Polycrystalline silicon thin films for photovoltaics.

摘要

Selective nucleation and solid phase epitaxy offers a low temperature method to fabricate large grain, polycrystalline silicon on foreign substrates. Undoped and highly doped silicon films were nucleated with nickel or indium and annealed at 600°C. Indium nucleated crystallization proceeded by conventional solid phase epitaxy. Undoped silicon had grain sizes of 1–2 μm. With doping, although there was enhancement of the growth rate, the grain size did not increase, since the incubation time correspondingly decreased. The exception was the phosphorus-doped silicon that had a maximum grain size of 10 μm.; In nickel-nucleated samples, the amorphous silicon layer fully crystallized before the onset of random nucleation, achieving grain sizes on order of tens of microns. Within each grain, however, were many low angle, sub-grain boundaries that came from the needle-like crystal growth. Epitaxy on these layers resulted in strained columnar crystals with dislocations.; Positron annihilation spectroscopy (PAS) was used to study vacancies in solid phase crystallized silicon in four doping cases: undoped, B-doped, P-doped, and P&B-doped. Oxygen-vacancy complexes were seen in all samples and phosphorus-vacancy complexes in the P- and P&B-doped samples. Progressive etchback of a subset of the samples was achieved, and a defect concentration on order of 1015 cm−3 was estimated for all samples.