Real-time thermal decomposition kinetics of GaAs nanowires and their crystal polytypes on the atomic scale.

Nanowires (NWs) offer unique opportunities for tuning the properties of III-V semiconductors by simultaneously controlling their nanoscale dimensions and switching their crystal phase between zinc-blende (ZB) and wurtzite (WZ). While much of this control has been enabled by direct, forward growth, the reverse reaction, i.e., crystal decomposition, provides very powerful means to further tailor properties towards the ultra-scaled dimensional level. Here, we use in situ transmission electron microscopy (TEM) to investigate the thermal decomposition kinetics of clean, ultrathin GaAs NWs and the role of distinctly different crystal polytypes in real-time and on the atomic scale. The whole process, from the NW growth to the decomposition, is conducted in situ without breaking vacuum to maintain pristine crystal surfaces. Radial decomposition occurs much faster for ZB- compared to WZ-phase NWs, due to the development of nano-faceted sidewall morphology and sublimation along the entire NW length. In contrast, WZ NWs form single-faceted, vertical sidewalls with decomposition proceeding only via step-flow mechanism from the NW tip. Concurrent axial decomposition is generally faster than the radial process, but is significantly faster (∼4-fold) in WZ phase, due to the absence of well-defined facets at the tip of WZ NWs. The results further show quantitatively the influence of the NW diameter on the sublimation and step-flow decomposition velocities elucidating several effects that can be exploited to fine-tune the NW dimensions.

Regulated Dynamics with Two Monolayer Steps in Vapor-Solid-Solid Growth of Nanowires.

The growth of ZnTe nanowires and ZnTe-CdTe nanowire heterostructures is studied by in situ transmission electron microscopy. We describe the shape and the change of shape of the solid gold nanoparticle during vapor-solid-solid growth. We show the balance between one monolayer and two monolayer steps, which characterizes the vapor-liquid-solid and vapor-solid-solid growth modes of ZnTe. We discuss the likely role of the mismatch strain and lattice coincidence between gold and ZnTe on the predominance of two monolayer steps during vapor-solid-solid growth and on the subsequent self-regulation of the step dynamics. Finally, the formation of an interface between CdTe and ZnTe is described.

Band-Gap Landscape Engineering in Large-Scale 2D Semiconductor van der Waals Heterostructures.

We present a growth process relying on pulsed laser deposition for the elaboration of complex van der Waals heterostructures on large scales, at a 400 °C CMOS-compatible temperature. Illustratively, we define a multilayer quantum well geometry through successive in situ growths, leading to WSe2 being encapsulated into WS2 layers. The structural constitution of the quantum well geometry is confirmed by Raman spectroscopy combined with transmission electron microscopy. The large-scale high homogeneity of the resulting 2D van der Waals heterostructure is also validated by macro- and microscale Raman mappings. We illustrate the benefit of this integrative in situ approach by showing the structural preservation of even the most fragile 2D layers once encapsulated in a van der Waals heterostructure. Finally, we fabricate a vertical tunneling device based on these large-scale layers and discuss the clear signature of electronic transport controlled by the quantum well configuration with ab initio calculations in support. The flexibility of this direct growth approach, with multilayer stacks being built in a single run, allows for the definition of complex 2D heterostructures barely accessible with usual exfoliation or transfer techniques of 2D materials. Reminiscent of the III-V semiconductors' successful exploitation, our approach unlocks virtually infinite combinations of large 2D material families in any complex van der Waals heterostructure design.

In situ TEM modification of individual silicon nanowires and their charge transport mechanisms.

Correlating the structure and composition of nanowires grown by the vapour-liquid-solid (VLS) mechanism with their electrical properties is essential for designing nanowire devices. In situ transmission electron microscopy (TEM) that can image while simultaneously measuring the current-voltage (I-V) characteristics of individual isolated nanowires is a unique tool for linking changes in structure with electronic transport. Here we grow and electrically connect silicon nanowires inside a TEM to perform in situ electrical measurements on individual nanowires both at high temperature and upon surface oxidation, as well as under ambient conditions. As-grown, the oxide-free nanowires have nonlinear I-V characteristics. We analyse the I-V measurements in terms of both bulk and injection limited transport models, finding Joule heating effects, bulk-limiting effects for thin nanowires and an injection-limiting effect for thick wires when high voltages are applied. When the nanowire surface is modified by in situ oxidation, drastic changes occur in the electronic properties. We investigate the relation between the observed geometry, changes in the surface structure and changes in electronic transport, obtaining information for individual nanowires that is inaccessible to other measuring techniques.

