Epitaxial growth of AB type compounds under quasi-equilibrium conditions

Single-crystal films of CdS, CdSe and CdTe have been grown in vacuum on mica (fluophlogopite and muscovite) under isothermal conditions, i.e. with T_ev ≈ T_ep ≈ T_gr where T_ev and T_ep are the evaporation and epitaxial temperatures respectively and T_gr is the growth temperature. The synthesis was carried out in the temperature range 430°–800°C in the case of CdS, 300°–650°C for CdSe and 270°–550°C for CdTe. It is shown that the growth rate of single crystal layers (V_gr) depends exponentially on the growth temperature: V_gr (Å/sec) = D exp (−E/RT_gr) Perfect epitaxial CdS, CdSe and CdTe films have a wide range of electrophysical properties. Co-evaporation of CdS and sulphur and of CdSe and selenium allowed high-resistance films of cadmium sulphide and cadmium selenide of both n- and p-types to be obtained.     

Chemical vapor deposition of aluminum from methylpyrrolidine alane complex

Chemical vapor deposition of aluminum from a recently developed precursor, methylpyrrolidine alane complex, has been studied. Aluminum films deposited on conducting surfaces (titanium nitride, copper, gold), but not on insulating surfaces (silicon, silicon dioxide, glass) at low substrate temperatures, showing deposition selectivity, while the deposition selectivity was lost at high substrate temperatures (> 210 °C). Al deposition rates on TiN and Cu were very close, but much higher than on Au. Deposition rates on all conducting substrates increased with the temperature and reached maximum at 180 °C. Al films deposited on as-sputtered TiN or Cu have no preferred orientations. Al–Au alloys and intermetallics were observed in the films deposited on Au. Surface morphology observation revealed that the film growth on TiN or Cu is different from that on Au. The surface roughness of Al films increased with the deposition time or the film thickness.     

Annealing effects on the thermoelectric power of thin copper and silver films

The effect of annealing on the thermoelectric power of thin copper and silver films has been investigated. Using Matthiessen's rule, the thermoelectric power is separated into three components: S₀ due to bulk lattice scattering, S_s due to surface scattering and S_i due to scattering by imperfections. The values of S₀ and S_s are independent of the film thickness, whereas S_i varies with film thickness. However, S_i approaches a constant value for both copper and silver when the film thickness is larger than 1000 Å. The values of S_s obtained for copper and silver films are 1.33 μV/°K and 3.23 μV/°K respectively. For thicker films (t>1000 Å), the values of S_i for copper and silver films are 3.89 μV/°K and 9.63 μV/°K respectively.     

Graphite formation on Ni film by chemical vapor deposition

Thin film formation of graphite by chemical vapor deposition using 2-methyl-1,2′-naphthyl ketone as a starting material was carried out on Ni film substrates. On Ni films directly deposited on quartz glass, the graphite films were obtained when the Ni film thickness was above 1 000 Å and above 5 000 Å at 700 °C and 1 000 °C, respectively. Depositions on thinner Ni film substrates comprise amorphous carbon (a-C) or graphite tubes which was owing to the thermal coagulation of the Ni film into droplets. On the other hand, graphite film was obtained on the Ni film with thickness 10 Å when a-C was inserted between the Ni film and the quartz glass. The coagulation of the Ni film is considered to be avoided by inserting a-C layer.     

A three-step process for epitaxial growth of (111)-oriented C60 films on alkali–halide substrates

Epitaxial (111)-oriented C₆₀ films have been grown on alkali–halide substrates, KCl (100), KBr (100) and NaCl (100) by a three-step process: (1), substrate surface cleaning by high temperature heating; (2), initial deposition with a low deposition rate to grow two or three monolayers (ML); and (3), deposition with a high deposition rate to grow a film with expected thickness. It was found that (111)-oriented epitaxial C₆₀ films could be grown at low temperatures in a wide temperature range, from 40 to 120°C. By this three-step process, we can also grow epitaxial C₆₀ films at deposition rates as high as 35 Å/min.     

