Abstract
Understanding and manipulating properties emerging at a surface or an interface require a thorough knowledge of structure-property relationships. We report a study of a prototype oxide system, grown on , by combining in situ angle-resolved x-ray photoelectron spectroscopy, ex situ x-ray diffraction, and scanning transmission electron microscopy/spectroscopy with electric transport measurements. We find that films thicker than 20 unit cells (u.c.) exhibit a universal behavior with no more than one u.c. intermixing at the interface but at least 3 u.c. of Sr segregation near the surface, which is (La/Sr)O terminated. The conductivity vs film thickness shows the existence of nonmetallic layers with thickness , which is independent of film thickness but mainly relates to the deviation of Sr concentration near the surface region. Below 20 u.c., the surface of the films appears mixed, (La/Sr)O with termination. Decreasing film thickness to less than 10 u.c. leads to the enhanced deviation of chemical composition in the films and eventually drives the film insulating. Our observation offers a natural explanation for the thickness-driven metal-nonmetal transition in thin films based on the variation of film stoichiometry.
- Received 9 June 2018
- Revised 25 January 2019
DOI:https://doi.org/10.1103/PhysRevMaterials.3.044407
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