Depth Profiling Charge Accumulation from a Ferroelectric into a Doped Mott Insulator

Abstract

The electric field control of functional properties is a crucial goal in oxide-based electronics.Non-volatile switching between different resistivity or magnetic states in an oxide channel can be achieved through charge accumulation or depletion from an adjacent ferroelectric. However, the way in which charge distributes near the interface between the ferroelectric and the oxide remains poorly known, which limits our understanding of such switching effects.Here we use a first-of-a-kind combination of scanning transmission electron microscopy with electron energy loss spectroscopy, near-total-reflection hard X-ray photoemission spectroscopy, and ab-initio theory to address this issue. We achieve a direct, quantitative, atomic-scale characterization of the polarization-induced charge density changes at the interface between the ferroelectric BiFeO3 and the doped Mott insulator Ca1-xCexMnO3, thus providing insight on how interface-engineering can enhance these switching effects. 3The ferroelectric control of functional properties in strongly correlated oxide nanostructures through electrostatic carrier density modulations has inspired intensive research efforts 1,2 .The field effect technique is particularly attractive for nanostructures consisting of complex oxides such as mixed-valence manganites, as these electron-correlated systems exhibit rich phase diagrams 3 .Hole-doped mixed-valence manganites with predominant Mn 3+ oxidation state have been extensively investigated [3][4][5][6][7][8] . In particular, recent works have been devoted on the interface between the half-metal La1-xSrxMnO3 (LSMO) manganite and ferroelectrics such as Pb(ZrxTi1- In these recent reports 13,14 , the resistivity change in the manganite channel induced by polarization switching in the ferroelectric BiFeO3 was limited to one order of magnitude while changes of several orders were reported for thin films of CaMnO3 tuned by different doping levels, strains or by electrolyte gating 12 . Typical electron injection of around 0.1 free electrons in the manganite channel were estimated by Hall measurements and associated with this resistivity change.A charge distribution measurement on a unit cell scale at the Ca manganite/ferroelectric interface is thus required to understand, control and possibly enhance the magnitude of the resistivity switching effects. In the present study, a first-of-a-kind combination of scanning transmission electron microscopy (STEM) coupled to electron energy loss spectroscopy (EELS) [15][16][17][18] , and grazing-incidence hard x-ray photoemission spectroscopy (HAXPES) in a near-total-reflection (NTR) condition have been used as site-sensitive valence probes to address the electronic and structural properties of BiFeO3 and Ca1-xCexMnO3 heterostructures (x=0, 2, 4 at.% nominal Ce concentrations) at the atomic scale. Our experimental results have also been compared to local density functional theory (DFT).We have used these two novel and complementary methods to evaluate electron densities in manganite oxides. ...

STEM-EELS Investigation of Charge and Strain Distributions in Perovskite Oxide Thin Films

“…These physical properties strongly depend on the strain and charge distribution in these devices. In this work, we report that charge distribution next to the BFO/CMO interface is highly inhomogeneous by combining grazing-incidence hard x-ray photoemission spectroscopy and STEM-EELS measurements [4]. Typical sheet carrier densities in the range of 10 14 cm -2 are obtained by EELS that are in line with Hall effect measurements but lower than expected to screen the BFO ferroelectric polarization.…”

“…Typical sheet carrier densities in the range of 10 14 cm -2 are obtained by EELS that are in line with Hall effect measurements but lower than expected to screen the BFO ferroelectric polarization. Ab-initio modeling concludes of the crucial role of plane termination [4]. The structure of ferroelectric domains in the BFO thin films will also be discussed with respect to the charge in the CMO channel.…”

Tunnel electroresistance in BiFeO3 junctions: size does matter

“…29,30 Hence, ferroelectric tunnel junctions offer a fantastic playground to explore electric-field-driven modifications at the nanoscale. 31 Top electrodes of Pt (10 nm) / Co (10 nm) with diameters ranging from 180 nm to 1200 nm ( Fig. 1(a)) are defined by electron-beam lithography, sputtering, and lift-off.…”

Engineering ferroelectric tunnel junctions through potential profile shaping

“…25 Recent cross section scanning transmission electron microscopy images evidence a sharp interface between CCMO and BFO with a large c/a ratio for the ultrathin ferroelectric film, which is characteristic of the T-phase. 26 In addition, analysis of the cation displacements in T-BFO suggests a large ferroelectric polarization pointing toward the CCMO in the virgin state. 26 We finally defined solid-state FTJs by e-beam lithography and lift-off of sputter-deposited TEs of W, Co, Ni, and Ir with submicron diameters.…”

“…26 In addition, analysis of the cation displacements in T-BFO suggests a large ferroelectric polarization pointing toward the CCMO in the virgin state. 26 We finally defined solid-state FTJs by e-beam lithography and lift-off of sputter-deposited TEs of W, Co, Ni, and Ir with submicron diameters. The metallic electrodes are 10 nm thick and capped by 10 nm of Pt.…”