Phase Selection in Self-catalyzed GaAs Nanowires.

Crystal phase switching between the zincblende and wurtzite structures in III-V nanowires is crucial from the fundamental viewpoint as well as for electronic and photonic applications of crystal phase heterostructures. Here, the results of in situ monitoring of self-catalyzed vapor-liquid-solid growth of GaAs nanowires by molecular beam epitaxy inside a transmission electron microscope are presented. It is demonstrated that the occurrence of the zincblende or wurtzite phase in self-catalyzed nanowires is determined by the sole parameter, the droplet contact angle, which can be finely tuned by changing the group III and V fluxes. The zincblende phase forms at small (<100°) and large (>125°) contact angles, whereas pure wurtzite phase is observed for intermediate contact angles. Wurtzite nanowires are restricted by vertical sidewalls, whereas zincblende nanowires taper or develop the truncated edge at their top. These findings are explained within a dedicated model for the surface energetics. These results give a clear route for the crystal phase control in Au-free III-V nanowires. On a more general note, in situ growth monitoring with atomic resolution and at the technological-relevant growth rates is shown to be a powerful tool for the fine-tuning of material properties at the nanoscale.

Selective Wet Etching of Silicon Germanium in Composite Vertical Nanowires.

Silicon germanium (SixGe1-x or SiGe) is an important semiconductor material for the fabrication of nanowire-based gate-all-around transistors in the next-generation logic and memory devices. During the fabrication process, SiGe can be used either as a sacrificial layer to form suspended horizontal Si nanowires or, because of its higher carrier mobility, as a possible channel material that replaces Si in both horizontal and vertical nanowires. In both cases, there is a pressing need to understand and develop nanoscale etching processes that enable controlled and selective removal of SiGe with respect to Si. Here, we developed and tested solution-based selective etching processes for SiGe in composite (SiNx/Si0.75Ge0.25/Si) vertical nanowires. The etching solutions were formed by mixing acetic acid (CH3COOH), hydrogen peroxide (H2O2), and hydrofluoric acid (HF). Here, CH3COOH and H2O2 react to form highly oxidizing peracetic acid (PAA or CH3 CO3H). The hydrofluoric acid serves both as a catalyst for PAA formation and as an etchant for oxidized SiGe. Our study shows that an increase in any of the two oxidizer (H2O2 and PAA) concentrations increases the etch rate, and the fastest etch rate of SiGe is associated with the highest PAA concentration. Moreover, using in situ liquid-phase TEM imaging, we tested the stability of nanowires during wet etching and identified the SiGe/Si interface to be the weakest plane; we found that once the diameter of the 160-nm-tall Si0.75Ge0.25 nanowire reaches ∼15 nm during the etching, the nanowire breaks at or very close to this interface. Our study provides important insight into the details of the nanoscale wet etching of SiGe and some of the associated failure modes that are becoming extremely relevant for the fabrication processes as the size of the transistors shrink with every new device generation.

Growth Dynamics of Gallium Nanodroplets Driven by Thermally Activated Surface Diffusion.

The growth of catalytic liquid-metal nanodroplets on flat substrates is essential for many technological applications. However, the detailed nucleation and growth dynamics of these nanodroplets remain unclear. Here, using in situ transmission electron microscopy (TEM) imaging, we track in real time the growth of individual Ga nanodroplets from a beam of Ga vapor. We show that the nucleation and growth are driven by thermally activated surface diffusion of Ga adatoms, with the diffusion activation energy of ED = 95 ± 10 meV on a SiNx surface. More importantly, our analysis shows that Ga dimers serve as the critical nucleation clusters and that the nanodroplet growth follows a power-law of the form R(t) ∝ e-ED/kBT(t - t0)1/2. These insights into the growth dynamics of metallic nanodroplets are essential for tailoring their size and density for their application in self-catalyzed growth of nanomaterials.

