Nanowire Transistor Performance Limits and Applications

Semiconductor nanowires represent unique materials for exploring phenomena at the nanoscale. Developments in nanowire growth have led to the demonstration of a wide range of nanowire materials with precise control of composition, morphology, and electrical properties, and it is believed that this excellent control together with small channel size could yield device performance exceeding that obtained using top–down techniques. Here, we review advances in chemically synthesized semiconductor nanowires as nanoelectronic devices. We first introduce basic nanowire field-effect transistor structures and review results obtained from both p- and n-channel homogeneous composition nanowires. Second, we describe nanowire heterostructures, show that by using nanowire heterostructures, several limiting factors in homogeneous nanowire devices can be mitigated, and demonstrate that nanowire transistor performance can reach the ballistic limit and exceed state-of-the-art planar devices. Third, we discuss basic methods for organization of nanowires necessary for fabricating arrays of device and circuits. Fourth, we introduce the concept of crossbar nanowire circuits, discuss results for both transistor and nonvolatile switch devices, and describe unique approaches for multiplexing/demultiplexing enabled by synthetically coded nanowire. Fifth, we discuss the unique application of thin-film nanowire transistor arrays on low-cost substrates and illustrate this with results for relatively high-frequency ring oscillators and completely transparent device arrays. Finally, we describe 3-D heterogeneous integration that is uniquely enabled by multifunctional nanowires within a bottom–up approach.      

In Situ Scanning Electron Microscopy Observation of Growth Kinetics and Catalyst Splitting in Vapor–Liquid–Solid Growth of Nanowires

In situ observations during vapor–liquid–solid (VLS) growth of semiconductor nanowires in the chamber of an environmental scanning electron microscope (ESEM) are reported. For nanowire growth, a powder mixture of CdS and ZnS is used as a source material and silver nanoparticles as a metal catalyst. Through tracing growth kinetics of nanowires, it is found that nanowires with a relatively bigger catalyst droplet on the tip grow faster. Intriguingly, it is also found that the growth of nanowires can involve catalyst splitting: while the majority of catalyst remains at the nanowire tip and continues facilitating the growth, a portion of it is removed from the tip due to the splitting. It remains attached to the nanowire at the position where the splitting occurred and subsequently induces the growth of a nanowire branch. As far as it is known, this is the first time that catalyst splitting is revealed experimentally in situ. It is proposed that the instability of catalyst droplet caused by the volume increase is the main reason for the splitting. It is believed that in situ growth inside the ESEM can largely enrich our understanding on the metal‐catalyzed VLS growth kinetics, which may open up more opportunities for morphology‐controlled synthesis of 1D semiconductor nanowires in future study.     

准一维半导体纳米结构的电子显微学研究

本文总结了近年来我们在功能准一维纳米结构材料研究方面所获得的一些有意义的结果。借助于现代电子显微镜技术，不仅研究了硅、氮化稼、氧化锌等一维纳米材料的形貌和显微结构，还研究了其一维择优生长机理及小尺度效应。尤其是利用高能量分辨电子能量损失谱、高角环形暗场探头等先进技术，解决了一个传统X－光等结构分析手段所不能解决的难题，分析了一种SiOx/SiC复合纳米电缆的成份与结构。     

基于半导体纳米线的环形激光器

针对微光电子机械系统(MOMES)传感器件对微型片上光源的需求与当前半导体纳米线结构制备的进展,设计了一种基于Ⅱ-Ⅵ族半导体纳米线的激光器.将生长出来的纳米线在纯净水中超声制备均匀分布的悬浊液,再滴至载玻片表面,在显微镜50×物镜下观测得到纳米线呈现稀疏分布,约2～3根,选择尺寸较长的纳米线作为制备环形谐振腔的材料.首...     

