Symmetry Driven Anomalous Growth of Epitaxial Anatase TiO2 on LAO (100) Substrates

Conventionally, thin film growth is known to follow any of the three well-studied growth mechanisms: layer-by-layer growth (Frank-van-der-Merwe), island growth (Volmer-Weber), and layer-by-layer followed by island growth (Stranski–Krastanov) [1]. Surprisingly, the growth of anatase TiO2 doesn’t follow any of the usual growth mechanism, even when it’s grown on closely lattice-matched substrate like lanthanum aluminate (LAO), for which, in general, layer-by-layer growth should be favored. In this work, we have monitored the initial growth dynamics of pulsed laser deposited (PLD) anatase TiO2 films of thicknesses ranging from as low as 1/4 of a monolayer to 40 nm on LAO (100) substrate using in-situ reflection high-energy electron diffraction (RHEED) diagnostic tool. We show that at very early stage, i.e., up to 10-unit cells, the film grows 3D island-type, forming so called “dead layer”. Above this thickness, we started seeing RHEED oscillations confirming layer-by-layer growth. We also found that the dead layer reorders itself as the film grew further, shrinking the dead layer and making the film closer to a single crystal near the interface. This kind of growth mechanism is quite contrary to the usual thin film growth schemes and could be explained based on which of the three segments of the anatase TiO2 unit cell starts growing on the substrate. We have characterized the films ex-situ using Scanning Transmission Electron Microscopy (STEM), which validates the presence of the interface “dead-layer” and its subsequent crystalline regrowth. In addition to this, we have shown that other materials like YBa2Cu3O7 (YBCO) and Sr3Al2O6 (SAO) also follow a similar growth process, making it a “general” thin film growth mechanism.<br/><br/>1. N. Kaiser, "Review of the fundamentals of thin-film growth," Appl. Opt. 41, 3053-3060 (2002).