Regaining a Spatial Dimension: Mechanically Transferrable Two-Dimensional InAs Nanofins Grown by Selective Area Epitaxy

Graphoepitaxial Self-Formation of Single Crystal Arrays Using Topologically Designed Electrostatic Fields and Charged Molecular Fluxes

The ability to locally engineer semiconducting structures enables the development of next generation nanoscale devices and photonics. Contrary to the widely used selective area epitaxy, an alternative approach for localized single crystal growth on noncrystalline substrates is presented. Individual growth positions are defined by topological design of an electrostatic field above the substrate guiding the material transport of unipolar charged molecules to the nucleation and growth positions. In this way, the topologically designed electric field imprints the geometrical pattern for graphoepitaxial crystallite growth. After nucleation, the growth evolves into vertical growth of three-dimensional high aspect ratio towers tipped with single crystal copper oxide. The demonstrated graphoepitaxial method allows for the facile growth of advanced 3D material structures on noncrystalline substrates.

Controlled formation of three-dimensional cavities during lateral epitaxial growth

Epitaxial growth is a fundamental step required to create devices for the semiconductor industry, enabling different materials to be combined in layers with precise control of strain and defect structure. Patterning the growth substrate with a mask before performing epitaxial growth offers additional degrees of freedom to engineer the structure and hence function of the semiconductor device. Here, we demonstrate that conditions exist where such epitaxial lateral overgrowth can produce complex, three-dimensional structures that incorporate cavities of deterministic size. We grow germanium on silicon substrates patterned with a dielectric mask and show that fully-enclosed cavities can be created through an unexpected self-assembly process that is controlled by surface diffusion and surface energy minimization. The result is confined cavities enclosed by single crystalline Ge, with size and position tunable through the initial mask pattern. We present a model to account for the observed cavity symmetry, pinch-off and subsequent evolution, reflecting the dominant role of surface energy. Since dielectric mask patterning and epitaxial growth are compatible with conventional device processing steps, we suggest that this mechanism provides a strategy for developing electronic and photonic functionalities.

Selective area growth of in-plane InAs nanowires and nanowire networks on Si substrates by molecular-beam epitaxy

In-plane InAs nanowires and nanowire networks show great potential to be used as building blocks for electronic, optoelectronic and topological quantum devices, and all these applications are keen to grow the InAs materials directly on Si substrates since it may enable nanowire electronic and quantum devices with seamless integration with Si platform. However, almost all the in-plane InAs nanowires and nanowire networks have been realized on substrates of III-V semiconductors. Here, we demonstrate the selective area epitaxial growth of in-plane InAs nanowires and nanowire networks on Si substrates. We find that the selectivity of InAs growth on Si substrates is mainly dependent on the growth temperature, while the morphology of InAs nanowires is closely related to the V/III flux ratio. We examine the cross-sectional shapes and facets of the InAs nanowires grown along the 〈110〉, 〈100〉 and 〈112〉 orientations. Thanks to the non-polar characteristics of Si substrates, the InAs nanowires and nanowire networks exhibit superior symmetry compared to that grown on III-V substrates. The InAs nanowires and nanowire networks are zinc-blende (ZB) crystals, but there are many defects in the nanowires, such as stacking faults, twins and grain boundaries. The crystal quality of InAs nanowires and nanowire networks can be improved by increasing the growth temperature within the growth temperature window. Our work demonstrates the feasibility of selective area epitaxial growth of in-plane InAs nanowires and nanowire networks on Si substrates.

Parallel Aluminum-Cobalt Oxide Nanosheet Arrays with High-Temperature Ferromagnetism

Parallel nanomaterials possess unique properties and show potential applications in industry. Whereas, vertically aligned 2D nanomaterials have plane orientations that are generally chaotic. Simultaneous control of their growth direction and spatial orientation for parallel nanosheets remains a big challenge. Here, a facile preparation of vertically aligned parallel nanosheet arrays of aluminum-cobalt oxide is reported via a collaborative dealloying and hydrothermal method. The parallel growth of nanosheets is attributed to the lattice-matching among the nanosheets, the buffer layer, and the substrate, which is verified by a careful transmission electron microscopy study. Furthermore, the aluminum-cobalt oxide nanosheets exhibit high-temperature ferromagnetism with a 919 K Curie temperature and a 5.22 emu g-1 saturation magnetization at 300 K, implying the potential applications in high-temperature ferromagnetic fields.

