Gas Mixture Of SiH4 Research Articles

Comparative Study of Silicon Quantum Dot Formation In-situ Grown with a Gas Mixture of SiH4+N2 and SiH4+NH3

Comparative Study of Silicon Quantum Dot Formation In-situ Grown with a Gas Mixture of SiH4+N2 and SiH4+NH3

Simulation of gas phase reactions for microcrystalline silicon films fabricated by PECVD

We present a numerical gas phase reaction model for hydrogenated microcrystalline silicon (μc-Si:H) films from SiH4 and H2 gas mixtures with plasma enhanced chemical vapor deposition (PECVD). Under the typical μc-Si:H deposition conditions, the concentrations of the species in the plasma are calculated and the effects of silane fraction (SF=[SiH4]/[H2+SiH4]) are investigated. The results show that SiH3 is the key precursor for μc-Si:H films growth, and other neutral radicals, such as Si2H5, Si2H4 and SiH2, may play some roles in the film deposition. With the silane fraction increasing, the precursor concentration increases, but H atom concentration decreases rapidly, which results in the lower H/SiH3 ratio.

UV–VISIBLE, FTIR AND RAMAN SPECTRA FROM LARGE BAND-GAP HYDROFLUORINATED AMORPHOUS SILICON–CARBON THIN FILMS

Hydrofluorinated amorphous silicon–carbon alloy films (a-Si,C:H,F) were grown by a CVD glow-discharge method. The samples were prepared from SiH4 , H2 and CF4 source gas mixtures. The structural and bonding characterstics of the materials were studied using FTIR and Raman spectroscopy. In addition to hydrogen passivation, the inclusion of fluorine in the films is found to further suppress undesirable disorder states. The optical band gap E opt of the materials was determined from the Tauc relation for the optical absorption spectra. They were found to increase from about 2 eV to 3.5 eV with increasing carbon content in the samples.

Amorphous silicon carbide photoelectrode for hydrogen production directly from water using sunlight

Results on the use of amorphous silicon carbide (a-SiC:H) as a photoelectrode in an integrated ‘hybrid’ photoelectrochemical (PEC) cell to produce hydrogen directly from water using sunlight are presented. A high quality a-SiC:H thin film with bandgap >2.0eV was fabricated by RF-PECVD technique using a SiH4, H2 and CH4 gas mixture. Incorporation of the carbon in the a-SiH film not only increased the bandgap, but also led to an increase in corrosion resistance in an electrolyte. Adding H2 during the fabrication process led to a decrease in the density of states (DOS) in the films. Immersing the a-SiC:H(p)/a-SiC:H(i) structure in a pH 2 electrolyte showed excellent durability up to 100 h (so far tested); the photocurrent increased and the photocurrent onset shifted anodically after the 100-h durability test, which is encouraging. It was also found that a SiO x layer is formed on the surface of a-SiC:H when exposed to air, which led to a decrease in the photocurrent, and the photocurrent onset shifted cathodically. We have found that this effect could be removed via a hydrofluoric acid dip and the photocurrent could be increased to over 6mA/cm2 at a potential of −1.4V (versus Ag/AgCl). To boost the photovoltage (potential) required for water splitting, the configuration of a-Si:H tandem (or ‘micromorph’)/a-SiC:H structure (PV/PEC) could be viable for water splitting. As a first step, we integrated amorphous silicon tandem solar cells with an a-SiC:H photoelectrode, which exhibited encouraging results. Finally, via simulation, we showed that using a-SiC:H as a photoelectrode could lead to a solar-to-hydrogen (STH) conversion efficiency >10%.

Investigation of Hydrogenated Amorphous SiNx:H Films Grown by Rapid Thermal Chemical Vapor Deposition

Hydrogenated silicon nitride films were deposited with NH3, SiH4 and N2 gas mixture at 700oC by rapid thermal chemical vapor deposition (RTCVD) system. Wafer curvature measurements were carried out at room temperature to study the SiNx:H film stress with various deposition parameters. The NH3/N2 flow ratio and deposition pressure are found to influence the film stress. The stress of SiNx:H films deposited by RTCVD is tensile, which can reach ~ 1.5 GPa in our study. The influence of the reactive gas mixture and working pressure on the film stress, in connection with its material properties, is demonstrated.

