Photoluminescence measurements from the two polar faces of ZnO

The crystal structure of ZnO is wurtzite and the stacking sequence of atomi c layers along the "c" axis is not symmetric. As a result, a ZnO crystal su rface that is normal to the c axis exposes one of two distinct polar faces, with (000 (1) over bar) being considered the O face and (0001) the Zn face . Photoluminescence (PL) measurements on the two faces reveal a striking di fference. Two transitions are observed in PL that are dominant from the O f ace and barely observed in PL from the Zn face. These lines are identified as phonon replicas of a particular D-0,X transition using energy separation s, excitation dependence, and time-resolved PL measurements. In addition, P L emission from free excitons is found to be more intense from the O face t han from the Zn face. (C) 2000 American Institute of Physics. [S0021-8979(0 0)02018-1].