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Rieger T.,Julich Research Center | Rieger T.,Julich Aachen Research Alliance for Fundamentals of Future Information Technology JARA FIT | Heiderich S.,Julich Research Center | Heiderich S.,Julich Aachen Research Alliance for Fundamentals of Future Information Technology JARA FIT | And 7 more authors.
Journal of Crystal Growth | Year: 2012

We present detailed results about the molecular beam epitaxy (MBE) growth of GaAs nanowires (NWs) on GaAs (111)B substrates prepared for the growth by a new method using hydrogen silsesquioxane (HSQ). Before the growth, HSQ is converted to SiO x by thermal treatment. The NWs are grown via the vapor-liquid-solid (VLS) mechanism. The influence of five growth parameters are described: SiO x thickness, growth time, substrate temperature and Ga and As 4 beam fluxes. It is shown that the nanowire density can be tuned by two orders of magnitude by adjusting the SiO x thickness. Additionally, the results demonstrate that the axial growth is controlled by the As 4 beam flux whereas the lateral growth is controlled by the Ga beam flux. The observed NW tapering is mainly determined by the V/III beam flux ratio. Our study gives important information about the VLS growth mechanism, which is extended by considering the secondary adsorption process of Ga adatoms. The nanowires have predominantly zinc blende crystal structure with rotational twins. A wurtzite segment is always found at the top of the NWs being associated with the growth after the Ga shutter has been closed. © 2012 Elsevier B.V. All rights reserved. Source


Rieger T.,Julich Research Center | Rieger T.,Julich Aachen Research Alliance for Fundamentals of Future Information Technology JARA FIT | Luysberg M.,Julich Research Center | Schapers T.,Julich Research Center | And 6 more authors.
Nano Letters | Year: 2012

We present results about the growth of GaAs/InAs core-shell nanowires (NWs) using molecular beam epitaxy. The core is grown via the Ga droplet-assisted growth mechanism. For a homogeneous growth of the InAs shell, the As4 flux and substrate temperature are critical. The shell growth starts with InAs islands along the NW core, which increase in time and merge giving finally a continuous and smooth layer. At the top of the NWs, a small part of the core is free of InAs indicating a crystal phase selective growth. This allows a precise measurement of the shell thickness and the fabrication of InAs nanotubes by selective etching. The strain relaxation in the shell occurs mainly via the formation of misfit dislocations and saturates at ∼80%. Additionally, other types of defects are observed, namely stacking faults transferred from the core or formed in the shell, and threading dislocations. © 2012 American Chemical Society. Source


Davydok A.,University of Siegen | Rieger T.,PGi | Rieger T.,Julich Aachen Research Alliance for Fundamentals of Future Information Technology JARA FIT | Biermanns A.,University of Siegen | And 6 more authors.
Journal of Applied Crystallography | Year: 2013

Vertically aligned InAs nanowires (NWs) doped with Si were grown self-assisted by molecular beam epitaxy on GaAs[111]B substrates covered with a thin SiO x layer. Using out-of-plane X-ray diffraction, the influence of Si supply on the growth process and nanostructure formation was studied. It was found that the number of parasitic crystallites grown between the NWs increases with increasing Si flux. In addition, the formation of a Ga0.2In0.8As alloy was observed if the growth was performed on samples covered by a defective oxide layer. This alloy formation is observed within the crystallites and not within the nanowires. The Ga concentration is determined from the lattice mismatch of the crystallites relative to the InAs nanowires. No alloy formation is found for samples with faultless oxide layers. © 2013 International Union of Crystallography Printed in Singapore - all rights reserved. Source


Rieger T.,Julich Research Center | Rieger T.,Julich Aachen Research Alliance for Fundamentals of Future Information Technology JARA FIT | Lepsa M.I.,Julich Research Center | Lepsa M.I.,Julich Aachen Research Alliance for Fundamentals of Future Information Technology JARA FIT | And 4 more authors.
Journal of Crystal Growth | Year: 2013

The control of the Ga droplet during the MBE growth and its impact on the crystal structure of self-catalyzed GaAs nanowires (NWs) were investigated. The consumption of the droplet proceeds in two steps. First, the contact angle decreases to 901 keeping the NW diameter constant. The crystal structure changes from zinc blende (ZB) to wurtzite (WZ). Then, the contact angle keeps constant while the top radius of the NW decreases and the NW grows again in ZB configuration. During the last step, {110}, {211}B and {100} facets develop at the top. Calculations show that the Ga desorption from the droplet has to be taken into account during its consumption. With this information, several WZ segments of different lengths were placed into ZB GaAs NWs via partial droplet consumption. For this purpose, we supplied As and Ga separately, in order to partially consume and refill the Ga droplet. The same mechanism was applied to self-catalyzed InAs NWs resulting in short WZ segments inserted in a ZB twinning superlattice. © 2012 Elsevier B.V. All rights reserved. Source


Rieger T.,Julich Research Center | Rieger T.,Julich Aachen Research Alliance for Fundamentals of Future Information Technology JARA FIT | Schapers T.,Julich Research Center | Schapers T.,Julich Aachen Research Alliance for Fundamentals of Future Information Technology JARA FIT | And 4 more authors.
Crystal Growth and Design | Year: 2014

We present a novel type of core-shell nanowires in which only certain parts of the core are covered by the shell. This is achieved by the crystal phase selective growth of the InAs shell on zinc blende GaAs nanowires with controlled wurtzite inclusions. The shell grows on the zinc blende phase, but its growth is hindered on the wurtzite crystal phase. Nucleation of InAs occurs exclusively on the zinc blende GaAs regions. The wurtzite segments are placed inside self-catalyzed GaAs nanowires by partially consuming and refilling the Ga droplet. The crystal phase selective growth of InAs on the side facets of the GaAs nanowires is explained by the local environment of each new In atom. Because of unbalanced neighbors on the wurtzite side facets, the growth of a highly lattice mismatched material is hindered. This happens not only on the wurtzite segments, but also on regions being characterized by a high density of twins. © 2014 American Chemical Society. Source

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