We studied optical manifestation of structural defects in unintentionally doped GaN. A series of sharp peaks (labeled
as Yi with i=1,2,y,11) was observed in the low-temperature photoluminescence spectrum in the photon energy range
between 2.6 and 3.46 eV. We attribute the majority of these peaks to excitons bound to yet unidentified structural
defects. A preliminary transmission electron microscopy study in one of the samples exhibiting strong Yi lines revealed
numerous inclusions, presumably inversion domains, which may be responsible for at least some of the Yi lines