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As semiconductor devices approach dimensions at the atomic scale, controlling the compositional grading across hetero-interfaces becomes paramount. Particularly in nanowire axial heterostructures, which are promising for a broad spectrum of nanotechnology applications, the achievement of sharp hetero-interfaces 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 semi-empirical 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 paves the way to advanced heterostructure schemes and a better exploitation of the nanowire platform; moreover, it is also considered expandable to other material systems and nanostructure types.

The elastic accommodation of lattice mismatch in core/shell nanowires presents unique features because both the shell and the core can be strained depending on their volume ratio. This allows the realization of heterostructure combinations with large lattice mismatch, as well as the extensive strain-mediated tailoring of their electronic properties [1]. Owing to the large stresses involved, any shell thickness asymmetry around the core may cause the bending of the nanowires, forming shapes like circular arcs, which can be a desired effect or not, depending on the targeted application. Here, we monitor the asymmetric shell growth, the asymmetric strain, and the resulting bending during the growth of GaAs/In0.4Al0.6As core/shell nanowires [2]. We show how the profile of the elemental beams in a conventional MBE system accounts for the asymmetric growth rate of the shell around the core, in spite of the substrate rotation during growth. The resulting asymmetric stresses around the core cause the bending of the nanowires to the side of the thinner shell, which, in turn, enhances further the growth rate and stress asymmetry, and thus the bending itself, in a self-feeding circle. Nevertheless, after a critical minimum shell thickness on all core sides has been reached, the bending is reversed and the nanowires straighten up. Analyzing cross sections normal to the axis of such straightened up nanowires by STEM, we developed a method to reconstruct, for the first time, the evolution of the shell thickness on every core side and the corresponding bending angle during growth. Furthermore, geometric phase analysis revealed the spatial distribution of strain in the core and the shell with respect to the uneven shell thicknesses around the core. Our results shine a light on the growth of lattice-mismatched core/shell nanowires and could serve as a guide to design deliberate curved structures or, instead, strategies to suppress the bending. Figure 1. (a) Side-view SEM image of as

Elastic accommodation of the high lattice mismatch in GaAs/In(Al,Ga)As core-shell nanowires holds great potential for applications in high-frequency electronics and optoelectronics. The unique core-shell geometry offers several advantages over planar systems. The reduced dimensionality of the core enables strain relaxation along the lateral dimension and strain partitioning, ultimately reducing the overall strain energy and expanding the limits of coherency [1]. Consequently, the GaAs core can undergo extreme elastic stretching, particularly along the nanowire axis, without the onset of plastic relaxation. Growing thick Inx(Al,Ga)1-xAs shells with high In-contents on narrow [111]-oriented GaAs cores leads to extreme elastic dilatation of the core that promotes a 40% bandgap reduction [2] as well as a 30-50% boost in electron mobility [3]. In order to elucidate the intricate 3D strain fields of such nanowires, we have employed transmission and scanning-transmission electron microscopy ((S)TEM) methods to study a series of nanowires featuring narrow cores (25 nm diameter) and thick shells (80 nm thickness) with composition x ranging from 0.20 up to 1. To enable the nanoscale investigation of all strain components, we developed elaborate sample preparation methods to facilitate observation along three zone axes, i.e. [111], <1-10> and <11-2>, as shown in Fig.1. For (S)TEM and high-resolution TEM (HRTEM) observations, a 200 kV JEOL JEM F200 CFEG microscope was used. High resolution STEM (HRSTEM) was performed in a 300 kV probe-corrected Thermo Fisher Scientific Titan Themis 60/300 microscope. In an integrated approach, experimental strain fields were compared with finite element (FE) calculations, performed using thermal expansivity to model the lattice mismatch, and with energetic calculations by molecular dynamics (MD) using the Tersoff interatomic potential [4]. (S)TEM observations revealed that for indium contents up to x≈50%, the shells were coherent and dislocatio

When lowering the dimensions of semiconductor heterostructures, controlled compositional changes become paramount owing to the increasing role of the heterointerfaces in the electronic properties of the heterostructures. A common issue faced in fabricating axial heterostructures in vapor-liquid-solid (VLS)-grown nanowires is the compositional grading, which is caused by the involved growth mechanisms and broadens the interfaces. Here, our previously developed nanowire growth technique called droplet-confined alternate pulsed-epitaxy (DCAPE) [1] (an adaptation of conventional molecular beam epitaxy), which grants precise control over the axial growth rate and droplet composition, is employed to grow AlxGa1-xAs axial insertions in self-catalyzed GaAs nanowires. Some examples showcasing the capability of DCAPE in growing systems of complex, abrupt heterostructures are shown in Fig. 1a, where AlxGa1-xAs is revealed as horizontal dark grey lines. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and a semi-empirical growth model [2] are utilized to gain an understanding of the compositional grading mechanism and its dependence on the growth temperature (TG), the nanowire radius (RNW), and the total Al content [2]. We show that pre-filling the Ga droplet with Al before the growth of an insertion is enough to sharpen the GaAs-to-AlxGa1-xAs interface at any TG. On the other hand, though, sharper AlxGa1-xAs-to-GaAs interfaces are obtained only at lower TG and/or smaller RNW, where the so-called reservoir effect is markedly reduced. In the best case, symmetric AlxGa1-xAs insertions with interface widths of only 2– 3 monolayers (comparable to, or better than, state-of-the-art thin film heterostructures) are achieved for the full range of x (Fig. 1b), proving the strength of VLS in the fabrication of sharp axial heterostructures. Figure 1: (a) HAADF-STEM images with 5 examples of GaAs/AlxGa1-xAs heterostructures grown in self-catalyzed GaAs na

