No description available.

Thesis (Ph.D)--Electrical and Computer Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Cressler, John; Committee Member: Deo, Chaitanya; Committee Member: Doolittle, Alan; Committee Member: Keezer, David; Committee Member: May, Gary; Committee Member: Papapolymerou, John. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (M. S.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Cressler, John; Committee Member: Papapolymerou, John; Committee Member: Ralph, Stephen.

Thesis (Ph. D.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2007. / Cressler, John, Committee Chair ; Laskar, Joy, Committee Member ; Papapolymerou, Ioannis, Committee Member ; Zhou, Haomin, Committee Member ; Milor, Linda, Committee Member.

No description available.

Ce mémoire présente une étude de la synthèse par la méthode Vapeur-Liquide-Solide (VLS) de nanofils de silicium et de germanium par Epitaxie par Jets Moléculaires ainsi que de l'effet de la présence de manganèse sur leur croissance. La croissance des nanofils est fortement modifiée par la présence de manganèse. Les nanofils de silicium élaborés sous un faible flux de manganèse présentent des propriétés morphologiques et structurales remarquables. La présence de manganèse modifie le diamètre d'équilibre des gouttes AuSi utilisées pour la croissance par voie VLS et permet l'élaboration de nanofils de silicium de longueurs élevées et de faibles diamètres. De plus, leur qualité cristalline est considérablement améliorée par rapport aux nanofils de silicium formés sans apport de manganèse. Dans ce mémoire nous proposons quelques explications à ce phénomène. Dans le cas des nanofils de germanium, l'incorporation de manganèse n'a pu être obtenue par codépôt. Aussi, (i) le dopage par implantation ionique de nanofils de germanium et (ii) la fonctionnalisation de nanofils de germanium par la formation d'hétérostructures type cœur/coquille Ge/GeMn ont été considérés : - les mesures d'aimantation effectuées sur des nanofils de germanium implantés au manganèse démontrent l'existence de propriétés ferromagnétiques avec des températures de Curie supérieures à 400K. Il s'agit d'un résultat très prometteur en vue d'applications utilisant des nanofils de germanium ferromagnétiques à température ambiante ; - pour accéder aux propriétés magnétiques des nanofils de germanium fonctionnalisés par dépôt de GeMn, nous avons mis au point une procédure de prises de contacts adaptée à la mesure de leurs propriétés de magnétotransport. Les caractéristiques électriques de ces dispositifs montrent que les propriétés de transport sont dominées par la présence de la couche coquille de GeMn, surtout à basse température. Des mesures de magnétotransport effectuées à 100K indiquent l'existence d'effets de magnétorésistance liés aux propriétés ferromagnétiques des nanofils de Ge ainsi fonctionnalisés. / This thesis presents a study of the Vapour-Liquid-Solid (VLS) synthesis of silicon and germanium nanowires by Molecular Beam Epitaxy and the effect of the presence of manganese on the growth properties. The presence of manganese strongly modifies the growth of nanowires and observed behaviours are very different for AuSi and AuGe systems. Silicon nanowires grown in the presence of manganese exhibit very interesting morphological and structural properties. The presence of manganese modifies AuSi droplets' diameter and allows manufacturing long nanowires with relatively small diameters. Moreover, the crystalline quality is dramatically improved as compared to that of silicon nanowires grown without manganese. In this manuscript we propose some explanation for the growth phenomena. In the case of germanium nanowires, manganese incorporation could not be obtained by concomitant deposition of germanium and manganese. Consequently, (i) the doping of germanium nanowires by ion implantation as well as (ii) germanium nanowires functionalization by core/shell Ge/GeMn heterostructures formation were considered: - magnetization measurements performed on implanted germanium nanowires demonstrate ferromagnetic properties with Curie temperatures above 400K. This result is very promising for the processing of devices using room-temperature ferromagnetic germanium nanowires ; - in order to access Ge/GeMn nanowires magnetic properties, we processed samples to probe nanowires magnetotransport properties. Electrical resistivities of devices show that transport properties are dominated by GeMn shell layer even more at low temperature. Magnetotransport measurements done at 100K indicate magnetoresistance effects linked with nanowires ferromagnetic properties.

