Transition from elastic to plastic strain release in core−shell nanowires revealed by in-plane x-ray diffraction

We investigate the strain evolution and relaxation process as function of increasing lattice mismatch between the GaAs core and surrounding InxGa$_{1-x}$As shell in core–shell nanowire heterostructures grown on Si(111) substrates. The dimensions of the core and shell are kept constant whereas the indium concentration inside the shell is varied. Measuring the $22\bar{4}$ and $2\bar{2}0$ in-plane Bragg reflections normal to the nanowire side edges and side facets, we observe a transition from elastic to plastic strain release for a shell indium content x > 0.5. Above the onset of plastic strain relaxation, indium rich mounds and an indium poor coherent shell grow simultaneously around the GaAs core. Mound formation was observed for indium contents x = 0.5 and 0.6 by scanning electron microscopy. Considering both the measured radial reflections and the axial 111 Bragg reflection, the 3D strain variation was extracted separately for the core and the InxGa$_{1-x}$As shell

Impact of Electrical Current on Single GaAs Nanowire Structure

The impact of electrical current on the structure of single free-standing Be-doped GaAs nanowires grown on a Si 111 substrate is investigated. Single nanowires have been structurally analyzed by X-ray nanodiffraction using synchrotron radiation before and after the application of an electrical current. The conductivity measurements on single nanowires in their as-grown geometry have been realized via W-probes installed inside a dual-beam focused ion beam/scanning electron microscopy chamber. Comparing reciprocal space maps of the 111 Bragg reflection, extracted perpendicular to the nanowire growth axis before and after the conductivity measurement, the structural impact of the electrical current is evidenced, including deformation of the hexagonal nanowire cross section, tilting, and bending with respect to the substrate normal. For electrical current densities below 30 A mm−2, the induced changes in the reciprocal space maps are negligible. However, for a current density of 347 A mm−2, the diffraction pattern is completely distorted. The mean cross section of the illuminated nanowire volume is reconstructed from the reciprocal space maps before and after the application of electrical current. Interestingly, the elongation of two pairs of opposing side facets accompanied by shrinkage of the third pair of facets is found. The variations in the nanowire diameter, as well as their tilt and bending, are confirmed by scanning electron microscopy. To explain these findings, material melting due to Joule heating during voltage/current application accompanied by anisotropic deformations induced by the W-probe is suggested

Beam damage of single semiconductor nanowires during X-ray nano beam diffraction experiments

Nanoprobe X-ray diffraction (nXRD) using focused synchrotron radiation is a powerful technique to study the structural properties of individual semiconductor nanowires. However, when performing the experiment under ambient conditions, the required high X-ray dose and prolonged exposure times can lead to radiation damage. To unveil the origin of radiation damage, we compare nXRD experiments carried out on individual semiconductor nanowires in their as grown geometry both under ambient conditions and under He atmosphere at the microfocus station of the P08 beamline at the 3rd generation source PETRA III. Using an incident X-ray beam energy of 9 keV and photon flux of 10$^{10}$s$^{-1}$, the axial lattice parameter and tilt of individual GaAs/In$_{0.2}$Ga$_{0.8}$As/GaAs core-shell nanowires were monitored by continuously recording reciprocal space maps of the 111 Bragg reflection at a fixed spatial position over several hours. In addition, the emission properties of the (In,Ga)As quantum well, the atomic composition of the exposed nanowires and the nanowire morphology are studied by cathodoluminescence spectroscopy, energy dispersive X-ray spectroscopy and scanning electron microscopy, respectively, both prior to and after nXRD exposure. Nanowires exposed under ambient conditions show severe optical and morphological damage, which was reduced for nanowires exposed under He atmosphere. The observed damage can be largely attributed to an oxidation process from X-ray induced ozone reactions in air. Due to the lower heat transfer coefficient compared to GaAs, this oxide shell limits the heat transfer through the nanowire side facets, which is considered as the main channel of heat dissipation for nanowires in the as-grown geometry

Nanomechanik: Mechanische Response-Analyse von Halbleiter-GaAs-Nanodrähten mit der Finite-Elemente-Methode und Röntgendiffraktionstechnik

