Catalyst orientation-induced growth of defect-free zinc-blende structured ⟨001̅⟩ InAs nanowires

Enhanced Photoemission Performance of InGaN Inclined Nanowire Array

We propose to improve the solar energy utilization by using InGaN inclined nanowire array photocathodes. We first study vertical nanowire array. On the basis of vertical nanowire array, we study inclined nanowires by changing the inclination angle of nanowires. The inclined nanowires exhibit higher quantum efficiency at larger period and larger inclination angle. However, the infinite expansion of period will cause its performance to degrade. The quantum efficiency of inclined nanowires with a period of 175 nm and an inclination angle of 5.35° is as high as 80.2% when the incident light angle is irradiated at 5°. In addition, applying an electric field can improve the collection efficiency of inclined nanowires and help them maintain a high collection efficiency over a longer wavelength range. The design principles proposed in this work will provide a theoretical reference for the performance improvement of InGaN photocathodes.

Non-〈111〉-oriented semiconductor nanowires: growth, properties, and applications

In recent years, non-〈111〉-oriented semiconductor nanowires have attracted increasing interest in terms of fundamental research and promising applications due to their outstanding crystal quality and distinctive physical properties. Here, a comprehensive overview of recent advances in the study of non-〈111〉-oriented semiconductor nanowires is presented. We start by introducing various growth techniques for obtaining nanowires with certain orientations, for which the growth energetics and kinetics are discussed. Attention is then given to the physical properties of non-〈111〉 nanowires, as predicted by theoretical calculations or demonstrated experimentally. After that, we review the advantages and challenges of non-〈111〉 nanowires as building blocks for electronic and optoelectronic devices. Finally, we discuss the possible challenges and opportunities in the research field of non-〈111〉 semiconductor nanowires.

Large-Composition-Range Pure-Phase Homogeneous InAs1-xSbx Nanowires

Narrow bandgap InAs_1-xSb_x nanowires show broad prospects for applications in wide spectrum infrared detectors, high-performance transistors, and quantum computation. Realizing such applications requires a fine control of the composition and crystal structure of nanowires. However, the fabrication of large-composition-range pure-phase homogeneous InAs_1-xSb_x nanowires remains a huge challenge. Here, we first report the growth of large-composition-range stemless InAs_1-xSb_x nanowires (0 ≤ x ≤ 0.63) on Si (111) substrates by molecular beam epitaxy. We find that pure-phase InAs_1-xSb_x nanowires can be successfully obtained by controlling the antimony content x, nanowire diameter, and nanowire growth direction. Detailed energy dispersive spectrum data show that the antimony is uniformly distributed along the axial and radial directions of InAs_1-xSb_x nanowires and no spontaneous core-shell nanostructures form in the nanowires. On the basis of field-effect measurements, we confirm that InAs_1-xSb_x nanowires exhibit good conductivity and their mobilities can reach 4200 cm² V^-1 s^-1 at 7 K. Our work lays the foundation for the development of InAs_1-xSb_x nanowire optoelectronic, electronic, and quantum devices.

Growth of MSe semiconductor nanowires on metal substrates through an Ag2Se-catalyzed solution–solid–solid mechanism (M = Zn, Cd and Mn)

Solution-phase growth of MSe nanowires on their respective metal foil or flakes (M = Zn, Cd and Mn) has been realized by a recently developed solution–solid–solid mechanism initiated by preexisting Ag2Se seeds.

In situ TEM observation of the vapor-solid-solid growth of <001[combining macron]> InAs nanowires

In situ transmission electron microscopy characterization is a powerful method in investigating the growth mechanism of catalyst-induced semiconductor nanowires. By providing direct evidence on the crystal growth at the atomic level, a real-time in situ heating investigation was carried out on Au-catalyzed <001[combining macron]> InAs nanowires. It was found that the Au catalyst maintained itself in the solid form during the nanowire growth, and maintained a fixed epitaxial relationship with its underlying InAs nanowire, indicating the vapor-solid-solid mechanism. Importantly, the growth of <001[combining macron]> InAs nanowires through a layer-by-layer manner at the catalyst/nanowire interface is evident. This study provides direct insights into the vapor-solid-solid growth and clarified the growth mechanism of <001[combining macron]> III-V nanowires, which provides pathways in controlling the growth of <001[combining macron]> semiconductor nanowires.

