Investigations Of The Transport Properties And Properties Of One-dimensional SiC Nanoscale Materials

As a typical wide band gap IV-IV compound semiconductor, silicon carbide (SiC)is the third-generation semiconductor of the recent years with the greatest potential,which is characterized with wide band gap, high electron saturation velocity, greatcritical breakdown field strength, high heat conductivity and strong radioresistance,showing potential applications in the fields of high temperature, high power, highfrequency, anti-radiation, short wavelength emitting materials and optoelectronicintegrated devices. This paper provides systematic researches into the electronicstructures and properties of SiC and nanomaterials based on the first-principles ofdensity functional theory (DFT). Morever, the electronic transport properties of SiCnanotubes and nanowires are studied, and carries out a theoretical analysis of theinfluence of such factors as different electrode combination and different voltage biaswould have on electronic transport properties of nano-materials. The researches alsoprovide theoretical reference for SiC nano-materials preparation and design the kindsof devices.The main contents and results are listed below：1. First-principles methods on the basis of density functional theory are adopted,and geometry and electronic properties of polytype SiC materials are investigated. The results indicate that the polytype SiC materials are all of indirect band-gapsemiconductors, and that Si-C bonds have strong s-p3hybrid distribution. Meanwhile,the fine structures of3C-SiC and2H-SiC are studied to use the B3LYPexchange-correlation functional methods. The results indicate that there is significantsplitting bands one the top v band, and that the top alence orbital energy levels splitinto a dual combinedp xandp ystates, and a single combinedp zstates, whichconsist with the experiment data, and the band gap is significantly improved, so thetheoretical results are close to the experiment results.2. The geometry and electronic properties of unsaturated SiC nanowires andhydrogen saturated SiC nanowires are investigated systematically. The resultsindicate that extensive relaxation has been occurred to, showing significant quantumsize effects, surface effects and small-size effects, but little relaxation has occurred toSi atom and C atom on the surface of hydrogen saturated SiC nanowires. Thecomputational Results of electronic structure indicate that unsaturated SiC nanowiresdisplay characteristics of the indirect semiconductor, and that hydrogen saturated SiCnanowires display characteristics of the direct wide band gap semiconductor. Theresults of SiC nano-wire optical properties indicate that with the increase inunsaturated SiC nano-wire size, dielectric peak in the low energy area move towardhigh energy area, and blueshift takes place; for hydrogen saturated SiC nanowires, thewhole spectrum tends to be localized, and dielectric spectrum drift toward the lowenergy, and redshifts takes place. Therefore, different emitting optoelectronic devicescould be prepared on the basis of the blueshifts and redshifts phenomena of the SiCnanowires.3. The single-walled SiC nanotubes, multiwall SiC nanotubes and Fe-doped SiCnanotube are investigated systematically. The results indicate that single-walled SiCnanotubes and multiwall SiC nanotubes exist stably on the nanoscale, thatsingle-walled SiC nanotube is the direct wide gap conductors, and multiwall SiCnanotubes is the indirect wide gap conductors; Moreover, Si and C atoms of SiC nanotubes are a typical compound bonds, with sp2and sp~3compound hybridization.Results of HOMO and LUMO orbitals indicate that single-walled SiC nano tube andmultiwall SiC nano tube show different orbit distribution, which would be of greatvalue as theoretical reference for SiC nanotube optoelectronic devices developmentand SiC nanotube electronic transport researches. SiC nanotube optical propertiesindicate that SiC nanotubes are an excellent optoelectronic material, with good blacklight and blue light photoemissivity. Electronic structures and magnetic properties ofFe-doped SiC nanotubes indicate that local symmetry of full relaxation Fe-doped SiCnanotubes show significant change, but one-dimension tube structure as a whole ismaintained. The calculation results of energy band structures show that Fesubstitution for C can form antiferromagnetic materials while Fe substitution for Si ismore likely to form ferromagnetism half-metal materials. Spin polarization occurredat ground state under two doping conditions. Also, intense p-d hybrid effects appearnear the Fermi level. These research results are of vital and great importance formaintaining magnetic properties of nanotube doping system4. First principle methods combined with Non-equilibrium Green Functions,electronic transport characteristics of (8.0) SiC nanotubes are systematicallyinvestigated. Using SiC nanotube as the electrode, transport spectrum of the isolated(8.0) SiC nanotube shows the characteristic of stepped spectrum, and a transmissionvalley about1.1eV could be found. Computation results indicate that I-V curve of the(8.0) SiC nanotube show symmetrical distribution. When voltage bias is greater than±2.2V，electric current would increase rapidly. Using gold atoms as the electrode, I-Vcurve shows the distribution not symmetrical as the computation results indicate, andelectric current shows linear increase with voltage increase. When voltage bias is at1.3V～1.7V, NDC would be displayed as gold electrode combined with differentatoms, but the electric current value obtained from computation is significantlydifferent, indicating the great influence when using different Si and C combination on SiC nano tube transmission, which would be of great importance to SiC nanotubetransport research and new devices designing.5. Electronic transport characteristics of SiC nanowires are systematicallyinvestigated. Using SiC nanowire as the electrode, computation results indicate thattransport spectrum of SiC nanowire with zero bias show characteristics of steppedspectrum near fermi level, and a transport valley about0.5eV could be found.Computation results indicate that I-V curve generally shows symmetrical distribution.When voltage bias is greater than±1.2V, electric current would increase rapidly.Using gold atoms as the electrode, I-V curve shows the distribution within the wholebias range not symmetrical, as the computation results indicate, and both ways ofcombination have led to NDC, but with significantly different bias value, whichwould be greatly implicational to nanounit research on the basis of SiC nanowire.Through contrast analysis, computation results indicate that SiC nanowires showbetter propertied in electronic transport than SiC nanotube, regardless of the ways ofcombination.