Improving the intrinsic conductance of selective area grown in-plane InAs nanowires with a GaSb shell

The nanoscale intrinsic electrical properties of in-plane InAs nanowires grown by selective area epitaxy are investigated using a process-free method involving a multi-probe scanning tunneling microscope. The resistance of oxide-free InAs nanowires grown on an InP(111)_Bsubstrate and the resistance of InAs/GaSb core-shell nanowires grown on an InP(001) substrate are measured using a collinear four-point probe arrangement in ultrahigh vacuum. They are compared with the resistance of two-dimensional electron gas reference samples measured using the same method and with the Van der Pauw geometry for validation. A significant improvement of the conductance is achieved when the InAs nanowires are fully embedded in GaSb, exhibiting an intrinsic sheet conductance close to the one of the quantum well counterpart.

Selective Area Epitaxy of Quasi-1-Dimensional Topological Nanostructures and Networks

Quasi-one-dimensional (1D) topological insulators hold the potential of forming the basis of novel devices in spintronics and quantum computing. While exposure to ambient conditions and conventional fabrication processes are an obstacle to their technological integration, ultra-high vacuum lithography techniques, such as selective area epitaxy (SAE), provide all the necessary ingredients for their refinement into scalable device architectures. In this work, high-quality SAE of quasi-1D topological insulators on templated Si substrates is demonstrated. After identifying the narrow temperature window for selectivity, the flexibility and scalability of this approach is revealed. Compared to planar growth of macroscopic thin films, selectively grown regions are observed to experience enhanced growth rates in the nanostructured templates. Based on these results, a growth model is deduced, which relates device geometry to effective growth rates. After validating the model experimentally for various three-dimensional topological insulators (3D TIs), the crystal quality of selectively grown nanostructures is optimized by tuning the effective growth rates to 5 nm/h. The high quality of selectively grown nanostructures is confirmed through detailed structural characterization via atomically resolved scanning transmission electron microscopy (STEM).

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.

Rotated domains in selective area epitaxy grown Zn3P2: formation mechanism and functionality

Zinc phosphide (Zn3P2) is an ideal absorber candidate for solar cells thanks to its direct bandgap, earth-abundance, and optoelectronic characteristics, albeit it has been insufficiently investigated due to limitations in the fabrication of high-quality material. It is possible to overcome these factors by obtaining the material as nanostructures, e.g. via the selective area epitaxy approach, enabling additional strain relaxation mechanisms and minimizing the interface area. We demonstrate that Zn3P2 nanowires grow mostly defect-free when growth is oriented along the [100] and [110] of the crystal, which is obtained in nanoscale openings along the [110] and [010] on InP(100). We detect the presence of two stable rotated crystal domains that coexist in the structure. They are due to a change in the growth facet, which originates either from the island formation and merging in the initial stages of growth or lateral overgrowth. These domains have been visualized through 3D atomic models and confirmed with image simulations of the atomic scale electron micrographs. Density functional theory simulations describe the rotated domains' formation mechanism and demonstrate their lattice-matched epitaxial relation. In addition, the energies of the shallow states predicted closely agree with transition energies observed by experimental studies and offer a potential origin for these defect transitions. Our study represents an important step forward in the understanding of Zn3P2 and thus for the realisation of solar cells to respond to the present call for sustainable photovoltaic technology.

High-Performance Room-Temperature UV-IR Photodetector Based on the InAs Nanosheet and Its Wavelength- and Intensity-Dependent Negative Photoconductivity

Low-dimensional narrow-band-gap III-V semiconductors have great potential in high-performance electronics, photonics, and quantum devices. However, high-performance nanoscale infrared photodetectors based on isolated two-dimensional (2D) III-V compound semiconductors are still rare. In this work, we demonstrate a new type of photodetector based on the InAs nanosheet. The photodetector has high optoelectronic response in the ultraviolet-infrared band (325-2100 nm) at room temperature. The high-performance photodetector has very high responsivity (∼1231 A/W), EQE (2.2 × 10⁵ %), and detectivity (5.46 × 10¹⁰ Jones) to 700 nm light at low operating voltage (∼0.1 V). These results indicate that 2D InAs nanosheet devices have great potential in nano-optoelectronic devices and integrated optoelectronic devices. In addition, we observe for the first time that the InAs nanosheet devices have a negative photoconductivity (NPC) that is not only affected by the wavelength but also related to the optical power intensity of the light. After analyzing experimental data, we propose that the origin of the NPC may come from electron trapping, and two competing mechanisms of optical absorption and the photogating effect in the photoelectric response process cause the dependence on the light wavelength and optical power intensity.

