Surface passivation of crystalline silicon by Cat-CVD amorphous and nanocrystalline thin silicon films

In this work, we study the electronic surface passivation of crystalline silicon with intrinsic thin silicon films deposited by Catalytic CVD. The contactless method used to determine the effective surface recombination velocity was the quasi-steady-state photoconductance technique. Hydrogenated amorphous and nanocrystalline silicon films were evaluated as passivating layers on n- and p-type float zone silicon wafers. The best results were obtained with amorphous silicon films, which allowed effective surface recombination velocities as low as 60 and 130 cm s⁻¹ on p- and n-type silicon, respectively. To our knowledge, these are the best results ever reported with intrinsic amorphous silicon films deposited by Catalytic CVD. The passivating properties of nanocrystalline silicon films strongly depended on the deposition conditions, especially on the filament temperature. Samples grown at lower filament temperatures (1600 °C) allowed effective surface recombination velocities of 450 and 600 cm s⁻¹ on n- and p-type silicon.     

掺硼nc-Si:H薄膜中纳米硅晶粒的择优生长

利用等离子体增强化学气相沉积(PECVD)生长的系列掺硼氢化纳米硅(nc-Si:H)薄膜中纳米硅晶粒(nanocrystallinesilicon,简称nc-Si)有择优生长的趋势。用HRTEM、XRD、Raman等方法研究掺硼nc-Si:H薄膜的微观结构时发现:掺硼nc-Si:H薄膜的XRD只有一个峰,峰位在2θ≈47o,晶面指数为(220),属于金刚石结构。用自由能密度与序参量的关系结合实验参数分析掺硼nc-Si:H薄膜择优生长的原因是:适当的电场作用引起序参量改变,导致薄膜在适当的自由能范围内nc-Si的晶面择优生长。随着掺硼浓度的增加,nc-Si:H薄膜的晶态率降低并逐步非晶化。nc-Si随硅烷的稀释比增加而长大,但晶态率降低。nc-Si随衬底温度升高而长大,晶态率提高。nc-Si随射频功率密度的增大而长大,晶态率增大的趋势平缓。但未发现掺硼浓度、稀释比、衬底温度、射频功率密度的变化引起薄膜中nc-Si晶面的择优生长。     

纳米硅薄膜及其太阳电池研究进展

纳米硅薄膜具有新颖的结构特征和一系列独特的物理性质,可望应用于新型光电子器件、量子功能器件、集成电路等领域.本文综述了纳米硅薄膜的研究现状及其优良的光电性能和纳米硅薄膜太阳电池的研究进展,指出在生产制备与性能方面纳米硅薄膜太阳电池所具有的优势,具有良好的发展前景.     

Surface effects in batch galvanizing of silicon containing steels

Abstract

There is a significant commitment by industry to understand the mechanism of the silicon induced modification of the Fe–Zn coating during batch galvanising. Considerable emphasis has been placed on solving this problem through investigation of substrate surface treatment. This article reviews pertinent literature dealing with surface effects associated with galvanising of silicon containing steels. Relevant observations were grouped together to help to establish the influence of crystallographic orientation, subsurface oxidation, strain energy, and topography on the formation of a reactive coating. This includes a critical discussion of the major factors and their interaction with thermodynamic instabilities informing reactive structure. It was found from the literature that subsurface oxidation and topographical effects had the greatest influence on the silicon reactivity phenomenon. However, these observations do not directly lead to economically viable solutions to the problem.

MST/1173     

Size modulation of nanocrystalline silicon embedded in amorphous silicon oxide by Cat-CVD

Different issues related to controlling size of nanocrystalline silicon (nc-Si) embedded in hydrogenated amorphous silicon oxide (a-SiO_x:H) deposited by catalytic chemical vapor deposition (Cat-CVD) have been reported. Films were deposited using tantalum (Ta) and tungsten (W) filaments and it is observed that films deposited using tantalum filament resulted in good control on the properties. The parameters which can affect the size of nc-Si domains have been studied which include hydrogen flow rate, catalyst and substrate temperatures. The deposited samples are characterized by X-ray diffraction, HRTEM and micro-Raman spectroscopy, for determining the size of the deposited nc-Si. The crystallite formation starts for Ta-catalyst around the temperature of 1700 °C.     

From nanoparticles to nanocrystalline bulk: percolation effects in field assisted sintering of silicon nanoparticles

Nanocrystalline bulk materials are desirable for many applications as they combine mechanical strength and specific electronic transport properties. Our bottom-up approach starts with tailored nanoparticles. Compaction and thermal treatment are crucial, but usually the final stage sintering is accompanied by rapid grain growth which spoils nanocrystallinity. For electrically conducting nanoparticles, field activated sintering techniques overcome this problem. Small grain sizes have been maintained in spite of consolidation. Nevertheless, the underlying principles, which are of high practical importance, have not been fully elucidated yet. In this combined experimental and theoretical work, we show how the developing microstructure during sintering correlates with the percolation paths of the current through the powder using highly doped silicon nanoparticles as a model system. It is possible to achieve a nanocrystalline bulk material and a homogeneous microstructure. For this, not only the generation of current paths due to compaction, but also the disintegration due to Joule heating is required. The observed density fluctuations on the micrometer scale are attributed to the heat profile of the simulated powder networks.     

