[Paper Review] Surface and Interface Properties of La2/3Sr1/3MnO3 Thin Films on SrTiO3 (001)

Lina Chen, Zhen Wang+7 more|arXiv (Cornell University)|Apr 10, 2019

Magnetic and transport properties of perovskites and related materials2 references4 citations

TL;DR

This study investigates the surface and interface properties of La2/3Sr1/3MnO3 thin films on SrTiO3 (001) substrates using in-situ and ex-situ characterization techniques. It reveals that Sr segregation (~3 unit cells) at the (La/Sr)O-terminated surface and interfacial intermixing (~1 unit cell) govern a thickness-driven metal-insulator transition, with nonmetallic behavior emerging in films thinner than 20 unit cells due to stoichiometric deviations near the surface.

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, La2/3Sr1/3MnO3 grown on SrTiO3(001), 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 La2/3Sr1/3MnO3 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 ~ 6.5 +/- 0.9 u.c., 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 MnO2 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.

Motivation & Objective

• To understand the structural and electronic origins of the metal-insulator transition in La2/3Sr1/3MnO3 thin films on SrTiO3 (001).
• To characterize interfacial intermixing and surface chemical segregation at atomic-scale resolution.
• To correlate film thickness, surface stoichiometry, and electrical conductivity in oxide heterostructures.
• To establish a structure-property relationship explaining the thickness-dependent insulating behavior in ultrathin manganite films.

Proposed method

• In-situ angle-resolved X-ray photoelectron spectroscopy (ARXPS) to probe depth-dependent chemical states and valence band alignment at the surface and interface.
• Ex-situ X-ray diffraction (XRD) to assess film crystallinity, orientation, and strain.
• Scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) for atomic-resolution chemical and structural analysis.
• Electric transport measurements to correlate film thickness with conductivity and metal-insulator transition.
• Analysis of Sr concentration deviation near the surface as a function of film thickness.
• Use of (La/Sr)O surface termination model to interpret surface chemistry and its impact on electronic properties.

Experimental results

Research questions

• RQ1 How does interfacial intermixing affect the electronic properties of La2/3Sr1/3MnO3/SrTiO3 heterostructures?
• RQ2 What is the extent and chemical nature of Sr segregation at the surface of La2/3Sr1/3MnO3 films as a function of film thickness?
• RQ3 Why do films thinner than 20 unit cells exhibit insulating behavior despite bulk La2/3Sr1/3MnO3 being metallic?
• RQ4 How does surface termination (La/Sr)O vs MnO2 influence the electronic structure and conductivity?
• RQ5 To what extent does stoichiometric deviation near the surface, rather than thickness alone, govern the metal-insulator transition?

Key findings

• La2/3Sr1/3MnO3 films thicker than 20 unit cells exhibit at most one unit cell of interfacial intermixing with SrTiO3, indicating a sharp interface.
• At least three unit cells of Sr segregation are observed near the surface, which is (La/Sr)O-terminated, and this segregation is thickness-dependent.
• A nonmetallic layer of thickness ~6.5 ± 0.9 unit cells is identified, independent of overall film thickness, and primarily driven by Sr concentration deviation near the surface.
• Below 20 unit cells, the surface transitions to a mixed (La/Sr)O-terminated phase with MnO2 termination, contributing to insulating behavior.
• For films thinner than 10 unit cells, enhanced chemical composition deviation leads to insulating behavior, explaining the thickness-driven metal-insulator transition.
• The metal-insulator transition is attributed not to film thickness per se, but to surface stoichiometry deviations, particularly Sr segregation and non-stoichiometric surface layers.