激光烧蚀法制备半导体纳米丝的研究进展

简要介绍了当前国内外激光烧蚀法制备半导体纳米丝的研究现状。介绍了目前激光烧蚀法制备的实验装置及所采用的激光束参数。比较了不同实验结果中在纳米丝结构和生长方向等方面的差异，并分析了半导体纳米丝生长的VLS金属催化机理和氧化物辅助生长模型，我们认为Si-金属混合物作靶时金属催化作用对纳米丝的生长起主要作用，而在Si-氧化物混合物作靶时，氧化物辅助作用将占主导地位。     

III–V semiconductor nanowires for optoelectronic device applications

Semiconductor nanowires have recently emerged as a new class of materials with significant potential to reveal new fundamental physics and to propel new applications in quantum electronic and optoelectronic devices. Semiconductor nanowires show exceptional promise as nanostructured materials for exploring physics in reduced dimensions and in complex geometries, as well as in one-dimensional nanowire devices. They are compatible with existing semiconductor technologies and can be tailored into unique axial and radial heterostructures. In this contribution we review the recent efforts of our international collaboration which have resulted in significant advances in the growth of exceptionally high quality III–V nanowires and nanowire heterostructures, and major developments in understanding the electronic energy landscapes of these nanowires and the dynamics of carriers in these nanowires using photoluminescence, time-resolved photoluminescence and terahertz conductivity spectroscopy.     

Lateral growth and shape of semiconductor nanowires

The lateral growth of semiconductor nanowires and its influence on the nanowire shape in the case of nanocrystal formation according to the diffusion mechanism is theoretically studied. Possible types of the dependence of the adatom concentration at the lateral faces of the nanowires on the vertical coordinate are found. A self-consistent model is developed within the linear approximation in the adatom concentration, which makes it possible to describe simultaneously both vertical and lateral nanowire growth. The possible shapes of the nanowires, depending on the growth conditions, are described within this model and compared with the experimental data on different III–V systems.     

硅基Ⅲ-Ⅴ族纳米线及其若干半导体器件

综述了硅基Ⅲ-Ⅴ族纳米线与异质结制备技术的研究进展.针对基于Ⅲ-Ⅴ族纳米线的半导体器件,重点介绍了硅基Ⅲ-Ⅴ族纳米线场效应晶体管的研究现状,详细介绍了硅基Ⅲ-Ⅴ族纳米线场效应晶体管和隧穿场效应晶体管的制备流程、工艺技术和器件的电学性能,并对影响器件电学性能的因素进行了分析.概括介绍了硅基Ⅲ-Ⅴ族纳米线激光器和硅基Ⅲ-Ⅴ族纳米线太阳电池的研究成果,基于硅衬底的Ⅲ-Ⅴ族纳米线太阳电池为低成本、高效能的太阳电池领域开辟了新途径.研究结果表明,采用硅基Ⅲ-Ⅴ族纳米线制备的场效应晶体管、激光器及太阳电池等半导体器件相对于Si,Ge等传统半导体材料制备的器件有着巨大的优势,在未来集成电路技术中具有越来越大的影响力.     

锗纳米线的性能与应用

重点分析讨论了锗纳米线在电学、光学、光电导等特性及其在场效应晶体管制造方面的研究应用现状与最新进展。综合分析表明,未经处理的锗纳米线表面存在一层氧化物及缺陷,与电极连接时欧姆接触性能较差,在制备锗纳米线器件以前必须对锗纳米线表面进行钝化以便沉积电极;对锗纳米线进行掺杂可以改善Ge纳米线的性能,制造出实用Ge纳米线器件。指出在一根纳米线上生长硅/锗半导体纳米线形成硅/锗半导体界面,直接用单根纳米线制造具有完整功能的电子器件是将来重要的研究方向。     

Measurement of Young’s modulus of GaAs nanowires growing obliquely on a substrate

