25 research outputs found
Molecular Beam Epitaxy of Catalyst-Free InAs nanowires on Si (111)
No full textSemiconductor nanowires (NWs) represent a unique system for exploring phenomena
at the nanoscale and are also expected to play a critical role in future electronic and
optoelectronic devices. For some functional NW-based devices, it is essential to have
control over position, size and directionality of NWs for homogeneous and predictive
performance. Moreover, the growth of device-quality NWs with high purity should
abstain from conventional nucleation schemes that employ foreign catalyst such
as gold. Recent progress in selective-area epitaxy (SAE) technique has allowed
position controlled catalyst-free growth of NWs, where semiconductor substrates
are masked by dielectrics with nano-aperture patterns. This is a kind of template
method, which involves a combination of bottom up (epitaxial growth) and top down
(lithography) approaches. However, the current approaches for the nano-patterning
are mainly based on electron beam lithography (e-beam), which is expensive and
not suitable for large scale productions. In this master’s thesis, we demonstrate the
molecular beam epitaxy (MBE) of catalyst free InAs NWs on Si (111) by different
techniques. The first technique adapts a fully bottom-up approach for selective-area
molecular beam epitaxy (SA-MBE) growth of catalyst-free InAs NWs. Specific to
this work, the inexpensive, versatile and simple colloidal lithography technique is
applied for opening nano-apertures in a SiO2 layer deposited on Si substrates, and
methods to have control over the size and density of patterns were studied. In the
second approach, epitaxy of non-ordered catalyst-free InAs NWs was demonstrated
on substrates without lithography patterning. This is realized by generating pinholes
in the SiO2 layer using wet etching
Fabrication of nanowire growth templates by forming pinholes in SiOx on Si
No full textInAs nanowire growth is carried out on a thin grainy layer of SiOx on Si (111), utilizing the openings of pinholes in the SiOx layer by isotropic wet etching. SiOx layers with different thicknesses were deposited and etched down to different thicknesses, to investigate how the initial layer roughness and the etching depth influence the formation of pinholes and thereafter the NW growth
Onset of uncontrolled polytypism during the Au-catalyzed growth of wurtzite GaAs nanowires
No full textThe optoelectronic properties of a semiconductor are determined by the combination of its elemental composition and the crystal structure. The vapor-liquid-solid nanowire growth mechanism offers the controlled epitaxy of the zinc-blende and wurtzite polytype for a number of semiconductors. Long, thin, and phase-pure wurtzite GaAs nanowires are desirable as epitaxial templates for the growth of hexagonal SiGe shells, but the growth of such nanowires remains a challenge. Here, we study the growth of wurtzite GaAs nanowires and find a diameter dependent critical length beyond which the crystal phase becomes mixed. The onset of uncontrolled polytypism is modeled with a small contribution of As diffusion during growth. Due to this increased supply of As during prolonged growth, Ga is depleted from the liquid catalyst, thereby decreasing the contact angle. We investigate possible pathways of As through diffusion on the facets and edges of the nanowire, and from the scaling of the critical length we deduce that edge diffusion has an important contribution. This study offers new insights for realizing long, phase-pure wurtzite GaAs nanowires with high aspect ratio.</p
Production of nano-holes patten on Si(111) by colloidal lithography for growth of InAs nanowires
No full textColloidal lithography is a simple, versatile and low-cost technique that can be used to pattern diverse nanostructures on a wafer scale. In this work, colloidal lithography utilizing polystyrene nanoparticles as a lift-off mask was used to produce nanohole patterns on Si (111) substrates. The hole size, which is determined by the size of the polystyrene particles, can be well controlled by oxygen plasma shrinking. Using this technique, we were able to obtain nanohole pattern with feature size down to 50 nm, which is close to the limit that conventional lithographic techniques can reach, in a time-efficient and cost effective manner. InAs nanowires were successfully grown on the patterned substrates using molecular beam epitaxy
Production of nano-holes patten on Si(111) by colloidal lithography for growth of InAs nanowires