Axial heterostructures have diverse functionality in electronic and optoelectronic devices. Implementing such systems in freestanding semiconducting nanowires further broadens the scope of potential applications, for example: distributed Bragg reflectors, high-efficiency light-emitting diodes, and quantum dot heterostructures. Although, as the physical dimensions of such devices approach the atomic scale, managing the compositional grading effect of the constituent materials at the heterointerface becomes critical and can be particularly challenging in nanowires grown in vapor-liquid-solid mode. We grow self-catalyzed GaAs nanowires with AlxGa1-xAs axial insertions on Si substrates using our previously developed nanowire growth technique called droplet-confined alternate pulsed-epitaxy (DCAPE) [1]. Here, nanowires are grown using alternating short beam pulses as opposed to the conventional molecular beam epitaxy analog where material is continuously supplied. With this approach, precise control over the axial growth rate, droplet composition and contact angle is granted along with the unique opportunity to grow nanowire heterostructures at CMOS compatible temperatures. To examine the compositional grading mechanism of Al in our AlxGa1-xAs axial insertions we employ high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Quantitative AlxGa1-xAs profiles are extracted from atomically resolved images to study in detail the profile’s interface sharpness and symmetry in relation to the profiles total Al content, heterostructure growth temperature and nanowire diameter. Additionally, building on a previously existing heterostructure growth model that describes the AlxGa1-xAs-to-GaAs interface while considering the solid-liquid thermodynamics and the so-called reservoir effect [2], we derive our own semi-empirical model that describes remarkably well the AlxGa1-xAs profile as a whole and allows for a rigorous analysis of both the profile’s lea

III-V semiconductor heterostructures have contributed to a wealth of studies in solid state physics, as well as to applied research and technology in electronics and photonics. More recently, the nanowire geometry introduced new possibilities, such as one-dimensional quantum transport, formation of Majorana modes, enhanced light-matter coupling, photon entanglement, as well as monolithic integration in Si-CMOS platforms for the realization of more-than-Moore hybrid systems. This talk will be focusing on the bottom-up fabrication and the structural and electronic properties of III-As nanowire heterostructures. The first type of heterostructures will be radial ones, comprising a GaAs core and a lattice-mismatched InxAl1-xAs or InxGa1-xAs shell. Molecular beam epitaxy and a combination of vapor-liquid-solid and vapor-solid growth modes are employed to grow free-standing nanowires on Si substrates [1, 2]. Owing to its high surface-to-volume ratio and the peculiar geometry, the thin core can be hydrostatically tensile strained to extremely high levels, depending on the In content x and the thickness of the shell. As a welcome effect, the bandgap of GaAs can be tuned to be anywhere between the strain-free value of 1.4 and 0.8 eV, allowing for potential applications in telecom photonics [3]. Furthermore, the electron mobility in the GaAs core is increased with increasing the tensile strain, as a result of a corresponding decrease in electron effective mass [4]. This is of major importance for the realization of transistors with high speed and low-power consumption. The second type of heterostructures will be axial ones, where the composition is modulated from GaAs to AlxGa1-x¬As and back to GaAs along the nanowire axis. Here, we develop a pulsed-growth technique [5], which grants precise control over the axial growth rate and droplet composition. Using advanced transmission electron microscopy methods and a thermodynamic equilibrium model, it becomes possible to quantitati

Strain engineering is a powerful tool for designing nanowires with tailored properties for a variety of applications. By carefully controlling the built-in strain in nanowires, it is possible to tune their bandgap to the near-infrared region, making them ideal for applications in telecommunication and imaging. In our previous work, we demonstrated that in GaAs/In x Al 1-x As core/shell nanowires, the bandgap of the core can be narrowed by up to 40%, for x up to 0.54, via strain due to the lattice mismatch between the shell [1]. Here, we explored the upper end of the lattice mismatch regime, extending the same concept to the contents of the shell towards x = 1, achieving unusually high strain values. The strain in the core and its effect on band structure are studied by a combination of spectroscopic methods and high-resolution transmission and scanning-transmission electron microscopy (HR(S)TEM). Raman spectroscopy showed that the tensile strain in the GaAs core increased linearly with increasing the In content in the shell (Fig. 1a), following the trend we reported in the past for lower values of x [1]. This behavior suggests the absence of plastic relaxation despite the very large lattice mismatch between the core and the shell. Using cross-sectional and longitudinal HR(S)TEM observations, we assessed the strain distribution normal and along the nanowire axis (Figs. 1b to 1d), which was found to be in good agreement with finite element and molecular dynamics simulations. Above a critical x value, plastic relaxation sets in via dislocations (Fig. 1b). We also correlated the photoluminescence emission properties with the strain distribution in the core and the shell, and the corresponding band alignment via band structure simulations. All in all, our results identified the limits of a coherent core and shell heterostructures and the potential application of tensile-strained GaAs nanowires for C- and O-band telecom photonics.

