Time-resolved photoluminescence experiments on high quality bulk GaN doped with Gd are presented. It was found that the decay time of Gd-related transitions observed for 4.2 K around 1.78 eV is of about 3 ms. Such a long decay time strongly supports the identification of this emission band as due to transitions between Gd 3+ (4f 7 ) levels. The decay time measured for Gd--related transitions observed in the UV spectral range, close to the GaN band-gap, was found to be much faster than 1 µs.

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... suggests that these emission lines could hardly be correlated with internal transitions within Gd 3+ (4f 7 ). Possible origin of the Gd-related UV luminescence is discussed.