Preparation, Properties And Mechanism Of SiC Nanowires

Semiconductor nanowires(NWs)have attracted much attention because of their versatile electrical,optical and mechanical properties and their potential applications in novel nanoscale photonic and electronic devices.As a third generation wide bandgap semiconductor,silicon carbide(SiC)has been developed accompanying the progress of elemental semiconductors,e.g.Si,of the first generation and compound semiconductors,e.g.GaAs,GaP and InP,of the second generation.Due to its wide energy bandgap,high breakout field,high thermal conductivity and high elelctron drift velocity.SiC crystals have potentially promsing applications in the fabrication of optic and electronic devices,especially those suitable for operation under harsh environment such as high frequency,high temperature,high power and high irradiation.Recently,the researches on SiC semiconductor are mainly focused on the growth of SiC bulk crystals and the synthesis of SiC NWs.Besides the wide energy bandgap of semiconductor,SiC NWs may exhibit unique mechanical, photoluminescence and field electron emission properties caused by quantum confinement effects,special morphologies and inner novel structures and defects.The studies on the synthesis,structure and properties of SiC NWs are of great importance to future nanoscale photonic and electronic devices as well as fundamental research.Compared with other reported synthesizing methods,A simple and low cost thermal evaporation technique was employed in this paper to prepare SiC NWs.β-SiC (3C-SiC)NWs and their array were synthesized on the different carbon substrates using different raw materials.The compositions,morphologies,and microstructure of SiC NWs were characterized with X-ray diffraction(XRD),field emission scanning electron microscopy(FE-SEM scanning electron microscopy(SEM),transmission electron microscopy(TEM),energy dispersion spectroscopy(EDS).The optical, electronic and energy bandgap properties of the SiC NWs were studied by Fourier transformed infrared spectroscopy(FT-IR),Raman spectroscopy,photoluminescence spectrum(PL),the field-emission measurement and UV-Vis spectroscopy.The growth mechanism and the relationships between the structure and properties have been studied based on the above investigation.Some innovative achievements have been obtained and important conclusions are drawn,establishing the fundamentals for the practical applications of SiC nanowires in nano-devices.A tube-brush shaped array of SiC NWs was synthesized on carbon fibers via the silicon thermal evaporation method in an Astro furnace.The effect of the vacuum, temperature and reaction time on the morphology and array of the NWs was discussed. The experiments suggested that a little oxygen is necessary to the growth of SiC NWs. The vapor-solid(VS)reaction mode of the SiC NWs is proposed.Solid carbon reacted with Si vapor or rather SiO to produce SiC embryos on the surface of the carbon fiber.The SiC embryos then grew in two ways:one was still the reaction of solid carbon and silicon and/or SiO vapors;the other one was the vapor-solid(VS) reaction between SiO and CO.The former lead the SiC particle aggregates to form on the surface of the carbon fiber to construct the axis of the tube brush.The latter, however,since Si and C were provided by two vapors the growth was operated along a preferential crystalline direction([111]ofβ-SiC),to form SiC NWs,i.e.the bristles of the tube brush.SiC NWs like a lawn planting on the surface of the polished graphite plate were obtained by the same thermal evaporation method.The VS reaction mode of the SiC NWs is also proposed.At the high-temperature processing,SiO and Si vapor reacted with carbon to form SiC embryos by heterogeneous nucleation on surface of the graphite plate.The SiC embryos should be two dimensional over(111)plane ofβ-SiC, with Si layer on the top and C layer on the bottom.The top Si layer of SiC embryos may still has Si-O bonds,which could absorb CO molecules,and react each other [SiO(g)+CO(g)=SiC(s)+O₂(g)]to form a C layer covering the Si layer.Similarly, the newly formed C layer will absorb SiO to form the second Si layer,and a layer-by-layer growth along[111]direction will be realized.The inner structure of the SiC nanowires is featured by two characters:One is that each individual SiC NW is composed of(111)periodic twins,and the other is that the tips of most SiC NWs show the typical screw dislocation morphologies.Based on these,a crystallographic growth mechanism for the formation of the periodic twin structure via screw dislocation growth is proposed.As the participation of oxygen in the reaction,the strain can be easily introduced into the(111)plane of the two-dimensional SiC nucleiation,and the screw dislocation vertical to the(111)plane can be formed due to the strain in the lattice.The Si and C atoms continuously deposited on the step of the screw dislocation so that the close-packed spiral growing mode is assumed.The stacking fault can thus be induced during the spiral stacking of Si-C double layers for the strain effect.The(111)periodic twinning then evolved to reduce the strain energy.{111} faceting along the surfaces of the NWs was also formed by the periodic twinning to reduce the surface energy.SiC NWs with periodic twinned structure and{111} faceting surfaces have the lowest strain and surface energy.A great number of SiC NWs grown vertically on the surface of the polished graphite plate constructed a lawn-like SiC nanowire array.The down-shifting feature of FT-IR spectrum and Raman spectroscopy of SiC NWs shows that there are some stacking faults in NWs.Raman spectrum of NWs at-100℃has a new peaks at 835 cm^-1,which was ascribed to the formation of 4H-SiC segments in 3C-SiC NWs at low temperature by the replacement of stacking faults.The Photoluminescence spectra of SiC NWs display several strong separated peaks around 470 nm and a weak shoulder peak around 485 nm.Compared with the band gap of bulk 3C-SiC(2.23 eV),the PL peaks of nanowires are blue-shifted.The phenomena can be explained by the quantum confinement effects of the quasi-periodically twinning superlattice structure formed by the twin stacking faults. We tentatively attribute the separated light emissions to the quantum confinement effects of 3C-SiC nano-scaled segments with different widths within the nanowires.The relationships between the stacking faults and twinning structure of SiC NWs are discussed.The region around a twinning boundary resembles the structure of 4H/6H-SiC.As the bandgap of 4H/6H-SiC wider than 3C-SiC,the twinning superlattice has quantum-well-like features perpendicular to the c-axis,i.e.[111] growth direction.Electron transport through these quantum wells may be an important issue for future work on the properties of SiC nanowires.The bandgap of the SiC NWs were also investigated by UV-Vis absorption spectroscopy.The absorption spectra show a perfect fit for the indirect transition and the extrapolation yields an Eg value of 2.85 eV which is wider than the bandgap of bulk 3C-SiC(2.23 eV),and is close to the bandgap calculated by photoluminescence (2.65eV).The difference may be caused by the Stokes shift.Field emission properties of lawny SiC nanowire array on the surface of the polished graphite plate were tested.The SiC nanowire emitters exhibit excellent macroscopic emission properties.It has low turn-on voltage(2.1V/μm)and high brightness.A novel metal silicide solution technique to grow SiC NWs was also developed by simple thermal evaporation.The supersaturation of the SiO of was controlled by using different metal silicides at the same temperature.NWs with different morphologies were obtained.Compared by the Co-Si₂ solution,the SiC NWs obtained by CoSi solution have a small average diameter(60 nm)and a low output.SiC NWs/whiskers with sub-micronmeter diameter were also observed on the surface of the freezed solution.It is believed that the formation of SiC NWs is a combination of solid-liquid-solid(SLS)reaction for nucleation and vapour-liquid-solid(VLS)process for nanowire growth.The hexagonal-shaped SiC NWs with 200 nm in diameter were also synthesized through a sol-gel and direct heating process.The growth of NWs was typical carbothermal reduction process.