The cathodoluminescence (CL) spectra were measured with a custom-made setup equipped with a pulsed "electron gun" and vacuum cryostat (510 -7 mm Hg). The spectra were recorded using a diffraction grating spectrometer (dispersion 5 Å/mm) and synchronous detector at temperatures of 77 and 300 K. The current and energy varied within 0.05-2 mA and 10-50 keV ranges, respectively.