Structural and optical properties of four-hexagonal polytype nanocrystalline silicon carbide films deposited by plasma enhanced chemical vapor deposition technique

Four-hexagonal polytype films of nanocrystalline silicon carbide (4H-nc-SiC) were deposited by plasma enhanced chemical vapor deposition method with more than 3×10⁴ W m⁻² threshold of power density, high hydrogen dilution ratio, and bias pretreatment. The source gases were silane, methane and hydrogen. Our work showed that under conditions similar to those used for the growth of μc-SiC—except a higher power densities over a threshold, a bigger bias pretreatment on substrates, and a moderate bias deposition—nc-SiC films could indeed be achieved. The Raman spectra and transmission electron microscopy diffraction patterns demonstrated that the as-grown films from the H₂-CH₄-SiH₄ plasma consist of amorphous network and phase-pure crystalline silicon carbide which has the 4H polytype structure. The microcolumnar 4H-SiC nanocrystallites of a mean size of approximately 1.6×10⁻⁸ m in diameter are encapsulated by amorphous SiC networks. The photoluminescence spectra of 4H-SiC at room temperature, peaking at 8.10×10⁻⁷ m using a wavelength of 5.145×10⁻⁷ m of argon ion laser, were obtained at room temperature; the luminescence mechanism is thought to be related to transitions in the energy band gap which could be ascribed to the surface states and defects in the structure of 4H-SiC nanocrystalline in these films due to its small size. The as-grown films showed an optical transmittance of 89% at 6.58×10⁻⁷ m. This higher transmittance is believed to be from the small size and amorphous matrix.     

Applications of microcrystalline hydrogenated cubic silicon carbide for amorphous silicon thin film solar cells

We demonstrated the fabrication of n-i-p type amorphous silicon (a-Si:H) thin film solar cells using phosphorus doped microcrystalline cubic silicon carbide (μc-3C-SiC:H) films as a window layer. The Hot-wire CVD method and a covering technique of titanium dioxide TiO₂ on TCO was utilized for the cell fabrication. The cell configuration is TCO/TiO₂/n-type μc-3C-SiC:H/intrinsic a-Si:H/p-type μc- SiC_x (a-SiC_x:H including μc-Si:H phase)/Al. Approximately 4.5% efficiency with a V_oc of 0.953 V was obtained for AM-1.5 light irradiation. We also prepared a cell with the undoped a-Si_1−xC_x:H film as a buffer layer to improve the n/i interface. A maximum V_oc of 0.966 V was obtained.     

Effects of high hydrogen dilution ratio on surface topography and mechanical properties of hydrogenated nanocrystalline silicon thin films

Hydrogenated nanocrystalline silicon thin films were deposited with high hydrogen dilution ratio by plasma enhanced chemical vapor deposition technique. The effects of high hydrogen dilution on the surface topography and mechanical properties of the films were studied with atomic force microscopy and TriboIndenter nano indenter. The results indicate that the average grain size in films deposited with high hydrogen dilution is about 3.18 ± 0.02 nm. The surface roughness and densification of the films decrease with the increase of hydrogen dilution ratio at certain range, resulting in the enhancement of the elastic modulus E and hardness H. Oppositely, the increase of hydrogen dilution can increase the surface roughness induced by the increase of the cavities on the film surfaces, and lead to the decrease of the elastic modulus and hardness correspondingly. In this paper, the detailed analysis and discussion were carried out to investigate the mechanism of the observed phenomena.     

Piezoresistive properties of nanocrystalline silicon thin films deposited on plastic substrates by hot-wire chemical vapor deposition

The piezoresistive property of n-type and p-type nanocrystalline silicon thin films deposited on plastic (PEN) at a substrate temperature of 150 °C by hot-wire chemical vapor deposition, is studied. The crystalline fraction decreased from 80% to 65% in p-type and from 84% to 62% in n-type films, as the dopant gas-to-silane flow rate ratio was increased from 0.18% to 3-3.5%. N-type films have negative gauge factor (− 11 to − 16) and p-type films have positive gauge factor (9 to 25). In n-type films the higher gauge factors (in absolute value) were obtained by increasing the doping level whereas in p-type films higher gauge factors were obtained by increasing the crystalline fraction.     

