Hot-wire chemical vapor deposition of WO3−x thin films of various oxygen contents

We present the synthesis of tungsten oxide (WO_3−x) thin films consisting of layers of varying oxygen content. Configurations of layered thin films comprised of W, W/WO_3−x, WO₃/W and WO₃/W/WO_3−x are obtained in a single continuous hot-wire chemical vapor deposition process using only ambient air and hydrogen. The air oxidizes resistively heated tungsten filaments and produces the tungsten oxide species, which deposit on a substrate and are subsequently reduced by the hydrogen. The reduction of tungsten oxides to oxides of lower oxygen content (suboxides) depends on the local water vapor pressure and temperature. In this work, the substrate temperature is either below 250 °C or is kept at 750 °C. A number of films are synthesized using a combined air/hydrogen flow at various total process pressures. Rutherford backscattering spectrometry is employed to measure the number of tungsten and oxygen atoms deposited, revealing the average atomic compositions and the oxygen profiles of the films. High-resolution scanning electron microscopy is performed to measure the physical thicknesses and display the internal morphologies of the films. The chemical structure and crystallinity are investigated with Raman spectroscopy and X-ray diffraction, respectively.     

Characteristics of SiOx thin films deposited by atmospheric pressure chemical vapor deposition as a function of HMDS/O2 flow rate

SiO₂-like thin films were deposited using a modified dielectric barrier discharge with a gas mixture of hexamethyldisilazane (HMDS)/O₂/He/Ar and their film characteristics were investigated as functions of the HMDS and O₂ flow rates. As the HMDS flow rate was increased, higher amounts of Si-(CH₃)_x bonds and lower amounts of Si-OH bonds were observed in the deposited SiO_x, due to the increase in the amount of the less dissociated HMDS, which also caused an increase of the surface roughness. The addition and increase of the oxygen flow to HMDS/He/Ar brought the stoichiometry of SiO_x close to SiO₂ and decreased the surface roughness by decreasing the amount of Si-(CH₃)_x bonds through the increased decomposition and oxidation of HMDS, even though the deposition rate was decreased. However, when the O₂ flow rate was higher than a certain threshold, the surface roughness increased again, possibly due to the decrease in the extent of HMDS dissociation caused by the decreased plasma density at the higher oxygen flow rate. By using an optimized gas mixture of HMDS (150 sccm)/O₂ (14 slm)/He (5 slm)/Ar (3 slm), SiO₂-like thin films with a very low impurity level and having a smooth surface could be obtained with a deposition rate of approximately 42.7 nm/min.     

Preparation and dielectric properties of polycrystalline films with dense nano-structured BaTiO3 by chemical vapor deposition using inductively coupled plasma

To clarify the dielectric properties of BaTiO₃ with nanometer size region, it is necessary to fabricate the dense structure composed of BaTiO₃ nanoparticles. In the present study, BaTiO₃ nanoparticles were directly deposited on Pt/Al₂O₃/SiO₂/Si substrate by introducing Ba(DPM)₂ and Ti(OiPr)₄ into an inductively coupled plasma (ICP). The optimal condition for preparing dense structure of BaTiO₃ nanoparticles was investigated by changing the substrate temperature. Single phase BaTiO₃ of perovskite structure was obtained at the substrate temperatures between 773 and 1173 K. The dense structure of BaTiO₃ nanoparticles with particle sizes of about 30 nm was successfully obtained at the substrate temperature of 773 K. At the substrate temperature>873 K, the deposited nanoparticles sintered to be the columnar structure. The ε_r and tan δ of the BaTiO₃ nanoparticles were estimated to be 285 and 6.6%, respectively (1 kHz and 100 mV). The phase of the BaTiO₃ nanoparticles were found to be paraelectric by the measurement of C-V curves. The breakdown field of the dense structure of BaTiO₃ nanoparticles was estimated to be 649 kV/cm according to I-V curves. These features are favorable for applying the structure to the dielectric layer of multilayer capacitors.     

Low temperature deposition of tin oxide films by inductively coupled plasma assisted chemical vapor deposition

Well-crystallized tin oxide films were successfully synthesized without additional heating by inductively coupled plasma assisted chemical vapor deposition (ICP-CVD). The degree of crystallization was affected by the ICP power and hydrogen flow rate. The substrate temperature was increased only up to 423-453 K by plasma heating, which suggests that the formation of the SnO₂ crystals was not caused by plasma heating, but by enhanced reactivity of precursors in high density plasma. The micro-hardness of deposited tin oxide films ranged from 5.5 to 11 GPa at different hydrogen flow rates.     

