基于气-液-固模式的Ⅲ-Ⅴ族一维半导体纳米线的生长研究概述

Ⅲ-Ⅴ族一维半导体纳米线由于具有独特的性能、丰富的科学内涵而被广泛应用于微机电、光电子、光伏电、传感等方面,并在未来纳米结构器件中占有重要的战略地位,近年来引起了人们极大的兴趣和关注。探索Ⅲ-Ⅴ族一维半导体纳米线新的结构调控手段,研究具有重要应用价值的Ⅲ-Ⅴ族一维半导体纳米线的可控生长方法和技术,从而获得可应用于器件和功能实现的高质量Ⅲ-Ⅴ族一维半导体纳米线是目前各研究组的主要目标。基于气-液-固模式的纳米线生长方法具有对纳米线形貌及晶体质量可控的优点,成为当前制备高质量Ⅲ-Ⅴ族一维半导体纳米线的主要生长技术。催化辅助生长是一种有金属催化剂参与的纳米线生长方式,它可以有效降低反应物裂解能量、提高材料成核质量、控制材料生长方向、提高反应效率、稳定材料晶体结构。自催化生长是指在纳米线生长过程中不添加其他物质作为催化剂,而由反应物自身起催化作用的生长。由于自催化生长在反应过程中未引入其他物质,所以生成物纯度较高。Ⅲ-Ⅴ族异质结构半导体纳米线常具有两种半导体各自不能达到的优良光电特性,其又可划分为纵向异质结构和横向异质结构。Ⅲ-Ⅴ族一维半导体纳米线除了可以在与其自身材料相同的基底表面上生长之外,还可在与其材料不同的基底表面上生长,即在异质基底表面生长。异质基底生长在材料兼容、光电集成等方面具有十分广阔的应用前景。本文对基于气-液-固模式的Ⅲ-Ⅴ族一维半导体纳米线的生长进行了综述,并对近些年基于催化辅助和自催化的纵向异质结构、横向异质结构以及异质基底的成长研究现状进行了总结,为推动Ⅲ-Ⅴ族一维半导体纳米线制备技术的发展提供了参考依据。     

GaAs(331)A面InAs自组织纳米结构形貌演化

采用分子束外延方法在CaAs(331)A高指数衬底上制备自对齐InAs纳米线(QWRs)或者三维(3D)岛状结构.InAs纳米线(QWRs)选择性生长在CaAs层的台阶边缘.通过原子力显微镜(AfM)仔细研究了InAs纳米微结构的表面形貌,发现不同的生长条件,包括衬底温度、生长速率、和InAs层厚度等,对InAa表面形貌有很大的影响.如,低温更容易导致线状纳米微结构的形成,而高温更利于3D岛状结构形成.表面形貌的转变归结于表面能同应变能之间的竞争.     

磁性金属Fe纳米线的制备及其性能

本文尝试采用纳米Au颗粒作为催化剂,利用化学气相沉积法合成铁纳米线。并着重研究了不同沉积温度对纳米线生长过程的影响。研究结果表明,过低沉积温度无法分解二茂铁,而过高沉积温度则会导致二茂铁高温分解副反应发生,生成大量碳颗粒,从而阻碍铁纳米线的生长。作者在600℃沉积温度下,以二茂铁为反应前驱体和蓝宝石作为基板,通过纳米Au颗粒的催化作用首次成功获得了大批量的铁单晶纳米线,并进一步揭示了纳米线的气-固生长机制。此外,本文还对合成的单晶Fe纳米线的磁各向异性进行了探讨。     

芯-壳结构/GaAs-AlGaAs纳米线中AlGaAs壳材料均匀性研究

Ⅲ-Ⅴ族半导体纳米线材料被认为是高速光电探测器最有前途的材料之一。采用金属有机化学气相沉积法在GaAs(111)B衬底上生长芯-壳结构/GaAs-AlGaAs纳米线材料,GaAs芯材料的生长机制为气相-液相-固相,而AlGaAs壳材料的生长机制为气相-固相。采用场发射扫描电子显微镜和微区光荧光谱仪等测试分析手段研究了AlGaAs壳材料横向、轴向和生长方向的均匀性,探讨了生长机制对均匀性的影响。在此基础上获得了组分均匀的高晶体质量的AlGaAs壳材料,其Al组分为0.14。     