Die vorliegende Dissertation befasst sich mit dem Wachstum und der Charakterisierung von Silizium- und Germanium-Nanodrähten. Diese Strukturen gelten als aussichtsreiche Komponenten für zukünftige Bauelemente. Für die Anwendung ist die genaue Kenntnis der Größe, der kristallographischen Orientierung und der Position der Nanodrähte erforderlich. Ziel dieser Arbeit war daher die Untersuchung von Si- und Ge-Nanodrähten im Hinblick auf ihre Größe, Orientierung und Position. Die Herstellung erfolgte durch Physikalische Gasphasenabscheidung (PVD) im Ultrahochvakuum nach dem Vapor-Liquid-Solid (VLS)-Verfahren, das auf dem Wachstum aus Lösungsmitteltröpfchen basiert. Die Größe der Nanodrähte konnte im Falle von Silizium auf Si(111) mit Gold als Lösungsmittel durch die Parameter des Experiments reproduzierbar bestimmt werden. Höhere Goldbedeckung und höhere Substrattemperaturen führten zu Tröpfchen mit größerem Duchmesser und somit zu dickeren Drähten. Längere Si-Verdampfungszeiten und höhere Si-Verdampfungsraten führten zu längeren Drähten. Dünnere Drähte wuchsen schneller als dickere. Als zweites Lösungsmittel wurde Indium untersucht, da es sich im Vergleich zu Gold nicht nachteilig auf die elektronischen Eigenschaften von Silizium auswirkt. Basierend auf den Ergebnissen zur Tröpfchenbildung konnten die besseren Wachstumsresultate mit Gold erklärt werden. Germanium-Nanodrähte, die aus Goldtröpfchen auf Ge(111) gezüchtet wurden, zeigten im Gegensatz zu den Si-Nanodrähten nicht die kristallographische [111]-Orientierung des Substrates, sondern eine -Orientierung, was durch Berechnungen von Keimbildungsenergien auf verschiedenen Kristallflächen erklärt werden konnte. Zur Anordnung von Metalltröpfchen und damit von Nanodrähten wurden Substrate mithilfe von fokussierten Ionenstrahlen (FIB) vorstrukturiert, um die Tröpfchenbildung an bestimmten Stellen zu begünstigen. Es gelang, aus angeordneten Goldtröpfchen epitaktisch gewachsene Si- und Ge-Nanodrähte zu züchten. / This dissertation deals with the growth and the characterization of silicon and germanium nanowhiskers, also called nanorods or nanowires. The investigation of these structures is of great interest as they represent promising building blocks for future electronic devices. With regard to a possible application, the knowledge of size, crystallographic orientation and position of the nanowhiskers is essential. The purpose of this work was, therefore, to investigate the growth of Si and Ge nanowhiskers with regard to their size, orientation and position. The nanowhiskers were grown via physical vapor deposition (PVD) in ultra-high vacuum using the vapor-liquid-solid (VLS) mechanism which is based on growth from solution droplets. The size of the nanowhiskers could be reproducibly determined by the experimental parameters in the case of Si nanowhiskers on Si(111) with gold as the solvent. A higher gold coverage as well as a higher substrate temperature led to larger droplet diameters and thus to thicker whiskers. A longer silicon evaporation time and a higher silicon rate led to longer whiskers. Thinner whiskers grew faster than thicker ones. A second material used as the solvent was indium as it is more suitable for electronic application compared to gold. Based on results of droplet formation of the two solvents on silicon, the better results of whisker growth using gold could be explained. Ge nanowhiskers grown from gold droplets on Ge(111) did not show the [111] orientation of the substrate as in the case of Si nanowhiskers on Si(111) but a orientation. By calculating nucleation energies on different crystal facets, the experimental findings could be explained. To position nanodroplets of the solvent material and thus to obtain a regular arrangement of nanowhiskers, substrates were pre-structured with nanopores by focused ion beams (FIB). Silicon and germanium nanowhiskers could be epitaxially grown from ordered arrays of gold droplets.

