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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 growth models [2, 3] to fit our experimental data we can extract valuable quantitative information regarding the interface characteristics. Our results show for the first time that not only TG, but also size effects, i.e. a decreasing RNW, play a large role in the thermodynamic stability at the liquid-solid interface, setting previously unknown limitations on the maximum attainable interface sharpness. These findings and their implications will be discussed in detail. The functionality of our insertions is successfully tested via their employment as axial barriers in quantum dot nanowire heterostructures. By growing the quantum dot heterostructure at 350 °C, consequently sharpening the quantum dot interfaces, we observe a one order of magnitude decrease in quantum dot emission linewidth (Figure 3) in comparison to a similar system grown at 550 °C. 1. Balaghi et al., Nano Lett. 16, 4032 (2016) 2. Luna et al., Phys. Rev. Lett. 109, 126101 (2012) 3. Priante et al., Nano Lett. 16, 1917 (2016)

Optical pump – THz probe spectroscopy has been established as a tool for contactless probing of electronic transport in semiconductor nanowires (NWs) [1]. Particularly in III-V NWs, scattering rates of charge carriers, as well as their plasmonic resonances for typical doping levels, are located in the THz range. The analysis of the optical conductivity spectra using the localized surface plasmon model provides an estimation of the carrier density and the mobility. Here, we employ THz spectroscopy to study electron mobility in the strained GaAs core of GaAs/InAlAs core/shell nanowires. Owing to the lattice mismatch between the core and the shell in these NWs, the bandgap energy in the strained GaAs core exhibits a reduction by up to 40% [2]. Our results demonstrate that this effect is accompanied by a notable increase in the electron mobility by 30-50% with respect to a bulk GaAs [3]. We discuss the role of various scattering mechanisms and their dependence on strain and temperature. In addition to the homogeneous plasmon broadening caused by the carrier scattering, we also observe an inhomogeneous broadening in dense ensembles of NWs as illustrated in Fig. 1(a). Our modelling demonstrates that such broadening stems from the plasmonic interaction between neighboring NWs leading to the shift of the plasmon frequency as shown in Fig. 1(b). This effect has to be considered in the analysis of THz response of NWs since it may result in a significant underestimation of the mobility values.

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 successfully tested via their employment as axial barriers in quantum dot nanowire heterostructures. By growing the quantum dot heterostructure at 350 °C, consequently sharpening the quantum dot interfaces, we observe a one order of magnitude decrease in quantum dot emission linewidth (Figure 1 (c)) in comparison to a similar system grown at 550 °C. [1] Balaghi et al., Nano Lett. 16, 4032 (2016) [2] Luna et al., Phys. Rev. Lett. 109, 126101 (2012) [3] Priante et al., Nano Lett. 16, 1917 (2016)

Optical pump – Terahertz (THz) probe (OPTP) spectroscopy has proven efficacy for contactless probing of electronic transport in semiconductor NWs [1]. Particularly in III-V NWs, scattering rates of charge carriers, as well as their plasmonic resonances for typical doping levels, are located in the THz range. The analysis of the optical conductivity spectra using the localized surface plasmon model allows estimating the carrier lifetime and the carrier mobility. Here, OPTP spectroscopy is employed to study two unique phenomena in GaAs/(In,Al,Ga)As core/shell nanowires. First, it is demonstrated that the mobility of electrons in the hydrostaticallystrained GaAs core (owing to the lattice mismatch between the core and the shell [2]) exceeds the mobility in bulk GaAs by 30-50% [3]. The role of the various scattering mechanisms is analyzed as a function of strain and temperature. Depending on the density of NWs in the probed sample, some of them can form bundles or touch each other, leading to an inhomogeneous broadening of the plasmon resonance. We discuss the role of this effect and its impact on the estimation of carrier mobility [3, 4]. Second, we demonstrate a strong THz nonlinearity using single-cycle intense THz pulses with peak electric fields reaching up to 0.6 MV/cm. With the increase of the driving THz field, we observe a systematic redshift of the plasmon frequency, accompanied by a gradual suppression of the spectral weight. Remarkably, the spectral weight does not remain proportional to the square of the plasmon frequency when the driving electric field exceeds 0.4 MV/cm, indicating an onset of a spatially inhomogeneous carrier distribution across the NW. The observed behavior can be ascribed to nonlinear effects caused by the scattering of electrons from the Gamma- to L-valley occurring in the high electric field regime. However, in contrast to bulk semiconductors, this effect initially sets in at hot spots of the NW, where the local electric field is enhanced by the plasmonic resonance [5]. All in all, our findings provide important guidelines for the exploitation of nanowires in high-frequency electronics, but also underline the unique strengths of OPTP spectroscopy for the study of electronic transport in nanowires. [1] H. J. Joyce et al., Semicond. Sci. Technol. 31, 103003 (2016). [2] L. Balaghi et al., Nat. Commun. 10, 2793 (2019). [3] L. Balaghi et al., Nat. Commun. 12, 6642 (2021). [4] I. Fotev et al., Nanotechnology 30, 244004 (2019). [5] R. Rana et al., Nano Lett. 20, 3225 (2020).

