Growth Of Amorphous Silicon Research Articles

Snow-Ice-Inspired Approach for Growth of Amorphous Silicon Nanotips.

The growth of one-dimensional nanostructures without a metal catalyst via a simple solution method is of considerable interest due to its practical applications. In this study, the growth of amorphous silicon (a-Si) nanotips was investigated using an aqueous solution dropped onto the Si substrate, followed by drying at room temperature or below for 24 h, resulting in the formation of a-Si nanotips on the Si substrate. Typically, the a-Si nanotips were up to 1.6 μm long, with average top and middle diameters of 30 and 80 nm, respectively, and contained no metal catalyst in their structure. The growth of a-Si nanotips can be explained in terms of the liquid–solid mechanism, where the supercritical Si solution (liquid) generated on the Si substrate (after reaction with the aqueous solution) promotes the nucleation of solid Si (acting as seeds) on the roughened surface, followed by surface diffusion of Si atoms along the side wall of the Si seeds. This is very similar to the phenomenon observed in the growth of snow ice crystals in nature. When photoexcited at 265 nm, the a-Si nanotips showed blue luminescence at around 435 nm (2.85 eV), indicating feasible applicability of the nanotips in optoelectronic functional devices.

Ni‐Catalyzed Growth of Silica Nanowires From Amorphous Silicon Films and Growth Mechanism

The fabrication of amorphous silica nanowires (NWs) using an amorphous silicon (a‐Si) film grown by plasma‐enhanced chemical vapor deposition is demonstrated. Fabrication is achieved by conventional high‐temperature annealing of an a‐Si layer coated with a thin Ni film. Annealing of the Ni (≈20 nm)/a‐Si (≈1 μm) structure at 1100 °C for 1 h is shown to clearly result in the formation of densely packed amorphous silica NWs with diameters in the range from 35 to 70 nm. A comparison of energy dispersive X‐ray analyses on the tip and body of an NW demonstrates that NW growth proceeded via the solid–liquid–solid mechanism.

Lateral Grain Growth of Amorphous Silicon Films With Wide Thickness Range by Blue Laser Annealing and Application to High Performance Poly-Si TFTs

We report high performance p-type poly-Si thin-film transistors (TFTs) achieved by lateral grain growth of amorphous silicon (a-Si) by using a continuous-wave blue diode laser of wavelength 445 nm. The blue laser beam is efficiently absorbed into the a-Si film, such that full melting and lateral crystallization is achieved in all thicknesses investigated, 50-200 nm. TFTs fabricated with 75-, 100-, and 125-nm-thick poly-Si films laterally grown by the blue laser annealing exhibited field-effect mobility of 108 ± 7, 104 ± 9, and 134 ± 12 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V · s, and subthreshold swing of 210±51, 191±16, and 193±53 mV/decade, respectively, and high ON/OFF current ratio of ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> .

Optimization of the Asymmetric Intermediate Reflector Morphology for High Stabilized Efficiency Thin n-i-p Micromorph Solar Cells

This paper focuses on our latest progress in n-i-p thin-micromorph solar-cell fabrication using textured back reflectors and asymmetric intermediate reflectors, both deposited by low-pressure chemical vapor deposition of zinc oxide. We then present microcrystalline bottom cells with high crystallinity, which yield excellent long wavelength response for relatively thin absorber thickness. In a 1.5-μm-thick μc-Si:H single-junction n-i-p solar cell, we thus obtain a short-circuit current density of 25.9 mA·cm <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX"> $^{-2}$</tex></formula> , resulting in an initial cell efficiency of 9.1%. Subsequently, the roughness of the intermediate reflector layer is adapted for the growth of high-performance amorphous silicon (a-Si:H) top cells. Combining bottom cells with high current, an optimal intermediate reflector morphology and a 0.22-μm-thick a-Si:H top cell, we reach high initial open-circuit voltages of 1.45 V, and we obtain a stabilized cell with an efficiency of 11.1%, which is our best stable efficiency for n-i-p solar cells.

