Microwave power dependence of the optical and structural properties of boron and phosphorus-doped SiC:H films prepared using ECR-CVD

Hydrogenated silicon carbide films (SiC:H) were deposited using the electron cyclotron resonance chemical vapour deposition (ECR-CVD) technique from a mixture of methane, silane and hydrogen, and using diborane and phosphine as doping gases. The effects of changes in the microwave power on the deposition rate and optical bandgap were investigated, and variations in the photo- and dark-conductivities were studied in conjunction with film analysis using the Raman scattering technique. In the case of boron-doped samples, the conductivity increased rapidly to a maximum, followed by rapid reduction at high microwave powers. The ratio of the photo- to dark-conductivity (σ_ph/σ_d) peaked at microwave power of 600 W. Under conditions of high microwave power, Raman scattering analysis showed evidence of the formation and increase in the silicon microcrystalline and diamond-like phases in the films, the former of which could account for the rapid increase and the latter the subsequent decrease in the conductivity. In the case of phosphorus-doped SiC:H samples, it was found that increase in the microwave power has the effect of enhancing the formation of the silicon microcrystalline phase in the films which occurred in correspondence to a rapid increase in the conductivity. The conductivity increase stabilised in samples deposited at microwave powers exceeding 500 W probably as a result of dopant saturation. Results from Raman scattering measurements also showed that phosphorus doping has the effect of enhancing the formation of the silicon microcrystals in the film whereas the presence of boron has the effect of preserving the amorphous structure.     

A Raman and photoconductivity analysis of boron-doped SiC : H films deposited using the electron cyclotron resonance method

Hydrogenated amorphous silicon carbide films (a-SiC:H) were deposited using the electron cyclotron resonance chemical vapour deposition technique from a mixture of methane, silane and hydrogen, with diborane as the doping gas. The effect of the microwave power on the deposition rate were studied, and variations in the photo and dark conductivities were investigated in conjunction with film analysis using the Raman scattering technique. The conductivity increases rapidly to a maximum, followed by rapid reduction at high microwave powers. The ratio of the photo to dark conductivity, σph/σd, peaks at microwave powers of ∼600 W. Under conditions of high hydrogen dilution and increasing microwave power, Raman scattering analysis showed evidence of the formation and increase of microcrystalline silicon and diamond-like components in the films, the former of which could account for the rapid increase and the latter the subsequent decrease in the conductivity. This revised version was published online in November 2006 with corrections to the Cover Date.     

Structural, electrical and optical characterization of N- and P-type SiC:H films deposited using ECR-CVD

Hydrogenated silicon carbide films (SiC:H) were deposited using the electron cyclotron resonance chemical vapour deposition (ECR-CVD) method from a mixture of methane, silane and hydrogen, and using diborane and phosphine as doping gases. The effects of changes in the diborane and phosphine levels on the optical bandgap and conductivity were investigated. In the case of boron-doped films, there is evidence from Raman scattering analysis to show that films deposited at a low microwave power of 150 W were largely amorphous and the bandgap decreases as the diborane levels are highly conductive and contains the whereas films deposited at a high microwave power of 800 W at low diborane levels are highly conductive and contains the silicon microcrystalline phase. These films become amorphous as the diborane level is increased, while the optical bandgap remains relatively unaffected throughout the entire range of diborane levels investigated. In the case of phosphorus-doped films, Raman scattering analysis showed that the deposition conditions strongly influence the structural, optical and electrical properties of the SiC:H films. Unlike boron doping, doping with phosphorus can have the effect of increasing the silicon microcrystalline phase in the SiC:H films which were prepared at low (150 W) and high (600 W) microwave powers. Films prepared at high microwave power showed only small variations in the optical bandgap, suggesting that good phosphorus doping efficiency can be achieved in films which contain the silicon microcrystalline phase (mc-SiC:H).     

The crystalline properties of nitrogen doped hydrogenated microcrystalline silicon thin films

The crystalline properties of nitrogen doped hydrogenated microcrystalline silicon thin films deposited by plasma enhanced chemical vapor deposition were studied. Gas phase doping density in the order of 10⁻² and 10⁻¹ leads to changes in the crystalline properties of the films. Raman scattering signals indicate that nitrogen doping causes a more significant reduction in crystallite size than does an increase in SiH₄ concentration. In addition, the size reduction occurs with a less significant increase in amorphous fraction volume than in the case of SiH₄ concentration increase. The N in the Si crystalline induces disorder or stress as a result of the higher electronegativity and smaller atomic size of N compared to Si. Thus, the crystallite size reduction is thought to occur to reduce the disorder in crystalline grain induced by doped nitrogen.     