High-quality epitaxial nanowires (NWs) based on III–V semiconductors such as (In)GaAs offer the possibility to fabricate ultrafast optical devices due to their direct bandgap and the high electron mobility. Contactless investigation of the average charge carrier concentration and mobility in NWs is enabled by terahertz time domain spectroscopy [1]. The determination of these properties locally on individual NWs can be carried out by scattering type scanning near-field optical microscopy (s-SNOM), which provides spatial resolution far beyond the diffraction limit. In optical-pump THz-probe experiments the response of photoexcited carriers has been investigated with 10 nm and 10 fs spatial and temporal resolution [2]. Time-resolved studies are still missing in both far-field and near-field spectroscopy for doped nanowires excited by THz radiation via intraband excitation. Here we report on THz-pump MIR-probe s SNOM studies on highly-doped GaAs/InGaAs core-shell NWs utilizing intense narrowband THz radiation from the free-electron laser (FEL) FELBE. The samples under study are Si-doped GaAs-InGaAs core-shell NWs grown by molecular beam epitaxy. They consist of a 25-nm-thick GaAs core and an 80-nm-thick In0.44Ga0.56As shell that is homogeneously doped with Si at a concentration of 9 × 1018 cm-3. For s-SNOM studies, these NWs are transferred to a (100) Si substrate and dispersed randomly over the substrate. The experiment was carried out with an s-SNOM setup from Neaspec GmbH equipped with a broadband difference-frequency generation (DFG) source (5 – 15 µm; 20 – 60 THz). For the pump-probe measurements the laser oscillator of the DFG source was synchronized to the FEL and the time delay between the pulses was varied by an optical delay line. A low-pass filter suppresses the scattered THz FEL radiation from the nano-FTIR unit (Fig 1a). In the unpumped case, a sharp plasma edge around 130 meV is observed. Upon intraband pumping with 13THz FEL radiation (pulse duration 2 – 5 ps, average power 15 mW), the near-field response of the plasma resonance changes dramatically. The spectrally integrated pump-probe signal exhibits a small negative component followed by a stronger positive signal that decays with the time constant (1/e) of ≈ 7 ps (Fig. 1b, insert). The nano FTIR studies reveal strong red shift (black curve) and then flattening (red curve) of the plasma resonance (Fig. 1b). We assign this effect to a substantial heating of the electrons in the conduction band and subsequent increase of the effective mass in the nonparabolic Γ-valley due to high peak electric field strengths up to several 10 kV cm−1 of pulsed FEL radiation [3]. Power-dependent nanoimaging pump-probe studies will be performed to conclude the nature of observed effects. In particular, the experiments should reveal if there is a contribution of carrier transfer to side valleys at high excitation fields. References [1] P. Parkinson, et al., Nano Lett. 7, 2162 (2007). [2] M. Eisele, et al., Nature Photon. 8, 841 (2014). [3] D. Lang, et al., Nanotechnology 30, 084003 (2019).

Novel transistor concepts based on semiconductor nanowires promise high performance, lower energy consumption and better integrability in various platforms in nanoscale dimensions. Concerning the intrinsic transport properties of electrons in nanowires, relatively high mobility values that approach those in bulk crystals have been obtained only in core/shell heterostructures, where electrons are confined inside the core and, thus, their scattering on the nanowire surface is suppressed. Here, we demonstrate that the large strain in core/shell nanowires with significant lattice-mismatch between the core and the shell can affect the effective mass and the scattering of electrons in a way that boosts their mobility to higher levels compared to results obtained by any other means. Specifically, we use GaAs/InAlAs core/shell nanowires with a lattice mismatch in the range of 3%, grown on Si substrates by molecular beam epitaxy. Overgrown with an 80-nm-thick shell, the 22-nm-thick core is hydrostatically tensile-strained as found by both Raman scattering and photoluminescence measurements [1, 2]. The transport properties and dynamics of electrons were probed at room temperature by optical-pump THz-probe spectroscopy, which is an established contactless method that circumvents challenges in the fabrication of electrical contacts on nanowires. We found that the mobility of electrons inside the strained GaAs core undergoes a remarkable enhancement despite the small core thickness, becoming 30 – 50 % higher than in unstrained GaAs/AlGaAs nanowires or bulk GaAs [2]. Our studies are extended to modulation-doped GaAs/InAlAs nanowires and the results will be presented. The reported strain-induced mobility enhancement is of major importance for the realization of transistors with high speed and low power consumption, having the potential to trigger major advancements in high-performance nanowire electronic devices monolithically integrated in Si platforms. [1] L. Balaghi, G. Bussone, R. Grifone, R. Hübner, J. Grenzer, M. Ghorbani-Asl, A. V. Krasheninnikov, H. Schneider, M. Helm, E. Dimakis, Nat Commun 10, 2793 (2019). [2] L. Balaghi, S. Shan, I. Fotev, F. Moebus, R. Rana, T. Venanzi, R. Hübner, T. Mikolajick, H. Schneider, M. Helm, A. Pashkin, E. Dimakis, Nat Commun 12, 6642 (2021).

