RAMAN-SPECTRA OF ISOTOPIC GAN

First-order Raman spectra of wurtzite GaN made from natural Ga and iso topically pure N-15 and natural N (99.63% N-14) were measured at low t emperature. The Raman frequencies of thr polar optical phonons [A(1)(T O, LO), E-1(TO,LO)] shift according to the inverse square root of the reduced masses, as expected. The isotope shifts of the two nonpolar E- 2 modes deviate significantly from the reduced-mass behavior as well a s from the dependence expected for pure N or Ga vibrations which might be expected in view of their large frequency difference. This indicat es that the E-2 modes involve mixed Ga and N vibrations. From fits to the experimental data with a coupled two-mode model we determine the m ode eigenvectors. They confirm the results of an ab initio calculation which we have done for the zone-center vibrations of wurtzite GaN. Ou r calculations also predict a vanishing coupling between the two B-1 s ilent modes. We also measured a (GaN0.5N0.5)-Ga-nat-N-14-N-15 alloy sa mple. Additional negative frequency shifts due to isotope disorder are observed for the A(1)(TO), E-1(TO), and the high-frequency E-2 modes as compared to the reduced-mass or coupled-mode behavior. An estimate from second-order perturbation theory provides a quantitative explanat ion of this result.