Photoluminescence measurements from the two polar faces of ZnO

The crystal structure of ZnO is wurtzite and the stacking sequence of atomic layers along the “c” axis is not symmetric. As a result, a ZnO crystal surface that is normal to the c axis exposes one of two distinct polar faces, with (0001̄) being considered the O face and (0001) the Zn face. Photoluminescence (PL) measurements on the two faces reveal a striking difference. Two transitions are observed in PL that are dominant from the O face and barely observed in PL from the Zn face. These lines are identified as phonon replicas of a particular D0,X transition using energy separations, excitation dependence, and time-resolved PL measurements. In addition, PL emission from free excitons is found to be more intense from the O face than from the Zn face.