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 constituent superlattice materials across the interfaces must be addressed. High-angle annular dark-field scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and nanowire growth models were utilized to gain an understanding of the compositional grading mechanism at the quantum dot/superlattice interface. We found that interfacial sharpness increases significantly by reducing the superlattice growth temperature and nanowire radius. Notwithstanding, limitations in what can be achieved ensue and possible strategies to overcome them will be presented. [1] Balaghi et al., Nano Lett. 16, 4032 (2016)

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-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. We confirmed with monolayer resolution, controlled Al contents in a wide compositional range and superlattice period widths of just a few monolayers (figure 1 (b)). The quality of our short-period superlattices was successfully tested via their employment as axial barriers in quantum dot nanowire heterostructures (figure 1 (c)). Additionally, by introducing a lattice-mismatched ternary alloy shell as the radial barrier (In(x)Al(1-x)As in this case), we demonstrated controlled strain-induced redshifts of the 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 180 meV as shown by the photoluminescence measurements in figure 1 (d). Notwithstanding, limitations in what can be accomplished are present and possible strategies to overcome them will be presented. [1] Balaghi et al., Nano Lett. 16, 4032 (2016)

We report the observation of the intersubband AC-Stark effect in a single wide GaAs/AlGaAs quantum well. In a three-level configuration, the n = 2 to n = 3 intersubband transition is resonantly pumped at 3:5 THz using a free-electron laser. The induced spectral changes are probed using THz time-domain spectroscopy with a broadband pulse extending up to 4 THz. We observe an Autler Townes splitting at the 1 -> 2 intersubband transition as well as an indication of a Mollow triplet at the 2 -> 3 transition, both evidencing the dressed states. For longer delay times, a relaxation of the hot-electron system with a time constant of around 420 ps is measured.

Single quantum dots in the core of freestanding semiconductor nanowires is a promising scheme for the realization of on-demand sources of single photons or entangled photon pairs in quantum technology systems. Here, we demonstrate that complex quantum-dots can be grown in self-catalyzed III-As nanowires and their emission can be tuned in a wide range of wavelengths. The quantum dots are formed inside self-catalyzed GaAs nanowires (grown on Si substrates by molecular beam epitaxy) by first growing an axial AlxGa1-xAs/GaAs/AlxGa1-xAs heterostructure in pulsed mode . The AlxGa1-xAs segments are grown as digital alloys with a precise control of the composition, the thickness, and the crystal structure (absence of stacking faults). Then, the nanowires are overgrown all-around with an InxAl1-xAs layer in a core/shell fashion. Owing to the large lattice-mismatch with the shell, the thin core develops tensile hydrostatic strain and the emission from the dot is strongly red-shifted. Furthermore, distinct exciton-biexciton features are identified in photoluminescence measurements.

We report a strong shift of the plasma resonance in highly doped GaAs/InGaAs core/shell nanowires for intense infrared excitation observed by scattering-type scanning near-field infrared microscopy. The studied nanowires show a sharp plasma resonance at a photon energy of about 125 meV in the case of continuous-wave excitation by a CO₂ laser. Probing the same nanowires with the pulsed free-electron laser with peak electric field strengths up to several 10 kV/cm reveals a power-dependent redshift to about 95 meV and broadening of the plasmonic resonance. We assign this effect to a substantial heating of the electrons in the conduction band and subsequent increase of the effective mass in the nonparabolic Γ-valley.

We present the electrical properties of GaAs/InxGa1-xAs core/shell nanowires measured by ultrafast optical pump - terahertz probe spectroscopy. This contactless technique was used to measure the photoconductivity of nanowires with shell compositions of x = 0.20, 0.30 and 0.44. The results were fitted with the model of localized surface plasmon in a cylinder in order to obtain electron mobilities, concentrations and lifetimes in the InxGa1-xAs NW shells. The estimated lifetimes are about 80 - 100 ps and the electron mobility reaches 3700 cm2/Vs at room temperature. This makes GaAs/InGaAs nanowires good candidates for the near-future realization of InGaAs based high-electronmobility transistor.

