/Si interface. The silicide thickness depends on the laser fluence.     

Fabrication and characterization of Au/SiO2 nanocomposite films grown by radio-frequency cosputtering

Au/SiO₂ nanocomposite films were prepared on Si wafers by cosputtering of SiO₂ and gold wires. Au/Si atomic ratios in Au/SiO₂ nanocomposite films were varied from 0.53 to 0.92 by controlling the length of gold wire to study the evolution of the crystallization of gold, the size of Au/SiO₂ nanocomposite particles, and the optical properties of as-deposited Au/SiO₂ nanocomposite films. An X-ray photoelectron spectroscopy reveals that Au exists as a metallic phase in the bulk of SiO₂ matrix. Dome-shaped Au/SiO₂ nanocomposite particles and both Au (1 1 1) and (2 0 0) planes were observed in a field-emission scanning electron microscopy and X-ray diffraction studies respectively. With an ultraviolet-visible, absorption peaks of Au/SiO₂ nanocomposite films were observed at 525 nm.     

Thickness-Dependent Phase Transformations in Implanted Iron

The dependence of iron nitride formation and phase transformations on the thickness of the nitrogen implanted Fe layers is investigated. The iron nitrides formation in N-implanted Fe-layers of various thickness (60–860 nm) implanted with 100 keV N₂ ⁺ ions with doses ranging from 1×10¹⁷ to 4.5×10¹⁷ at. N/cm² is studied by CEMS and supplemented by GXRD. It was found that nitride formation is strongly enhanced in thin Fe films as compared to thicker layers or the bulk samples. A given nitride phase is formed in thin (60, 130 nm) Fe layers at significantly lower N-doses and the transformation of the original Fe layer into iron nitrides is more complete than in the bulk -Fe. It is suggested that high stresses in thin Fe layers, revealed by the GXRD measurements, may enhance nitrides formation.     

Effects of substrate on surface morphology, crystallinity, and photoluminescence properties of sputtered ZnO nano films

Effects of substrate on crystallinity, surface morphology, and luminescence properties of radio frequency sputtered zinc oxide (ZnO) thin films were investigated. A variety of materials such as Si (100), Si (111), Al₂O₃, quartz, and silicon carbide (SiC) wafers were examined as substrates for deposition of ZnO thin films. The results showed smooth and uniform growth of c-axis orientation films. The thickness of the layers was about 50 nm. The average grain sizes of films were about 10, 13, and 12 nm for Si (111), quartz, and SiC samples, respectively. The deposited film on Al₂O₃ showed the largest grain size, about 500 nm. Grazing incidence x-ray diffraction patterns of the samples revealed that sputtered layers on Al₂O₃ and quartz had better crystallinity with higher peak at (002) orientation compared to Si and SiC substrates. Moreover, the Al₂O₃ sample exhibited a weak peak at position of (100) planes of ZnO too. The photoluminescence spectra of the samples showed a typical luminescence behavior with a broad UV band, including a main peak at around 388 nm and a weak shoulder peak at around 381 nm, corresponding with bound excitonic recombination and free excitonic recombination, respectively. The luminescence peak revealed that the intensity of UV emission is not necessarily dependent on the grain sizes and the micro-structural quality of ZnO films.     

Growth of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> films on the Si(111) surface covered by a thin SiO<Subscript>2</Subscript> layer

Magnetite polycrystalline films are grown by variously oxidizing a Fe film on the Si(111) surface covered by a thin (1.5 nm) SiO₂ layer. It is found that defects in the SiO₂ layer influence silicidation under heating of the Fe film. The high-temperature oxidation of the Fe film results in the formation of both Fe₃O₄ and iron monosilicide. However, the high-temperature deposition of Fe in an oxygen atmosphere leads to the growth of a compositionally uniform Fe₃O₄ film on the SiO₂ surface. It is found that such a synthesis method causes [311] texture to arise in the magnetite film, with the texture axis normal to the surface. The influence of the synthesis method on the magnetic properties of grown Fe₃O₄ films is studied. A high coercive force of Fe₃O₃ films grown by Fe film oxidation is related to their specific morphology and compositional nonuniformity.     

Reaction products and oxide thickness formed by Ti out-diffusion and oxidization in poly-Pt/Ti/SiO2/Si with oxide films deposited

In the paper, we present experimental results to enhance the understanding of Ti out-diffusion and oxidization in commercial poly-Pt/Ti/SiO₂/Si wafers with perovskite oxide films deposited when heat-treated in flowing oxygen ambient. It indicates that when heat-treated at 550 and 600 °C, PtTi₃+PtTi and PtTi are the reaction products from interfacial interaction, respectively; while heat-treated at 650 °C and above, the products become three layers of titanium oxides instead of the alloys. Confirmed to be rutile TiO₂, the first two layers spaced by 65 nm encapsulate the Pt surface by the first layer with 60 nm thick forming at its surface and by the next layer with 35 nm thick inserting its original layer. In addition, the next layer is formed as a barrier to block up continuous diffusion paths of Ti, and thus results in the last layer of TiO_2−x formed by the residual Ti oxidizing.     