The Influence of the Substrate on the Mechanical Properties of Si-Doped DLC Thin Films

In this work, Si-doped DLC films were deposited on stainless steel (316SS) and polycarbonate (PC) substrates by RF-PACVD in gas mixtures of SiH4+CH4, with 2, 5 and 10 vol.% SiH4. The increase of the Si content in the films led to a progressive drop in the hardness from 30 GPa down to 23 GPa whereas the elastic modulus increased from 124 GPa up to 146 GPa, as measured in the SS coated substrates. In the case of coated PC samples pop-in was observed in the loading curve which was interpreted by finite element simulation and nanoscratching techniques.

Structural and morphological studies on SiOxNy thin films

In this work, the structure and morphology of silicon oxynitride films deposited by the PECVD technique were studied. The films were deposited under two different conditions: (a) SiOxNy with chemical compositions varying from SiO2 to Si3N4 via the control of a N2O+N2+SiH4 gas mixture, and (b) Si-rich SiOxNy films via the control of a N2O+SiH4 gas mixture. The analyses were performed using X-ray near edge spectroscopy (XANES) at the Si-K edge, transmission electron microscopy (TEM) and Rutherford backscattering spectroscopy (RBS). For samples with chemical composition varying from SiO2 to Si3N4, the diffraction patterns obtained by TEM exhibited changes with the chemical composition, in agreement with the XANES results. For silicon-rich silicon oxynitride samples, the formation of a-Si clusters was observed and the possibility of obtaining Si nanocrystals after annealing depending on the composition and temperature was realized.

Application of hydrogenated amorphous silicon oxide layers to c-Si heterojunction solar cells

Wide-gap hydrogenated amorphous silicon oxide (a-SiO:H), fabricated by plasma process using SiH4 and CO2 gas mixture, has been applied to crystalline silicon (c-Si) heterojunction solar cells. It has been demonstrated that incorporation of an a-SiO:H p layer, instead of a hydrogenated amorphous silicon (a-Si:H) p layer, improves the conversion efficiency slightly. Moreover, when an a-SiO:H i layer is formed on the c-Si substrate, Si epitaxial growth that occurs at an a-Si:H∕c-Si heterointerface at high deposition temperatures can be prevented entirely. Accordingly, high-efficiency solar cells are fabricated more easily by applying a-SiO:H p-i layers to n-type c-Si heterojunction solar cells.

Optical and structural properties of polycrystalline 3C-SiC films

In this letter, polycrystalline 3C-SiC (111) films were deposited by plasma enhanced chemical vapor deposition system at a temperature of 670°C using a gas mixture of SiH4∕CH4∕H2∕(CF4). The optical properties of deposited films with different feed gases and different structures were investigated. In these studies, a broad photoluminescence band was observed for films with lower crystallinity and the radiative transitions between the conduction and valance band tails were suggested as the origin of the observed peak. The band gap of these polycrystalline SiC films was estimated at around 2.10eV.

Pulsed radio frequency plasma deposition of a-SiNx:H alloys: Film properties, growth mechanism, and applications

In this work, we propose a fabrication process of a-SiNx:H alloys by pulsing the radio frequency (rf) signal in a low pressure plasma-enhanced chemical vapor deposition (PECVD) system. The characteristics of the films can be controlled simply by adjusting the duty cycle of the pulsed rf power, while keeping the N2∕SiH4 gas mixture constant. Spectroscopic ellipsometry analysis in the ultraviolet-visible-near infrared and far infrared ranges, atomic force microscopy, and elastic recoil detection reveal strong variations in the optical properties (1.88⩽n⩽2.75, 10−4⩽k⩽5×10−2 at 550nm), optical gap (4.01eV⩽Eg⩽1.95eV), microstructural characteristics (1.3nm⩽surfaceroughness⩽8.3nm), and chemical composition (0.47⩽x⩽1.35) of the coatings as a function of duty cycle. This behavior is interpreted in terms of radical concentration changes in the gas phase, as well as variation in the average ion bombardment energy at the film surface, leading to modifications of both chemical and physical mechanisms that sustain the film growth. Using the control of duty cycle, we fabricated two types of a-SiNx:H-based thin film devices, namely, (i) a model Fabry-Perot optical filter deposited on plastic substrate and (ii) a superlattice structure displaying a photoluminescence signal four times higher than the reference single layer. These two examples of applications point out the main advantages of this pulsed rf PECVD process, in particular, low deposition temperature, reproducibility, versatility, and ease of use.