Doped semiconductor systems have for decades provided an excellent platform to study novel concepts in solid state physics such as quantum hall effect, metal-to-insulator transition (MIT), weak localization and many body interaction effects. Doped Si, in particular and doped Ge has been studied extensively to study MIT as a function of dopant concentration or uniaxial stress. Spin transport phenomena have also been probed in bulk doped Si. All the previous studies involved bulk doped semiconductors where the dopants are spread through the bulk of the material. However spatial confinement of dopants in one or more dimensions may lead to a range of exotic quantum phenomena such as an absence of Anderson localization in one and two dimensions, hole-mediated (Nagaoka) ferromagnetism and new modes of quantum transport, when the Fermi energy lies at or close to centre of the band. Since many of these phenomena are inherent to lower dimensions, it has been hard to observe these experimentally in bulk doped crystals of Si and Ge. Recent advances in the monolayer doping techniques with atoms that closely pack on a surface, has made it possible to design a new class of 2D electron systems (2DES) in elemental semiconductors, such as Si and Ge, where the dopant (P) atoms are confined within a few atomic planes. The uniqueness of these systems lies not merely in the planar doping profile in bulk semiconductors that allow versatile designs of nanodevices, such as 1D wires, tunnel gaps and quantum dots, but also that it is now possible to study the interplay of wavefunction overlap and commensurability effects in 2D with unprecedented control. From an application perspective as well these systems are technologically important as they are aimed at being the building blocks of a solid state quantum computer. This thesis deals with investigating the electrical transport properties, both average (resistance) and dynamic (noise) of doped semiconductor systems in 2D delta layers, 1D wires and 0D quantum dots. We find that the 2D δ-layers shows suppressed low frequency noise and the Hooge parameter of delta doped Si is about five to six orders of magnitude lower when compared to bulk doped Si in metallic regime. At low temperatures, the noise arises in these systems due to universal conductance fluctuations. For 1D wires as well we find that the Hooge parameter is one of the lowest among various 1D systems including carbon nanotubes. We identify that charge traps in the Si/SiO2 are responsible for causing noise in δ-doped systems. Then we study the noise and transport in 2D delta layers as a function of doping density (and hence carrier density and interaction). Weak localization corrections to the conductivity and the universal conductance fluctuations were both found to decrease rapidly with decreasing doping in the Si:P and Ge:P delta layers, suggesting a spontaneous breaking of time reversal symmetry driven by strong Coulomb interactions. At low doping density we observe metal-like dependence of resistance on temperature at low temperatures, raising the possibility of a metallic ground state in 2D at 0 K in doped semiconductors. Finally we probe the low density devices (with broken time reversal symmetry) using superconducting Al as ohmic contacts. Anomalous increase in resistance below the superconducting transition temperature of Al and magnetoresistance with a sharp peak at 0 T is observed. Additionally we find that when the Al is superconducting, there exists a non-local resistance in low doped devices.