Effects of Ni catalyst–substrate interaction on carbon nanotubes growth by CVD

An investigation of the effects of substrate type and various treatments on carbon nanotubes (CNT) growth, using an evaporated Ni thin film as a catalyst, is presented. Barrier layers of SiO₂, Si₃N₄, and TiN on Si were used as substrates. The catalyst-insulating substrate systems have been processed in several gaseous atmospheres (Ar, NH₃ and H₂) and in the temperature range 700–900 °C, in order to obtain the most appropriate morphology, size and density of catalyst particles as seeds for the subsequent CNT growth. On this kind of substrates, the smallest nanoparticles were obtained on SiO₂ layers, in H₂ or NH₃ atmosphere even at 700 °C. However, the best vertically aligned and well-graphitized CNT resulted from the NH₃ annealing process, followed by the CNT deposition at 900 °C in C₂H₂ and H₂.On TiN conducting substrates, the best vertically aligned CNT were deposited using a shorter annealing step and a deposition process at reduced pressure. The samples were characterized by means of scanning electron microscopy (SEM) and Raman spectroscopy analysis.     

Mössbauer study of oxide films of Fe-, Sn-, Cr- doped zirconium alloys during corrosion in autoclave

Mössbauer investigations were used to compare iron atom states in oxide films of binary Zr-Fe, ternary Zr-Fe-Cu and quaternary Zr-Fe-Cr-Sn alloys. Oxide films are received in an autoclave at a temperature of 350–360 °C and at pressure of 16.8 MPa. The corrosion process decomposes the intermetallic precipitates in alloys and forms metallic iron with inclusions of chromium atoms α–Fe(Cr), α–Fe(Cu), α–Fe ₂O₃ and Fe ₃O₄ compounds. Some iron ions are formed in divalent and in trivalent paramagnetic states. The additional doping influences on corrosion kinetics and concentration of iron compounds and phases formed in oxide films. It was shown the correlation between concentration of iron in different chemical states and corrosion resistance of alloys.     

Influence of MgO and ZrO2 buffer layers on dielectric properties of Ba(Zr0.20Ti0.80)O3 thin films prepared by sol-gel processing

Ba(Zr_0.20Ti_0.80)O₃ (BZT) thin films are deposited on Pt(1 1 1)/Ti/SiO₂/Si, MgO and ZrO₂ buffered Pt(1 1 1)/Ti/SiO₂/Si substrates by a sol-gel process. The BZT thin films directly grown on Pt(1 1 1)/Ti/SiO₂/Si substrates exhibit highly (1 1 1) preferred orientation, while the films deposited on Pt(1 1 1)/Ti/SiO₂/Si substrates with MgO and ZrO₂ buffer layers show highly (1 1 0) preferred orientation. At 100 kHz, dielectric constants are 417, 311 and 321 for the BZT thin films grown on Pt(1 1 1)/Ti/SiO₂/Si, MgO and ZrO₂ buffered Pt(1 1 1)/Ti/SiO₂/Si substrates, respectively. The difference in dielectric properties of three BZT films can be attributed to the series capacitance effect, interface conditions and their orientations.     

Effects of laser pulse wavelength and laser fluence on the characteristics of silver nanoparticle generated by laser ablation

Electrically pumped ultraviolet random lasing was achieved in metal-insulator-semiconductor (MIS) diodes based on ZnO films at room temperature. The ZnO films were grown by plasma assisted molecular beam epitaxy. Two different kinds of insulator layers, SiO _x (0<x≤2) and AlO _x (0<x≤1.5) were deposited by electron beam evaporation. X-ray diffraction experiments found these oxide layers were amorphous (or microcrystals), and X-ray photoelectron spectroscopy confirmed the Si and Al were fully oxidized. Compared with devices using SiO _x as the insulator layer, diodes with evaporated AlO _x layers showed a lower working threshold forward current (～20 mA to ～26 mA) and higher emission intensity. Periodic features indicating formation of closed-loop paths were deduced by the power Fourier transform of electroluminescence spectra. The cavity length of both devices increased as forward currents increased, while a larger cavity length was always obtained in the AlO _x -involved device under the same working current. The improved performance was attributed to larger hole amount in AlO _x layers. These results revealed that evaporated AlO _x can serve as good electron blocking and hole supplying layers for hetero-structures.     

Al2O3/SiO2 films prepared by electron-beam evaporation as UV antireflection coatings on 4H-SiC

Al₂O₃/SiO₂ films have been deposited as UV antireflection coatings on 4H-SiC by electron-beam evaporation and characterized by reflection spectrum, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The reflectance of the Al₂O₃/SiO₂ films is 0.33% and 10 times lower than that of a thermally grown SiO₂ single layer at 276 nm. The films are amorphous in microstructure and characterize good adhesion to 4H-SiC substrate. XPS results indicate an abrupt interface between evaporated SiO₂ and 4H-SiC substrate free of Si-suboxides. These results make the possibility for 4H-SiC based high performance UV optoelectronic devices with Al₂O₃/SiO₂ films as antireflection coatings.