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.

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Reaction mechanisms during the growth of multinary compounds by atomic layer deposition can be complex, especially for sulfide materials. For instance, the deposition of copper indium disulfide (CuInS2) shows a non-direct correlation between the cycle ratio, the growth per cycle of each binary growth cycles, i.e., CuxS and In2S3, and the film composition. This evidences side reactions that compete with the direct Atomic Layer Deposition (ALD) growth reactions and makes the deposition of large films very challenging. To develop a robust upscalable recipe, it is essential to understand the chemical surface reactions. In this study, reaction mechanisms in the Cu-In-S ternary system were investigated in-situ by using a quartz crystal microbalance system to monitor mass variations. Pure binary indium sulfide (In2S3) and copper sulfide (CuxS) thin film depositions on Al2O3 substrate were first studied. Then, precursors were transported to react on CuxS and In2S3 substrates. In this paper, gas-phase ion exchanges are discussed based on the recorded mass variations. A cation exchange between the copper precursor and the In2S3 is highlighted, and a solution to reduce it by controlling the thickness deposited for each stack of binary materials during the CuInS2 deposition is finally proposed.

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