Knowledge Bank

Infrared transmission spectra of ZnO single crystals have been investigated from 500 to $1600cm-1$, and tentative assignment of the 25 observed absorption maxima to multiphonon processes leads to the following characteristic phonon frequencies (in $cm-1:LO=489,TO1=441,TO2=426,LA=243,TA1=137$) and $TA2=101$, belonging to the Brillouin zone boundary. In wurtzite ZnO the degeneracy of the transverse modes breaks down as was also observed in CdS and $ZnS.1$ The assignments were consistent with temperature dependence and polarization of the observed bands. The characteristic phonon frequencies have been discussed in terms of crystal structure and effective ionic $charge.2$ The assignment is consistent with the established trends for the II-VI $compounds.3$