Ordering effects in Raman spectra of coherently strained GaAs1-xNx

Raman spectra of coherently strained layers of GaAs_1-xN_x grown on (001) GaAs with x=0-0.05 by metalorganic molecular-beam epitaxy are reported. The optical phonons of the GaAs and GaN types, as well as disorder-activated acoustical phonons, are observed. A strongly confined GaAs optical mode at ~255 cm^-1, indicating the ordering of As and N atoms, is also detected. The GaAs- and GaN-type optical phonons exhibit strong diagonal components, forbidden for the zinc-blende structure. A bond polarizability analysis of the Raman selection rules shows that these components are activated by the trigonal distortion of the alloy lattice. The trigonal distortion arises from the formation of ordered \{111\}-(GaN)_m(GaAs)_n clusters with n=m=1.