Structural changes of hot-wire CVD silicon carbide thin films induced by gas flow rates

Silicon carbide (SiC) thin films were prepared by hot-wire chemical vapor deposition in a CH₄ gas flow rate of 1 sccm, and the influence of the gas flow rates of SiH₄ and H₂ gases on the film structure and properties were investigated. In the case of a H₂ gas flow rate below 100 sccm, the SiC:H films obtained in SiH₄ gas flow rates of 3 and 4 sccm were amorphous. On the other hand, when the H₂ gas flow rate was above 150 sccm, SiH₄ gas flow rates of 4 and 3 sccm resulted in a Si-crystallite-embedded amorphous SiC:H film and a nanocrystalline cubic SiC film, respectively. It was found that gas flow rates were important parameters for controlling film structure.     

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.     

Microstructure of highly crystalline silicon carbide thin films grown by HWCVD technique

Highly crystalline silicon carbide films were synthesised by HWCVD technique. Raman spectroscopic studies show that the SiC films contain crystalline SiC and also carbon phases. Carbon is graphitic at higher chamber pressures (≥ 50 Pa) and resembles diamond-like carbon at low pressure (5 Pa). Cross-section TEM results show a columnar morphology of the crystallites with typical column diameters up to ∼ 50 nm. Transmission electron diffraction patterns reveal SiC in its cubic and hexagonal SiC phases and the C diamond phase at low pressure. Annealing at 1000 °C for 1 h results in enhancement of crystallite size without nucleation of new phases.     

Surface nano-structured silicon carbide thin film produced using hot filament decomposition of ethylene at low temperature on silicon wafer

The structure and spectroscopic properties of nano-structured silicon carbide (SiC) thin films were studied for films obtained through deposition of decomposed ethylene (C₂H₄) on silicon wafers via hot filament chemical vapor deposition method at low temperature followed by annealing at various temperatures in the range 300-700 °C. The prepared films were analyzed with focus on the early deposition stage and the initial growth layers. The analysis of the film's physics and structural characteristics was performed with Fourier transform infrared spectroscopy and Raman spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy, and X-ray diffraction. The conditions for forming thin layer of cubic SiC phase (3C-SiC) are found. X-ray diffraction and Raman spectroscopy confirmed the presence of 3C-SiC phase in the sample. The formation conditions and structure of intermediate SiC layer, which reduces the crystal lattice mismatch between Si and diamond, are essential for the alignment of diamond growth. This finding provides an easy way of forming SiC intermediate layer using the Si from the substrate.     

Tungsten filament effect on electronic properties of hot-wire CVD silicon films for heterojunction solar cell application

In this study, we describe the correlation between cell efficiency and wire aging during hot-wire chemical vapor deposition in detail. The new and aged tungsten (W) filaments were used to deposit the n-type microcrystalline silicon (μc-Si) films for heterojunction (HJ) Si solar cell applications. Tungsten silicide (WSi_x) was coated on the W catalyzer surface (center and end regions) after each deposition, and which was investigated and determined by scanning electron microscopy and electron probe microanalysis. The wire age has an effect on the resulting electronic properties of the grown film, thought to be related to differences in dark conductivity with aged versus new wires. It was found that the aging process is related to the formation of a silicide at the surface. A limited amount of silicon was observed in the bulk of catalyzer, suggesting that silicon diffusion into the wire has occurred. The original single-side HJ solar cell with efficiency of 15.3% has been fabricated using the new wires. The quality of n-type μc-Si films and efficiency of HJ solar cells were reduced when the aged W filament was employed. The quality of silicon films and the efficiency of HJ solar cell could be improved after regeneration process.     

Microcrystalline silicon carbide thin films grown by HWCVD at different filament temperatures and their application in n-i-p microcrystalline silicon solar cells

To optimize the performance of microcrystalline silicon carbide (µc-SiC:H) window layers in n-i-p type microcrystalline silicon (µc-Si:H) solar cells, the influence of the rhenium filament temperature in the hot wire chemical vapor deposition process on the properties of µc-SiC:H films and corresponding solar cells were studied. The filament temperature T_F has a strong effect on the structure and optical properties of µc-SiC:H films. Using these µc-SiC:H films prepared in the range of T_F = 1800-2000 °C as window layers in n-side illuminated µc-Si:H solar cells, cell efficiencies of above 8.0% were achieved with 1 µm thick µc-Si:H absorber layer and Ag back reflector.     

