The formation of Ge nanowires in V-grooves has been studied experimentally as well as theoretically. As substrate oxide covered Si V-grooves were used formed by anisotropic etching of (001)Si wafers and subsequent oxidation of their surface. Implantation of 1E17 Ge+ cm^-2 at 70 keV was carried out into the oxide layer covering the V-grooves. Ion irradiation induces shape changes of the V-grooves, which are captured in a novel continuum model of surface evolution. It describes theoretically the effects of sputtering, redeposition of sputtered atoms, and swelling. Thereby, the time evolution of the target surface is determined by a nonlinear integro-differential equation, which was solved numerically for the V-groove geometry. A very good agreement is achieved for the predicted surface shape and the shape observed in XTEM images. Surprisingly, the model predicts material (Si, O, Ge) transport into the V-groove bottom which also suggests an Ge accumulation there proven by STEM-EDX investigations. In this Ge rich bottom region, subsequent annealing in N2 atmosphere results in the formation of a nanowire by coalescence of Ge precipitates shown by XTEM images. The process of phase separation during the nanowire growth was studied by means of kinetic 3D lattice Monte-Carlo simulations. These simulations also indicate the disintegration of continuous wires into droplets mediated by thermal fluctuations. Energy considerations have identified a fragmentation threshold and a lower boundary for the droplet radii which were confirmed by the Monte Carlo simulation. The here given results indicate the possibility of achieving nanowires being several nanometers wide by further growth optimizations as well as chains of equally spaced clusters with nearly uniform diameter.

>Magister Scientiae - MSc / Phase change material (PCM) through latent heat of molten salt, is a convincing way for thermal energy storage in CSP applications due to its high volume density. Molten salt, with (60% NaNO3 and 40% KNO3) has been used extensively for energy storage however; the low thermal conductivity and specific heat have limited its large implementation in solar applications. For that, molten salt with the additive of silver nanowires (AgNWs) was synthesized and characterized. This research project aims to investigate the thermophysical properties enhancement of nanosalt (Mixture of molten salt and silver nanowires). The results obtained showed that by simply adjusting the temperature, Silver nanowires with high aspect ratio have been synthesized through the enhanced PVP polyol process method. SEM results revealed a network of silver nanowires and TEM results confirmed the presence of silver nanowires with an average diameter of 129 nm and 16 μm in length.

Nanowires with Rashba spin-orbit coupling represent a promising platform for the realization of one-dimensional topological superconductivity and Majorana bound states. In this work we investigate Majorana bound states in hybrid normal-superconductor and short superconductor-normal-superconductor junctions based on nanowires with Rashba spin-orbit coupling. In particular, we explore consequences of the topological phase transition as well as the non-locality and self conjugation properties of the Majorana states on the low-energy spectrum and the Josephson effect in the case of superconductor-normal-superconductor junctions. Our work shows the great potential of hybrid junctions as a platform for the study of topological superconductivity and Majorana bound states.

This work presents the bottom-up fabrication of DNA origami-assembled metal nanowires and metal-semiconductor junctions, and their electrical characterization. Integration of metal and semiconductor nanomaterials into prescribed sites on self-assembled DNA origami has facilitated the fabrication of electronic nanomaterials, whereas use of conventional tools in their characterization combines bottom-up and top-down technologies. To expand the contemporary DNA-based nanofabrication into nanoelectronics, I performed site-specific metallization of DNA origami to create arbitrarily arranged gold nanostructures. I reported improved yields and conductivity measurements for Au nanowires created on DNA origami tile substrates. I measured the conductivity of C-shaped Au nanowires created on DNA tiles (∼130 nm long, 10 nm diameter, and 40 nm spacing between measurement points) with a four-point measurement technique which revealed the resistivity of the gold nanowires was as low as 4.24 × 10-5 Ω m. Next, I fabricated DNA origami-templated metal-semiconductor junctions and performed electrical characterization. Au and Te nanorods were attached to DNA origami in an alternating fashion. Electroless gold plating was used to create nanoscale metal--semiconductor interfaces by filling the gaps between Au and Te nanorods. Two-point electrical characterization indicated that the Au--Te--Au junctions were electrically connected, with non-linear current--voltage curves. Finally, I formed metal-semiconductor nanowires on DNA origami by annealing polymer-encased nanorods. Polymer-coated Au and Te nanorods pre-attached to ribbon-shaped DNA origami were annealed at 170°C for 2 min. Gold migration occurred onto Te nanorods during annealing and established electrically continuous interfaces to give Au/Te nanowires. Electrical characterization of these Au/Te/Au assemblies revealed both nonlinear current-voltage curves and linear plots that are explained. The creation of electronic nanomaterials such as metal nanowires and metal-semiconductor junctions on DNA origami with multiple techniques advances DNA nanofabrication as a promising path toward future bottom-up fabrication of nanoelectronics.

