Pressure dependent structural and optical properties of silicon carbide thin films deposited by hot wire chemical vapor deposition from pure silane and methane gases

Silicon carbide (SiC) thin films were deposited using hot wire chemical vapor deposition technique from silane (SiH₄) and methane (CH₄) gas precursors. The effect of deposition pressure on structural and optical properties of SiC films was investigated. Various spectroscopic methods including Fourier transform infrared spectroscopy, Raman scattering spectroscopy, Auger electron spectroscopy, and UV–Vis–NIR spectroscopy were used to study these properties. Films deposited at low deposition pressure were Si-rich, and were embedded with nano-crystals of silicon. These films showed strong absorption in the visible region and had low energy band gaps. Near stoichiometric SiC film, were formed at intermediate deposition pressure and these films were transparent in the visible region and exhibited a wide optical band gap. High deposition pressures caused inhomogeneity in the film as reflected by the increase in disorder parameter and low refractive index of the films. This was shown to be due to formation of sp ² carbon clusters in the film structure.     

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.     

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.     

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.     

Microstructures of microcrystalline silicon thin films prepared by hot wire chemical vapor deposition

Undoped hydrogenated microcrystalline silicon (μc-Si:H) thin films were prepared at low temperature by hot wire chemical vapor deposition (HWCVD). Microstructures of the μc-Si:H films with different H₂/SiH₄ ratios and deposition pressures have been characterized by infrared spectroscopy X-ray diffraction (XRD), Raman scattering, Fourier transform (FTIR), cross-sectional transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). The crystallization of silicon thin film was enhanced by hydrogen dilution and deposition pressure. The TEM result shows the columnar growth of μc-Si:H thin films. An initial microcrystalline Si layer on the glass substrate, instead of the amorphous layer commonly observed in plasma enhanced chemical vapor deposition (PECVD), was observed from TEM and backside incident Raman spectra. The SAXS data indicate an enhancement of the mass density of μc-Si:H films by hydrogen dilution. Finally, combining the FTIR data with the SAXS experiment suggests that the Si---H bonds in μc-Si:H and in polycrystalline Si thin films are located at the grain boundaries.     

Homogeneous nanocrystalline cubic silicon carbide films prepared by inductively coupled plasma chemical vapor deposition

Silicon carbide films with different carbon concentrations x(C) have been synthesized by inductively coupled plasma chemical vapor deposition from a SiH(4)/CH(4)/H(2) gas mixture at a low substrate temperature of 500?°C. The characteristics of the films were studied by x-ray photoelectron spectroscopy, x-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared absorption spectroscopy, and Raman spectroscopy. Our experimental results show that, at x(C) = 49?at.%, the film is made up of homogeneous nanocrystalline cubic silicon carbide without any phase of silicon, graphite, or diamond crystallites/clusters. The average size of SiC crystallites is approximately 6?nm. At a lower value of x(C), polycrystalline silicon and amorphous silicon carbide coexist in the films. At a higher value of x(C), amorphous carbon and silicon carbide coexist in the films.     

Optical and structural properties of silicon oxynitride deposited by plasma enhanced chemical vapor deposition

We present an overview of the properties of silicon oxynitride material (SiON) deposited by plasma enhanced chemical vapor deposition (PECVD) for photovoltaic applications. SiON films were deposited using silane (SiH₄), ammonia (NH₃) and nitrogen protoxide (N₂O) as precursor gases in a low frequency PECVD. Varying the gas flow mixture leads to a whole range of SiON layers starting from the silicon oxide to the silicon nitride with unique stoichiometries and properties. Thanks to spectroscopic ellipsometry measurements we have confirmed the suitability of SiON for antireflection coating layers due to the range of the refractive indexes attainable. SiON structure was analyzed by X-ray photo-electron spectroscopy. We have thus highlighted the critical role of oxygen behavior on the SiON network and the progressive replacement of nitrogen by oxygen atoms when the oxygen precursor increases. The type of chemical bonds present in SiON layers was also investigated by infrared spectroscopy. The SiON layers also contain a non-negligible amount of hydrogen which might be useful for passivation applications. The behavior of hydrogen content was thus analyzed by elastic recoil decay analysis and desorption characterization. A typical rapid thermal annealing was performed on the SiON samples in order to simulate the solar cells contact annealing and to investigate its impact on the dielectric film properties. It was found that hydrogen becomes weakly bonded to the films and strongly decreases in quantity with the annealing. The surface passivation effect is presented in the last part of this paper. The trend before and after a rapid thermal annealing showed opposite results which could be explained by the high porosity of the layers and the formation of Si-O bonds.     

