It is shown that bulk MOVPE-grown GaN can exhibit dramatically increased ne
ar-band-edge and below-bandgap emission after argon plasma etching. The tem
perature dependence of the near-band-edge emission shows that donor-bound e
xcitons are the dominant species in Ar-etched samples while free excitons d
ominate spectra from unetched samples. Under UV illumination and at low tem
peratures metastable behaviour is apparent from the defect-related blue and
yellow luminescence bands. The clear linkage between the yellow and blue e
mission bands indicates that any microscopic model of the defect(s) associa
ted with the yellow luminescence must also be able to explain the blue lumi
nescence. At room temperature, UV illumination over a period of time enhanc
es the yellow emission by a factor of 20 and it is shown that this effect c
an be employed for room-temperature optical data storage and retrieval.