Deep levels (V, Cr, UD-1 and UD-3) have been studied with time resolved photoluminescence. The decay time is determined as 193 ns, 58 mus, 247 ns and 51.5 mus for the V, Cr, UD-1 and UD-3 centers, respectively, at low temperature (2 K). The relatively long decay times indicate that the radiative recombination is dominant at low temperatures. By fitting the temperature dependence of the decay time, the activation energies of the V and UD-1 centers are determined as 160 meV and 50 meV respectively.