Polycrystalline silicon film growth in a SiF4/SiH4/H2 plasma

The growth of polycrystalline silicon (polysilicon) films from SiF₄/SiH₄/H₂ gas mixtures is reported. The polysilicon films have been deposited in a multi process reactor by a PECVD process. The effect of r.f. power, chamber temperature and gas flow ratios on grain size and deposition rate have been determined. The fluorine concentration and the grain sizes of the films have been determined by SIMS and atomic force microscopy (AFM), respectively. Grain sizes in excess of 900 Å are reported for layers deposited at 300°C.     

Deposition of silicon nitride from SiCl4 and NH3 in a low pressure r.f. plasma

Silicon nitride coatings were deposited in a low pressure (1–10 Torr) r.f. plasma from SiCl₄ and NH₃ in the presence of argon onto stainless martensitic steel grounded and floating substrates at 300 °C and 440 °C respectively. The heating of the substrates depends mainly on the position and the induced r.f. power. The coatings were identified as silicon nitride by X-ray investigation and were found to contain chlorine by energy-dispersive analysis of X-rays. The growth rate, the microhardness and the chlorine concentration of the coatings were determined as a function of the total gas pressure, the r.f. power input and the NH₃-to-SiCl₄ ratio. It was observed that the coatings on the floating substrates have higher deposition rates and are of superior quality.     

Characterization of pyrolytic carbon deposited on graphite substrates in inductive R.F. plasmas of propylene and argon

Coatings of pyrolytic carbon (PyC) on graphite substrates were obtained in an inductive r.f. plasma in mixtures of propylene and argon at pressures up to 10 Torr. The temperature of the graphite substrate in the inductive r.f. system was kept at 350 ± 50°C. The PyC coatings were analysed for anisotropy, for composition by small angle X-ray scattering and for density. The results obtained (varying the gas pressure and the propylene concentration in the r.f. plasma system) were partly compared with PyC coatings obtained in a chemical vapour deposition fluidized bed system.     

On the properties of AlN thin films grown by low temperature reactive r.f. sputtering

In this paper we report on the growth of c-axis-oriented AlN thin films by low temperature reactive r.f. sputtering and the results of their examination and analysis in a variety of ways. In addition to using other substrates, we fabricated c-axis- oriented AlN films on aluminium film substrates. A hexagonal column structure was observed in the morphology of replicas of the natural surfaces of films. As well as the dielectric constant ε, the resistivity ϱ, the breakdown field E_p and the refractive index n, curves of ε versus frequency f, ε versus temperature T and conductivity σ versus T were measured. The IR absorption spectrum of an AlN film sputtered at low temperature coincides with that of an AlN film sputtered onto a high temperature substrate and of a bulk crystal, which remains unchanged after annealing. The band gap E_g = 5.9–6.0 eV, which remains unchanged after annealing in N₂ at 900°C. Theoretical calculation of the dispersion curve of surface acoustic waves by an AlN/glass structure shows that the curve is very level in the range hk = 0.2–0.6. The capacitance-voltage curve of an Al/AlN/Si structure is given in this paper.     

Study of Ar + O2 deposition pressures on properties of pulsed laser deposited CdTe thin films at high substrate temperature

Cadmium telluride (CdTe) thin films deposited by pulsed laser deposition (PLD) on fluorine–tin–oxide substrates under different pressures of argon (Ar) + oxygen (O₂) at high substrate temperature (T_s = 500 °C) was reported in this paper. In our work, the CdTe thin films were prepared successfully at high T_s by inputting Ar + O₂. As reported, PLD-CdTe thin films were almost prepared at low substrate temperatures (<300 °C) under vacuum conditions. The deposition of CdTe thin films at high T_s by PLD is rarely reported. The influence of the Ar + O₂ gas pressure on thickness, structural performance, surface morphology, optical property and band gap (E_g) had been investigated respectively by Ambios probe level meter, X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–Vis spectrometer. Strong dependence of properties on the deposition pressures was revealed. In the range of Ar + O₂ gas pressure from 5 to 12 Torr, the deposition rate and the E_g of CdTe films vary in the range of 41.9–57.66 nm/min then to 35.26 nm/min and 1.51–1.54 eV then to 1.47 eV, respectively. The XRD diagrams showed that the as-deposited films were polycrystalline, and the main phase was cubic phase. However, the preferred orientation peak disappeared when the deposition pressure was higher. SEM images indicated that the CdTe film deposited at a higher deposition pressure was more uniform and had a higher compactness and a lower pinhole density. Furthermore, based on this thorough study, FTO/PLD-CdS (100 nm)/PLD-CdTe (~1.5 μm)/HgTe:Cu/Ag solar cells with an efficiency of 6.68 % and an area of 0.64 mm² were prepared successfully.     

