AZO (Al:ZnO) thin films with high figure of merit as stable indium free transparent conducting oxide

Abstract

This work presents a broad study of transparent and conducting Al-doped ZnO (AZO) thin films that could replace indium tin oxide (ITO) as transparent electrode in optoelectronic devices. AZO thin films are deposited by pulsed DC magnetron sputtering at a sputtering power of 80 W with different pulse frequencies in the range 50–100 kHz. Diffraction data confirm formation of doped ZnO and AZO thin films exhibit preferred orientation in the c-direction. The crystallite sizes of Al:ZnO are in the range 16–21 nm. The field emission scanning electron microscopy and atomic force microscopy of AZO thin films reveal nucleation and growth of uniform and dense films with better quality films deposited at a pulse frequency 75 kHz. A simple and non-destructive optical method is adopted to determine thickness and dispersion parameters such as dispersion energy, carrier concentration to effective mass ratio, and plasma frequency. AZO thin films offer excellent visible light transparency and limited transparency in the near-infrared region due to free carrier absorption. The sheet resistance of AZO thin films is recorded in the range 9–45 Ω/sq making these transparent conducting oxides (TCOs) suitable for optoelectronic applications. The figure of merit as high as 1.99 × 10−2 Ω−1 is achieved for AZO thin film deposited at a pulse frequency of 75 kHz. AZO thin film sputtered with a pulse frequency of 75 kHz is quite stable in ambient oxidizing environment and surface adsorption sites might govern the initial oxidation of films when exposed to atmosphere. Excellent figure of merit and good stability of sputtered AZO thin films as TCO fulfill the requirements of a transparent electrode in photovoltaics.