Low-thermal budget Si nanocrystals in SiC

Si NCs embedded in a wide band gap material are promising candidates for all-Si tandem solar cells thanks to the possibility of tuning the NC band gap by varying their size, offered by the NC quantum confinement. The high temperature treatment required for Si crystallization represents a concern for the integration of this material in a tandem solar cell production route. The limiting step for solid phase crystallization is grain nucleation. A high temperature nucleation seeding followed by grain growth at a lower temperature is therefore a promising method to reduce the thermal budget. To this aim, we report on different annealing routes pursued to decrease the thermal budget: in all cases the germs of the c-Si nucleation are created by a high temperature (1150 °C) spike, then the crystallization follows at different temperatures, above and below the 3C-SiC solid phase epitaxial growth temperature (800 °C). The results are discussed in terms of the structural features produced by each process: the time required for complete dehydrogenation, mandatory to avoid disruptive hydrogen evolution during the high temperature spike; Si and SiC crystallization; defect nucleation in c-SiC, correlated to the rate of temperature rise during the annealing; role of the c-Si/SiC interface energy in affecting the growth process. The photovoltaic properties of the resulting structures are estimated based on the measured optical absorption, electronic transitions, and electrical transport.