In den letzten zwei Jahrzehnten wurden durch die Integration von quasi eindimensionalen Nanodrähten enorme Fortschritte bei der Miniaturisierung von opto-elektronischen Bauelementen und sensorbasierten nano-elektromechanischen Systemen erzielt. Für die Entwicklung von Nanodraht basierten Bauelementen der nächsten Generation ist es unerlässlich, die zugrundeliegenden Deformationsmechanismen und deren Mechanik zu untersuchen. Die vorliegende Arbeit demonstriert die Analyse des mechanischen Verhaltens von Halbleiter Nanodrähten aus Galliumarsenid (GaAs), die mittels Molekularstrahlepitaxie auf Siliziumsubstrat gewachsen sind. Das mechanische Verhalten der Nanodrähte wurde durch in-situ Biegeversuche im Rasterelektronenmikroskop und in Kombination mit Röntgenbeugung untersucht. Infolge der rassanten Entwicklung von Röntgenfokussieroptiken stehen inzwischen an verschiedenen Synchrotronanlagen nanometergroße Röntgenstrahlen zur Verfügung, die es ermöglichen, einen einzelnen Nanodraht zu untersuchen. Das erste Ziel dieses Manuskripts war es, die Auswirkung einer systematischen dynamischen Belastung, d.h. elektromechanisch induzierter Vibrationen, auf einzelne Nanodrähte mit Hilfe der Bragg Beugungsabbildung zu untersuchen. Die parametrische Studie wurde entweder durch die Variation der Amplitude der Resonanz oder der Verweilzeit durchgeführt. Das ex-situ-Experiment, das die kristalline Struktur und die Phasen dieser vibrierenden NWs untersuchen sollte, war für März 2020 geplant, aber aufgrund der Ausbreitung der COVID-19-Pandemie wurde das Experiment von der Synchrotronanlage abgesagt. Das zweite Ziel dieser Arbeit war es, die anelastische Dehnungsrelaxation der Nanodrähte zu identifizieren, die als direkte Folge von Cantilever-Biegeversuchen und Knickversuchen an freistehenden Be-dotierten GaAs-Nanodrähten beobachtet wurde. Die anelastische Dehnung wurde mit Hilfe eines digitalen Bildkorrelationsalgorithmus abgeleitet. Die Ergebnisse wurden mit FEM-Simulationen unter Berücksichtigung eines Systems von hochgradig gekoppelten nichtlinearen partiellen Differentialgleichungen verglichen. Die Übereinstimmung zwischen FEM-Simulationen und gemessenen Daten bringen die anelastische Relaxation schlüssig mit dem Gorsky-Effekt in Nanodrähten in Verbindung. Be-dotierte GaAs-Nanodrähte wurden weiter in der lateralen Dreipunkt-Biegekonfiguration mit Hilfe des Rasterkraftmikroskops für in-situ nanofokussierte Röntgenbeugung an der Beamline P23 untersucht. Die Biegung der Nanodrähte wurde durch die laterale Bewegung des Rasterkraftmikroskops induziert und in-situ mittels nanofokussierte Röntgenbeugung detektiert. Die Nanodrähte zeigten elastische Deformation, plastische Deformation und eine zeitabhängige anelastische Relaxation. Die anelastische Relaxation hat einen Diffusionskoeffizienten von 2.71 x 10^{-13} cm^{2}/s , was mit dem FEM Modell übereinstimmt und auf das Vorhandensein eines durch den Gorsky-Effekt angetriebenen Mechanismus hinweist.During the last two decades, tremendous advances have been made in the miniaturization of opto-electronic devices and sensor-based nano-electromechanical systems by the integration of quasi one-dimensional nanowires. For the development of future generation nanowire-based devices, it is essential to investigate the underlying deformation mechanisms and their mechanics The present work focuses on the mechanical response analysis of semiconductor gallium arsenide (GaAs) nanowires grown on silicon substrate via molecular beam epitaxy. The mechanical behavior of the nanowires is characterized via in-situ bending tests in a scanning electron microscope and in combination with x-ray diffraction. With the major development of x-ray focusing optics, sub-100 nm sized beams are readily available at synchrotron facilities enabling the study of single nanowires. The first aim of this work is to investigate the impact of systematic dynamic loading, i.e., electromechanically induced vibrations on single nanowires by using Bragg diffraction imaging. A parametric study is carried out either by varying the amplitude of the vibration or the dwell time. The ex-situ experiment, planned to examine the crystalline structure of these vibrated NWs, was planned for March 2020, but due to the COVID-19 pandemic spread, the experiment has been shifted to a later time by the synchrotron facility. The second aim of this work is to identify the anelastic strain relaxation of the nanowires which was observed as a direct consequence of cantilever bending tests and buckling tests on free standing Be-doped GaAs nanowires. The anelastic strain is derived by using a digital image correlation algorithm. The results are compared with FEM simulations used to solve a system of highly coupled nonlinear partial differential equations (elasticity and diffusion). The agreement between FEM simulations and measured data conclusively relates the anelastic relaxation in the investigated nanowires to the Gorsky effect, i.e. the coupling between point defects diffusion and stress gradient. Be doped GaAs nanowires are further examined in the lateral three-point bending configuration by employing the Scanning Force Microscope for in situ Nanofocused X-ray diffraction (SFINX) and x-ray diffraction at beamline P23 at PETRA III. The bending of the nanowires was induced by the lateral movement of the tip of SFINX . The nanowires demonstrate elastic deformation, plastic deformation, and time-dependent anelastic relaxation. The anelastic relaxation yields a diffusion coefficient of 2.71 x 10^{-13} cm^{2}/s and is consistent with a Gorsky effect