Hydrogenated microcrystalline silicon films prepared by VHF-PECVD and single junction solar cell

Intrinsic microcrystalline silicon films have been prepared with very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) from silane/hydrogen mixture at 180°C. The effect of silane concentration and discharge power on the growth of silicon films was investigated. Samples were investigated by Fourier transform infrared spectroscopy, Raman scattering and X-ray diffraction. The Raman spectrum shows that the morphological transition from microcrystalline to amorphous occurs under conditions of high silane concentration and low discharge power. X-ray diffraction spectra indicate a preferential growth direction of all microcrystalline silicon films in the (111) plane. In addition, a solar cell with an efficiency of 5.1% has been obtained with the intrinsic microcrystalline layer prepared at 10W.     

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.     

Raman studies of aluminum induced microcrystallization of n Si:H films produced by PECVD

We performed a Raman scattering study of aluminum induced microcrystallization of thin films of phosphorous-doped hydrogenated amorphous silicon (n⁺ a-Si:H). These thin films of heavily doped n⁺ a-Si:H were prepared by plasma enhanced chemical vapor deposition. Afterwards, aluminum was deposited and followed by an annealing process at 523 K in a nitrogen environment during several hours. Raman results reveal the formation of microcrystalline regions distributed in the amorphous matrix, induced by the film annealing in the presence of the aluminum. We have used the spatial correlation model to estimate from the Raman signal the microcrystallite size and its relation with the annealing time. The estimated crystallite size was found to be between 6.8 and 9.5 nm and the broadening and downshift of the signals are explained in terms of the crystallite size and lattice expansion effects due to the annealing process. Conductivity values of the samples as a function of the annealing time are explained in terms of the contributions from the amorphous and from the microcrystalline phases.     

Low-pressure synthesis and characterization of multiphase SiC by HWCVD using CH4/SiH4

Silicon carbide (SiC) thin films were deposited by low-pressure hot wire chemical vapor deposition (HWCVD) technique using SiH₄ and CH₄ gas precursors with no hydrogen dilution. Spectroscopic and structural properties of the films deposited at various methane flow rate (10-100 sccm) and low silane flow rate of 0.5 sccm were investigated. The use of low methane flow rate resulted in a sharp and intense Si-C peak in the Fourier transform infrared (FTIR) absorption spectra. The XRD spectra of the films showed the formation of SiC crystallites at low methane flow rate. The Raman spectroscopy measurements showed the coexistence of a-Si and SiC phases in the films. Increase in methane flow rate increased the carbon incorporation and deposition rate of the SiC films but also promoted the formation of amorphous Si and SiC phases in the films.     

Evolution of structural and optical properties of nanostructured silicon carbon films deposited by plasma enhanced chemical vapour deposition

Nanostructured silicon carbon films composed of silicon nanocrystallites embedded in hydrogenated amorphous silicon carbon matrix have been deposited by plasma enhanced chemical vapour deposition technique using silane and methane gas mixture highly diluted in hydrogen. The structural and optical properties of the films have been investigated by X-ray diffraction, Raman, Fourier transform infrared, ultra violet-visible-near infrared and photoluminescence spectroscopies while the composition of the films has been obtained from nuclear techniques. The study has demonstrated that the structure of the films evolves from microcrystalline to nanocrystalline phase with the increase in radio frequency (rf) power. Further, it is shown that with increasing the rf power the size of silicon nanocrystallites decreases while the optical gap increases and a blueshift of visible room temperature photoluminescence peak can be observed.     

