金属辅助化学刻蚀法制备硅纳米线的研究进展

Progress in Fabrication of Silicon Nanowires by Metal-assisted Chemical Etching

硅纳米线(Si NWs)由于具有独特的一维结构、热电导率、光电性质、电化学性能等特点,被广泛应用于热电与传感器件、光电子元器件、太阳能电池、锂离子电池等领域。金属辅助化学刻蚀法(MACE)是制备Si NWs的常用方法之一,具有操作简便、设备简单、成本低廉和高效等优点,可大规模商业化应用,因而近年来被广泛研究。金属辅助化学刻蚀制备硅纳米线的过程可以分为两步:首先在洁净的硅衬底表面沉积一层金属(Ag、Au、Pt等)纳米颗粒,以催化、氧化它附近的硅原子;然后利用HF溶解氧化层,从而对硅晶片进行刻蚀,形成纳米线阵列。然而,这种简单高效的制备硅纳米线的方法存在一些难以控制的缺点:(1)金属纳米颗粒聚集、相连后造成Si NWs之间的缝隙比较大,从而导致Si NWs密度较低;(2)由于金属纳米颗粒沉积的随机性,在硅晶片表面分布不均匀,不仅导致刻蚀出的纳米线直径范围(50～200 nm)较宽,而且使制得的纳米线阵列排列无序且间距不易调控;(3)当刻蚀出的硅纳米线太长时,范德华力等作用会造成纳米线顶端出现严重的团簇现象。针对常规法存在的一些问题以及不同的器件对硅纳米线的形貌、类型和直径等的要求,近年来的研究主要集中在如何减少纳米线顶端团簇、调控纳米表面粗糙度和直径、低成本制备有序硅纳米线等方面。目前一些改进常规金属辅助化学刻蚀的方法取得了进展,比如:(1)用酸溶液或UV/Ozone对硅晶片预处理,在表面形成氧化层,可以使纳米线的均匀性得到改善并增大其密度(从18%提高到38%);(2)使用物理气相沉积法在硅晶片表面沉积一层金属纳米薄膜,然后再刻蚀,这种方法能够减少纳米线顶端团簇和有效调控纳米线直径;(3)利用模板法(聚苯乙烯小球模板、氧化铝模板、二氧化硅模板和光刻胶模板等)可以制备出有序的硅纳米线阵列。本课题组用离子束刻蚀的方法制备了直径范围可以控制在30～90 nm的聚苯乙烯小球模板,为小尺寸有序硅纳米线的制备打下了坚实的基础。本文简要介绍了常规MACE的原理和制备流程,总结了硅晶片的类型、刻蚀溶液的浓度、温度和刻蚀时间等因素对Si NWs形貌、尺度、表面粗糙度、刻蚀方向以及刻蚀速率的影响,用相关的机制解释了H2O2过量时刻蚀路径偏离垂直方向的机理以及刻蚀速率随溶液浓度变化的原因,重点综述了氧化层预处理、物理法沉积贵金属纳米薄膜、退火处理和模板法等改进方法在减少纳米线顶部团簇、改善均匀性、制备有序且直径和间距可控纳米线中的研究进展。

Silicon nanowire arrays(Si NWs)have been extensively used in nanoscale thermoelectric and biosensors,optoelectronic devices,solar cells,lithium-ion battery and other fields,thanks to its unique structure,high thermal conductivity,photoelectric properties and electrochemical performance.The metal-assisted chemical etching(MACE)provides a simple,convenient,low-cost and high efficient approach to fabricate Si NWs,which is most likely to achieve large-scale commercial application and has aroused considerable attention in the past decades.The processes of preparing silicon nanowires by metal-assisted chemical etching can be separated into two steps.First,a layer of metal(Ag,Au,Pt,et al.)nanoparticles is deposited on the surface of clean silicon wafer to oxidize the nearby silicon atoms.Then dissolve the oxide layer by HF and etch the silicon wafer to form an array of Si NWs.Nevertheless,this simple and efficient approach for preparing Si NWs presents several uncontrollable defects.Firstly,the deposited metal nanoparticles on silicon wafer surface would gradually grow larger and agglomerate together,resulting in low density of Si NWs.Secondly,nonuniformity distribution of metal nanoparticles on silicon wafer not only leads to relatively wide diameter range(50-200 nm)of the fabricated nanowires,but also causes the disorder of nanowire arrays and the difficulty to control their spacing.Thirdly,it is commonly observed that long nanowire clusters at tips,due to van der Waals.In view of the problems in conventional approaches and the requirements of diverse devices for the morphology,types and diameters of the silicon nanowires,therefore,recent studies have focused on reducing of clusters at the top of nanowires,regulating the surface roughness and diameter of nanowires,and preparing ordered silicon nanowires at low cost.Notable progress has been achieved in optimizing the conventional MACE methods,which can be concluded as follow.Ⅰ.Pre-surface treatment of silicon wafer using acid solution or UV/Ozone will improve the uniformity of nanowire arrays as well as increase the density(from 18%to 38%).Ⅱ.Depositing a layer of metal nano-film on the surface of silicon wafer by physical vapor deposition,followed by etching,can effectively reduce the clusters and improve the uniformity of nanowire diameter.Ⅲ.Template methods(polystyrene spheres templates,porous anodic alumina templates,silica templates,and photoresist templates,etc.)can realize the fabrication of ordered nanowire arrays.The controllable diameter of polystyrene nanoparticles,ranging from 30 nm to 90 nm,were successfully prepared by ion beam etching technology in our research group,which pave an effective way for preparing ordered Si NWs with small size(less than 100 nm).In this article,the basic synthetic processes and fundamental etching mechanism of MACE are briefly introduced.Specifically,the impact of type of silicon substrate,the concentration of etching solution,temperature and etching time on the morphology,surface roughness,etching direction and etching rate of Si NWs are discussed.The mechanism of etching path deviation from the vertical direction under excessive amount of H 2O 2 and the reason for the variation of etching rate with the solution concentration are explained by the relevant theories.Emphasis is put on the research progress of reducing the cluster at the top of nanowires,improving uniformity,preparing ordered nanowires with controllable diameters and spacing by approaches including pretreatment of silicon wafer using oxidation layer,deposition of metal nanofilms using physical method,the treatment of annealing and template method.