OSE Dissertation Defense with Mr. Mahmoud Behzadirad, Ph.D. Candidate
Posted: November 27, 2018
Date: Friday, November 30, 2018
Time: 1:00 PM
Location: CHTM, Room 103
Map to CHTM:
Parking passes are available at the receptionist desk. The CHTM building is equipped with ADA accommodations as well as parking spaces.
GaN nanowires are promising for optical and optoelectronic applications because of their waveguiding properties and large optical bandgap. Recent researches have shown superior mechanical properties of GaN nanowires which promises their use in new research areas e.g. nanometrology. In this work, we develop a scalable two-step top-down approach using interferometric lithography as well a bottom up growth on NWs using MOCVD, to manufacture highly-ordered arrays of nanowires with atomic quality and desired aspect-ratios to be used in nano-lasers and atomic precision metrology and lithography. Using this method, uniform nanowire arrays were achieved over a large-area (~1 mm2) with aspect-ratio as large as 50, radius as small as 17 nm, and atomic-scale sidewall roughness. The mechanisms involved in the wet-etch process are thoroughly investigated and FDTD modeling is employed to study modal properties of the fabricated GaN NWs. It is shown that HE11 is the dominant transverse mode in the nanowires with radius sub-100 nm. Single-mode lasing with high Q-factors of ~1139-2443 were obtained in nanowire array lasers, corresponding to a linewidth of 0.32-0.15 nm. We also demonstrate the preliminary result of including nanoporous AlxGa1-xN DBRs in the GaN nanowire structures grown on Si, not only to enhance modal reflectivity, but also to alleviate accumulating tensile stress due to the lattice and thermal expansion constants mismatch between overgrown GaN and Si.
Top-down fabricated GaN NWs and sharp bottom-up grown GaN NWs were used to fabricate AFM and STM tips for nanometrology. The advantages of these tips are to assist nanometrology in scanning high-aspect-ratio structures, provide cost-effective and durable tips for scanning probe lithography for the creation of sub-10 nm features, and enabling tip-based nanometrology with Raman spectroscopy. GaN NWs as a new material for AFM tips, enhanced image resolution in scanning high-aspect-ratio structures with straight sidewalls. Using sharp GaN NWs as STM tips, we could demonstrate atomic resolution imaging and sub-10 nm lithography with a high stability over large-area scanning tests. Employing sharp GaN NWs in active AFM cantilevers demonstrated improvement in the image resolution compared to the standard Si-tips and accomplished ~10 nm line-width structures under applied bias in field emission lithography.