Microstructure And Energy Gap Characteristics Of Nano-silicon Carbide Thin Films

Nanocrystalline silicon carbide (nc-SiC) thin films have been deposited by the helicon wave plasma-enhanced chemical vapor deposition (HWP-CVD) technique on single crystalline silicon and quartz substrates from SiH4, CH4 and H2. A variety of techniques, including Fourier transform infrared spectroscopy, Raman scattering, X-ray diffraction, ultraviolet–visible transmittance and reflection spectroscopy and atomic force microscopy are used to analyze the crystalline fraction, bonding configurations, hydrogen contents, optical gap and the degree of order in the films. The effects of the experiment parameters such as coupling magnetic field and substrate temperature on the structure and optical characteristics of the films are studied. It has been shown that the applied magnetic field enhance the film deposition rate, which can make the crystalline fraction of the films increase, result in the optical gap and the degree of order of the deposited films increase with magnetic field strength. Increasing the substrate temperature can improve the mobility of the reactive species on growth surface, which enhance the film deposition rate, make the degree of order of the films and their crystalinity increase. The quantum confinement effect results in a large optical gap of the films.In addition, the optical absorption technique is used to investigate the energy gap characteristics of nc-SiC films. The energy gap model of nc-SiC films are found on the base of simulating and analyzing the density of the states (DOS) in the energy gap of Si-rich and C-rich amorphous SiC films. The results indicate that the gap states distribution of Si-rich amorphous SiC films is similar to that of a-Si:H; while the C-rich SiC films can be interpreted using the DOS of a-C:H. Nc-SiC films have a inlaid structure, when the crystalinity of the films is low, the films contains nano-SiC crystallites embedded in amorphous matrix. The amorphous SiC with a wide optical gap is the controbutor to the extended states and localized states in the energy gap of the films, nanocrystallites exist as localized states; when the crystalinity of the films is high, the structure of the films is nanocrystallites SiC matrix with amorphous SiC embedded in or located at the nanocrystallites interface. The nanocrystallites SiC with a wide optical gap is the controbutor to the extended states in the energy gap of the films, while the amorphous SiC energy state only form the surface states located in the nanocrystallites.