Proceedings Article | 6 March 2014
KEYWORDS: Annealing, Amorphous silicon, Picosecond phenomena, Crystals, Optical simulations, Thin films, Absorption, Plasma enhanced chemical vapor deposition, Solar cells, Scanning electron microscopy
In this work, a picosecond DPSS and a nanosecond Nd:YAG laser have been used for the annealing and the partial nanocrystallization
of an amorphous silicon layer. These experiments were conducted in order to improve the characteristics
of a micromorph tandem solar cell. The laser annealing was attempted at 1064nm in order to obtain the desired
crystallization’s depth and ratios. Preliminary annealing-processes, with different annealing parameters, have been
tested, such as fluence, repetition rate and number of pulses. Irradiations were applied in the sub-melt regime, in order to
prevent significant diffusion of p- and n-dopants to take place within the structure. The laser experimental work was
combined with simulations of the laser annealing process, in terms of temperature distribution evolution, using the
Synopsys Sentaurus Process TCAD software. The optimum annealing conditions for the two different pulse durations
were determined. Experimentally determined optical properties of our samples, such as the absorption coefficient and
reflectivity, were used for a more realistic simulation. From the simulations results, a temperature profile, appropriate to
yield the desired recrystallization was obtained for the case of ps pulses, which was verified from the experimental
results described below. The annealed material was studied, as far as it concerns its structural properties, by XRD, SEM
and micro-Raman techniques, providing consistent information on the characteristics of the nanocrystalline material
produced by the laser annealing experiments. It was found that, with the use of ps pulses, the resultant polycrystalline
region shows crystallization’s ratios similar to a PECVD developed poly-Silicon layer, with slightly larger nanocrystallite’s
size.
