Influence of substrate properties on the growth of titanium films: Part II

In situ internal stress measurements were used to investigate the influence of the chemical properties of the substrate on the growth of a titanium film deposited under UHV conditions. The substrate films used were titanium films evaporated at different water partial pressures. When the titanium film is evaporated at substrate temperatures of 130 °C and higher the shape of the stress vs thickness curve is interpreted to indicate island growth. Comparing the internal stress curve of titanium on Al₂O₃ and Ti/H₂O substrates it is seen that the initial tensile stress is significantly larger on the Ti/H₂O substrate film. This larger tensile stress is interpreted to originate from a redistribution of oxygen at the substrate interface during the early growth stage of the clean titanium film. A compressive stress contribution at higher thickness of the titanium film is interpreted to be due to hydrogen interdiffusion from the substrate into the titanium film. Annealing of the Ti/H₂O substrate films at 350 °C for 20 min reduces the concentration of the surface oxygen species and the amount of hydrogen incorporated in the films.

Dosing of previously annealed Ti/H₂O substrate films with water affects both the oxygen concentration on the surface of these substrate films and the amount of hydrogen incorporated in the films. Oxygen dosing of these films only increases the concentration of the oxygen species on the substrate surface; thus only the initial tensile stress built up in the titanium film is affected. Dosing the films with hydrogen, on the other hand, only increases the amount of hydrogen incorporated in the substrate film, which by way of interdiffusion into the growing titanium film gives rise to a larger compressive stress at higher film thickness     

Competitive effects of film thickness and growth rate in spray pyrolytically deposited fluorine-doped tin dioxide films

Here we report efforts to understand the competitive roles of film thickness (up to ∼ 650 nm) and growth rate (up to ∼ 190 nm/min) in spray pyrolytically deposited fluorine-doped tin dioxide films. This was achieved by varying the time of deposition and/or the precursor concentration. Film properties were investigated using X-ray diffraction technique and Hall effect measurements. The thickness evolution involved textured growth along [200] orientation followed by a secondary prominence of [110] and [101] orientations. The growth rate induced effects accelerated this change. The carrier concentration and carrier mobility showed that the [200] oriented growth is technologically advantageous and can be obtained using a wider growth rate range ∼ 50-130 nm/min.     

Effect of niobium on the structure of titanium dioxide thin films

This paper reports the effect of niobium on the structure of titanium dioxide (TiO₂) thin films deposited on glass. The results obtained indicated that the direct current (DC) co-sputtering of Ti and Nb onto glass substrates in the presence of oxygen results in the formation of Nb-doped anatase thin films with strong preferential orientation. In the concentration range between 0 and 40 at.% Nb, niobium is incorporated into the TiO₂ lattice according to a substitution mechanism, entering Ti sites in the cation sub-lattice. No evidence exists for a solubility limit < 40 at.% of Nb under the applied deposition conditions, however, it is not believed that an equilibrium situation prevails. Lattice charge compensation was concluded to occur by the formation of cation vacancies for samples with up to 10 at.% Nb, and by Ti³⁺ or Nb⁴⁺ ions for the samples with ≥ 15 at.% Nb, the latter in conjunction with cation vacancy compensation.     

STM/STS investigations of titanium oxide nanostructures on au substrate

Au substrate was prepared by Au evaporation on Si(111) surface. Au surface was composed of Au grains with typical diameter of about 50 nm with atomically flat terraces. TiO₂ nanostructures were created by electron gun evaporation of Ti while simultaneous dosing of high purity O₂ gas. The pressure of oxygen was kept at 5 · 10^− 8 mbar and controlled by means of residual gas analyzer (RGA). Scanning tunneling microscopy (STM) images showed that TiO₂ nanocrystallites had grown between Au grains in cavities between them. This may suggest that such spots are preferred TiO₂ nucleation sites. I-V curves measured above Au showed metallic properties while those measured above TiO₂ exhibit energy gap characteristics for semiconducting material.     