Axial heterostructures have diverse functionality in electronic and optoelectronic devices. Implementing such systems in freestanding semiconducting nanowires further broadens the scope of potential applications, for example: distributed Bragg reflectors, high-efficiency light-emitting diodes, and quantum dot heterostructures. The challenge, however, lies in reducing the compositional grading effect of the constituent heterostructure materials across the interfaces in nanowires grown in vapor-liquid-solid mode. Here, our previously developed nanowire growth technique called droplet-confined alternate pulsed-epitaxy [1] (an adaptation of conventional molecular beam epitaxy), which grants precise control over the axial growth rate and droplet composition, is employed to grow Al(x)Ga(1-x)As axial insertions in self-catalyzed GaAs nanowires. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and nanowire growth models are utilized to gain an understanding of the compositional grading mechanism and its dependence on the growth temperature (TG), the nanowire radius (RNW), and the amount of supplied Al. Figure 1 (a) is an example HAADF-STEM image showing two Al(x)Ga(1-x)As insertions embedded in a zincblende GaAs nanowire, whereas Figure 1 (b) shows the extracted Al-content (x) axial profiles for three selected insertions with different TG and RNW (the same amount of supplied Al). We show that lower TG and/or smaller RNW result in sharper interfaces, with a more profound improvement for the AlxGa1-xAs-to-GaAs interface. In the best case, almost symmetric insertions with interface thicknesses of only 2 – 3 monolayers are achieved, approaching the absolute limit of atomically sharp interfaces. The fitting of our experimental data with existing heterostructure growth models [2, 3] is suggestive of different mechanisms behind the compositional grading of the two interfaces and will be discussed in detail. The functionality of our insertions is

Axial heterostructures have diverse functionality in electronic and optoelectronic devices. Implementing such systems in freestanding semiconducting nanowires further broadens the scope of potential applications, for example: distributed Bragg reflectors, high-efficiency light-emitting diodes, and quantum dot heterostructures. The challenge, however, lies in reducing the compositional grading effect of the constituent heterostructure materials across the interfaces in nanowires grown in vapor-liquid-solid mode. Here, our previously developed nanowire growth technique called droplet-confined alternate pulsed-epitaxy [1] (an adaptation of conventional molecular beam epitaxy), which grants precise control over the axial growth rate and droplet composition, is employed to grow Al(x)Ga(1-x)As axial insertions in self-catalyzed GaAs nanowires. A full nanowire with six insertions is shown in Figure 1 (top). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and nanowire growth models are utilized to gain an understanding of the compositional grading mechanism and its dependence on the growth temperature (TG), the nanowire radius (RNW), and the amount of supplied Al. Figure 1 (bottom) is an example HAADF-STEM image showing two atomically resolved Al(x)Ga(1-x)As insertions embedded in a zincblende GaAs nanowire, whereas Figure 2 shows the extracted Al-content (x) axial profiles for three selected insertions with different TG and RNW (the same amount of supplied Al). We show that lower TG and/or smaller RNW result in sharper interfaces, with a more profound improvement for the AlxGa1-xAs-to-GaAs interface. In the best case, almost symmetric insertions with interface thicknesses of only 2 – 3 monolayers are achieved, approaching the absolute limit of atomically sharp interfaces. Thermodynamics, kinetics, and nanowire geometry are all factors considered during the formation of our Al(x)Ga(1-x)As insertions. Using two existing heterostructure gr

Axial heterostructures have diverse functionality in electronic and optoelectronic devices. Implementing such systems in freestanding semiconducting nanowires further broadens the scope of potential applications, for example: distributed Bragg reflectors, high-efficiency light-emitting diodes, and quantum dot heterostructures. The challenge, however, lies in reducing the compositional grading effect of the constituent heterostructure materials across the interfaces in nanowires grown in vapor-liquid-solid mode. Here, our previously developed nanowire growth technique called droplet-confined alternate pulsed-epitaxy [1] (an adaptation of conventional molecular beam epitaxy), which grants precise control over the axial growth rate and droplet composition, is employed to grow Al(x)Ga(1-x)As axial insertions in self-catalyzed GaAs nanowires. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and nanowire growth models are utilized to gain an understanding of the compositional grading mechanism and its dependence on the growth temperature (TG), the nanowire radius (RNW), and the amount of supplied Al. Figure 1 (a) is an example HAADF-STEM image showing two Al(x)Ga(1-x)As insertions embedded in a zincblende GaAs nanowire, whereas Figure 1 (b) shows the extracted Al-content (x) axial profiles for three selected insertions with different TG and RNW (the same amount of supplied Al). We show that lower TG and/or smaller RNW result in sharper interfaces, with a more profound improvement for the AlxGa1-xAs-to-GaAs interface. In the best case, almost symmetric insertions with interface thicknesses of only 2 – 3 monolayers are achieved, approaching the absolute limit of atomically sharp interfaces. The fitting of our experimental data with existing heterostructure growth models [2, 3] is suggestive of different mechanisms behind the compositional grading of the two interfaces and will be discussed in detail. The functionality of our insertions is

Axial heterostructures have diverse functionality in electronic and optoelectronic devices. Implementing such systems in freestanding semiconducting nanowires further broadens the scope of potential applications, for example: distributed Bragg reflectors, high-efficiency light-emitting diodes, and quantum dot heterostructures. The challenge, however, lies in reducing the compositional grading effect of the constituent heterostructure materials across the interfaces in nanowires grown in vapor-liquid-solid mode. Here, our previously developed nanowire growth technique called droplet-confined alternate pulsed-epitaxy [1] (an adaptation of conventional molecular beam epitaxy), which grants precise control over the axial growth rate and droplet composition, is employed to grow Al(x)Ga(1-x)As axial insertions in self-catalyzed GaAs nanowires. A full nanowire with six insertions is shown in Figure 1 (top). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and nanowire growth models are utilized to gain an understanding of the compositional grading mechanism and its dependence on the growth temperature (TG), the nanowire radius (RNW), and the amount of supplied Al. Figure 1 (bottom) is an example HAADF-STEM image showing two atomically resolved Al(x)Ga(1-x)As insertions embedded in a zincblende GaAs nanowire, whereas Figure 2 shows the extracted Al-content (x) axial profiles for three selected insertions with different TG and RNW (the same amount of supplied Al). We show that lower TG and/or smaller RNW result in sharper interfaces, with a more profound improvement for the AlxGa1-xAs-to-GaAs interface. In the best case, almost symmetric insertions with interface thicknesses of only 2 – 3 monolayers are achieved, approaching the absolute limit of atomically sharp interfaces. Thermodynamics, kinetics, and nanowire geometry are all factors considered during the formation of our Al(x)Ga(1-x)As insertions. Using two existing heterostructure gr