Fabrication of nanocrystalline silicon carbide thin film by helicon wave plasma enhanced chemical vapour deposition

Nanocrystalline cubic silicon carbide thin films have been fabricated by helicon wave plasma enhanced chemical vapour deposition on Si substrates using the mixture of SiH₄, CH₄, and H₂ at a low substrate temperature of 300 °C. The infrared absorption spectroscopy analyses and microstructural characteristics of the samples deposited at various magnetic fields indicate that the high plasma intensity in helicon wave mode is a key factor to the success of growing nanocrystalline silicon carbide thin films at a relative low substrate temperature. Transmission electron microscopy measurements reveal that the films consist of silicon carbide nanoparticles with an average grain size of several nanometers, and the light emission measurements show a strong blue photoluminescence at room temperature, which is considered to be caused by the quantum confine effect of small size silicon carbide nanoparticles.     

Structural characterization of boron doped hydrogenated nanocrystalline silicon films

Structural properties of boron doped hydrogenated nanocrystalline silicon films deposited by plasma enhanced chemical vapor deposition method were mainly characterized with Raman and X-ray diffraction methods. The experimental Raman data were fitted better by Fano effect profiles than those by phonon confinement effect line shapes chiefly due to high efficiency doping in grown films. The measured Raman spectra were deconvoluted into three-Gaussian profile components: around the peak positions 520 and 480 cm⁻¹ which contribute from crystalline and amorphous tissues separately, as well as a curve centered at about 500 cm⁻¹, which is attributed to the presence of grain boundaries. The average crystalline grain size and crystalline volume fraction were valued with Raman and X-ray diffraction techniques, respectively, while the error derived from different methods was elucidated. Accordingly, the structural changes including crystallites, grain boundaries and amorphous matrices in doped films with boron doping level were analyzed.     

N2 post-deposition treatment on silicon thin films with a hot-wire chemical vapor method at a low wire temperature

For nitride layer formation on a hydrogenated microcrystalline silicon film surface, post-deposition treatments were carried out using a tungsten wire heated to 1700 °C in N₂ (12 Torr) or N₂/H₂ (4 Torr) atmospheres. The nitride layers were investigated with an X-ray photoelectron spectroscopy. The intense peaks due to the Si-N bonds were observed. Those in N₂ treatment were larger with increasing the treating time and decreased with depth direction, while those in N₂/H₂ treatment were virtually unchanged with the treating time and the depth up to about 20 nm. These findings indicate that even at a low wire temperature of 1700 °C, N₂ molecules decompose sufficiently and nitride layers can be formed when high gas pressures are used.     

Effect of filament and substrate temperatures on the structural and electrical properties of SiC thin films grown by the HWCVD technique

The effect of varying filament and substrate temperatures on the structure and electrical conductivity of crystalline SiC films prepared by HWCVD technique are described in this paper. At a constant filament temperature, the electrical conductivity of the SiC films increases with increasing substrate temperature. However, TEM studies show that there is no change in the size of the SiC columnar grains. On the other hand, a significant variation in filament temperature at constant substrate temperature leads to a variation of structure and conductivity. Raman spectroscopy and TEM studies reveal that crystallinity improves with increase in filament temperature. Furthermore, a μc-Si phase exists alongside SiC at low filament temperature (1750 °C).     

Chemical vapour deposition of zirconium carbide and silicon carbide hybrid whiskers

Zirconium carbide and silicon carbide hybrid whiskers were codeposited by chemical vapour deposition using methyl trichlorosilane, zirconium chloride, methane and hydrogen as the precursors. The zirconium carbide and silicon carbide whiskers were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. The results indicate that the codeposition process is more effective in the presence of methane than in the absence of methane. The codeposition process and the growth of zirconium carbide in the whiskers can be accelerated at high temperature in the presence of methane. A growth model was proposed based on the deposition model of carbon, zirconium carbide and silicon carbide.     