Nanomécanique : analyse de la réponse mécanique de nanofils de GaAs semi-conducteurs par la méthode des éléments finis et les techniques de diffraction des rayons X

Au cours des deux dernières décennies, d’énormes progrès ont été réalisés dans la miniaturisation des dispositifs optoélectroniques et des systèmes nanoélectromécaniques à base de capteurs grâce à l'intégration de nanofils quasi unidimensionnels. Le présent travail porte sur l'analyse de la réponse mécanique de nanofils GaAs semi-conducteurs préparés sur un substrat de silicium par épitaxie par jet moléculaire. Le comportement mécanique du nanofil est caractérisé par des essais de flexion in situ dans un MEB et en combinaison avec la diffraction des rayons X. L'objectif de ce travail est d'identifier le mécanisme responsable de la relaxation anélastique qui a été observée après des essais de flexion ou flexion ou de flambage effectués sur des nanofils de GaAs dopés avec du Be. La déformation anélastique est quantifiée en utilisant un algorithme de corrélation d'images numériques. L'accord entre les simulations FEM et les données mesurées montre que la relaxation anélastique peut être attribuée à l’ effet Gorsky dans les nanofils, c’est à dire au couplage entre la diffusion des défauts ponctuels et le gradient de contrainte. Les nanofils de GaAs dopé avec du Be ont été sollicités in situ en flexion latérale trois points en utilisant SFINX et la XRD sur la ligne de lumière P23 à PETRA III. La flexion a été induite dans les nanofils par le mouvement latéral de la pointe du SFINX. Les nanofils présentent une déformation élastique, une déformation plastique et une relaxation anélastique dépendant du temps. La relaxation anélastique donne lieu à un coefficient de diffusion de 2.71 x 10 puissance -13 cm puissance 2 et est en accord avec un effet GorskyDuring the last two decades, tremendous advances have been made in the miniaturization of opto-electronic devices and sensor-based nano-electromechanical systems by the integration of quasi one-dimensional nanowires. The present work focuses on the mechanical response analysis of semiconductor gallium arsenide (GaAs) nanowires grown on silicon substrate via molecular beam epitaxy. The mechanical behavior of the nanowires is characterized via in-situ bending tests in a scanning electron microscope and in combination with x-ray diffraction. The aim of this work is to identify the anelastic strain relaxation of the nanowires which was observed as a direct consequence of cantilever bending tests and buckling tests on free standing Be-doped GaAs nanowires. The anelastic strain is derived by using a digital image correlation algorithm. The agreement between FEM simulations and measured data conclusively relate the anelastic relaxation in the investigated nanowires to the Gorsky effect, i.e. the coupling between point defects diffusion and stress gradient. Be doped GaAs nanowires are further examined in the lateral three-point bending configuration by employing the Scanning Force Microscope for in situ Nanofocused X-ray diffraction (SFINX) and x-ray diffraction at beamline P23 at PETRA III. The bending of the nanowires was induced by the lateral movement of the tip of SFINX . The nanowires demonstrate elastic deformation, plastic deformation, and time-dependent anelastic relaxation. The anelastic relaxation yields a diffusion coefficient of 2.71 x 10 puissance -13 cm puissance 2 and is consistent with a Gorsky effec

Impact of Electrical Current on Single GaAs Nanowire Structure

The impact of electrical current on the structure of single free-standing Be-doped GaAs nanowires grown on a Si 111 substrate is investigated. Single nanowires have been structurally analyzed by X-ray nanodiffraction using synchrotron radiation before and after the application of an electrical current. The conductivity measurements on single nanowires in their as-grown geometry have been realized via W-probes installed inside a dual-beam focused ion beam/scanning electron microscopy chamber. Comparing reciprocal space maps of the 111 Bragg reflection, extracted perpendicular to the nanowire growth axis before and after the conductivity measurement, the structural impact of the electrical current is evidenced, including deformation of the hexagonal nanowire cross section, tilting, and bending with respect to the substrate normal. For electrical current densities below 30 A mm−2, the induced changes in the reciprocal space maps are negligible. However, for a current density of 347 A mm−2, the diffraction pattern is completely distorted. The mean cross section of the illuminated nanowire volume is reconstructed from the reciprocal space maps before and after the application of electrical current. Interestingly, the elongation of two pairs of opposing side facets accompanied by shrinkage of the third pair of facets is found. The variations in the nanowire diameter, as well as their tilt and bending, are confirmed by scanning electron microscopy. To explain these findings, material melting due to Joule heating during voltage/current application accompanied by anisotropic deformations induced by the W-probe is suggested