Short-period superlattices have diverse functionality in electronic and optoelectronic devices. Implementing such systems as axial heterostructures in freestanding semiconducting nanowires further broadens the scope of potential applications, for example: distributed Bragg reflectors, high-efficiency light-emitting diodes, and quantum dot heterostructures. The challenge, however, lies in reducing the compositional grading effect of the constituent superlattice materials across the interfaces in nanowires grown in vapor-liquid-solid mode. Here, our previously developed nanowire growth technique called droplet-confined alternate pulsed-epitaxy[1] (an adaptation of conventional molecular beam epitaxy), which grants precise control over the axial growth rate and droplet composition, was employed to grow Al(x)Ga(1-x)As/Al(y)Ga(1-y)As axial superlattices in self-catalyzed GaAs nanowires with diameters as thin as 25 nm. High-angle annular dark-field scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and growth models were utilized to gain an understanding of the compositional grading mechanism. By varying several growth parameters involving growth temperature, nanowire diameter, and droplet contact angle, the link between them and the superlattice characteristics was explored. We found that interfacial abruptness increases significantly by reducing the superlattice growth temperature and nanowire radius. Moreover, we studied the impact of an unstable contact angle on the superlattice growth rate, showing good agreement with analytical growth models and demonstrating the importance of growth rate stability in obtaining reproducible Al contents across successive superlattice periods. Finally, we confirmed with monolayer resolution, controlled Al contents in the whole compositional range and superlattice period widths of just a few monolayers. Notwithstanding, limitations in what can be accomplished are present and possible strategies to overcome

Single quantum dots embedded in the core of freestanding semiconductor nanowires grown directly on Si offer a novel and promising scheme for the realization of on-demand sources of single photons or entangled photon pairs in quantum technology systems. Here, we have investigated the growth mechanism and optical properties of axial quantum dots embedded in GaAs nanowires grown in self-catalyzed vapor-liquid-solid mode while demonstrating the tuneabilty of their emission energy across a wide range of wavelengths with the potential for telecom band access. We have incorporated GaAs complex quantum dots inside GaAs/In(x)Al(1-x)As and GaAs/Al(x)Ga(1-x)As core/shell nanowires grown via our nanowire growth technique called droplet-confined alternate pulsed-epitaxy (DCAPE)[1] (an adaptation of conventional molecular beam epitaxy (MBE)), which grants precise control over the axial growth rate and droplet composition. Using a combination of conventional MBE and DCAPE the growth of highly symmetrical quantum dots, as little as 10 nm in diameter and just a few nanometers in height, were made possible. Strong axial confinement was achieved in the form of a double axial heterostructure by incorporating two Al(x)Ga(1-x)As/Al(y)Ga(1-y)As short-period superlattices separated by a thin GaAs segment (figure 1(a)). The complexity of our quantum dots is derived from the unique possibility of using different ternary alloys for the axial and radial confinement. By introducing a latticed mismatched ternary alloy shell as the radial barrier (In(x)Al(1-x)As), we demonstrated controlled hydrostatic strain induced redshifts of the highly polarized quantum dot emission energy by adjusting the In content of the shell. For 39% In we measured a redshift in the quantum dot emission of 320 meV, from an unstrained quantum dot reference (Al(x)Ga(1-x)As shell), as shown by the photoluminescence measurements in figure 1 (b). To optimize the emission quality, the compositional grading effect of the const

Single quantum dots embedded in the core of freestanding semiconductor nanowires grown directly on Si offer a novel and promising scheme for the realization of on-demand sources of single photons or entangled photon pairs in quantum technology systems. The primary challenge in using the nanowire growth medium lies in reducing the compositional grading effect of the axial barrier’s constituent materials. Here, we have investigated the Ga-catalyzed vapor-liquid-solid growth mechanism and optical properties of axial GaAs quantum dots confined between two Al(x)Ga(1-x)As/Al(y)Ga(1-y)As short-period superlattices inside GaAs/InxAl(1-x)As and GaAs/Al(x)Ga(1-x)As core/shell nanowires. By increasing the interfacial abruptness between the axial barrier and quantum dot, its relevance was highlighted by significant improvements in the quantum dot emission linewidth. Using tensile strain, the tuneabilty of the quantum dot emission energy was clearly demonstrated across a wide range of wavelengths by employing lattice mismatched InxAl1-xAs shells as radial barriers, showing the potential for telecom band access.

Single quantum dots embedded in the core of freestanding semiconductor nanowires grown directly on Si offer a novel and promising scheme for the realization of on-demand sources of single photons or entangled photon pairs in quantum technology systems. The primary challenge in using the nanowire growth medium lies in reducing the compositional grading effect of the axial barrier’s constituent materials. Here, we have investigated the Ga-catalyzed vapor-liquid-solid growth mechanism and optical properties of axial GaAs quantum dots confined between two Al(x)Ga(1-x)As/Al(y)Ga(1-y)As short-period superlattices inside GaAs/InxAl(1-x)As and GaAs/Al(x)Ga(1-x)As core/shell nanowires. By increasing the interfacial abruptness between the axial barrier and quantum dot, its relevance was highlighted by significant improvements in the quantum dot emission linewidth. Using tensile strain, the tuneabilty of the quantum dot emission energy was clearly demonstrated across a wide range of wavelengths by employing lattice mismatched InxAl1-xAs shells as radial barriers, showing the potential for telecom band access.

