Thickness effect on structure and properties of ZAO thin films by RF magnetron sputtering at different substrate temperatures

Al-doped ZnO (ZAO) films, having film thickness of about 50-1200 nm, were deposited at substrate temperature of 100-300 degrees C by radio-frequency magnetron sputtering. Structural, electrical, and optical properties of as-deposited ZAO films have been studied as a function of film thickness and substrate temperature. The investigation of X-ray diffraction indicates that the crystalline quality of the film improves and its stress relaxes with increasing film thickness or substrate temperature, and preferred (0 0 2) orientation is found at substrate temperature above 100 degrees C. By the observation under scanning electron microscope, columnar structure can be obviously observed from cross-section of the films with increasing film thickness at substrate temperature above 100 degrees C; the surface morphology can exhibit nanocrystalline, honeycomb, or hillock structure, depending on film thickness and substrate temperature. Hall effect measurements reveal that the decrease of resistivity of the film is generally accompanied by the increase of carrier concentration and mobility with increasing film thickness or substrate temperature. It is speculated that the main scattering mechanism in as-deposited ZAO films is intercrystallite boundary scattering. The transmission spectra measurements of ZAO films indicate that average transmittance between 400 and 800 nm decreases from about 86% to 70% with increase in film thickness. The obtained energy gap (E-g) of the films at different substrate temperatures is found in order of 200 degrees C > 300 degrees C > 100 degrees C, which can be attributed to high carrier concentration and compressive stress at 200 degrees C, but markedly low carrier concentration at 100 degrees C. With increase in film thickness, the decrease in compressive stress and/or increase in crystallite size result in the decreased or unchanged tendency of Eg of the films although the carrier concentration increases. (C) 2011 Elsevier B.V. All rights reserved.