III-As semiconductors in the form of free-standing nanowires have exhibited new potentials for a wide variety of future applications in nanotechnology, ranging from energy-efficient electronic switches to entangled-photon-pair sources for quantum information technology, including the possibility for monolithic integration in the mainstream Si technology. Using molecular beam epitaxy, we developed an in situ procedure (substrate annealing – Ga deposition – substrate annealing) in order to modify the surface of Si substrates and, thus, to achieve highly synchronized nucleation of self-catalyzed GaAs nanowire ensembles with well controlled dimensions and number density. Specifically, the radius can be as low as 10 nm, the distribution of lengths can be sub-Poissonian (due to the so-called nucleation anti-bunching), and the number density can be varied from 10 ⁶ to 10 ⁹ cm-2. Furthermore, the GaAs nanowires can be hydrostatically strained when they are overgrown all-around with lattice-mismatched shells. The high surface-to-volume ratios allow for growing highly mismatched combinations without dislocations, beyond to what is possible in thin-film heterostructures. Here, we show that the mismatch strain inside the GaAs core can be engineered via the composition and the thickness of an (In, Ga)As or (In, Al)As shell. As a result, the electronic properties of GaAs can be widely tuned; the band gap and the electron effective mass can be reduced down to 60% of the strain-free values, rendering GaAs nanowires suitable for photonic devices across the near-infrared (800 – 1400 nm) range or for high-speed transistors.

Short period superlattices (SPSs) of InxGa1-xN/GaN quantum wells (QW) with a thickness of one up to just a few atomic monolayers (MLs) are promising for bandgap and strain engineering towards advanced optoelectronics devices and novel topological insulators. We have considered 5-period InxGa1-xN/GaN SPSs deposited by plasma-assisted molecular beam epitaxy (PAMBE) on c-GaN/Al2O3 templates under metal-rich conditions. The nominal QW thickness was 1 ML and the GaN barriers were 10 nm thick. The SPSs were grown at various growth temperatures keeping the same temperature for both QWs and GaN barriers. Cs-corrected high resolution transmission electron microscopy (HRTEM), and probe-corrected scanning TEM (HRSTEM) observations were carried out in order to elucidate the effect of growth temperature on the structural quality, composition, and strain state of the QWs. Cross-sectional observations were conducted along the and projection directions. Atomic positions were identified on images using a peak finding algorithm and were employed in order to extract nanoscale strain maps. Furthermore, quantification of the Z-contrast of atomic columns on the HRSTEM observations was employed for the direct determination of the indium content in QWs. The thin foil relaxation phenomenon was considered in the analysis. Composition dependent strain graphs were calculated theoretically in order to associate the experimental strain measurements to the indium content. For that purpose, a series of energetically relaxed InxGa1-xN/GaN supercells were constructed taking into account several indium contents, and the QW thickness limited to 1 ML. For the relaxation of the supercells a ternary empirical interatomic potential was utilized using molecular dynamics simulations.

Free-standing nanowires are promising platforms for hosting three-dimensional heterostructures such as single quantum dots for quantum photonics, modulation doped heterostructures for gate-all-around high mobility transistors, etc. The peculiarity of heteroepitaxy in nanowires is the existence of multiple growth interfaces with different crystallographic orientations (usually one top- and six side-facets), where the growth can take place in different modes (e.g. vapor-liquid-solid on the top- and vapor-solid on the side-facets) and can be controlled independently. Furthermore, strained epilayers in nanowires can relax elastically both at the lateral free surface and by stretching the substrate, which in this case are the thin nanowires. All these features open up new possibilities for complex three-dimensional heterostructures with loose strain restrictions. Here, we have investigated the growth of radial and axial heterostructures, as well as combinations of the two, within III-As nanowires. The radial ones consist of thin GaAs nanowires overgrown all-around with an InxAl1-xAs layer in a core/shell fashion. In agreement with theory, the small volume of the core allows for strain engineering both in the core and the shell, and for realization of highly-mismatched/strained heterostructures (with misfits up to 4%) without dislocations. The manipulation of the growth kinetics is necessary in order to suppress strain-driven phenomena, such as the preferential shell-growth and In-incorporation on one side of the core. The axial heterostructures consist of single or multiple GaAs/AlxGa1-xAs quantum dots. The width and the thickness of these dots can be controlled independently. The AlxGa1-xAs segments were grown as digital alloys using pulsed epitaxy in order to achieve sharper interfaces and to avoid the formation of stacking faults. Finally, we realized complex heterostructures with single GaAs/AlxGa1-xAs quantum dots inside the core of core/shell nanowires, aiming at engineering the electronic properties of the dots depending on the composition and thickness of the shell. Besides MBE experiments, our investigations involved transmission electron microscopy (HR-TEM, STEM, EDX, STEM tomography), X-ray diffraction, Raman scattering spectroscopy, and photoluminescence spectroscopy.

