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29th European Photovoltaic Solar Energy Conference and Exhibition; 1971-1974, The aim of this work is to examine the properties and the post oxidation process of high rate deposited μc-Si thin films by maens of FTIR spectroscopy. High crystalline materials were deposited in an ECWR reactor with deposition rates between 1.2-1.8 nm/sec. The analysis of IR spectra, in the region of silicon hydrides stretching modes, revealed that the absorption band assigned to silicon hydrides on vacancies (HSM) is dominant. Moreover, the presence of three narrow peaks (NHSMs) attributed to the mono- , di- and tri-hydrides on the crystallites surface suggests that there is no sufficient amorphous tissue in the intergranual space. Films deposited with growth rates up to 1.8 nm/sec were very sensitive to oxidation, which starts immediately after the exposure to atmosphere.As the oxidation proceeds, the area of OxSi-Hy peaks increases while the intensity and the area of NHSMs and HSM peaks decrease indicating that the crystalline and the amorphous phases are both oxidized. Oxidized materials post treatment with hydrogen did not affect the spectra revealing that the oxidation could not be reversed.

28th European Photovoltaic Solar Energy Conference and Exhibition; 2604-2607, We introduce a new self-consistent simulation approach for rapid estimation of deposition uniformity in large-area high frequency capacitively coupled plasma (CCP) discharges. The model integrates the numerical solution of Maxwell equations in a fluid plasma model, by continuous modification of an appropriate analytical formula describing the electromagnetic standing wave effect in axisymmetric cylindrical and rectangular CCP reactors.The reliability of the computationally effective model is successfully validated through comparison to experimental data and simulation results produced from a considerably more complicated self-consistent approach for a 150 mTorr Argon discharge in a 40 cm diameter cylindrical CCP reactor. As a final point, the simulation procedure is applied to a 1 Torr hydrogen-diluted silane discharge in a cylindrical reactor operated at 40 MHz, allowing for a fast assessment of electromagnetic effect in plasma parameters and deposited film profiles.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2665-2668, In this paper we investigate post oxidation phenomena in high rate microcrystalline silicon grown by means of PECVD for solar cell applications. Microcrystalline silicon thin films were deposited by PECVD with rates higher than 1.5 nm/s and up to 2.4 nm/s on silicon wafers and glass. The RF power of the PECVD process varied from 300 W to 1000 W in order to provide layers within the amorphous to microcrystalline transition. The resulting films exhibited a maximum crystallinity of 60 % when grown at high deposition rate of 2.4 nm/s. Fourier Transform Infrared (FTIR) spectroscopy show that the oxidation occurs in ambient conditions at the crystalline phase as well as in the amorphous phase depending on the deposition rate, thus in the possible existence of porosity within the bulk of the film. Moreover, the post-oxidation is time-dependent and affects the electrical properties of the film. More precisely, it was found that the photoconductivity to dark conductivity ratio as well as the activation energy are affected by the post-oxidation mechanisms.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2615-2619, A detailed Raman spectroscopic study of the amorphous-to-microcrystalline phase transition in a-Si:H films on glass substrates is presented. In order to induce crystallization of the films a continuous wave CO2-laser working at 10.6 μm was utilized at constant irradiation time/variable power density and at constant power density /variable irradiation time. The analysis of the Raman spectra made it possible to follow quantitatively the induced crystallization. The crystallite size distribution and the film stress are estimated directly from spectroscopic data after a fitting procedure of the experimental data. The Raman line shapes are evaluated in view of the effect of the finite nanocrystal size on the phonons by investigating the CO2-laser induced shift of the c-Si peak. The results are discussed in the context of recent experimental and theoretical works concerning the bond polarizability.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2669-2672, In this work we present a hybrid kinetic Monte Carlo (kMC) methodology for the simulation of microcrystalline silicon film, grown by plasma enhanced chemical vapor deposition (PECVD). All chemical reactions, besides the very fast diffusion of adsorbed radicals, are treated via an n-fold MC algorithm. For the latter events the kMC is coupled with a markovian random walk scheme. Our computational scheme involves a rich chemical model that is able to simulate voids and vacancies formation spanning several hundreds of monolayers. Predicted film growth characteristics such as growth rate obtained from simulations using our hybrid method are in a good agreement with experimental data.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2624-2627, A detailed 2D model of SiH4-H2 discharges was implemented in order to investigate the effect of the total gas pressure and flow on the SiH4 consumption and depletion. Simulations of an industrially relevant medium scale reactor were carried out and the results were compared to experimental measurements of silane depletion in the center of the plasma and consumption in the exhaust port of the reactor. The limited electron – SiH4 dissociation rates, due to short residence times or low electron – molecule collision frequencies (high flows - low pressures), result in reduced silane consumption and depletion. This indicates that a first step in optimizing SiH4 consumption should be to maximize the electron population with sufficient energy to dissociate SiH4 towards neutral species. This can be achieved by tuning process parameters such as pressure and input flow. In addition, a good agreement between model and experiments for the effect of total gas pressure on silane consumption and depletion efficiency was found when SiH4 vibrational states reactions and particle formation reactions were included in the model. This underlines the importance of these microscopic processes on the mechanisms of SiH4 consumption and depletion and the deposition efficiency.

