5 research outputs found
Structure and electrical response of GaAs nanowires : looking for a correlation at the nano-scale
For applications of nanowires in optoelectronic devices their controlled positioning onto substrates during growth is often required, as well as the control of their polytype mixture and strain state. Therefore a deep comprehension of the growth and preparation mechanisms is essential. Multiple investigation techniques at the nano-scale have been developed nowadays with the goal to correlate the structure and the physical response of nanostructures. The first aim of this work is the structural investigation of individual nanowires grown on GaAs substrates by molecular beam epitaxy. The specific positioning of the nanowires during growth has been achieved by Au-implantation into the substrate. X-ray nano-diffraction in grazing incidence diffraction geometry was used to investigate the in-plane strain at different positions, along the growth axis of individual nanowires. It was found that the nanowires are twisted about their growth axis with respect to the substrate. As expected, the crystal lattice of the nanowires is totally relaxed. Under surface diffraction conditions, the upper most layers of the growth substrate could be investigated at the same time. Here, regions at different distances from the Au-implanted area revealed the presence of a compressive strain, decreasing further away from the nanowire base. In prior studies of semiconductor nanowires it has been shown that the value of specific physical parameters strongly differ frombulk materials. For example, the high aspect ratio and the mixture of polytypes can significantly influence electrical and optical properties of GaAs nanowires. Moreover, nanowires standing in upright position onto their growth substrate revealed their importance in modern devices. The second aim of this thesis is to correlate the electrical and structural parameters of individual as-grown nanowires onto their growth substrate. This was achieved by measuring the electrical characteristics of single nanowires in a focused ion beam chamber; and by determining their polytype composition using coplanar X-ray nano-diffraction. The nanowires measured showed differing electrical characteristics. These differences have been successfully correlated with the number of zinc-blende and twinned zinc-blende units detected within single nanowires. The combination of the described techniques represents the main challenge of this work. Besides the difficulties of identifying individual nanowires in different experimental configurations; all electrical characteristics, linked to the contacting procedure of the nanowire, had to be understood and controlled. Electronic and optoelectronic applications often require specific sample preparations. Those may include the embedding of the nanostructures in a polymer matrix and the application of a top contact. As a third aim, symmetric and asymmetric X-ray diffraction revealed indirectly the presence of a preparation-induced average initial strain in the Benzocyclobutene polymer, by measuring an ensemble of embedded GaAs nanowires. This resulted in uniaxial strained embedded nanostructures, and was found to originate from the thermal processing of the polymer. The detected compression was shown to decrease under X-ray illumination and application of an external static electric field, which lead to the formation and reorientation of polar sub-molecules in the polymer.Um NanodrĂ€hte gezielt fĂŒr optoelektronische Schaltungen anzuwenden, muss bereits beim Wachstum auf deren genaue Positionierung auf dem Substrat, auf die Polytype Verteilung im Nanodraht und auf dessen Verzerrungszustand geachtet werden. Deshalb ist ein tieferes VerstĂ€ndis der Wachstums und PrĂ€parationsmechanismen erforderlich. Der Einsatz einer Reihe von Untersuchungstechniken auf der Nanosskala wurden entwickelt, um physikalische Eigenschaften der Nanoobjekte mit deren Struktur zu korrelieren. Das erste Ziel der vorliegenden Arbeit liegt daher in der strukturellen Untersuchung einzelner, mittels Molekular Strahl Epitaxie auf GaAs Substraten gewachsener, NanodrĂ€hte. In diesem Fall wurde die Positionierung der DrĂ€hte ĂŒber eine rĂ€umlich selektive Implantation von Gold Atomen im Substrat erreicht. Röntgenbeugung unter streifendem Einfall mittels eines Nanostrahls wurde eingesetzt, um den Verzerrungszustand in der Substratebene an verschiedenen Positionen auf dem Substrat und entlang des Drahtes zu bestimmen. Dabei wurde festgestellt, dass die NanodrĂ€hte entlang der Wachstumsrichtung leicht verdreht gegenĂŒber dem Substrat wachsen. Wie erwartet ist dabei das Kristallgitter der NanodrĂ€hte verzerrungsfrei. Infolge der extremen OberflĂ€chensensitivitĂ€t konnten gleichzeitig die obersten Gitterebenen des Substrats untersucht werden. Hierbei zeigte sich eine mit gröĂer werdendem Abstand zum Au implantierten Gebiet kleiner werdende kompressible Deformation. Aus frĂŒheren Untersuchungen an Halbleiter NanodrĂ€hten ist bekannt, dass die GröĂe vieler physikalischer Eigenschaften von der des Volumenmaterials abweicht. Zum Beispiel beeinfluĂt das groĂe SeitenverhĂ€ltnis und die strukturelleMischung verschiedener Polytypen signifikant die elektrischen und optischen Eigenschaften von GaAs NanodrĂ€hten.Weiterhin spielt die Tatsache, dass die DrĂ€hte in aufrechter Position auf dem Substrate verbleiben können, eine groĂe Rolle fĂŒr deren