钴、镍纳米线及其异质结的制备与磁性研究

采用脉冲电沉积技术在氧化铝模板中制备了单晶钴、镍纳米线阵列和镍/钴纳米线异质结阵列.分别用场发射扫描电镜、透射电镜、X射线衍射仪、物理性能测试系统对纳米线阵列的微观形貌、结构和性能进行了表征与研究.结果表明,所制备的磁性纳米线有很大的长径比,易磁化方向均为纳米线长轴方向.纳米线异质结阵列在易磁化方向具有较大的矫顽力和矩形比,可用作高密度垂直磁记录材料.     

一维纳米结构氧化锌材料的气相沉积制备及生长特性研究

ZnO是一种可用于室温或更高温度下的紫外发光材料,纳米结构的ZnO(如单晶薄膜、纳米粒子膜、纳米线和纳米带等)则更是在紫外激光发射领域显示了独到的优势,被认为是有望构造短波长半导体激光器的理想材料.本文对一维ZnO纳米结构(纳米线、纳米管和纳米带)的真空物理气相沉积制备技术及生长机理进行了初步探讨.     

InAs/GaInSb应变层超晶格材料的界面结构

介绍了InAs／GaInSb应变层超晶格材料的界面结合类型及其对材料界面结构和光电性能的影响,分析了在超晶格界面处的原子置换和扩散现象。同时,总结了分子束外延生长过程中控制InAs／GaInSb超晶格界面结构的生长工艺,指出采用原子迁移增强（MEE）外延生长技术,可以有效地抑制界面处原子的置换和扩散现象。     

硅纳米线的制备及其生长条件探究

Si纳米材料自问世以来便受到研究者的重视，其不同于宏观块体材料的特殊性质可使其应用于各个领域。如何制备形貌较好且具有良好光电性能的纳米材料是在纳米材料应用前必须解决的问题。以Ni膜作为催化剂直接在Si衬底上制备了密集的Si纳米线，获得了Si纳米线较强的蓝紫波段的发光，研究了退火温度、退火气氛N₂流速、Si膜厚度等制备条件对Si纳米线形貌、光致发光强度的影响，并讨论了双层膜制备Si纳米线形成和生长机理。实验结果显示，退火温度、N₂流速对Si纳米线的生长起到关键性的作用，N₂流速能够影响Si纳米线的光致发光强度，且较大的N₂流速能够使Si纳米线定向生长。而在Ni膜催化剂上预沉积一层适当厚度的Si膜也有助于Si纳米线的生长，且有效改善了Si纳米线的光致发光强度。     

1.5微米波段GaAs基异变InAs量子点分子束外延生长

1.55微米波段GaAs基近红外长波长材料在光纤通讯,高频电路和光电集成等领域有潜在的应用价值。本文用分子束外延方法研究了GaAs基异变InAs量子点材料的生长,力图实现在拓展量子点发光波长的同时保持或增加InAs量子点的密度。在实验中,首先优化了In0.15GaAs异变缓冲层的生长,研究了生长温度和退火对减少穿通位错的作用。在此基础上,优化了长波长InAs量子点的生长。最终在GaAs基上获得了温室发光波长在1491nm,半高宽为27.73meV,密度达到4×1010cm-2的InAs量子点。     

氧化锌基发光二极管的研究进展

概述了ZnO材料的发光性质以及ZnO基发光二极管(LED)器件的发展历程、工作原理和技术路线。详细介绍了各种结构的ZnO基同质结、异质结LED及一维ZnO纳米线/棒阵列异质结LED的最新研究成果,存在的问题以及改进的方法。通过改进器件结构、提高材料质量和采用新型纳米结构材料,使得ZnO基LED的光谱质量和电致发光效率有了本质提高。     

