Luminescence Mechanism Of Nano-Silicon Carbide Thin Films

Nanocrystalline 3C silicon carbide (SiC) thin films have been deposited by the technique of helicon wave plasma enhanced chemical vapor deposition (HWP-CVD) on single crystalline silicon and Corning 7059 glass substrates. Fourier transform infrared spectroscopy (FTIR), Raman scattering, transmission electron microscopy (TEM) and X-ray diffraction (XRD) are used to analyze the microstructure characteristics of the films, such as bonding configurations, crystalline fraction, component, crystal size and crystal form. Ultraviolet-visible (UV-Vis) transmission and reflection measurements are performed to investigate the optical absorption properties. The optical parameters of the films including optical bandgap and disorder degree are obtained. Combining with the microstructure characterization and the optical absorption properties, the luminescence mechanism and the dynamics processes are discussed in detail by photoluminescence (PL) spectra, photoluminescence excitation (PLE) spectra, PL intensity decay spectra and temperature dependence of PL spectra.The microstructure characterization results clearly show that the films are in a typical structure of nanocrystals embedded in the matrix of amorphous counterpart. The size distribution of nanocrystals ranges from 1 nm to 6 nm and the crystal form is manily in 3C-SiC. In addition, crystalline structure similar to 6H-SiC polytype exists in the films due to the presence of stacking faults introduced in the process of deposition. The analysis of optical absorption properties indicates that the maximum of the optical bandgap appears in the stoichiometric film. While the disorder degree and the band tail width of the films increase with the carbon content.The study of luminescence mechanism shows that due to the quantum confinement effect, the bandgap is widened, the radiant recombination probability increases, and an efficent light emission visible to naked eyes can be obtained. The carrier photogeneration mainly occurs in quantum states of 3C-SiC nanocrystals and that at stacking faults plays an additional effect. For carrier recombination, there are also two main routes contributing to the PL process. The excited carriers recombining directly in the quantized states of 3C-SiC nanocrystals gives rise to the short-wavelength luminescence with the decay time of about 200 ps. While the carriers thermalizing to surface defects and recombining there form a low energy PI band with the decay time of near 1 ns. The temperature dependence of PL intensity indicates that the non-radiation recombination can be restrained at room temperature due to the large active energy of the film. In addition, the change of surface structure and content of nanocrystal under the condition of Si-rich and C-rich, plays an important role in the shift of PL peak originated from the quantum confinement effect. At the same time, the Si-rich and C-rich defect can act as the center of radiation recombination.