Introduction/Purpose: Short period superlattices (SPSs) comprising ultrathin InGaN/GaN quantum wells (QW) with thicknesses ranging from one to few (0002) monolayers (MLs) are promising for novel applications ranging from band gap engineering in optoelectronic devices to topological insulators. The strain relaxation behavior of a range of samples grown by varying the growth temperatures for the QWs and GaN barriers has been considered. Methods: In(Ga)N/GaN SPSs were deposited by plasma-assisted molecular beam epitaxy (PAMBE) on c-GaN/Al2O3 templates. Structural characterization was performed by high resolution transmission and scanning transmission electron microscopy (HRTEM/HRSTEM). Results: Strain relaxation through formation of stacking fault domains was observed with decreasing growth temperature. For the quantification of strain versus composition, peak finding with a recently established approach was implemented. This involves quantification of Z-contrast from HRSTEM observations by comparison with calculated composition-dependent graphs of InxGa1-xN/GaN atomic column intensity ratios obtained from multislice image simulations of energetically relaxed supercells under the frozen lattice approximation. Energetical relaxation was performed by molecular dynamics using an empirical interatomic potential, considering the ordered, disordered, and island models of QW structure. Comparison to the experimental observations was performed along the a-type zone axis that is appropriate to deduce average values for the QW composition and strain. Conclusions: The investigation concluded to the influence of growth temperature in the composition and structural properties of ultra-thin In(Ga)N/GaN QWs.

We report terahertz response of photoexcited core/shell nanowires. The obtained parameters of the localized surface plasmon mode allow us to estimate electron mobilities, concentrations and recombination lifetimes. The extracted mobilities reach 4000 cm²/V·s at room temperature, while the carrier lifetimes range from 80 to 300 ps, depending on the shell composition and the photoexcitation level.

We have investigated the plasmonic response of GaAs/In0.20Ga0.80As core/shell nanowires driven resonantly by strong THz fields with the amplitude of few MV/cm. The plasmon mode exhibits a systematic redshift with the suppression of the spectral weight with the increase of the driving THz field. Interestingly, the scaling of the plasmon parameters does not follow the usual quadratic behavior, indicating an inhomogeneous intervalley electron scattering across the nanowire.

We present nanoscopic infrared-optical investigations on highly n-type doped GaAs-based nanowires, revealing interesting nonlinear phenomena such as a pronounced redshift of the plasma resonance by the strong THz fields of a free-electron laser.

III-V compound semiconductors have fueled many breakthroughs in physics and technology owing to their direct band gap and high electron mobility. It has also been very important that these fundamental properties can be tailored in ternary or quaternary alloys by selecting the chemical composition appropriately. Here we explore the great possibilities for strain engineering in core/shell nanowires as an alternative route to tailor the properties of III-V semiconductors without changing their chemical composition. In particular, we demonstrate that the GaAs core in GaAs/InₓGa₁₋ₓAs or GaAs/InₓAl₁₋ₓAs core/shell nanowires can sustain unusually large misfit strains that would have been impossible in equivalent thin-film heterostructures, and undergoes a significant modification of its electronic properties. Core/shell nanowires were grown in the self-catalyzed mode on SiOₓ/Si(111) substrates by molecular beam epitaxy. Strain analysis was performed using synchrotron X-ray diffraction and Raman scattering spectroscopy, and showed that for a thin enough core, the magnitude and the spatial distribution of the built-in misfit strain can be regulated via the composition and the thickness of the shell. Beyond a critical shell thickness, we obtain a heavily tensile-strained core and an almost strain-free shell. The tensile strain of the core exhibits a predominantly-hydrostatic character and causes the reduction of the GaAs band gap energy in accordance with our theoretical predictions using deformation-potential theory and first-principle calculations. For 7 % of strain (x = 0.54), the band gap energy was reduced to 0.87 eV at 300 K, i.e. a remarkable reduction of 40 %. Signatures of valence-band splitting were also identified in polarization-resolved photoluminescence measurements, as a result of the strain anisotropy in GaAs. Presuming a reduced effective mass of electrons in the tensile-strained core of GaAs/InₓAl₁₋ₓAs nanowires (core diameter = 22 nm, x = 0.39 - 0.49), the corresponding electron mobility was measured by time-domain terahertz spectroscopy to be in the range of 4000 cm²/V·s at 300 K. These values are the highest reported, even in comparison to GaAs/AlₓGa₁₋ₓAs nanowires with double the core thickness. Our results demonstrate that strained GaAs in core/shell nanowires can resemble the electronic properties of InₓGa₁₋ₓAs, surmounting issues with phase separation, surface segregation or alloy disorder that typically exist in ternary alloys and limit the device performance.

We utilize ultrafast optical pump – terahertz probe spectroscopy in order to investigate charge carrier response of GaAs nanowires with InxGa1-xAs and InxAl1-xAs shells. The estimated electron mobilities reach 4000 cm2V−1s−1 and the carrier lifetimes range from 100 to 300 ps depending on the type of the shell.