Low temperature synthesis of spherical nano-diamond

Thin film nano-diamonds have been synthesised using CH₄/H₂ mixture by micro-wave plasma enhanced CVD (MW-PECVD) at a substrate temperature as low as 250°C. These are found to be spherically shaped, homogeneously distributed on the film surface (size ～100?nm) and are related to ball-like or ballas-like nano-diamonds. With the increment of temperature from 200°C to 250°C, these spherical structures are found to form in increasing number density, but the same could not be grown with the variation of power and pressure. However, the bulk material is found to improve in crystallinity with the increment of pressure and highly polycrystalline nano-diamonds (111), along with Graphitic [(100), (004)] inclusions, have been grown using a gas pressure of 70?Torr and 500?W of MW power. The preliminary results on FE-SEM, Raman and HR-TEM studies are discussed here. Further developments of the material and characterisation are in progress.     

Ultrafast thermal plasma physical vapor deposition of thick SiC films

Thick silicon carbide films have been successfully deposited at a deposition rate of 125 nm/s on stationary graphite substrates by the thermal plasma physical vapor deposition technique, with ultrafine SiC powder fed into a hybrid plasma jet and completely evaporated. The relationship between the processing parameters and the morphology, deposition rate, composition and crystal structure has been investigated under the typical conditions of substrate temperature in the range of 1400–1700 °C and chamber pressure of 250 Torr, and compared with the results of rotating substrate deposition at the substrate temperature of around 750 °C. It was found that the deposition rate and composition showed different processing parameter dependences for rotating substrate deposition and stationary substrate deposition. The films showed dense cross-sections or cauliflower-like structures depending on the deposition conditions.

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Structural investigation of silicon films chemically vapour deposited onto amorphous SiO2 substrates

The influence of substrate temperature and the silane-to-nitrogen ratio on the structure of silicon films 0.5–0.6 μm thick deposited onto amorphous SiO₂ substrates was investigated by X-ray diffraction. The investigations were carried out for silicon films deposited at various temperatures in the range 500–750 °C and with various silane-to-nitrogen ratios in the range 3.04 × 10^-4-2.84 × 10^-3 by volume. The silicon films deposited at 500 °C were amorphous while the films deposited at 550 °C were randomly oriented polycrystalline. The films deposited in the temperature range 600–700 °C were polycrystalline with a preferred orientation that changed from 〈110〉 through 〈100〉 to 〈111〉. The structure of the films deposited at 750 °C was randomly oriented polycrystalline. Investigations of the influence of the silane-to-nitrogen ratio on the silicon film structure revealed that the structure of films deposited at a substrate temperature of 500 °C was independent of the silane-to-nitrogen ratio. The structure of the films deposited at 600 °C depended on the silane-to-nitrogen ratio and changed from polycrystalline with a 〈110〉 preferred orientation to randomly oriented polycrystalline when the ratio was increased. The structure of films deposited at 700 °C also depended on the silane-to-nitrogen ratio and changed from randomly oriented polycrystalline to polycrystalline with double preferred orientation (〈100〉 and 〈111〉) when the ratio was increased.     

Preparation of nanocrystalline nickel oxide thin films by sol–gel process for hydrogen sensor applications

Preparation of nanocrystalline NiO thin films by sol–gel method and their hydrogen (H₂) sensing properties were investigated. The thin films of NiO were successfully deposited on the glass and SiO₂/Si substrate by a sol–gel coating method. The films were characterized for crystallinity, electrical properties, surface topography and optical properties as a function of calcination temperature and substrate material. It was found that the films produced by this method were polycrystalline and phase pure NiO. The H₂ gas sensitivity of these films was studied as a function of H₂ concentration and calcination temperature. The results indicated that the sol–gel derived NiO films could be used for the fabrication of H₂ gas sensors to monitor low concentration of H₂ in air quantitatively at low temperature range (< 200 °C).     

Structural,electrical and optical properties of CuxS layers produced by reactive r.f. sputtering

Thin films of Cu_xS (0.3–0.5μm thick) were produced by r.f. sputtering using hydrogen and H₂S as additives to the sputter gas argon. The electrical resistivity and the optical properties of the films show marked changes when the concentration of the reactive part of the sputter gas exceeds 1%. These changes correspond to a variation in the chemical composition of the Cu_xS layers. The films are polycrystalline and consist of a mixture crystallographic phases that are not in thermodynamic equilibrium in all of the sputtered layers.     

