Abstract
First-order Raman spectra of wurtzite GaN made from natural Ga and isotopically pure and natural N (99.63% ) were measured at low temperature. The Raman frequencies of the polar optical phonons [, ] shift according to the inverse square root of the reduced masses, as expected. The isotope shifts of the two nonpolar modes deviate significantly from the reduced-mass behavior as well as from the dependence expected for pure N or Ga vibrations which might be expected in view of their large frequency difference. This indicates that the modes involve mixed Ga and N vibrations. From fits to the experimental data with a coupled two-mode model we determine the mode eigenvectors. They confirm the results of an ab initio calculation which we have done for the zone-center vibrations of wurtzite GaN. Our calculations also predict a vanishing coupling between the two silent modes. We also measured a alloy sample. Additional negative frequency shifts due to isotope disorder are observed for the , , and the high-frequency modes as compared to the reduced-mass or coupled-mode behavior. An estimate from second-order perturbation theory provides a quantitative explanation of this result.
- Received 19 August 1997
DOI:https://doi.org/10.1103/PhysRevB.56.14399
©1997 American Physical Society