BiFeO3 thin films prepared using metalorganic chemical vapor deposition

BiFeO₃ thin films were grown on (001) SrTiO₃ and (001) ZrO₂(Y₂O₃) substrates by single source metalorganic chemical vapor deposition in the temperature range T = 500 ÷ 800 °C using Fe(thd)₃ and Bi(C₆H₅)₃ as volatile precursors. X-ray diffraction analysis shows cube-on-cube epitaxial growth of BiFeO₃ on (001) SrTiO₃. The strongly reduced bismuth transfer into the film due to the high thermal stability of Bi(C₆H₅)₃ was counterbalanced by the increase of the total pressure as well as of the residence time of the precursor flow in the reactor; the Bi/Fe ratio in the film thus becomes close to that in the precursor mixture. Optical second harmonic generation measurements have evidenced the ferroelectric ordering in BiFeO₃ films and the apparent decrease of the Curie temperature of the strained films as compared to BiFeO₃ single crystal.     

Microstructure and physical properties of nanofaceted antimony doped tin oxide thin films deposited by chemical vapor deposition on different substrates

Antimony doped tin oxide SnO₂: Sb thin films have been fabricated by atmospheric pressure chemical vapour deposition at substrate temperature varying between 350 °C and 420 °C in a horizontal reactor, from a mixture of hydrated SnCl₂, SbCl₃ and O₂ gas. The films were grown on glass substrates and onto polished and porous n-type silicon. Doped films fabricated with various Sb (Sb/Sn %) contents ranging from undoped 0% to 4% were characterised employing different optical characterisation techniques, like X-ray diffraction, transmittance and reflectance in the wavelength range of 300 to 2500 nm and ellipsometry. The films exhibit the usual cassiterite diffraction pattern with high crystalline structure. Examination of the surface by scanning electron microscopy (SEM) showed that the films are textured made up of many pyramidal crystallites with nanofaceted surfaces, indicating highly stabilised material. The presence of inverted pyramids indicates that the crystallites grown by coalescence. The surface morphology was found to be independent on the kind of the substrate. From X-Ray spectra and SEM observations we get the texture the lattice constant and the grain size. The optical results provide information on film thickness, optical parameters and transmittance upon antimony concentration. The microstructure of the films, the grain growth topics (nucleation, coalescence…) depend strongly on deposition conditions and doping concentration. The observed variations of both the resistivity ρ and transmittance T are correlated to antimony atoms concentration which induced variation in the microstructure and in the size of SnO₂ nanograins (typically 20-40 nm). In this work, we have determined the feasibility of incorporating the correct amount of Sb atoms in tin oxide film by means of resistivity and transmission. SEM observations showed that the substrate do not affect the morphology.     

Growth of silicon carbide thin films by hot-wire chemical vapor deposition from SiH4/CH4/H2

Silicon carbide (SiC) thin films were prepared by hot-wire chemical vapor deposition from SiH₄/CH₄/H₂ and their structural properties were investigated by X-ray diffraction, Fourier transform infrared absorption and Raman scattering spectroscopies. At 2 Torr, Si-crystallite-embedded amorphous SiC (a-Si_1 − xC_x:H) grew at filament temperatures (T_f) below 1600 °C and nanocrystalline cubic SiC (nc-3C-SiC:H) grew above T_f = 1700 °C. On the other hand, At 4 Torr, a-Si_1 − xC_x:H grew at T_f = 1400 °C and nc-3C-SiC grew above T_f = 1600 °C. When the intakes of Si and C atoms into the film per unit time are almost the same and H radicals with a high density are generated, which takes place at high T_f, nc-3C-SiC grows. On the other hand, at low T_f the intake of Si atoms is larger than that of C atoms and, consequently, Si-rich a-Si_1 − xC_x:H or Si-crystallite-embedded a-Si_1 − xC_x:H grow.     

Large-scale initiated chemical vapor deposition of poly(glycidyl methacrylate) thin films

The initiated chemical vapor deposition (iCVD) of poly(glycidyl methacrylate) (PGMA) was scaled up using dimensionless analysis. In the first stage, PGMA was deposited onto a large stationary substrate and a deposition rate as high as 85 nm/min was achieved. It was found that the deposition rate increases with increasing filament temperature, whereas the deposition rate and the number-average molecular weight decrease with increasing substrate temperature. In the second stage, PGMA was deposited onto a moving substrate. At speeds between 20 mm/min and 60 mm/min, the deposition rate on the moving substrate was found to be equal to the deposition rate on the stationary substrate. Fourier transform infrared spectroscopy showed that the epoxide functionality of the PGMA films was retained during the iCVD process. Since the iCVD polymerization of different vinyl monomers all use similar parameters, this scale up can be applied to the scale up of other vinyl monomers such as 2-hydroxyethyl methacrylate and perfluoroalkyl ethyl methacrylate.     