No full textColloidal lithography is a simple, versatile and low-cost technique that can be used to pattern diverse nanostructures on a wafer scale. In this work, colloidal lithography utilizing polystyrene nanoparticles as a lift-off mask was used to produce nanohole patterns on Si (111) substrates. The hole size, which is determined by the size of the polystyrene particles, can be well controlled by oxygen plasma shrinking. Using this technique, we were able to obtain nanohole pattern with feature size down to 50 nm, which is close to the limit that conventional lithographic techniques can reach, in a time-efficient and cost effective manner. InAs nanowires were successfully grown on the patterned substrates using molecular beam epitaxy
Growth‐Related Formation Mechanism of I3‐Type Basal Stacking Fault in Epitaxially Grown Hexagonal Ge‐2H
No full textInternational audienceThe hexagonal-2H crystal phase of Ge has recently emerged as a promising direct bandgap semiconductor in the mid-infrared range providing new prospects of additional opto-electronic functionalities of group-IV semiconductors (Ge and SiGe). The controlled synthesis of such hexagonal-2H Ge phase is a challenge that can be overcome by using wurtzite GaAs nanowires as a template. However, depending on growth conditions, unusual basal stacking faults (BSFs) of I3-type are formed in the metastable 2H structure. The growth of such core/shell heterostructures is observed in situ and in real time by means of environmental transmission electron microscopy using chemical vapor deposition. The observations provide the first direct evidence of a step-flow growth of Ge-2H epilayers and reveal the growth-related formation of I3-BSFs during unstable growth. Their formation conditions are dynamically investigated. Through these in situ observations, a scenario can be proposed for the nucleation of I3-type BSFs that is likely valid for any metastable hexagonal 2H or wurtzite structures grown on m-plane substrates. Conditions are identified to avoid their formation for perfect crystalline synthesis of SiGe-2H
Observation of Conductance Quantization in InSb Nanowire Networks
No full textMajorana zero modes (MZMs) are prime candidates for robust topological quantum bits, holding a great promise for quantum computing. Semiconducting nanowires with strong spin orbit coupling offer a promising platform to harness one-dimensional electron transport for Majorana physics. Demonstrating the topological nature of MZMs relies on braiding, accomplished by moving MZMs around each other in a certain sequence. Most of the proposed Majorana braiding circuits require nanowire networks with minimal disorder. Here, the electronic transport across a junction between two merged InSb nanowires is studied to investigate how disordered these nanowire networks are. Conductance quantization plateaus are observed in most of the contact pairs of the epitaxial InSb nanowire networks: the hallmark of ballistic transport behavior.QRD/Kouwenhoven LabQN/Conesa-Boj LabQN/Bakkers La
Probing Lattice Dynamics and Electronic Resonances in Hexagonal Ge and SixGe1-x Alloys in Nanowires by Raman Spectroscopy
No full textRecent advances in nanowire synthesis have enabled the realization of crystal
phases that in bulk are attainable only under extreme conditions, i.e. high
temperature and/or high pressure. For group IV semiconductors this means access
to hexagonal-phase SixGe1-x nanostructures (with a 2H type of symmetry), which
are predicted to have a direct band gap for x up to 0.5 - 0.6 and would allow
the realization of easily processable optoelectronic devices. Exploiting the
quasi-perfect lattice matching between GaAs and Ge, we synthesized hexagonal
phase GaAs-Ge and GaAs-SixGe1-x core-shell nanowires with x up to 0.59. By
combining position-, polarization- and excitation wavelength-dependent u-Raman
spectroscopy studies with first-principles calculations, we explore the full
lattice dynamics of these materials. In particular, by obtaining
frequency-composition calibration curves for the phonon modes, investigating
the dependence of the phononic modes on the position along the nanowire, and
exploiting resonant Raman conditions to unveil the coupling between lattice
vibrations and electronic transitions, we lay the grounds for a deep
understanding of the phononic properties of 2H-SixGe1-x nanostructured alloys
and of their relationship with crystal quality, chemical composition, and
electronic band structure