Features of photoacoustic transformation in microporous nanocrystalline silicon

Results of a study of the photoacoustic transformation in microporous nanocrystalline silicon are described. The amplitude-frequency and phase-frequency characteristics of the photoacoustic signal from microporous silicon samples on a monocrystalline substrate exposed to illumination at various wavelengths are experimentally determined. Informative response was measured by the gas-microphone and piezoelectric detection methods. In terms of the proposed mathematical model, it is shown that the difference in the parameters of the photoacoustic signal for different wavelengths of exciting radiation is attributed to a shift of the fundamental absorption edge in nanocrystalline silicon. It is pointed out that the piezoelectric detection method is more sensitive to changes in the thermophysical and optical parameters of the porous layer.     

The quantum size effects on the surface potential of nanocrystalline silicon thin film transistors

The impact of the grain size of nc-Si (nanocrystalline silicon) on the surface potential of doped nc-Si TFTs (thin film transistors) is discussed. Quantum size effects cause the change in both band gap and dielectric constant of nc-Si. Numerical calculation of the surface potential in nc-Si TFTs shows that the diameter of nc-Si has a larger effect on the surface potential of nc-Si TFTs. The results demonstrate that, for medium size (7 - 50 nm), the change in the band gap of nc-Si should be considered, whereas, for small size (< 7 nm), the change in the dielectric constant of nc-Si should be considered. A simplified surface potential equation for nc-Si TFTs under strong inversion condition is proposed, and shows good agreement with the original equation via numerical calculation.     

Novel aspects in thin film silicon solar cells-amorphous, microcrystalline and nanocrystalline silicon

The improvement of photodegradation of a-Si:H has been studied on the basis of controlling the subsurface reaction and gaseous phase reaction. We found that higher deposition temperature, hydrogen dilution and triode method are effective to reduce the SiH₂ density in the film and to suppress the photodegradation of solar cells. These results are explained in terms of the hydrogen elimination reaction in the subsurface region and the contribution of the higher silane radicals to the film growth. The high-rate deposition of μc-Si:H was obtained by means of a high-pressure method and further improvement in deposition rate and the film quality was achieved in combination with the locally high-density plasma, which enables effective dissociation of source gases without thermal damage. It was also found that the deposition pressure is crucial to improve the film quality for device. This technique was successfully applied to the solar cells and an efficiency of 7.9% was obtained at a deposition rate of 3.1 nm/s. The potential application of nanocrystalline silicon is also discussed.     

Photoluminescence properties of nanocrystalline ZnS on nanoporous silicon

This paper embodies the report on the microwave solvothermal synthesizing of nanocrystalline ZnS particles for optoelectronic device. The effect of different parameters such as time, temperature, solvents, molar ratio of zinc and thiourea on the phase(s) formation of nanocrystalline Zinc Sulphide was investigated. The obtained nanosize ZnS materials were characterized by the X-ray diffraction, Optical absorption measurements, TEM and Photoluminescence studies. The crystallite size of the ZnS nanoparticles was estimated from the X-ray diffraction pattern by using Scherrer's formula. The as prepared material was obtained in the cubic phase, which showed a perfect match with the earlier reports. The Optical absorption edge of ZnS were blue shifted from the absorption edge of bulk ZnS. The estimated band gap value of ZnS was 4.01 eV. The ZnS nano materials were coated on nano porous silicon by screen-printing technique. Luminescence studies indicated room temperature emission in the wavelength ranges from 422.6 to 612 nm, which cover the blue emission to red emission. The emitted light that depending on the created pore size from porous silicon and the size of the ZnS nano particles.     

Photoluminescence characteristics of oxidized hydrogenated nanocrystalline silicon film

We have observed visible photo-luminescence (PL) from an oxidized hydrogenated nanocrystalline silicon (nc-Si:H) film which was prepared in a plasma enhanced chemical vapor deposition (PECVD) system and post-treated by thermal oxidization processes. At low oxidization temperature (T_ox) below 500 °C, silicon oxyhydrides and silicon oxides are formed at the surface of grains; while at high T_ox above 500 °C, the surface of grains is covered by α-SiO₂. PL around 650 nm-750 nm is observed as T_ox ranges from 100 °C to 700 °C during which the grain size (d_c) varies from 2.7 nm to 5.1 nm. At T_ox > 700 °C, the d_c is larger than 5.1 nm and PL peak shifts to 920 nm. The quantum size effect and surface states model was employed to explain our experimental results.     

Formation mechanism of nanocrystalline high-pressure phases in silicon during nanogrinding

The phase transformations of Si under nanogrinding have been studied by transmission electron microscopy and Raman spectroscopy. Nanocrystalline high-pressure phases (Si-III/Si-XII) were found in the amorphous layer of the subsurface of heavily ground Si. The sequence of the phase transformation in nanogrinding has been found to be different to that in nanoindentation. The formation mechanism of the nanocrystalline high-pressure phases in nanogrinding is proposed based on experimental results.     

