Phase and surface structures-optical transmittance correlation of sputtered Al-doped ZnO film on glass substrate

Abstract

The electrical and optical properties of sputtered Al-doped ZnO films prepared on a glass substrate with different thicknesses (97, 127, 161, 211, and 276 nm) were systematically investigated. The 97 nm film showed only the main peak of the AZO (002) phase, whereas the rest films exhibited AZO (002) and (004) phases, and the peak intensities were obviously increased with increasing thickness. The films displayed a granular grain surface and columnar-like structure with different sizes and distributions depending on film thickness. Surface roughness was increased, whereas the electrical resistance was decreased with increasing film thickness. The smallest crystallite size of about 26 nm with the highest resistivity and lowest carrier concentration was observed on a 127 nm film, whereas the crystallite size of about 29 nm was observed on the 97, 161, 211, and 276 nm films. All AZO films exhibited good electrical properties and transparency with an averaged optical transmittance higher than 80% in the visible wavelength. The 162 nm film showed the highest transmittance of 86% in the wavelength range of 350–900 nm and a wide energy band gap of 3.52 eV because of the highest mobility and crystallite size with a columnar structure and random size distribution. The figure of merit (FOM) was strongly related to the optical band gap and tended to increase with increasing thickness. The results are attributed that the optical energy band gap was altered by film thickness by improving phase structure and surface morphology.