Metal-templated crystallization of germanium for optoelectronic applications [electronic resource]
Keywords: Applied Physics.

Crystalline germanium (Ge) structures, such as nanowires (NWs) and thin films, have been investigated intensively in recent years due to their unique properties emerging from germanium's large absorption coefficient, high carrier mobilities, lattice match with photovoltaic material GaAs, and compatibility with standard silicon-processing technology. The dynamics of electrons, photons, and phonons in crystalline Ge strongly depend on the geometrical factors of the different device structures in which it is fabricated. A better understanding of Ge growth mechanisms in different nano- and micro-structures, and their connection to fundamental optical and electronic properties is essential in order to better exploit Ge in the design of nanoscale optoelectronic devices. However, such investigations in semiconductor NWs and thin films are limited and have mainly focused on a small set of crystal growth processes. This thesis focuses on the investigations of both the synthesis and properties of two types of structures, NWs and poly-crystalline thin films. In order to achieve large-scale arrays of relatively defect-free vertically aligned NWs, it is essential to understand the spontaneous kinking during growth. In Chapter 2, two fundamental mechanisms underlying the spontaneous kinking of Ge NWs during vapor-liquid-solid (VLS) growth will be discussed. The diameter of NWs, sidewall facets of NWs, and the capillary stability of the Au-Ge catalyst droplet play important roles in spontaneous kinking. 3-D phase field model simulations by our collaborators are combined with experimental results to confirm the kinking mechanisms. The high electron and hole mobilities and high optical absorption in visible and IR wavelengths of Ge makes it a promising candidate for ultrafast optoelectronic device applications. There are few investigations of arrays of Ge NWs in this context, and most such studies have focused on a qualitative analysis of relevant phenomena. Chapter 3-5 will present a study of ultrafast optical, acoustic and electronic properties of vertically aligned < 111> Ge NW arrays through ultrafast, optical pump-probe transient absorption measurements on dense arrays of single-crystal and relatively uniform-diameter Ge NWs. Two coexisting physical phenomena governing the spectral and temporal dependence of the detected probe signal will be discussed. In Chapter 4, ultrafast dynamics of electrons and holes, especially their strong dependence on NW diameters and photoexcitation powers, are investigated. The different interaction of electrons and holes with surface states of Ge NWs will be demonstrated, thereby leading to different methods to extend electron and hole lifetime in such nanostructures. Chapter 6 will first introduce a method to produce poly-crystalline Ge (poly-Ge) thin films via low-temperature Al-induced-crystallization. Electron backscatter diffraction (EBSD), Hall mobility, and photoluminescence (PL) measurements of the poly-Ge films and poly-GaAs films epitaxially deposited on these poly-Ge templates indicate that templates with relatively large Ge crystallites (up to 124 um2) are a low-cost alternative to single-crystal Ge wafers, albeit exhibiting somewhat reduced performance. The poly-Ge templates can function well for seeding epitaxial growth of overlying Ge nanostructures or GaAs thin film absorbers for application in photodetectors or solar cells.