26th European Photovoltaic Solar Energy Conference and Exhibition; 2515-2518, In September 2010 the PEPPER project started after receiving a grant under the Framework Programme FP7 for research of the European Union, coordinated by the DG Energy of the European Commission (no. 249782). The PEPPER project tackles major factors relating to micromorph module efficiency and production cost by assessing the influences of glass, TCO and silicon deposition (including in-situ cleaning). The project bridges the gap between research and industrial application by executing new developments and improvements in the field of TCO and PECVD reactors and processes and transferring them to production plants where the full impact on module efficiency and costs can be evaluated. The joint goal of this project is the demonstration of a 157Wp (stable) micromorph module with a cost of ownership (CoO) of 0.5€/Wp. Succeeding in this project will ensure the competitiveness of the Micromorph technology and further approach the final goal of grid parity. The first results of the project have already shown efficiency improvement by both narrow gap reactor and advanced TCO concepts.

26th European Photovoltaic Solar Energy Conference and Exhibition; 2695-2698, A 2D model of gas phase chemistry of SiH4/H2 discharges was combined to a detailed surface model of μc Si:H thin film growth. The model was used for the investigation of edge effects on the deposition uniformity in a bench chamber similar to those used for micromorph solar panel manufacturing. RF electrode and substrate holder edges were found to affect all plasma properties as electric potential, electron density distribution and species production leading finally to a non-uniform flux of species to the surface and non-uniform deposition rate. Negative ions are produced and accumulated preferentially on the edges probably initiating particle formation in these areas. In addition, material grows rougher and less crystalline on the edges due to the change in the composition of the species reaching the surface and also due to the change in their angle of incidence and the consequent shadowing effects.

26th European Photovoltaic Solar Energy Conference and Exhibition; 2580-2583, The effect of very small disilane addition in silane-hydrogen mixtures on the growth rate of silicon thin films is investigated. Different ways of gas introduction into the plasma zone were tested including single gas entrance configuration as well as two different showerhead designs. Variation of flow and mixing conditions were critical for the effect of disilane addition on the electrical properties of the discharge and the film growth rate. Electrical measurements showed that the power density of the discharge is slightly affected by the disilane addition but affects much stronger the phase impedance leading to more resistive plasmas. The way the growth rate is influenced by the disilane addition depends on the RF electrode design and the proper choice of the process parameters.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 2724-2727, A continuum model of nucleation and growth of microcrystalline silicon thin films from SiH4/H2 discharges is presented. The simulation counts for the mass balance and surface coverage of adspecies and silicon clusters up to size where they can be considered as thermodynamically stable. The model was combined to a plasma gas phase simulator and a simulator of thin film morphology and was applied in order to study the differences in the growth between the center and the edge of a medium scale plasma reactor. The predictions concerning the film growth rate, the film crystallinity and surface roughness in the center and the edges are presented and discussed together with the main processes and the slow step for stable nanocrystal formation.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 2842-2845, A study of the initial growth stage and related process drifts during the deposition of microcrystalline silicon thin films from high pressure – high power SiH4/H2 RF discharges was performed. A 2D time dependent fluid simulator of SiH4/H2 discharges was used for this investigation. As a first step, the model calculations are compared to the experimental findings concerning the variation of species density and temperature during the deposition. In addition, the variation of electrical properties, gas phase chemistry and neutral species properties for 30 min of deposition are presented. The main origin of process drifts that affect gas and substrate temperature and consequently the species density, species fluxes and deposition rate is then revealed and discussed.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 2475-2478, In the present work, we apply a 2D self consistent model of SiH4/H2 discharges in order to investigate the transition from microcrystalline to amorphous silicon growth. The model is used to examine the relative importance of ions and atomic hydrogen in the film growth process. This examination also involves monitoring the changes that take place in the electrical properties of the plasma, the gas phase as well as the surface chemistry near the transition between the two growth regimes. Based on these results, the discussion is extended to the mechanism of a-Si:H to μc-Si:H growth transition and the use of plasma diagnostics that can be used to monitor this transition.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 2451-2454, In the present work, we investigate the changes that are produced in the SiH4/H2 plasma phase from the increase of total gas pressure. Plasma diagnostics as electrical measurements, ultra-fast (nanosecond resolution) time and space resolved emission, film growth rate and film crystallinity measurements were implemented. The experiments were performed under constant applied RF (13.56 MHz) voltage and the electrical diagnostics were used for studying the effect of pressure on the coupling between the RF generator and the plasma and on the discharge microscopic parameters as electron density and temperature, and electric field intensity. The emission measurements were implemented in order to monitor variations in the electron heating mechanism and excitation rates. A significant change of the electron heating mechanism was found for pressures higher than 1 Torr and the beneficial effect of the total gas pressure on either film growth rate or crystallinity is discussed in relation to this change. Keywords: Micro Crystalline Si, deposition, PECVD, thin film