Purely Intrinsic Poly-Silicon Films by Hot Wire Chemical Vapor Deposition

Abstract

Poly-silicon films have been prepared by hot-wire chemical vapor deposition (HWCVD) from hydrogen diluted silane gas at a low temperature (430 °C). The optical gap of the poly-silicon films is 1.1 eV, though with a higher optical absorption than c-Si. The grains have a preferential orientation (220) perpendicular to the substrate with an average crystallite size of 70 nm. The crystalline volume fraction is 95% with complete coalescence of grains. Large structures up to 0.5 μm could be observed in the AFM micrograph. The activation energy (0.54 eV) and the low carrier concentration (1011 cm−3) indicate a fully intrinsic nature of the films. The μτ product of carriers is 7.1×10−7 cm2V−1 whereas the ambipolar diffusion length (LD) is 334 nm. The excellent photo-conductive properties are attributed to the low (∼1017 cm−3) defect density. The HWCVD poly-silicon films showed a very small temperature dependence of mobility, indicating negligible trapping of carriers at the grain boundaries. Preliminary n-i-p cells incorporating poly-silicon i-layer yielded 3.15 % efficiency.

Nanocystalline silicon solar cells

“…Whereas the material made at high hydrogen dilution is completely crystalline with no noticeable amorphous incubation phase, it is very defective and porous, characterised by the typical 2100 cm −1 mode of vibration in the IR spectrum and this material goes through fast post deposition oxidation (becomes n-type). On the other hand, the films made at low hydrogen dilution have the characteristics of a compact structure, and hydrogen atoms in the film are at compact sites [24] (2000 cm −1 in IR spectrum [25]). These sorts of materials showed the device quality, but suffered from amorphous incubation phase, being in a deposition regime near to the amorphous transition regime [26].…”

Fast deposited microcrystalline silicon solar cells

“…Note that this situation which applies to intrinsic µc-Si:H is in contrast to the situation prevailing for the very thin doped µc-Si:H layers which are used as contact layers in amorphous and other thin-film silicon solar cells [8]. When using hot-wire CVD, very high deposition rates up to 50 Å/s for silicon (µc-Si:H / poly-Si) have been reported [9]; this fact illustrates that high deposition rates should not be excluded a priori for µc-Si:H. Indeed, encouraging first results on cells showed 3 % conversion efficiency for the hot wire technique at a deposition rate of 5 Å/s [10]. Within the field of plasma-enhanced CVD (PECVD), it has been shown that the VHF-GD technique, where the plasma excitation frequency is increased to the VHF-range, is very favourable for the growth of µc-Si:H [11][12][13][14][15].…”

Electrical evaluation of the crystallization characteristics of excimer laser annealed polycrystalline silicon active layer

“…21) We measured the generation lifetime of the MOS capcitor and used it as an indicator to estimate the crystallinity and impurities level in the grain boundary. Since the conductivity of poly-Si is lower than single crystal silicon, [22][23][24] we detected more sensitively the behavior of carriers generated from impurities in deep depletion state. Also, the high roughness of poly-Si reacted at the physical deep position of SiO 2 and generated slow state traps (border traps).…”