The optical metastability in unintentionally doped GaN (0001) films grown on AlN/SiC substrates has been investigated as a function of thermal annealing conditions using photoluminescence and optical microscopy. Annealing at 800 degrees C for 48 h in 1 atm of flowing nitrogen produced no change in the metastability. Annealing at 800 degrees C in ultrahigh vacuum for 48 h eliminated the phenomenon. Exposure of the sample to ultraviolet light during the latter anneal reduced the time to eliminate the metastability. This phenomenon was restored by subsequently annealing in ammonia at 775 degrees C for 3 h. These results suggest that the presence and elimination of the optical metastability are related to the presence in and the elimination of hydrogen from the GaN. (c) 2006 American Vacuum Society.