RAMAN-ACTIVE MODES IN WURTZITE (GAN)(M)(ALN)(N) SUPERLATTICES

We have established that the symmetry of wurtzite (GaN)(m)(AlN)(n) sup erlattices grown along the symmetry axis depends on the numbers of mon olayers m and n: the appropriate space groups are C-6v(4) and C(3v)(1 )for odd and even values of m+n, respectively. The difference in the s uperlattice point symmetry drastically influences the first-order vibr ational spectra: the number of Raman-active modes depends nonmonotonic ally on m+n whereas the dependence for the IR-active modes is monotoni c. A simple Raman test to distinguish the superlattices with even and odd m+n values is proposed.