Simulation of the Growth, Structure and Crystallization of the Amorphous Silicon Thin Film

In this paper, using classical molecular dynamics, the growth of the amorphous silicon thin film deposited on the single crystal silicon is simulated and studied by Stillinger-Weber potential. The radial distribution functions of particles are calculated and the Voronoi diagrams of the films are given. The regular and irregular structures in the film are analyzed and the part crystallization condition is discussed. It is found that the features of the film are related with the ratio of the substrate temperature and the temperature of the incident atoms. The density of the deposited silicon film is obtained. The value of the density is about 2.2515g/cm3 which is consistent with the experiment data.

Monte-Carlo simulation of hydrogenated amorphous silicon growth

A Monte-Carlo simulation of the growth of hydrogenated amorphous silicon (a-Si:H) thin films deposited by plasma enhanced chemical vapour deposition technique is presented in this work which is based on four fundamental processes that determine the layer growth: (i) random deposition of SiH3 radicals, (ii) desorption, (iii) “H” abstraction and (iv) sticking on a dangling bond. The essential goal of the model is to predict the bulk and the surface properties of films (hydrogen content, dangling bonds, surface roughness...) having different thicknesses and deposited at different substrate temperatures. The effects on the film properties of the incident radical flux of SiH3, directed towards the surface isotropically, on the surface are examined. A rate of content of hydrogen (8–12%) in the bulk is found. We find that surface roughness increases with increasing film thickness, though thin films (<20 mono-layers) have large hydrogen fractions on surface layer with a thickness approximately equal to the surface roughness. We also find a correlation between the average thickness and the incident radical flux of SiH3.

Evaluation of Optical Waveguide by Solid Phase Epitaxial Growth of Amorphous Silicon

This study reports the evaluation of an optical waveguide fabricated on a bulk-Si platform by using an epitaxially grown Si layer. The epitaxial growth of Si is obtained by the solid phase epitaxy (SPE) using a hot furnace annealing method. It is observed that an optical propagation loss of the waveguide decreases with increasing the epitaxially grown area in the waveguide. In this study, an epitaxial growth rate is greatly enhanced by an (100) growth orientation due to the lower atomic close packing density along the (100) orientation. The optical propagation loss is also improved with the (100) growth orientation because of the higher crystalline quality and full epitaxial growth of Si.

氢稀释对FTS沉积a-Si∶H薄膜结构特性的影响

Hydrogenated amorphous silicon films were deposited by facing target sputtering with different hydrogen dilution ratio with H2 as reaction gas.The growth rate and the optical properties of a-Si∶H films with different hydrogen dilution ratio were characterized by surface profiler,Fourier-transform infrared spectroscopy,Raman scattering and ultraviolet-visible transmittance spectrum.It was found that,the low temperature and high deposition of a-Si∶H films can be deposited by facing target sputtering method.With increasing hydrogen dilution,a trend that the deposition rate of a-Si∶H films first decreases and then increases is showed.Fourier transform infrared transmission spectra show that hydrogen content of a-Si∶H films increases first and then becomes smaller.While,through analysising the Raman spectra and UV-visible light transmission spectrum,the degree of order and the optical band gap also first increase and then decrease.Obviously,the film structure can be effectively controlled by the change of the hydrogen dilution with this technology.

Remote plasma deposition of microcrystalline silicon thin-films: Film structure and the role of atomic hydrogen

Microcrystalline silicon films grown in an expanding thermal plasma, i.e. in the absence of ion bombardment, are found to be porous and rich in nano-sized voids. By carrying out an extensive investigation on the material quality of films deposited in the amorphous-to-microcrystalline transition regime, on the microcrystalline silicon growth development, and on the influence of the substrate temperature, it is concluded that the inferior material quality is related to the lack of a sufficient amount of amorphous silicon tissue. As possible cause for the insufficient amount of amorphous silicon tissue, the interaction of atomic hydrogen with amorphous silicon films has been studied in order to highlight a possible competition between film growth and H-induced etching of amorphous silicon, and between film growth and H-induced surface/film modification. The etch rates obtained are too low to compete with film growth. Furthermore, atomic H cannot be considered responsible for the poor quality of amorphous tissue present in the microcrystalline silicon films, as the H up-take mainly takes place in divacancies. These results suggest that ion bombardment may be a necessary condition to provide good quality microcrystalline silicon films.

Evaluation of Optical Waveguide by Solid Phase Epitaxial Growth of Amorphous Silicon

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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.