Effect of substrate temperature on the texture and structure of polycrystalline Si0.7Ge0.3 films deposited on SiO2 by molecular beam deposition

The evolution of microstructure and texture of molecular beam deposited Si_0.7Ge_0.3 films on SiO₂ at the deposition temperature range of 400–700°C was investigated by X-ray diffraction and transmission electron microscopy. At deposition temperatures between 400 and below 500°C, the films were directly deposited as a mixed-phase on SiO₂ and have a inversely cone-shaped structure. In this temperature range deposited as a mixed-phase, the grain size increases as the temperature increases, so that the grains not only grow up by deposition, but also laterally grow by the solid phase crystallization, furthermore, the texture is changed from a {110} texture to mixed {311} and {110} textures. At 500°C, the film was deposited as only a crystalline phase and has a columnar structure with a strong {110} texture. In the temperature range of 500–700°C, as the temperature increases, the {311} and {111} textures develop whereas the {110} texture reduces. The film deposited at 700°C has a random orientation and structure.     

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.     

Preparation and characterisation of silver particulate films on softened polystyrene substrates

The preparation of silver particulate films on softened polystyrene (PS) substrates and their characterisation using Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and optical absorption spectroscopy is reported in this paper. Silver films of 150 nm thickness were vacuum deposited onto PS coated glass substrates held at temperatures in the range 415–475 K at different deposition rates of 4 to 12 Å/s. SEM studies indicate that films deposited at 415 K are close to a semicontinuous structure and the structure is discontinuous at higher temperatures. The film morphology is strongly dependent on the deposition rate at any given substrate temperature. The film agglomeration increases with increasing rate of deposition. In the XPS studies, considerable attenuation of the signal corresponding to silver is observed at lower electron take of angles (ETOAs). This indicates that Ag is formed beneath the PS surface. Optical absorption studies showed an interesting red shift of the plasmon resonance wavelength for lower deposition rates again indicating that a sub-surface particulate structure is formed at lower deposition rates. These results are consistent with reported observations.     

Alkoxide route to La0.5Sr0.5CoO3 epitaxial thin films on SrTiO3

An all alkoxide based sol–gel route was investigated for preparation of epitaxial La_0.5Sr_0.5CoO₃ (LSCO) films on 100 SrTiO₃ (STO) substrates. Films with 20–30 to 80–100 nm thickness were prepared by spin-coating 0.2–0.6 M (metal) solutions on the STO substrates and heat treatment to 800 °C at 2 °C min^− 1, 30 min, in air. The films were epitaxial with a cube-on-cube alignment and the LSCO cell was strained to match the STO substrate of 3.905 Å closely; a and b = 3.894 Å and 3.897 Å for the 20–30 and 80–100 nm films, respectively. The c-axis was compressed to 3.789 Å and 3.782 Å for the 20–30 and 80–100 nm films, respectively, which resulted in an almost unchanged cell volume as compared to polycrystalline film and nano-phase powders prepared in the same way. The SEM study showed mainly very smooth, featureless surfaces, but also some defects. A film prepared in the same way on an -Al₂O₃ substrate was dense and polycrystalline with crystallite sizes in the range 10–50 nm and gave cubic cell dimensions of a_c = 3.825 Å. The conductivity of the ca 30–40 nm thick polycrystalline film was 1.7 mΩcm, while the epitaxial 80–100 nm film had a conductivity of around 1.9 mΩcm.     

Influence of thickness and growth temperature on the properties of zirconium oxide films grown by atomic layer deposition on silicon

ZrO₂ films of thicknesses varied in the range of 3–30 nm were atomic layer deposited from ZrI₄ and H₂O–H₂O₂ on p-Si(100) substrates. The effects of film thickness and deposition temperature on the structure and dielectric properties of ZrO₂ were investigated. At 272 and 325 °C, the growth of ZrO₂ started with the formation of the cubic polymorph and continued with the formation of the tetragonal polymorph. The ratio between the lattice parameters increased with the film thickness and growth temperature. The effective permittivity, determined from the accumulation capacitance of Hg/ZrO₂/Si capacitors, increased with the film thickness, reaching 15–17 in 25-nm-thick films. The permittivity decreased with the increasing growth temperature. The hysteresis of the capacitance–voltage curves was the narrowest for the films deposited at 325 °C, and increased towards both lower and higher deposition temperatures.     

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.     

