In Situ Heat-Induced Replacement of GaAs Nanowires by Au

High-Performance Multiwavelength GaNAs Single Nanowire Lasers

In this study, we report a significant enhancement in the performance of GaNAs-based single nanowire lasers through optimization of growth conditions, leading to a lower lasing threshold and higher operation temperatures. Our analysis reveals that these improvements in the laser performance can be attributed to a decrease in the density of localized states within the material. Furthermore, we demonstrate that owing to their excellent nonlinear optical properties, these nanowires support self-frequency conversion of the stimulated emission through second harmonic generation (SHG) and sum-frequency generation (SFG), providing coherent light emission in the cyan-green range. Mode-specific differences in the self-conversion efficiency are revealed and explained by differences in the light extraction efficiency of the converted light caused by the electric field distribution of the fundamental modes. Our work, therefore, facilitates the design and development of multiwavelength coherent light generation and higher-temperature operation of GaNAs nanowire lasers, which will be useful in the fields of optical communications, sensing, and nanophotonics.

Microscopic Understanding of the Growth and Structural Evolution of Narrow Bandgap III-V Nanostructures

III–V group nanomaterials with a narrow bandgap have been demonstrated to be promising building blocks in future electronic and optoelectronic devices. Thus, revealing the underlying structural evolutions under various external stimuli is quite necessary. To present a clear view about the structure–property relationship of III–V nanowires (NWs), this review mainly focuses on key procedures involved in the synthesis, fabrication, and application of III–V materials-based devices. We summarized the influence of synthesis methods on the nanostructures (NWs, nanodots and nanosheets) and presented the role of catalyst/droplet on their synthesis process through in situ techniques. To provide valuable guidance for device design, we further summarize the influence of structural parameters (phase, defects and orientation) on their electrical, optical, mechanical and electromechanical properties. Moreover, the dissolution and contact formation processes under heat, electric field and ionic water environments are further demonstrated at the atomic level for the evaluation of structural stability of III–V NWs. Finally, the promising applications of III–V materials in the energy-storage field are introduced.

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.

Towards the understanding of the gold interaction with AIII-BV semiconductors at the atomic level

AIII-BV semiconductors have been considered to be a promising material for decades in overcoming the limitations of silicon semiconductor devices. One of the important aspects within the AIII-BV semiconductor technology is gold-semiconductor interactions on the nanoscale. We report on the investigations into the basic chemical interactions of Au atoms with AIII-BV semiconductor crystals by the investigation of the nanostructure formation in the process of thermally-induced Au self-assembly on various AIII-BV surfaces by means of atomically resolved High Angle Annular Dark Field (HAADF) Scanning Transmission Electron Microscopy (STEM) measurements. We have found that the formation of nanostructures is a consequence of the surface diffusion and nucleation of adatoms produced by Au induced chemical reactions on AIII-BV semiconductor surfaces. Only for InSb crystals we have found that there is efficient diffusion of Au atoms into the bulk, which we experimentally studied by Machine Learning HAADF STEM image quantification and theoretically by Density Functional Theory (DFT) calculations with the inclusion of finite temperature effects. Furthermore, the effective number of Au atoms needed to release one AIII metal atom has been estimated. The experimental finding reveals a difference in the Au interactions with the In- and Ga-based groups of AIII-BV semiconductors. Our comprehensive and systematic studies uncover the details of the Au interactions with the AIII-BV surface at the atomic level with chemical sensitivity and shed new light on the fundamental Au/AIII-BV interactions at the atomic scale.

Kinetics of Au-Ga Droplet Mediated Decomposition of GaAs Nanowires

Particle-assisted III-V semiconductor nanowire growth and applications thereof have been studied extensively. However, the stability of nanowires in contact with the particle and the particle chemical composition as a function of temperature remain largely unknown. In this work, we use in situ transmission electron microscopy to investigate the interface between a Au-Ga particle and the top facet of an ⟨1̅1̅1̅⟩-oriented GaAs nanowire grown via the vapor-liquid-solid process. We observed a thermally activated bilayer-by-bilayer removal of the GaAs facet in contact with the liquid particle during annealing between 300 and 420 °C in vacuum. Interestingly, the GaAs-removal rates initially depend on the thermal history of the sample and are time-invariant at later times. In situ X-ray energy dispersive spectroscopy was also used to determine that the Ga content in the particle at any given temperature remains constant over extended periods of time and increases with increasing temperature from 300 to 400 °C. We attribute the observed phenomena to droplet-assisted decomposition of GaAs at a rate that is controlled by the amount of Ga in the droplet. We suggest that the observed transients in removal rates are a direct consequence of time-dependent changes in the Ga content. Our results provide new insights into the role of droplet composition on the thermal stability of GaAs nanowires and complement the existing knowledge on the factors influencing nanowire growth. Moreover, understanding the nanowire stability and decomposition is important for improving processing protocols for the successful fabrication and sustained operation of nanowire-based devices.