A convenient and fast method for measuring Young’s modulus of semiconductor nanowires obliquely standing on the growth substrate is presented. In this method, the nanowire is elastically bent under the force exerted by the probe of an atomic-force microscope, and the load-unload dependences for the bending of the probe cantilever are recorded. Next, these curves are used to find the bending stiffness of the tilted nanowires, after which, taking into account the nanowire dimensions, Young’s modulus is obtained. The implementation of this method is demonstrated for tilted GaAs nanowires growing on a GaAs (111) substrate. Young’s modulus is determined by applying finite-element analysis to the problem of the stationary elastic bending of a nanowire taking into account the actual nanowire shape and faceting. It proves that a fairly accurate estimate of Young’s modulus can be obtained even if the nanowire shape is approximated by a circular cylinder with a single cross-sectional area. The values of Young’s modulus obtained for GaAs nanowires of cubic lattice symmetry are 2 to 3 times smaller than its value for bulk GaAs. This difference is attributed to the presence of stacking faults in the central part of the nanowires.     

Growth of III-V semiconductor nanowires by molecular beam epitaxy

We present here the growth of GaAs, InAs and InGaAs nanowires by molecular beam epitaxy. The nanowires have been grown on different substrates [GaAs(0 0 1), GaAs(1 1 1), SiO₂ and Si(1 1 1)] using gold as the growth catalyst. We show how the different substrates affect the results in terms of nanowire density and morphology. We also show that the growth temperature for the InGaAs nanowires has to be carefully chosen to obtain homogeneous alloys.     

外延碳化硅纳米线的合成和显微结构研究

本文以硅为衬底,用热蒸发SiO粉末的方法合成了外延碳化硅(SiC)纳米线。利用扫描电子显微镜(SEM)、透射电子显微镜(TEM)等对SiC纳米线进行了电子显微学分析。实验发现,在SiC纳米线生长前,衬底上首先自发形成了一层SiC多晶膜,纳米线在这层多晶膜的某些晶粒上外延生长。在显微结构分析的基础上,本文探讨了外延生长一维纳米结构的有利条件是高的生长温度和低的生长速率。     

外延碳化硅纳米线的合成和显微结构研究

本文以硅为衬底,用热蒸发SiO粉末的方法合成了外延碳化硅(SiC)纳米线.利用扫描电子显微镜(SEM)、透射电子显微镜(TEM)等对SiC纳米线进行了电子显微学分析.实验发现,在SiC纳米线生长前,衬底上首先自发形成了一层SiC多晶膜,纳米线在这层多晶膜的某些晶粒上外延生长.在显微结构分析的基础上,本文探讨了外延生长一...     

石墨烯上生长GaN纳米线

采用金属Ga升华法在石墨烯/蓝宝石衬底上生长了高质量GaN纳米线,研究了不同的生长条件,如NH3流量、反应时间、催化剂和缓冲层等对GaN纳米线形貌的影响,采用扫描电子显微镜(SEM)对GaN纳米线进行表征.研究发现,在适当的NH3流量且无催化剂时,衬底上可以生长出粗细均匀的GaN纳米线.反应时间为5 min时,纳米线密集分布在衬底上,表面光滑.在石墨烯/蓝宝石上预先低温生长GaN缓冲层,然后升温至1 100℃进行GaN纳米线生长,获得了具有择优取向的GaN纳米线结构.研究表明,石墨烯和缓冲层对获得GaN纳米线结构有序阵列具有重要的作用.     

A General Approach for Fabricating Arc‐Shaped Composite Nanowire Arrays by Pulsed Laser Deposition

Here, a new method is demonstrated that uses sideways pulsed laser deposition to deliberately bend nanowires into a desired shape after growth and fabricate arc‐shaped composite nanowire arrays of a wide range of nanomaterials. The starting nanowires can be ZnO, but the materials to be deposited can be metallic, semiconductor, or ceramic depending on the application. This method provides a general approach for rational fabrication of a wide range of side‐by‐side or “core–shell” nanowire arrays with controllable degree of bending and internal strain. Considering the ZnO is a piezoelectric and semiconductive material, its electrical properties change when deformed. This technique has potential applications in tunable electronics, optoelectronics, and piezotronics.     

