Optoelectronic properties of hexagonal boron nitride epilayers

This paper summarizes recent progress primarily achieved in authors' laboratory on synthesizing hexagonal boron nitride (hBN) epilayers by metal organic chemical vapor deposition (MCVD) and studies of their structural and optoelectronic properties. The structural and optical properties of hBN epilayers have been characterized by x-ray diffraction (XRD) and photoluminescence (PL) studies and compared to the better understood wurtzite AlN epilayers with a comparable energy bandgap. These MOCVD grown hBN epilayers exhibit highly efficient band-edge PL emission lines centered at around 5.5 eV at room temperature. The band-edge emission of hBN is two orders of magnitude higher than that of high quality AlN epilayers. Polarization-resolved PL spectroscopy revealed that hBN epilayers are predominantly a surface emission material, in which the band-edge emission with electric field perpendicular to the c-axis (E-emi perpendicular to c) is about 1.7 times stronger than the component along the c-axis (E-emi//c). This is in contrast to AlN, in which the band-edge emission is known to be polarized along the c-axis, (E-emi//c). Based on the graphene optical absorption concept, the estimated band-edge absorption coefficient of hBN is about 7x10(5) cm(-1), which is more than 3 times higher than the value for AlN (similar to 2x10(5) cm(-1)). The hBN epilayer based photodetectors exhibit a sharp cut-off wavelength around 230 nm, which coincides with the band-edge PL emission peak and virtually no responses in the long wavelengths. The dielectric strength of hBN epilayers exceeds that of AlN and is greater than 4.5 MV/cm based on the measured result for an hBN epilayer released from the host sapphire substrate.