Hydrogenated silicon carbon nitride films obtained by HWCVD, PA-HWCVD and PECVD techniques

Hydrogenated silicon carbon nitride (SiCN:H) thin film alloys were produced by hot wire (HWCVD), plasma assisted hot wire (PA-HWCVD) and plasma enhanced chemical vapor (PECVD) deposition techniques using a Ni buffer layer as catalyst for inducing crystallization. The silicon carbon nitride films were grown using C2H4, SiH4 and NH3 gas mixtures and a deposition temperature of 300°C. Prior to the deposition of the SiCN:H film a hydrogen etching of 10min was performed in order to etch the catalyst material and to facilitate the crystallization. We report the influence of each deposition process on compositional, structural and morphological properties of the films. Scanning Electron Microscope-SEM and Atomic Force Measurement-AFM images show their morphology; the chemical composition was obtained by Rutherford Backscattering Spectrometry-RBS, Elastic Recoil Detection-ERD and the structure by Infrared-IR analysis. The thickness of the catalyst material determines the growth process and whether or not islands form. The production of micro-structured SiCN:H films is also dependent on the gas pressure, gas mixture and deposition process used.

Low-temperature electron cyclotron resonance plasma-enhanced chemical-vapor deposition silicon dioxide as gate insulator for polycrystalline silicon thin-film transistors

Silicon dioxide films have been deposited at temperatures below 270°C in an electron cyclotron resonance (ECR) plasma reactor from O2, SiH4, and He gas mixture. Pinhole density analysis as a function of substrate temperature for different microwave powers was carried out. Films deposited at higher microwave power and at room temperature show defect densities (<7pinhole∕mm2), ensuring low-temperature process integration on large area. From Fourier transform infrared analysis and thermal desorption spectrometry we also evaluated very low hydrogen content if compared to conventional rf-plasma-enhanced chemical-vapor-deposited (PECVD) SiO2 deposited at 350°C. Electrical properties have been measured in metal-oxide-semiconductor (MOS) capacitors, depositing SiO2 at RT as gate dielectric; breakdown electric fields >10MV∕cm and charge trapping at fields >6MV∕cm have been evaluated. From the study of interface quality in MOS capacitors, we found that even for low annealing temperature (200°C), it is possible to considerably reduce the interface state density down to 5×1011cm−2eV−1. To fully validate the ECR-PECVD silicon dioxide we fabricated polycrystalline silicon thin-film transistors using RT-deposited SiO2 as gate insulator. Different postdeposition thermal treatments have been studied and good device characteristics were obtained even for annealing temperature as low as 200°C.

Condensation of Argon, Monosilane and Their Mixtures in a Pulse Free Jet

Pulse supersonic outflow of Ar, SiH4 and Ar + SiH4 gas mixture (where monosilane is a small admixture) was studied experimentally by the method of molecular beam mass-spectrometry. Using argon as an example we have shown that condensation processes at the quasi-stationary region of a pulse flow and within a stationary jet are similar. In the flows of pure gases clusters of argon and silane (hydrogenated silicon) and in the mixture argon – silane complexes were registered. The dependencies of the intensities of monomer and cluster ions on stagnation pressure were investigated. It was shown that in the mixture jet at low stagnation pressures the condensation process with the formation of monosilane clusters takes place and at high pressures mixed argon-silane complexes are formed. The parameters of flow transition into the regime of developed condensation were determined for pure gases and their mixture.