Chalcogenide glasses are a class of covalent amorphous semiconductors with interesting properties. The presence of short-range order and the pinned Fermi level are the two important properties that make them suitable for many applications. With flash memory technology reaching the scaling limit as per Moore’s law, alternate materials and techniques are being researched at for realizing next generation non-volatile memories. Two such possibilities that are being looked at are Phase Change Memory (PCM) and Programmable Metallization Cell (PMC) both of which make use of chalcogenide materials. This thesis starts with a survey of the work done so far in realizing PCMs in reality. For chalcogenides to be used as a main memory or as a replacement to FLASH technology, the electrical switching parameters like switching voltage, programming current, ON state and OFF state resistances, switching time and optical parameters like band gap are to be considered. A survey on the work done in this regard has revealed that various parameters such as chemical composition of the PC material, nature of additives used to enhance the performance of PCM, topological thresholds (Rigidity Percolation Threshold and Chemical Threshold), device geometry, thickness of the active volume, etc., influence the electrical switching parameters. This has motivated to further investigate the material and experimental parameters that affect switching and also to explore the possibility of multi level switching. In this thesis work, the feasibility of using two chalcogenide systems namely Si15Te85-xGex and Ge15Te85-xAgx in the form of amorphous thin films for PCM application is explored. In the process, electrical switching experiments have been carried out on thin films belonging to these systems and the results obtained are found to exhibit some interesting anomalies. Further experiments and analysis have been carried out to understand these anomalies. Finally, the dynamics of electrical switching has been investigated and presented for amorphous Si15Te85-xGex thin films. From these studies, it is also seen that multi state switching/multiple resistance levels of the material can be achieved by current controlled switching, the mechanisms of which have been further probed using XRD analysis and AFM studies. In addition, investigations have been carried out on the electrical switching behavior of amorphous Ge15Te85-xAgx thin film devices and optical band gap studies on amorphous Ge15Te85-xAgx thin films. Chapter one of the thesis, gives a brief introduction to the limitations in existing memory technology and the alternative memory technologies that are being researched, based on which it can be inferred that PCM is a promising candidate for the next generation non volatile memory. This chapter also discusses the principle of using PCM to store data, realization of PCM using chalcogenides, the material properties to be considered in designing PCM, the trade offs in the process of design and the current trends in PCM technology. Chapter two provides a brief review of the electrical switching phenomenon observed in various bulk chalcogenide glasses, as electrical switching is the underlying principle behind the working of a PCM. In the process of designing a memory, many parameters like read/write operation speed, data retentivity and life, etc., have to be optimized for which a thorough understanding on the dependence of electrical switching mechanism on various material parameters is essential. In this chapter, the dependence of electrical switching on parameters like network topological thresholds and electrical and thermal properties of the material is discussed. Doping is an efficient way of controlling the electrical parameters of chalcogenides. The nature of dopant also influences switching parameters and this also is briefly discussed. Chapter three provides a brief introduction to the different experimental techniques used for the thesis work such as bulk chalcogenide glass preparation, preparation of thin amorphous films, measurement of film thickness, confirmation of amorphous nature of the films using X-Ray Diffraction (XRD), electrical switching experiments using a custom made setup, crystallization study using XRD and Atomic Force Microscopy (AFM) and optical band gap studies using UV-Vis spectrometer. Vt is an important parameter in the design of a PCM. Chapter four discusses the dependence of Switching voltage, Vt, on input energy. It is already established that the Vt is influenced by the composition of the base glass, nature of dopants, thickness of films and by the ambient temperature. Based on the results of electrical switching experiments in Si15Te74Ge11 amorphous thin films a comprehensive analysis has been done to understand the kinetics of electrical switching. Chapter five discusses a current controlled crystallization technique that can be used to realize multi-bit storage with a single layer of chalcogenide material. In case of PCM, data is stored as structural information; the memory cell in the amorphous state is read as data ‘0’ and the memory cell in crystalline state is read as data ‘1’. This is accomplished through the process of electrical switching. In order to increase the memory density or storage density, multi-bit storage is being probed at by having multiple layers of chalcogenide material. However, with this technique, the problems of inter-diffusion between different layers cannot be ruled out. In this thesis work, a current controlled crystallization technique has been used to achieve multiple stable resistance states in Si15Te75Ge10 thin films. Chapter six discusses the mechanism behind multi state switching exhibited by certain compositions of Si15Te85-xGex thin films. Crystallization studies on certain Si15Te85-xGex films have been carried out using XRD and AFM to understand the phenomenon of multiple states. The results of these experiments and analysis are presented in this chapter. Chapter seven discusses the results of electrical switching experiments and optical band gap studies on amorphous Ge15Te85-xAgx thin films. Chapter eight gives the conclusion and scope for future work.