Solid-state thermal neutron detectors with improved detection efficiencies are highly sought after for many applications. Hexagonal boron nitride (hBN) epilayers have been synthesized by metal organic chemical vapor deposition on sapphire substrates. Important material parameters including the mobility-lifetime (μτ) product and the thermal neutron absorption length (λ) have been measured. For hBN epilayers with a room temperature resistivity of 5.3 Â 10 10 Ω cm, the measured μτ product of electrons is 4.46 Â 10 À 8 cm 2 /V and of holes is 7.07 Â 10 À 9 cm 2 /V. The measured λ values are 277 μm and 77 μm for natural and 10 B enriched hBN epilayers, respectively. Metal-semiconductor-metal detectors incorporating 0.3 mm thick hBN epilayers were fabricated. The reaction product pulse-height spectra were measured under thermal neutron irradiation produced by a 252 Cf source moderated by high density polyethylene block. The measured pulse-height spectra revealed distinguishable peaks corresponding to the product energies of 10 B and neutron reaction with the 0.84 MeV 7 Li peak being the most prominent. The detectors exhibited negligible responses to gamma rays produced by 137 Cs decay. Our results indicate that hBN epilayers are highly promising for realizing highly sensitive solid-state thermal neutron detectors with expected advantages resulting from semiconductor technologies, including compact size, light weight, ability to integrate with other functional devices, and low cost.

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Photoluminescence emission spectroscopy and electrical transport measurements have been employed to study the origin and roles of oxygen impurities in hexagonal boron nitride (h-BN) epilayers grown on sapphire substrates. The temperature dependence of the electrical resistivity revealed the presence of a previously unnoticed impurity level of about 0.6 eV in h-BN epilayers grown at high temperatures. The results suggested that in addition to the common nitrogen vacancy (V N) shallow donors in h-BN, oxygen impurities diffused from sapphire substrates during high temperature growth also act as substitutional donors (O N). The presence of O N gives rise to an additional emission peak in the photoluminescence spectrum, corresponding to a donoracceptor pair recombination involving the O N donor and the C N (carbon occupying nitrogen site) deep level acceptor. Moreover, due to the presence of O N donors, the majority charge carrier type changed to electrons in epilayers grown at high temperatures, in contrast to typical h-BN epilayers which naturally exhibit "p-type" character. The results provided a more coherent picture for common impurities/defects in h-BN as well as a better understanding of the growth mediated impurities in h-BN epilayers, which will be helpful for finding possible ways to further improve the quality and purity of this emerging material.

We report the realization of 1 cm2 hexagonal boron nitride (h-BN) thermal neutron detectors with an unprecedented detection efficiency of 59%. This was achieved through improvements in material quality, as reflected in a sixfold enhancement in the electron mobility and lifetime product and a threefold reduction in the surface recombination field, which resulted in a higher detection efficiency at a lower applied electric field over that of a previous state-of-the-art lateral detector with a detection area of 30 mm2. The attainment of 1 cm2 h-BN neutron detectors capable of retaining a high detection efficiency represents a significant milestone toward the practical applications of h-BN detectors.

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