Nanowire geometry allows for realising defect-free heterostructures with large lattice mismatch, in addition to the possibility for their monolithic integration on foreign substrates. Engineering of the built-in strain can be employed to tailor the electronic properties and fit them to the needs of photonic or electronic devices. This talk focuses on the epitaxial growth, the built-in strain and the modified electronic properties of free-standing GaAs/InxGa1-xAs and GaAs/InxAl1-xAs core/shell nanowires on Si substrates. The thin GaAs core can be hydrostatically tensile strained to a level that depends on the chemical composition and the thickness of the shell. As a result, the bandgap of the GaAs core can be tuned to be anywhere between 1.4 and 0.8 eV, with potential applications in telecom photonics. The same mechanism is employed to shift also the emission of GaAs/AlxGa1-xAs quantum dots that can be grown inside the core, in a scheme that could be employed for photon sources in quantum technology. Furthermore, the reduced effective mass of electrons inside the strained GaAs core results in increased mobility values (higher than those in unstrained GaAs nanowires or in bulk GaAs), which is promising for the advancement of gate-all-around transistors.

Shrt abstract: Using our nanowire growth technique called droplet-confined alternate pulsed-epitaxy, which provides precise control over the axial growth rate and droplet composition, the growth mechanism of self-catalyzed GaAs epitaxial nanowires containing AlxGa1-xAs/AlyGa1-yAs axial short-period superlattices, was systematically examined. Significant increases in interfacial abruptness were confirmed by reducing the nanowire diameter and superlattice growth temperature. Additionally, we found that an unstable droplet contact angle impacts the superlattice growth rate considerably. The unique versatility of our short-period superlattices was tested by using them as axial barriers for GaAs quantum dot heterostructures embedded in nanowires. Furthermore, encapsulating the nanowire in lattice-mismatched shells operating as radial barriers, we demonstrated the tuneabilty of the quantum dot emission energy via strain engineering. Long abstract Short-period superlattices have diverse functionality in electronic and optoelectronic devices. Implementing such systems as axial heterostructures in freestanding semiconducting nanowires further broadens the scope of potential applications, for example: distributed Bragg reflectors, high-efficiency light-emitting diodes, and quantum dot heterostructures. The challenge, however, lies in reducing the compositional grading effect of the constituent superlattice materials across the interfaces in nanowires grown in self-catalyzed vapor-liquid-solid mode. Here, our previously developed nanowire growth technique called droplet-confined alternate pulsed-epitaxy[1] (an adaptation of conventional molecular beam epitaxy), which grants precise control over the axial growth rate and droplet composition, was employed to grow Al(x)Ga(1-x)As/Al(y)Ga(1-y)As axial superlattices in self-catalyzed GaAs nanowires with diameters as thin as 20 nm. High-angle annular dark-field scanning transmission electron microscopy (figure 1(a)), energy-dispersiv

Single quantum dots embedded in the core of freestanding semiconductor nanowires grown directly on Si offer a novel and promising scheme for the realization of on-demand sources of single photons or entangled photon pairs in quantum technology systems. Here, we have investigated the growth mechanism and optical properties of axial quantum dots embedded in GaAs nanowires grown in self-catalyzed vapor-liquid-solid mode while demonstrating the tuneabilty of their emission energy across a wide range of wavelengths with the potential for telecom band access. We have incorporated GaAs complex quantum dots inside GaAs/In(x)Al(1-x)As and GaAs/Al(x)Ga(1-x)As core/shell nanowires grown via our nanowire growth technique called droplet-confined alternate pulsed-epitaxy (DCAPE)[1] (an adaptation of conventional molecular beam epitaxy (MBE)), which grants precise control over the axial growth rate and droplet composition. Using a combination of conventional MBE and DCAPE the growth of highly symmetrical quantum dots, as little as 10 nm in diameter and just a few nanometers in height, were made possible. Strong axial confinement was achieved in the form of a double axial heterostructure by incorporating two Al(x)Ga(1-x)As/Al(y)Ga(1-y)As short-period superlattices separated by a thin GaAs segment (figure 1(a)). The complexity of our quantum dots is derived from the unique possibility of using different ternary alloys for the axial and radial confinement. By introducing a latticed mismatched ternary alloy shell as the radial barrier (In(x)Al(1-x)As), we demonstrated controlled hydrostatic strain induced redshifts of the highly polarized quantum dot emission energy by adjusting the In content of the shell. For 39% In we measured a redshift in the quantum dot emission of 320 meV, from an unstrained quantum dot reference (Al(x)Ga(1-x)As shell), as shown by the photoluminescence measurements in figure 1 (b). To optimize the emission quality, the compositional grading effect of the const

Short-period superlattices have diverse functionality in electronic and optoelectronic devices. Implementing such systems as axial heterostructures in freestanding semiconducting nanowires further broadens the scope of potential applications, for example: distributed Bragg reflectors, high-efficiency light-emitting diodes, and quantum dot heterostructures. The challenge, however, lies in reducing the compositional grading effect of the constituent superlattice materials across the interfaces in nanowires grown in vapor-liquid-solid mode. Here, our previously developed nanowire growth technique called droplet-confined alternate pulsed-epitaxy[1] (an adaptation of conventional molecular beam epitaxy), which grants precise control over the axial growth rate and droplet composition, was employed to grow Al(x)Ga(1-x)As/Al(y)Ga(1-y)As axial superlattices in self-catalyzed GaAs nanowires with diameters as thin as 25 nm. High-angle annular dark-field scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and growth models were utilized to gain an understanding of the compositional grading mechanism. By varying several growth parameters involving growth temperature, nanowire diameter, and droplet contact angle, the link between them and the superlattice characteristics was explored. We found that interfacial abruptness increases significantly by reducing the superlattice growth temperature and nanowire radius. Moreover, we studied the impact of an unstable contact angle on the superlattice growth rate, showing good agreement with analytical growth models and demonstrating the importance of growth rate stability in obtaining reproducible Al contents across successive superlattice periods. Finally, we confirmed with monolayer resolution, controlled Al contents in the whole compositional range and superlattice period widths of just a few monolayers. Notwithstanding, limitations in what can be accomplished are present and possible strategies to overcome

