Temperature dependence of the surface diffusion distance of bismuth atoms adsorbed on mica,carbon and silicon monoxide surfaces

The mean distance of surface diffusion of bismuth adatoms on mica, carbon and silicon monoxide surfaces has been determined at different temperatures by measurement of the instantaneous sticking coefficient and the nucleus density.The surface diffusion distance has been found to increase with decreasing temperature in accordance with the formula

$\text{X=}\text{1}\text{2}\text{a}{}_0\text{v}{}_{\mathrm{0d}}\text{v}{}_{\mathrm{0a}}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{exp}\text{E}{}_{\mathrm{a}}\text{? E}{}_{\mathrm{d}}\text{2RT}$

at temperatures above 413, 373 and 383 K for mica, carbon and silicon monoxide respectively. Here X is one-half of the diffusion distance, E_a is the adsorption energy, E_d the activation energy for surface diffusion, a₀ the diffusion jump distance and v_0a and v_0d the vibrational frequencies associated with re-evaporation and with surface diffusion respectively. Below these temperatures it has been found that the temperature dependence of the diffusion distance deviates from the above formula; this can be explained by the presence of residual gas molecules adsorbed on the surfaces.From the temperature dependence of the diffusion distance, the respective values of the pre-exponential term $\text{a}{}_0\text{(}\text{v}{}_{\mathrm{0d}}\text{v}{}_{\mathrm{0a}}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and the difference of energies E_a?E_d have been estimated as 7.6 Å and 5.8 kcal mol^?1 for mica, 17 Å and 3.2 kcal mol^?1 for carbon and 58 Å and 1.3 kcal mol^?1 for silicon monoxide.     

Growth of long aligned carbon nanotubes on amorphous hydrogenated silicon nitride by thermal chemical vapor deposition

Vertically aligned long carbon nanotubes in the range of 80-100 µm have been synthesized on amorphous hydrogenated silicon nitride (a-SiN_x:H) coated silicon substrate by thermal chemical vapor deposition of ferrocene and xylene. It is observed that high temperature annealing in oxygen ambient results in formation of crystalline silicon dioxide in the matrix of amorphous silicon nitride due to out diffusion of hydrogen. It is suggested that active sites created on silicon dioxide and a-SiN_x:H clusters provide mechanical support for the alignment of long carbon nanotubes. It is proposed that a thin layer of a-SiN_x:H prevents silicide formation between the catalyst (Fe) and silicon thus lengthening the catalyst life.     

Thermochemistry of silicon carbide growth by chemical transport reactions

Chemical Vapour Transport is a well known process widely used for the growth of monocrystals. This paper is a thermodynamic overview of different heterogeneous chemical systems, promising for the growth of silicon carbide by means of chemical transport reactions. The systems are Si-C-Y where Y is oxygen or a chalcogen (S, Se) and Si-C-H-X where X is an halogen (Cl, Br, I). We studied in a first step the gas phase composition obtained from SiC etching with the transporting agent as a function of temperature. In a second step, we report the conditions for the formation of silicon carbide from such a vapour at a different temperature. Finally we discuss optimal conditions of temperatures and thermal gradients required for SiC transport with each systems.     

Growth of oriented aluminium nitride films on silicon by chemical vapour deposition

Aluminium nitride films were grown on silicon substrates using the chemical vapour deposition (CVD) method. The properties of the films were studied by scanning electron microscopy (SEM), atomic force microscope (AFM) measurements, X-ray diffraction and Raman scattering. The resulting films were strongly textured and had a preferential orientation with the c-axis normal to the surface, the Raman spectra showed two peaks at 607 and 653 cm^–1 and two large bands at 750 and 900 cm^–1 of smaller intensity. Both the macro- and micro-Raman spectra showed the same peaks.     

Analysis on the interfacial properties of transparent conducting oxide and hydrogenated p-type amorphous silicon carbide layers in p-i-n amorphous silicon thin film solar cell structure

Quantitative estimation of the specific contact resistivity and energy barrier at the interface between transparent conducting oxide (TCO) and hydrogenated p-type amorphous silicon carbide (a-Si_1 − xC_x:H(p)) was carried out by inserting an interfacial buffer layer of hydrogenated p-type microcrystalline silicon (μc-Si:H(p)) or hydrogenated p-type amorphous silicon (a-Si:H(p)). In addition, superstrate configuration p-i-n hydrogenated amorphous silicon (a-Si:H) solar cells were fabricated by plasma enhanced chemical vapor deposition to investigate the effect of the inserted buffer layer on the solar cell device. Ultraviolet photoelectron spectroscopy was employed to measure the work functions of the TCO and a-Si_1 − xC_x:H(p) layers and to allow direct calculations of the energy barriers at the interfaces. Especially interface structures were compared with/without a buffer which is either highly doped μc-Si:H(p) layer or low doped a-Si:H(p) layer, to improve the contact properties of aluminum-doped zinc oxide and a-Si_1 − xC_x:H(p). Out of the two buffers, the superior contact properties of μc-Si:H(p) buffer could be expected due to its higher conductivity and slightly lower specific contact resistivity. However, the overall solar cell conversion efficiencies were almost the same for both of the buffered structures and the resultant similar efficiencies were attributed to the difference between the fill factors of the solar cells. The effects of the energy barrier heights of the two buffered structures and their influence on solar cell device performances were intensively investigated and discussed with comparisons.     