We conclusively explain the different lasing mode energies in ZnO nano- and microcavities observed by us and reported in literature. The limited penetration depth of usually used excitation lasers results in an inhomogeneous spatial gain region depending on the structure size and geometry. Hence, weakly or even nonexcited areas remain present after excitation, where modes are instantaneously suppressed by excitonic absorption. We compare the effects for ZnO microwires, nanowires, and tetrapod-like structures at room temperature and demonstrate that the corresponding mode selective effect is most pronounced for whispering-gallery modes in microwires with a hexagonal cross section. Furthermore, the absorptive lasing mode suppression will be demonstrated by correlating the spot size of the excitation laser and the lasing mode characteristic of a single ZnO nanowire.

The emission properties of bent ZnO microwires with diameters ranging from 1.5  μm to 7.3  μm are systematically investigated by cathodoluminescence spectroscopy at T ≈ 10 K. We induced uniaxial strains along the c-axis of up to ±2.9 %. At these high strain values, we observe a nonlinear shift of the emission energy with respect to the induced strain, and the magnitude of the energy shift depends on the sign of the strain. The linear and non-linear deformation potentials were determined to be D1=−2.50±0.05 eV and D2=−15.0±0.5 eV, respectively. The nonlinearity of the energy shift is also reflected in the observed spectral broadening of the emission peak as a function of the locally induced strain, which decreases with increasing strain on the compressive side and increases on the tensile side.

An emerging branch of electronics, the optospintronics, would be highly boosted if the control of magnetic order by light is implemented in magnetic semiconductors’ nanostructures being compatible with the actual technology. Here, we show that the ferromagnetic magnetization of low Fe-doped ZnO nanowires prepared by carbothermal process is enhanced under illumination up to temperatures slightly below room temperature. This enhancement is related to the existence of an oxygen vacancy VO in the neighborhood of an antiferromagnetic superexchange Fe3+-Fe3+ pair. Under illumination, the VO is ionized to to V+O giving an electron to a closeFe3+ ion from the antiferromagnetic pair. This light excited electron transition allows the transition of Fe3+ to Fe2+ forming stable ferromagnetic double exchange pairs, increasing the total magnetization. The results presented here indicate an efficient way to influence the magnetic properties of ZnO based nanostructures by light illumination at high temperatures.

