Lateral Epitaxial Growth of Ge Films on Si Via a Vapor-liquid-solid Mechanism PDF Download

Author: Weizhen Wang
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Languages : en
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Book Description
"In recent years, there has been an increasing interest in developing alternative semiconductor materials since traditional Si-based devices have approached their physical limits. Given that Si has many advantages over other materials as substrate, the use of Si wafer is expected to continue. It is therefore important to grow high-quality semiconductor materials, e.g., thin films or nanowires, epitaxially on Si. The past thirty years have seen rapid advances in the field of heteroepitaxial Ge growth on Si because Ge-based devices can add functionality to Si chips and may be the key to enable next-generation computer systems or solar cells. The direct growth of Ge on Si, however, suffers from high-density threading dislocations that are formed during the growth process; these defects act as scattering and recombination centers that degrade the device performance. In this thesis, a novel growth approach, metal-catalyzed, lateral epitaxial growth, is demonstrated to grow Ge films on Si with reduced threading dislocation density (TDD). In contrast to the traditional blanketed film growth on Si, this technique starts with a small crystal nucleation at a specific position on Si followed by a Ge lateral growth in the horizontal direction. It has been hypothesized that during the lateral growth process the lattice mismatch between Si and Ge can be accommodated by the extension of the preexisting misfit dislocations from the initial growth region or the nucleation of dislocations from the film sidewalls instead of generating additional threading dislocations from the film surface. In this thesis, SEM and TEM were two primary characterization techniques to study the film morphologies, the growth process, and the relaxation mechanisms. One important finding was that the first nucleation areas of the films often have a higher Si concentration or may be thinner compared to the lateral overgrowth areas. This is important because such differences made it possible to figure out where the first growth occurred in electron microscopy. From here we could compare the dislocation morphologies in different areas. In plan-view and cross-sectional TEM micrographs, high-density threading dislocations were found to be present in the initial growth areas while the lateral overgrowth areas demonstrated substantially reduced TDD or even defect-free areas. Moreover, the XRD results showed that the Ge films were almost fully relaxed with little Si incorporation. Given that the growth occurred at a low temperature, 375-500 °C, we suggest the presence of new relaxation mechanism since the previous mechanism, dislocation nucleation and glide, would require a much higher temperature to fully relax the lattice mismatch strain. Therefore, we hypothesize that the strain induced by the lattice mismatch can be relaxed by extending the preexisting misfit dislocations and that lateral growth can "build in" dislocations as it grows. Furthermore, this thesis proposes a simple model to describe the relation between the catalyst (Au) area and the area of the film in plan-view SEM; a film's size can thus be estimated by the size and shape of the Au catalyst. It has been found that a large Au catalyst can absorb more Ge due to its larger vapor-liquid interface, but more Ge is required to saturate the Au. This thesis also presents a study of the optimum growth condition of Ge lateral growth. The results show that high temperature and high GeH4 partial pressure can boost the growth rate and thus increase the film size within a specified period of time. However, high growth rate can cause more uncatalyzed, vapor-solid Ge growth on Si as well. A relatively lower growth rate and a longer growth time are thus required to simultaneously increase the size of the films and reduce the uncatalyzed growth." --