【作者】 许金友；

【导师】 潘安练；

【作者基本信息】 湖南大学 ， 材料科学与工程， 2014， 博士

【摘要】 纳米结构的独特物化性质不仅有利于半导体器件的小型化和集成化，而且在新型纳米光电子器件研发领域有着重要的潜在应用价值。带隙是半导体光电子器件中最重要的本征参数之一，原则上决定了半导体光电器件的光谱响应特征。然而众所周知，所有天然半导体可供利用的带隙值组成的是一个十分有限的离散集。为了克服天然半导体带隙的这种局限性，人们利用合金化和量子尺寸效应等多种手段开展了半导体带隙调控工作，极大地丰富了半导体可供利用的带隙值，甚至成功地在单个衬底上集成了一系列递变的带隙。鉴于单个纳米结构中的带隙调控能够在纳米尺度空间集成丰富的带隙值，有望为纳米光电器件研发提供全新的材料平台，单纳米结构带隙调控在纳米科技日新月异的发展过程中日益重要。有鉴于此，本论文精心设计和改进了传统热蒸发纳米材料生长技术并在此基础上生长了多种特殊半导体纳米结构。最终成功地在单个纳米结构中通过合金组分调控实现了单纳米结构带隙调控。更加重要的是，利用这些特殊的纳米结构，我们设计和研究了几种重要的新型纳米光子学概念器件并测试评估了它们的性能表现。主要代表性研究成果归纳如下：（1）率先将磁力助推系统整合到了传统的热蒸发纳米材料生长系统中，成功地在高温生长过程中实现了固态反应源材料置换或收集衬底位移。在此基础上，生长了沿纳米线轴向具有全组分梯度CdSxSe1-x(x=0~1)和ZnxCd1-xSySe1-y(x, y=0~1)合金纳米线，实现了沿单根纳米线轴向的梯度带隙调控。它们均可用作集成了丰富波长的高品质纳米光源。尤其是ZnxCd1-xSySe1-y纳米线，其发光波长几乎覆盖整个可见光谱区且尺寸小于人眼的分辨极限，可直接用作高品质纳米白光光源。（2）我们从理论上预测了组分梯度型半导体纳米线中的非对称光波导现象，并以组分梯度CdSxSe1-x纳米线为基础，首次在半导体纳米线中阐明了具有结构简单、成本低廉、工作波长可调、工作阀值低，纳米尺寸和方便集成等优秀特征的非对称光传播。研究表明，光在组分递变型半导体纳米线中的传输损耗与传输方向密切相关。沿带隙增加方向传输时的损耗来自结构无序诱导的带尾态吸收和非理想表面泄漏。相反，沿带隙减小方向传输时除上述损耗外，总传输损耗由于每个吸收-发射过程中伴随的大量无辐射跃迁而急剧增加。正是这种传输方向依赖的巨大损耗差异提供了半导体纳米线非对称光传输的基本原理。（3）系统研究了组分递变型CdSxSe1-x半导体纳米线中的光波导行为，首次发现了沿这种纳米线的带隙减小方向的波长转换光波导过程。研究表明，当聚集激光束在窄带隙端激发纳米线时，纳米线对于激发出来的光是一种无源波导介质，光在其中通过全反射沿纳米线带隙增加方向往前传播，整个传播过程中保持传输光光子频率不变。相反，当在宽带隙端激发纳米线时，纳米线对于激发出来的光是一种有源波导介质，激发出来的光在纳米线中通过不断重复的带间吸收-发射过程沿带隙减小的方向传播，其光波长将随传播的距离而不断改变，此时纳米线实现了波长转换器件的功能。在此基础上，利用梯度纳米线这些有趣的性质，通过将多根常规CdS纳米线搭在单根CdSxSe1-x纳米线上不同位置构成纳米线分支结构，首次成功演绎了基于半导体纳米线的波分器功能。（4）利用多步源移动式热蒸发法，可控的制备了一种类三明治纳米带横向异质结。这种异质结中间组分是CdSxSe1-x合金，两边外延生长了CdS，因此实现了沿单个纳米带宽度方向的突变带隙调控。在紫外光照射下，单个这种纳米带的发光颜色沿其宽度方向呈现典型的三明治结构，其中两边为绿色，中间为红色，均来自于相应组分的带边发光。在脉冲激光激发下，首次报导了基于这种单纳米带的室温双色激光发射并且两工作波长之间的间隔可通过控制中间CdSxSe1-x合金的组分实现可调（5）基于改进的热蒸发法，成功控制了锡在CdS纳米线中掺杂浓度，生长了具有不同束缚态发光强度的高质量CdS纳米线并以此为基础有效的降低了CdS纳米线中有源光波导的损耗，有效提高了其整体波导效率。此外，由于束缚态发光的存在，CdS的带边发射在这种纳米线中传输一定距离后将被彻底吸收，实现对半导体带边发光的过滤功能。这是首个利用掺杂来避免发光被半导体带隙吸收从而实现增强波导效率的例子。更加重要的是，利用这种高质量掺杂纳米线，首次报导了工作波长遍布540~750nm的低阀值束缚态回音壁多模半导体纳米激光器。更多还原

