Advancements in Hexagonal Boron Nitride for Thermal Neutron Detection Applications

Hexagonal boron nitride (h-BN) possesses many unique physical properties which combined have made h-BN an incredibly attractive material for a vast array of technologically significant applications. Some of these properties include extremely wide bandgap, strong optical absorption of above bandgap photons, a layered structure with a close lattice match to graphite, and critical to this research effort, a large thermal neutron capture cross section when 100% isotopically enriched with 10B. However, as with all emerging materials, many important physical properties have yet to be fully explored and documented. Discussed in this dissertation are major achievements in the development of 10B enriched h-BN (h-10BN) based thermal neutron detectors and the exploration of the physical properties of this important material. This work is divided into three main themes 1) detector fabrication and testing, 2) exploration of the electrical transport properties of h-BN, and 3) probing of the defects and impurities of h-BN. As for any material under development, it is necessary to establish device processing techniques to realize active devices based on h-BN. Advancements in dry etching techniques for devices based on h-BN epilayers via inductively coupled plasma (ICP) were investigated and reported. Neutron detection electronics were optimized to maximize compatibility with this highly resistive material and a Californium-252 based thermal neutron source for in house testing was designed and fabricated. The electrical transport properties of h-BN are difficult to measure due to the extreme nature of this material’s resistivity and bandgap. For the first time, the room temperature carrier drift mobilities (µ) and lifetimes (τ) of both electrons and holes in h-10BN epilayers were probed using a modified time-of-flight (TOF) technique. The values for surface recombination velocity (S) for both electrons and holes in h-10BN were also extracted. Photoluminescence (PL) emission spectroscopy, electrical transport measurements, and photocurrent excitation spectroscopy were employed to study the origin and roles of defects and impurities in h-BN epilayers. Photoluminescence (PL) emission spectroscopy results suggest that oxygen impurities diffused from sapphire substrates during high temperature growth act as substitutional donors (ON). The observed energy thresholds present in the photocurrent excitation spectra indicated that oxygen impurities occupying nitrogen sites (ON) and boron vacancy - hydrogen complexes (VB-H) are, respectively, the dominant donor and acceptor impurities in h-BN epilayers. These impurities determine overwhelmingly the transport properties and charge collection efficiency of these h-BN epilayers. Combined, the presented results in this dissertation represent major achievements towards the realization of practical h-BN based thermal neutron detectors and provide important insights into strategies for further improving the material quality of this emerging ultrawide bandgap semiconductor.

Name thesis:degree_name

Doctor of Philosophy

Grantor

Texas Tech University

Year dc:date.issued

2020

Author dc:creator

• Grenadier, Samuel Joseph

Advisors dc:contributor.advisor

• Jiang, Hongxing
• Lin, Jingyu

Committee member dc:contributor.committeemember

• Li, Jing

Handle dc:identifier.uri

https://hdl.handle.net/2346/104606

OAI identifier oai:identifier

oai:ttu-ir.tdl.org:2346/104606