Development of millimeter-thick hexagonal boron nitride wafers and fast neutron detectors

We report the attainment of millimeter-thick neutron detectors fabricated from quasi-bulk hexagonal boron nitride (h-BN) produced by halide vapor phase epitaxy (HVPE). Detection efficiencies of 0.7% and 0.5% in response to neutrons emitted from bare AmBe and Cf-252 sources, respectively, have been achieved, corresponding to a charge collection efficiency of about 38%. These results mark a significant improvement over our previous single-stack h-BN detectors, which were 90 μm thick and exhibited a detection efficiency of 0.1%. This enhancement is primarily attributed to the increased thickness of the h-BN layer, leading to a higher intrinsic detection efficiency. We also observed that the carrier mobility-lifetime (μτ) product increases as layers of h-BN are successively removed from the top by polishing, indicating that a degradation in h-BN's electronic properties with thickness is now a major limiting factor for achieving high charge collection efficiency. This finding highlights the need for further refinement in HVPE growth processes to produce h-BN wafers with both larger thicknesses and improved electronic properties. Nevertheless, the fabrication of millimeter-thick single-stack h-BN neutron detectors represents a major milestone in the application of h-BN for fast neutron detection.