We utilize ultrafast optical pump – terahertz probe spectroscopy in order to investigate charge carrier response of GaAs/In𝑥Ga1−𝑥As core/shell nanowires (NWs) produced by molecular beam epitaxy. The NWs were ≈2 𝜇m long. The GaAs core diameter was 25nm and the InGaAs shell thickness was 80 nm. We studied the shells with different compositions, from 𝑥 = 0.20 to 𝑥 = 0.44. From the pump-probe measurements we extracted terahertz photoconductivity of NWs and used the localized surface plasmon model to fit the results. The charge carrier lifetimes were estimated to be around 80–100 ps while the extracted electron mobilities reach 3700 cm2V−1s−1 at room temperature. Even without a surface passivation shell, these values are higher than those in previously studied GaAs/AlGaAs core/shell nanowires, but still lower than the ones for bulk InGaAs. Possible reasons (sources of electron scattering) which affect the mobility will be discussed.

The achievement of sub-Poissonian length distributions (LDs) in VLS-grown III-V nanowire (NW) ensembles, as theoretically predicted by Glas and Dubrovskii, requires synchronized nucleation of all NWs on their substrate. This is especially challenging for self-catalyzed GaAs NWs on a natively-oxidized Si(111) substrate because their nucleation involves a sequence of different physical mechanisms: the formation of Ga droplets at random positions on the substrate, their interaction with SiOx and the formation of nano-sized holes, and finally the droplet-assisted nucleation of GaAs inside these holes. Here, we demonstrate that it is possible to achieve highly-synchronized nucleation of MBE-grown GaAs NWs and, thus, very narrow LDs if a simple in situ procedure is employed prior to the growth in order to decouple the formation of SiOx holes from the subsequent nucleation of NWs. This procedure consists of three steps (substrate annealing – Ga deposition – substrate annealing) and produces SiOx holes (free of Ga droplets) of controlled size and number density. Our study compares the LD of GaAs NWs grown on Si substrates with different size or number density of SiOx holes. The results were fitted with a continuum-growth theoretical model that accounts for nucleation fluctuations, kinetic fluctuations and nucleation antibunching in individual NWs. We have found that the formation of large-enough holes before the initiation of the NW growth can shorten the characteristic nucleation time of the NWs by one order of magnitude and narrow the LD by a factor of 2. The LD was further improved by decreasing the number density of SiOx holes/GaAs NWs, which is attributed to the suppression of beam-shadowing effects. In the best case, we obtained GaAs NWs that exhibit a remarkably short characteristic nucleation time of 10 ms and a sub-Poissonian LD. All in all, our results (unpublished) not only prove the validity of theoretical considerations about the sub-Poissonian LD for self-catalyzed NWs, but also demonstrate a simple route to low-cost fabrication (without substrate patterning) of GaAs NW-based devices with controllable number density and length uniformity.

© 2017 American Chemical Society. The unique electronic band structure of indium nitride InN, part of the industrially significant III-N class of semiconductors, offers charge transport properties with great application potential due to its robust n-type conductivity. Here, we explore the water sensing mechanism of InN thin films. Using angle-resolved photoemission spectroscopy, core level spectroscopy, and theory, we derive the charge carrier density and electrical potential of a two-dimensional electron gas, 2DEG, at the InN surface and monitor its electronic properties upon in situ modulation of adsorbed water. An electric dipole layer formed by water molecules raises the surface potential and accumulates charge in the 2DEG, enhancing surface conductivity. Our intuitive model provides a novel route toward understanding the water sensing mechanism in InN and, more generally, for understanding sensing material systems beyond InN.

Liquid Ga droplets play a double role in the self-catalyzed growth of GaAs nanowires on Si(111) substrates covered with a native SiOx layer: they induce the formation of nano-sized holes in SiOx and then drive the uniaxial nanowire growth directly onto the underlying Si. The independent control of the two mechanisms is a prerequisite for mastering the growth of nanowires, but it is challenging in a conventional growth procedure where they both take place under the same droplets. To that end, we have developed an in situ procedure where the Ga droplets used for the formation of SiOx holes are removed before new Ga droplets drive the growth of GaAs nanowires. In that way, it was made possible to study the interaction between Ga droplets and SiOx, to create holes in SiOx with controlled number density and size and, finally, to grow GaAs nanowires only within those holes. Our results show unprecedented control of the nanowire nucleation with unique possibilities: (1) deliberate control of the number density of nanowires within three orders of magnitude (106-109 cm-2) without patterning the substrate and without changing the growth conditions, (2) highly synchronous nucleation events and, thus, exceptionally narrow nanowire length distributions (standard deviation < 1 % for 3 mm long nanowires), (3) high yield of vertical nanowires up to 80 % (against GaAs islands), (4) highly reproducible results, and (5) independent control of the nanowire diameter from the number density. We anticipate that our methodology could be also exploited for different materials or other types of nanostructures.

