Structural Properties Of Semiconductor Nanowire Heterostructures

As a one-dimensional nano-material,semiconductor nanowires have a vast range of potential applications due to its novel physic properties,including photonic(laser,photodetector),electronic(filed effect transistor,logic device),energy(solar cell,thermoelectric generator)devices.Modulation of the properties(e.g.band gap,carrier mobility)can be easier through fabricating heterostructures which expand the applications of nanowires.The structural characteristics of nanowire heterostructures,such as morphology,heterointerface,strain,have a great influence on the devices.Hence,it is necessary to do a systematic study on the structural characteristics of nanowire heterostructures.The research work in this paper focus on the structural characteristics of nanowire heterostructures on the basis of the continuum elasticity theory,lattice dynamic theory,etc.The evolution of the shape and composition profile are investigated during the growth of nanowire heterostructures.The influences of composition and strain on Raman spectra and X-ray diffraction are analyzed through calculations.These analyses will build a theoretical foundation for the growth and characterization of nanowire heterostructures.The main contents are as follows:(1)The radial growth of core-shell nanowire heterostructure is investigated.Based on the diffusion equation and Vapor-Liquid-Solid growth mechanism,the reason of tapering which occurs in the growth of nanowire is analyzed.The adsorption and atom diffudison length on the nanowire's sidewall are taken into consideration.We attribute the tapering to the non-uniform distribution of radial growth rate,which is induced by the diffusion of atom on the axial direction of nanowire when the atom diffusion is dominant in the radial growth of nanowire.Based on the atom exchange mechanism between the bulk and surface of the nanowire's sidewall,the evolution of the composition profile in shell and the local composition peak at heterointerface are analyzed.Our analysis provides a theoretical basis for the growth of core-shell nanowire heterostructures.(2)The strain field in nanowire heterostructures is calculated in the framework of continuum elasticity theory.Based on lattice dynamic theory,the influences of composition and shell thickness on Raman spectra in GeSi core-shell nanowire heterostructure are investigated.In the case of uniform composition profile,the change of strain-induced Raman shift shows linear relation with concentration and nonlinear relation with shell thickness.The relationship between Raman shift and structural parameters in core-shell nanowire heterostructure is built through data fitting.In the analysis of Raman spectra,several kinds of non-uniform composition profiles are taken into consideration.Our research provides a theoretical basis for the analysis of structural information of core-shell nanowire heterostructure through Raman spectra.(3)The strain model of GeSn-Ge dual-nanowire heterostructure is built based on the experimental data.The effective strain relaxation of dual-nanowire heterostructures is verified based on the analysis of strain energy density.In the investigation of Raman spectra,the composition profile and strain distribution in heterostructure are taking into consideration.We find that the composition is dominant in the change of Raman spectra of dual-nanowire heterostructure,which can also prove the effective relaxation in the heterostructure.Based on the growth mechanism of dual-nanowire heterostructure,the composition and diameter of GeSn nanowire are considered in parameter scanning.The relation between structural parameters and Raman shift in dual-nanowire heterostructure is built through the analysis of parameter scanning,based on which the structural analysis of dual-nanowire heterostructure through Raman spectra is feasible.(4)Based on the kinematic scattering theory,the influences of nano wire' s shape on strain distribution and X-ray diffraction profile arc investigated.The relation between strain distribution and diameter of nanowire is analyzed by taking the mismatch between nanowire and substrate into consideration.The influences of shell thickness and shape on axial strain in core-shell nanowire heterostructure are studied.Based on the strain distribution,the changes in X-ray diffraction profile are analyzed.The theoretical analysis will serve as a support for the analysis of axial strain in nanowire heterostructures through X-ray diffraction in experiment.