Influence of substrate properties on the growth of titanium films: part III

The growth of thin Ti-oxide films (12 nm) on alumina substrate films formed by reactive evaporation of Ti in an oxygen atmosphere was studied by in situ internal stress measurements under ultra high vacuum conditions and transmission electron microscopy. Oxygen pressure and substrate temperatures were the varied parameters of the reactive evaporation. These Ti-oxide-films with different oxygen content (O₂/Ti-films) were then used as substrate films for the deposition of a clean titanium film. The growth stress of the titanium film on the as-deposited O₂/Ti-substrate films is comparable with that previously found for H₂O/Ti-substrates and indicates island growth and the formation of polycrystalline titanium films. Annealing (400°C, 20 min) of the as-deposited – amorphous – O₂/Ti-films gives rise to the formation of crystalline TiO₂. The amount of TiO₂ formed during annealing is strongly dependent on the oxygen content of the O₂/Ti-film. The oxygen content, in return, is dependent on oxygen partial pressure and substrate temperature during O₂/Ti-film deposition. The corresponding changes in the substrate film properties (oxygen content, crystallinity, etc.) are reflected in significant changes in the growth stress of the titanium film. The stress vs. thickness curve of these titanium films appears to indicate a superposition of the growth stress of two different growth modes, i.e. growth of a polycrystalline film with island growth on the as-deposited, amorphous oxide substrate and epitaxial growth of a quasi single crystalline film on the crystalline TiO₂-substrate.     

Fabrication of TiO2-based transparent conducting oxide on glass and polyimide substrates

We report on preparation and properties of anatase Nb-doped TiO₂ transparent conducting oxide films on glass and polyimide substrates. Amorphous Ti_0.96Nb_0.04O₂ films were deposited at room temperature by using sputtering, and were then crystallized through annealing under reducing atmosphere. Use of a seed layer substantially improved the crystallinity and resistivity (ρ) of the films. We attained ρ = 9.2 × 10^− 4 Ω cm and transmittance of ~ 70% in the visible region on glass by annealing at 300 °C in vacuum. The minimum ρ of 7.0 × 10^− 4 Ω cm was obtained by 400 °C annealing in pure H₂.     

Neural cell growth on TiO2 anatase nanostructured surfaces

Titanium oxides have anti-inflammatory activity and tunable electrochemical properties that make them attractive materials for biomedical applications. This work investigated the compatibility of nanometric coatings of low-temperature phases of TiO₂ with neurons in 4-day and 10-day cultures, using different cell densities to quantify cell survival and neurite extension. TiO₂ films were prepared by sol–gel and thermal treatment (250–450 °C) of hydrolyzed titanium tetra-isopropoxide on electrically conducting or insulating slides. The conducting substrates were not passivated by the nanometric oxide layer and could be used as electrodes. Characterization of the films showed nano-structured TiO₂ containing exclusively Ti⁺⁴ valence in anatase and amorphous phases. When coated with polylysine, all films permitted good neuron attachment and survival for at least ten days in culture. However, they generally reduced neurite growth compared to cell culture borosilicate glass, with dendrites more affected than axons. The analyses of surface topography, hydrophilicity, charge and chemical composition revealed that TiO₂ chemistry was the main factor responsible for neurite inhibition.     

Fabrication of Co-Based Coatings on Titanium Alloy by Laser Cladding with CeO2 Addition

In this study, Co-based laser cladding coatings reinforced by multiple phases were fabricated on titanium alloy. Co42 Co-based self-fluxing alloy, B₄C, and CeO₂ mixed powders were used as the precursor materials. The coatings were mainly composed of γ-Co/Ni, CoTi₂, CoTi, NiTi, TiC, Cr₇C₃, TiB₂, and TiB phases. A typical TiB₂/Cr₇C₃/TiC composite structure was chosen. It was found that CeO₂ did not influence the phase types of the coating significantly, but was effective in refining the microstructure and enhancing the microhardness and dry sliding wear resistance. Compared with the Ti-6Al-4V titanium alloy, the microhardness and wear resistance of the composite coatings were enhanced by 3.44–4.21 times and 14.26–16.87 times, respectively.     

Electrodeposition and growth mechanism of preferentially orientated nanotwinned Cu on silicon wafer substrate

Homogeneous columnar Cu film with fully embedded nanotwins was successfully fabricated on Ti/Cu seed layer on silicon wafer. The nanotwins with thickness of tens of nanometers are generally parallel to the silicon surface, showing a strong (111) preferred orientation. The acid concentration was found to be important in influencing the formation of nanoscale twins. By adjusting the acid concentration, the nanotwins can be induced from the top columnar grain to middle columnar grain and reach the bottom equiaxed grain, and a microstructural transformation model was given. A theory focusing on the cathode overpotential was proposed to reveal the effect of acid concentration on the growth mechanism of nanoscale twins. An appropriate adsorption proportion of hydrogen on cathode (acid concentration 17 ml L^?1) could increase the overpotential which supplies adequate nucleation energy for nanoscale twins formation.     