The geometry and high surface-to-volume ratio of nanowires offer unique possibilities for strain engineering in epitaxial semiconductor heterostructures with large lattice mismatch. In addition, the possibility to grow nanowires of high crystal quality epitaxially on Si substrates adds to their technological significance. In this work, we have investigated the growth of free-standing GaAs/InxGa1-xAs and GaAs/InxAl1-xAs core/shell nanowires on Si(111) substrates by molecular beam epitaxy, the accommodation of the lattice mismatch therein, and its effect on the nanowire properties. Very thin GaAs core nanowires (20-25 nm in diameter) were grown in the self-catalyzed mode with a sufficiently low number density (to avoid beam shadowing effects) on SiOx/Si(111) substrates, after an in situ treatment of the latter with Ga droplets. This resulted in zinc blende nanowires with their axis along the [111] crystallographic direction and six {1-10} sidewalls. Subsequently, conformal overgrowth of the InxGa1-xAs or InxAl1-xAs shell was obtained only under kinetically limited growth conditions that suppressed mismatch-induced bending phenomena. The strain in the core and the shell was studied systematically as a function of the shell composition and thickness. To that end, we used Raman scattering spectroscopy, transmission electron microscopy and synchrotron X-ray diffraction, and compared the results with theoretical predictions based on continuum elasticity and density functional theories. Our results demonstrate that highly mismatched core/shell nanowires with defect-free interface can be obtained beyond what is possible in thin film heterostructures. More interestingly, nanowires with strain-free shell and fully strained core can be grown under certain conditions. The large strain in the GaAs core is expected to have a strong effect on its fundamental properties. Here, we demonstrate a large shrinkage of the band gap, which can be as high as 35 % depending on the composition of the shell.

Compound semiconductors are versatile materials due to the possibility to tailor their (opto)electronic properties by selecting their composition appropriately. When grown heteroepitaxially, though, this possibility is constrained by the lattice mismatch with the substrate. InxGa1-xAs is a good example because it can have, depending on x, a suitable direct optical band gap for optoelectronic applications in the infrared (e.g. telecommunication wavelengths) or high electron mobility for high-speed transistors. However, the practical choices of x are limited by the available substrates, typically GaAs for low x or InP for x≈0.53. Nanowires are a promising alternative for the realization of epitaxial heterostructures with high lattice mismatch due to their unique geometry and high surface-to-volume ratio. In addition, the possibility of monolithic integration in Si-CMOS platforms adds to their technological significance. In this work, we have investigated the growth of free-standing GaAs/InxGa1-xAs core/shell nanowires on Si(111) substrates by molecular beam epitaxy and the accommodation of lattice mismatch therein. Specifically, we have concentrated on highly lattice mismatched heterostructures (x=0.20-0.80) and very thin cores (diameter < 25 nm). Self-catalyzed growth of very thin GaAs core nanowires with a sufficiently low number density (to avoid beam shadowing during the shell growth) was possible on native-oxide/Si(111) substrates, after an in situ treatment of the latter with Ga droplets. This resulted in zinc blende nanowires with their axis along the [111] crystallographic direction and six {1-10} sidewalls. Subsequently, conformal overgrowth of the InxGa1-xAs shell was obtained only under kinetically limited growth conditions that suppressed mismatch-induced bending phenomena. The strain in the core and the shell was studied systematically as a function of the shell composition and thickness. To that end, we used Raman scattering spectroscopy, transmission electron microscopy and (synchrotron/lab source) X-ray diffraction, and compared the results with theoretical predictions based on continuum elasticity and density functional theories. All findings point to the existence of anisotropic tensile strain in the core that increases (as quantified by Raman measurements) with increasing the shell thickness, whereas the corresponding compressive strain in the shell decreases to zero. Our work demonstrates the opportunity to grow not only relaxed InxGa1-xAs shells with high structural quality (as adopted from the GaAs core) in a wide, if not the whole, compositional range, but also highly strained (tensile) GaAs cores with (opto)electronic properties that remain to be explored.