Nanoscale, low-phase noise, tunable transmitter-receiver links are key for enabling the progress of wireless communi-cation. We demonstrate that vortex-based spin-torque nano-oscillators, which are intrinsically low-noise devices due to their topologically-protected magnetic structure, can achieve frequency tunability when submitted to local ion im-plantation. In the experiments presented here, the gyrotropic mode is excited with spin-polarized alternating currents and anisotropic magnetoresistance measurements yield discreet frequencies from a single device. Indeed, chromium-implanted regions of permalloy disks exhibit different saturation magnetisation than the surrounding, non-irradiated areas, and thus different resonance frequency, corresponding to the specific area where the core is gyrating. Our study proves that such devices can be fabricated without the need of further lithographical steps, suggesting ion irradiation can be a viable and cost-effective fabrication method for densely-packed networks of oscillators.

Interfaces separating ferromagnetic (FM) layers from non-ferromagnetic layers offer unique properties due to spin-orbit coupling and symmetry breaking, yielding effects such as exchange bias, perpendicular magnetic anisotropy, spin-pumping, spin-transfer torques, conversion between charge and spin currents and vice-versa. These interfacial phenomena play crucial roles for magnetic data storage and transfer applications, which require forming FM nano-structures embedded in non-ferromagnetic matrices. Here, we investigate the possiblity of creating such nano-structures by ion-irradiation. We study the effect of lateral confinement on the ion-irradiation-induced reduction of non-magnetic metal oxides (e.g., antiferro- or paramagnetic) to ferromagnetic metals. Our findings are later exploited to form 3-dimensional magnetic interfaces between Co, CoO and Pt by spatially-selective irradiation of CoO/Pt multilayers. We demonstrate that the mechanical displacement of the O atoms plays a crucial role during their reduction from insulating, non-ferromagnetic Co oxides to metallic Co. Metallic Co yields both perpendicular magnetic anisotropy in the generated Co/Pt nano-structures, and, at low temperatures, exchange bias at vertical interfaces between Co and CoO. If pushed to the limit of ion-irradiation technology, this approach could, in principle, enable the creation of densely-packed, atomic scale ferromagnetic point-contact spin-torque oscillator (STO) networks, or conductive channels for current-conned-path based current perpendicular-to-plane giant magnetoresistance read-heads.

Interfaces separating ferromagnetic (FM) layers from non-ferromagnetic layers offer unique properties due to spin-orbit coupling and symmetry breaking, yielding effects such as exchange bias, perpendicular magnetic anisotropy, spin-pumping, spin-transfer torques, conversion between charge and spin currents and vice-versa. These interfacial phenomena play crucial roles for magnetic data storage and transfer applications, which require forming FM nano-structures embedded in non-ferromagnetic matrices. Here, we investigate the possiblity of creating such nano-structures by ion-irradiation. We study the effect of lateral confinement on the ion-irradiation-induced reduction of non-magnetic metal oxides (e.g., antiferro- or paramagnetic) to ferromagnetic metals. Our findings are later exploited to form 3-dimensional magnetic interfaces between Co, CoO and Pt by spatially-selective irradiation of CoO/Pt multilayers. We demonstrate that the mechanical displacement of the O atoms plays a crucial role during their reduction from insulating, non-ferromagnetic Co oxides to metallic Co. Metallic Co yields both perpendicular magnetic anisotropy in the generated Co/Pt nano-structures, and, at low temperatures, exchange bias at vertical interfaces between Co and CoO. If pushed to the limit of ion-irradiation technology, this approach could, in principle, enable the creation of densely-packed, atomic scale ferromagnetic point-contact spin-torque oscillator (STO) networks, or conductive channels for current-conned-path based current perpendicular-to-plane giant magnetoresistance read-heads.

The demand on high data transfer and storage capacities requires smaller devices to transmit or save data. Forming well-defined ferromagnetic and electrically conducting volumes in a non-magnetic and insulating matrix in nanometer dimensions can pave a way to the production of such small devices. Oxygen reduction in Co3O4/Pd multilayers is possible by local proton irradiation resulting in ferromagnetic and conducting Co embedded in a nonmagnetic and insulating Co3O4 matrix [1]. To understand the mechanism behind this, we analysed in-plane single- and out-of-plane multilayer cobalt oxide films after H+ irradiation. We also confined irradiated areas on films in the range of microns to sub-micron to ascertain the lateral distribution of displaced oxygen, while establishing vertical Co/CoO interfaces which would lead to exchange bias across said interfaces. Irradiated films were characterized by SQUID magnetometry to estimate the effective O removal. Figure 1 (a) shows hysteresis loops for single layers irradiated with various doses and (b) multilayer systems irradiated with a fixed dose. In (a) we see that irradiating single layer films results in minimal O removal by measuring the saturation magnetization Ms. Geometrical confinement of the irradiated region indeed increases the Ms suggesting lateral O displacement, although this value is still only about 10% of bulk Co metal ((a) inset). The effect is much more pronounced in 0.8 nm CoO multilayers as indicated by the presence of perpendicular magnetic anisotropy (b). Figure 2 shows a loop shift for the multilayer (green) after field cooling demonstrating the formation of vertical Co/CoO interfaces post irradiation. This result is not seen in a single layer system (orange) as the layer is too thick to maintain a well-defined interface. These findings present new opportunities of device fabrication in single and bilayer systems.

