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This book covers virtually all aspects of semiconductor nanowires, from growth to related applications, in detail. First, it addresses nanowires’ growth mechanism, one of the most important topics at the forefront of nanowire research. The focus then shifts to surface functionalization: nanowires have a high surface-to-volume ratio and thus are well-suited to surface modification, which effectively functionalizes them. The book also discusses the latest advances in the study of impurity doping, a crucial process in nanowires. In addition, considerable attention is paid to characterization techniques such as nanoscale and in situ methods, which are indispensable for understanding the novel properties of nanowires. Theoretical calculations are also essential to understanding nanowires’ characteristics, particularly those that derive directly from their special nature as one-dimensional nanoscale structures. In closing, the book considers future applications of nanowire structures in devices such as FETs and lasers.





Vapor–Liquid–Solid Growth of Semiconductor Nanowires

We discuss the growth of semiconductor nanowires, with an emphasis on the vapor–liquid–solid growth of III–V nanowires. Special attention is paid to modeling of growth and the resulting morphology, crystal phase, composition, nanowire heterostructures, and statistical properties within the nanowire ensembles. We give a general overview of the vapor–liquid–solid growth of nanowires by different epitaxy techniques and the bases for nanowire growth modeling. We discuss the role of surface energetics in the formation of GaAs nanowires, which has an important impact on the nanowire morphology and crystal phase. A detailed description of the nanowire growth kinetics is presented, including the transport-limited growth, chemical potentials, nucleation and growth of two-dimensional islands, and self-consistent growth models combining the material transport equations with the nucleation rate. The nanowire length and diameter distributions are considered along with the methods for narrowing them to sub-Poissonian values. Ternary III–V nanowires and heterostructures based on such nanowires are discussed, including the relaxation of elastic stress at the free sidewalls and the sharpening of the heterointerfaces. We consider polytypism of III–V nanowires and possibilities to control their crystal phase by tuning the growth parameters.
Vladimir G. Dubrovskii, Frank Glas



Surface Functionalization of III–V Nanowires

The physical and chemical properties of semiconductor nanowires are significantly influenced by their surface structure and morphology. This can be understood in that surfaces make out a much larger part of the total structure as compared to macroscale objects. An immediate consequence is that the lack of surface control can result in poor performance and reproducibility of any nanowire device. It is clear that bad performance is problematic, but it must be stressed that without performance reproducibility across millions of nanowires they can never become a useful real technology. This is indeed why many promising nanostructures and materials lost interest of both the scientific and commercial communities. However, surface control also can be used to strongly enhance nanowire performance and even introduce new functionality. As a result, surface functionalization is a key issue for nanowire science and technology. In this chapter, we describe in detail how standard surface science techniques such as Scanning Tunneling Microscopy (STM) and X-ray Photoemission Spectroscopy (XPS) can be modified for effective studies of 1D nanowires despite that they have been originally invented only for large and flat 2D surfaces. We go on to give a number of examples on how these techniques have revealed the precise structure–function relationship in particular of III–V semiconductor nanowires and their surfaces. We further discuss, how this can be used to control the structure and chemistry of the wires down to the atomic scale enabling new functionality for (opto)electronics, sensors, and many other device types. While we focus on III–V nanowires, the examples and techniques put forward should be applicable to many other material systems and types of nanostructures.
Rainer Timm, Anders Mikkelsen

Impurity Doping in Semiconductor Nanowires

Due to their novel physical properties, semiconductor nanowires are of great research interest. To realize novel devices that feature them, however, they need to be modified. One of the most well-known, important and effective functionalization techniques is impurity doping: this will be the key to the application of nanowires to metal oxide field-effect transistors, solar cells, sensors, and similar devices. To control doping and obtain superior properties, it is important to fine-tune the impurity doping process to a high level of precision. In this chapter, several doping methods such as ex situ, in situ, and surface doping are described. Each method has its own advantages and disadvantages for impurity doping of nanowire structures. The characterization of dopant atoms is also important to understand the status and behaviors of dopant atoms in nanowire structures and to control their properties.
Naoki Fukata



X-ray Methods for Structural Characterization of III-V Nanowires: From an ex-situ Ensemble Average to Time-resolved Nano-diffraction

