Renat Bilyalov

Self-Standing Porous Silicon Films by One-Step Anodizing

A novel technique for the formation and lift-off of thin porous silicon films from starting substrates in a single step by electrochemical etching in hydrofluoric acid-based solutions is described. Lift-off or separation of porous silicon (PS) film occurs under specific sets of current density and HF concentration, which also determines the PS film thickness that can vary from a few micrometers to a few tens of micrometers. A model based on a diffusion-limited mass transfer of HF molecules from the bulk of the solution to the point of reaction at the pore tip is proposed to explain the lift-off phenomena. Based on this an expression for expected PS film thickness and separation time as a function of current density and hydrofluoric acid concentration has been derived. The experimental results are in agreement with the proposed medel.

Comparative Analysis of Chemically and Electrochemically Formed Porous Si Antireflection Coating for Solar Cells

From the viewpoint of the porous Si (PS) application as an antireflection coating (ARC) in crystalline Si solar cells, a comparative study has been made on the morphological, kinetic, and optical properties of both chemically and electrochemically etched PS. These results are correlated and interpreted in terms of a simplified empirical model for the simultaneous PS formation and dissolution of Si. The fact that PS mainly carries potential as an ARC stresses the importance of the reflectance study. Chemically etched (stain-etched) PS has a pronounced porosity gradient in depth which is the result of a simultaneous etching at the PS/Si interface and Si dissolution throughout the existing porous layer. This implies that a part of the solar cell emitter is removed during stain etching. The predominant etching at the PS/Si interface in the case of electrochemically etched PS only converts part of the solar cell emitter into a porous layer and results in a nearly constant porosity in the thickness range of interest (up to 200 nm) for solar cell applications. Multicrystalline Si (mc-Si) behaves strongly similar to monocrystalline Si when applying an electrochemical porous etching, while stain etching is intrinsically more sensitive to the Si surface structure. The typical reflectivity behavior of both types of PS is interpreted in terms of the morphological differences. The integrated reflectance of both electrochemically formed and stain-etched porous layers is comparable to the value obtained for a silicon nitride coating on textured mc-Si, which stresses the potential of a PS ARC for crystalline Si solar cells.

New Approach for the Formation and Separation of a Thin Porous Silicon Layer

A new one-step method is developed for the formation and separation of a thin porous silicon film (PSF) by electrochemical etching of silicon in hydrofluoric (HF) acid based solution. This provides advantages over the existing techniques, which are requiring multiple-steps for fabrication and separation of a thin layer from the substrate. The in-situ change in the fluoride ion concentration results in the formation of a high porosity layer beneath the low porosity layer under the same formation condition, which is confirmed by detailed morphological analysis. In-situ separation of a thin porous silicon film from the substrate is obtained by carrying out an anodization for sufficiently long time. A two-step approach is also proposed which consists of an electrochemical etching followed by an electrochemical polishing step and provides a better control over the separation condition than the one-step approach.

Transmission electron microscopy investigation of the crystallographic quality of silicon films grown epitaxially on porous silicon

Epitaxial growth of thin crystalline layers on porous silicon can provide opportunities for silicon-on-insulator applications and silicon-based solar cells provided that the epitaxial layer has a sufficient crystallographic quality. Transmission electron microscopy (TEM) is used in this work to study the microstructural properties of porous silicon (PS) and of epitaxial Si layers grown on top of the PS. A more dense silicon layer exists in the upper part of the as-prepared porous silicon. The quality of the epitaxial layers is found to depend strongly on the morphology of the initial porous Si layers, and on the deposition temperature of the epitaxial silicon growth. The porous structure is completely destroyed after thermal CVD deposition of Si at too high temperature, resulting in a highly defective epitaxial layer. A high-quality epi-Si layer is obtained when depositing on a low porosity layer at 725°C. A stacked porous layer with a low porosity in the upper-part and a high porosity in the bulk can be formed by changing the conditions during the formation of the layer. On such a dual porous layer, an epitaxial silicon layer with a low defect density can be grown.

