Elucidating the Growth Kinetics of hMBE-Grown SrTiO₃ via ReaxFF

Titanium(IV) isopropoxide (TTIP), a commonly used metal-organic substance, has tremendous industrial applications since its thermal decomposition produces TiO₂ as a final product. Therefore, it has come into focus as an indispensable component of hybrid molecular beam epitaxy (<i>h</i>MBE) systems to grow titanate structures such as SrTiO₃.¹ To have a better understanding of the reaction kinetics of the growth process, reactive force field molecular dynamic simulations (ReaxFF-MD) constitute as a powerful tool which is based on a bond order-based force field that provides a unique opportunity to observe chemical reactions with the continuous bond formation and breaking together with allowing for large simulation sizes.^2,3 The decomposition pathways and surface interactions can realistically be captured and tracked in the low-pressure conditions present in the deposition experiments. In this study, a SrTiO₃(001) structure was simulated using ReaxFF molecular dynamics simulations in such a way that one surface of SrTiO₃ is TiO₂ terminated while the other surface is SrO terminated. TTIP molecules and Sr atoms were placed randomly around the SrTiO₃structure to mimic the <i>h</i>MBE growth conditions. Performing the ReaxFF-MD simulations in the presence of TiO₂- and SrO-terminated SrTiO (001) surfaces revealed that the particular surface chemistry of the SrTiO₃ growth front dramatically affected the initialization of the TTIP decomposition process. While the metal-organic precursor TTIP was found to be rather stable with a pronounced tendency to thermally desorb from TiO terminated SrTiO (001) surfaces, its decomposition on SrO terminated surfaces was much pronounced, providing first insights into the intricacies of the hybrid MBE growth process. The major observations that were made based on these simulations are as follows:<br/><br/>1) Sr atoms had a clear tendency to go to the surface with TiO₂ termination, 2) TTIP molecules were attracted by the SrO surface and no TTIP can be observed in the vicinity of TiO₂-termination which confirmed the experimental finding that TTIP breaks down at a 40% higher rate on SrO than on bare TiO₂.⁴ 3) As soon as the Sr atoms started making bonds with the TiO₂ surface, TTIP molecules were attracted by Sr atoms and got incorporated into the surface to decompose and to grow the SrTiO₃ structure.<br/><br/>The conclusions that were drawn from the simulations of TTIP decomposition on SrTiO₃ play a critical role in understanding the growth kinetics of <i>h</i>MBE in more detail. At this level, one can ask questions such as what are the decomposition reactions of TTIP on SrTiO₃? Do the bonds of the TTIP molecule tend to break between C-O or Ti-O leaving the remaining molecule oxygen-deficient? Does the temperature have any contribution to the types of bond dissociation reactions? How does the steric hindrance of TTIP affect the growth? Can TTIP be replaced by a better-suited Ti-based metal-organic precursor? What is the ideal amount of TTIP molecules that need to be supplied on an 8x8 atom surface? All these questions can be answered via ReaxFF-MD simulations and will be discussed in this talk in detail.<br/><br/>1. Jalan, B., Moetakef, P. & Stemmer, S. Molecular beam epitaxy of SrTiO3 with a growth window. <i>Appl. Phys. Lett.</i> <b>95</b>, (2009).<br/>2. Van Duin, A. C. T., Dasgupta, S., Lorant, F. & Goddard, W. A. ReaxFF: A reactive force field for hydrocarbons. <i>J. Phys. Chem. A</i> <b>105</b>, 9396–9409 (2001).<br/>3. Senftle, T. P. <i>et al.</i> The ReaxFF reactive force-field : development, applications and future directions. <i>Nat. Publ. Gr.</i> (2016) doi:10.1038/npjcompumats.2015.11.<br/>4. Brahlek, M. <i>et al.</i> Frontiers in the Growth of Complex Oxide Thin Films: Past, Present, and Future of Hybrid MBE. <i>Adv. Funct. Mater.</i> <b>28</b>, 1–41 (2018).