InAs/GaAs Sharply Defined Axial Heterostructures in Self-Assisted Nanowires

At the Limit of Interfacial Sharpness in Nanowire Axial Heterostructures

As semiconductor devices approach dimensions at the atomic scale, controlling the compositional grading across heterointerfaces becomes paramount. Particularly in nanowire axial heterostructures, which are promising for a broad spectrum of nanotechnology applications, the achievement of sharp heterointerfaces has been challenging owing to peculiarities of the commonly used vapor-liquid-solid growth mode. Here, the grading of Al across GaAs/AlxGa1-xAs/GaAs heterostructures in self-catalyzed nanowires is studied, aiming at finding the limits of the interfacial sharpness for this technologically versatile material system. A pulsed growth mode ensures precise control of the growth mechanisms even at low temperatures, while a semiempirical thermodynamic model is derived to fit the experimental Al-content profiles and quantitatively describe the dependences of the interfacial sharpness on the growth temperature, the nanowire radius, and the Al content. Finally, symmetrical Al profiles with interfacial widths of 2-3 atomic planes, at the limit of the measurement accuracy, are obtained, outperforming even equivalent thin-film heterostructures. The proposed method enables the development of advanced heterostructure schemes for a more effective utilization of the nanowire platform; moreover, it is considered expandable to other material systems and nanostructure types.

Axial Growth Characteristics of Optically Active InGaAs Nanowire Heterostructures for Integrated Nanophotonic Devices

III-V semiconductor nanowire (NW) heterostructures with axial InGaAs active regions hold large potential for diverse on-chip device applications, including site-selectively integrated quantum light sources, NW lasers with high material gain, as well as resonant tunneling diodes and avalanche photodiodes. Despite various promising efforts toward high-quality single or multiple axial InGaAs heterostacks using noncatalytic growth mechanisms, the important roles of facet-dependent shape evolution, crystal defects, and the applicability to more universal growth schemes have remained elusive. Here, we report the growth of optically active InGaAs axial NW heterostructures via completely catalyst-free, selective-area molecular beam epitaxy directly on silicon (Si) using GaAs(Sb) NW arrays as tunable, high-uniformity growth templates and highlight fundamental relationships between structural, morphological, and optical properties of the InGaAs region. Structural, compositional, and 3D-tomographic characterizations affirm the desired directional growth along the NW axis with no radial growth observed. Clearly distinct luminescence from the InGaAs active region is demonstrated, where tunable array-geometry parameters and In content up to 20% are further investigated. Based on the underlying twin-induced growth mode, we further describe the facet-dependent shape and interface evolution of the InGaAs segment and its direct correlation with emission energy.

Band Edges Engineering of 2D/2D Heterostructures: The C3 N4 /Phosphorene Interface

We investigate the interface between carbon nitride (C₃ N₄ ) and phosphorene nanosheets (P-ene) by means of Density Functional Theory (DFT) calculations. C₃ N₄ /P-ene composites have been recently obtained experimentally showing excellent photoactivity. Our results indicate that the formation of the interface is a favorable process driven by Van der Waals forces. The thickness of P-ene nanosheets determines the band edges offsets and the charge carriers' separation. The system is predicted to pass from a nearly type-II to a type-I junction when the thickness of P-ene increases, and the conduction band offset is particularly sensitive. Last, we apply the Transfer Matrix Method to estimate the efficiency for charge carriers' migration as a function of the P-ene thickness.

Quantum confinement in chalcogenides 2D nanostructures from first principles

We investigated the impact of quantum confinement on the band gap of chalcogenides 2D nanostructures by means of density functional theory. We studied six different systems: MoS₂, WS₂, SnS₂, GaS, InSe, and HfS₂and we simulated nanosheets of increasing thickness, ranging from ultrathin films to ∼10-13 nm thick slabs, a size where the properties converge to the bulk. In some cases, the convergence of the band gap with slab thickness is rather slow, and sizeable deviations from the bulk value are still present with few nm-thick sheets. The results of the simulations were compared with the available experimental data, finding a quantitative agreement. The impact of quantum confinement can be rationalized in terms of effective masses of electrons and holes and system's size. These results show the possibility of reliably describing quantum confinement effects on systems for which experimental data are not available.

