ZnO Thin Films Grown by Plasma Sputtering Process for Optoelectronic Applications: Effect of Substrate Type

In the present work, zinc oxide (ZnO) thin films were deposited at the same time by Radio Frequency (RF) magnetron sputtering technique on glass, quartz and silica-on-silicon (SiO2/Si) substrates. The effect of substrate type on the microstructure, surface morphology, optical and luminescence properties inspected by X-ray diffraction (XRD), scanning electronic microscopy (SEM), atomic force microscopy (AFM), ultraviolet-visible (UV-Vis) and photoluminescence (PL) spectroscopies. XRD patterns illustrates that all prepared films crystallized in a hexagonal wurtzite structure with a (002) preferential orientation. The film deposited on SiO2/Si substrate exhibit better crystalline quality and higher c-axis orientation. SEM and AFM images demonstrate that grain size, surface morphology and roughness of the films are substrate type dependent. A smoothest surface morphology with the smallest root-mean-square roughness (Rrms) value around 2.00 nm is obtained for the ZnO film prepared on quartz substrate. UV-Vis measurements reveal that ZnO thin film deposited on quartz substrate is highly transparent, in the visible region, with more than 84% average optical transmittance. However, a decrease in the average transmittance (81.26%) is observed for the film prepared on glass substrate. Moreover, the values of optical band gap (Eg) are found to be 3.23 and 3.24 eV for ZnO films prepared on glass and quartz substrates, respectively. Room temperature PL spectra of ZnO films grown on glass and quartz substrates show intense UV emission as well as two main weak bands centered at blue and red regions. However, the film prepared on SiO2/Si substrate exhibit very weak emission comprising UV and several visible bands.