Understanding and controlling the highly dynamical self-catalyzed vapor–liquid–solid growth of GaAs nanowires remains being a challenge. In order to gain deeper insight into these processes, our approach is the analysis by in-situ synchrotron X-ray diffraction during MBE-growth. This allows recording time-resolved information about crystal structure and shape of the nanowires. Within this chapter, we give a detailed overview of how this goal can be achieved. Starting from crucial basics about the crystal structure of GaAs nanowires and the signal obtained by X-ray diffraction, we describe how an asymmetric three-dimensional mapping of the reciprocal space is possible using a fixed angle of incidence—a geometry that is highly advantageous for time-resolved in-situ characterization. Additionally, the experimental setup is introduced and accompanying challenges are discussed. Applying the described methods, we were able to directly observe and distinguish several different effects during the growth of GaAs nanowires as well as of (In,Ga)As core-shell heterostructures. Firstly, an increasing transition probability from wurtzite to zinc-blende is observed during the course of growth, which is converted into the corresponding energy difference of the nucleation barriers of both phases. Secondly, analyzing the shape of the nanowires during growth, the evolution of the Ga-droplet on top of the nanowire was derived and the contributions of facet growth and tapering to the total radial growth were distinguished. Additionally, the effect of interfacial strain during the epitaxial growth of an (In,Ga)As shell layer around GaAs nanowires is monitored by X-ray diffraction and described by applying basic model representations. Finally, most recent results on nanowire arrays grown on Si substrates patterned by electron beam lithography are discussed.
Ludwig Feigl, Philipp Schroth

Characterisation of Semiconductor Nanowires by Electron Beam Induced Microscopy and Cathodoluminescence

Nowadays the realization of market-competitive devices based on nanomaterials is a major challenge. Optimization of the device performance requires a deep understanding of the physical phenomena at the nanoscale. In this context, electron beam-based techniques become indispensable. Several instruments have been developed in the past decades to provide versatile diagnostic solutions for improving materials, designs, and device fabrication. These characterization techniques applied to nanostructured semiconductors can help filling the gap between material science and engineering by bringing in light important physical parameters. In this Chapter, the family of electron beam-based techniques is briefly introduced. First, the electron beam/matter interaction is described both in physical and operational terms. In particular, different phenomena occurring when a flux of electron collides with a semiconductor material are discussed. Then, two main electron beam scanning techniques are discussed in the following sections: electron beam induced current microscopy and cathodoluminescence. After a short description of the fundamentals, for each technique, a bibliographic review is presented to illustrate its applications to analyses of semiconductor nanowires.
Maria Tchernycheva, Gwénolé Jacopin, Valerio Piazza

Photoluminescence Spectroscopy Applied to Semiconducting Nanowires: A Valuable Probe for Assessing Lattice Defects, Crystal Structures, and Carriers’ Temperature

Photoluminescence (PL) spectroscopy is a reliable, non-invasive tool widely employed to investigate the electronic properties of semiconductors and their nanostructures near the band-gap edge states. PL is particularly relevant for determining the energy and symmetry properties of excitons as well as the nature and relative abundances of defects in a semiconductor material. In this chapter, we will present PL measurements on InP nanowires (NWs), a notable material system for NW structures. We address the electronic and defect properties of wurtzite NWs, and provide a comparison with the zincblende counterpart. PL as a function of various external parameters, such as photoexcited carrier density and temperature, allows us to assign the origin of various recombination bands typically observed in InP NWs grown by selective area epitaxy or by vapor–liquid–solid method. The possibility to explore the density of states of NWs is implemented by PL-excitation measurements as a function of polarization, which unveil the optical selection rules pertinent to the wurtzite crystal phase. Finally, a careful analysis of the PL lineshape provides also access to carriers’ temperature and thus precious insight on carrier relaxation phenomena that occur in thin NWs.
Davide Tedeschi, Marta De Luca, Antonio Polimeni

Addressing Crystal Structure in Semiconductor Nanowires by Polarized Raman Spectroscopy