Understanding Shape Evolution and Phase Transition in InP Nanostructures Grown by Selective Area Epitaxy

There is a strong demand for III-V nanostructures of different geometries and in the form of interconnected networks for quantum science applications. This can be achieved by selective area epitaxy (SAE) but the understanding of crystal growth in these complicated geometries is still insufficient to engineer the desired shape. Here, the shape evolution and crystal structure of InP nanostructures grown by SAE on InP substrates of different orientations are investigated and a unified understanding to explain these observations is established. A strong correlation between growth direction and crystal phase is revealed. Wurtzite (WZ) and zinc-blende (ZB) phases form along <111>A and <111>B directions, respectively, while crystal phase remains the same along other low-index directions. The polarity induced crystal structure difference is explained by thermodynamic difference between the WZ and ZB phase nuclei on different planes. Growth from the openings is essentially determined by pattern confinement and minimization of the total surface energy, regardless of substrate orientations. A novel type-II WZ/ZB nanomembrane homojunction array is obtained by tailoring growth directions through alignment of the openings along certain crystallographic orientations. The understanding in this work lays the foundation for the design and fabrication of advanced III-V semiconductor devices based on complex geometrical nanostructures.

Postgrowth Shaping and Transport Anisotropy in Two-Dimensional InAs Nanofins

We report on the postgrowth shaping of free-standing two-dimensional (2D) InAs nanofins that are grown by selective-area epitaxy and mechanically transferred to a separate substrate for device fabrication. We use a citric acid-based wet etch that enables complex shapes, for example, van der Pauw cloverleaf structures, with patterning resolution down to 150 nm as well as partial thinning of the nanofin to improve local gate response. We exploit the high sensitivity of the cloverleaf structures to transport anisotropy to address the fundamental question of whether there is a measurable transport anisotropy arising from wurtzite/zincblende polytypism in 2D InAs nanostructures. We demonstrate a mobility anisotropy of order 2-4 at room temperature arising from polytypic stacking faults in our nanofins. Our work highlights a key materials consideration for devices featuring self-assembled 2D III-V nanostructures using advanced epitaxy methods.

Wurtzite InP microdisks: from epitaxy to room-temperature lasing

Metastable wurtzite crystal phases of conventional semiconductors comprise enormous potential for high-performance electro-optical devices, owed to their extended tunable direct band gap range. However, synthesizing these materials in good quality and beyond nanowire size constraints has remained elusive. In this work, the epitaxy of wurtzite InP microdisks and related geometries on insulator for advanced optical applications is explored. This is achieved by an elaborate combination of selective area growth of fins and a zipper-induced epitaxial lateral overgrowth, which enables co-integration of diversely shaped crystals at precise position. The grown material possesses high phase purity and excellent optical quality characterized by STEM and µ-PL. Optically pumped lasing at room temperature is achieved in microdisks with a lasing threshold of 365 µJ cm^-2. Our platform could provide novel geometries for photonic applications.

Integrated bioelectronic proton-gated logic elements utilizing nanoscale patterned Nafion

A central endeavour in bioelectronics is the development of logic elements to transduce and process ionic to electronic signals. Motivated by this challenge, we report fully monolithic, nanoscale logic elements featuring n- and p-type nanowires as electronic channels that are proton-gated by electron-beam patterned Nafion. We demonstrate inverter circuits with state-of-the-art ion-to-electron transduction performance giving DC gain exceeding 5 and frequency response up to 2 kHz. A key innovation facilitating the logic integration is a new electron-beam process for patterning Nafion with linewidths down to 125 nm. This process delivers feature sizes compatible with low voltage, fast switching elements. This expands the scope for Nafion as a versatile patternable high-proton-conductivity element for bioelectronics and other applications requiring nanoengineered protonic membranes and electrodes.