Axial GaAs/Ga(As, Bi) nanowire heterostructures

Bi-containing III-V semiconductors constitute an exciting class of metastable compounds with wide-ranging potential optoelectronic and electronic applications. However, the growth of III-V-Bi alloys requires group-III-rich growth conditions, which pose severe challenges for planar growth. In this work, we exploit the naturally-Ga-rich environment present inside the metallic droplet of a self-catalyzed GaAs nanowire (NW) to synthesize metastable GaAs/GaAs_1-x Bi _x axial NW heterostructures with high Bi contents. The axial GaAs_1-x Bi _x segments are realized with molecular beam epitaxy by first enriching only the vapor-liquid-solid (VLS) Ga droplets with Bi, followed by exposing the resulting Ga-Bi droplets to As₂ at temperatures ranging from 270 °C to 380 °C to precipitate GaAs_1-x Bi _x only under the NW droplets. Microstructural and elemental characterization reveals the presence of single crystal zincblende GaAs_1-x Bi _x axial NW segments with Bi contents up to (10 ± 2)%. This work illustrates how the unique local growth environment present during the VLS NW growth can be exploited to synthesize heterostructures with metastable compounds.

Synthesis and Applications of III-V Nanowires

Low-dimensional semiconductor materials structures, where nanowires are needle-like one-dimensional examples, have developed into one of the most intensely studied fields of science and technology. The subarea described in this review is compound semiconductor nanowires, with the materials covered limited to III-V materials (like GaAs, InAs, GaP, InP,...) and III-nitride materials (GaN, InGaN, AlGaN,...). We review the way in which several innovative synthesis methods constitute the basis for the realization of highly controlled nanowires, and we combine this perspective with one of how the different families of nanowires can contribute to applications. One reason for the very intense research in this field is motivated by what they can offer to main-stream semiconductors, by which ultrahigh performing electronic (e.g., transistors) and photonic (e.g., photovoltaics, photodetectors or LEDs) technologies can be merged with silicon and CMOS. Other important aspects, also covered in the review, deals with synthesis methods that can lead to dramatic reduction of cost of fabrication and opportunities for up-scaling to mass production methods.

Understanding semiconductor nanostructures via advanced electron microscopy and spectroscopy

Transmission electron microscopy (TEM) offers an ample range of complementary techniques which are able to provide essential information about the physical, chemical and structural properties of materials at the atomic scale, and hence makes a vast impact on our understanding of materials science, especially in the field of semiconductor one-dimensional (1D) nanostructures. Recent advancements in TEM instrumentation, in particular aberration correction and monochromation, have enabled pioneering experiments in complex nanostructure material systems. This review aims to address these understandings through the applications of the methodology for semiconductor nanostructures. It points out various electron microscopy techniques, in particular scanning TEM (STEM) imaging and spectroscopy techniques, with their already-employed or potential applications on 1D nanostructured semiconductors. We keep the main focus of the paper on the electronic and optoelectronic properties of such semiconductors, and avoid expanding it further. In the first part of the review, we give a brief introduction to each of the STEM-based techniques, without detailed elaboration, and mention the recent technological and conceptual developments which lead to novel characterization methodologies. For further reading, we refer the audience to a handful of papers in the literature. In the second part, we highlight the recent examples of application of the STEM methodology on the 1D nanostructure semiconductor materials, especially III-V, II-V, and group IV bare and heterostructure systems. The aim is to address the research questions on various physical properties and introduce solutions by choosing the appropriate technique that can answer the questions. Potential applications will also be discussed, the ones that have already been used for bulk and 2D materials, and have shown great potential and promise for 1D nanostructure semiconductors.

Length measurement and spatial orientation reconstruction of single nanowires

The accurate determination of the geometrical features of quasi one-dimensional nanostructures is mandatory for reducing errors and improving repeatability in the estimation of a number of geometry-dependent properties in nanotechnology. In this paper a method for the reconstruction of length and spatial orientation of single nanowires (NWs) is presented. Those quantities are calculated from a sequence of scanning electron microscope (SEM) images taken at different tilt angles using a simple 3D geometric model. The proposed method is evaluated on a collection of SEM images of single GaAs NWs. It is validated through the reconstruction of known geometric features of a standard reference calibration pattern. An overall uncertainty of about 1% in the estimated length of the NWs is achieved.

