Visualizing atomistic formation process of SrOx thin films on SrTiO3.

Metallic conductivity observed in the heterostructure of LaAlO3/SrTiO3 has attracted great attention, triggering a debate over whether the origin is an intrinsic electronic effect or a defect-related phenomenon. One of the issues to be solved is the role of SrO layer, which turns the conductive interface into an insulator when inserted between LaAlO3 and SrTiO3. To understand the origins of this oxide interface phenomenon and to further explore unconventional functionalities, it is necessary to elucidate how SrO layers are formed during the initial growth process at the atomic level. Here, we atomically resolve growth processes of heteroepitaxial SrOx films on SrTiO3(001)-(√13×√13)-R33.7° substrate using scanning tunneling microscopy/spectroscopy. On the sub-unit-cell SrOx film surface, no periodic structure was observed as a result of random Ti incorporation into the SrOx islands, indicating the importance of the control of excess Ti atoms on the substrate prior to deposition. This random arrangement of Ti atoms is a marked contrast to the homoepitaxy on SrTiO3(001)-(√13×√13)-R33.7°. Furthermore, the formation of SrOx islands introduced defects in the surrounding SrTiO3 substrate surface. Such atom-by-atom engineering and characterizations of oxide heterostructures not only provide microscopic understanding of formation process of interfaces in metal-oxides, but also would lead to the creation of exotic electronic phenomena and novel functionalities at these interfaces.