δ-Doping of oxygen vacancies dictated by thermodynamics in epitaxial SrTiO₃ films

Journal Article · Thu Jun 01 00:00:00 EDT 2017 · AIP Advances

DOI:https://doi.org/10.1063/1.4985048· OSTI ID:1430203

Li, Fengmiao ^[1]; Yang, Fang ^[2]; Liang, Yan ^[2]; Li, Shanming ^[2]; Yang, Zhenzhong ^[2]; Zhang, Qinghua ^[2]; Li, Wentao ^[2]; Zhu, Xuetao  ^[2]; Gu, Lin ^[2]; Zhang, Jiandi ^[3]; Plummer, E. W. ^[3]; Guo, Jiandong ^[4]

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1. Beijing National Laboratory for Condensed Matter Physics & Institute of Physics, Chinese Academy of Sciences (China); Univ. of British Columbia, Vancouver, BC (Canada). Stewart Blusson Quantum Matter Institute, Department of Physics and Astronomy
2. Beijing National Laboratory for Condensed Matter Physics & Institute of Physics, Chinese Academy of Sciences (China)
3. Louisiana State Univ., Baton Rouge, LA (United States). Department of Physics and Astronomy
4. Beijing National Laboratory for Condensed Matter Physics & Institute of Physics, Chinese Academy of Sciences (China); Collaborative Innovation Center of Quantum Matter, Beijing (China); University of Chinese Academy of Sciences, Beijing (China). School of Physical Science

Homoepitaxial SrTiO₃(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 LaAlO₃/SrTiO₃ interface. V_O^s induce ferromagnetism in the epitaxial LaCoO₃ 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 O₂, O₃ 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 SrTiO₃ (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 SrTiO₃ 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.

• Research Organization: Louisiana State Univ., Baton Rouge, LA (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: SC0002136
• OSTI ID: 1430203
• Journal Information: AIP Advances, Vol. 7, Issue 6; ISSN 2158-3226
• Publisher: American Institute of Physics (AIP)
• Country of Publication: United States
• Language: English

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75 CONDENSED MATTER PHYSICS
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36 MATERIALS SCIENCE