@phdthesis{Lange2021, author = {Lange, Felix}, title = {MBE growth and investigation of Si, Ge, and SiₓGe₁₋ₓ nanowires}, doi = {10.26127/BTUOpen-5751}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-57510}, school = {BTU Cottbus - Senftenberg}, year = {2021}, abstract = {Within this work, the growth of out-of-plane Si and SiᵪGe₁₋ᵪ and in-plane Ge nanowire have been investigated. For this purpose, nanowires have been grown ccording to the vapor-liquid-solid mechanism by means of molecular beam epitaxy with Au as metallic solvent on Si(111) and nano-structured Si(001)/SiO₂ substrates. Each stage from surface preparation to final nanowire growth has been examined in order to determine relevant parameters which influence the nucleation process nanowire elongation. Particular attention has been given to describe the variation of Au droplet size distribution on Si(111) to evaluate the subsequent nanowire growth and to obtain optimal growth conditions for an site-selective nucleation on the nano-structured substrates. Due to increased surface diffusion velocity with raising temperature, the droplet diameter distribution shifts from a positive skewed distribution with a high fraction of smaller droplets to a negative skewed distribution with a high fraction of larger droplets. The temperature dependency of the most probable droplet diameter and the number of Au droplets per area has been determined, which can be applied to predict the resulting nanowire diameters. The out-of-plane Si and SiᵪGe₁₋ᵪ on Si(111) predominantly grow along the surface normal, exhibiting the characteristic sawtooth-like sidewall faceting. During the nanowire formation, the Au surface diffusion velocity increased and causes an increase of the most frequently observed diameter. Furthermore, the total number of droplets/nanowires decreases by a constant factor due to an increase of the contact angle during Si/Ge deposition. The specific diameter range for an possible nanowire formation is reduced by the incorporation of Ge into SiᵪGe₁₋ᵪ nanowires. To obtain a regular and uniform nanowire growth, a nano-structured substrate consisting of Si(001) terminated pillars surrounded by a SiO₂ matrix has been utilized. The initial growth of Ge nanowire starting from Si-Au droplets with SiᵪGe₁₋ᵪ nucleation from ternary alloy is discussed from a thermodynamic point of view and a model based on the Si-Ge-Au ternary phase diagram has been developed to predict the SiᵪGe₁₋ᵪ concentration gradient in the nanowire base. The fully relaxed in-plane Ge nanowires occur within one of the four distinct in-plane ⟨110⟩ directions and nanowires are mainly bounded by two 55° inclined {111} facets and a less pronounced planar (001) top facet. High-resolution scanning X-ray diffraction microscopy reveals a slightly tilted growth of individual nanowires with respect to each other, causing an abrupt change in the orientation at junction points of interconnected nanowires.}, subject = {MBE; Silicon; Germanium; Nanowires; Epitaxy; MBE; Silizium; Germanium; Nanodr{\"a}hte; Epitaxie; Nanodraht; Molekularstrahlepitaxie; Germanium; Silicium; Tropfengr{\"o}ßebestimmung}, language = {en} } @phdthesis{Grzela2015, author = {Grzela, Tomasz}, title = {Comparative STM-based study of thermal evolution of Co and Ni germanide nanostructures on Ge(001)}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-36234}, school = {BTU Cottbus - Senftenberg}, year = {2015}, abstract = {Since 1947, when Bardeen and Brattain initiated the era of microelectronics by constructing the first Germanium (Ge) transistor, semiconductors have become the main material platform for advanced integrated circuit (IC) technologies. Later on, given in particular the electrical stability of its native oxide, IC technology shifted from Ge to Silicon (Si) substrates and the dominance of Si-based complementary metal oxide semiconductor (CMOS) microelectronics is today unquestionable. However, as the semiconductor industry is approaching the limits of traditional Si CMOS scaling, the integration of new materials into Si micro- and nano-electronics is required to extend the performance and functionality of future CMOS-based IC technologies. Recently, Ge due to its superior optoelectronic properties and compatibility with conventional Si CMOS technology has re-emerged as an alternative semiconductor material on the mainstream Si technology platform. Many of the Ge integration challenges, such as e.g. doping, epitaxial quality etc., have been recently solved or minimized to an acceptable level. However, the fabrication of low resistance, thermally stable metal/Ge contacts is still one of the main barriers towards the full use of the potential offered by Ge. In particular, the formation of ohmic contacts is relevant for applications where high current densities are of importance (i.p. Ge p-MOSFET and Ge laser applications). Consequently, intensive investigations of metal/Ge contacts are imperative for future applications of Ge. Various metal/Ge contact systems were studied and demonstrated good thermal stability and promising electrical properties. However, given their widespread use in Si CMOS technologies in form of their respective silicides, Co- and Ni-germanides seem to be an obvious choice for electrical contacts in Ge-based devices. Both metal/Ge systems exhibit a complex bulk phase diagrams with a wide range of different physical properties. It is generally acknowledged that the stoichiometric CoGe2 and NiGe phases are best suited for ohmic metal contact formation, mainly due to their low resistivity. It is worth noting that the bulk phase diagram is limited in its use for nanoscience due to an increased surface/volume ratio as well as by the strong nanostructure/substrate interface influence. This PhD thesis sheds light on the formation process at the atomic level of Co and Ni germanide nanostructures on clean, reconstructed Ge(001) substrates. The main part of the presented research is based on in-situ scanning tunneling microscopy (STM) studies on the influence of subsequent, post-evaporation annealings at various temperatures in order to follow and investigate on the nano-scale the structural evolution of a few monolayers of Co and Ni metal (deposited at RT and in UHV conditions) on an atomically clean, reconstructed Ge(001) surface. Furthermore, additional techniques like LEED, (S)TEM-EDX and XPS were used to corroborate and complement the STM derived insights. It was demonstrated that - for both investigated systems - room temperature deposition of a few metal monolayers on clean Ge(001) results in a Volmer Weber growth mode. Starting with annealing treatments at relatively low temperature ranges, the formation of a continuous MetalxGey wetting layer from as-deposited 3D metal clusters on Ge(001) was detected. It should be noted that a very flat wetting layer was observed for the Co/Ge(001) system, which is different for the Ni/Ge(001) system where inhomogeneous terraced domains were formed. Finally, the 2D wetting layer gradually evolves with increasing temperature into well-ordered 3D MetalxGey nanostructures, surrounded by clean, reconstructed Ge(001). Analysis of these Co and Ni germanide nanostructures shows that the growth mechanism is different: in particular the Ni/Ge system is more reactive by means of Ni bulk diffusion and results in 3D Ni germanide nanostructures which show a strong tendency to be embedded into the Ge(001) substrate. In contrast, Co germanide nanostructures are situated initially on top of the Ge(001) substrate due to the fact that Ge diffusion dominates in the low temperature range. Only at higher annealing temperatures, Co diffusion into the bulk occurs and Co germanide nanostructures penetrate into the Ge substrate. For the Co- as well as Ni-Germanide system, the nanostructures undergo Ostwald ripening phenomena in the high temperature range. The present PhD thesis thus allows to understand on the nano-scale the main growth and reaction mechanisms of the Walser and Ben{\`e} rule set up about 40 years ago to describe metal/semiconductor interface reaction on the macro-scale.}, subject = {Scanning Tunneling Microscope; STM; Germanium; Cobalt germanides; Nickel germanides; Rastertunnelmikroskop; Nickel Germanide; Kobalt Germanide; Intermetallische Verbindungen; Schichtwachstum; Nanostruktur; Germanide; Rastertunnelmikroskop; Halbleiter}, language = {en} }