Self-positioned And Homo/hetero-epitaxy In-plane Growth Of Silicon Nanowires

Self-assembly silicon nanowires (SiNWs) are becoming populous building blocks for developing a new generation of electronics and optoelectronics, where precise position or orientation control of the self-assembled SiNWs, ideally in a convenient planar architecture, is a prerequisite. In the last decade, research efforts have been devoted to transferring self-assembly SiNWs grown via a vapor-liquid-solid (VLS) process upon planar substrate, with the aid of post-growth manipulation, elaborated nano holes or channels templates. In order to define the growth routine of self-assembly SiNWs, a straightforward strategy is to direct the movement of the metal catalyst droplets that catalyze or mediate the growth of SiNWs. Precise alignment and positioning of such VLS-grown SiNWs relies heavily on the use of pre-defined alumina nano porous membrane, electro-beam lithography defined guiding channels or other post-growth manipulations, which are mostly by themselves complicated fabrication procedures or involved costly nano-operations.In the meantime, a controlled in-plane growth of SiNWs can bring in important opportunity to deploy and integrate the self-assembled SiNW functionalities. Recently, we discovered that the use of a thin film of hydrogenated amorphous Si (a-Si:H) deposited on substrate surface can be adopted as a solid precursor medium, which enables us to confine naturally the growth of SiNWs into an in-plane or on-surface growth mode, mediated by surface-running indium catalyst droplets. This feature can help to dictate the growth course of the in-plane SiNWs by very simple and convenient surface patterning that requires no high resolution lithography. The purpose of this work is thus to address the fundamental aspects of the step-edge guided growth behavior, epitaxy growth on silicon wafer and sapphire substrate and then we will examine these growth control principles against direct experimental observations, and explore them to demonstrate a series of interesting operation of the in-plane SiNWs.The main results and innovations of the thesis are shown below1. Guided growth of SiNWs based on an in-plane solid-liquid-solid mechanism are activated to grow and be guided into predefined patterns by effective controlling the movement of the catalyst droplets. The guided growth ratio of SiNWs is about 98%. Meanwhile the growth of guided SiNWs could be achieved in all kind of curved step-edge strategy. These results provide a design principle for future SiNWs-based nanodevices.2. The SiNWs alignment were used to fabricate field effect transistors in a simple bottom-gate or top-gate configuration, and demonstrate an on/off ratio>106 and hole mobility of>200 cm2/Vs. These results lay an important basis for direct integration of in-plane SiNW FETs for high performance planar display and flexible electronics.3. A new and interesting phenomenon of epitaxy growth of the in-plane SiNWs upon Si(100) wafer, emphasizing the capability to achieve ultra-long self-aligned SiNWs following the crystallographic orientations of underlying Si substrate without the need of any pre-surface patterning. We show that a rich set of growth dynamics for epitaxy growth of in-plane SiNWs with different morphologies and growth modes can be triggered by variant growth balance conditions. Conductive atomic force microscopy (CAFM) characterization was also used to reveal a rectified transport between the epitaxial SiNWs (with intrinsic p-type doping) and the n-type c-Si substrate, indicating a well-defined p-n junction formation. This epitaxy growth of in-plane SiNWs provides an interesting testing-bed to understand and engineer nanoscale epitaxial growth dynamics, junction formation and doping profiles for developing novel nanoelectronics.4. We demonstrate that a tiny catalyst droplet, running on R-plane sapphire substrate, can be exploited to deposit/or write-down aligned matrix of SiNWs, along the major crystallographic orientations in the substrate due to the formation of a coherent heteroepitaxial interface between c-Si/sapphire. More interestingly, we show that the heteroepitaxial growth is determined by a nucleation competition and interplay between two nucleation interfaces, that is, the catalyst/SiNW interface in the axial direction and the catalyst/sapphire interface on the substrate, and the top layer of sapphire R(1-102) plane must be rearranged and the top layer aluminum could reconfigure bonding to oxygen with unitary strength and similar periodicities as those of the R plane. The heteroepitaxy growth can be driven into two distinct SP(-1101) and SP(2-110) orientations, with the least lattice mismatch between c-Si and the exposed surface of R-plane sapphire substrate. This orderly arrangement of SiNWs-on-insulating heteroepitaxy opens up a new and convenient strategy for circuitry design, devising and integration of SiNW-based nanoelectronics.