1. Introduction
One important advantage of ZnO is that it is a II–VI
semiconductor of wurtzite structure with a wide
direct-bandgap of 3.3 eV [1] at room temperature. The
most unique property of ZnO is its large exciton
binding energy, 60 meV, which is about three times
larger than that of ZnSe and of GaN [2]. Because of
this large binding energy, the exciton is stable at room