Microstructure and characterization of Al-doped ZnO films prepared by RF power sputtering on Al and ZnO targets

Abstract

Al-doped zinc oxide (AZO) transparent conductive films were prepared on a glass substrate using a magnetron sputtering system with a pure zinc oxide (ZnO) target and a pure Al target sputtered using radio frequency (RF) power. The RF power was set at 100W for the ZnO target and varied from 20 to 150W for the Al target. The morphology of the thin films was examined by field-emission scanning electron microscope (FE-SEM), and their composition was analyzed by the equipped energy-dispersive X-ray spectroscopy (EDS). The cross section of the films determined through FE-SEM indicated that their thickness was around 650nm. EDS analysis revealed that the Al-dopant concentration of the AZO films increased in the following order: 0.85at.% (20W)<1.60at.% (40W)<3.52at.% (100W)<4.34at.% (150W). Analysis of the films using X-ray diffractometer (XRD) indicated that all films had a wurtzite structure with a texture of (002). High-resolution transmission electron microscopy (HRTEM) revealed a number of defects in the films, such as stacking faults and dislocations. Ultraviolet photoelectron spectroscopy (UPS) was used to estimate the optical energy gap (Eg) for the AZO thin films. The energy gap increases from 3.39 to 3.58eV as the RF power applied to the Al target increase. The electrical resistivity of the films decreased from 3.43×10−2Ωcm to 3.29×10−3Ωcm as the RF power increased from 20 to 150W when a four-point probe was used to investigate. Atomic force microscope (AFM) revealed that the surface roughness of the films increased with increasing RF power. The average optical transmittance of the films was determined by UV–visible spectrometer. The films are suitable for use as transparent conductive oxide films in the optoelectronic industry. A decrease in the electrical resistivity of the film with increasing Al-dopant concentration was ascribed to an increase in the carrier concentration and density of stacking faults in the films.