Spatially resolved Raman spectroscopy on indium-catalyzed core-shell germanium nanowires: size effects

The structure of indium-catalyzed germanium nanowires is investigated by atomic force microscopy, scanning confocal Raman spectroscopy and transmission electron microscopy. The nanowires are formed by a crystalline core and an amorphous shell. We find that the diameter of the crystalline core varies along the nanowire, down to few nanometers. Phonon confinement effects are observed in the regions where the crystalline region is the thinnest. The results are consistent with the thermally insulating behavior of the core-shell nanowires.

GaAs nanowires and related prismatic heterostructures

Anna Fontcuberta i Morral

The growth of GaAs nanowires by the gallium-assisted method with molecular beam epitaxy (MBE) is presented in this review article. The structure of the grown nanowires was investigated by means of scanning and transmission electron microscopy as well as Raman spectroscopy. Their optical properties were revealed by performing photoluminescence measurements at the single nanowire level. Furthermore, by tuning the MBE conditions to planar growth, quantum heterostructures on the side facets of the nanowires were achieved. High-resolution transmission electron microscopy proved that the grown heterostructures have epitaxial precision, while photoluminescence measurements showed that they possess excellent optical quality. These quantum heterostructures constitute templates for developing novel nanowire based devices, such as a high electron mobility one-dimensional transistor or third generation solar cells.

Institute of Physics2009

The growth and optical properties of III-V nanostructures grown by Molecular Beam Epitaxy

Gözde Tütüncüoglu

This thesis is dedicated to the growth and characterization of the optoelectronic properties of III-V semiconductor nanostructures namely nanowires and nanoscale membranes. III-V semiconductors possess promising intrinsic properties like direct band gap, high electron/- hole mobility and spin-orbit interaction which makes them interesting for a wide range of applications such as high speed electronics, optoelectronics and photovoltaics. Nanostruc- tures enable the exploitation and further functionalization of the inherent semiconductor properties. The nanostructures we study in the scope of this thesis are grown with molecular beam epitaxy which enables us to obtain ultra-pure nanostructures with high crystalline quality. In the first part of this thesis we investigate and optimize the growth of GaAs nanowires both on silicon and GaAs substrates. We employ the self-catalyzed growth technique in order to avoid the risk of foreign metal contamination. Growth on Si substrates is interesting in views of enabling the integration of existing Si microtechnology and III-V technology. GaAs nanowire growth on (111) Si substrates is achieved with self-assisted and position controlled nanowire growth techniques. In both techniques, the effects of the silicon oxide thickness and composition along with the nature of the openings are investigated. GaAs nanowires grown on (111)B GaAs substrates are employed in optomechanical and optoelectronics applications. Pristine GaAs nanowires grown on (111)B GaAs substrates are employed in scanning force microscopy thanks to asymmetric orthogonal modes they exhibit. Furthermore, GaAs/AlGaAs heterostructure nanowires demonstrate lasing when their dimensions and AlGaAs capping are optimized. The rest of this thesis is dedicated to the growth of defect-free structures. Two methods are presented to create defect-free pure zinc-blende GaAs nanostructures. The first one is to modify the polarity of GaAs nanowires. We optimize the growth parameters to obtain a high yield of (111)A nanowires on (100) GaAs. GaAs nanowires grown in (111)A direction exhibited a defect-free structure in contrast to the nanowires grown in (111)B direction. Our second approach is to grow elongated nanostructures and control their orientation to âlock outâ the defects. When these nanostructures, GaAs nanoscale membranes, are oriented in direction on a (111)B GaAs substrate they exhibit pure zinc-blende crystalline structure. Their superior crystalline quality is confirmed with transmission electron microscopy and optical characterization techniques, i.e. photoluminescence and cathodoluminescence. Their growth mechanism and parameter window is investigated in detail. Next, GaAs nanoscale membranes are used as templates for quantum heterostructures. GaAs quantum wells are embedded in an AlGaAs shell around the nanoscale membranes. They are found to exhibit bright luminescence with narrow linewidth. Additionally, local alloy fluctuations in the AlGaAs shell are investigated. They exhibited sharp and localized luminescence characteristics like self-assembled quantum dots.

EPFL2017

Wafer-Scale Fabrication of Nanopore Devices for Single-Molecule DNA Biosensing using MoS2

Andrey Chernev, Jochem Deen, Michael Graf, Andras Kis, Martina Lihter, Michal Daniel Macha, Aleksandra Radenovic, Mukeshchand Thakur, Mukesh Kumar Tripathi

Atomically thin (2D) nanoporous membranes are an excellent platform for a broad scope of academic research. Their thickness and intrinsic ion selectivity (demonstrated for example in molybdenum disulfide-MoS2) make them particularly attractive for single-molecule biosensing experiments and osmotic energy harvesting membranes. Currently, one of the major challenges associated with the research progress and industrial development of 2D nanopore membrane devices is small-scale thin-film growth and small-area transfer methods. To address these issues, a large-area protocol including a wafer-scale monolayer MoS2 synthesis, Si/SiNx substrate fabrication and wafer-scale material transfer are demonstrated. First, the 7.62 cm wafer-scale MOCVD growth yielding homogenous monolayer MoS2 films are introduced. Second, a large number of devices are fabricated in one batch by employing the wafer-scale thin-film transfer method with high transfer efficiency (>70% device yield). The growth, the transfer quality and cleanliness are investigated using transmission electron microscopy, atomic force microscopy and Raman spectroscopy. Finally, the applicability and robustness of the large-area protocol is demonstrated by performing a set of double-stranded DNA translocation experiments through as-fabricated MoS2 nanopore devices. It is believed that the shown approach will pave the way toward wafer-scale, high-throughput use of 2D nanopores in various applications.

2020

The growth and optical properties of III-V nanostructures grown by Molecular Beam Epitaxy

Gözde Tütüncüoglu

EPFL2017