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

Micro-Raman spectroscopy has been applied to isotopically,pure (GaN)-N-i (i = 14, 15) epitaxial films in which structural disorder was introduced by i on implantation or mechanical damage. A pair of Raman peaks appearing at ab out 300 (300) and 667 (646) cm(-1) is consistent with the anion mass 14 (15 ) suggesting disorder activation of the silent B-1 modes, where the low-fre quency mode is related to the Ga vibration (no N isotope shift) and the hig h-frequency mode to the N vibration (full N isotope shift). Isotopically pu re (Al0.5Ga0.5N)-N-i indicates respective activation of the silent B-1 mode s due to alloying disorder and confirms a two-mode behavior of the E-2 mode . In agreement with recent IR measurements and theoretical predictions, we additionally provide evidence for an E-1(TO) two-mode behavior in Raman sca ttering as obtained from polarisation dependent measurements on AlxGa1-xN e pilayers.