Single GaAs Nanowire/Graphene Hybrid Devices Fabricated by a Position-Controlled Microtransfer and an Imprinting Technique for an Embedded Structure

We developed a new technique to fabricate single nanowire devices with reliable graphene/nanowire contacts using a position-controlled microtransfer and an embedded nanowire structure in a planar junction configuration. A thorough study of electrical properties and fabrication challenges of single p-GaAs nanowire/graphene devices was carried out in two different device configurations: (1) a graphene bottom-contact device where the nanowire-graphene contact junction is formed by transferring a nanowire on top of graphene and (2) a graphene top-contact device where the nanowire-graphene contact junction is formed by transferring graphene on top of an embedded nanowire. For the graphene top-contact devices, graphene-nanowire-metal devices, where graphene is used as one electrode and metal is the other electrode to a nanowire, and graphene-nanowire-graphene devices, where both electrodes to a nanowire are graphene, were investigated and compared with conventional metal/p-GaAs nanowire devices. Conventional metal/p-GaAs nanowire contact devices were further investigated in embedded and nonembedded nanowire device configurations. A significantly improved current in the embedded device configuration is explained with a "parallel resistors model" where the high-resistance parts with the metal-semiconductor Schottky contact and the low-resistance parts with noncontacted facets of the hexagonal nanowires are taken into consideration. Consistently, the nonembedded nanowire structure is found to be depleted much easier than the embedded nanowires from which an estimation for a fully depleted condition has also been established.

Stability of Schottky and Ohmic Au Nanocatalysts to ZnO Nanowires

Manufacturable nanodevices must now be the predominant goal of nanotechnological research to ensure the enhanced properties of nanomaterials can be fully exploited and fulfill the promise that fundamental science has exposed. Here, we test the electrical stability of Au nanocatalyst-ZnO nanowire contacts to determine the limits of the electrical transport properties and the metal-semiconductor interfaces. While the transport properties of as-grown Au nanocatalyst contacts to ZnO nanowires have been well-defined, the stability of the interfaces over lengthy time periods and the electrical limits of the ohmic or Schottky function have not been studied. In this work, we use a recently developed iterative analytical process that directly correlates multiprobe transport measurements with subsequent aberration-corrected scanning transmission electron microscopy to study the electrical, structural, and chemical properties when the nanowires are pushed to their electrical limits and show structural changes occur at the metal-nanowire interface or at the nanowire midshaft. The ohmic contacts exhibit enhanced quantum-mechanical edge-tunneling transport behavior because of additional native semiconductor material at the contact edge due to a strong metal-support interaction. The low-resistance nature of the ohmic contacts leads to catastrophic breakdown at the middle of the nanowire span where the maximum heating effect occurs. Schottky-type Au-nanowire contacts are observed when the nanowires are in the as-grown pristine state and display entirely different breakdown characteristics. The higher-resistance rectifying I-V behavior degrades as the current is increased which leads to a permanent weakening of the rectifying effect and atomic-scale structural changes at the edge of the Au interface where the tunneling current is concentrated. Furthermore, to study modified nanowires such as might be used in devices the nanoscale tunneling path at the interface edge of the ohmic nanowire contacts is removed with a simple etch treatment and the nanowires show similar I-V characteristics during breakdown as the Schottky pristine contacts. Breakdown is shown to occur either at the nanowire midshaft or at the Au contact depending on the initial conductivity of the Au contact interface. These results demonstrate the Au-nanowire structures are capable of withstanding long periods of electrical stress and are stable at high current densities ensuring they are ideal components for nanowire-device designs while providing the flexibility of choosing the electrical transport properties which other Au-nanowire systems cannot presently deliver.

Regrowth mechanism for oxide isolation of GaAs nanowires

Oxide-isolated GaAs nanowires (NWs) were obtained through a lithography-free method in which axial growth of NWs coated in aluminum oxide (Al₂O₃) is restarted using an annealing step. NWs are grown using the vapor-liquid-solid method and coated in nanometer thin oxide films using atomic layer deposition. Continued growth at the oxide-coated nanoparticle (NP) occurs after the thermally-induced fracture of the oxide during annealing. This oxide fracture is observed to depend on NP diameter, oxide thickness and annealing temperature. A thermal expansion mismatch model for stresses on the oxide shell is put forward to explain these results.

