Interface electronic structure in a metal/ferroelectric heterostructure under applied bias

The effective barrier height between an electrode and a ferroelectric (FE) depends on both macroscopic electrical properties and microscopic chemical and electronic structure. The behavior of a prototypical electrode/FE/electrode structure, Pt/BaTiO${}_3$/Nb-doped SrTiO${}_3$, under in-situ bias voltage is investigated using x-ray photoelectron spectroscopy. The full band alignment is measured and is supported by transport measurements. Barrier heights depend on interface chemistry and on the FE polarization. A differential response of the core levels to applied bias as a function of the polarization state is observed, consistent with Callen charge variations near the interface.

Figure 1

(a) Schematic of capacitor; (b) Pt $4f$ intensity map for the Pt/BTO/NSTO sample showing 20 identical Pt/BTO/NSTO capacitors ($300\times 300$ $\mu {\mathrm{m}}^2$) on the $5\times 5$ mm${}^2$ surface, allowing location of the wired capacitor.

Figure 2

(a) (002) and (b) (202) peaks for BTO and STO. BTO is relaxed onto STO with a tetragonality ratio $c/a$ of 1.040. (c) Reflectivity curve indicating the BTO thickness. (d) LEED image of BTO surface before Pt deposition showing $c(2\times 2)$ surface reconstruction.

Figure 3

XPS spectra before (top) and after (bottom) Pt deposition.

Figure 4

(a) C-V measurements for Pt/BTO/NSTO (black circles) and Pd/Al${}_2$O${}_3$/BTO/NSTO (blue squares). (b) J-V measurements for Pt/BTO/NSTO [black triangles, upward (downward) triangles for increasing (decreasing) voltage sweep], Pd/Al${}_2$O${}_3$/BTO/NSTO (blue squares), and Pd/Pt (inset). (c) $\ln \left|J\right|-|V_{\mathrm{top}}{|}^{1/2}$ showing conduction limited by Schottky emission (arrows indicates sweep direction). (d) J-V curve showing Ohmic behavior for $V_{\mathrm{top}}>V_{\mathrm{c}}$.

Figure 5

(a) Capacitance without $C_{\mathrm{dark}}$ (black circles) and with $C_{\mathrm{photon}}$ (red squares) synchrotron irradiation, (b) capacitor under illumination, short circuit. Photogenerated electrons (holes) are filled (empty) circles.

Figure 6

Photoemission spectra for the $\mathrm{P}+$ (top) and $\mathrm{P}-$ (bottom) FE states for the Ti $2p_{3/2}$–$2p_{1/2}$ (left) and Ba $3d_{5/2}$ (right). The bottom spectra have been shifted by 0.70 eV towards lower BE using Pt $4f$ spectra.

Figure 7

Band lineup for the $\mathrm{P}+$ (left) and $\mathrm{P}-$ (right) polarization states.

Figure 8

Titanium $2p$ spectra for $P+$ to $\mathrm{P}-$ and their intensity difference. $\mathrm{P}-$ spectra have been shifted in binding energy using ${\Delta }_{\mathrm{BE}}$ from Table . The two spectra have identical shapes.