Impact of Electrical Current on Single GaAs Nanowire Structure

The impact of electrical current on the structure of single free-standing Be-doped GaAs nanowires grown on a Si 111 substrate is investigated. Single nanowires have been structurally analyzed by X-ray nanodiffraction using synchrotron radiation before and after the application of an electrical current. The conductivity measurements on single nanowires in their as-grown geometry have been realized via W-probes installed inside a dual-beam focused ion beam/scanning electron microscopy chamber. Comparing reciprocal space maps of the 111 Bragg reflection, extracted perpendicular to the nanowire growth axis before and after the conductivity measurement, the structural impact of the electrical current is evidenced, including deformation of the hexagonal nanowire cross section, tilting, and bending with respect to the substrate normal. For electrical current densities below 30 A mm−2, the induced changes in the reciprocal space maps are negligible. However, for a current density of 347 A mm−2, the diffraction pattern is completely distorted. The mean cross section of the illuminated nanowire volume is reconstructed from the reciprocal space maps before and after the application of electrical current. Interestingly, the elongation of two pairs of opposing side facets accompanied by shrinkage of the third pair of facets is found. The variations in the nanowire diameter, as well as their tilt and bending, are confirmed by scanning electron microscopy. To explain these findings, material melting due to Joule heating during voltage/current application accompanied by anisotropic deformations induced by the W-probe is suggested

Beam damage of single semiconductor nanowires during X-ray nanobeam diffraction experiments

Nanoprobe X-ray diffraction (nXRD) using focused synchrotron radiation is a powerful technique to study the structural properties of individual semiconductor nanowires. However, when performing the experiment under ambient conditions, the required high X-ray dose and prolonged exposure times can lead to radiation damage. To unveil the origin of radiation damage, a comparison is made of nXRD experiments carried out on individual semiconductor nanowires in their as-grown geometry both under ambient conditions and under He atmosphere at the microfocus station of the P08 beamline at the third-generation source PETRA III. Using an incident X-ray beam energy of 9 keV and photon flux of 1010 s-1, the axial lattice parameter and tilt of individual GaAs/In0.2Ga0.8As/GaAs core-shell nanowires were monitored by continuously recording reciprocal-space maps of the 111 Bragg reflection at a fixed spatial position over several hours. In addition, the emission properties of the (In,Ga)As quantum well, the atomic composition of the exposed nanowires and the nanowire morphology were studied by cathodoluminescence spectroscopy, energy-dispersive X-ray spectroscopy and scanning electron microscopy, respectively, both prior to and after nXRD exposure. Nanowires exposed under ambient conditions show severe optical and morphological damage, which was reduced for nanowires exposed under He atmosphere. The observed damage can be largely attributed to an oxidation process from X-ray-induced ozone reactions in air. Due to the lower heat-transfer coefficient compared with GaAs, this oxide shell limits the heat transfer through the nanowire side facets, which is considered as the main channel of heat dissipation for nanowires in the as-grown geometry

In-situ X-ray analysis of misfit strain and curvature of bent polytypic GaAs-Inx_{x}Ga1−x_{1-x}As core-shell nanowires

Misfit strain in core–shell nanowires can be elastically released by nanowire bending in case of asymmetric shell growth around the nanowire core. In this work, we investigate the bending of GaAs nanowires during the asymmetric overgrowth by an InxGa1−xAs shell caused by avoiding substrate rotation. We observe that the nanowire bending direction depends on the nature of the substrate\u27s oxide layer, demonstrated by Si substrates covered by native and thermal oxide layers. Further, we follow the bending evolution by time-resolved in situ x-ray diffraction measurements during the deposition of the asymmetric shell. The XRD measurements give insight into the temporal development of the strain as well as the bending evolution in the core–shell nanowire

X-ray diffraction reveals the amount of strain and homogeneity of extremely bent single nanowires

Core–shell nanowires (NWs) with asymmetric shells allow for strain engineering of NW properties because of the bending resulting from the lattice mismatch between core and shell material. The bending of NWs can be readily observed by electron microscopy. Using X-ray diffraction analysis with a micro- and nano-focused beam, the bending radii found by the microscopic investigations are confirmed and the strain in the NW core is analyzed. For that purpose, a kinematical diffraction theory for highly bent crystals is developed. The homogeneity of the bending and strain is studied along the growth axis of the NWs, and it is found that the lower parts, i.e. close to the substrate/wire interface, are bent less than the parts further up. Extreme bending radii down to ∼3 µm resulting in strain variation of ∼2.5% in the NW core are found