【Abstract】 The unique physical and chemical properties of nanostructures are not only helpfulfor miniaturization and integration of semiconductor devices but also possesspromising applications in novel nano-optoelectronic components. Bandgap is one ofthe most important intrinsic parameters for semiconductor-based optoelectronicdevices and it fundamentally decides the basic spectral-response feature of the devices.Unfortunately, it is well know that the bandgaps available in nature semiconductorsconsist of a very finite discrete set. In order to address this limitation, bandgapmodulation was developed by viarous means, like alloy structures and quantumconfinement effect, which significantly enriches the available bandgaps ofsemiconductors, even integration of a series of graded bandgaps along single substrate.Bandgap modulation within single-nanostructures is increasingly important for therapid development of nanotechnology considering the facts that it can enrich bandgapsdown to nanometer scale and provide brand new material platform for R&D ofnano-optoelectronic components.With these ideas in mind, in this dissertation, we well designed and improved thetraditional thermal evaporation growth method of nanostructures and accordinglyvarious special nanostructures were fabricated. Ultimately, bandgap modulation wassuccessfully achieved within single-nanostructures via variation of alloy composition.More importantly, several kinds of novel proof-of-concept nanophotonic devices wereconstructed using these special nanostructures and their performances were evaluated.The main achievements are summarized as follows.(1) We firstly succeeded in exchange of solid-state source reagents and movement ofcollection substrate during the growth of nanostructures at high temperature byintegrating pulling system of magnetic force into traditional thermal evaporationgrowth system. Based on these improved-strategy, CdSxSe1-x(x=0~1) andZnxCd1-xSySe1-y(x, y=0~1) nanowires with a completely compositional gradient alongaxial direction of the nanowire were fabricated, and accordingly graded bandgapmodulation along axial direction of single-nanowire was achieved. Both of them arehigh-quality integrated nanoscale light sources with abundant wavelengths. Especiallyfor those ZnxCd1-xSySe1-ynanowires, since their emission wavelengths cover almost theentire visible spectrum region and their nanometer scale footprints are beyond theresolution of human naked eyes, they can be directly used as high-quality nanoscale white light source.(2) We theoretically predicted the asymmetric light propagation alongcompositionally-graded semiconductor nanowires and further demonstratedasymmetric light propagation in semiconductor nanowires features simple structure,low cost, tunable work wavelength, ultra-low operation power, and nanoscale footprintbased on compositionally-graded CdSxSe1-xnanowires. Our results show that thepropagation optical loss along compositionally-graded semiconductor nanowires ishighly dependent on the guided direction. The propagation loss alongbandgap-increasing direction is dominated by the disorder-induced band-tailabsorption and non-perfect geometry-caused leakage. In comparison, the total lossalong bandgap-decreasing direction would be significantly increased, coming from thehuge nonradiative loss involved in each band-to-band re-absorption and re-emittingprocess, except for the same propagation loss along the opposite direction. It is thishuge propagation direction-dependent loss contrast that provides the physical base ofthe asymmetric light propagation along semiconductor nanowires.(3) We comprehensively investigated the optical waveguide in compositionally-graded CdSxSe1-xnanowires and presented the first wavelength conversion waveguidealong the bandgap-decreasing direction of these nanowires. Study revealed that whenthe narrow bandgap end of the wire is excited with a focused laser beam, the nanowireitself acts as a passive cavity for the emitting light of the nanowire, hence it can beguided passively along the bandgap-increasing direction of the nanowire by totalinternal reflection, keeping the photonic energy of the guided light almost unchangedduring the whole propagation. In comparison, when the wide bandgap end of thenanowire is excited, the nanowire itself acts as an active cavity for the emitting light ofthe nanowire, and it can be guided actively through incessantly repeated band-to-bandre-absorption and re-emitting processes along the bandgap-decreasing direction,resulting in a gradual wavelength conversion during propagation. In this case, thenanowire can work as optical wavelength converter. On the basis of thiswavelength-converted waveguide, a concept of nanoscale wavelength splitter isdemonstrated by assembling a graded nanowire with several compositionally-uniformnanowires into branched nanowire structure.(4) We reported the first controllable growth of sandwitch-like nanoribbon lateralheterostructures made of a CdSxSe1-xcentral region and two epitaxial CdS lateral sidesusing a multistep thermal evaporation route with moving sources, accordingly abruptbandgap modulation along the width direction of single-nanoribbons was achieved. Under excitation of an ultrviolet laser, the emission of these ribbons indicates finesandwich-like structures along width direction, with characteristic red emission in thecenter and green emission at both edges. Under pump of a pulse laser, a roomtemperature dual-color lasing with tunable wavelengths was demonstrated based onthese single-nanoribbon heterostructures for the first time. Moreover, the wavelengthspacing of this dual-color lasing is adjustable by viaring the composition of the centralCdSxSe1-xalloy.(5) We successfully controlled the doping concentration of Sn in CdS nanowiresby the improved thermal evaporation route, and accordingly high-quality dilutetin-doped CdS nanowires with controlled trap-state emission were synthesized. Usingthese nanowires, the active guiding loss of CdS nanowires was significantly reducedand the guiding efficiency was considerably increased. In addition, the bandedgeemission of CdS will be totally absorbed after a certain transport distance due to theexistence of trap-state emission, which could be discounted as a bandedge emissionfilter. This study is the first example of using dopants to shift emission away fromsemiconductor bandgap absorption for the purpose of enhanced waveguide. Moreimportantly, low threshold trap-state whispering gellory modes semiconductornanolaser with work wavelength spanning from540nm to750nm was reported usingthese high-quality doping nanowires for the first time.更多还原