Growth of straight InAs-on-GaAs nanowire heterostructures

One of the main motivations for the great interest in semiconductor nanowires is the possibility of easily growing advanced heterostructures that might be difficult or even impossible to achieve in thin films. For III-V semiconductor nanowires, axial heterostructures with an interchange of the group III element typically grow straight in only one interface direction. In the case of InAs-GaAs heterostructures, straight nanowire growth has been demonstrated for growth of GaAs on top of InAs, but so far never in the other direction. In this article, we demonstrate the growth of straight axial heterostructures of InAs on top of GaAs. The heterostructure interface is sharp and we observe a dependence on growth parameters closely related to crystal structure as well as a diameter dependence on straight nanowire growth. The results are discussed by means of accurate first principles calculations of the interfacial energies. In addition, the role of the gold seed particle, the effect of its composition at different stages during growth, and its size are discussed in relation to the results observed.     

Faceting, composition and crystal phase evolution in III-V antimonide nanowire heterostructures revealed by combining microscopy techniques

III-V antimonide nanowires are among the most interesting semiconductors for transport physics, nanoelectronics and long-wavelength optoelectronic devices due to their optimal material properties. In order to investigate their complex crystal structure evolution, faceting and composition, we report a combined scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning tunneling microscopy (STM) study of gold-nucleated ternary InAs/InAs(1-x)Sb(x) nanowire heterostructures grown by molecular beam epitaxy. SEM showed the general morphology and faceting, TEM revealed the internal crystal structure and ternary compositions, while STM was successfully applied to characterize the oxide-free nanowire sidewalls, in terms of nanofaceting morphology, atomic structure and surface composition. The complementary use of these techniques allows for correlation of the morphological and structural properties of the nanowires with the amount of Sb incorporated during growth. The addition of even a minute amount of Sb to InAs changes the crystal structure from perfect wurtzite to perfect zinc blende, via intermediate stacking fault and pseudo-periodic twinning regimes. Moreover, the addition of Sb during the axial growth of InAs/InAs(1-x)Sb(x) heterostructure nanowires causes a significant conformal lateral overgrowth on both segments, leading to the spontaneous formation of a core-shell structure, with an Sb-rich shell.     

Growth of InAs/InP core-shell nanowires with various pure crystal structures

We have studied the epitaxial growth of an InP shell on various pure InAs core nanowire crystal structures by metal-organic vapor phase epitaxy. The InP shell is grown on wurtzite (WZ), zinc-blende (ZB), and {111}- and {110}-type faceted ZB twin-plane superlattice (TSL) structures by tuning the InP shell growth parameters and controlling the shell thickness. The growth results, particularly on the WZ nanowires, show that homogeneous InP shell growth is promoted at relatively high temperatures (～500?°C), but that the InAs nanowires decompose under the applied conditions. In order to protect the InAs core nanowires from decomposition, a short protective InP segment is first grown axially at lower temperatures (420-460?°C), before commencing the radial growth at a higher temperature. Further studies revealed that the InP radial growth rate is significantly higher on the ZB and TSL nanowires compared to WZ counterparts, and shows a strong anisotropy in polar directions. As a result, thin shells were obtained during low temperature InP growth on ZB structures, while a higher temperature was used to obtain uniform thick shells. In addition, a schematic growth model is suggested to explain the basic processes occurring during the shell growth on the TSL crystal structures.     

InAs1-xPx nanowires for device engineering

We present the growth of homogeneous InAs(1-x)P(x) nanowires as well as InAs(1-x)P(x) heterostructure segments in InAs nanowires with P concentrations varying from 22% to 100%. The incorporation of P has been studied as a function of TBP/TBAs ratio, temperature, and diameter of the wires. The crystal structure of the InAs as well as the InAs(1-x)P(x) segments were found to be wurtzite as determined from high-resolution transmission electron microscopy. Furthermore, temperature-dependent electrical transport measurements were performed on individual heterostructured wires to extract the conduction band offset of InAs(1-x)P(x) relative to InAs as a function of composition. From these measurements we extract a value of the linear coefficient of the conduction band versus x of 0.6 eV and a nonlinear coefficient, or bowing parameter, of 0.2 eV. Finally, homogeneous InAs(0.8)P(0.2) nanowires were shown to have a nondegenerate n-type doping and function as field-effect transistors at room temperature.     

