Correlating in situ RHEED and XRD to study growth dynamics of polytypism in nanowires

Design of novel nanowire (NW) based semiconductor devices requires deep understanding and technological control of NW growth. Therefore, quantitative feedback over the structure evolution of the NW ensemble during growth is highly desirable. We analyse and compare the methodical potential of reflection high-energy electron diffraction (RHEED) and X-ray diffraction reciprocal space imaging (XRD) for in situ growth characterization during molecular-beam epitaxy (MBE). Simultaneously recorded in situ RHEED and in situ XRD intensities show strongly differing temporal behaviour and provide evidence of the highly complementary information value of both diffraction techniques. Exploiting the complementarity by a correlative data analysis presently offers the most comprehensive experimental access to the growth dynamics of statistical NW ensembles under standard MBE growth conditions. In particular, the combination of RHEED and XRD allows for translating quantitatively the time-resolved information into a height-resolved information on the crystalline structure without a priori assumptions on the growth model. Furthermore, we demonstrate, how careful analysis of in situ RHEED if supported by ex situ XRD and scanning electron microscopy (SEM), all usually available at conventional MBE laboratories, can also provide highly quantitative feedback on polytypism during growth allowing validation of current vapour–liquid–solid (VLS) growth models

Exploiting flux shadowing for strain and bending engineering in core - shell nanowires

Here we report on the non-uniform shell growth of In$_x$Ga$_{1−x}$As on the GaAs nanowire (NW) core by molecular beam epitaxy (MBE). The growth was realized on pre-patterned silicon substrates with the pitch size (p) ranging from 0.1 μm to 10 μm. Considering the preferable bending direction with respect to the MBE cells as well as the layout of the substrate pattern, we were able to modify the strain distribution along the NW growth axis and the subsequent bending profile. For NW arrays with a high number density, the obtained bending profile of the NWs is composed of straight (barely-strained) and bent (strained) segments with different lengths which depend on the pitch size. A precise control of the bent and straight NW segment length provides a method to design NW based devices with length selective strain distribution

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

In-situ-Untersuchung des Flusses Abschattungseffekts auf Polytypismus und Dehnungsentwicklung in selbst-katalysierten Kern und Kern-Schale Nanodraht-Systemen

Halbleiter-Nanodrähte mit quasi-eindimensionaler Geometrie haben in den letzten Jahrzehnten große Aufmerksamkeit erlangt. Die einzigartige Geometrie dieser Objekte trägt zu ihren unverwechselbaren optischen und elektrischen Eigenschaften mit vielversprechenden Möglichkeiten für neuartige Bauelemente bei. Die Bandlücke der Nanodrähte hängt von ihren Kristalleigenschaften wie den Kristallphasen und der Verspannung ab, die wiederum in hohem Maße modifizierbar sind. Daher ist die Herstellung und das Studium von Nanodrahtkristallen wesentlich für die Modifizierung und Kontrolle ihrer Eigenschaften. In dieser Arbeit wird eine Molekularstrahlepitaxie-Kammer zur Herstellung von GaAs-Nanodrähten verwendet und die Veränderungen der Nanodraht-Kristallstruktur während des Wachstums als Funktion des Abstands zwischen den benachbarten Nanodrähten untersucht. Die Untersuchung der Nanodrähte während des Wachstums in Arrays mit unterschiedlichen Drahtdichten mittels zeitaufgelöste Röntgenbeugung zeigt eine starke Abhängigkeit der Kristallstruktur vom Nanodrahtabstand. Diese Abhängigkeit wird zum Teil auf den Abschattungseffekt der Materialflüsse zurückgeführt, auf den sich der erste Teil dieser Studie konzentriert. Weiterhin wird die Entwicklung der Gitterverspannung in den Nanodrähten während des asymmetrischen Wachstums einer gitterfehlangepassten InxGa1−xAs-Schale untersucht. Das asymmetrische Wachstum der Schale auf den Nanodrahtfacetten führt zu einer asymmetrischen Variation der Gitterverspannung über den Nanodrahtquerschnitt, die eine Biegung des Nanodrahtes hervorruft. Die über den Nanodrahtquerschnitt variierende Gitterverspannung kann für die Modifikation der Bandlücke sowie der optischen Eigenschaften des Nanodrahtes genutzt werden. Aus diesem Grund wird eine detaillierte Untersuchung der Gitterverspannung und der Nanodrahtbiegung durchgeführt. Die Entwicklung der Gitterverspannung und der Nanodrahtbiegung während des Schalenwachstums wird mit Hilfe von zeitaufgelöste in-situ Röntgenbeugung an einem einzelnen Nanodraht sowie an einem Nanodraht-Ensemble beobachtet. Diese Untersuchung offenbart eine nicht-lineare Abhängigkeit der Gitterverspannung und der Nanodrahtbiegung von der Dauer des Schalenwachstums, was auf Veränderungen derWachstumsdynamik hinweist. Zuletzt wird der Abschattungseffekt der Materialflüsse durch die benachbarten Nanodrähte genutzt um die Verteilung des Schalenmaterials entlang der Wachstumsachse der Nanodrähte zu steuern, was zu einem variierenden Verspannungsfeld entlang derWachstumsachse der Nanodrähte führt. Diese Methode kann für die Modifikation von Verspannungsgradienten und für Bauteile auf Nanodraht-Basis mit neuartigen Geometrien eingesetzt werden.Semiconductor nanowires with a quasi-one-dimensional geometry have gained great attention during the past decades. The unique geometry of these objects contributes to their distinctive optical and electrical properties that are promising for novel devices. The configuration of the band gap of the nanowires depends on their crystal properties such as the crystal phase and the strain which in turn are highly controllable. Therefore, the realization and the study of the nanowire crystal are essential for tuning and controlling their properties. In this work we use a molecular beam epitaxy chamber for fabricating GaAs nanowires and we investigate the changes of the nanowire crystal structure during growth as a function of the interspacing between the neighboring nanowires. By means of time-resolved X-ray diffraction technique, monitoring the nanowires during growth at arrays with different densities shows a high dependency of the crystal structure on the nanowire interspacing. This dependency is partially attributed to the shadowing effect of the growth material fluxes which we focus on in the first part of this study. Further, we investigate the strain evolution in the nanowires during an asymmetric growth of a lattice- mismatched InxGa1−xAs shell. The asymmetric growth of the shell materials on the nanowire facets results in an asymmetric strain variation across the nanowire cross section, which induces nanowire bending. The varying strain across the nanowire cross-section can be utilized for engineering the band gap and the optical properties of the nanowire. Therefore, we perform a detailed study of the strain and nanowire bending. The evolution of the strain and nanowire bending during shell growth is observed by means of time-resolved in-situ X-ray diffraction technique on a single nanowire as well as on a nanowire ensemble. This investigation revealed a non-linear dependency of the strain and nanowire bending to the shell growth time, indicating changes of the growth dynamics. Lastly, we exploit the shadowing effect of material fluxes by the neighboring nanowires to control the distribution of the shell material along the nanowire growth axis, which results in a varying strain field along the nanowire growth axis. This method can be employed for strain gradient engineering and nanowire-based devices with novel geometries