Effect of negatively charged species on the growth behavior of silicon films in hot wire chemical vapor deposition

The deposition behavior of silicon in hot wire chemical vapor deposition was investigated, focusing on the generation of negatively charged species in the gas phase using a gas mixture of 20% SiH₄ and 80% H₂ at a 450 °C substrate temperature under a working pressure of 66.7 Pa. A negative current of 6–21 µA/cm² was measured on the substrate at all processing conditions, and its absolute value increased with increasing wire temperature in the range of 1400 °C–1900 °C. The surface roughness of the films deposited on the silicon wafers increased with increasing wire temperature in the range of 1510 °C–1800 °C. The film growth rate on the positively biased substrates (+ 100 V, + 200 V) was higher than that on the neutral (0 V) and negatively biased substrates (− 100 V, − 200 V, − 300 V). These results indicate that the negatively charged species are generated in the gas phase and contribute to deposition. The surface roughness evolved during deposition was attributed to the electrostatic interaction between these negatively charged species and the negatively charged growing surface.     

Transmission electron microscope study of the topotactic reaction of (0 0 1), (0 1 1) and (1 1 1) Ag films and Te

The formation, structure and morphology of silver telluride was investigated in the reaction of (0 0 1), (0 1 1) and (1 1 1) single crystalline Ag films with vacuum deposited Te. Silver films 30–40 nm in thickness were deposited by thermal evaporation onto water- and chlorine-treated NaCl. Onto this silver 1–40 nm of tellurium were deposited at 100 and 200 °C. The Ag–Te reaction occurred during Te deposition. Accordingly, formation of the compound phase was investigated from the nucleation stage through complete tellurization on either side of the polymorphic phase transformation temperature (T_c=150 °C). Transmission electron microscope and selected area electron diffraction showed that monoclinic silver telluride (Ag₂Te) of different morphology and texture was always formed. The orientation of silver and monoclinic phase upon differently oriented monocrystalline Ag films and at deposition temperatures around T_c is discussed.     

Epitaxy of atomically flat CaF2 films on Si(1 1 1) substrates

The growth of CaF₂ films with a thickness of approximately 3–4 nm on well-oriented Si(1 1 1) substrates by molecular beam epitaxy at temperatures between 410 and 560 °C were investigated by ex vacuo atomic force microscopy. Layer-by-layer growth producing atomically flat CaF₂ surfaces has been observed in a very narrow growth temperature window between approximately 430 and 470 °C. Perfect triangular shaped islands of one CaF₂ layer height are found on the surface with all corners aligned with the Si directions, indicating a pure B-stacking of the CaF₂ film. Surprisingly, also the substrate steps have been overgrown without visible defects. Below 410 °C, two different island orientations revealed a mixture of A- and B-stacking areas in the films. Above 520 °C non-wetting of the CaF interface layer leads to epitaxial films with a rough surface morphology.     

IR spectra and etch rates of plasma-enhanced chemically vapour-deposited phosphosilicate glass films

IR transmission spectra of phosphosilicate glass (PSG) films with 8 wt.% P prepared by plasma-enhanced chemical vapour deposition (PECVD) and CVD are compared. The differential IR spectrum of a PECVD PSG film differs from that of a CVD PSG film: the P=O peak has a lower intensity than the corresponding peak of the CVD film with the same phosphorus content; no peaks are evident at 980 and 500 cm⁻¹—the characteristic frequencies for P---O---P stretching and bending vibrations. The differential IR spectra of PECVD and CVD PSG films become very similar after annealing for 4 h in water vapour at 850°C. The etch rate of a PECVD film in p-etchant, which is constant throughout the film thickness, is 400 Å min⁻¹. However, the etch rate recorded after the film is subjected to annealing in water vapour at 850°C varies with the depth in the film, attaining values as high as 800 Å min⁻¹ in the region near the outer surface of the film. The results are explained as due to the oxidation of P₂O₃ to P₂O₅.     