Growth and characterization of InP nanowires with InAsP insertions

We report on the fabrication by Au-assisted molecular beam epitaxy of InP nanowires with embedded InAsP insertions. The growth temperature affects the nucleation on the nanowire lateral surface. It is therefore possible to grow the wires in two steps: to fabricate an axial heterostructure (at 420 degrees C), and then cover it by a shell (at 390 degrees C). The InAsP alloy composition could be varied between InAs0.35P0.65 and InAs0.5P0.5 by changing the As to P flux ratio. When a shell is present, the InAsP segments show strong room-temperature photoluminescence with a peak wavelength tunable from 1.2 to 1.55 mum by adjusting the As content. If the axial heterostructure has no shell, luminescence intensity is drastically reduced. Low-temperature microphotoluminescence performed on isolated single wires shows narrow peaks with a line width as small as 120 microeV.     

Realizing Zinc Blende GaAs/AlGaAs Axial and Radial Heterostructure Nanowires by Tuning the Growth Temperature

Vertical zinc blende GaAs/AlGaAs heterostructure nanowires were grown at different temperatures by metalorganic chemical vapor deposition via Au-assisted vapor-liquid-solid mechanism.It was found that radial growth can be enhanced by increasing the growth temperature.The growth of radial heterostructure can be realized at temperature higher than 500℃,while the growth temperature of axial heterostructure is lower than 440℃.The room temperature photoluminescence properties of the nanowires were investigated and the relevant growth mechanism was discussed.     

Coexistence of Vapor–Liquid–Solid and Vapor–Solid–Solid Growth Modes in Pd‐Assisted InAs Nanowires

During the growth of InAs nanowires from Pd catalyst particles on InAs(111)A substrates, two distinct classes of nanowires are observed with smooth or zigzagged sidewalls. It is shown that this is related to a bimodal distribution of the wire‐tip diameter: above a critical diameter wires grow with smooth sidewalls, and below with zigzagged morphology. Transmission electron microscopy analysis shows that the catalyst particles at the tip of zigzagged wires are smooth and have a higher aspect ratio than those at the tip of smooth wires. Zigzagged wires grow from liquid particles in the vapor–liquid–solid (VLS) mode whereas the smooth ones grow from solid particles in the vapor–solid–solid (VSS) mode.     

Position-controlled interconnected InAs nanowire networks

We demonstrate here a method for controlled production of complex self-assembled three-dimensional networks of InAs nanowires on a substrate, based on sequentially seeded epitaxial nanowire structures, or "nanotrees". A position-controlled array of trunk nanowires is first produced using lithographically defined Au particles as seeds. With these wires positioned along the proper crystallographic directions with respect to each other, nanotree branches grow toward neighboring trunks, connecting them together. Finally, we investigate the crystal structure of the interconnected nanotrees, demonstrating that branch growth after the contact with the second trunk has an epitaxial relationship to that trunk.     

Diameter dependent growth rate and interfacial abruptness in vapor-liquid-solid Si/Si1-xGex heterostructure nanowires

A strong diameter dependence is observed in the interfacial abruptness and growth rates in Si/Si 1- x Ge x axial heterostructure nanowires grown via Au-mediated low pressure CVD using silane and germane precursors. The growth of these nanowires has similarities to that of heterostructure thin films with similar compositional interfacial broadening, which increases with and is on the order with diameter. This broadening may reveal a fundamental challenge to fabrication of abrupt heterostructures via VLS growth.     

Strain and shape of epitaxial InAs/InP nanowire superlattice measured by grazing incidence X-ray techniques

Quantitative structural information about epitaxial arrays of nanowires are reported for a InAs/InP longitudinal heterostructure grown by chemical beam epitaxy on an InAs (111)B substrate. Grazing incidence X-ray diffraction allows the separation of the nanowire contribution from the substrate overgrowth and gives averaged information about crystallographic phases, epitaxial relationships (with orientation distribution), and strain. In-plane strain inhomogeneities, intrinsic to the nanowires geometry, are measured and compared to atomistic simulations. Small-angle X-ray scattering evidences the hexagonal symmetry of the nanowire cross-section and provides a rough estimate of size fluctuations.