Preparation of CuInSe2 thin films through metal organic chemical vapor deposition method by using di-μ-methylselenobis(dimethylindium) and bis(ethylisobutyrylacetato) copper(II) precursors

Highly polycrystalline copper indium diselenide (CuInSe₂) thin films on molybdenum substrate were successfully grown at 330 °C through two-stage metal organic chemical vapor deposition (MOCVD) method by using two precursors at relatively mild conditions. First, phase pure InSe thin film was prepared on molybdenum substrate by using a single-source precursor, di-μ-methylselenobis(dimethylindium). Second, on this InSe/Mo film, bis(ethylisobutyrylacetato) copper(II) designated as Cu(eiac)₂ was treated by MOCVD to produce CuInSe₂ films. The thickness and stoichiometry of the product films were found to be easily controlled in this method by adjusting the process conditions. Also, there were no appreciable amounts of carbon and oxygen impurities in the prepared copper indium diselenide films.     

Fabrication of PTFE thin films by dual catalytic chemical vapor deposition method

Dependence of catalyzing materials on deposition of polytetrafluoroethylene (PTFE = ”Teflon” in commercial) films by catalytic chemical vapor deposition (Cat-CVD) method is investigated. It has been clarified that Ni-containing catalyzers has a catalyzing effect that can decompose hexafluoropropylene-oxide (HFPO) to form PTFE films. A novel method named Dual Cat-CVD is also proposed. In the method, carbonized and fluorinated surface of Ni-containing catalyzer is removed and refreshed using atomic hydrogen generated by additionally introduced tungsten (W) catalyzer in the same chamber. This Dual Cat-CVD method enables to recover the deposition rate of PTFE films drastically.     

Electrical conduction in 10-20 nm thick polycrystalline tin oxide thin films deposited by chemical vapor deposition

Electrical properties were studied for chemical vapor deposited fluorine doped tin oxide films that were less than 20 nm thick. The electrical properties of the coatings were found to be affected by the type of additive alcohol used in the deposition process. Conductivity was superior for ethanol or isopropyl alcohol (IPA) compared to methanol. Hall effect measurements showed that mobility and carrier concentration were best for IPA, less for ethanol, and least for methanol. Influence of carrier scattering factors to electrical properties was speculated. Potential barrier for carrier scattering at grain boundaries was estimated to be lower in an IPA-added film compared to methanol-added films. Experimental results suggested electrical properties were influenced by size and density of tin oxide micro-grains. It was concluded that interconnections between the micro-grains increased mobility and carrier concentration of very thin films.     

Photo-induced properties of thin TiO2 films deposited using the radio frequency plasma enhanced chemical vapor deposition method

Thin titanium oxide films were deposited with the help of radio frequency plasma enhanced chemical vapor deposition technique. The RF power of deposition was applied as the operational parameter of the process, and optical properties, transmittance in particular, of the films were utilized as criterion for their selection.Photo-induced properties of the films, including a change of water wettability under the effect of illumination and photocatalytic activity, were studied. A substantial decrease of water contact angle was observed upon the irradiation of the films with ultraviolet (UV) light. The largest increase of water wettability was obtained for the surfaces of the films exhibiting the highest value of index of refraction. Testing of the photo-catalytic activity of the titanium oxide films comprised UV light-induced decomposition of benzene and aniline dissolved in water and bactericidal action against the Escherichia coli strain DH5α. The largest bactericidal efficiency was observed in the case of the film characterized by the highest index of refraction. Auger Electron Spectroscopy measurements have shown that the film composition is that of a nearly stoichiometric TiO₂, with a small chlorine contamination.     

Fabrication of thin films of bismuth selenide using novel single-source precursors by metal organic chemical vapor deposition

A metal-organic compound, Bi[(SePⁱPr₂)₂N]₃ has been synthesized and used as a single-source precursor for the deposition of bismuth selenide thin films via low-pressure metal-organic chemical vapor deposition. Crystalline thin films of rhombohedral Bi₂Se₃ have been deposited on glass substrates. The films have been characterized by X-ray powder diffraction, scanning electron microscopy and energy dispersive analysis of X-rays.     

Hydrogen storage of a mechanically milled carbon material fabricated by plasma chemical vapor deposition

Abstract

Carbon materials were prepared by plasma chemical vapor deposition at different pressures without catalyst, and the structure and hydrogen storage characteristics of milled and unmilled samples of the materials were evaluated. Using this approach, we were able to fabricate graphite microcrystals with a crystallite size of several nanometers, and the crystallite size and surface area could be controlled by changing the pressure during plasma chemical vapor deposition. The hydrogen storage capacity of the unmilled materials was 0.3?wt%, but milling increased this value to 1.0?wt% by reducing the crystallite size and increasing the crystallite surface area.     

