Low temperature-pyrosol-deposition of aluminum-doped zinc oxide thin films for transparent conducting contacts

Abstract

Aluminum doped-zinc oxide (ZnO:Al) thin films with thickness ~1000nm have been deposited by the ultrasonic spray pyrolysis technique using low substrate temperatures in the range from 285 to 360°C. The electrical and optical properties of the ZnO:Al (AZO) films were investigated by Uv–vis spectroscopy and Hall effect measurements. The crystallinity and morphology of the films were analyzed using X-ray diffraction (XRD), atomic force microscopy (AFM), and high resolution scanning electron microcopy (SEM). XRD results reveal that all the films are nanocrystalline with a hexagonal wurtzite structure with a preferential orientation in the (002) plane. The size of the grains calculated from Scherrer's formula was in the range from 28 to 35nm. AFM and SEM analysis reveals that the grains form round and hexagonal shaped aggregates at high deposition temperatures and larger rice shaped aggregates at low temperatures. All the films have a high optical transparency (~82%). According to the Hall measurements the AZO films deposited at 360 and 340°C had resistivities of 2.2×10−3–4.3×10−3Ωcm, respectively. These films were n-type and had carrier concentrations and mobilities of 3.71–2.54×1020cm−3 and 7.4–5.7cm2/V s, respectively. The figure of merit of these films as transparent conductors was in the range of 2.6×10−2Ω−1–4.1×10−2Ω−1. Films deposited at 300°C and 285°C, had much higher resistivities. Based on the thermogravimetric analysis of the individual precursors used for film deposition, we speculate on possible film growing mechanisms that can explain the composition and electrical properties of films deposited under the two different ranges of temperatures.