On the Growth of Complex Oxides by Molecular Beam Epitaxy

Functional materials based on complex oxides in thin film form offer new and exciting strategies for meeting many of our outstanding energy challenges through systematic control of layer sequencing, strain, etc. However, the synthesis of such oxide films can be a major challenge even when utilizing reactive molecular-beam epitaxy (MBE), a powerful deposition technique that allows the construction of materials atomic plane by atomic plane. To understand the fundamental physics of oxide growth by reactive MBE, we present \textit{in situ} surface x-ray diffraction results on the growth of SrTiO$_{\mathrm{3}}$ and SrO-SrTiO$_{\mathrm{3}}$ thin films on (001)-oriented SrTiO$_{\mathrm{3}}$ substrates. For homoepitaxy, we compare sequential deposition (alternating Sr and Ti monolayer doses) with that of co-deposition of Sr and Ti, both in a background of oxygen pressure, and observe drastically different growth pathways due to the presence of a TiO$_{\mathrm{2}}$ double layer. For heteroepitaxial growth of Ruddlesden-Popper SrO-SrTiO$_{\mathrm{3}}$ films, we find that layers rearrange dynamically, resulting in layer sequences distinct from the shutter sequence. In general, the starting surface structure and composition, in combination with local thermodynamic considerations, strongly influence our ability to atomically construct new complex oxides.