DOE OSTI2017
Homoepitaxial SrTiO 3 (110) film is grown by molecular beam epitaxy in ultra-high vacuum with oxygen diffusing from substrate as the only oxidant. The resulted oxygen vacancies (V O s ) are found to be spatially confined within few subsurface layers only, forming a quasi-two-dimensional doped region with a tunable high concentration. Such a δ-function distribution of V O s is essentially determined by the thermodynamics associated with the surface reconstruction, and facilitated by the relatively high growth temperature. Here, our results demonstrate that it is feasible to tune V O s distribution at the atomic scale by controlling the lattice structure of oxide surfaces. Transition metal oxide interfaces have exhibited a variety of novel phenomena, including the high-mobility two-dimensional electron gas, superconductivity and unusual magnetism. While these phenomena are inherently related to the artificially designed architecture, oxygen vacancies (V O s ) are recognized to be particularly important in determining the exotic behaviors of the oxides systems. As electron donors in oxides, V O s were observed to be one of the major factors for the high carrier density at the LaAlO 3 /SrTiO 3 interface. V O s induce ferromagnetism in the epitaxial LaCoO 3 films via the ordering of excess electrons in Co 3d orbit. It was also suggested that the existence of V O s plays an important role in the formation of undesirable insulating phase in ultrathin films of many metallic oxide materials, referred to as the “dead layer” behavior. How V O s are involved in such complex phenomena is still unclear. One essential issue toward the design of emergent properties of oxide films is how to characterize and control V O s at the atomic scale. Recently, quantitative measurements of V O concentration have been realized with high spatial resolution benefited by the development of the state-of-the-art aberration-corrected scanning transmission microscope (STEM) and related spectroscopy techniques. However, tuning the V O density precisely by growth control is still extremely challenging. Different oxidants, i.e., molecular O 2 , O 3 and atomic O, have been used with different feeding pressure during the growth. This approach is limited by thermodynamics of the material since the pressure of oxidant has to coordinate with temperature and other parameters in order to ensure the formation of the desired phase. In many cases the optimal condition for oxygen stoichiometry cannot be reached, and residual V O s are inevitably formed. It has been reported that V O s even form inhomogeneous clusters in SrTiO 3 (STO) films. Post annealing is another thermodynamic approach and seems to be effective to reduce the V O density. However, it does not allow the precise control of V O s distribution at the atomic scale either. In the current work, to study the control of V O s at the atomic scale, we grow SrTiO 3 homoepitaxial films along the polar [110] direction. Atomically well-defined (4 × 1) reconstruction is maintained on the surface all through the growth [see Fig. 1 (a) and (b)], which compensates the polarity. More importantly, the energy configuration associated with the surface reconstruction results in the spatial confinement of V O s within only few subsurface layers of the film, while their concentration remains extremely low on the topmost surface and in the bulk of STO substrate. In conclusion, we show that such a quasi-two-dimensional (2D) V O -doped layer, mimicking the δ-doping case in conventional semiconductors, can be precisely controlled.