Raman-active modes in wurtzite (GaN)m(AlN)n superlattices

We have established that the symmetry of wurtzite (GaN)(m)(AlN)(n) superlattices grown along the symmetry axis depends on the numbers of monolayers 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 superlattice point symmetry drastically influences the first-order vibrational spectra: the number of Raman-active modes depends nonmonotonically on m+n whereas the dependence for the IR-active modes is monotonic. A simple Raman test to distinguish the superlattices with even and odd m+n values is proposed.