Photoelectron Spectroscopy and Optical Properties of Al-Doped ZnO Films Prepared by Sputtering with Radio Frequency Power Applied to Al Target

• Published in: Japanese Journal of Applied Physics 2013-02, Vol.52 (2), p.025801-025801-8
• Main Authors: Tseng, Chun-An, Lin, Jing-Chie, Weng, Wei-Heng, Lin, Chen-Chun
• Format: Article
• Language: English
• Subjects: Aluminum; Azo; Concentration (composition); Electric power generation; Optics; Photoelectron spectroscopy; Physics; Radio frequencies; Texture; Zinc oxide
• ISSN: 0021-4922; 1347-4065
• DOI: 10.7567/JJAP.52.025801

Al-doped ZnO (AZO) films were prepared on a glass substrate using a magnetron sputtering system. The metallic aluminum and ZnO targets were bombarded by radio frequency power source. The Al-dopant concentration of the films, analyzed by energy-dispersive spectroscopy, increased with increasing RF power. The electrical resistivity of the films, measured by four-point probe, revealed a decrease from $3.43 \times 10^{-2}$ to $3.29 \times 10^{-3}$ $\Omega$ cm with increasing Al-content from 0.85 to 4.34 at. %. The crystal structure analyzed by grazing incidence X-ray diffraction indicated that all of the films prepared by sputtering with RF power preferred a stronger texture on (002) orientation than those with DC power applied to Al target. The optical transmittance, measured by UV--visible, indicated an average transmittance higher than 82% for all the films in visible region, and increased with Al-content; however, it reversed at 4.34 at. %. The band gap of the films increased from 3.39 to 3.58 eV with increasing RF power. Ultraviolet photoelectron spectroscopy (UPS) analysis revealed that a characteristic peak at approximately 5--7 eV was found in the AZO films due to the O 2p valence electrons. Meanwhile, the work functions of the undoped ZnO film and various AZO films were measured by UPS decreased from 4.9 to 4.5 eV with increasing Al-content. The chemical composition of the films was analyzed by X-ray photoelectron spectroscopy with Gaussian-resolved fitting. According to XPS analysis of O 1s for the films, we inferred that the decrease in electrical resistivity of the film with increasing Al-dopant concentration was predominated by the concentration of oxygen vacancies.