Structural and optical properties of ZnO thin films grown by using atomic layer deposition on O2-plasma-treated flexible PES substrates

Zinc oxide (ZnO) thin films have been grown by using atomic layer deposition (ALD) on O-2-plasma-treated polyethersulfone (PES) substrates. We investigated the effects of the duration of O-2 plasma pretreatment from 0 to 60 min on the characteristics of ZnO thin films grown on the substrates. When the duration of the plasma treatment increases, the surface roughness of the PES substrate increases. X-ray diffraction (XRD) measurements reveal an increasing tendency toward preferential c-axis orientation of the grains in the ZnO thin films with increasing duration of plasma pretreatment, which probably results from the improved crystallinity of the ZnO thin films grown on the roughened PES substrates after O-2 plasma pretreatment. The roughened surfaces of PES substrates promote many nucleation sites, leading to perpendicular c-axis growth of ZnO thin films and, therefore, improved crystallinity of the ZnO thin films. The peak intensity of the NBE emission around 3.0 eV decreases with increasing duration of O-2 plasma pretreatment up to 30 min. Then, it rapidly increases up to 60 min. The variations of the PL intensity and the NBE peak position strongly depend on the crystallinity of the ZnO thin films.