L’objet de cette thèse est d’étudier le dopage ex-situ de nanofils semiconducteurs IV pour des applications en électronique, spintronique ou encore thermoélectricité. Deux techniques de dopage ont été explorées : l’implantation par faisceaux d’ions et le Spin-On-Doping (SOD).L’implantation d’ions Mn a été testée dans les nanofils de Ge avec l’objectif de synthétiser un matériau semi-conducteur ferromagnétique dilué. Une concentration en Mn de quelques pourcents peut être atteinte sans amorphisation du fil ni formation de précipités, ce qui est très encourageant. Lors d’expériences d’implantation réalisées in situ dans un microscope électronique en transmission, une forte exaltation de la pulvérisation sous irradiation électronique a été constatée.La technique SOD consiste à faire diffuser thermiquement les impuretés de type p ou n contenues une résine de type HSQ (Hydrogen silsesquioxane) qui enrobe les nanofils. Le recuit de la HSQ (dopée ou non) engendre une modification structurale des nanofils (bien que cette technique soit considérée comme non destructive). Lors du recuit, une transformation partielle de la phase diamant 3C vers la phase hexagonale 2H, a en effet été observée dans les nanofils de Si et de Ge, au-delà de 500 et 400°C respectivement. Les paramètres essentiels de la transformation de phase sont la contrainte de cisaillement résultant de la densification de la résine et le budget thermique. Les nanofils de Ge deviennent amorphes au-delà de 650°C, ce qui interdit en pratique leur dopage par SOD.Les caractérisations électriques ont été réalisées sur des nanofils de Si réalisés par gravure ionique réactive sur substrats orientés (111) et contactés en matrice ou individuellement. Pour le contactage de nanofils uniques en configuration NW-FET (nanowire field effect transistors), un procédé technologique basé sur la lithographie électronique a dû être développé. Les difficultés à surmonter étaient relatives à la faible longueur des nanofils. Diverses techniques de caractérisation ont été mises en œuvre (I-V en configuration verticale ou horizontale de type TLM (Transient Linear Measurement), SSRM (scanning spreading resistance microscopy), EBIC (electron beam induced current). Les mesures collectives concernent des ensembles de nanofils de type p enrobés dans une résine qu’elle soit dopante ou non. Pour observer un courant notable dans la structure, un recuit est nécessaire. Au-delà d’une température de recuit de 600°C, une polarisation négative du substrat induit un comportement conforme au mécanisme SCLC (space charge limited current) attendu pour des nanofils faiblement dopés enrobés dans une matrice isolante. En positif, on observe une caractéristique I(V) ohmique et une densité de courant jusqu’à 500 fois plus élevée dans les nanofils que dans le substrat. Ce comportement pourrait être dû à l’influence des états d’interface provenant de la technique de gravure. Cette hypothèse est confortée par le fait qu’après recuit à 900°C, le courant en direct s’explique en considérant dans les fils un dopage proche de celui du substrat, et surtout par l’observation en SSRM d’une couche conductrice interfaciale entre fils et HSQ. Elle permet aussi d’interpréter les mesures sous pointes faites sur les fils de type n. Le mode de transport SCLC a également été observé pour des nanofils individuels contactés sous pointe ou par lithographie. Ces mesures n’ont pas mis directement en évidence l’influence de la transformation de phase.Le dopage de type n ou p par SOD s’avère efficace après recuit à 900°C. Dans ce cas, les comportements observés, contacts ohmiques et jonctions p-n, peuvent être interprétés plus simplement en considérant des niveaux de dopage supérieurs à 3×10¹⁶ cm⁻³ en type p et 2×10¹⁶ cm⁻³ en type n. Ces valeurs déduites des résistivités mesurées sont sans doute très sous-estimées puisque la mobilité dans les fils est sans doute inférieure à celle du volume. / This thesis aims at studying the ex-situ doping of semiconducting nanowires (NWs) for applications in electronics, spintronics or thermoelectricity. Two widely used techniques have been envisaged: ion beam implantation and Spin-On-Doping (SOD).The ion beam implantation of Mn ions has been tested in Ge NWs in an attempt to form a 1D diluted magnetic semiconductor structure. A Mn concentration of few percents can be achieved without amorphization of the nanowire nor clustering, what is very promizing. During implantation done in situ in a transmission electron microscope, a strong enhancement of the sputtering under electron irradiation has been observed.The doping by SOD results from the thermal diffusion of p-type or n-type impurities contained in a HSQ (Hydrogen silsesquioxane) resist in which the NWs are embedded. The curing of the HSQ resist (doped or not) leads to a structural modification of nanowires (while SOD is generally assumed to be non-destructive). As a result of the annealing, a partial transformation of the 3C diamond phase towards the 2H hexagonal phase is observed in Si and Ge nanowires, above 500 et 400°C respectively. The main parameters of that phase transformation are the shear stress due to the HSQ densification and the thermal budget. Ge NWs are found to turn to amorphous above 650°C, what renders SOD practically unusable for Ge NWs. Two methods are currently used for the fabrication of nanowires, the VLS (vapor-liquid-solid) growth and reactive ion etching of (111) Si wafers. For practical reasons, etched NWs were used for the study of their electrical properties.The electrical characterizations were done on arrays of Si NWs embedded in a HSQ matrix or on single NWs. For contacting single NWs in the NW-FET(nanowire field effect transistors) configuration , a process based on electron beam lithography has been developed. The issues to be solved were related to the low length of NWs. Various measurement techniques were used: I-V in two tips or TLM (Transient Linear Measurement) arrangement, SSRM (scanning spreading resistance microscopy), EBIC (electron beam induced current). Collective measurements were done on arrays of p-type NWs embedded in a HSQ resist, doped or not. It was firstly observed that an annealing is needed to observe a noticeable current in the structure. Above an annealing temperature of 600°C, for a negative bias applied to the substrate, the observed behavior can be described by the SCLC (space charge limited current) mechanism expected for poorly doped NWs in an isolating matrix, while a positive bias applied to the substrate results in an ohmic characteristic and in a current density up to 500 times higher in the NWs than in the substrate. This unexpectedly high intensity in direct bias may be attributed to electrically active surface states resulting from the etching process. This hypothesis is conforted by the fact that an annealing at 900°C (without extra doping) the measured intensity can be explained by assuming the same doping level in NWS than in the substrate. In addition, an interfacial conductive between resist and nanowires can be observed by SSRM. These interfacial states can be also involved for understanding the measurements done on n-type NWS. The SCLC mechanism of transport has been also observed for single NWs contacted by tips or by lithographied contacts. These measurements were not able to evidence the effect of the phase transformation on the electrical properties.P-type and n-type doping by SOD becomes effective after annealing at 900°C. After doping, ohmic or rectifying behaviours on p-type substrate are observed as expected. That renders more easy the interpretation of results, by assuming doping levels in the NWs of 3×10¹⁶ cm⁻³ and 2×10¹⁶ cm⁻³ for p-type and n-type respectively. These values as deduced from resistivities are probably very underestimated as the mobilities in NW are probably much lower than in the bulk.