Effect of hydrogen on the structure of high-rate deposited SiC on Si by atmospheric pressure plasma chemical vapor deposition using high-power-density condition

Using the atmospheric pressure plasma chemical vapor deposition (AP-PCVD) technique, SiC films were fabricated from the gas mixture of He, H₂, SiH₄ and CH₄ on silicon substrates. High-power-density condition was adopted to sufficiently activate the reactive gas molecules in the plasma. The structure, composition and crystallinity of the films were investigated as functions of the H₂ concentration in the gas mixture and substrate temperature. It was shown that increase in H₂ concentration in the plasma atmosphere reduced the growth temperature of polycrystalline SiC film. As a result, polycrystalline 3C-SiC film of which grain size was of the order of 10 nm could be grown at a substrate temperature of 820 K with a deposition rate of approximately 6.7 nm/s. It was suggested that atomic hydrogen generated with addition of H₂ in the gas mixture considerably affects not only the reaction process at the film-growing surface but also the form of precursors in the atmospheric pressure plasma. The results indicated the possibility of realizing the columnar growth of large 3C-SiC grains on Si substrate when the H₂ concentration and the VHF power were simultaneously increased in the AP-PCVD process.     

Crystalline diamond/graphite nanoflake hybrid films

Freestanding crystalline diamond/graphite nanoflake hybrid films have been deposited in H₂/CH₄ gas mixtures using a high pressure (1.3 × 10⁴ Pa) direct current plasma discharge. Sacrificial layers of close-packed silica microspheres were used as a matrix to produce dual gas chemistries on the plasma-facing and reverse sides of the microspheres. A continuous polycrystalline diamond film was formed on the front surface whilst graphite was deposited in the form of nanoflakes as a thinner hemispherical layer on the reverse side of the silica spheres respectively. Chemical etching of the silica matrix yielded crystalline diamond/well-aligned graphite nanoflakes hybrid films.     

Influence of ion-beam sputtering deposition parameters on highly photosensitive and transparent CdZnO thin films

CdZnO thin films with a nominal thickness of ~200 nm were grown on c-plane sapphire substrates by dual ion-beam sputtering deposition technique. The effect of substrate temperature (300–600 °C) and gas ambience on structural, morphological, compositional and opto-electronic properties was studied. X-ray diffraction patterns confirmed that all the films were polycrystalline in nature and were preferentially oriented along the c-axis. It was revealed that the films grown at Ar/O₂ ratio of 4:1 were structurally more ordered and the film quality was found to be the best at 500 °C. The compositional studies specify that approximately 11.8 at.% of cadmium were present in the film deposited at 300 °C in Ar–O₂ mixture. Investigations on optical properties by photoluminescence and absorption studies indicate band gap shrinkage with the increase in argon partial pressure and substrate temperature. It was found that photosensitivity of the deposited films was highly dependent on growth conditions. The photosensitivity was found to be 5000-fold higher for CdZnO film grown at 600 °C in Ar–O₂ ambience compared to the best reported result, and this was promising to realize high-performance opto-electronic devices on such CdZnO films.     

Ultrafast thermal plasma physical vapor deposition of thick SiC films

Thick silicon carbide films have been successfully deposited at a deposition rate of 125 nm/s on stationary graphite substrates by the thermal plasma physical vapor deposition technique, with ultrafine SiC powder fed into a hybrid plasma jet and completely evaporated. The relationship between the processing parameters and the morphology, deposition rate, composition and crystal structure has been investigated under the typical conditions of substrate temperature in the range of 1400–1700 °C and chamber pressure of 250 Torr, and compared with the results of rotating substrate deposition at the substrate temperature of around 750 °C. It was found that the deposition rate and composition showed different processing parameter dependences for rotating substrate deposition and stationary substrate deposition. The films showed dense cross-sections or cauliflower-like structures depending on the deposition conditions.     

Stress generation and relaxation in (Al,Ga)N/6<Emphasis Type="Italic">H</Emphasis>-SiC heterostructure grown by plasma-assisted molecular-beam epitaxy

In situ stress generation and relaxation in Al_0.25Ga_0.75N/GaN/AlN heterostructure with an overall thickness exceeding 3 μm in the process of its growth on a 6H-SiC substrate by low-temperature plasma-assisted molecular-beam epitaxy at substrate temperatures ranging from 690 to 740°C was studied. At room temperature, AlN and GaN layers revealed residual compressive stresses of–2.3 and–0.1 GPa, respectively. This made it possible to avoid cracking during postgrowth cooling of the structure.     

