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The optical and electrical properties of amorphous- nano-crystalline Si

Nano-crystalline Si is a promising material for high efficiency, “third generation” of solar cells. In particularly is interesting, do to industrial relevance, to produce it using standard, plasma enhanced chemical vapor deposition (PECVD). Thin amorphous –nano-crystalline Si films were deposited by plasma enhanced chemical vapor deposition, using radio frequency discharge in gas mixture of silane and hydrogen. Two hydrogen to silane partial pressure ratios were used, 0, 1 and 20, resulting in amorphous and amorphous-nano- crystalline samples, respectively. The structure of obtained films was measured by high resolution electron microscopy (HRTEM) and Raman spectroscopy. HRTEM micrographs showed that films contained isolated nano-sized ordered domains, nano-crystals, with log-normal size distribution and average value between 5 and 8 nm. The Raman crystal fraction was between 20 and 40%, depending on details of preparation. Optical properties were calculated from transmittance measurements in uv-visible part of spectrum, photo-conductivity and photo deflection spectroscopy. These measurements showed that absorption for amorphous-nano-crystalline samples in visible part of spectrum remained close to pure amorphous material while optical gap showed “blue shift” in respect to amorphous and crystalline materials. This shift was larger for smaller crystal sizes analogue to effects related to “quantum dots”. The Urbach slopes for amorphous- nano-crystalline samples were larger than for pure amorphous, as expected due to nano-crystals. Electrical properties of obtained films were characterized by impedance spectroscopy in a wide frequency range (from 0.01 Hz to 1 MHz) and at temperatures from -100 oC to 120 oC. The dependence of the dark conductivity upon inverse temperature (Arrhenius plot) for amorphous sample showed nearly linear behavior in the whole temperature range. For amorphous nano-crystalline samples, Arhenius plot showed two slopes. High temperature one was the same as for amorphous while slope for the low temperature regime was lower.

The optical and electrical properties of amorphous-nano-crystalline Si

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