The optical properties of solar cells greatly affect their efficiencies. Decreasing the broadband and directional reflectance of solar cells increases the solar irradiance transmitted and absorbed by the cell, thereby increasing the production of electron-hole pairs. Traditional optical enhancements such as light trapping and anti-reflection coatings reduce the reflectance of silicon at an optimized wavelength and angle of incidence. They do not perform as well at high angles of incidence or over the broadband solar spectrum. Theoretical studies suggest that layers with a suitable gradient-index of refraction can create both a broadband and directional anti-reflective coating. Through their variations in height and tapered growth, Zinc oxide (ZnO) nanowires can create a gradient index anti-reflection coating. ZnO is a wide-band gap semiconductor that is non-absorbing over most of the solar spectrum. With low cost, low temperature techniques, ZnO nanowires can be grown with a variety of morphologies. ZnO nanowires were grown by aqueous chemical growth and by electrodeposition on silicon to create a gradient-index anti-reflective coating for solar cell applications. The nanowire arrays were characterized using SEM images, goniometer scattering measurements, and integrating sphere total reflectance measurements. ZnO nanowires grown by aqueous chemical growth on silicon had average diameters between 60 nm and 100 nm and average lengths between 800 nm and 1100 nm. The nanowires had vertical alignment. They exhibited relatively small diffuse reflectivities and relatively large specular reflectivities. ZnO nanowires grown by electrodeposition had greater variances in length and diameter, with average diameters between 85 nm and 180 nm and average lengths between 500 nm and 1200 nm. Electrodeposited ZnO nanowires were randomly arrayed and exhibited relatively large diffuse reflectivities and relatively small specular reflectivities. Total reflectance measurements showed that all nanowire arrays reduced the broadband reflectance of silicon. Smaller nanowire arrays outperformed the larger crystal growths. A five-fold decrease in the broadband reflectance of silicon was obtained from both vertical and randomly oriented nanowire arrays. The reflectances were constant for angles of incident below 35°. Measurements at angles of incidence greater than 35° are required to determine whether ZnO nanowires can perform as directional anti-reflective coatings and whether the morphology of the nanowires affects the directional reflectances.

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