Correlating in situ RHEED and XRD to study growth dynamics of polytypism in nanowires

Design of novel nanowire (NW) based semiconductor devices requires deep understanding and technological control of NW growth. Therefore, quantitative feedback over the structure evolution of the NW ensemble during growth is highly desirable. We analyse and compare the methodical potential of reflection high-energy electron diffraction (RHEED) and X-ray diffraction reciprocal space imaging (XRD) for in situ growth characterization during molecular-beam epitaxy (MBE). Simultaneously recorded in situ RHEED and in situ XRD intensities show strongly differing temporal behaviour and provide evidence of the highly complementary information value of both diffraction techniques. Exploiting the complementarity by a correlative data analysis presently offers the most comprehensive experimental access to the growth dynamics of statistical NW ensembles under standard MBE growth conditions. In particular, the combination of RHEED and XRD allows for translating quantitatively the time-resolved information into a height-resolved information on the crystalline structure without a priori assumptions on the growth model. Furthermore, we demonstrate, how careful analysis of in situ RHEED if supported by ex situ XRD and scanning electron microscopy (SEM), all usually available at conventional MBE laboratories, can also provide highly quantitative feedback on polytypism during growth allowing validation of current vapour–liquid–solid (VLS) growth models

Impact of the Shadowing Effect on the Crystal Structure of Patterned Self-Catalyzed GaAs Nanowires

The growth of regular arrays of uniform III–V semiconductor nanowires is a crucial step on the route toward their application-relevant large-scale integration onto the Si platform. To this end, not only does optimal vertical yield, length, and diameter uniformity have to be engineered, but also, control over the nanowire crystal structure has to be achieved. Depending on the particular application, nanowire arrays with varying area density are required for optimal device efficiency. However, the nanowire area density substantially influences the nanowire growth and presents an additional challenge for nanowire device engineering. We report on the simultaneous in situ X-ray investigation of regular GaAs nanowire arrays with different area density during self-catalyzed vapor–liquid–solid growth on Si by molecular-beam epitaxy. Our results give novel insight into selective-area growth and demonstrate that shadowing of the Ga flux, occurring in dense nanowire arrays, has a crucial impact on the evolution of nanowire crystal structure. We observe that the onset of Ga flux shadowing, dependent on array pitch and nanowire length, is accompanied by an increase of the wurtzite formation rate. Our results moreover reveal the paramount role of the secondary reflected Ga flux for VLS NW growth (specifically, that flux that is reflected directly into the liquid Ga droplet)

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_xGa1−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 In$_x$Ga$_{1−x}$As shell caused by avoiding substrate rotation. We observe that the nanowire bending direction depends on the nature of the substrate's 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

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