Growth of PbS and CdS thin films by low-pressure chemical vapour deposition using dithiocarbamates

Thin films of cadmium and lead sulphides grown by chemical vapour deposition (CVD) and remote plasma enhanced chemical vapour deposition (RPECVD) using dithiocarbamates as precursors were prepared on fused silica, sapphire, (111)Si and (111)InP substrates. These films were deposited in the temperature range 473–873 K. It was established that the activation energy of the CVD process is 191.5±1.5 kJ mol⁻¹. The structure of polycrystalline films was halenide for PbS and wurtzite for CdS. It was also found that r.f.-plasma activation of the gas phase decreases remarkably the growth temperature and orders the film structure. RPECVD sulphide films had a high degree of preferred orientation.     

Influences of interfacial resistances on gas transport through carbon nanotube membranes

Carbon nanotubes have significant promise as gas separation membranes. Gas permeation through nanopores involves mass transfer resistances from molecules entering and leaving pores (so-called surface resistances) and diffusion within the pores. We use molecular simulations to give the first estimates of surface resistances for gas transport through nanotubes. For CH4 transport through (20,0) carbon nanotubes at 300 K, surface resistances are small for nanotubes 5-10 mum in length but can be significant for shorter nanotubes.     

Some electrical properties of mixed amorphous thin films of germanium and silicon monoxide before electroforming

The co-evaporated SiO _x -Ge system was studied. Thin-film MIM sandwich structures were deposited by vacuum evaporation at a pressure of 10^–4 Pa and were measured at a pressure of 10^–3 Pa. The conductivity at low temperature and under d.c. fields has been found to be governed by a combination of an electronic hopping process and free-band conduction. At fields greater than 2 × 10⁶ V m^–1, it is concluded that the conduction process is governed by the Poole-Frenkel effect. Comparison with earlier results on SiO _x -GeO₂ films showed small differences in activation energy for conduction for samples of broadly similar overall composition.     

A study of the optical properties of amorphous thin films of germanium and silicon monoxide

Amorphous films of Ge-SiO have been co-evaporated and some of their optical properties are reported. The optical constants have been measured and estimated. At the high absorption end of the absorption edge, an equation due to non-direct transitions inK-space is found to match the optical absorption data. The variation of the optical band gapE _opt with film composition is reported. The infrared spectrum of a mixed layer is presented and a simple conclusion is drawn.     

Aluminum-induced crystallization of amorphous silicon films deposited by hot wire chemical vapor deposition on glass substrates

Aluminum-induced crystallization of amorphous silicon films is discussed. Amorphous Si films were deposited by hot wire chemical vapor deposition onto Al coated glass substrates at 430 °C. Complete crystallization of a-Si films was achieved during a-Si deposition by controlling Al and Si layer thicknesses. The grain structure of the poly-Si films formed on glass substrate was evaluated by optical and electron microscopy. Continuous poly-Si films were obtained using Al layers with a thickness of 500 nm or less. The average grain size was found to be 10-15 μm, corresponding to a grain size/thickness ratio greater than 20.     

Adherent amorphous hydrogenated carbon films on metals deposited by plasma enhanced chemical vapor deposition

This paper reports the findings of a study of the structural, mechanical, and tribological properties of amorphous hydrogenated carbon (a-C:H) coatings for industrial applications. These thin films have proven quite advantageous in many tribological applications, but for others, thicker films are required. In this study, in order to overcome the high residual stress and low adherence of a-C:H films on metal substrates, a thin amorphous silicon interlayer was deposited as an interface. Amorphous silicon and a-C:H films were grown by using a radio frequency plasma enhanced chemical vapor deposition system at 13.56 MHz in silane and methane atmospheres, respectively. The X-ray photoelectron spectroscopy technique was employed to analyze the chemical bonding within the interfaces. The chemical composition and atomic density of the a-C:H films were determined by ion beam analysis. The film microstructure was studied by means of Raman scattering spectroscopy. The total stress was determined through the measurement of the substrate curvature, using a profilometer, while micro-indentation experiments helped determine the films' hardness. The friction coefficient and critical load were evaluated by using a tribometer. The results showed that the use of the amorphous silicon interlayer improved the a-C:H film deposition onto metal substrates, producing good adhesion, low compressive stress, and a high degree of hardness. SiC was observed in the interface between the amorphous silicon and a-C:H films. The composition, the microstructure, the mechanical and tribological properties of the films were strongly dependent on the self-bias voltages. The tests confirmed the importance of the intensity of ion bombardment during film growth on the mechanical and tribological properties of the films.     

