Photoelectronic Properties Of Nanocrystalline SiC Thin Films

Hexagonal nano-silicon carbide (nano-SiC) thin films doped and undoped with phosphorus which possess a characteristic of the ultraviolet photoluminescence (PL) have been deposited by helicon wave plasma enhanced chemical vapour deposition technique under the high hydrogen condition in this work. The optical and electric properties of the doped nano-SiC films together with the electric and photovoltage properties of the n-p nc-SiC/Si heterojunction is studied and analyzed.The structure characteristics of n-type nano-SiC and the doping mechanism is investigated by Fourier transform infrared spectroscopy, Raman scattering, transmission electron microscope, atomic force microscopy and X-ray Diffraction. The results show that the crystalline volume fraction of these films is increased with doping proportion and the doping mechanism is Si or C substituted by phosphorus. The optical absorption and PL analysis show that with increasing the doping proportion, the band tail absorption and the PL intensity is enhanced. The conductivity analysis shows that the greatest conductivity is 10-2 S·cm-1 obtained at the 1% doping proportion. It has been evidenced that there are two different conduction mechanisms in the n-type nano-SiC films: one is the extended states conduction which is dominated at a higher temperature; the other is the hopping conduction through the localized states near the Fermi level under a lower temperature. The I-V character of n-p nc-SiC/Si heterojunction diode is also investigated, we obtain that the best rectification ratio is about 350 and the ideality factor is 1.89, which suggest that the forward current at low voltage is a generation–recombination processes likely due to defects at the nc-SiC/Si interface. This forward current comes from the emission and diffuse process under high voltage but the reverse current is from a Muti-step tunneling process through the interface states.The research on the photovoltage character of this nc-SiC/Si heterojunction indicates that SiC can serve for the window layer for Silicon based photoelectric apparatus. The photovoltage is mainly from the interface space charge region (SCR) and it is greatly depended on the intensity of this interface electric field. The interface electric field is improved under a reverse bias and becomes weaker under forward bias or under the illumination. There are mainly two different recombination mechanisms of the electron-hole pairs in this p-n junction: One is the direct recombination in the SCR, another is the recombination through interface states. Under a forward electric field the direct recombination in this SCR is restrained and the recombination rate through the interface states becomes slow but the reverse electric field can increase both recombination rate and increase the probability of the direct recombination in the SCR.