Field Emission Properties Of SiC Nanowires

In the thesis,we systematically investigated SiC nanowires including their synthesis and field emission properties,discussed their field emission mechanism,and simply compared them with carbon nanotubes in field emission performance.SiC nanowires were successfully synthesized by carbon-nanotubes-template method,and the key process was following:carbon nanotubes grew by plasma enhanced chemical vapor deposition,followed by depositing Si on carbon nanotubes surface;At last the as-deposited specimen was annealed at 1100℃.Morphologies of SiC nanowires could be controlled by adjusting technical parameters of carbon nanotubes growth.As-synthesized SiC nanowires were characterized by SEM,Raman spectroscopy and XPS.SiC nanowires exhibit good field emission properties such as low turn-on field of 8.0V/μm and threshold field of 13.0V/μm.Whereas our SiC nanowires are bad-oriented,we pointed out that electron also can emit from sides of SiC nanowires except their tips. Investigating field emission properties of SiC nanowires with various morphologies showed aspect ratio unimportantly affects field enhancement factor for bad-oriented SiC nanowires.There are two kinds of discontinuity in the field emission I-V characteristics curves of SiC nanowires.One is due to carbon nanotubes'being incompletely transformed into SiC nanowires,and the other is the quantization of the field emission current resulted from electric field penetration and the quantum confinement effect.Accordingly we proposed a field emission mechanism of SiC nanowires.With electric field,electrons accumulate in the tip and side of SiC nanowire.Combination of electric field and special nanostructure of SiC nanowire confines the electrons movement,which results in quantization of their energy.In case of various field strength,the electrons population on the energy levels is different,and their transparent coefficient of the electrons populating the different energy levels is evidently different.Consequently the quantization of the field emission current occurs.The cathode-anode gap can affect the field emission behavior of SiC nanowires.The practical emission area increases with the increasing gap,and finally remains constant.There is a similar relationship between the field enhancement factor and the cathode-anode gap. In case of high electric field strength, SiC nanowires exhibit negative differential conductance during their field emission.By suggesting a"collision-ionization"model,we successfully explained the phenomenon.Electrons in SiC nanowires gain enough energy from external field becoming hot electron.The hot electrons collide defect centers and crystal lattice,and ionize them.As a consequence,conductance of SiC nanowires greatly increases which results in descending of the voltage falling on the vacuum and SiC nanowires with output voltage of high-voltage source increasing.And the"collision-ionization"model can present how electrons contribute to the high emission current in SiC nanowires.By comparing SiC nanowires with carbon nanotubes similar to the SiC nanowires in shape,electron emission turn-on field of SiC nanowires is lower.On the assumption that their field enhancement factor is equal, the effective work function of SiC nanowires was estimated to be 2.0eV,and lower than the work function of carbon nanotubes.