A comparison of grain nucleation and grain growth during crystallization of HWCVD and PECVD a-Si:H films

From TEM, XRD and Raman measurements, we compare the crystallization kinetics when HWCVD and PECVD a-Si:H films, containing different initial film hydrogen contents (C_H), are crystallized by annealing at 600 °C. For the HWCVD films, the nucleation rate increases, and the incubation time and the full width at half maximum (FWHM) of the XRD (111) peak decrease with decreasing film C_H. However, the crystallization kinetics of HWCVD and PECVD films of similar initial film C_H are quite different, suggesting that other factors beside the initial film hydrogen content affect the crystallization process. Even though the bonded hydrogen evolves very early from the film during annealing, we suggest that the initial spatial distribution of hydrogen plays a critical role in the crystallization kinetics, and we propose a preliminary model to describe this process.     

Studying early time HWCVD growth of a-Si:H by real time spectroscopic ellipsometry

We have applied real time spectroscopic ellipsometry and secondary ion mass spectrometry to study the growth of amorphous silicon by hot-wire chemical vapor deposition. Differences in temperature and hydrogen content affect the optical properties of the film. These effects provide valuable insight into the growth process. We have compared a-Si:H films grown at two different temperatures to better understand these effects. Our studies reveal the presence of a distinct 100–200-thick layer at the top of the growing film. The properties of this layer are primarily determined by the ambient conditions in the growth chamber and appear relatively independent of substrate temperature. In contrast, the properties of the bulk of the film are strongly influenced by substrate temperature. These results imply that differences in film properties associated with substrate temperature are the result of subsurface reconstruction and diffusion processes.     

An alternative method to determine the steady state nucleation rate in thermally annealed HWCVD a-Si:H films

A determination of the steady state nucleation rate r_n in thermally annealed a-Si:H has typically been performed using TEM, where the increase in grain density with isothermal sample anneal time can be directly observed for samples with small crystalline volume fractions. Using the classical model of crystallite nucleation and grain growth, this paper presents an alternative technique for determining r_n using in situ XRD measurements of the crystallization time and EBSD measurements of the final grain size, the latter in fully annealed samples. HWCVD a-Si:H samples containing different as-grown film H contents C_H have been examined by both techniques, and the agreement between these techniques is excellent. R_n is seen to decrease with increasing as-grown film C_H. Differences in the values of r_n are suggested as being due to variations in the transition rate per atom at the amorphous/crystalline interface.     

Rapid crystallization of a-Si:H films with various silicon-to-hydrogen bonding configurations using rapid energy transfer annealing

Hydrogenated amorphous silicon (a-Si:H) films were prepared by changing substrate temperature of plasma-enhanced chemical vapor deposition to induce different contents of monohydride and polyhydride bonds, which were then crystallized into polysilicon (poly-Si) films by rapid energy transfer annealing. Fourier transform infrared and transmission spectra show that the formation of numerous polyhydride bonds increases the hydrogen content and reduces the refractive index of a-Si:H films. The rise in the concentration of polyhydride bonds in as-deposited a-Si:H films can result in the increase of ultraviolet reflectance, small peak shift, and change in full width at half maximum of Raman scattering and X-ray diffraction peaks of the obtained poly-Si films after annealing. These results demonstrate that high-concentration polyhydride bonds can promote the rapid crystallization of a-Si:H and obtain high-crystallinity poly-Si films. Transmission electron microscopy identifies that the poly-Si films have the typical dendrite-like grain structure.     

热丝CVD制备微晶硅薄膜的研究

采用热丝化学气相沉积技术制备微晶硅薄膜,利用X射线衍射谱、Raman散射谱、透射光谱和扫描电子显微镜等检测手段,系统地研究沉积过程中沉积气压对微晶硅薄膜晶粒尺寸、晶态比和光学带隙的影响,运用Tacu法则对薄膜的透射率和厚度进行计算分析,得到薄膜光学带隙与沉积气压之间的关系,结果表明薄膜的光学带隙随着沉积气压的升高而单调下降。     

Effect of substrate temperature on HWCVD deposited a-SiC:H film

Hydrogenated amorphous silicon carbon (a-SiC:H) films were deposited using pure SiH₄ and C₂H₂ without hydrogen dilution by hot wire chemical vapor deposition (HWCVD) technique. The photoluminescence, optical, and structural properties of these films were systematically studied as a function of substrate temperature (T_s). a-SiC:H films deposited at lower substrate temperature (T_s) show degradation in their structural, optical and network properties. The hydrogen content (C_H) in the films was found to be increased with decrease of T_s studied. Photoluminescence spectra shift to higher energy and less FWHM at high T_s. Raman spectroscopic analysis showed that structural disorder increases with decrease in the T_s.     

