Rational Molecular Design for Non-aqueous Atomic Layer Deposition of Zinc Oxide

Zinc oxide (ZnO) is a transparent wide band gap semiconductor material with various possible applications in form of thin films. Most previous studies on atomic layer deposition (ALD) of ZnO thin films utilized a few well-known processes with diethylzinc (DEZ) and counter-reactants such as H2O and O3. However, O3 and H2O reactants have relatively strong reactivity, so that they are not suitable for substrates sensitive to oxidation. Therefore, development of milder non-aqueous alternative ALD process for ZnO is highly desired. In this study, we introduce ALD of ZnO using alcohols with theoretically optimized molecular structure. To discover suitable alcohol reactants for ZnO ALD, reaction pathways and reactivity between various types of alcohol reactants with surface-ethyl groups were evaluated through density functional theory calculations. It was found that unsaturated allylic alcohols would have the lowest activation energy for ZnO ALD via an allylic rearrangement mechanism. Experimental, novel ALD processes for ZnO using DEZ with alcohol as oxygen sources are set up. Ethanol as a typical simple alcohol is compared to 2-methyl-3-buten-2-ol (MBO) as an alcohol with optimal molecular structure setup. ALD ZnO films using MBO showed processes and material properties comparable to those of H2O-ALD ZnO. ZnO thin films as transparent conducting oxide could be obtained, and device performances of thin-film transistors based on alcohol-ALD processes are evaluated.