CONTROLLED GROWTH AND ORIENTATION IN ALUMINUM-MEDIATED CRYSTALLIZATION OF SILICON NANOWIRES AND THIN FILMS
Open Access
- Author:
- Hainey, Mel Forrest
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 29, 2017
- Committee Members:
- Dr. Joan Redwing, Dissertation Advisor/Co-Advisor
Dr. Joan Redwing, Committee Chair/Co-Chair
Dr. Roman Engel-Herbert, Committee Member
Dr. Suzanne Mohney, Committee Member
Dr. Jeffery Brownson, Outside Member - Keywords:
- Silicon
Aluminum
Nanowire
Thin Film
Chemical Vapor Deposition
Aluminum-induced Crystallization - Abstract:
- Metal-mediated silicon crystallization has received extensive study as a means to form silicon wires and thin films for electronic and photovoltaic applications. Typical metals used in these processes, such as gold, silver, nickel, and copper, are expensive and act as deep-level traps in silicon, making incorporation into the final silicon crystal undesirable. In contrast, aluminum is earth-abundant and acts as a p-type dopant in silicon, and the low Al-Si eutectic temperature (577°C) enables silicon crystallization under conditions compatible with a wide variety of substrates. In this thesis, aluminum-mediated silicon nanowire and thin film growth is investigated, and the effects of growth parameters such as temperature, pressure, and substrate surface energy on nanowire orientation and morphology and thin film morphology are investigated. In particular, through controlled growth temperature, reactor pressure, and silane partial pressure, growth on aluminum-catalyzed silicon nanowires in high energy growth directions such as <110> and <100> can be realized. Wires grown in high-energy growth directions have unique morphologies that suggest a different growth mechanism than the standard vapor-liquid-solid nanowire growth mechanism. Because these wires are grown in a region with partially depleted silane concentrations, this regime is described as silane-depleted vapor-liquid-solid growth. Along with promoting growth in high energy growth directions, reactor temperature and pressure can be used to change the shape of <111> wires to pyramids. These pyramids have improved anti-reflective properties compared to vertical nanowire arrays, enabling black silicon textures to be grown on silicon substrates. Because the wires and pyramids are p-type, growth on n-type substrates enables black silicon solar cells to be fabricated in a process that combines texturing and junction formation into a single step. Aluminum-induced crystallization of silicon thin films offers a unique method for producing highly (111) oriented polycrystalline thin films on amorphous substrates. Al and a-Si are deposited on glass or other substrates, and then annealed below the Al-Si eutectic temperature. For film thicknesses below 50nm, the a-Si diffuses through the Al film and nucleates at the Al/substrate interface. By using plasma surface treatments to change the surface energies of the fused quartz substrates, silicon crystallization rates and grain sizes can be manipulated. Furthermore, by combining multiple surface treatments on a single substrate, preferential crystallization at the low-energy interface can be realized, allowing for the formation of patterned AIC-films from uniform, continuous initial a-Si and Al layers. Finally, along with pattern formation, these AIC-Si films are able to act as seed layers for III-nitride semiconductor growth on fused quartz and other substrates. Through use of an AlN buffer layer, highly c-axis oriented GaN films can be grown using metalorganic chemical vapor deposition on AIC-Si substrates. Post growth characterization indicates that the GaN films follow the template provided by the AIC-Si films, with uniform surface normal orientation and random in-plane orientation. Defect analysis suggests that threading dislocation densities within grains are comparable to GaN grown on bulk Si (111) substrates. Additional studies have extended the GaN on AIC-Si process to other substrates, including oxidized Si (001) and polycrystalline diamond, with GaN films showing similar morphologies to those grown on AIC-Si on fused quartz. Overall, this thesis demonstrates how aluminum-mediated crystallization of silicon can be used to fabricate thin films and nanowires with a variety of orientations and morphologies. Furthermore, the initial demonstration of these wires and thin films in photovoltaic and electronic applications is also presented. Ultimately, aluminum-mediated silicon crystallization is demonstrated to be a flexible, controllable approach for producing a variety of technologically relevant nanowires and thin films.