Highly sensitive hexagonal boron nitride thermal neutron detectors

Hexagonal boron nitride (h-BN) has emerged as an alternative solid-sate material for thermal neutron detection. Large cross-section of 3480 barns for thermal neutrons for boron-10 (10B) isotope coupled with high density of 10B atoms in h-BN material provides very short absorption length (~47.3 µm) for thermal neutrons (with a kinetic energy ~25 meV). Moreover, in an h-BN detector, absorption of neutrons, charge carrier generation, and charge transport all occur in the same layer, which potentially could enable a detection efficiency of 100%. Due to its wide bandgap ~ 6.4 eV and hence high electrical resistivity, h-BN possesses superior charge transport properties with the potential to provide excellent charge collection efficiencies. Though earlier research works have demonstrated the excellent prospects of h-BN for neutron detectors, the detection efficiency and sensitivity were limited by the thin materials of thicknesses < 5 µm. This dissertation describes the development of high efficiency and high sensitivity solid-state thermal neutron detectors based on 10B-enriched freestanding h-BN (h-10BN) epilayers with large thicknesses, with the ultimate aim to replace the helium-3 (3He) gas-filled detectors, which are currently in use for radiation portal monitoring, geothermal, and well-logging applications. This dissertation discusses how to overcome the challenges involved in realizing highly efficient h-BN thermal neutron detectors and strategies to further scale up the detector size. By utilizing a vertical detector geometry, we fabricated detectors from 43 µm thick h-10BN with an area 1 mm2 with a record high thermal neutron detection efficiency at 51.4%. However, it was soon realized that increasing the detector size while maintaining the high detection efficiency of the h-10BN detectors will be critical for practical applications. In the process of scaling up the detector size, we were able to identify key physical parameters, which determine the performance of h-10BN detectors. Among these, increased detection electronic noise due to increased dark current and capacitance with an increase in the detector size was identified to be a critical issue. Initially, through the improvements in the overall material quality, i.e. increased electrical resistivity and carrier mobility-lifetime product helped to reduce the dark current and subsequent noise level. Hence, we were able to increase the sensitivity of h-10BN vertical detectors by a factor of 9 times by successfully fabricating a detector with a detection area of 9 mm2 and efficiency of  ~ 53%. Furthermore, the optimization of the vertical device structures was performed in terms of radiation direction, metal contact type, and bias voltage polarity. This optimization processes pushed the detection efficiency to 58% for 1 mm2 detectors. It was recognized that during high temperature growth, oxygen impurities diffused into h-BN from sapphire substrate. Further analysis suggested that the charge collection efficiency in vertical detectors was primarily limited by surface recombination of charge carriers due to presence of surface traps. These findings paved the way for the implementation of lateral (or horizontal) detector design which overcomes all three constraining parameters for scaling up the detector size: increased high dark current, capacitance, and surface recombination field with increasing detector size while simultaneously benefits from the superior lateral transport properties of layer structured h-BN. The new lateral device architecture enabled us to achieve detectors with an area of 100 mm2 and thermal neutron detection efficiency of 59%, providing an enhancement in the detection sensitivity by more than two orders of magnitude over those of 1 mm2 vertical detectors. The results of this dissertation also provided useful insights into strategies for further improving the material growth and device processes and thereby contributed to the further advancement of h-BN neutron detectors for practical uses.