Selective Growth of Stacking Fault Free ⟨100⟩ Nanowires on a Polycrystalline Substrate for Energy Conversion Application

Cubic semiconductor nanowires grown along ⟨100⟩ directions have been reported to be promising for optoelectronics and energy conversion applications, owing to their pure zinc-blende structure without any stacking fault. But, until date, only limited success has been achieved in growing ⟨100⟩ oriented nanowires. Here we report the selective growth of stacking fault free ⟨100⟩ nanowires on a commercial transparent conductive polycrystalline fluorine-doped SnO₂ (FTO) glass substrate via a simple and cost-effective chemical vapor deposition (CVD) method. By means of crystallographic analysis and density functional theory calculation, we prove that the orientation relationship between the Au catalyst and the FTO substrate play a vital role in inducing the selective growth of ⟨100⟩ nanowires, which opens a new pathway for controlling the growth directions of nanowires via the elaborate selection of the catalyst and substrate couples during the vapor-solid-liquid (VLS) growth process. The ZnSe nanowires grown on the FTO substrate are further applied as a photoanode in photoelectrochemical (PEC) water splitting. It exhibits a higher photocurrent than the ZnSe nanowires do without preferential orientations on a Sn-doped In₂O₃ (ITO) glass substrate, which we believe to be correlated with the smooth transport of charge carriers in ZnSe ⟨100⟩ nanowires with pure zinc-blende structures, in distinct contrast with the severe electron scattering happened at the stacking faults in ZnSe nanowires on the ITO substrate, as well as the efficient charge transfer across the intensively interacting nanowire-substrate interfaces.

Free-Standing InAs Nanobelts Driven by Polarity in MBE

In this study, we demonstrated the Au-catalyzed growth of free-standing defect-free zinc-blende structured InAs nanobelts on the GaAs {111}_B substrate by molecular beam epitaxy. Through detailed morphological, chemical, and structural characterizations using advanced electron microscopy, it was found that the nanobelts grew along the ⟨001̅⟩ direction, induced by Au catalysts via vapor-solid-solid mechanism, with features of {001̅} catalyst/nanobelt interfaces and extensive {11̅0} surfaces. The formation of the belt-shaped morphology of our nanostructures resulted from a faster lateral growth rate along the ±[110] direction than that along the ±[11̅0] direction, driven by polarity. This study provides insights into understanding the growth of free-standing zinc-blende structured <001̅> InAs nanobelts.

Wurtzite/Zinc-Blende 'K'-shape InAs Nanowires with Embedded Two-Dimensional Wurtzite Plates

The prediction that Majorana Fermions obey nonabelian exchange statistics can only be tested by interchanging such carriers in "Y'- or 'X'- (or 'K'-) shaped nanowire networks. Here we report the molecular beam epitaxy (MBE) growth of 'K'-shaped InAs nanowires consisting of two interconnected wurtzite wires with an additional zinc-blende wire in between. Moreover, occasionally, the growth results in formation of a purely wurtzite two-dimensional plate between the zinc-blende nanowire and one (sometimes both) intersecting wurtzite arm. By modeling the crystal structure we explain the transformation from wurtzite to zinc-blende and the coexistence of both crystallographic phases in such nanowire structures. To the best of our knowledge neither the MBE growth of an InAs nano-object showing combination of wurtzite and zinc-blende crystal structures nor the growth of pure wurtzite InAs nanoplates in this geometry has been reported before.

Growth of InAs Quantum Dots on GaAs (511)A Substrates: The Competition between Thermal Dynamics and Kinetics

The growth process of InAs quantum dots grown on GaAs (511)A substrates has been studied by atomic force microscopy. According to the atomic force microscopy studies for quantum dots grown with varying InAs coverage, a noncoherent nucleation of quantum dots is observed. Moreover, due to the long migration length of In atoms, the Ostwald ripening process is aggravated, resulting in the bad uniformity of InAs quantum dots on GaAs (511)A. In order to improve the uniformity of nucleation, the growth rate is increased. By studying the effects of increased growth rates on the growth of InAs quantum dots, it is found that the uniformity of InAs quantum dots is greatly improved as the growth rates increase to 0.14 ML s(-1) . However, as the growth rates increase further, the uniformity of InAs quantum dots becomes dual-mode, which can be attributed to the competition between Ostwald ripening and strain relaxation processes. The results in this work provide insights regarding the competition between thermal dynamical barriers and the growth kinetics in the growth of InAs quantum dots, and give guidance to improve the size uniformity of InAs quantum dots on (N11)A substrates.

