Impact on Ferroelectricity and Band Alignment of Gradually Grown Au on BaTiO₃

Abstract

The competition between interface barrier in the Schottky-Mott limit and polarization driven mechanism is established during gradual formation of metal (Au)ferroelectric (BaTiO 3 ) interface. X-ray photoelectron spectroscopy provides core level energies and valence band positions in the contact region, to monitor the band alignment from the very first stages of metal deposition on BaTiO 3 . The band bending at metal/ferroelectric (FE) interface is extracted from the shift of core levels (Ba 3d, Ti 2p) as a function of the metal thickness. It is shown that the interface band alignment mechanism involves a well-defined polarization orientation washing out the bending expected from the work function difference. The sudden modification of the binding energies within ferroelectric at the first 2 ÅAu indicates that the ferroelectric compensation mechanism triggered by the metal overlayer initiates already at ultrathin top layer, while subsequent growth contributes only at a gradual correction of the potential in the FE. The emerging picture is confirmed in first-principle calculation indicating the preferences of Au to grow preferentially to different terminated regions and to stabilize distinct ferroelectric polarization.Ferroelectric materials exhibit a spontaneous polarization, which can be reversed by applying a voltage exceeding the coercive field. [1] They are used in a wide number of applications such as sensors and actuators, [2,3] in ferroelectric capacitors [4] and nonvolatile memories. [5,6] In most of these applications, the ferroelectric (FE) system is in direct contact with a metal electrode, and the device physics is strongly influenced by the metal-ferroelectric interface. Consequently, understanding the mechanism of metal/FE interface formation is of central importance in device design. Ferroelectric thin films may present a particular single-domain ferroelectric state with the polarization perpendicular to the film, which is connected to the existence of free charge carriers into the film. [7] Electrons and holes from the ferroelectric thin film may then be transferred by the internal field (P=e, where P ¼ polarization perpendicular to the surface of the FE, e ¼ e 0 e r ¼ dielectric permittivity of the FE)

Experimental Band Structure of Pb(Zr,Ti)O3: Mechanism of Ferroelectric Stabilization

“…These results are consistent with previous findings when metal/FE interfaces feature positive or negative charging in order to stabilize opposed FE states. [ 25,34,46,47,57,60 ]…”

“…Hence, the electronic states of PZT in the UP sample are displaced toward higher BE compared to PZT in DW. Such variation of the potential inside PZT is the standard indication of the opposite, out of plane FE states [25,[44][45][46][47][48][49] in free-standing ferroelectrics. It occurs in principle when negative/positive charge carriers accumulate close to the surfaces in order to screen the accompanying DF established in the material with opposed P+/P− FE states.…”

“…[25,34] The band alignment picture and the resulting potential profile across the FE defines the regions with positive/negative charge accumulation at the extremities of the FE. Under applied bias, it controls how and if the required electrons and/or holes are injected across the interface and how, by diffusing through material, accumulate at the surface/interfaces of the FE [16,25,46,47,57] in order to compensate for the DF. This further translates into material-dependent potential inside the FE as well as in the joining contacts; and ii) on the polar character of the interface where the electric dipoles possibly resulting at the interface may locally account for additional charges required to stabilize the FE state.…”

“…These results are consistent with previous findings when metal/FE interfaces feature positive or negative charging in order to stabilize opposed FE states. [25,34,46,47,57,60] The substrate-induced band bending at the bottom interface appears as an essential condition to stabilization of the FE state, facilitating the accumulation of negative/positive charge sheets at the surface/bottom interface to stabilize the P+/P− FE polarization.…”

First-principles calculations of Schottky barrier height at barium titanate/metal interface

“…[14][15][16][17] However, few theoretical studies have focused on the interface insulating properties in BTO/metal systems. [18][19][20] In this study, we performed first-principles calculations to analyze the SBH at the interface between BTO and metals with a cubic crystal structure. Section 2 presents the details of the calculation method, Sect.…”