Novel Manufacturing Methods and Mechanical Properties of Semiconductor Nanowires

Sun, Zhiyuan

doi:https://doi.org/10.21985/n2-wdhv-4e30

Work

Novel Manufacturing Methods and Mechanical Properties of Semiconductor Nanowires

Public

Download PDF

Download All Files (.zip)

Semiconductor nanowires, such as group IV and III-V nanowires, shows distinct electrical, optical and mechanical properties from their bulk counterparts due to their nanoscale size and 1-D morphology. For example, the quantum confinement effect modulates the band gap of a semiconductor nanowire when its diameter approaches or below the exciton Bohr radius. Nanowires also exhibit higher flexibility and strength because of their high aspect ratio and low defects density, which make them ideal building blocks for flexible electronics. However, despite their advantages over bulk materials, there are several challenges of utilizing nanowires for various applications. First, the traditional process technologies developed for bulk materials cannot be used on nanowires directly due to their special shape and small size. Second, the bottom-up assembly method cannot fabricate nanomaterials in a well-controlled manner like the traditional top-down method for bulk materials. Third, the reliability issues and failure modes of nanowire-based flexible electronics have rarely been studied. This thesis describes researches that shed light on solving those three challenges in nanoscience and nanotechnology. The research topics described in this thesis include 1) modulate the doping profile of the nanowire using ex-situ and in-situ methods, 2) assembly of a highly ordered metal nanoparticle array within nanowire via a dewetting method, 3) the failure mode of Si and GaAs nanowire upon bending. Dopants play a critical role in modulating the electric properties of semiconducting nanowires. The application of traditional doping methods developed for bulk materials involves additional considerations for nanoscale semiconductors because of the influence of surfaces and stochastic fluctuations, which may become significant at the nanometer-scale level. For example, placing ten phosphorus atoms into a 10-nm silicon cube yields a dopant concentration of 10^19 cm^-3, which is considered to be a high doping level and thus influences dramatically the electronic properties. In this thesis, I investigated several novel dopant control methods for semiconductor nanowires, especially silicon nanowires as silicon is still the predominate materials for the semiconductor industry, including monolayer contact doping (MLCD) and vapor-liquid-solid in-situ doping. Besides doping control, doping measurement is another challenge due to dopant’s extremely small concentration within limited spaces, which requires both high chemical resolution (serval atomic-ppm) and spatial resolution (at least 1 nm). Here we use atom-probe tomography (APT), which detects millions of atoms within nanomaterials one-by-one, combined with other technologies such as scanning tunneling microscopy (STM), transmission electron microscopy (TEM), and Raman spectroscopy to measure the dopant distributions and further explore the dopant diffusion kinetics, and thermodynamics. Metal nanoparticle arrays are excellent candidates for a variety of applications due to the versatility of their morphology and structure at the nanoscale. Bottom-up self-assembly of metal nanoparticles provides an important complementary alternative to the traditional top-down lithography method and makes it possible to assemble structures with higher-order complexity, for example, nanospheres, nanocubes, and core-shell nanostructures. Here we present a mechanism study of the self-assembly process of 1-D noble metal nanoparticles arrays, composed of Au, Ag, and AuAg alloy nanoparticles. These are prepared within an encapsulated germanium nanowire, obtained by oxidation of a metal-germanium nanowire hybrid-structure. The resulting structure is a 1-D array of equidistant metal nanoparticles with the same diameter, the so-called nanobead (NB) array structure. Atom-probe tomography and transmission electron microscopy were utilized to investigate the details of the morphological and chemical evolution during the oxidation of the encapsulated metal-germanium nanowire hybrid-structures. The self-assembly of nanoparticles relies on the formation of a metal-germanium liquid alloy, and the migration of the liquid alloy into the nanowire, followed by dewetting of the liquid during shape-confined oxidation where the liquid column breaks-up into nanoparticles due to the Plateau-Rayleigh instability. Our results demonstrate that the encapsulating oxide layer serves as a structural scaffold, retaining the overall shape during the eutectic liquid formation, and demonstrates the relationship between the oxide mechanical properties and the final structural characteristics of the 1-D arrays. The mechanistic details revealed here provide a versatile tool-box for the bottom-up fabrication of 1-D arrays nano-patterning that can be modified for multiple applications according to the RedOx properties of the material system components. Strain is an important engineering degree of freedom in semiconductors, which is used to modulate carrier mobility, tune the energy bandgap, and drive growth of self-assembled nanostructures. Understanding strain energy relaxation mechanisms including phase transformations, dislocation generation and motion, and cracking is essential to both exploit this degree of freedom and avoid degradation of electronic properties including carrier lifetime and mobility, particularly in pre-strained electronic devices174 and flexible electronics that undergo large changes in strain during operation. Raman spectroscopy and transmission electron microscopy and diffraction were utilized to identify strain-energy release mechanisms of bent diamond-cubic silicon and zinc-blende GaAs nanowires, which were elastically strained to > 6% at room temperature and then annealed at an elevated temperature to activate relaxation mechanisms. High-temperature annealing of bent Si nanowires leads to the nucleation, glide, and climb of dislocations, which align themselves to form grain boundaries that reduce the strain energy. For the first time, silicon nanowires are observed to undergo polygonization, which is the formation of polygonal-shaped grains separated by grain-boundaries consisting of aligned edge dislocations. In contrast, GaAs nanowires release strain-energy by forming nanocracks in regions of tensile strain due to the weakening of As-bonds. These new insights into relaxation of highly strained perfect crystals can inform the design of novel nanoelectronic devices and provide guidance on mitigating degradation.

Creator