Surface Crystallization of Liquid Au-Si and Its Impact on Catalysis.

In situ transmission electron microscopy reveals that an atomically thin crystalline phase at the surface of liquid Au-Si is stable over an unexpectedly wide range of conditions. By measuring the surface structure as a function of liquid temperature and composition, a simple thermodynamic model is developed to explain the stability of the ordered phase. The presence of surface ordering plays a key role in the pathway by which the Au-Si eutectic solidifies and also dramatically affects the catalytic properties of the liquid, explaining the anomalously slow growth kinetics of Si nanowires at low temperature. A strategy to control the presence of the surface phase is discussed, using it as a tool in designing strategies for nanostructure growth.

Atomic Step Flow on a Nanofacet.

Crystal growth often proceeds by atomic step flow. When the surface area available for growth is limited, the nucleation and progression of the steps can be affected. This issue is particularly relevant to the formation of nanocrystals. We examine the case of Au-catalyzed GaAs nanowires, which we grow in a transmission electron microscope. Our in situ observations show that atomic layers nucleate at the periphery of the interface between the nanowire and the catalyst droplet. From this starting location, the atomic step flows within a restricted area of hexagonal shape. At specific partial coverages, the monolayer configuration changes abruptly. A simple model based on the geometry of the system and its edge energies explains these observations. In particular, we observe an inversion of the step curvature which reveals that the effective energy per unit length of monolayer edge is much lower at the interface periphery than inside the catalyst droplet.

Controlling nanowire growth through electric field-induced deformation of the catalyst droplet.

Semiconductor nanowires with precisely controlled structure, and hence well-defined electronic and optical properties, can be grown by self-assembly using the vapour-liquid-solid process. The structure and chemical composition of the growing nanowire is typically determined by global parameters such as source gas pressure, gas composition and growth temperature. Here we describe a more local approach to the control of nanowire structure. We apply an electric field during growth to control nanowire diameter and growth direction. Growth experiments carried out while imaging within an in situ transmission electron microscope show that the electric field modifies growth by changing the shape, position and contact angle of the catalytic droplet. This droplet engineering can be used to modify nanowires into three dimensional structures, relevant to a range of applications, and also to measure the droplet surface tension, important for quantitative development of strategies to control nanowire growth.

Nanowire growth kinetics in aberration corrected environmental transmission electron microscopy.

We visualize atomic level dynamics during Si nanowire growth using aberration corrected environmental transmission electron microscopy, and compare with lower pressure results from ultra-high vacuum microscopy. We discuss the importance of higher pressure observations for understanding growth mechanisms and describe protocols to minimize effects of the higher pressure background gas.

Interface dynamics and crystal phase switching in GaAs nanowires.

Controlled formation of non-equilibrium crystal structures is one of the most important challenges in crystal growth. Catalytically grown nanowires are ideal systems for studying the fundamental physics of phase selection, and could lead to new electronic applications based on the engineering of crystal phases. Here we image gallium arsenide (GaAs) nanowires during growth as they switch between phases as a result of varying growth conditions. We find clear differences between the growth dynamics of the phases, including differences in interface morphology, step flow and catalyst geometry. We explain these differences, and the phase selection, using a model that relates the catalyst volume, the contact angle at the trijunction (the point at which solid, liquid and vapour meet) and the nucleation site of each new layer of GaAs. This model allows us to predict the conditions under which each phase should be observed, and use these predictions to design GaAs heterostructures. These results could apply to phase selection in other nanowire systems.

Creating New VLS Silicon Nanowire Contact Geometries by Controlling Catalyst Migration.

The formation of self-assembled contacts between vapor-liquid-solid grown silicon nanowires and flat silicon surfaces was imaged in situ using electron microscopy. By measuring the structural evolution of the contact formation process, we demonstrate how different contact geometries are created by adjusting the balance between silicon deposition and Au migration. We show that electromigration provides an efficient way of controlling the contact. The results point to novel device geometries achieved by direct nanowire growth on devices.