engleski

Large area, high rate deposition of mc-Si by microwave PECVD

The aim of the film-Si solar cell research at ECN is the development of technologies that are suitable for large-scale production of high efficiency solar cells based on amorphous and microcrystalline silicon. Main issue presently in this program is further improvement of the deposition rate of intrinsic mc-Si layers. In previous papers we have shown that Microwave PECVD offers good perspectives to solve this issue, but it was also noted that high deposition rates of mc-Si often are accompanied with low material quality (high porosity and defect density). In this paper we discuss our approach to obtain device quality intrinsic mc-Si by MW PECVD at high deposition rates. This approach has lead to recipes that will be transferred to a large area multi-chamber rollto-roll system for fabrication of complete mc-Si solar cells, which will become operational at ECN in the autumn of 2005. We have systematically varied deposition conditions (substrate temperature, gas flows, pressure, microwave power) for the growth of intrinsic mc-Si in our large area single chamber MW PECVD reactor. The width of the deposition area is 60 cm. The structural quality (e.g. void fraction and presence of contaminations) was examined by reflection/transmission (R/T) measurements, FTIR and Raman spectroscopy, Small Angle X-ray Spectroscopy (SAXS) and SEM-XDS. The electronic quality was examined by dark and photo-conductivity measurements and Fourier Transform Photocurrent Spectroscopy (FTPS). We used Optical Emission Spectroscopy (OES) to monitor presence of Si, SiH and H atoms in the plasma during deposition. Key parameters in our optimization procedure are the refractive index n (as obtained from interference fringes in FTIR and R/T spectra), as an indicator of the void fraction, and the sub-bandgap absorption. @0.8 eV as obtained by FTPS, as a measure of the defect density. Microwave PECVD is technique with good perspectives for high rate deposition of mc-Si at low temperatures. Its plasma characteristics (high dissociation rate, low ion energies, semi-remote character) leads to fundamentally different growth mechanisms then for more established techniques as e.g. RF-PECVD. Our study therefore not only contributes to the practical issue of large area deposition of mc-Si for PV applications, but also contributes to more generic insight in the process of mc-Si deposition. The three dominating parameters determining the structural quality of the mc-Si layers grown by MW-PECVD are: substrate temperature Ts, residence time of gas molecules tr and the applied microwave power P. These three parameters define a deposition regime in which device quality mc-Si can be grown at rates up to 0.3 nm/s. This rate, however, is limited by the maximum flow rate of the present mass flow controllers in our system. OES shows that this regime is characterized by minimum Si/H ratios in the plasma for a given SiH4/Hz gas flow ratio. The refractive index of the layers is in the range 3.2-3.3 and the ratio of photo/dark conductivity is typically larger than 100. FTPS measurements so far yield data for' @0.8 eV in the range of 1-5 cm-1, but it should be noted that these data refer to 'apparent' absorption, i.e. they are not yet corrected for scattering, and the true absorption thus will be smaller. Beyond this regime we were able to grow mc-Si at much higher rates, up to 2 nm/s, but these layers suffer from high void fraction and high defect density. SAXS measurements indicate that voids in such layers usually have elongated shapes in the growth direction, which probably explains the high post oxidation rate that has been observed for this type of layers. Microwave PECVD is a promising technique for large area deposition of mc-Si for photovoltaic devices. Deposition rate of device quality material is up to 0.3 nm/s. Applying larger flow rates then presently possible probably can increase this rate.

mc-Si; microwave PECVD

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