Correlating Growth Characteristics in Atomic Layer Deposition with Precursor Molecular Structure: The Case of Zinc Tin Oxide

The growth characteristics in atomic layer deposition (ALD) of mixed oxide thin films have been investigated by DFT calculations and in situ quadrupole mass spectrometry (QMS) using zinc tin oxide (ZTO) ALD from diethylzinc (DEZn), tetrakis(dimethylamido)tin (TDMASn), and H2O as a case study. The DFT-calculated Gibbs free energies of reaction for binding TDMASn on OH-terminated ZnO surfaces demonstrate the reaction to be feasible and provide evidence for a reduction in surface reaction site density upon mixing a small number of SnO x cycles into the ZnO ALD process. The in situ QMS experiments verify the reduction in surface reaction site density during the SnO x cycle, and demonstrate restoration of reaction site density during the subsequent ZnO cycles. The reduction in reaction site density, which is a consequence of the four exchangeable ligands of the TDMASn precursor, is shown to provide an atomic-level explanation for experimentally observed ZTO ALD growth characteristics. The correlation between precursor molecular structure and material growth established here for ZTO ALD applies to other ALD processes where precursors follow ligand-exchange surface chemistries, and thus, it provides general guiding principles useful in understanding and developing ALD processes.