Correlation of electrolyte-derived inclusions to crystallization in the early stage of anodic oxide film growth on titanium

Pure titanium has been subjected to anodization in sulfuric and phosphoric acid. For a better understanding of the oxide growth and properties of the final film, with a particular interest focused on the solution anions in the early stage of crystallization, microstructural analyses (Raman, Transmission Electron Microscopy [TEM]) of the oxide films were correlated to chemical depth profiling by glow discharge optical emission spectroscopy (GDOES).Raman spectroscopy shows that crystallization of the oxide films starts at potentials as low as 10-20 V. The onset of crystallization and the ongoing increase in crystallinity with increasing anodization potentials had already earlier been correlated to ac-impedance measurements [Jaeggi et al., Surf. Interface Anal. 38 (2006) 182]. TEM observations show a clear difference in the early phase of crystallization between oxides grown in 1 M sulfuric acid compared to 1 M phosphoric acid. Moreover, independent of electrolyte type, nano-sized pores from oxygen bubbles formation were revealed in the central part of the films. Until now, oxygen bubbles inside an anodically grown oxide have not been observed before without the presence of crystalline regions nearby. A growth model is proposed, in which the different starting locations of crystallization inside the films are correlated to the presence of the acid anions as residues in the film, as found by GDOES chemical depth-profiling.     

Microstructure evolution of in-situ nanoparticles and its comprehensive effect on high strength steel

A novel steel strengthened by nanoparticles was investigated in this study. A Fe-based high-strength steel was developed by the trace-element regional supply method during deoxidization to generate in situ nanoparticles with a high number density in the matrix. The results show that the endogenous nanoparticles are aluminum oxide (Al₂O₃) and titanium oxide (Ti₃O₅) formed in the liquid melt. Al₂O₃ functioned as a heterogeneous nucleation site for MnS during solidification; the size of the resultant complex inclusions was approximately 1–2 μm. Furthermore, 13 nm Nb(C,N) precipitates grew with the Ti₃O₅ during the tempering process. These in situ nanoparticles strongly affected refining of the grain and inclusions. The investigated steel was strengthened more than 200 MPa by precipitation strengthening and more than 265 MPa by grain refinement strengthening according to the Ashby–Orowan mechanism and the Hall–Petch relationship, respectively.     

MBE growth and characterization of buried silicon oxide films on Si(100)

Silicon molecular beam epitaxy (Si-MBE) has been used to produce silicon oxide (SiO_x) films by evaporating Si on a heated Si(100) substrate in an ultra high vacuum system with an O₂ pressure of 10⁻⁶ to 10⁻⁴ mbar. Then the SiO_x films were overgrown with pure Si. The influence of the substrate temperature, the O₂ pressure and the Si deposition rate on the oxygen content in the SiO_x films and on the crystalline quality of the Si top-layer was investigated by Rutherford backscattering spectrometry and ion channeling. Epitaxial growth of the Si top-layer was observed up to a maximum concentration of ≈20 at.% oxygen content in the SiO_x film. Cross-sectional transmission electron microscopy shows that the structure of the SiO_x film changes duringa subsequent annealing procedure. Electron energy loss spectrometry proves that amorphous SiO₂ is formed and the development of holes indicates that the density of the as-grown SiO_x film is much lower than that of SiO₂. The specific for the as-grown SiO_x films was determined by I–V measurements.     

Evidence of hexagonal WO3 structure stabilization on mica substrate

WO₃ nanorods are grown by a simple vapor deposition method on a mica substrate and characterized by Selected Area Electron Diffraction and Energy Dispersive X-rays Spectroscopy. Experimental results show the clear evidence of an unexpected WO₃ hexagonal structure as well as an epitaxial growth on the mica substrate. Besides, potassium is evidenced inside the nanorods. It is thus deduced that a metastable WO₃ hexagonal phase is stabilized by epitaxy through a tungsten bronze interlayer having same hexagonal structure.     

The effect of defect location on coating fragmentation patterns under biaxial tension

Fragmentation of a coating possessing orthogonal preferential crack propagation directions is modeled for equibiaxial tensile loading. Two plausible cracking scenarios are compared, caused by flaws randomly distributed over the area of the coating or along the coating fragment edges. The two fragmentation scenarios considered are shown to yield qualitatively different fragment patterns.     

Structural investigation of thin TiO2 films prepared by evaporation and post‐heating

Thin films of TiO₂ have been prepared by reactive evaporation of Ti₂O₃ at substrate temperatures from 150 °C to 350 °C and by post‐heating at 150 °C to 850 °C. The mass density of the films increases with increasing substrate and annealing temperature. The crystalline structure of the film prepared at 350 °C is anatase and becomes rutile upon annealing at 850 °C. All other films are amorphous as‐prepared and become anatase upon annealing above 250 °C. The crystallinity is higher for films prepared at lower temperature and does not increase with annealing temperature. Coatings with reproducible optical properties are obtained when deposited and post‐annealed at 250 °C.     

