Effect of N isotopic mass on the photoluminescence and cathodoluminescence spectra of gallium nitride

GaN is a wurtzite-type semiconductor at ambient conditions whose natural composition consists of almost pure ¹⁴N (99.63% ¹⁴N and 0.37% ¹⁵N) and a mixture of 60.1% ⁶⁹Ga, and 39.9% ⁷¹Ga. We report a low-temperature photoluminescence and cathodoluminescence study of GaN thin films made from natural Ga and N, and from natural Ga and isotopically pure ¹⁵N. The contribution of the nitrogen vibrations to the bandgap renormalization by electron-phonon interaction has been estimated from the nitrogen isotopic mass coefficient of the bound exciton energy. The temperature dependence of the bandgap of GaN can be explained with the measured isotopic mass coefficients of Ga and N. We have estimated the aluminum and indium contribution to the bandgap renormalization in AlN and InN from the temperature dependence of the AlN and InN bandgap up to 300 K, assuming that the N contribution is similar to that found in GaN. The similar bandgap isotopic mass coefficients of C, N, and O, of Al, Si and P, of Zn, Ga and Ge, and of Cd and In suggests that elements of the same row of the periodic table have similar bandgap isotopic mass coefficients.