STRUCTURAL AND OPTICAL PROPERTIES OF STRAINED SILICON NANOCRYSTALS IN SI/SIO_2 UPERLATTICES FOR SILICON TANDEM SOLAR CELLS

摘要

Silicon quantum dot superlattice is a promising material for all-silicon tandem cells. 10 periods of amorphous-Si(3nm)/ amorphous-SiO_2(3nm) layers are deposited by high vacuum RF magnetron sputtering on quartz and sapphire substrate at different temperatures. The samples are then post-annealed in N_2 atmosphere at 1100°C for 1 hour for Si crystallization. At high growth temperature, the Raman frequency of Si nanocrystal (NC) peak up-shift to around 522 cm~(-1) reveals strong compressive stress existing in the samples grown on sapphire substrates, while the obvious confliction between the estimated Si NC size from Raman shift by bond polarization model and HRTEM indicates tensile stress existing in the counterparts grown on quartz substrates. The strain is attributed to the thermal expansion coefficient between substrate and Si/SiO_2 SL film. While such substrate-induced strain has little effect on the optical bandgap of Si NC and its crystallinity, it can obviously reduce the photoluminescent intensity of Si NC and increase the absorption in 280-650nm region, which indicates less recombination centers and wider size distribution of Si NC with smaller ones. The strain may be used to tune the carrier mobility of Si nanocrystals, important to carrier transport in Si quantum dot solar cell.