Impact of the sequence of precursor introduction on the growth and properties of atomic layer deposited Al-doped ZnO films

Abstract

Atomic layer deposition relies on surface chemical reactions which implies that the order of the precursor pulses (so-called “sequence”) impacts the growth, especially for multinary compounds. In the case of Al-doped zinc oxide (AZO) thin films, the sequence of introduction of precursors tri-methyl aluminum (TMA)/diethylzinc (DEZ)/H2O has been reported to impact their growth and some of their properties. Here, five different Al sequences for doping the AZO films in Al have been tested at a constant deposition temperature of 160 °C and a TMA:DEZ ratio of 1:10, and the film growth and properties are investigated by in situ quartz crystal microbalance (QCM) measurements and ex situ characterizations. This paper provides evidence of the impact of the Al sequence on AZO material and proposes an explanation of the macroscopic properties based on the nature of chemical surface reactions evidenced by QCM. The growth rate, composition, electrical and optical properties, and, to a lesser extent, structural properties are affected by the TMA/DEZ/H2O pulse sequence. In particular, better electrical properties are obtained by reducing the Al content incorporated per cycle when the TMA pulse follows a DEZ pulse and, second, the optical band gap size is increased when the TMA pulse is between two DEZ pulses. Mass variations during surface reactions of TMA on hydroxyl and zinc-alkyl surface groups have also been proven to be temperature dependent when comparing growth mechanisms at 160 and 200 °C. It is also observed that the increase in temperature has a similar impact on the mass variations as the increase in the TMA:DEZ ratio. This is probably because of an improved intermixing between Al species and ZnO layers.