Isotopically enriched gallium nitride films, Ga<sup>14</sup>N and Ga<sup>15</sup>N, have been fabricated by molecular-beam epitaxy to study
the effect of nitrogen atomic mass on structures and properties of GaN. Due to the isotopic substitution, the phonon
frequency shift has been clearly observed by using Raman spectroscopy. The lattice constants of Ga<sup>15</sup>N differed from
Ga<sup>14</sup>N, and, actually, the unit cell volume of Ga<sup>15</sup>N was approximately 0.07% less than that of Ga<sup>14</sup>N. Temperature-dependent
photoluminescence measurements revealed that recombination mechanism in Ga<sup>14</sup>N and Ga<sup>15</sup>N was the same
to each other in the temperature range of 4-50 K, and the band gap energy difference between them was found to be E<sub>g15</sub>-E<sub>g14</sub>=6.0±0.1 meV. This E<sub>g</sub> difference is discussed in terms of volume shrinkage and change in phonon-electron
interaction due to isotopic substitution.