Massive stress changes in plasma-enhanced chemical vapor deposited silicon nitride films on thermal cycling

Massive irreversible increases in tensile stress (up to 2 GPa) on thermal cycling are demonstrated for plasma-enhanced chemical vapor deposited (PECVD) silicon nitride films. Results give further evidence for the claim that this phenomenon is generic to PECVD films and is attributable to the removal of bonded hydrogen: the magnitude of stress increase is independent of the film stress and can be accounted for with a calculation involving the amount of evolved hydrogen. The massive stress changes cause film fracture in most of the films discussed here, with a large diversity of fracture behavior exhibited. The effects of deposition conditions (temperature, plasma frequency, substrate) on film modulus, hardness and coefficient of thermal expansion, as well as stress and stress hysteresis are also examined.     

Ultrafast deposition of silicon nitride and semiconductor silicon thin films by hot wire chemical vapor deposition

The technology of Hot Wire Chemical Vapor Deposition (HWCVD) or Catalytic Chemical Vapor Deposition (Cat-CVD) has made great progress during the last couple of years. This review discusses examples of significant progress. Specifically, silicon nitride deposition by HWCVD (HW-SiN_x) is highlighted, as well as thin film silicon single junction and multijunction junction solar cells. The application of HW-SiN_x at a deposition rate of 3 nm/s to polycrystalline Si wafer solar cells has led to cells with 15.7% efficiency and preliminary tests of our transparent and dense material obtained at record high deposition rates of 7.3 nm/s yielded 14.9% efficiency. We also present recent progress on Hot-Wire deposited thin film solar cells. The cell efficiency reached for (nanocrystalline) nc-Si:H n-i-p solar cells on textured Ag/ZnO presently is 8.6%. Such cells, used in triple junction cells together with Hot-Wire deposited proto-Si:H and plasma-deposited SiGe:H, have reached 10.9% efficiency. Further, in our research on utilizing the HWCVD technology for roll-to-roll production of flexible thin film solar cells we recently achieved experimental laboratory scale tandem modules with HWCVD active layers with initial efficiencies of 7.4% at an aperture area of 25 cm².     

Low temperature chemical vapor deposition of nanocrystalline V2O5 thin films

Spatially uniform, carbon-free thin films of V₂O₅ were deposited on silicon by chemical vapor deposition using vanadium oxide triisopropoxide and water as gaseous precursors, in the temperature range of 100-300 °C. Films with substantial crystallinity were obtained for deposition temperatures as low as 180 °C. The “neat” chemistry that nominally leaves no fragments of ligand or water in the solid promotes film purity and reduces the deposition temperature needed for crystallization. Such deposition temperatures also open up additional possibilities for using crystalline vanadia on fragile substrates such as polymers for electronics and optical applications.     

Deposition and optical studies of silicon carbide nitride thin films

Thin films of silicon carbide nitride (SiCN) have been prepared by reactive radioactive frequency (r.f.) sputtering using SiC target and nitrogen as the reactant gas. Deposition rates are studied as a function of deposition pressures and argon-nitrogen flow ratios. The optical absorption studies indicated the band edge shifting of the films when the nitrogen ratios are increased during deposition. Fourier transform infrared spectroscopy (FTIR) analysis on the films indicated several stretching modes corresponding to SiC, SiN and CN compositions.     

Microstructure and electronic properties of microcrystalline silicon carbide thin films prepared by hot-wire CVD

An overview on microstructural and electronic properties of stoichiometric microcrystalline silicon carbide (μc-SiC) prepared by Hot-Wire Chemical Vapor Deposition (HWCVD) at low substrate temperatures will be given. The electronic properties are strongly dependent on crystalline phase, local bonding, strain, defects, impurities, etc. Therefore these quantities need to be carefully investigated in order to evaluate their influence and to develop strategies for material improvement. We will particularly address the validity of different experimental methods like Raman spectroscopy and IR spectroscopy to provide information on the crystalline volume fraction by comparing the results with Transmission Electron Microscopy (TEM) and X-Ray diffraction data. Finally the electronic properties as derived from optical absorption and transport measurements will be related to the microstructure.