Understanding of the metal-insulator transition (MIT) in correlated transition-metal oxides is a fascinating topic in condensed matter physics and a precise control of such transitions plays a key role in developing novel electronic devices. Here we report an effective tuning of the MIT in epitaxial SrVO3 (SVO) films by expanding the out-of-plane lattice constant without changing in-plane lattice parameters, through helium ion irradiation. Upon increase of the ion fluence, we observe a MIT with a crossover from metallic to insulating state in SVO films. A combination of transport and magnetoresistance measurements in SVO at low temperatures reveals that the observed MIT is mainly ascribed to electron-electron interactions rather than disorder-induced localization. Moreover, these results are well supported by the combination of density functional theory and dynamical mean field theory (DFT+DMFT) calculations, further confirming the decrease of the bandwidth and the enhanced electron-electron interactions resulting from the expansion of out-of-plane lattice constant. These findings provide insights into the understanding of MIT in correlated oxides and perspectives for the design of unexpected functional devices based on strongly correlated electrons.

The demand on high data transfer and storage capacities requires smaller devices to transmit or save data. Forming well-defined ferromagnetic and electrically conducting volumes in a non-magnetic and insulating matrix in nanometer dimensions can pave a way to the production of such small devices. Oxygen reduction in Co3O4/Pd multilayers is possible by local proton irradiation resulting in ferromagnetic and conducting Co embedded in a nonmagnetic and insulating Co3O4 matrix [1]. To understand the mechanism behind this, we analysed in-plane single- and out-of-plane multilayer cobalt oxide films after H+ irradiation. We also confined irradiated areas on films in the range of microns to sub-micron to ascertain the lateral distribution of displaced oxygen, while establishing vertical Co/CoO interfaces which would lead to exchange bias across said interfaces. Irradiated films were characterized by SQUID magnetometry to estimate the effective O removal. Figure 1 (a) shows hysteresis loops for single layers irradiated with various doses and (b) multilayer systems irradiated with a fixed dose. In (a) we see that irradiating single layer films results in minimal O removal by measuring the saturation magnetization Ms. Geometrical confinement of the irradiated region indeed increases the Ms suggesting lateral O displacement, although this value is still only about 10% of bulk Co metal ((a) inset). The effect is much more pronounced in 0.8 nm CoO multilayers as indicated by the presence of perpendicular magnetic anisotropy (b). Figure 2 shows a loop shift for the multilayer (green) after field cooling demonstrating the formation of vertical Co/CoO interfaces post irradiation. This result is not seen in a single layer system (orange) as the layer is too thick to maintain a well-defined interface. These findings present new opportunities of device fabrication in single and bilayer systems.

Spintronic devices are often patterned from continuous films into micro- or nanostructures. Fabrication of those nano-devices is self-limited and depends on the lateral resolution of the chosen fabrication method. Ion irradiation offers an alternative route to introduce smaller magnetic patterns limited by the size of the ion beam. Irradiation of oxide materials can cause chemical reduction and lead to the local formation of metallic species. By using the oxide family of ferromagnets (e.g., Fe, Ni and Co), reduction leads to the formation of ferromagnetic and conducting volumes limited by the size of the ion irradiated area that are embedded into a non-magnetic and insulating matrix. On the other hand, the physical mechanism behind ion irradiation-induced oxide reduction could not be explained. Therefore, our studies focus on ion (H, He, Ne, O) irradiated cobalt-oxide (CoO or Co3O4) systems in order to explain the physics behind the process. Also, the knowledge is being exploited to tune exchange-bias direction, prepare nano contacts for synchronized spin torque oscillators, and to form topographically stabilized magnetic skyrmions.

Spintronic devices are often patterned from continuous films into micro- or nanostructures. Fabrication of those nano-devices is self-limited and depends on the lateral resolution of the chosen fabrication method. Ion irradiation offers an alternative route to introduce smaller magnetic patterns limited by the size of the ion beam. Irradiation of oxide materials can cause chemical reduction and lead to the local formation of metallic species. By using the oxide family of ferromagnets (e.g., Fe, Ni and Co), reduction leads to the formation of ferromagnetic and conducting volumes limited by the size of the ion irradiated area that are embedded into a non-magnetic and insulating matrix. On the other hand, the physical mechanism behind ion irradiation-induced oxide reduction could not be explained. Therefore, our studies focus on ion (H, He, Ne, O) irradiated cobalt-oxide (CoO or Co3O4) systems in order to explain the physics behind the process. Also, the knowledge is being exploited to tune exchange-bias direction, prepare nano contacts for synchronized spin torque oscillators, and to form topographically stabilized magnetic skyrmions.

Spintronic devices are often patterned from continuous films into micro- or nanostructures. Fabrication of those nano-devices is self-limited and depends on the lateral resolution of the chosen fabrication method. Ion irradiation offers an alternative route to introduce smaller magnetic patterns limited by the size of the ion beam. Irradiation of oxide materials can cause chemical reduction and lead to the local formation of metallic species. By using the oxide family of ferromagnets (e.g., Fe, Ni and Co), reduction leads to the formation of ferromagnetic and conducting volumes limited by the size of the ion irradiated area that are embedded into a non-magnetic and insulating matrix. On the other hand, the physical mechanism behind ion irradiation-induced oxide reduction could not be explained. Therefore, our studies focus on ion (H, He, Ne, O) irradiated cobalt-oxide (CoO or Co3O4) systems in order to explain the physics behind the process. Also, the knowledge is being exploited to tune exchange-bias direction, prepare nano contacts for synchronized spin torque oscillators, and to form topographically stabilized magnetic skyrmions.

