Disorder-activated scattering and two-mode behavior in Raman spectra of isotopic GaN and AlGaN

Micro-Raman spectroscopy has been applied to isotopically pure GaiN (i = 14, 15) epitaxial films in which structural disorder was introduced by ion implantation or mechanical damage. A pair of Raman peaks appearing at about 300 (300) and 667 (646) cm—1 is consistent with the anion mass 14 (15) suggesting disorder activation of the silent B1 modes, where the low-frequency mode is related to the Ga vibration (no N isotope shift) and the high-frequency mode to the N vibration (full N isotope shift). Isotopically pure Al0.5Ga0.5iN indicates respective activation of the silent B1 modes due to alloying disorder and confirms a two-mode behavior of the E2 mode. In agreement with recent IR measurements and theoretical predictions, we additionally provide evidence for an E1(TO) two-mode behavior in Raman scattering as obtained from polarisation dependent measurements on AlxGa1—xN epilayers.