Hydrogenated amorphous silicon(a-Si:H) films were deposited by reactive facing target sputtering(FTS) technique with a mixture of Ar and H2 reaction gas.Fourier transform infrared(FTIR) absorption,Raman scattering and ultraviolet-visible optical absorption are used to investigate the microstructure and optical properties of the deposited films.The decrease of the concentration of bonded hydrogen,especially that of(Si-H2)n with increasing substrate temperature(Ts),was observed in FTIR spectra,suggesting the atomic density increases and the concentration of microvoids decrease in a-Si:H films.The increase of both the short range order and the intermediate range order of amorphous network for a-Si:H films were verified by Raman scattering spectra,in which increasing Ts decreasing the band width of TO and the scattering intensity ratio ITA/ITO were obtained.All above results clarify the effect of increasing Ts on the microstructure amelioration for a-Si:H films.The reduction of disordered domains is correlated with the film growing process,where the increased surface diffusion mobility and etching of weak bonds is induced by increasing Ts.Furthermore,analysis of optical absorption indicates that the films with a lower optical band gap and a narrower band edge can be obtained by this FTS technique.
YU Wei,MENG LingHai,YUAN Jing,LU HaiJiang,WU ShuJie & FU GuangSheng College of Physics Science and Technology,Hebei University,Baoding 071002,China
Alternating multilayer films of hydrogen diluted hydrogenated protocrystalline silicon (pc-Si:H) were prepared using a plasma-enhanced chemical vapor deposition technique.The microstructure of the deposited films and photoresponse characteristics of their Schottky diode structures were investigated by Raman scattering spectroscopy,Fourier transform infrared spectroscopy and photocurrent spectra.Microstructure and optical absorption analyses suggest that the prepared films were pc-Si:H multilayer films with a two-phase structure of silicon nanocrystals (NCs) and its amorphous counterpart and the band gap of the films showed a decreasing trend with increasing crystalline fraction.Photocurrent measurement revealed that silicon NCs facilitate the spatial separation of photo-generated carriers,effectively reduce the non-radiative recombination rate,and induce a photoresponse peak value shift towards the short-wavelength side with increasing crystallinity.However,the carrier traps near the surface defects of silicon NCs and their spatial carrier confinement result in a significant reduction of the diode photoresponse in the longwavelength region.An enhancement of the photoresponse from 350 to 1000 nm was observed when applying an increased bias voltage in the diode,showing a favorable carrier transport and an effective collection of photo-generated carriers was achieved.Both the spatial separation of the restricted electron-hole pairs in silicon NCs and the de-trapping of the carriers at their interface defects are responsible for the red-shift in photoresponse spectra and enhancement of external quantum efficiency.The results provide fundamental data for the carrier transport control of high-efficiency pc-Si:H solar cells.