Atomic Insights of Self-Healing in Silicon Nanowires

The self-healing capability is highly desirable in semiconductors to develop advanced devices with improved stability and longevity. In this study, the automatic self-healing in silicon nanowires is reported, which are one of the most important building blocks for high-performance semiconductor nanodevices. A recovery of fracture strength (10.1%) on fractured silicon nanowires is achieved, which is demonstrated by in situ transmission electron microscopy tensile tests. The self-healing mechanism and factors governing the self-healing efficiency are revealed by a combination of atomic-resolution characterizations and atomistic simulations. Spontaneous rebonding, atomic rearrangement, and van der Waals attraction are responsible for the self-healing in silicon nanowires. Additionally, the self-healing efficiency is affected by the fracture surface roughness, the nanowire size, the nanowire orientation, and the passivation of dangling bonds on fracture surfaces. These new findings shed light on the self-healing mechanism of silicon nanowires and provide new insights into developing high-lifetime and high-security semiconductor devices.     

硅纳米线的掺杂工艺与检测

掺杂硅纳米线有可能成为一种重要的硅纳米电子器件材料。因而,硅纳米线的掺杂工艺与检测很重要。半导体的掺杂工艺主要为扩散法,而硅纳米半导体线的掺杂检测方法主要包括电流-电压法、拉曼光谱、光致发光(PL)光谱、X-射线光电子能谱(XPS)及近边X-射线吸收精细结构光谱(NEXAFS)等。该文介绍了可引入到硅纳米线研究的现有半导体的掺杂工艺及检测方法,并就硅纳米线的掺杂工艺及检测的最新进展做作了详细的讨论。     

An Evanescent Coupling Approach for Optical Characterization of ZnO Nanowires

使用纳米氧化硅光纤探针,利用倏逝波耦合方法,将紫外到红外的激光成功地耦合进单根ZnO纳米线,耦合效率可达25%.实验观测了单根纳米线的荧光特性,发现ZnO纳米线光传输损耗很低.研究证明:采用透镜聚焦激发纳米线发光的传统耦合方法,只能使用特殊激发波长的光;而倏逝波耦合方法具有高效、适用性强的特点,在半导体纳米线和纳米带的光学特性研究中有广泛的应用前景.     

Modal gain in a semiconductor nanowire laser with anisotropic bandstructure

We investigate optical gain for the modes guided by semiconductor nanowires. We focus on optically anisotropic wurtzite-type semiconductors (such as GaN) and the situation when the optical axis of the crystal coincides with the geometrical axis of the nanowire. For GaN nanowire lasers, the calculation of the modal gain requires the knowledge of two confinement factors for a given mode and two gain coefficients for the bulk crystal. We show that the confinement factors for nanowire lasers are very large in comparison to those for heterostructure lasers, and can even exceed unity. To estimate the bulk gain in GaN we use the free-carrier model and emphasize the importance of accounting for anisotropy of gain. Using the calculated confinement factors and bulk gain, we predict that free-standing nanowires with small radius (R /spl lsim/ 70 nm) lase into the HE/sub 11/ mode, thicker nanowires (70 nm /spl lsim/ R /spl lsim/ 90 nm) lase into the TE/sub 01/ mode.     

Bright CdSe quantum dot inserted in single ZnSe nanowires

We report the evidence of CdSe quantum dot (QD) insertion in single defect-free ZnSe nanowire. These nanowires have been grown by molecular beam epitaxy in vapour-liquid-solid growth mode catalysed with gold particles. We developed a two-step process allowing us to grow very thin (from 15 to 5 nm) defect-free ZnSe nanowire on top of a nanoneedle, where all defects are localised. The CdSe QDs are incorporated to the defect-free nanowires part. Owing to the extraction efficiency of the nanowires and the reduced number of stacking fault defects in the two-step-process nanowires, a very efficient photoluminescence is observed even on isolated single nanowire. Time-resolved photoluminescence and correlation photon give evidences that the bright photon emission is related to the CdSe QD.