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Title: δ-Doping of oxygen vacancies dictated by thermodynamics in epitaxial SrTiO 3 films

Abstract

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 Os) 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 Os 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 Os 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 Os) are recognized to be particularly important in determining the exotic behaviors of the oxides systems. As electron donors in oxides, V Os were observed to be one of the major factors for the high carrier density at the LaAlO 3/SrTiO 3 interface. V Os induce ferromagnetism in the epitaxial LaCoO 3 films via the ordering of excess electrons in Co 3d orbit. It wasmore » also suggested that the existence of V Os 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 Os 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 Os 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 Os are inevitably formed. It has been reported that V Os 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 Os distribution at the atomic scale either. In the current work, to study the control of V Os 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 Os 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.« less

Authors:
 [1];  [2];  [2];  [2];  [2];  [2];  [2]; ORCiD logo [2];  [2];  [3];  [3];  [4]
  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
Publication Date:
Research Org.:
Louisiana State Univ., Baton Rouge, LA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1430203
Grant/Contract Number:  
SC0002136
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
AIP Advances
Additional Journal Information:
Journal Volume: 7; Journal Issue: 6; Journal ID: ISSN 2158-3226
Publisher:
American Institute of Physics (AIP)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; 36 MATERIALS SCIENCE

Citation Formats

Li, Fengmiao, Yang, Fang, Liang, Yan, Li, Shanming, Yang, Zhenzhong, Zhang, Qinghua, Li, Wentao, Zhu, Xuetao, Gu, Lin, Zhang, Jiandi, Plummer, E. W., and Guo, Jiandong. δ-Doping of oxygen vacancies dictated by thermodynamics in epitaxial SrTiO3 films. United States: N. p., 2017. Web. doi:10.1063/1.4985048.
Li, Fengmiao, Yang, Fang, Liang, Yan, Li, Shanming, Yang, Zhenzhong, Zhang, Qinghua, Li, Wentao, Zhu, Xuetao, Gu, Lin, Zhang, Jiandi, Plummer, E. W., & Guo, Jiandong. δ-Doping of oxygen vacancies dictated by thermodynamics in epitaxial SrTiO3 films. United States. doi:10.1063/1.4985048.
Li, Fengmiao, Yang, Fang, Liang, Yan, Li, Shanming, Yang, Zhenzhong, Zhang, Qinghua, Li, Wentao, Zhu, Xuetao, Gu, Lin, Zhang, Jiandi, Plummer, E. W., and Guo, Jiandong. Thu . "δ-Doping of oxygen vacancies dictated by thermodynamics in epitaxial SrTiO3 films". United States. doi:10.1063/1.4985048. https://www.osti.gov/servlets/purl/1430203.
@article{osti_1430203,
title = {δ-Doping of oxygen vacancies dictated by thermodynamics in epitaxial SrTiO3 films},
author = {Li, Fengmiao and Yang, Fang and Liang, Yan and Li, Shanming and Yang, Zhenzhong and Zhang, Qinghua and Li, Wentao and Zhu, Xuetao and Gu, Lin and Zhang, Jiandi and Plummer, E. W. and Guo, Jiandong},
abstractNote = {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 (VOs) 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 VOs 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 VOs 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 (VOs) are recognized to be particularly important in determining the exotic behaviors of the oxides systems. As electron donors in oxides, VOs were observed to be one of the major factors for the high carrier density at the LaAlO3/SrTiO3 interface. VOs 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 VOs 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 VOs 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 VOs at the atomic scale. Recently, quantitative measurements of VO 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 VO 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 VOs are inevitably formed. It has been reported that VOs even form inhomogeneous clusters in SrTiO3 (STO) films. Post annealing is another thermodynamic approach and seems to be effective to reduce the VO density. However, it does not allow the precise control of VOs distribution at the atomic scale either. In the current work, to study the control of VOs 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 VOs 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) VO-doped layer, mimicking the δ-doping case in conventional semiconductors, can be precisely controlled.},
doi = {10.1063/1.4985048},
journal = {AIP Advances},
number = 6,
volume = 7,
place = {United States},
year = {Thu Jun 01 00:00:00 EDT 2017},
month = {Thu Jun 01 00:00:00 EDT 2017}
}

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