Study on amorphous silicon thin film by aluminum-induced crystallization

With the rapid development photovoltaic industry, crystallization techniques play an important role in silicon thin film solar cells. In this paper, Amorphous silicon thin film and aluminum films were prepared by electron beam evaporation technique. The films after different annealing crystallization processes were characterized by means of X-ray diffraction, Raman spectra and scanning electron microscopy. We have the emphasis on discussing the influence parameters for the amorphous silicon crystallization quality and growth mechanism. In addition, Al/Si interface was also studied. A layer exchange process occurs between Al layer and Si layer.

Amorphous silicon and silicon nitride channel optical waveguides

AbstractIn this work we report on the characterisation of multimode rectangular ridge waveguides fabricated by Plasma Enhanced Chemical Vapor Deposition and anisotropic etching using different cores, consisting of hydrogenated amorphous silicon and amorphous silicon nitride. The propagation losses have been measured at several wavelengths by an end‐fire coupling set‐up and compared to intrinsic absorption of the core materials.Two‐dimensional photonic crystal structures have been carved onto the channel waveguides by Focused Ion Beam lithography demonstrating the possibility to fabricate complex optical elements merging the consolidated amorphous silicon growth technology with ion‐beam nanomachining. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

High Quality, Low Bandgap a-Si Films and Devices Produced Using Chemical Annealing

AbstractWe report on the growth and properties of novel amorphous Silicon (a-Si:H) p-i-n devices prepared using chemical annealing with argon gas. The i layer in the p-i-n devices was grown using a layer by layer approach, where the growth of a very thin a-Si:H layer (7-30 angstroms) grown using a silane:argon mixture was followed by chemical anneal by argon ions. Repeated cycling of such growth/anneal cycles was used to produce the desired total thickness of the i layer. The thickness of the a-Si layer, and duration of the anneal time, were varied systematically. Pressure and power of the plasma discharge were also systematically varied. It was found that a thin a-Si layer, &lt;10 angstroms, and low pressures which led to relatively high ion flux on the surface, gave rise to a significantly smaller bandgap in the device, as indicated by a significant lateral shift in the quantum efficiency vs. photon energy curve to lower energies. The smallest Tauc gap observed was in the range of 1.62 eV. Corresponding to this smaller bandgap, the current in the solar cell increased, and the voltage decreased. The Urbach energies of the valence band tail were also measured in the device, using the quantum efficiency vs. energy curve, and found to be in the range of45 meV, indicating high quality devices. Too much ion bombardment led to an increase in Urbach energy, and an increase in defect density in the material. Raman spectra of the device i layer indicated an amorphous structure. When hydrogen was added to argon during the annealing cycle, some materials turned microcrystalline, as indicated by the Raman spectrum, and confirmed using x-ray diffraction.

Phase evolution in nanocrystalline silicon films: Hydrogen dilution and the cone kinetics model

Our cone kinetics model of heterogeneous thin film growth explains the evolution of the crystalline inclusions that form during chemical vapor deposition (CVD) growth of silicon films. Various morphologies, including isolated crystallites and conical nanocrystalline formations, form during plasma-enhanced (PE) and other CVD techniques when isotropic growth is coupled with the point nucleation of a second phase with a higher growth rate. Cone angles are determined by the relative growth rates alone. By generalizing the physics of cone formation, we create a qualitative phase diagram that predicts film morphology from two factors during deposition: the relative amorphous and crystalline growth rates and the rate of crystallite nucleation. It is well known that both of these factors are influenced by the H-dilution of the film precursor gases. Analysis of the statistics of cone geometries and densities in published experimental data on PECVD-grown nanocrystalline silicon shows that the cone angle increases monotonically with H-dilution. In the context of the cone kinetics model, this implies that increased H-dilution results in an increase of the ratio of the nanocrystalline silicon growth rate (νnc) to the amorphous silicon growth rate (νa). At a specific H-dilution, νnc/νa exceeds unity and nanocrystalline cone formation results.