Characterisation of silicon carbide films deposited by plasma-enhanced chemical vapour deposition

The paper presents a characterisation of amorphous silicon carbide films deposited in plasma-enhanced chemical vapour deposition (PECVD) reactors for MEMS applications. The main parameter was optimised in order to achieve a low stress and high deposition rate. We noticed that the high frequency mode (13.56 MHz) gives a low stress value which can be tuned from tensile to compressive by selecting the correct power. The low frequency mode (380 kHz) generates high compressive stress (around 500 MPa) due to ion bombardment and, as a result, densification of the layer achieved. Temperature can decrease the compressive value of the stress (due to annealing effect). A low etching rate of the amorphous silicon carbide layer was noticed for wet etching in KOH 30% at 80 °C (around 13 A/min) while in HF 49% the layer is practically inert. A very slow etching rate of amorphous silicon carbide layer in XeF₂ -7 A/min- was observed. The paper presents an example of this application: PECVD-amorphous silicon carbide cantilevers fabricated using surface micromachining by dry-released technique in XeF₂.     

Annealing effects on structural and electrical properties of fluorinated amorphous carbon films deposited by plasma enhanced chemical vapor deposition

The annealing effects on the structural and electrical properties of fluorinated amorphous carbon (a-C:F) thin films prepared from C₆F₆ and Ar plasma are investigated in a N₂ environment at 200 mTorr. The a-C:F films deposited at room temperature are thermally stable up to 250 °C, but as the annealing temperature is increased beyond 300 °C, the fluorine incorporation in the film is reduced, and the degree of crosslinking and graphitization in the film appears to be enhanced. At the annealing temperature of 250 °C, the chemical bond structures of the film are unchanged noticeably, but the interface trapped charges between the film and the silicon substrate are reduced significantly. The increased annealing temperature contributes the decrease of both the interface charges and the effective charge density in the a-C:F film. Higher self-bias voltage is shown to reduce the charge density in the film.     

螺旋波等离子体沉积纳米硅薄膜结构特性

采用螺旋波等离子体化学气相沉积(HWPCVD)技术,以SiH4作为源反应气体在Si(100)和玻璃衬底上制备了纳米Si薄膜。通过X射线衍射(XRD)、Raman光谱、原子力显微镜(AFM)对所制备的材料结构和形貌等特性进行表征,分析了纳米Si薄膜结构随衬底温度变化的规律。实验结果表明,在较低的衬底温度(100-300℃)范围内,可以实现高晶化度纳米Si薄膜的沉积,颗粒大小在4-8nm之间,样品的晶化度随着衬底温度升高而升高,晶粒大小也随之增大,样品表面光滑,晶粒分布均匀。     

Electrical characterization of amorphous silicon carbide thin films deposited via polymeric source chemical vapor deposition

Polymeric source chemical vapor deposition (PS-CVD) was used to synthesize amorphous silicon carbide (a-SiC) thin films. The PS-CVD process was conducted at temperatures between 750 and 1000 °C. The substrates used were silicon single crystal wafers of p-type and n-type, and thermally grown silicon dioxide substrates. The chemical and electrical properties of the films were studied by various techniques, including Fourier transform infrared spectroscopy, elastic recoil detection (ERD), and capacitance-voltage technique. A correlation was observed between the average concentration of oxygen in the films and the deposition temperature, linking a low oxygen concentration to a high deposition temperature. However, the concentration of oxygen in the films deposited at the same temperature is independent of the substrate. The thin films deposited at low temperature showed insulating behaviour, while the semiconducting behaviour is obtained at high deposition temperatures. Ohmic contacts were obtained on the deposited semiconductor thin film by evaporating nickel contacts, followed by annealing of the sample at 800 °C for 2 min.     