Growth of Catalyst-Free Epitaxial InAs Nanowires on Si Wafers Using Metallic Masks

Development of heteroepitaxy growth of catalyst-free vertical III-V nanowires on Si wafers is highly desirable for future nanoscale Si-based electronic and optoelectronic devices. In this study, a proof-of-concept approach is developed for catalyst-free heteroepitaxy growth of InAs nanowires on Si wafers. Before the growth of InAs nanowires, a Si-compatible metallic film with a thickness of several tens of nanometers was predeposited on a Si wafer and then annealed to form nanosize openings so as to obtain a metallic mask. These nano-openings exposed the surface of the Si wafer, which allowed subsequent nucleation and growth of epitaxial InAs nanowires directly on the surface of the Si wafer. The small size of the nano-openings limits the lateral growth of the nanostructures but promotes their axial growth. Through this approach, catalyst-free InAs nanowires were grown on both Si (111) and (001) wafers successfully at different growth temperatures. In particular, ultralong defect-free InAs nanowires with the wurtzite structure were grown the Si (111) wafers at 550 °C using the Ni mask. This study offers a simple, cost-effective, and scalable method to grow catalyst-free III-V nanowires on Si wafers. The simplicity of the approach opens a new avenue for the growth and integration of catalyst-free high-quality heteroepitaxial III-V nanowires on Si wafers.

InAs/GaSb core-shell nanowires grown on Si substrates by metal-organic chemical vapor deposition

We report the growth of InAs/GaSb core-shell heterostructure nanowires with smooth sidewalls on Si substrates using metal-organic chemical vapor deposition with no assistance from foreign catalysts. Sb adatoms were observed to strongly influence the morphology of the GaSb shell. In particular, Ga droplets form on the nanowire tips when a relatively low TMSb flow rate is used, whereas the droplets are missing and the radial growth of the GaSb is enhanced due to a reduction in the diffusion length of the Ga adatoms when the TMSb flow rate is increased. Moreover, transmission electron microscopy measurements revealed that the GaSb shell coherently grew on the InAs core. The results obtained here show that the InAs/GaSb core-shell nanowires grown using the Si platform have strong potential in the fabrication of future nanometer-scale devices and in the study of fundamental quantum physics.

Orientation Dependence of Electromechanical Characteristics of Defect-free InAs Nanowires

Understanding the electrical properties of defect-free nanowires with different structures and their responses under deformation are essential for design and applications of nanodevices and strain engineering. In this study, defect-free zinc-blende- and wurtzite-structured InAs nanowires were grown using molecular beam epitaxy, and individual nanowires with different structures and orientations were carefully selected and their electrical properties and electromechanical responses were investigated using an electrical probing system inside a transmission electron microscope. Through our careful experimental design and detailed analyses, we uncovered several extraordinary physical phenomena, such as the electromechanical characteristics are dominated by the nanowire orientation, rather than its crystal structure. Our results provide critical insights into different responses induced by deformation of InAs with different structures, which is important for nanowire-based devices.

Defect-free thin InAs nanowires grown using molecular beam epitaxy

In this study, we designed a simple method to achieve the growth of defect-free thin InAs nanowires with a lateral dimension well below their Bohr radius on different substrate orientations. By depositing and annealing a thin layer of Au thin film on a (100) substrate surface, we have achieved the growth of defect-free uniform-sized thin InAs nanowires. This study provides a strategy to achieve the growth of pure defect-free thin nanowires.

Tunable Polarity in a III-V Nanowire by Droplet Wetting and Surface Energy Engineering

Controllable axial switching of polarity in GaAs nanowires with minimal tapering and perfect twin-free ZB structure based on the fundamental understanding of nanowire growth and kinking mechanism is presented. The polarity of the bottom segment is confirmed to be (111)A by atomically resolved scanning transmission electron microscopy.

Defect-free zinc-blende structured InAs nanowires realized by in situ two V/III ratio growth in molecular beam epitaxy

In this study, we devised a two-V/III-ratio procedure to control the Au-assisted growth of defect-free InAs nanowires in molecular beam epitaxy. The demonstrated two V/III ratio procedure consists of a first high V/III ratio growth step to prepare the nanowire foundation on the substrate surface, followed by a low V/III ratio step to induce the nanowire growth. By manipulating the V/III ratios in different steps, we have achieved the controlled growth of pure defect-free zinc-blende structured InAs nanowires on the GaAs {1̄1̄1̄} substrates. This study provides an approach to control not only the crystal structure of semiconductor nanowires, but also their structural qualities.

Influence of substrate orientation on the structural quality of GaAs nanowires in molecular beam epitaxy

In this study, the effect of substrate orientation on the structural quality of Au-catalyzed epitaxial GaAs nanowires grown by a molecular beam epitaxy reactor has been investigated. It was found that the substrate orientations can be used to manipulate the nanowire catalyst composition and the catalyst surface energy and, therefore, to alter the structural quality of GaAs nanowires grown on different substrates. Defect-free wurtzite-structured GaAs nanowires grown on the GaAs (110) substrate have been achieved under our growth conditions.