Effects of p(O2) and p(CO2) on epitaxial growth of BaTiO3 thin films on MgO(100) substrates by using metal organic acid salts

BaTiO₃ thin films were prepared by using metal organic acid salts on MgO(100) substrates, which have large lattice-misfit with BaTiO₃. Amorphous films prefired at 470°C were crystallized to BaTiO₃ phase by heat treatment at higher temperature. Crystallinity and in-plane alignment of the prepared films were found to depend on the heat-treatment conditions. BaTiO₃ films with high crystallinity but poor (100)-orientation were obtained in air at higher than 1200°C. Whereas, (100)-oriented epitaxial BaTiO₃ film was fabricated by annealing at 900°C under low oxygen partial pressure (p(O₂)). Low carbon dioxide partial pressure (p(CO₂)) is also found to be essential for preparation of epitaxial BaTiO₃ films on MgO substrates by using metal organic acid salts.     

Characterization of titanium dioxide atomic layer growth from titanium ethoxide and water

Atomic layer growth of titanium dioxide from titanium ethoxide and water was studied. Real-time quartz crystal microbalance measurements revealed that adsorption of titanium ethoxide is a self-limited process at substrate temperatures 100–250°C. A relatively small amount of precursor ligands was released during titanium ethoxide adsorption while most of them was exchanged during the following water pulse. At temperatures 100–150°C, incomplete reaction between surface intermediates and water hindered the film growth. Nevertheless, the deposition rate reached 0.06 nm per cycle at optimized precursor doses. At substrate temperatures above 250°C, the thermal decomposition of titanium ethoxide markedly influenced the growth process. The growth rate increased with the reactor temperature and titanium ethoxide pulse time but it insignificantly depended on the titanium ethoxide pressure. Therefore reproducible deposition of thin films with uniform thickness was still possible at substrate temperatures up to 350°C. The films grown at 100–150°C were amorphous while those grown at 180°C and higher substrate temperature, contained polycrystalline anatase. The refractive index of polycrystalline films reached 2.5 at the wavelength 580 nm.     

Anodic oxide growth on aluminium surfaces modified by cathodic deposition of Ni and Co

Surface conditions similar to those found in aluminium alloys of practical use were assessed by cathodic deposition of transitions metals (Ni and Co) from different electrolytes. Fundamental aspects concerning with the growth of anodic oxide films at potentials lower than 10 V in neutral acetate buffer solution on these modified surfaces were analysed by common electrochemical techniques complemented with scanning electron microscopy and transmission electron microscopy. In both potentiodynamic and galvanostatic modes, the growth of aluminium oxide competes with the dissolution of deposited metal particles. The formation of a thin aluminium barrier oxide film beneath them shifts the dissolution potential over to 1.5 V towards more positive values. Some particles get progressively embedded in the matrix of the growing alumina and act as cation sources, increasing the film conductivity and diminishing the established electric field in the oxide. This effect is more pronounced with Co deposits due to its high active dissolution rate, before passivation occurs. Then, the generation of a two-layer film is explained in terms of the precipitation of metal hydroxide at the solution side on the oxide barrier film.     

Study of the substrate treatment effect on initial growth of indium tin-oxide films on polymer substrate using in situ conductance measurement

Changes in the initial growth mode of ion beam sputtered indium tin oxide (ITO) films on polycarbonate (PC) substrates were investigated by an in situ measurement of electrical conductance. The PC substrates were irradiated with l keV Ar ions in an oxygen environment (ion assisted reaction: IAR), prior to the film deposition for changing the surface energy. The electrical conduction modes in ITO films were discussed in terms of the film thickness and the surface energy of PC substrates. It was found that, in the initial part of the film growth, ITO nucleation density increased with the increase of the surface energy of PC. The change of the growth mode was discussed in both viewpoints of thermodynamics and atomic kinetics theories and verified by AFM (atomic force microscope) observations. Thermal stability of ITO films was investigated to observe the effect of the growth mode change by IAR pre-treatment of polymer substrate.     

β-FeSi2 growth on Cu-mediated Si substrate and enhancement of photoluminescence

We have investigated the growth of β-FeSi₂ on Cu-mediated (100) Si substrates and photoluminescence (PL) behavior. X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations revealed that the mediated surface became amorphous-like Si layers due to Cu atomic diffusion from the surface to the inside of Si and recrystallized during β-FeSi₂ deposition, and that the recrystallization may contribute to the improvement of crystallinity of β-FeSi₂ and the hetero-interface. We have observed pronounced enhancement of PL intensity from β-FeSi₂ grown on the Cu-mediated Si substrate. This implies that non-radiative recombination centers may be decreased by the improvement of hetero-interface.