Novel silicon nanocrystal materials for photovoltaic applications
In order to find an appropriate absorber material for the top cell of a crystalline all-silicon tandem solar cell we investigate silicon nanocrystals (Si NCs) surrounded by a silicon carbide matrix. The films are produced by plasma enhanced chemical vapour deposition of amorphous Si-rich SiC with 67% of Si from the precursor gases SiH4 and CH4. The formation of Si NCs takes place during a subsequent annealing step which is usually performed at 1100°C. In order to reduce the thermal budget for future cell processing and to investigate the crystallisation process, the annealing temperature was reduced stepwise to 800°C. Grazing incidence X-ray diffraction (GIXRD) and Fourier transform infrared spectroscopy analysis reveal that both Si and SiC crystallization starts at 900°C. By extending the annealing time from 30 min up to 24 h at this temperature we show by careful GIXRD peak analysis that there is no significant change, neither in crystal size nor in the amount of the crystalline phases. In contrast, both the Si NC and the SiC NC grain size show a significant growth from 2.0 ± 0.5 nm at 900° up to a grain size of 3.3 ± 0.5 nm and 4.1 ± 0.5 nm at 1100°C, respectively. Also the proportion of crystalline Si increases relative to the quantity of crystalline SiC at higher temperatures. This is interpreted in terms of different activation energies for the formation of Si and SiC NCs that arise from the nature of the amorphous film before the onset of crystallization.