Facet-dependent growth of InAsP quantum wells in InP nanowire and nanomembrane arrays

Selective area epitaxy is a powerful growth technique that has been used to produce III-V semiconductor nanowire and nanomembrane arrays for photonic and electronic applications. The incorporation of a heterostructure such as quantum wells (QWs) brings new functionality and further broadens their applications. Using InP nanowires and nanomembranes as templates, we investigate the growth of InAsP QWs on these pure wurtzite nanostructures. InAsP QWs grow both axially and laterally on the nanowires and nanomembranes, forming a zinc blende phase axially and wurtzite phase on the sidewalls. On the non-polar {11[combining macron]00} sidewalls, the radial QW selectively grows on one sidewall which is located at the semi-polar 〈112[combining macron]〉 A side of the axial QW, causing the shape evolution of the nanowires from hexagonal to triangular cross section. For nanomembranes with {11[combining macron]00} sidewalls, the radial QW grows asymmetrically on the {11[combining macron]00} facet, destroying their symmetry. In comparison, nanomembranes with {112[combining macron]0} sidewalls are shown to be an ideal template for the growth of InAsP QWs, thanks to the uniform QW formation. These QWs emit strongly in the near IR region at room temperature and their emission can be tuned by changing their thickness or composition. These findings enrich our understanding of the QW growth, which provides new insights for heterostructure design in other III-V nanostructures.

Impact of invasive metal probes on Hall measurements in semiconductor nanostructures

Recent advances in bottom-up growth are giving rise to a range of new two-dimensional nanostructures. Hall effect measurements play an important role in their electrical characterization. However, size constraints can lead to device geometries that deviate significantly from the ideal of elongated Hall bars with currentless contacts. Many devices using these new materials have a low aspect ratio and feature metal Hall probes that overlap with the semiconductor channel. This can lead to a significant distortion of the current flow. We present experimental data from InAs 2D nanofin devices with different Hall probe geometries to study the influence of Hall probe length and width. We use finite-element simulations to further understand the implications of these aspects and expand their scope to contact resistance and sample aspect ratio. Our key finding is that invasive probes lead to significant underestimation of measured Hall voltage, typically of the order 40-80%. This in turn leads to a subsequent proportional overestimation of carrier concentration and an underestimation of mobility.

Geometry-tailored freestanding epitaxial Pd, AuPd, and Au nanoplates driven by surface interactions

Freestanding epitaxial metal nanoplates can be utilized as advanced three-dimensional platforms for various novel applications. Here we report the vapor-phase epitaxial growth of freestanding Pd, AuPd, and Au nanoplates on an a-cut sapphire substrate as well as the comprehensive study of their growth mechanisms and geometry tailoring. All as-grown Pd, AuPd, and Au nanoplates possess twin-free single crystallinity as well as are aligned three-dimensionally on the substrate with the same orientation. Interestingly, depending on their composition, they have the following three distinct geometries: trapezoid (Pd), hexagon (AuPd), or rhombus (Au). By analyzing the correlation of the geometry and orientation of the as-synthesized nanostructures, we reveal that all the nanoplates grow from square pyramidal seed crystals. The interfacial lattice mismatch between the bottom plane of the square pyramidal seeds and a-cut sapphire substrate increases in the following order: Pd < AuPd < Au. Consequently, the length of the interface between the bottom of the nanoplate and the substrate decreases in the following order: Pd > AuPd > Au; this leads to the resulting geometries of the synthesized nanoplates. Such a fundamental understanding of the growth mechanism would aid the growth of epitaxial metal nanostructures with the desired geometry, which is very attractive for building macroscale functional nanoarchitectures.

High-quality epitaxial wurtzite structured InAs nanosheets grown in MBE

In this study, we have grown epitaxial wurtzite structured InAs nanosheets using Au catalysts on a GaAs{111}_B substrate by molecular beam epitaxy. Through detailed electron microscopy characterization studies on grown nanosheets, it was found that these wurtzite structured InAs nanosheets grew epitaxially on the GaAs{111}_B substrate, with {0001[combining macron]} catalyst/nanosheet interfaces and extensive {112[combining macron]0} surfaces. It was anticipated that the epitaxially grown InAs nanosheet can be triggered by a high supersaturation in catalysts, leading to an inclined growth leaving the substrate surface, and driven by the small lattice mismatch between the nanosheets and the substrate, with the orientation relationship of (0001[combining macron])_InAs//(112[combining macron])_GaAs. This study provides insights into achieving epitaxial free-standing III-V nanosheet growth.