Revisiting the B-factor variation in a-SiC:H deposited by HWCVD

In order to understand material properties in a better way, it is always desirable to come up with new variables that might be related to the film properties. The B-parameter is such a variable, which relates to the quality of a-SiC:H films both in terms of electronic and optical properties. B (scaling factor) is essentially the slope of the straight-line part of the (E)^1/2–E (Tauc plot). Due to dependence on a large number of parameters and no detailed research, many previous authors have surmised that B has an ambiguous correlation with carbon content. We have made an attempt to establish the relation between the B-parameter as a quality-indicating factor of a-SiC:H films in both carbon- and silicon-rich material. For this we studied a-SiC:H films deposited by the HWCVD method with broad deposition parameters of substrate temperature (T_s), filament temperature (T_F) and C₂H₂ fraction. Our results indicate that the B-parameter varies considerably with process conditions such as T_F, total gas pressure and carbon content. An attempt is made to correlate the B-parameter with an opto-electronic parameter, such as the mobility edge, which has relevance to the device-quality aspects of a-SiC:H films prepared by HWCVD.     

Variation of microstructure and transport properties with filament temperature of HWCVD prepared silicon thin films

Thin films of hydrogenated silicon are prepared by varying the filament temperature (T_F) (1600-1900 °C) at a deposition rate of 8-12 Å/s without using any hydrogen dilution. While the films deposited at low T_F are amorphous in nature, those deposited at higher T_F (≥ 1800 °C) contain nanocrystallites embedded in the amorphous network. The optical band gap (E₀₄) of the films (~ 1.89-1.99 eV) is slightly higher compared to the regular films, which is attributed to the improved short and medium range order as well as the presence of low density amorphous tissues in the grain boundary regions. The films show improved stability under long term light exposure due to more ordered structure and presence of hydrogen mostly as strong Si-H bonds.     

Simulated body fluid (SBF) adsorption onto a-SiC:H thin films deposited by hot wire chemical vapor deposition (HWCVD)

Hydrogen amorphous silicon carbon (a-SiC:H) film deposited by the Hot Wire Chemical Vapor Deposition (HWCVD) technique on silicon substrates were soaked in simulated body fluid (SBF). Characterization of the film with different soaking durations in SBF was carried out by Fourier Transform Infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and contact angle measurements. It was found that the relative amounts of carbon in the a-SiC:H film surface decreases with increase in soaking period. XPS results showed the adsorption of Ca and Mg on the a-SiC:H surface. This indicates the formation of negatively charged surface possibly due to formation of silanol groups or dissolution of carbon to SBF confirming the bioactivity of the material. Contact angle decreased from 74° to 65° during 30 days of soaking in the body fluid. Present study is an attempt to observe the interaction of a-SiC:H film prepared by HWCVD technique with the body environment for its future suitability as artificial heart valve and stent coating materials.     

In situ TEM observations of fracture in nanolaminated metallic thin films

Fracture of single crystal nanolaminated thin films has been investigated through in situ straining of cross-sectional samples of Cu/Ni nanolaminates grown on Cu (001) single crystal substrates. The earlest stages of deformation exhibits a confined layer slip mechanism. With continued straining, unstable fracture occurs creating a mixed-mode crack that propagates across the nanolaminate, roughly perpendicular to the interfaces. Eventually, stable crack growth with intense plastic deformation ahead of the crack tip occurs over many bilayers in the direction of crack growth. Simultaneously, plasticity was seen to spread only 1 or 2 bilayer distances normal to the crack, creating an extremely localized plastic zone. Transmission electron microscopic (TEM) examination after the test did not reveal the presence of dislocations in the crack wake, except where severe crack deflection was observed. By comparison, the plastic zone size in the substrate was greater by several of orders of magnitude.     