玻璃及不锈钢衬底N型微晶硅薄膜的制备

利用射频等离子体增强化学气相沉积(RF-PECVD)技术,以PH3为掺杂剂,在浮法玻璃衬底和不锈钢衬底上制备了纳米级的n型微晶硅薄膜。采用Wvase32型椭圆偏振光谱仪、Reinishaw2000型拉曼(Raman)光谱仪和高阻仪对薄膜进行测试,以研究沉积条件对薄膜沉积速率、晶化率和暗电导特性的影响,并对沉积功率和沉积气压进行双因素优化,绘制了双因素相图。     

Improved transport properties of microcrystalline silicon films grown by HWCVD with a variable hydrogen dilution process

The structure and the transport properties of microcrystalline silicon films prepared by hot-wire/catalytic chemical vapor deposition (HWCVD/Cat-CVD), using different dilution ratios of silane in hydrogen, were investigated. Spectroscopic ellipsometry analysis revealed an increase in the thickness of amorphous incubation layer formed before nucleation and a reduction of the void volume fraction when hydrogen dilution decreases. Thus, a specific microcrystalline silicon film growth process was proposed, based on a variable dilution of silane in hydrogen. For films prepared in such conditions, the formation of the incubation layer was inhibited, which led to a drastic improvement in carrier transport along the growth direction as proved by the diffusion-induced time-resolved microwave conductivity data.     

Raman spectra of intrinsic and doped hydrogenated nanocrystalline silicon films

Raman scattering characteristics of intrinsic and doped hydrogenated nanocrystalline silicon films which prepared by a plasma-enhanced chemical vapor deposition system are investigated. Results indicate that Raman spectra depend intensively on microstructure and impurity in the films. Taking into account phonon confinement effect and tensile strain effect in Si nanocrystals, peak redshift of measured transverse optical modes in Raman spectra of intrinsic films can be well interpreted. With respect to Raman scattering from doped samples, besides phonon confinement effect, the peak of experimental transverse optical mode further downshifts with heightening doping level, which can be primarily assigned to impurity effect from doping. In addition, the increase in relative integral intensity ratio of transverse acoustic branch to transverse optical mode and that of longitudinal acoustic branch to transverse optical mode with decreasing mean dimension of nanocrystals and heightening doping ratio, respectively, can be ascribed to disorder. Furthermore, at the same doping level, incorporation of boron can induce higher disorder than incorporation of phosphorus in nc-Si:H films.     

Silane versus silicon tetrafluoride in the growth of microcrystalline silicon films by standard radio frequency glow discharge

Properties of microcrystalline silicon (μc-Si) films produced by standard radio frequency glow discharge from SiH₄ or SiF₄ precursors were studied. Spectroscopic ellipsometry (SE), Raman spectroscopy and time resolved microwave conductivity (TRMC) were used for their characterisation. Although modelling of SE spectra shows that μc-Si films with crystalline fractions up to 100% can be deposited from both types of precursors, we have found that SiF₄ allows obtaining films with improved electronic properties. A detailed analysis of the SE data reveals that such improvement relates to the larger grain size for films deposited from SiF₄ in comparison with the ones deposited from SiH₄. Interestingly, the deposition rate, the fraction of large grains, and the mobility of carriers for the films grown from SiF₄ display close dependencies on the product of pressure and inter-electrode distance. These results are discussed with respect to the contribution of the plasma-synthesised nanocrystals to deposition.     

SiO2层上沉积的纳米多晶硅薄膜及其特性

采用低压化学气相沉积(LPCVD)系统以高纯SiH4为气源,在p型10.16 cm<100>晶向单晶硅衬底SiO2层上制备纳米多晶硅薄膜,薄膜沉积温度为620℃,沉积薄膜厚度分别为30 nm、63 nm和98 nm.对不同薄膜厚度的纳米多晶硅薄膜分别在700℃、800℃和900℃下进行高温真空退火.通过X射线衍射(XRD)、Raman光谱、扫描电子显微镜(SEM)和原子力显微镜(AFM)对SiO2层上沉积的纳米多晶硅薄膜进行特性测试和表征,随着薄膜厚度的增加,沉积态薄膜结晶显著增强,择优取向为<111>晶向.通过HP4145B型半导体参数分析仪对沉积态掺硼纳米多晶硅薄膜电阻I-V特性测试发现,随着薄膜厚度的增加,薄膜电阻率减小,载流子迁移率增大.     