Crystalline silicon thin films with porous Si backside reflector

Thin film crystalline Si solar cells on cheap Si-based substrates have a large potential in PV technology. Optical light confinement is a very crucial point of such thin film structures. Porous Si (PS) as a perfect light diffuser could be used as a backside reflector if its multi-layer structure would be preserved during the deposition of a thin Si film. That is why low-energy plasma enhanced chemical vapor deposition (LEPECVD) was chosen to deposit a thin Si film on a PS multilayer structure at low temperature and high deposition rate. This technique allows one to deposit a Si film with epitaxial quality on the top of PS without destroying its multi-layer structure as revealed by high-resolution X-ray diffraction and cross-sectional transmission electron microscopy (TEM). The epi-layers of 10 μm are grown at very high deposition rates (approx. 3 nm/s) at 590°C. TEM-analysis reveals that during the deposition a high density of defects forms at the PS/epi-Si interface and spreads through the whole epi-layer. The defect density is decreased when the deposition temperature is increased to 645°C. LEPECVD appears to be an appropriate deposition technique to grow thin Si films on cheap Si based substrates with PS reflector.

Transfer of a thin silicon film on to a ceramic substrate

Abstract In a scenario of large volume photovoltaic production, the cost of the Si starting substrate is almost 70% of the total module level cost. In such cases thin film crystalline silicon technology has large potential to reduce the cost of solar cells if a method to combine a high-quality Si film with a low-cost substrate is found. A process of transferring a thin porous silicon layer (PSL) onto a ceramic substrate like alumina is described. Separation of PSL from its parent Si substrate is obtained either by double porosity layer formation followed by high temperature annealing at 1050°C in H2 or by carrying out electrochemical etching for a sufficiently long time at constant formation parameters. The transfer of PSL to alumina substrate is carried out using spin-on oxide or pyrolytic oxide as an intermediate layer. The quality of the transfer is checked by means of a scratch test. Reflectance characteristics of PSL transferred onto alumina substrate reveal an effective light passing.

Microcrystalline silicon films for solar cells obtained by gas-jet electron-beam PECVD method

The characteristics of thin microcrystalline silicon films obtained by gas-jet electron-beam plasma enhanced chemical vapor deposition (GJ-EB PECVD) method are investigated from the viewpoint of their use for solar cell technology. This method provides a very high deposition rate of 5-10 nm/s in the temperature range of 150-430/spl deg/C. Morphological quality of the films is investigated by means of transmission electron microscopy and Raman spectroscopy. It is shown that a morphological structure of the films consists a mixture of amorphous and microcrystalline fractions. Preliminary electrical characterization of the films is performed using steady-state photoconductivity measurements. The results show a certain potential of GJ-EB PECVD for a solar cell application.

Firing through Porous Silicon Antireflection Coating for Silicon Solar Cells

The new idea to realize contacts through a porous silicon (PS) antireflection coating (ARC) is tested for the first time from the viewpoint of PS eligibility towards a rapid high-temperature process. The morphological analysis (TEM and SE) reveals that the thickness of PS after the thermal treatment is slightly increased accompanied by an increase of its porosity. As a result no significant changes in reflectance characteristics are observed. Therefore it can be successfully used as an alternative way for preparation of PS ARC for silicon solar cells.

A new high-rate deposition method for thin film crystalline Si solar cells

A new gas-jet electron beam plasma enhanced chemical-vapor deposition (GJEB PECVD) method for high-rate deposition of crystalline silicon films is presented. The method is based on activation of initial gas molecules in an electron beam plasma and convective transfer of the radicals to a substrate by means of a supersonic free jet. The deposition of micro- and polycrystalline Si films is done at a growth rate of 10-20 nm/sec on standard ceramic and stainless steel substrates in the temperature range of 400-650/spl deg/C. The morphological (TEM, SEM, Raman spectroscopy) and optical (spectroscopic ellipsometry) analysis reveals that the grains have a columnar structure and an average size between 300-1000 nm depending on the growth conditions. Since the conditions in a supersonic gas jet and in the deposition zone are relatively independent on the conditions in a vacuum chamber, the deposition can be made without using an ultrahigh vacuum chamber. This feature in combination with a high deposition rate makes this gas-jet method very attractive for high throughput deposition of micro- and polycrystalline Si thin films on foreign substrates for further solar cell application.

Comparison between SiNx:H and hydrogen passivation of electromagnetically casted multicrystalline silicon material

Abstract This work intends to compare two different passivation methods for electromagnetically continuous pulling silicon (EMCP): remote plasma hydrogenation and remote plasma enhanced CVD of SiN followed by high-temperature sintering. All experiments are carried out on textured and non-textured EMCP samples from the same ingot. To check the effect of high-temperature diffusion on EMCP, a n+-emitter is formed on one group of the samples using POCl3 diffusion. Passivation capabilities of both techniques are checked using measurements of minority carrier lifetime by means of microwave photoconductance decay mapping. Solar cells are made to compare lifetime measurement with cell parameters.