Potential technological applications of periodic nanostructure arrays range from photovoltaics augmented by light trapping [1] to high-sensitivity biomolecule detection using plasmonic signal enhancement [2] and high-speed low-energy information technology based on magnonic crystals [3]. Industrial-scale fabrication of such devices requires nanopatterning processes which are cost-effective, scalable, and highly reproducible. These demands can be met by a versatile bottom-up approach based on ion irradiation of semiconductor surfaces and well-established thin film deposition techniques. Reverse epitaxy, i.e. the non-equilibrium self-assembly of vacancies and ad-atoms under ion irradiation, leads to nanoscale surface patterning with well-defined lateral periodicity on semiconductor substrates [4]. The GaAs(001) and InAs(001) surfaces exhibit highly regular faceting and therefore lend themselves to transferring this pattern regularity to other materials. The nanofaceted surfaces with a sawtooth profile can for instance be employed as substrates for molecular beam epitaxy under grazing incidence, producing periodic arrays of nanowires, periodically corrugated thin films, or combinations thereof by geometrical shading. They can also induce long-range chemical ordering in diblock copolymer thin films, which may then serve as highly ordered chemical templates for metal nanostructure growth in a variety of pattern morphologies [5]. Furthermore, separated semiconductor nanostructures can be fabricated by introducing an interlayer before ion irradiation. In this contribution, we outline the reverse epitaxy mechanism and present examples of how it can be employed in the fabrication of large-area nanostructure arrays. We hope to stimulate discussion of further applications by emphasizing the simplicity and versatility of this bottom-up approach. [1] H.A. Atwater and A. Polman, Nature Materials 9 (2010); [2] J. Vogt et al., Phys. Chem. Chem. Phys. 17 (2015); [3] D. Grundler, Nature Physics 11 (2015); [4] X. Ou et al., Nanoscale 7 (2015); [5] D. Erb et al., Science Advances 1 (2015)

Liquid Ga droplets have been widely used in molecular beam epitaxy to induce the nucleation of nanostructures as well as to remove surface oxides from the substrate prior to epitaxial growth. In this work, we have employed Ga droplets to etch nanoholes into the native surface oxide of Si(111) substrates for the subsequent growth of III-V nanostructures. To that end, we studied the nucleation kinetics of Ga droplets on SiOx and the interaction between Ga adatoms/droplets and SiOx. For given Ga flux and deposition time, we found that the number density of Ga droplets depends not only on the substrate temperature during deposition, but also on the annealing history of the substrate. This is attributed to the dependence of Ga adatom diffusivity on the surface morphology and/or the composition of the native oxide layer, both of which can be modified during the annealing. With an appropriate selection of substrate temperatures for the annealing and the Ga deposition steps, it was possible to vary deliberately the number density of Ga droplets within four orders of magnitude, i.e. 107-1010 cm-2. Interestingly, the droplet size and contact angle were found to be independent of the number density. This finding implies that low number densities of droplets are accompanied by an extensive loss of Ga atoms from the substrate surface. We discuss various explanations, such as desorption of Ga adatoms from the SiOx surface or reaction of Ga adatoms with SiOx and formation of volatile Ga2O. All Ga droplets induce local modifications in the SiOx surface that evolve into nanoholes during a subsequent thermal annealing (which also results in complete evaporation of the Ga droplets). Depending on their size (which is a function of annealing temperature and duration), nanoholes can accommodate the nucleation of nanostructures on the underlying Si surface. As an example, we demonstrate the epitaxial growth of GaAs nanowires in the self-catalyzed mode. The number density of nanowires was controlled precisely and reproducibly within the range of 107-109 cm-2, whereas their length distribution was exceptionally narrow.

Strain distribution in GaAs/InxGa1-xAs core/shell nanowires grown by molecular beam epitaxy on Si(111) substrates

We present nanoscopic studies on MBE-grown GaAs-InGaAs core-shell nanowires (NWs) with various Si-doped shells. For higher dopings and charge carrier densities, the plasmonic resonance shifts into the mid-infrared wavelength range, that can be fully probed using IR radiation from the FELBE free-electron laser source at the Helmholtz-Center Dresden-Rossendorf. Exploring these plasmonic resonance peaks at different wavelengths allows mapping the local charge carrier density and distribution along the NW with a high spatial resolution of better than 50 nm. Preliminary results using a CO2 laser scanned from 9.7-11.4 μm indicated the resonant behavior for the highest shell doping, in clear contrast to lower or undoped NWs. In the resonant case, the near-field (NF) emerging from the NW is strongly increased as compared to the substrate, in accordance with theory. The NF profiles of particular NWs show characteristic modifications at different wavelengths, which indicate an inhomogeneous distribution of the charge carrier density.

The core/shell nanowire (NW) geometry is suitable for the pseudomorphic growth of highly mismatched semiconductor heterostructures, where the shell thickness can exceed significantly the critical thickness in equivalent planar heterostructures. We have investigated the accommodation of misfit strain in self-catalyzed GaAs/(In,Ga)As core/shell NWs grown on Si (111) substrates by molecular beam epitaxy. The NWs have their axis along the [111] crystallographic direction, six {11 ̅0} sidewalls, and their crystal structure is predominantly zinc blende. For strain analysis, we used Raman scattering spectroscopy, transmission electron microscopy, X-ray diffraction and photoluminescence spectroscopy. Within a certain range of core/shell dimensions and shell composition, our findings reveal that the elastic energy in NWs without misfit dislocations can be confined exclusively inside the core, allowing for the shell to be strain-free. The experimental results are also compared with theoretical simulations of the strain (continuum elasticity theory) and phonon energy (density functional theory).