Recordings and Analysis of Atomic Ledge and Dislocation Movements in InGaAs to Nickelide Nanowire Phase Transformation

The formation of low resistance and self-aligned contacts with thermally stable alloyed phases is a prerequisite for realizing reliable functionality in ultrascaled semiconductor transistors. Detailed structural analysis of the phase transformation accompanying contact alloying can facilitate contact engineering as transistor channels approach a few atoms across. Original in situ heating transmission electron microscopy studies are carried out to record and analyze the atomic scale dynamics of contact alloy formation between Ni and In_0.53 Ga_0.47 As nanowire channels. It is observed that the nickelide reacts on the In_0.53 Ga_0.47 As (111) || Ni₂ In_0.53 Ga_0.47 As (0001) interface with atomic ledge propagation along the Ni₂ In_0.53 Ga_0.47 As [101¯0] direction. Ledges nucleate as a train of strained single-bilayers and propagate in-plane as double-bilayers that are associated with a misfit dislocation of b→=2c3[0001]. The atomic structure is reconstructed to explain this phase transformation that involves collective gliding of three Shockley partials in In_0.53 Ga_0.47 As lattice to cancel out shear stress and the formation of misfit dislocations to compensate the large lattice mismatch in the newly formed nickelide phase and the In_0.53 Ga_0.47 As layers. This work demonstrates the applicability of interfacial disconnection (ledge + dislocation) theory in a nanowire channel during thermally induced phase transformation that is typical in metal/III-V semiconductor reactions.

Crystal Phases in Hybrid Metal-Semiconductor Nanowire Devices

We investigate the metallic phases observed in hybrid metal-GaAs nanowire devices obtained by controlled thermal annealing of Ni/Au electrodes. Devices are fabricated onto a SiN membrane compatible with transmission electron microscopy studies. Energy dispersive X-ray spectroscopy allows us to show that the nanowire body includes two Ni-rich phases that thanks to an innovative use of electron diffraction tomography can be unambiguously identified as Ni₃GaAs and Ni₅As₂ crystals. The mechanisms of Ni incorporation leading to the observed phenomenology are discussed.

Atomic Scale Dynamics of Contact Formation in the Cross-Section of InGaAs Nanowire Channels

Alloyed and compound contacts between metal and semiconductor transistor channels enable self-aligned gate processes which play a significant role in transistor scaling. At nanoscale dimensions and for nanowire channels, prior experiments focused on reactions along the channel length, but the early stage of reaction in their cross sections remains unknown. Here, we report on the dynamics of the solid-state reaction between metal (Ni) and semiconductor (In_0.53Ga_0.47As), along the cross-section of nanowires that are 15 nm in width. Unlike planar structures where crystalline nickelide readily forms at conventional, low alloying temperatures, nanowires exhibit a solid-state amorphization step that can undergo a crystal regrowth step at elevated temperatures. In this study, we capture the layer-by-layer reaction mechanism and growth rate anisotropy using in situ transmission electron microscopy (TEM). Our kinetic model depicts this new, in-plane contact formation which could pave the way for engineered nanoscale transistors.

Modifying the Interface Edge to Control the Electrical Transport Properties of Nanocontacts to Nanowires

Selecting the electrical properties of nanomaterials is essential if their potential as manufacturable devices is to be reached. Here, we show that the addition or removal of native semiconductor material at the edge of a nanocontact can be used to determine the electrical transport properties of metal-nanowire interfaces. While the transport properties of as-grown Au nanocatalyst contacts to semiconductor nanowires are well-studied, there are few techniques that have been explored to modify the electrical behavior. In this work, we use an iterative analytical process that directly correlates multiprobe transport measurements with subsequent aberration-corrected scanning transmission electron microscopy to study the effects of chemical processes that create structural changes at the contact interface edge. A strong metal-support interaction that encapsulates the Au nanocontacts over time, adding ZnO material to the edge region, gives rise to ohmic transport behavior due to the enhanced quantum-mechanical tunneling path. Removal of the extraneous material at the Au-nanowire interface eliminates the edge-tunneling path, producing a range of transport behavior that is dependent on the final interface quality. These results demonstrate chemically driven processes that can be factored into nanowire-device design to select the final properties.