Preparation And Properties Of Ordered Nanowire Arrays With Controllable Diameter To Pitch Ratio

Owing to the special structures and unique physical and chemical properties of semiconductor nanowires(NWs),Group IV and III-V NWs have a broad application prospect in chemical sensors,lithium batteries,thermoelectric converter and photo-voltaic devices.In order to realize various application of semiconductor NWs in the f'uture,it is a major challenge that prepares vertically ordered NWs arrays with con-trollable diameter and periodicity.It is of great significance to understand the prepara-tion mechanism of one-dimensional nanomaterials for controllable preparation of NWs.Therefore,this dissertation is mainly focused on the following aspects:1.By depositing Ti/Au film as catalyst on polystyrene spheres coated Si sub-strates,well-separated Si NWs with good periodicity and lower porosity density are fabricated using thermal oxidization and HF acid etching after forming Si NW arrays with high diameter-to-pitch ratio(D/P)by metal-assisted chemical etching(MACE)based on nanosphere lithography.The influence of oxidation temperature and oxida-tion time on the morphology of Si NWs was studied and the relationship between oxide layer thickness and oxidation time was calculated according to the extended Deal-Grove model.The factors responsible for the special morphology features of Si NW during the oxidation process are understood by thermal oxidation process simulation and stress distribution of NW during oxidation process is discussed,the theoretical results are con-sistent with the experimental results.A high-temperature oxidation process is proposed to facilitate the alleviation of necking and the oxidation accelerating of NWs.Fur-thermore,to reduce the tapering trend,an oxygen diffuse barrier layer is proposed to be pre-deposited on the top of Si NWs before the high-temperature thermal oxidation treatment.By using these methods,periodic arrays of Si NWs with a large pitch and small diameter have been fabricated.This approach can substantially reduce porosities and surface defects on the outer surface of Si NWs.2.Based on the synthesis of vertically aligned Si and Si/Si02 NW arrays,the optical properties of Si and Si/Si02 NW arrays was studied systematically.The optical reflection of the NW arrays can be substantially suppressed by addition of transparent shell.Meanwhile,using finite difference time-domain(FDTD)method,we have found that absorption enhancement in core Si NW can be obtained by adding transparent shell.The special absorption enhancement of Si NW arrays with a core-shell structure can be theoretically understood with modal analysis method.Theoretical analysises reveal that the absorption in such Si NWs array structures is very sensitive to thickness of transparent coating.Furthermore,significant absorption enhancement and broadband antireflection effects can be achieved by diluted Si NWs combined with single dielectric shell and it is useful for guiding the Si NW based solar cell devices.3.On n-type Si(111)substrate,disordered GaAs NWs with 100%vertical produc-tion were achieved by gallium droplet self-catalysis method.On the basis of previous work,we propose a simple and convenient method for preparation of ordered GaAs NWs.Si/Si02 core-shell structure NWs arrays with controllable diameter are fabri-cated using MACE combine with thermal oxidization method and then remove the top of Si/Si02 NWs for exposing internal Si NWs,thus,Si NWs substrate with ordered controllable diameter was obtained.Vertically ordered GaAs/Si axial heterostructure NWs were grown on prepared substrate by gallium droplet self-catalyzed method in MBE system.In this dissertation,We mainly use three methods for preparation of Si/-Si02 core-shell NW substrate and growth of ordered GaAs/Si heterostructure NWs,but the experimental results of ordered GaAs/Si NW failed to fully meet our expectations,further studies on ordered heterostructure NWs is needed.4.Effect of surface states on Photoluminescence(PL)spectrum was studied by sulfur passivation and radial growth of shell structure.Experiment results indicated GaAs NW surface state density decreased significantly after passivation,and the peak PL intensity was also greatly improved.By passivating GaAs NWs in Se+P2S5/(NH4)2S+t-C4H9OH solution for 5min,we obtained thicker passivation layer and good passiva-tion effect.We theoretically calculated the effect of surface defect density and NWs diameter on PL intensity,and theoretical results show recombination and depletion of carriers at GaAs surface increase and PL intensity decrease with increase of defects on the surface.At low surface trap density(<108cm-2),the PL intensity is proportional to the NW volume(or the square of the NW diameter),and surface recombination begins to dominate with surface trap density increases.