Switching of single-electron oscillations in dual-gated nanocrystalline silicon point-contact transistors

Switching of single-electron transport is observed in point-contact transistors fabricated in nanocrystalline silicon thin films, where the grain size is /spl sim/10 to 40 nm. The effects may be associated with electrostatic coupling between the grains. At 4.2 K, single-electron oscillations in the device current are switched as a function of the voltages on two separate gates. This is investigated further using single-electron Monte Carlo simulation of a model with two charging grains in parallel and intergrain capacitive coupling. A change in the electron number of a grain occurs due to charging of the other grain by a single electron, causing bistable regions in charge stability versus gate voltage. These effects depend not only on the coupling capacitance but also on the cross capacitances between the grains and the two gates.     

Surface stress effects on the critical buckling strains of silicon nanowires

The objective of this paper is to quantify how nanoscale surface stresses impact the critical buckling strains of silicon nanowires. These insights are gained by using nonlinear finite element calculations based upon a multiscale, finite deformation constitutive model that incorporates nanoscale surface stress and surface elastic effects to study the buckling behavior of silicon nanowires that have cross sectional dimensions between 10 and 25 nm under axial compressive loading. The key finding is that, in contrast to existing surface elasticity solutions, the critical buckling strains are found to show little deviation from the classical bulk Euler solution. The present results suggest that accounting for axial strain relaxation due to surface stresses may be necessary to improve the accuracy and predictive capability of analytic linear surface elastic theories.     

Enhancement of electroluminescent properties in ethanol dispersible nanocrystalline silicon particles

We have fabricated two kinds of electroluminescent (EL) devices of which the luminous layer differs in order to improve an amount of injected carriers into the nanocrystalline silicon (nc-Si) particles. These EL devices showed red luminescence by the injection of carriers into the nc-Si particles after applying the direct current voltage. A large amount of carriers could be injected in the EL device prepared by the adhesion of ethanol dispersible nc-Si particles onto the dimples of Si substrate. The increase in the amount of injected carriers led to the enhancement of luminescent intensity. These results were achieved by the reduction of oxide layer surrounding nc-Si particles and the use of nc-Si particles without the nonradiative recombination centers (Pb centers).     

Surface plasmon resonance in nanocrystalline gold-copper alloy films

Nanocrystalline Au(x)Cu(1-x) films were synthesized by depositing Cu/Au/Cu multilayer in nanocrystalline thin film form with requisite thickness of individual layers onto fused silica substrates by high pressure sputtering technique. The absorbance spectra showed only one surface plasmon peak for all the compositions with the exception that the peak position did not indicate gradual shift as gold concentration was increased. Peak position for the two compositions corresponding to the two superlattice structures, AuCu3 and AuCu, deviated significantly from linear variation. The experimental results have been discussed in light of the existing Mie theory and the Core-shell model.     

Amorphous and nanocrystalline silicon growth on carbon nanotube substrates

In this study, smooth and conformal hydrogenated silicon thin films are examined and analyzed on various multi-walled carbon nanotube (MWCNT) substrates. The films are deposited using radio-frequency plasma-enhanced chemical vapor deposition with He dilution and parameters that are heavily in the γ regime. It is proposed that high-energy plasmas with limited penetration depth can induce crystallization to occur on MWCNT substrates of varying active surface areas. The samples presented exhibit properties that are promising for energy applications, including photovoltaics and lithium-ion batteries and have been studied using scanning electron microscopy, Raman spectroscopy, X-ray diffraction, UV-Vis spectrophotometry, four-point probe measurements, and Fourier transform infrared spectroscopy.     

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.     

Growing nanocrystalline silicon on sapphire by molecular beam epitaxy

It is established that an array of silicon nanoislands is formed on the surface of a sapphire substrate at the initial stages of molecular beam epitaxy. Silicon islands formed at low temperatures of the substrate (below 650°C) exhibit a pyramidal shape, while the islands grown at T > 650° possess a dome shape. The maximum density of islands was 2 × 10¹¹ cm^?2, their lateral dimensions were within 20 nm, and their heights did not exceed 3 nm.     

Preparation of ITO thin films applied in nanocrystalline silicon solar cells

ITO thin films were prepared by changing the experimental parameters including gas flow ratio, sputtering pressure and sputtering time in DC magnetron sputtering equipment. The stable experimental parameters of Ar flow at 70 sccm, O₂ flow at 2.5 sccm ∼ 3.0 sccm, sputtering pressure around 0.5 Pa, and sputtering time of 80 s were obtained. Under these parameters, we had achieved the ITO thin films with low resistivity (<4 × 10⁻⁴ Ω ∙ cm) and high average transmissivity (95.48%, 350 nm ∼ 1100 nm). These ITO thin films were applied in nanocrystalline silicon solar cells as top transparent conductive layer. The solar cell test result showed that the open circuit voltage (Voc) was up to 534.9 mV and the short circuit current density (Jsc) was 21.56 mA/cm².