Anwendung in modernen Bauelementen. Das zweite Ziel dieser Arbeit besteht in der Korrelation von elektrischen und strukturellen Eigenschaften einzelner NanodrĂ€hte in ihrer Position auf dem Substrat. Diese Korrelation wurde erreicht, indem die elektrische Charakteristik ausgewĂ€hlter NanodrĂ€hte in einer Fokussierten Ionenstrahl Kammer gemessen und die Polytypzusammensetzung der gleichen DrĂ€hte mittels Röntgennanodiffraktion bestimmt wurde. Die gemessenen NanodrĂ€hte zeigen eine unterschiedliche elektrische Charakteristik. Diese Unterschiede konnten erfolgreich mit der unterschiedlichen Zahl von Zinkblende und verzwillingter Zinkblende Struktureinheiten innerhalb der NanodrĂ€hte korreliert werden. Die Kombination dieser beiden Techniken stellte die gröĂte Herausforderung dieser Arbeit dar. Neben der Schwierigkeit der Identifizierung individueller NanodrĂ€hte in unterschiedlichen experimentellen Konfigurationen, muĂten die EinfluĂparameter der elektrischen Charakteristik, einschieĂlich der Kontaktierungsprobleme, verstanden und kontrolliert werden. Elektronische und optoelektronische Anwednungen fordern sehr oft eine spezielle ProbenprĂ€paration. Das schlieĂt die Einbettung von Nanostrukturen in eine Polymermatrix und die Kontaktierung derselben mit ein. Als drittes Ziel dieser Arbeit wurde mit Hilfe der symmetrischen und asymmetrischen Röntgenbeugung nachgewiesen, dass Benzocyclobutene im Resultat des Einbettungsprozesses gegenĂŒber demeingebetteten GaAs NanodrĂ€hten elastisch verzerrt ist. Das Auftreten dieser einachsig deformierten eingebetten Nanostruktur wurde auf den thermischen Prozessschritt bei der Einbettung zurĂŒckgefĂŒhrt. Es zeigte sich, dass die festgestellte Kompression unter Bestrahlung mit Röntgenlicht und bei gleichzeitig angelegtem elektrischen Feld zurĂŒckging. Dieser Effekt konnte mit der Formierung und Reoriertierung von polaren Sub-MolekĂŒlen im Polymer erklĂ€rt werden
Grazing-incidence X-ray diffraction of single GaAs nanowires at locations defined by focused ion beams
Complete structural and strain analysis of single GaAs/(In,Ga)As/GaAs coreâshellâshell nanowires by means of in-plane and out-of-plane X-ray nanodiffraction
Typically, coreâshellâshell semiconductor nanowires (NWs) made from IIIâV materials with low lattice mismatch grow pseudomorphically along the growth axis, i.e. the axial lattice parameters of the core and shell materials are the same. Therefore, both the structural composition and interface strain of the NWs are encoded along directions perpendicular to the growth axis. Owing to fluctuations in the supplied growth species during molecular beam epitaxy (MBE) growth, structural parameters such as local shell thickness, composition and strain may differ between NWs grown onto the same substrate. This requires structural analysis of single NWs instead of measuring NW ensembles. In this work, the complete structure of single GaAs/(In,Ga)As/GaAs coreâshellâshell NW heterostructures is determined by means of X-ray nanodiffraction using synchrotron radiation. The NWs were grown by MBE on a prepatterned silicon (111) substrate with a core diameter of 50â
nm and an (In,Ga)As shell thickness of 20â
nm with a nominal indium concentration of 15%, capped by a 30â
nm GaAs outer shell. In order to access single NWs with the X-ray nanobeam being incident parallel to the surface of the substrate, a single row of holes with a separation of 10â
”m was defined by electron-beam lithography to act as nucleation centres for MBE NW growth. These well separated NWs were probed sequentially by X-ray nanodiffraction, recording three-dimensional reciprocal-space maps of Bragg reflections with scattering vectors parallel (out-of-plane) and perpendicular (in-plane) to the NW growth axis. From the out-of-plane 111 Bragg reflection, deviations from hexagonal symmetry were derived, together with the diameters of probed NWs grown under the same conditions. The radial NW composition and interface strain became accessible when measuring the two-dimensional scattering intensity distributions of the in-plane and reflections, exhibiting well pronounced thickness fringes perpendicular to the NW side planes (truncation rods, TRs). Quantitative values of thickness, composition and strain acting on the (In,Ga)As and GaAs shells were obtained via finite-element modelling of the coreâshellâshell NWs and subsequent Fourier transform, simulating the TRs measured along the three different directions of the hexagonally shaped NWs simultaneously. Considering the experimental constraints of the current experiment, thicknesses and In content have been evaluated with uncertainties of ±2â
nm and ±0.01, respectively. Comparing data taken from different single NWs, the shell thicknesses differ from one to another
Correlation of Electrical and Structural Properties of Single As-Grown GaAs Nanowires on Si (111) Substrates
We present the results of the study
of the correlation between the electrical and structural properties
of individual GaAs nanowires measured in their as-grown geometry.
The resistance and the effective charge carrier mobility were extracted
for several nanowires, and subsequently, the same nano-objects were
investigated using X-ray nanodiffraction. This revealed a number of
perfectly stacked zincblende and twinned zincblende units separated
by axial interfaces. Our results suggest a correlation between the
electrical parameters and the number of intrinsic interfaces