Structure And Optical Properties Of Si-Based Films

Since the observation of strong photoluminescence (PL) of porous silicon, a great interest in nanocrystalline silicon (nc-Si) has been stimulated because of its potential applications as light-emitting devices compatible with silicon-based optoelectronic integrated circuits. Scientists have spent many years changing the light-emitting characteristics of Si-based materials and improving the light-emitting efficiency to several orders larger than that of Si.The properties of luminescence from Si-SiO2, Ge-SiO2, Al-Si-SiO2 , Al-Ge-SiO2 thin films and the mechanisms of light emission have been studied. Films were deposited on p-Si substrates by radio frequency magnetron sputtering technique with a Si-SiO2 or Ge-SiO2 composite target or some pieces of Al wafers on the target. Some films were annealed in N2 ambience for 30 minutes at different temperature. The microstructure of films were characterized by XRD,XPS, FTIR , TEM spectra. From the optical absorption spectra ,the absorption edges of filmsclearly exhibited blue shift with the decrease of the size of Ge nanocrystallites. The calculations with the effective-mass approximation and the dielectric confinement effect have been carried out respectively. But there is some differences between calculated value and experimental value.The differences between them are discussed. We draw a conclusion that the cause of the phenomenon was focused our attention on the effect of the dielectric confinement effect on the excitonic states and so on. The widening of optical band gap of the amorphous films seems to be related to the function of impurities, defects or the quantum confinement on the clusters in the films.Under ultraviolet excitation, all the samples emit the comparable bands at 370nm, 410nm ,470nm, 510nm but Al-Ge-SiO2 films haven't the emission band at 510nm. With the aid of PLE spectra and the dependence of PL on the annealing temperature, the possible mechanisms of light emission have been discussed. They all come from the luminescence centers(LCs) in the matrix of SiOx.To improve the luminescence efficiency and the intensity of luminescence, attempts by doping with Al element to keep it in the form of atom clusters under certain deposition condition have been proved useful.