Deep ultraviolet photoluminescence (PL) emission spectroscopy has been employed to investigate the origin of the widely observed deep level impurity related donor-acceptor pair (DAP) transition with an emission peak near 4.1 eV in hexagonal boron nitride (h-BN). A set of h-BN epilayers were grown by metal-organic chemical vapor deposition (MOCVD) under different ammonia (NH 3) flow rates to explore the role of nitrogen vacancies (V N) in the deep-level transitions. The emission intensity of the DAP transition near 4.1 eV was found to decrease exponentially with an increase of the NH 3 flow rate employed during the MOCVD growth, implying that impurities involved are V N. The temperature-dependent PL spectra were measured from 10 K up to 800 K, which provided activation energies of $0.1 eV for the shallow impurity. Based on the measured energy level of the shallow impurity ($0.1 eV) and previously estimated bandgap value of about 6.5 eV for h-BN, we deduce a value of $2.3 eV for the deep impurity involved in this DAP transition. The measured energy levels together with calculation results and formation energies of the impurities and defects in h-BN suggest that V N and carbon impurities occupying the nitrogen sites, respectively, are the most probable shallow donor and deep acceptor impurities involved in this DAP transition.
We report the achievement of highly efficient 10 B enriched hexagonal boron nitride (h-10 BN) direct conversion neutron detectors. These detectors were realized from freestanding 4-in. diameter h-10 BN wafers 43 lm in thickness obtained from epitaxy growth and subsequent mechanical separation from sapphire substrates. Both sides of the film were subjected to ohmic contact deposition to form a simple vertical "photoconductor-type" detector. Transport measurements revealed excellent vertical transport properties including high electrical resistivity (>10 13 X cm) and mobilitylifetime (ls) products. A much larger ls product for holes compared to that of electrons along the c-axis of h-BN was observed, implying that holes (electrons) behave like majority (minority) carriers in undoped h-BN. Exposure to thermal neutrons from a californium-252 (252 Cf) source moderated by a high density polyethylene moderator reveals that 43 lm h-10 BN detectors possess 51.4% detection efficiency at a bias voltage of 400 V, which is the highest reported efficiency for any semiconductor-based neutron detector. The results point to the possibility of obtaining highly efficient, compact solid-state neutron detectors with high gamma rejection and low manufacturing and maintenance costs.
Recent advances in epitaxial growth and demonstration of p-type conductivity in hexagonal boron nitride (hBN) epilayers represent an exceptional opportunity to revolutionize player approach and overcome the intrinsic problem of low p-type conductivity in Al-rich AlGaN for deep ultraviolet (DUV) device applications. Nevertheless, the ability of epitaxial growth of hBN on AlGaN is a prerequisite for the incorporation of p-type hBN in AlGaN DUV device structures. We report on the epi-growth of hBN on Al-rich AlGaN/AlN/Al 2 O 3 templates using metal organic chemical vapor deposition. X-ray diffraction measurement revealed a 2h peak at 26.5 which indicates that the BN epilayers are hexagonal and consist of a single phase. Mg doped hBN epilayers were also grown on highly insulating AlN and n-type AlGaN templates with an attempt to demonstrate hBN/AlGaN p-n junctions. Mg doped hBN epilayers grown on insulating templates were p-type with an in-plane resistivity of $2.3 X cm. Diode behavior in the p-n structures of p-hBN/ n-Al x Ga 1Àx N (x $ 0.62) has been demonstrated. The results here reveal the feasibility of using highly conductive p-type hBN as an electron blocking and p-contact layers for AlGaN deep UV emitters. V
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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