Ultraviolet Luminescence from Hexagonal BN Heteroepitaxial Layers
Yasuyuki Kobayashi, Tetsuya Akasaka, and Toshiki Makimoto
Materials Science Laboratory
Hexagonal boron nitride (h-BN) is a promising material system for exciton-based quantum information processing and for optical device applications in the ultraviolet spectral region. For investigating the optical properties and fabricating optical devices with a p-n junction and quantum well structures, a high-quality h-BN layer on a suitable substrate is indispensable. During the past few decades, BN thin films have been deposited by a number of methods. However, there has been no report of ultraviolet luminescence at room temperature (RT) from these BN thin films.
Recently, we have achieved growth of single crystal h-BN (0001) heteroepitaxial layers on Ni (111) substrate by flow-rate modulation epitaxy (FME) using triethylboron and ammonia (NH3) . Here, we report RT observation of near-band-gap (NBG) luminescence at a wavelength of 227 nm in cathodoluminescence (CL) from the h-BN heteroepitaxial layers .
Figure 1 shows the CL spectra for h-BN layers grown under NH3 supply times of 1, 2, and 3 s with NH3 flow rate of 700 sccm. A NBG ultraviolet emission peak centered at energy of 5.47 eV (227 nm) and one broad deep-level emission peak centered at around 3.85 eV (322 nm) are clearly observed for h-BN layer grown under NH3 supply times of 3 s. To our knowledge, this dominant NBG ultraviolet emission feature at RT has never reported for h-BN films deposited on any substrates. The luminescence intensity at the NBG peak increases monotonically with increasing NH3 supply time, indicating that h-BN grown by FME with longer NH3 supply time is preferable for obtaining stronger NBG luminescence.
Figure 2 shows the NH3 supply time dependence of c-axis lattice constant and full width at half maximum (FWHM) of (0002) h-BN X-ray rocking curve (XRC) for these three h-BN samples. The c-axis lattice constant of all three is identical to that of bulk h-BN samples. The FWHM of the (0002) h-BN XRC decreases from 1.5 to 0.7o with increasing NH3 supply time from 1 to 3 s. The FWHM of 0.7o is the narrowest value ever reported for a h-BN layer. The lower h-BN growth rate and larger amount of NH3 with the longer NH3 supply time may reduce the lattice defects in h-BN heteroepitaxial layers, resulting in the stronger NBG ultraviolet emission from h-BN samples.
The present result is a vital step toward the ultimate goal of h-BN-based optical devices.
 Y. Kobayashi, et al., J. Cryst. Growth 298 (2007) 325.
 Y. Kobayashi, et al., Phys. Stat. Sol. (b) 244 (2007) 1789.
Fig. 1. CL spectra at RT from h-BN thin films grown by FME.
Fig. 2. The lattice constant and the FWHM of XRC as a function of the NH3 supply time.
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