Chemical vapor deposition of boron- and nitrogen-containing graphene thin films

Boron and nitrogen-incorporated graphene thin films were grown on polycrystalline Ni substrates by thermal chemical vapor deposition using separate boron- and nitrogen-containing feedstocks. Boron and nitrogen atoms were incorporated in the film in almost equal amounts and the total content reached ∼28%. The film predominantly consisted of separate graphene and boron nitride domains. Carrier concentration in the graphene domains was estimated to be about 1 × 10⁻³ e/atom (3.8 × 10¹² cm⁻²) from G band shift in Raman spectra.     

Deposition of CNx thin films by plasma-activated chemical vapour deposition using various precursors as carbon source

CNx-thin films have been deposited by plasma-activated chemical vapour deposition with capacitively or inductively coupled r.f. plasma. Acetylene, methane, carbon monoxide and tetracyanoethylene have been used as carbon precursor. A strong dependence of the layer properties on the precursor was found. In some films the nitrogen to carbon ratio was close to that of C3N4. The highest nitrogen content was observed in films made from carbon monoxide as precursor in an inductively coupled argon/nitrogen plasma. The nitrogen was mainly incorporated with covalent nitrogen–carbon single bonds. X-ray diffraction measurements showed no reflections indicating crystallinity. In small grains (length ∼10 μm) found on the layer surface the stoichiometry corresponded nearly to that of C3N4, the oxygen content is very low. Further characterizations by TEM are intended. This revised version was published online in November 2006 with corrections to the Cover Date.     

Plasma-enhanced chemical vapor deposition of TiO2 thin films for dielectric applications

Amorphous TiO₂ thin films were formed by plasma-enhanced chemical vapor deposition (PECVD) from mixtures of titanium IV isopropoxide (Ti(O-i-C₃H₇)₄) and oxygen. The deposition rate was found to be weakly activated, with an apparent activation energy of 4.5 kJ/mol. The deposition rate increased with equivalence ratio and decreased with plasma power. This dependence on atomic oxygen density was consistent with behavior observed in other metal oxide PECVD systems. Metal-insulator-silicon devices were fabricated, and characterized using capacitance-voltage measurements. The apparent dielectric constant of the TiO₂ thin films increased from 15 to 82 with film thickness. The observed variations were consistent with the formation of an interfacial SiO₂ layer. Assuming that a TiO₂/SiO₂ bilayer behaves as two capacitors in series, an intrinsic TiO₂ dielectric constant of 82 ± 10 and an interfacial SiO₂ layer thickness of 3 ± 1 nm were extracted from electrical measurements.     

Properties of amorphous silicon thin films synthesized by reactive particle beam assisted chemical vapor deposition

Amorphous silicon thin films were formed by chemical vapor deposition of reactive particle beam assisted inductively coupled plasma type with various reflector bias voltages. During the deposition, the substrate was heated at 150 °C. The effects of reflector bias voltage on the physical and chemical properties of the films were systematically studied. X-ray diffraction and Raman spectroscopy results showed that the deposited films were amorphous and the films under higher reflector voltage had higher internal energy to be easily crystallized. The chemical state of amorphous silicon films was revealed as metallic bonding of Si atoms by using X-ray photoelectron spectroscopy. An increase in reflector voltage induced an increase of surface morphology of films and optical bandgap and a decrease of photoconductivity.     

Epitaxial growth of RuO2 thin films by metal-organic chemical vapor deposition

Conductive RuO₂ thin films were epitaxially grown on LaAlO₃(100) and MgO(100) substrates by metal-organic chemical vapor deposition (MOCVD). The deposited RuO₂ films were crack-free, and well adhered to the substrates. The RuO₂ film is (200) oriented on LaAlO₃ (100) substrates at deposition temperature of 600°C and (110) oriented on MgO(100) substrates at deposition temperature of 350°C and above. The epitaxial growth of RuO₂ on MgO and LaAlO₃ is demonstrated by strong in-plane orientation of thin films with respect to the major axes of the substrates. The RuO₂ films on MgO(100) contain two variants and form an orientation relationship with MgO given by RuO₂(110)//MgO(100) and RuO₂[001]//MgO[011]. The RuO₂ films on LaAlO₃(100), on the other hand, contain four variants and form an orientation relationship with LaAlO₃ given by RuO₂(200)//LaAlO₃(100) and RuO₂[011]//LaAlO₃[011]. Electrical measurements on the RuO₂ thin films deposited at 600°C show room-temperature resistivities of 40 and 50 μΩ cm for the films deposited on the MgO and LaAlO₃ substrates, respectively.