Characteristics of SiO2/Si Quantum Dots Super Lattice Structure Prepared by Magnetron Co-Sputtering Method

Abstract

Solar cells have been more intensely studied as part of the effort to find alternatives to fossil fuels as power sources. The progression of the first two generations of solar cells has seen a sacrifice of higher efficiency for more economic use of materials. The use of a single junction makes both these types of cells lose power in two major ways: by the non-absorption of incident light of energy below the band gap; and by the dissipation by heat loss of light energy in excess of the band gap. Therefore, multi junction solar cells have been proposed as a solution to this problem. However, the <TEX>$1^{st}$</TEX> and <TEX>$2^{nd}$</TEX> generation solar cells have efficiency limits because a photon makes just one electron-hole pair. Fabrication of all-silicon tandem cells using an Si quantum dot superlattice structure (QD SLS) is one possible suggestion. In this study, an <TEX>$SiO_x$</TEX> matrix system was investigated and analyzed for potential use as an all-silicon multi-junction solar cell. Si quantum dots with a super lattice structure (Si QD SLS) were prepared by alternating deposition of Si rich oxide (SRO; <TEX>$SiO_x$</TEX> (x = 0.8, 1.12)) and <TEX>$SiO_2$</TEX> layers using RF magnetron co-sputtering and subsequent annealing at temperatures between 800 and <TEX>$1,100^{\circ}C$</TEX> under nitrogen ambient. Annealing temperatures and times affected the formation of Si QDs in the SRO film. Fourier transform infrared spectroscopy (FTIR) spectra and x-ray photoelectron spectroscopy (XPS) revealed that nanocrystalline Si QDs started to precipitate after annealing at <TEX>$1,100^{\circ}C$</TEX> for one hour. Transmission electron microscopy (TEM) images clearly showed SRO/<TEX>$SiO_2$</TEX> SLS and Si QDs formation in each 4, 6, and 8 nm SRO layer after annealing at <TEX>$1,100^{\circ}C$</TEX> for two hours. The systematic investigation of precipitation behavior of Si QDs in <TEX>$SiO_2$</TEX> matrices is presented.