Epitaxial bilayered Nb–Mo(001) films: growth, characterization and size effect in electron conductivity

Mo–Nb and Nb–Mo epitaxial thin (10–200 nm) films growth on the r- sapphire plane under ultra high vacuum by laser ablation deposition at the growth temperature 750°C were tested. Grown films were characterized by reflected high energy electron diffraction (RHEED), X-ray diffraction (XRD), scanning atomic force microscopy and electron transport measurements. It was found that bilayered films are high quality heteroepitaxial single-crystalline (001) films with low surface roughness (0.3–0.5 nm) and large residual electron mean free path, compared to the film thickness. The superconducting transition temperature of Nb–Mo films with equal layer thickness varies from 5 up to 9 K as the thickness increases from 10 to 100 nm. Physical properties of the films are close to each other both for Nb/Mo and Mo/Nb films of equal layer thickness and do not depend on the alternation of Mo and Nb layers. This suggests the same film structure quality and mutual epitaxy of the metals. The size dependence of electron conductivity of bilayered Mo–Nb films showed that it is determined by the dimension of individual layers, constituting the film, where the effect of ‘contact’ potential formed at the interface between Mo and Nb layers and a fluctuation of film bending is important.     

Characterization of cobalt annealed on silicon-germanium epilayers

Cobalt-coated single-crystal Si-Ge layers grown epitaxially by ultrahigh vacuum chemical vapor deposition on silicon substrates were annealed by rapid thermal annealing in the temperature range from 450 °C to 800 °C for periods ranging from 1 to 3 min. The measured sheet resistivities of the films exhibit strong dependence on the annealing conditions. The Co-SiGe film annealed at 700 °C for 3 min had the lowest sheet resistivity (3Ω/p). Structural studies using cross-sectional transmission electron microscopy showed that the cobalt films reacted with the SiGe layer and the thickness of the resulting film increases with increasing annealing temperature or time. Electron diffraction and X-ray microanalysis using energy-dispersive spectrometry showed that CoSi₂ was formed during initial annealing. The detection of germanium in the reacted layer and the deviation of the reacted layer's lattice constant from that of CoSi₂ indicated that germanium diffused into the CoSi₂ and formed ternary compounds (Co_xSi_yGe_z) during further annealing.     

2.24 Structural and electrical properties of discontinuous gold films on glass

In the present work thin gold films are investigated in the thickness interval 10–160 Å. The films are deposited in UHV on to glass substrates at room temperature and with an electric field in the substrate plane. In the thickness interval 10–45 Å the film is discontinuous and the size distribution of the islands as determined from electron micrographs is described with a log-normal distribution function. The onset of the in situ electrical conduction at the average thickness 45 Å makes it possible to measure the film resistance as a function of the film thickness during the condensation. At the thickness 53 Å stable metallic continuous paths are formed and the film achieves metallic properties. The fraction of the surface covered with material and the island density are measured 20 h after the deposition. Both these entities change rapidly at the stage when the film grows to be metallic continuous.     

Evolution of hole size and shape in {100}, {110} and {111} monocrystalline thin films of gold

Monocrystalline thin films of gold, containing controlled distributions of small holes, were produced by an epitaxial flash deposition process on heated {100}, {110} and {111} monocrystalline substrates of sodium chloride. These films, ranging from 10 to 20 nm in thickness, were then removed from their substrates, annealed for various periods at temperatures ranging from 180 to 290 °C and subsequently examined by transmission electron microscopy in order to record the evolution of hole size and shape as a function of crystallographic orientation and annealing conditions. During annealing, these holes either grow or shrink, depending on the ratio of hole diameter to film thickness, with growing holes developing clearly defined crystallographic facets aligned normal to the film surface. The evolution of hole size is in satisfactory agreement with a kinetic analysis based on atomic surface mobility, whereas the evolution of hole shape is consistent with anisotropy of the surface energy, as computed from a nearest neighbor bond model.     

Dielectric properties of vacuum-evaporated films of tungsten oxide

The dielectric constant of vacuum-evaporated films of tungsten oxide has been measured in the frequency range 300 Hz–10 MHz at various temperatures (25°C–155°C) and with various thickness (300–-5000Å). The frequency response exhibits a dispersion region between 10 and 100 kHz at room temperature (25°C) which shifts to higher frequencies as the temperature rises. The thickness dependence of the dielectric constant shows a rapid rise at low thicknesses but gradually attains a saturation value at thicknesses at which the density of the films is seen to approach its bulk value. Electron diffraction patterns show that these films are stoichiometric, possibly due to the very slow rate of evaporation. The films have a dielectric strength of the order of 5.5 × 10⁶ V cm⁻¹.