Strong Modification of Excitons and Optical Conductivity for Different Dielectric Environments in ZnO Films

We study the evolution of excitons and optical conductivity of ZnO films on two different substrates (Pt and quartz) using three different Cu doping concentrations known as Cu:ZnO. The concentrations used in this paper were 0% (pure ZnO), 2%, and 8% of Cu. The results show that the excitons are strongly observed in the ZnO film on quartz (ZOQ). However, they are significantly suppressed in the ZnO film on Pt (ZOP). These imply that the interactions of the valence itinerant electrons in Pt with ZnO film are responsible for the electronic blocking of electron (e)-hole (h) pairs or excitons. Furthermore, as a function of Cu doping, the screening effects on excitonic states and the new localized interband transition were observed for the ZOP and ZOQ systems. Our results confirm the different origins of the reduction of excitonic effects and the importance of the dielectric environments to strongly modify excitons and optical conductivity in a wide band-gap semiconductor.