Vibrational spectroscopy study of Ar+-ion irradiated Si-rich oxide films grown by plasma-enhanced chemical vapor deposition

SiO x thin films with different stoichiometry degree were obtained by plasma-enhanced chemical vapor deposition on crystalline silicon substrates from SiH4 and N2O gas mixtures. Two twin sets of samples were irradiated by 380 keV Ar+ ions at a fluence of 5×1016ions∕cm2 at room temperature and at 500 °C, respectively, and then annealed in vacuum at different temperatures, between 500 and 1100 °C. A set of unirradiated samples has been annealed in the same conditions in order to discriminate the contribution of ion irradiation and of thermal treatments to the changes of the film microstructure. The structural modification of the oxide network and the growth of Si nanoclusters have been studied by vibrational spectroscopy techniques. Fourier transform infrared absorption spectra evidenced that ion irradiation induces a hydrogen loss of about 50%, and that postirradiation thermal treatments lead to the recovery of the irradiation defects and to the out diffusion of the residual hydrogen. After heating at 800 °C, irradiated and unirradiated samples exhibit substantially the same structure from the point of view of infrared-absorption spectra. In the meanwhile, the Si–O–Si stretching peak blue shifts, but never reaches the wavenumber value of pure silica owing to the presence of nitrogen into the network. Raman spectra of as-irradiated films reveal the presence of an amorphous silicon phase within the damaged layer of the oxide matrix. Raman spectra of irradiated samples undergoing thermal treatments at high temperature indicate a rearrangement of the film microstructure with the progressive clustering of the amorphous silicon phase. However, no clear spectroscopic evidence is gained about the crystallization of silicon nanoclusters, even after annealing at the highest temperature. In fact, the Raman scattering from silicon nanocrystals is partially hidden by the Raman peak of the c-Si substrate.

Development of low temperature silicon oxide thin films by photo-CVD for surface passivation

Low temperature (250°C) silicon oxide (a-SiOx:H) films have been developed for surface passivation as well as antireflection coating in silicon solar cell. Films have been fabricated by ion damage free photochemical vapor decomposition technique using SiH4, N2O, and H2 gas mixture. In this paper we have reported the effect of N2O to SiH4 ratio (R) on optoelectronic and structural properties of the films. The bonding configurations of Si and H were investigated in detail by IR absorption measurement. The Si–H stretching mode supports the presence of H–(Si3-nOn)(n=0–3) structural unit, which is also present in the Si–O stretching mode. Developed silicon oxide films also have been studied on the c-Si solar cell. A substantial enhancement (11.2%) in efficiency has been achieved.

Field Emission Properties of Nanocrystalline and Amorphous Silicon Carbon Nitride Prepared from Microwave Plasma Chemical Vapor Deposition

Silicon carbon nitride (SiCN) films have been deposited on silicon wafers by microwave plasma chemical vapor deposition (MPCVD). Gas mixture of H2, CH4, N2, and SiH4 was used as precursors, in which the flow rate of N2 was changed. X-ray photoelectron spectroscopy (XPS) and micro-Raman spectroscopy were employed to characterize the composition and bonding structures, while field-emission scanning electron microscopy were used to investigate the microstructure of the films. With increasing the flow rate of N2 from 50 sccm to 300 sccm, the SiCN films changed from amorphous to nanocrystalline. Characteristic current-voltage measurements indicate a low turn-on field of 10.8 V/µm. Field emission current density of 4.5 mA/cm2 has been observed at 20 V/µm.

Modeling of microcrystalline silicon film deposition in a capacitively coupled radio-frequency plasma reactor

We present a numerical model of plasma-enhanced chemical-vapor deposition of hydrogenated microcrystalline silicon (μc-Si:H) film from SiH4 and H2 gas mixtures in a capacitively coupled radio-frequency plasma reactor. The model takes into account electron-impact, gas-phase, and surface reactions within a well-mixed reactor model. Plasma parameters such as the electron density, the electron temperature, and the electron-impact reaction rates are determined through a discharge model and used as inputs for the reactor model. The gas-phase reactions include electron-impact and neutral–neutral reactions. Some of the surface reaction rates are determined using quantum chemical calculations and transition state theory. In the reactor model, concentrations of each chemical species are calculated at steady state using mass conservation equation uniformed throughout the reactor. Numerical results of the deposition rate as a function of the plasma reactor operating parameters show good agreement with experiments. Based on the model, the correlation between μc-Si:H properties, such as the crystal grain orientation and the hydrogen content, and deposition operating parameters has been studied using a design of experiment. Finally, optimal operating parameters are investigated using optimization techniques.