Electron microscopy studies of sprayed thin tin dioxide films

Electron microscopy studies of thin tin dioxide films (about 80 nm thick) prepared by spraying from a mixture of SnCl₄ and ethyl acetate indicate that such films are amorphous at substrate temperatures of 340–350°C, partially crystallized at 360–410°C and polycrystalline at 420–500°C. Grains about 40 nm in size were found for all polycrystalline structures, independent of the substrate temperature. Only SnO₂, having values of the interplanar spacing d in agreement with the ASTM data, was clearly identified by electron diffraction. Scanning electron microscopy studies indicate smooth surfaces with pits of varying size and density depending on the preparation conditions.     

Joining of AlN and graphite disks using interlayer tapes by spark plasma sintering

AlN and graphite disks were successfully joined using a polymer plasticized ceramic tape as the interlayer by spark plasma sintering (SPS). The tape contains either composite powders of AlN and graphite or AlN powders without graphite. Both tapes contained 5 mass% Y₂O₃ as the sintering aid of AlN. The joining was carried out at 1700–1900 °C and 30 MPa for 5 min. No other reaction phase except for Al₂Y₄O₉ was identified in the joints. The maximum tensile strength of the joints was obtained when the AlN–graphite composite interlayer tape was used. The joining mechanism is attributed not to the chemical bonding, but to the physical bonding of the Al₂Y₄O₉ phase, which is solidified from the molten Al–Y–O squeezing into the porous graphite under pressure during SPS.     

The microstructure and orientation of silver on NaCl: The influence of surface conditions

Silver films up to 1000 Å thick (coalescence stage and continuous films) were evaporated onto vacuum-cleaved NaCl at constant temperature and constant deposition rate but under various vacuum conditions (ultrahigh vacuum (UHV), high vacuum (oil pumped and ion pumped), a partial pressure of contaminant (10^-10–10^-6 Torr) etc.). The films were characterized by low energy electron diffraction, Auger electron spectroscopy, transmission electron microscopy and transmission electron diffraction to determine their orientation, microstructure and cleanliness. The UHV-grown films were always polycrystalline. In contrast with previous observations, however, an increase in the partial pressure of H₂O alone did not improve the epitaxy. Hence the influence of other materials such as O₂, H₂O + O₂, hydrocarbons etc. was investigated. In addition, films were grown in UHV after pre-irradiation of the substrate with electrons and prenucleation at room temperature. If the prenucleated silver clusters were small and if their orientation was improved by annealing, continuous films were perfectly epitaxial even when the deposition of silver was carried out at room temperature after prenucleation. This indicates a post-nucleation mechanism of epitaxy.     

An economic CVD technique for pure SnO 2 thin films deposition: Temperature effects

A modified new method of CVD for formation of pure layers of tin oxide films was developed. This method is very simple and inexpensive and produces films with good electrical properties. The effect of substrate temperature on the sheet resistance, resistivity, mobility, carrier concentration and transparency of the films has been studied. The best sheet resistance obtained at substrate temperature of 500 °C was about 27 Ω/cm². X-ray diffraction showed that the structure of deposited films was polycrystalline with a grain size between 150–300 Å. The preferred orientation was (211) for films deposited at substrate temperature of about 500 °C. FESEM micrographs revealed that substrate temperature is an important factor for increasing grain size and modifies electrical parameters. UV-visible measurement showed reduction of transparency and bandgap of the layers with increasing substrate temperature.     

R.f.-sputtered SiC coatings on carbon fibres

R.f. sputtering as a new method for the deposition of SiC layers onto carbon fibre substrates was applied at temperatures below 400°C and deposition rates higher than 1 μm h^-1. Optimum conditions for high quality SiC films were selected by variation in the r.f. power, r.f. peak voltage, substrate temperature and gas pressure. The SiC layers were characterized using electron probe microanalysis, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, transmission high energy electron diffraction, IR and UV transmission spectroscopy as well as Vickers' hardness measurements. Amorphous SiC layers were obtained. In the case of discontinuous SiC deposition onto carbon monofilaments the film thickness precalculated from the sputtering parameters was achieved, whereas the continuous process results in film thickness of about one-third of the precalculated value. When fibre bundles with different numbers of monofilaments were used no influence on the resulting SiC layer thickness could be observed. Increasing the SiC film thickness, however, led to a strong decrease in the fibre tensile strength for layers more than 50 nm thick.