Chalcogen passivation: an in-situ method to manipulate the morphology and electrical property of GaAs nanowires

Recently, owing to the large surface-area-to-volume ratio of nanowires (NWs), manipulation of their surface states becomes technologically important and being investigated for various applications. Here, an in-situ surfactant-assisted chemical vapor deposition is developed with various chalcogens (e.g. S, Se and Te) as the passivators to enhance the NW growth and to manipulate the controllable p-n conductivity switching of fabricated NW devices. Due to the optimal size effect and electronegativity matching, Se is observed to provide the best NW surface passivation in diminishing the space charge depletion effect induced by the oxide shell and yielding the less p-type (i.e. inversion) or even insulating conductivity, as compared with S delivering the intense p-type conductivity for thin NWs with the diameter of ~30 nm. Te does not only provide the surface passivation, but also dopes the NW surface into n-type conductivity by donating electrons. All of the results can be extended to other kinds of NWs with similar surface effects, resulting in careful device design considerations with appropriate surface passivation for achieving the optimal NW device performances.

Dislocation-free axial InAs-on-GaAs nanowires on silicon

We report on the growth of axial InAs-on-GaAs nanowire heterostructures on silicon by molecular beam epitaxy using 20 nm diameter Au catalysts. First, the growth parameters of the GaAs nanowire segment were optimized to achieve a pure wurtzite crystal structure. Then, we developed a two-step growth procedure to enhance the yield of vertical InAs-on-GaAs nanowires. We achieved 90% of straight InAs-on-GaAs nanowires by further optimizing the growth parameters. We investigated the composition change at the interface by energy dispersive x-ray spectroscopy and the nanowire crystal structure by transmission electron microscopy. The composition of the nominal InAs segment is found to be In _x Ga_1-x As with x = 0.85 and corresponds to 6% of lattice mismatch with GaAs. Strain mapping performed by the geometrical phase analysis of high-resolution images revealed a dislocation-free GaAs/In_0.85Ga_0.15As interface. In conclusion, we successfully fabricated highly mismatched heterostructures, confirming the prediction that axial GaAs/In_0.85Ga_0.15As interfaces are pseudomorphic in nanowires with a diameter smaller than 40 nm.

Ga crystallization dynamics during annealing of self-assisted GaAs nanowires

In As atmosphere, we analyzed the crystallization dynamics during post-growth annealing of Ga droplets residing at the top of self-assisted GaAs nanowires grown by molecular beam epitaxy. The final crystallization steps, fundamental to determining the top facet nanowire morphology, proceeded via a balance of Ga crystallization via vapor-liquid-solid and layer-by-layer growth around the droplet, promoted by Ga diffusion out of the droplet perimeter, As desorption, and diffusion dynamics. By controlling As flux and substrate temperature the transformation of Ga droplets into nanowire segments with a top surface flat and parallel to the substrate was achieved, thus opening the possibility to realize atomically sharp vertical heterostructures in III-As self-assisted nanowires through group III exchange.

Methods of Ga droplet consumption for improved GaAs nanowire solar cell efficiency

We describe methods of Ga droplet consumption in Ga-assisted GaAs nanowires, and their impact on the crystal structure at the tip of nanowires. Droplets are consumed under different group V flux conditions and the resulting tip crystal structure is examined by transmission electron microscopy. The use of GaAsP marker layers provides insight into the behavior of the Ga droplet during different droplet consumption conditions. Lower group V droplet supersaturations lead to a pure zincblende stacking-fault-free tip crystal structure, which improved the performance of a nanowire-based photovoltaic device.

Silver as Seed-Particle Material for GaAs Nanowires--Dictating Crystal Phase and Growth Direction by Substrate Orientation

Here we investigate the feasibility of silver as seed-particle material to synthesize GaAs nanowires and show that both crystal phase and growth direction can be controlled by choice of substrate orientation. A (111)B substrate orientation can be used to form vertically aligned wurtzite GaAs nanowires and a (100) substrate orientation to form vertically aligned zinc blende GaAs nanowires. A 45-50% yield of vertical nanowire growth is achieved on the (100) substrate orientation without employing any type of surface modification or nucleation strategy to promote a vertical growth direction. In addition, photoluminescence measurements reveal that the photon emission from the silver seeded wurtzite GaAs nanowires is characterized by a single and narrow emission peak at 1.52 eV.

Ternary In2S3/In2O3 heterostructures and their cathodoluminescence

Three-dimensional In₂S₃/In₂O₃ heterostructures are obtained through a CVD process. The heterostructures have a broad emission band covering the whole visible range, which holds a promise in photocatalytic applications.