Micropore density in a-Si:H and a-Si:H:Cl films

a-Si:H films deposited from various silane-containing gas mixtures have been studied regarding their porosity. Micropore densities of 2.O × 10²cm^-2, 5.0 × 10⁴cm^-2 and 5.0 × 10⁴cm^-2 were determined for a-Si:H:C1, a-Si:H(H₂) and a-Si:H(Ar) films, respectively. It is suggested that these values correlate with the structural properties of the films, so that a-Si:H:Cl films seem to be the most uniform on the microstructural scale.     

Quantitative hydrogen measurements in PECVD and HWCVD a-Si:H using FTIR spectroscopy

A series of hydrogenated amorphous silicon films have been deposited using plasma enhanced chemical vapour deposition (PECVD) and hot-wire chemical vapour deposition (HWCVD) techniques. The total concentration of bonded hydrogen in the films was varied between 3% and 18% as determined by hydrogen effusion measurements. Fourier transform infra-red (FTIR) spectra of the PECVD and HWCVD samples exhibit strong absorption peaks that correspond to Si–H bend and stretch modes, and Si–H₂ stretch modes. A quantitative fit of the FTIR peak areas to the hydrogen effusion concentrations reveals that there is reasonable agreement between the required proportionality constants in PECVD and HWCVD material for the Si–H bend and stretch modes. The uncertainty error for the FTIR proportionality constants is consistently greater for the HWCVD data set, however, which may indicate that the effective dynamical charge of the Si–H dipoles is perturbed in the HWCVD material by bonded impurities that are sourced from the tungsten wire.     

Comparison of growth mechanisms of silicon thin films prepared by HWCVD with PECVD

Hot-wire chemical vapor deposition (HWCVD) and plasma-enhanced chemical vapor deposition (PECVD) of Si thin films show different growth kinetic processes. According to the fractal analysis, the root-mean-square surface roughness δ and the film thickness d have the relation of δ ∼ d^β, where β is the dynamic scaling exponent related to the film growth mechanism. It was found that β is 0.44 for Si films prepared by HWCVD and 0.24 by PECVD. The former refers to a stochastic deposition while the latter corresponds to the finite diffusion of the radicals. Monte Carlo simulations indicate that the sticking process of growth radicals play an important role in determining the morphology of Si films.     

Preparation of zeolite-coated cordierite honeycombs prepared by an in situ crystallization method

Zeolite thin films were prepared on cordierite (Mg₂Al₄Si₅O₁₈) honeycomb substrates by in situ crystallization using soft hydrothermal conditions. The synthesized zeolite films (zeolite A and ZSM-5) were characterized by XRD, FTIR, SEM, NH₃-TPD, and Ns₂ and propane gas adsorption. Zeolite-A films were prepared by dip-coating on a cordierite substrate in a precursor of molar composition of Na₂O:Al₂O_3: SiO_2: H₂O = 4:1:2:100 and heating in an autoclave at 80 °C for 6–10 h. The resultant zeolite-A films consisted of cubic crystals about 2–3 μm in size, achieving a thickness of 20 μm after re-coating. ZSM-5 films were similarly formed using a microporous silica precursor obtained by selective leaching of metakaolinite with tetrapropylammonium hydroxide (TPAOH) as the templating agent. The molar composition of the precursor was NaOH:microporous silica:TPAOH:H₂O = 1:10:1:200. The dipped substrate covered with the wet precursor gel was heated in an autoclave at 150 °C for 24 h. The resultant films were composed of short prismatic <1 μm crystals achieving thickness of several to 10 μm after re-coating. The microstructure and porous properties of the ZSM-5 films were found to change according to the chemical composition and surface treatment of the cordierite substrates. The presence of a SiO₂-rich interfacial layer between the substrate and zeolite film increased the amount of zeolite formed and the physical adsorption but decreased the solid acidity and amount of chemisorption.     