N-type crystalline silicon films free of amorphous silicon deposited on glass by HCl addition using hot wire chemical vapour deposition

Since n-type crystalline silicon films have the electric property much better than those of hydrogenated amorphous and microcrystalline silicon films, they can enhance the performance of advanced electronic devices such as solar cells and thin film transistors (TFTs). Since the formation of amorphous silicon is unavoidable in the low temperature deposition of microcrystalline silicon on a glass substrate at temperatures less than 550 degrees C in the plasma-enhanced chemical vapour deposition and hot wire chemical vapour deposition (HWCVD), crystalline silicon films have not been deposited directly on a glass substrate but fabricated by the post treatment of amorphous silicon films. In this work, by adding the HCl gas, amorphous silicon-free n-type crystalline silicon films could be deposited directly on a glass substrate by HWCVD. The resistivity of the n-type crystalline silicon film for the flow rate ratio of [HCl]/[SiH4] = 7.5 and [PH3]/[SiH4] = 0.042 was 5.31 x 10(-4) ohms cm, which is comparable to the resistivity 1.23 x 10(-3) ohms cm of films prepared by thermal annealing of amorphous silicon films. The absence of amorphous silicon in the film could be confirmed by high resolution transmission electron microscopy.     

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.     

Cold deposition of large-area amorphous hydrogenated silicon films by dielectric barrier discharge chemical vapor deposition

Amorphous hydrogenated silicon films were deposited on glass substrates at room temperature. This cold deposition process was operated in a dielectric barrier discharge CVD reactor with a fixed strip-shaped plasma matched with a moving substrate holder. The maximum film area was 300 × 600 mm². The film deposition rate as a function of applied peak voltage of DBD power was investigated under different hydrogen-diluted silane concentrations, and the film surface smoothness, continuity, and film/glass adherence were also studied. The maximum deposition rate was 12.2 Å/s, which was performed under the applied peak voltage of 16 kV and a hydrogen-diluted silane concentration of 50%. IR measurements reveal that the silane concentration plays a key role in determining the hydrogen-silicon bonding configurations. With increasing hydrogen-diluted silane concentration, the H-Si bonding configurations shift gradually from Si-H₃ to Si-H. The variation of photo/dark conductivity ratio and optical bandgap versus hydrogen-diluted silane concentration were investigated. The use of DBD-CVD for deposition of a-Si:H films offers certain advantages, such as colder substrate, faster film growth rate, and larger deposition area. However, the consumption of silane for the DBD-PECVD procedure is much greater than for the RF-PECVD process.     

Hydrogenated amorphous carbon and carbon nitride films deposited at low pressure by plasma enhanced chemical vapor deposition

Hydrogenated amorphous carbon (a-C:H) films were deposited by plasma enhanced chemical vapor deposition from methane, argon diluted methane, and nitrogen diluted methane at 26.7 Pa with a 13.56 MHz RF power supply. In this pressure regime, multiple-scattering of carbon species within the plasma phase is expected during the transport to the substrates placed on both the driven and the earthed electrodes. These films were analyzed using UV-VIS optical transmittance, monochromatic ellipsometry, Raman spectroscopy and current-voltage measurements. From these results, the effect of the plasma conditions and the effective flux of the carbon species controlled by the input power through the negative self bias are found to be important in the deposition process. The growth conditions at the higher pressure regime are important to synthesize a-C:H films from low energetic carbon species, since it reduces the defect density and improves the quality of the films. Furthermore, the effect of nitrogen on the growth conditions of a-C:H:N films is observed.     

Thermal analysis of hydrogenated amorphous carbon films prepared by plasma enhanced chemical vapour deposition

Hydrogenated amorphous carbon (a-C∶H) films were produced from propane and argon by an inductively coupled radio frequency (r.f.) glow discharge process under a particular deposition condition. Thermal analysis for the deposit by GC, DSC, DTA, and TG gave information for the structural changes upon heating. Most C-H vibration spectra disappeared by heating up to 600 °C. The gas desorption began above 300 °C and reached maxima above 650 °C with several peaks. The desorption reaction was endothermic. Up to 600 °C the desorbed gases was not the hydrogen. The large weight change was observed without the thickness reduction. The weight change rate was maximum at 480 °C. Hydrocarbons are believed to desorb below 600 °C. A hydrocarbon desorption model is suggested. Hydrocarbons are formed in the inner surface of a microvoid and effuse out through the interconnected microvoids in the column boundaries. The proposed desorption reaction is also endothermic.     

Surface passivation of crystalline silicon by Cat-CVD amorphous and nanocrystalline thin silicon films

In this work, we study the electronic surface passivation of crystalline silicon with intrinsic thin silicon films deposited by Catalytic CVD. The contactless method used to determine the effective surface recombination velocity was the quasi-steady-state photoconductance technique. Hydrogenated amorphous and nanocrystalline silicon films were evaluated as passivating layers on n- and p-type float zone silicon wafers. The best results were obtained with amorphous silicon films, which allowed effective surface recombination velocities as low as 60 and 130 cm s⁻¹ on p- and n-type silicon, respectively. To our knowledge, these are the best results ever reported with intrinsic amorphous silicon films deposited by Catalytic CVD. The passivating properties of nanocrystalline silicon films strongly depended on the deposition conditions, especially on the filament temperature. Samples grown at lower filament temperatures (1600 °C) allowed effective surface recombination velocities of 450 and 600 cm s⁻¹ on n- and p-type silicon.