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) [1]. 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 [2].
 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.

[1] Y. Kobayashi, et al., J. Cryst. Growth 298 (2007) 325.
[2] 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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