Phosphorus-doped nanocrystalline silicon-oxycarbide thin films

Abstract

P-doped nc-SiOxCy:H films are grown at moderately low substrate temperature ~250°C and rf power ~200 W via controlled incorporation of C and O from CH4 and CO2 precursor gases by PECVD. The optoelectronic and structural properties of the films have been studied in detail. On gradual increase in CH4 flow rate (F) in the plasma, systematic formations of the Si-C and Si-O-C bonds in a-SiOxCy:H network are evident from the gradual shift of the Si-H wagging and Si-O-Si stretching vibrational modes towards higher and lower wavenumbers, respectively. XPS studies have confirmed on the presence of Si-O-C bonds and the P-atoms as dopants in the network. The Si-C bond density has increased from 1.56×1021 to 4.1×1021 cm−3 and the grain size of the nanocrystals has reduced from 6 to 3.2 nm with the increase in F from 0 to 0.3. An enhancement in the optical band gap is accomplished via the addition of stronger Si-C bonds in the a-SiOxCy:H matrix and also due to quantum confinement effect arising from the reduced size of the nanocrystals, together. The optimum n-type nc-SiOxCy:H film possessing ~50.5% crystallinity with nanocrystal grain size ~5 nm has demonstrated an optical band gap ~1.96 eV and high dark conductivity ~6.41 × 10−1 S cm−1, which altogether appear promising for the window layer of all-Si tandem solar cells. A switching of the electrical characteristics from the degenerate-like crystalline network to its non-degenerate amorphous-like configuration is identified at higher C incorporation in the P-doped SiOxCy:H complex network, corresponding to the CH4 flow rate above 0.2sccm.