铝诱导非晶硅薄膜快速低温晶化研究

采用金属铝诱导晶化非晶硅薄膜的方法制备多晶硅薄膜。研究了不同的退火温度对a-Si薄膜晶化的影响，采用XRD，Raman，SEM等测试手段分析了实验结果。实验结果发现非晶硅薄膜在400℃下退火20min薄膜仍为非晶结构（a-Si），在450℃下退火20min后非晶硅开始晶化且随着温度的升高，且晶化程度加强。     

Detwinning through migration of twin boundaries in nanotwinned Cu films under in situ ion irradiation

The mechanism of radiation-induced detwinning is different from that of deformation detwinning as the former is dominated by supersaturated radiation-induced defects while the latter is usually triggered by global stress. In situ Kr ion irradiation was performed to study the detwinning mechanism of nanotwinned Cu films with various twin thicknesses. Two types of incoherent twin boundaries (ITBs), so-called fixed ITBs and free ITBs, are characterized based on their structural features, and the difference in their migration behavior is investigated. It is observed that detwinning during radiation is attributed to the frequent migration of free ITBs, while the migration of fixed ITBs is absent. Statistics shows that the migration distance of free ITBs is thickness and dose dependent. Potential migration mechanisms are discussed.     

多晶硅薄膜的铝诱导晶化法制备及其晶粒的择优取向特性

采用铝诱导非晶硅薄膜晶化技术制备了多晶硅薄膜,并研究了多晶硅的成核和生长特性。非晶硅薄膜采用等离子体增强化学气相沉积法制备,其表面沉积铝薄膜后经不同温度的氮氛围退火处理。结果表明,退火后的硅薄膜层与铝层发生置换,所生长的多晶硅颗粒的平均尺寸约为150nm。X射线衍射分析结果揭示,薄膜的晶向显著依赖于退火温度,较低温度下,铝诱导晶化速率较慢,薄膜的优化晶向与非晶硅薄膜中团簇的初始原子排列趋势紧密相关。而较高温度下,铝诱导晶化促使多晶硅(111)择优成核及随后的固相生长。     

In situ synthesis of nanostructured carbon reinforcement in aluminum powders

A novel approach was developed to obtain uniformly dispersed nanostructured carbon reinforcement in aluminum matrix composite powders. This process involves in situ synthesis of carbon nanoflakes (CNFs) with thickness of 30-50 nm on the surface of aluminum powders through Friedel-Crafts alkylation of benzene under its supercritical conditions, at 300 °C and 5.7 MPa with benzene and C₂Cl₆ as carbon sources, and AlCl₃ as catalyst. In this process, all the reactants and catalyst are in gaseous state, which play an important role in inducing homogeneous reaction and dispersion of the CNFs. Meanwhile, Al powders play an important role in inducing the morphology of the CNFs by providing the heterogeneous nucleation sites.     

The silicon neighborhood across the a-Si:H to μc-Si transition by X-ray absorption spectroscopy (XAS)

We report a synchrotron X-ray absorption spectroscopy study of the average neighborhood of Si near the transition from a-Si:H to μc-Si on wedge-shaped samples prepared by hot-wire CVD in a chamber using a movable shutter. The thickness of the wedge varies from 30 to 160 nm. Nucleation of μc-Si occurs at a critical thickness of approximately 100 nm. X-Ray absorption was measured at the Si K-edge (1.84 keV) by total electron photoemission yield. The absorption oscillations in the EXAFS region are very similar to all along the wedge. Analysis indicates an average tetrahedral first neighbor shell with radial disorder decreasing with crystallization. In the near-edge (XANES) region multiple scattering effects appear at the onset of crystallinity. Unlike single crystal silicon, these effects involve only double scattering within the first neighbor shell, indicating an ill-formed second shell in μc-Si.     

Stability of microcrystalline silicon solar cells with HWCVD buffer layer

Microcrystalline silicon solar cells deposited by VHF-PECVD with or without HWCVD grown p/i interface buffer layer were investigated. We studied long-term stability under storage in ambient atmosphere and performed light soaking experiments. Cells with i-layers covering a wide range of crystalline volume fractions were studied. All cells were stable or degraded slightly after storage for 2 years in air, regardless of crystalline volume fraction or presence of p/i buffer interface. Upon light soaking all cells show efficiency degradation to more or less extent depending on crystal volume fraction of the i-layer and the presence of the buffer layer: the solar cell with high crystal volume fraction are nearly stable, cells with high amorphous volume fraction degrade by up to 20%. The solar cell with HWCVD buffer layer shows better stability in the high efficiency range of relative efficiency degradation typically less than 10% after 1000 h AM 1.5 light soaking. The efficiency degradation is mainly caused by Voc and FF deterioration while Jsc is almost stable.