Optoelectronic and Structural Properties of Undoped Microcrystalline Silicon Thin Films: Dependence on Substrate Temperature in Very High Frequency Plasma Enhanced Chemical Vapor Deposition Technique

The effects of substrate temperature on optoelectronic and structural properties of undoped microcrystalline silicon thin films have been investigated. The undoped silicon films have been deposited by the very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) technique using a SiH4 and H2 gas mixture at 105 MHz plasma excitation frequency and moderately low power density of 70 mW/cm2. The effect of the systematic variation of substrate temperature (from 180°C to 370°C) on film properties has been studied, while keeping the other parameters constant. The deposition rate is considerably high (2.6 Ås-1) at 180°C and remains almost constant over the whole temperature range. Dark conductivity for all the films lies around ∼10-6 Scm-1. Low subband gap absorption has been observed by photothermal deflection spectroscopy for these microcrystalline films. Increase of substrate temperature improves the microcrystallinity of the film, which is confirmed from structural studies. Crystalline grain size also increases with increase of substrate temperature and a maximum of 460 Å has been achieved at 370°C. A satisfactory correlation is observed among the results of different structural studies: Raman spectroscopy, infrared spectroscopy, X-ray diffraction, transmission electron microscopy and atomic force microscopy.

Influence of H on the composition and atomic concentrations of “N-rich” plasma deposited SiOxNyHz films

The influence of H on the composition and atomic concentrations of Si, O, and N of plasma deposited SiOxNyHz films was investigated. The bonding scheme of H was analyzed by Fourier-transform infrared spectroscopy. The composition and absolute concentrations of all the species present in the SiOxNyHz, including H, was measured by heavy-ion elastic recoil detection analysis (HI–ERDA). Samples were deposited from SiH4, O2, and N2 gas mixtures, with different gas flow ratios in order to obtain compositions ranging from SiNyHz to SiO2. Those samples deposited at higher SiH4 partial pressures show both Si–H and N–H bonds, while those deposited at lower SiH4 partial pressures show N–H bonds only. The Si–H and N–H bond concentrations were found to be proportional to the N concentration. The concentration of H was evaluated from the Si–H and N–H stretching absorption bands and compared to the HI–ERDA results, finding good agreement between both measurements. The deviation from H-free stoichiometric SiOxNy composition due to the presence of N–H bonds results in an effective coordination number of N to produce Si–N bonds lower than 3. By fitting the experimental composition data to a theoretical model taking into account the influence of N–H bonds, the actual concentration of N–H bonds was obtained, making evident the presence of nonbonded H. The presence of Si–H and Si–Si bonds was found to partially compensate the effect of N–H bonds, from the point of view of the relative N and Si contents. Finally, the presence of N–H bonds results in a lower Si atom concentration with respect to the stoichiometric film, due to a replacement of Si atoms by H atoms. This decrease of the Si concentration is lower in those films containing Si–H and Si–Si bonds. A model was developed to calculate the Si, O, and N atom concentrations taking into account the influence of N–H, Si–H, and Si–Si bonds, and was found to be in perfect agreement with the experimental data measured by HI–ERDA.

RBS study of amorphous silicon carbide films deposited by PECVD

We present properties of nitrogen-doped amorphous silicon carbide films that were grown by a plasma enhanced chemical vapour deposition (PECVD) technique and annealed by pulsed electron beam. Samples with different amounts of N were achieved by a small addition of ammonia NH3 into the gas mixture of silane SiH4 and methane CH4, which were directly introduced into the reaction chamber. The actual amount of nitrogen in the SiC films was determined by Rutherford backscattering spectrometry (RBS). A simulation of the RBS spectra was used to calculate the concentration of carbon, silicon and nitrogen.