VLS Growth Model Of Si Nanowires And Structural Dynamics Of Nanowire Networks

Nanowires(NWs)have many potential applications in advanced semiconductor materials,advanced energetic materials,functional materials and nano-reinforced composites due to the combination of traditional properties and special 1-D quasi nanostructures.Chemical vapor deposition(CVD)is an important approach to the synthesis of high quality single(aligned)nanowire.The growth mechanisms of nanowires synthesized via CVD method are mainly involved in two categories:the“vapor-liquid-solid(VLS)”mechanism and the“vapor-solid”mechanism.These two mechanisms are usually used to explain the growth with or not with assistant of catalyst respectively.Since the introduction of catalyst can decrease the energy barrier of the growth system and improve the quality of synthesized nanowires,research and modeling on VLS mechanism are the precondition of growth control of nanowire structures and morphologies.There are many sub-processes in VLS mechanism.Previous researches were mainly focused on the modeling of one or several sub-processes,leaving the studies on unified model which combines most of the sub-processes and their interactions.The present work devotes to integrate most of the VLS sub-processes and to develop the axial and radial growth model for Sinanowires grown via VLS mechanism.In addition,the formation and evolution of nanowire networks are also achieved in this work.The major contents and conclusions are shown as follows:Thermodynamic calculations of diagrams and surface tensions of Au-Ge,Au-Siand Ag-Sieutectic droplets have been achieved based on Calculation of Phase Diagrams(CALPHAD)and Butler model.The R-squared index of calculated L/L+Ge liquidus line(Au-Ge system)is 0.988 comparing with Au-Ge diagram;the R-squared index of calculated L/L+Siliquidus lines in Au-Sior Ag-Sisystem are 0.993 and 0.996respectively comparing with the diagrams;surface tension of Au-Ge or Ag-Sieutectic droplet is the decreasing function of temperature and Ge(Si)concentration;the equilibrium partial pressure of Siin Ag-Sisystem is lower than the vapor pressure of pure Si,causing the supersaturation in vapor phase.Axial VLS growth model,which combines the evolution of geometrical parameters and mass transport process,were proposed for Sinanowires by analysis on geometrical relations,energy balances and inter-phase mass transport.The model can give predictions on evolutions of contact radius,contact angle,inclination angle,liquid component and axial growth rate during the axial growth of Sinanowire via VLS mechanism.The model can also describe the size-dependent axial growth rate during steady state growth of SiNWs.The as-proposed model was applied on the predictions of VLS growth of Ge and Sinanowires reported in previous researches.Results show that the calculations were in good agreement with the conclusions in references.The values of R-squared range from 0.925 to 0.992.Mass transport modeling from vapor to substrate,substrate to sidewall,sidewall to liquid and vapor to sidewall has been achieved based on surface diffusion equations of substrate and nanowire sidewall.In addition,radial growth model is also proposed according to mass balance conditions and sidewall diffusion model.The as-proposed models can give descriptions to surface mass concentrations,sidewall shapes,radial growth rate and vapor-sidewall flux varying as the function of nanowire length and growth time.The as-proposed axial and radial growth model was verified by the VLS growth data of SiNWs obtained from experiment.SiNWs,which have diameter of 20 nm?40 nm and are covered by SiO_x layer,were synthesized with assistant of Ag nanoparticle at 950??1200?.Predictions on nanowire elongation(with R-squared of 0.923?0.955),sidewall shapes(with R-squared of 0.923?0.955)and size dependence of growth rate(with relative error of 3.8%?12%)show good consistence with the measurements.Simulations for VLS growth of SiNW show significant influence of energy balance on the shape of nanowire base.At initial growth stage,the radius of nanowire narrows down to reduce the increased surface energy which is caused by the newly grown vapor-solid interface.Then,Gibbs-Thomson effect caused by radius decrease breaks the mass balance inside the droplet,triggering the unsteady growth of nanowire.The radial growth process is manly controlled by closed-loop feedback formed by interactions among nanowire shape,vapor-sidewall mass transport,mass distribution on sidewall and radial growth kinetic.Otherwise,the activity of sidewall-liquid mass transport can significantly change the size dependence of nanowire axial growth rate.Simulated morphologies which have similar structures to the ones formed by nanowire networks were generated by Monte Carlo method.Parameter called“theoretical area ratio”was defined to describe the relations among quantity,length and radius of nanowire networks.Fractal dimension calculations for simulated morphologies show that fractal dimension is linear distributed with theoretical area ratio in lg-lg coordinates.The width of the distribution narrows down with increase of nanowire radius.Statistical analysis for microstructure of simulated morphologies has been carried out according to box counting model and results show the uniform geometric structure independent with directions.Histograms of the covered fractions of boxes covered by simulated morphology spears a transition from positive skew to normal with the increase of nanowire quantity,length or radius.A method for quantity and length estimations of nanowire networks was proposed by means of similarity analysis with simulated morphologies.The average relative error of length estimations for synthesized messily grown SiNWs is 7.8%?12.3%.Structural evolution of nanowire networks is studied.Parameter called“actual area ratio”is defined to describe the density and fraction of nanowire networks.A model describing the relationship between actual area ratio and nanowire growth kinetics has been proposed based on the induction from simulations.Predictionss on actual area ratio growth of SiNWs synthesized at 1000? and 1100? were proposed.The calculations were in the best consistence with experiments only if the saturated growth of nanowire quantity is assumed.Further simulations show that the growth dynamic of actual area ratio is consistent with“S”type growth model.The pattern of actual area ratio growth is dependent on the growth modes of messily grown nanowires.