Low-temperature preparation of phosphorus doped μc-Si:H thin films by low-frequency inductively coupled plasma assisted chemical vapor deposition

The phosphorus doped n-type hydrogenated microcrystalline silicon (n-μc-Si:H) thin films are prepared, at the two low substrate temperatures of room temperature and 200 °C, through a low-frequency inductively coupled plasma assisted chemical vapor deposition. The effect of the substrate temperature on the structural properties of the thin films, such as the X-ray Diffraction (XRD) patterns and the Raman spectra, is studied. The XRD measurements show that the diffraction orientations of the thin films present an obvious change when the radio frequency power is increased from 1300 W to 2300 W. The Raman spectra of the thin films deposited at room temperature unambiguously present a phase transition from the amorphous structure to microcrystalline structure whereas no structural phase transition is observed for the thin films deposited at 200 °C. The effect of the substrate temperature on the crystalline volume fraction of the thin films presents a large difference for the radio frequency power in the range of 1300 W-1700 W, while the difference becomes small when the power is increased from 1700 W to 2300 W. The deposition rate and the radio frequency power-sheet resistance curve of the thin films deposited at room temperature are obviously different from those of the thin films prepared at 200 °C. It is attributed to the joint effect of the radio frequency power and substrate temperature on the doping concentration. The electron energy distribution function of the species in the chamber is mainly distributed in a low energy range.     

Influence of filament-to-substrate distance on the spectroscopic,structural and optical properties of silicon carbide thin films deposited by HWCVD technique

Silicon carbide (SiC) thin films were deposited using hot wire chemical vapor deposition (HWCVD) technique from pure silane and methane gas mixture. The effect of filament distance to the substrate on the structural and optical properties of the films was investigated. Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), Raman scattering spectroscopy and UV–Vis–NIR spectroscopy were carried out to characterize SiC films. XRD patterns of the films indicated that the film deposited under highest filament-to-substrate distance were amorphous in structure, while the decrease in distance led to formation and subsequent enhancement of crystallinity. The Si–C bond density in the film structure obtained from FTIR data, showed significant increment with transition from amorphous to nano-crystalline structure. However, it remained almost unchanged with further improvement in crystalline volume fraction. From Raman data it was observed that the presence of amorphous silicon phase and sp ² bonded carbon clusters increased with the decrease in distance. This reflected in deterioration of structural order and narrowing the optical band gap of SiC films. It was found that filament-to-substrate distance is a key parameter in HWCVD system which influences on the reactions kinetics as well as structural and optical properties of the deposited films.     

Study on the mechanism of rapid solid-phase recrystallization of hydrogenated amorphous silicon film by rapid thermal processing

High-quality hydrogenated amorphous silicon films (a-Si:H) were deposited on quartz glass substrates by radio-frequency plasma-enhanced chemical vapor deposition method. The films were then annealed at 800 °C for 3 min by rapid thermal processing (RTP). As confirmed by X-ray diffractometry and Raman spectrometry, hydrogenated microcrystalline silicon films were obtained after the annealing procedure. The mechanism of the rapid solid-phase recrystallization of a-Si:H film by RTP was theoretically mainly attributed to the interaction between short-wavelength photons and ground-state precursor radicals (silicon, SiH₂ and SiH₃).     

用等离子体增强化学气相沉积制备微晶硅薄膜

以Ar+SiH_4作为反应气体,用电子回旋共振等离子体化学气相沉积(ECR PECVD)方法制备微晶硅薄膜,研究了微波功率对薄膜中H含量、薄膜的沉积速率、择优取向和结晶度的影响。结果表明,在300℃制备低温微晶硅薄膜,随着微波功率的增大,薄膜的沉积速率先增大后减小,微波功率为600 W时达到最大;而结晶度和薄膜中的H含量则分别呈现单调增大和单调减少的趋势;使用不同的微波功率,薄膜的择优取向均为(111)方向。     

Enhancement of thermal conductivity of hydrogenated silicon film by microcrystalline structure growth

Hydrogenated silicon film is fabricated by plasma enhanced chemical vapor deposition method, and the enhancement of thermal conductivity of hydrogenated silicon film by microcrystalline structure growth is investigated. The thermal conductivity of films is measured based on Fourier thermal transmitting law by using platinum electrode. Raman spectroscopy characterization reveals the crystalline volume fraction (X _c) of microcrystalline silicon (μc-Si:H) and demonstrates it is embedded with nanocrystals. Spectroscopic ellipsometry with Forouhi–Bloomer model is used to obtain the thickness of films. The measurement results show that the thermal conductivity of μc-Si:H is much higher than amorphous silicon (a-Si:H).     

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.