Raman scattering is a powerful inelastic light scattering technique able to probe the vibrational properties of materials. This technique has been successfully employed in semiconductor nanowires to provide information on their fundamental properties, such as the phononic properties, the crystal composition, and the electronic band structure. When performed in a polarization-resolved manner on a single nanowire, Raman spectroscopy can even allow addressing the nanowire’s crystal structure. This is a fact of pivotal importance, as crystal phase is emerging as a novel degree of freedom in the bandgap engineering and phonon engineering of materials, and the control of the crystal phase is a possibility uniquely offered by nanowires. Indeed, recent advances in the synthetic growth of nanowires have given access to crystal phases (e.g., hexagonal phase in Si and Ge) that in the bulk can only be obtained under extreme pressure conditions, and it is possible to controllably switch between different crystal phases during the growth of nanowires. The realization and, even more, the interpretation of polarized Raman experiments on nanowires can be non-trivial, as several issues have to be considered. Therefore, in this chapter, we provide the basic theoretical background necessary to calculate Raman selection rules and interpret polarization-resolved Raman spectra of semiconductor nanowires. We also discuss the main ingredients of a Raman setup, with a focus on the scattering geometries typically used for nanowires. We highlight the main differences in the Raman spectra of nanowires with cubic and hexagonal crystal symmetries, and we treat also the case of the most challenging type of heterostructure: a nanoscale crystal-phase homostructure. Finally, we discuss resonant Raman experiments that allow the determination of the energy of some electronic transitions in nanowires. We focus mostly on a very new material system, namely Ge nanowires with controlled crystal phase, but the general procedure that we establish can be applied to several types of nanostructures.
Claudia Fasolato, Ilaria Zardo, Marta De Luca

Theoretical Aspects of Point Defects in Semiconductor Nanowires

We review several fundamental and practical aspects of the theoretical treatment of neutral and charged point defects—i.e., vacancies, interstitials, substitutional impurities, and complexes—in bulk and nanowires (NWs). In particular, we show how a few of the issues preventing the straightforward application of bulk methods to NWs can be partially remedied. With this, we show how standard substitutional doping yields too high activation energies for ultrathin NWs, and that substrate bias should also be an important parameter to control impurity incorporation into the NW during growth.
Riccardo Rurali, Maurizia Palummo, Xavier Cartoixà



Nanowire Field-Effect Transistors

Vertical field-effect transistors (FETs) using semiconductor nanowires (NWs) formed by bottom-up approach are expected to outperform conventional planar transistors owing to their compatibility with surrounding-gate (SG) structures, possible heterogeneous integration with various kinds of materials including Si, and so on. In this chapter, basics, current status, and the possibility of vertical NW-FETs are described. We first introduce the importance of SG geometry from a viewpoint of the device application as well as the general advantage of vertical NW-FETs. Next, the fabrication of the NWs, where the site controllability is important for FET applications, are described with the main emphasis on the selective-area (SA) growth. Then, the current status and some of the issues for further development of their performances are explained. Some of the new directions including tunnel FETs (TFETs) and spin FETs to go beyond the classical limit of the FET will also be described before the summary of this chapter.
Junichi Motohisa, Shinjiro Hara

InP/InAs Quantum Heterostructure Nanowires Toward Telecom-Band Nanowire Lasers

One challenge for compound semiconductor nanowires has been the development of a complementary metal–oxide–semiconductor (CMOS)-compatible synthesis approach which can produce semiconductor heterostructure nanowires with excellent optical properties in technologically important telecom-band range. This remains challenging mostly because gold (Au), which is widely used as a catalyst particle for nanowire synthesis with the vapor–liquid–solid (VLS) approach [1], is not allowable in the mainstream CMOS process [2, 3]. Here, we describe the growth, structural, and optical properties of InP/InAs heterostructure nanowires by developing an Au-free self-catalyzed VLS approach. Indium material, one of the elements of InP and InAs materials, is used as the catalyst particle to catalyze the growth of InP, InAs, and further InP/InAs heterostructure nanowires. The heterostructure nanowire exhibits excellent optical property, enabling lasing operation with a tunable lasing wavelength in telecom band at room temperature. An approach is then described to form site-defined InP/InAs nanowires by combining bottom-up self-assembly process with top-down photolithography techniques. A unique growth phenomenon of self-catalyzed VLS approach is then revealed that the catalyst particle size (thus, the nanowire diameter) can be tailored by modulating V/III flow ratio during growth. Finally, we give a summary of InP/InAs quantum heterostructure nanowires.
Guoqiang Zhang, Kouta Tateno, Takehiko Tawara, Hideki Gotoh
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