Hexagonal BN (h-BN) films codoped with Mg and O atoms were grown on n-type Si and quartz substrates heated at 500 degrees C by the sputtering of h-BN targets containing MgO. It is confirmed from Fourier transform infrared spectroscopic measurements that h-BN films are formed by sputtering h-BN targets containing 0.25 mol% MgO in an atmosphere of Ar + 1% O-2. Absorption is seen at wavelengths <400 nm in transmission spectrum of the film prepared from the h-BN target in an Ar atmosphere, whereas absorption is seen at wavelengths <220 nm for the film prepared from the h-BN target containing 0.25 mol% MgO in an atmosphere of Ar + 1% O-2. The calculated energy levels of defects and impurities of O and Mg indicate that the absorption near 400 nm is originated from nitrogen vacancies, and that the absorption near 220 nm is ascribed to O-N-Mg-B-O-N complexes, where O-N and Mg-B denote O atoms at N sites and Mg atoms at B sites, respectively. The film prepared from the h-BN target (0.5 mol% MgO) in an Ar atmosphere exhibits a low turn-on electric field of 2.2 V/mu mand a gentle slope in the Fowler-Nordheim plots. The gentle slope in the Fowler-Nordheim plots is not ascribed to an increase of a field-enhancement factor, but to a decrease of the work function. (c) 2013 Elsevier B.V. All rights reserved.