Molecular beam epitaxy (MBE) is renowned for its potential for atomic layer control, but unexpected growth mechanisms can potentially compromise this level of precision. In this study, we employ first-principles calculations to investigate the atomistic processes governing the MBE growth of perovskite ${\mathrm{SrTiO}}_3$ and Ruddlesden-Popper ${\mathrm{Sr}}_2{\mathrm{TiO}}_4$ films on a ${\mathrm{SrTiO}}_3$ substrate. We systematically explore the potential molecular species in the gas phase and their reactions and diffusion dynamics on the film surfaces and layer edges. Our analyses uncover three mechanisms of importance for understanding this type of growth. First, oxygen vacancies can be dynamically induced during surface diffusion on defect-free substrate and noticeably accelerate the diffusion processes. Second, while the SrO layer is expected to grow in a single-layer growth mode, the presence of potential ${\mathrm{Ti}}_{\mathrm{Sr}}$ defects may promote the formation of SrO islands. Lastly, adsorbed Ti atoms and ${\mathrm{TiO}}_2$ molecules on the SrO bilayer can insert into SrO bilayers, resulting in an unexpected growth sequence. These findings may have broader implications for the MBE growth of metal oxide films and provide guidance for achieving improved control over their growth processes.

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• Growth
• Surface diffusion
• Perovskites
• Thin films
• Density functional theory
• First-principles calculations
• Molecular beam epitaxy