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.     

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.     

Effect of thermal annealing on the structural and mechanical properties of amorphous silicon carbide films prepared by polymer-source chemical vapor deposition

We report on the effect of thermal annealing on the structural and mechanical properties of amorphous SiC thin films prepared by means of a polymer-source chemical vapor deposition process. The chemical bondings of the a-SiC:H films were systematically examined by means of Fourier transform infrared spectroscopy (FTIR). The film composition was measured by X-ray photoelectron spectroscopy, while X-ray reflectivity measurements were used to account for the film density variations caused by the post-annealing treatments over the 750-1200 °C range. In addition, their mechanical properties (hardness and Young's modulus) were investigated by using the nano-indentation technique. FTIR measurements revealed that not only the intensity of a-SiC absorption band linearly increases but also its position is found to shift to a higher wave number as a result of annealing. In addition, the bond density of Si―C is found to increase from (101.6-224.5) × 10²¹ bond·cm^− 3 accompanied by a decrease of Si―H bond density from (2.58-0.46)× 10²¹ bond·cm^− 3 as a result of increasing the annealing temperature (T_a) from 750 to 1200 °C. Annealing-induced film densification is confirmed, as the a-SiC film density is found to increase from 2.36 to ∼ 2.75 g/cm^− 3 when T_a is raised from 750 to 1200 °C. In addition, as T_a is increased from 750 to 1200 °C, both hardness and Young's modulus are found to increase from 15.5 to 17.6 GPa and 155 to 178 GPa, respectively. Our results confirm the previously established linear correlation between the mechanical properties of the a-SiC films and their bond densities.     

Effect of substrate temperature on structural, optical and electrical properties of ZnO thin films deposited by pulsed laser deposition

ZnO thin films were grown by the pulse laser deposition (PLD) method using Si (100) substrates at various substrate temperatures. The influence of the substrate temperature on the structural, optical, and electrical properties of the ZnO thin films was investigated. All of the thin films showed c-axis growth perpendicular to the substrate surface. At a substrate temperature of 500 °C, the ZnO thin film showed the highest (002) peak with a full width at half maximum (FWHM) of 0.39°. The X-ray Photoelectron Spectroscopy (XPS) study showed that Zn was in excess irrespective of the substrate temperature and that the thin film had a nearly stoichiometrical composition at a substrate temperature of 500 °C. The photoluminescence (PL) investigation showed that the narrowest UV FWHM of 15.8 nm and the largest ratio of the UV peak to the deep-level peak of 32.9 were observed at 500 °C. Hall effect measurement systems provided information about the carrier concentration, mobility and resistivity. At a substrate temperature of 500 °C, the Hall mobility was the value of 37.4 cm²/Vs with carrier concentration of 1.36 × 10¹⁸ cm⁻³ and resistivity of 2.08 × 10⁻¹ Ω cm.     

Role of argon in hot wire chemical vapor deposition of hydrogenated nanocrystalline silicon thin films

