PhD Oral Exam - Marc Collette, Physics
When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
We built an atomically engineered laboratory inside a silicon nanowire (SiNW) to study fundamental transport mechanics and correlate results with crystal structure. We quantify the effects of ordered stacking faults (OSFs) present in SiNWs on their electrical transport capabilities.
We use Raman spectroscopy to characterize the hexagonal-phase core structure of the Si crystal in our novel nanowires caused by the OSFs.
Our results indicate that electrical current is prevented from flowing within the hexagonal-phase core. Using OSFs to tune crystal structure in SiNWs can be used to control the effective cross-section of the nanowire without the need to change its physical dimensions.
We find that the channel conductivity of field-effect transistors formed using these nanowires is decreased substantially compared to the familiar cubic phase counterpart (from roughly 100 to 1 μS/cm). This result indicates that modulating crystal phase can be effective in tuning material conductivity, offering an additional degree of freedom in device engineering.
We also show that hexagonal-core SiNWs have larger effective Schottky barriers with gold electrode contacts (from 0.48 to 0.67 eV), which increases device contact resistance.
Having a cubic-phase portion and a hexagonal-phase portion in series within a single kinked SiNW exploits this barrier asymmetry to create excellent gate-controlled and temperature-dependent rectifiers with rectifying ratios exceeding 100. Our transport model explains how the kink region also acts as a 10-nm scale diode.
These results indicate that controlling OSF density could be exploited in new device architectures and help optimize SiNWs for applications in high-impedance Schottky barrier rectifying transistors.