Spintronic devices are often patterned from continuous films into micro- or nanostructures. Fabrication of those nano-devices is self-limited and depends on the lateral resolution of the chosen fabrication method. Ion irradiation offers an alternative route to introduce smaller magnetic patterns limited by the size of the ion beam. Irradiation of oxide materials can cause chemical reduction and lead to the local formation of metallic species. By using the oxide family of ferromagnets (e.g., Fe, Ni and Co), reduction leads to the formation of ferromagnetic and conducting volumes limited by the size of the ion irradiated area that are embedded into a non-magnetic and insulating matrix. On the other hand, the physical mechanism behind ion irradiation-induced oxide reduction could not be explained. Therefore, our studies focus on ion (H, He, Ne, O) irradiated cobalt-oxide (CoO or Co3O4) systems in order to explain the physics behind the process. Also, the knowledge is being exploited to tune exchange-bias direction, prepare nano contacts for synchronized spin torque oscillators, and to form topographically stabilized magnetic skyrmions.

We investigated how to control the growth of vertically aligned graphene on C-face SiC by varying the processing conditions. It is found that, the growth rate scales with the annealing temperature and the graphene height is proportional to the annealing time. Temperature gradient and crystalline quality of the SiC substrates influence their vaporization. The partial vapor pressure is crucial as it can interfere with further vaporization. A growth mechanism is proposed in terms of physical vapor transport. The monolayer character of vertically aligned graphene is verified by Raman and X-ray absorption spectroscopy. With the processed samples, d0 magnetism is realized and negative magnetoresistance is observed after Cu implantation. We also prove that multiple carriers exist in vertically aligned graphene.

The aim of the study was to investigate the effects of a dipeptidyl peptidase-4 (DPP-4) inhibitor, of metformin, and of the combination of the two agents, on incretin hormone concentrations. Active and inactive (or total) incretin plasma concentrations, plasma DPP-4 activity, and preproglucagon (GCG) gene expression were determined after administration of each agent alone or in combination to mice with diet-induced obesity (DIO) and to healthy human subjects. In mice, metformin increased Gcg expression in the large intestine and elevated the plasma concentrations of inactive glucagon-like peptide 1 (GLP-1) (9-36) and glucagon. In healthy subjects, a DPP-4 inhibitor elevated both active GLP-1 and glucose dependent insulinotropic polypeptide (GIP), metformin increased total GLP-1 (but not GIP), and the combination resulted in additive increases in active GLP-1 plasma concentrations. Metformin did not inhibit plasma DPP-4 activity either in vitro or in vivo. The study results show that metformin is not a DPP-4 inhibitor but rather enhances precursor GCG expression in the large intestine, resulting in increased total GLP-1 concentrations. DPP-4 inhibitors and metformin have complementary mechanisms of action and additive effects with respect to increasing the concentrations of active GLP-1 in plasma.

The aim of the study was to investigate the effects of a dipeptidyl peptidase-4 (DPP-4) inhibitor, of metformin, and of the combination of the two agents, on incretin hormone concentrations. Active and inactive (or total) incretin plasma concentrations, plasma DPP-4 activity, and preproglucagon (GCG) gene expression were determined after administration of each agent alone or in combination to mice with diet-induced obesity (DIO) and to healthy human subjects. In mice, metformin increased Gcg expression in the large intestine and elevated the plasma concentrations of inactive glucagon-like peptide 1 (GLP-1) (9-36) and glucagon. In healthy subjects, a DPP-4 inhibitor elevated both active GLP-1 and glucose dependent insulinotropic polypeptide (GIP), metformin increased total GLP-1 (but not GIP), and the combination resulted in additive increases in active GLP-1 plasma concentrations. Metformin did not inhibit plasma DPP-4 activity either in vitro or in vivo. The study results show that metformin is not a DPP-4 inhibitor but rather enhances precursor GCG expression in the large intestine, resulting in increased total GLP-1 concentrations. DPP-4 inhibitors and metformin have complementary mechanisms of action and additive effects with respect to increasing the concentrations of active GLP-1 in plasma.

A fully automated measurement system has been developed that combines many properties previously realized with separate techniques. This system is an extension of the dual mixer time difference technique, and maintains its important features: zero dead time, absolute phase different measurement, very high precision, the ability to measure oscillators of equal frequency and the ability to make measurements at the time of the operator's choice. For one set of design parameters, the theoretical resolution is 0.2 ps, the measurement noise is 2 ps rms and measurements may be made within 0.1 s of any selected time. The dual mixer technique has been extended by adding scalers which remove the cycle ambiguity experienced in previous realizations. In this respect, the system functions like a divider plus clock, storing the epoch of each device under test in hardware., This article is from the Proceedings of the Symposium on Frequency Control (36th Annual), 2-4 Jun 82, Philadelphia, PA., AD-A130 811, p314-320.

The water chemistry of this facility indicated that during the period of maximum photosynthetic activity (May through August), ammonium compounds and phosphate phosphorus appeared to be readily utilized by the algae, but during the colder months these compounds built up significantly. Similarly, the biological oxygen demand (BOD) was generally low during the warmer months (30 mg/l or less) but increased appreciably with the advent of ice cover. The pH and dissolved oxygen content both indicate a photosynthetically healthy regime during the warmer months, but the latter parameter became depressed with occurrence of ice. Pond odor was not detectable until after ice cover had formed. Plankton analyses indicated 18 taxa of photosynthetically active algae, belonging principally to Chlorococcales and Volvocales. Of the last named order, 11 species are new for North America and presumably are new records as sewage-tolerant organisms. The Alaskan pond was compared with an experimental pond receiving comparable BOD loading at Fayette, Missouri, and the results are discussed.