Preparation of microcrystalline silicon solar cells on microcrystalline silicon carbide window layers grown with HWCVD at low temperature

N-type microcrystalline silicon carbide layers prepared by hot-wire chemical vapor deposition were used as window layers for microcrystalline silicon n–i–p solar cells. The microcrystalline silicon intrinsic and p-layers of the solar cells were prepared with plasma-enhanced chemical vapor deposition at a very high frequency. Amorphous silicon incubation layers were observed at the initial stages of the growth of the microcrystalline silicon intrinsic layer under conditions close to the transition from microcrystalline to amorphous silicon growth. ‘Seed layers’ were developed to improve the nucleation and growth of microcrystalline silicon on the microcrystalline silicon carbide layers. Raman scattering measurement demonstrates that an incorporation of a ‘seed layer’ can drastically increase the crystalline volume fraction of the total absorber layer. Accordingly, the solar cell performance is improved. The correlation between the cell performance and the structural property of the absorber layer is discussed. By optimizing the deposition process, a high short-circuit current density of 26.7mA/cm2 was achieved with an absorber layer thickness of 1μm, which led to a cell efficiency of 9.2%.

Modeling and experiments of high-pressure VHF SiH 4/H 2 discharges for higher microcrystalline silicon deposition rate

The effect of the total gas pressure (1–3 Torr) on the growth rate and the crystalline volume fraction (Xc) of films deposited from 70 MHz SiH 4/H 2 plasmas is investigated. The films were grown in conditions near the transition from microcrystalline to amorphous silicon growth at high deposition rates (10 Å/s). Simultaneously, a two-dimensional self-consistent fluid model of diluted SiH 4 in H 2 discharges was used to simulate the deposition process. The model integrates a detailed gas phase and surface chemistry including 25 species, 85 gas phase reactions and 27 surface reactions. A good agreement was found between model and experiments concerning the deposition rate. On this basis, the effects of pressure on the electron density and temperature, discharge impedance, electric field distribution as well as on the distribution and importance of the films precursors is discussed.

Real-time spectroscopic ellipsometry studies of the growth of amorphous and epitaxial silicon for photovoltaic applications

In situ monitoring of material properties during thin-film deposition provides researchers with a valuable tool for maximizing solar cell performance, while also enabling efficient exploration of deposition parameter space. This article describes how our research team has utilized in situ real-time spectroscopic ellipsometry (RTSE) to maximize our productivity in two related projects. We are using hot wire chemical vapor deposition (HWCVD) for low-temperature (90–235 °C) deposition of very thin films of amorphous hydrogenated silicon for amorphous Si∕crystal-Si heterojunction (SHJ) solar cells. We are also using HWCVD for low-temperature (200–440 °C) deposition of epitaxial films of silicon on crystal-Si substrates. We utilize RTSE as both an in situ diagnostic and a postgrowth analysis tool for SHJ solar cells and epi-Si films grown by HWCVD. Using input from RTSE analysis we have achieved a photovoltaic energy conversion efficiency of 17.1% on an Al-backed p-type float-zone c-Si wafer. Epi-Si films have been grown as thick as 500 nm utilizing parameter optimization based on RTSE analysis.

Improving stability of amorphous silicon using chemical annealing with helium

We report on the growth and properties of amorphous silicon (a-Si:H) materials and devices prepared using a chemical annealing technique. The chemical annealing technique consists of a layer-by-layer growth, where a thin a-Si:H layer is followed by annealing in a plasma beam of inert ions (helium). We show that high quality materials and devices can be fabricated using chemical annealing with helium. Infrared absorption data show that chemical annealing significantly improves the hydrogen microstructure and this improved hydrogen microstructure leads to improved stability of both materials and devices.

Effect of plasma parameters on the amorphous to microcrystalline silicon transition

A study of the role of the main plasma parameters on the transition from microcrystalline to amorphous silicon growth was performed, considering also the possibility to use plasma diagnostics for the prediction of this transition. Experimental measurements have shown that charged species do not play a significant role in the transition while Spatially Resolved Emission Spectroscopy was found to present a potential to be used as an in-situ diagnostic tool for the prediction of the transition. Calculations of the ion and hydrogen atom fluxes have shown that they both decrease in the transition region; however the hydrogen atoms flux was found to be about three orders of magnitude higher compared to the ion flux, implying a less important role of the later species in the transition. Possible scenarios concerning the energy transfer from hydrogen atoms and ions to the surface are presented and the limited case were ions can play a significant role on the deposition process is discussed.