Structural, optical and electrical properties of hydrogenated nanocrystalline silicon (nc-Si:H) films, deposited from silane (SiH₄) and argon (Ar) gas mixture without hydrogen by hot wire chemical vapor deposition (HW-CVD) method were investigated. Film properties are carefully and systematically studied as a function of argon dilution of silane (R_Ar). We observed that the deposition rate is much higher (4-23 Å/s) compared to conventional plasma enhanced chemical vapor deposited nc-Si:H films using Ar dilution of silane (0.5-0.83 Å/s). Characterization of these films with Raman spectroscopy revealed that Ar dilution of silane in HW-CVD endorses the growth of crystallinity and structural order in the nc-Si:H films. The Fourier transform infrared spectroscopic analysis showed that with increasing Ar dilution, the hydrogen bonding in the films shifts from di-hydrogen (Si-H₂) and (Si-H₂)_n complexes to mono-hydrogen (Si-H) bounded species. The hydrogen content in the films increases with increasing Ar dilution and was found to be < 4 at.% over the entire range of Ar dilutions of silane studied. However, the band gap shows decreasing trend with increase in Ar dilution of silane and it has been attributed to the decrease in the percentage of the amorphous phase in the film. The microstructure parameter was found to be > 0.4 for the films deposited at low Ar dilution of silane and ~ 0.1 or even less for the films deposited at higher Ar dilution, suggesting that there is an enhancement of structural order and homogeneity in the film. From the present study it has been concluded that the Ar dilution of silane is a key process parameter to induce the crystallinity and to improve the structural ordering in the nc-Si:H films deposited by the HW-CVD method.     

Hot wire chemical vapor deposited multiphase silicon carbide (SiC) thin films at various filament temperatures

Influence of filament temperature (T_Fil) on the structural, morphology, optical and electrical properties of silicon carbide (SiC) films deposited by using hot wire chemical vapor deposition technique has been investigated. Characterization of these films by low angle XRD, Raman scattering, XPS and TEM revealed the multiphase structure SiC films consisting of 3C–SiC and graphide oxide embedded in amorphous matrix. FTIR spectroscopy analysis show an increase in Si–C, Si–H, and C–H bond densities and decrease in hydrogen content with increase in T_Fil. The C–H bond density was found higher than the of Si–H and Si–C bond densities suggesting that H preferably get attached to C than Si. AFM investigations show decrease in rms surface roughness and grain size with increase in T_Fil. SEM studies show that films deposited at low T_Fil has spherulites-like morphology while at high T_Fil has cauliflower-like structure. Band gap values E_Tauc and E₀₄ increases from 1.76 to 2.10 eV and from 1.80 to 2.21 eV respectively, when T_Fil was increased from 1500 to 2000 °C. These result show increase in band tail width (E₀₄–E_Tauc) of multiphase SiC films. Electrical properties revealed that σ_Dark increases from ~7.87 × 10^?10 to 1.54 × 10^?5 S/cm and E_act decreases from 0.67 to 0.41 eV, which implies possible increase in unintentional doping of oxygen or nitrogen due to improved crystallinity and Si–C bond density with increase in T_Fil. The deposition rate for the films was found moderately high (21 < r_dep < 30 Å/s) over the entire range of T_Fil studied.     

温度对化学气相沉积碳化硅涂层的影响

以三氯甲基硅烷和氢气为气源,研究了化学气相沉积碳化硅过程中,温度(850-1350℃)对沉积速率、反应物消耗效应、涂层形貌和相结构的影响.用磁悬浮天平在线实时称量基体质量变化进行动力学研究;采用扫描电镜和X射线衍射对样品做了表征.结果表明,沉积过程存在四个控制机理:a区(<1000℃)为表面反应动力学控制;b区(1000-1050℃)主要是HCl对沉积的抑制作用;c区(1050-1300℃)是表面化学反应和传质共同控制;d(>1300℃)为传质为限速步骤.由于不同的控制机制,导致所得涂层的形貌存在差异.含碳含硅中间物质浓度的减小、HCl增多和MTS的分解共同导致反应物消耗效应.涂层由热解碳和碳化硅两相组成,温度的升高使热解碳相减少,碳硅比接近1.     

Two-step annealing of hot wire chemical vapor deposited a-Si:H films

A two-step annealing process was used to investigate the effect of dehydrogenation on crystallization and grain growth of low and high hydrogen content hot wire chemical vapor deposited (HWCVD) a-Si:H films. A low temperature pre-annealing followed by a rapid thermal annealing step at 600 °C was carried out. For the high hydrogen content film XRD (111) peak narrowed quite a bit, while opposite effect was observed for the low hydrogen content film. According to the grain sizes as calculated from TEM images, grain sizes of both of the two-step annealed high and low hydrogen content films are smaller than that of the single stage annealed film.