Band alignment at memristive metal-oxide interfaces investigated by hard x-ray photoemission spectroscopy

The electronic structure and band alignment at metal/oxide interfaces for nonvolatile memory applications are investigated by hard x-ray photoelectron spectroscopy (HAXPES) and DC transport measurements, using acceptor doped ${\mathrm{SrTiO}}_3$ as a model memristive oxide. Metal-insulator-metal (MIM) structures with a noble metal (Pt) top electrode form a Schottky barrier and exhibit rectifying properties, while a reactive metal (Ti) as top electrode shows symmetric I(V) characteristics and a flat band situation at the interface. The transition from rectifying to ohmic I(V) relations with increasing Ti thickness is discussed with respect to the electrochemical reaction at the interface, the band alignment at the electrode/oxide interface, and the slope of the energy bands across the MIM structure.

Figure 1

(a)–(c) I(V) characteristics of pristine MIM structures based on 20 nm STFO films grown on Nb:STO, with top electrodes of 30 nm Pt, 25 nm Pt/5 nm Ti, and 20 nm Pt/10 nm Ti, respectively. Insets depict the sample and measurement geometry. (d)–(f): Switching curves of MIM structures corresponding to the pristine states (a), (b), and (c), respectively. The switching polarity is indicated by the red arrows in (d) and is the same for all three samples.

Figure 2

a) Core level emission lines of ${\mathrm{SrTi}}_{}{\mathrm{Fe}}_x{\mathrm{O}}_{3-\delta }$ thin films ($x=0$, 0.02). b) Valence band maximum, as determined by the Kraut method, for different doping concentrations. The band gap energy ${\mathrm{E}}_{\mathrm{Gap}}=3.25\mathrm{eV}$ and the thermal energy $3kT=75$ meV are indicated in red and blue dashed lines, respectively.

Figure 3

(a), (b): I(V) characteristics of two MIM devices in the pristine state, based on a 20 nm and a 10 nm STFO film, respectively, with a 25 nm Pt/5 nm Ti top electrode. The top electrodes of both samples were deposited at the same time to ensure equal film thickness. (c) Ti 2p core level spectra recorded on samples with 2 nm Pt/4 nm Ti electrodes, deposited on a 10 nm ${\mathrm{SrTi}}_{0.95}{\mathrm{Fe}}_{0.05}{\mathrm{O}}_{3-\delta }$ (STFO5) and 20 nm STFO film (blue and red, respectively), plus a 2 nm Pt/20 nm STFO sample (black line) and a Nb:STO (0.5 wt%) single crystal for reference. The Pt/STFO spectrum was shifted in energy by +0.9 eV, and the Nb:STO by $-0.2$ eV to facilitate the comparison of the spectra.

Figure 4

Core level and valence band emission from the Pt reference (blue), the STFO reference (red), the 2 nm Pt/STFO interface (black), and the 2 nm Pt/4 nm Ti /STFO (green). The Au Fermi edge for photon energy calibration is shown in gray.

Figure 5

Schematic band diagram for (a) Pt/STFO and (b) ${\mathrm{Pt}/\mathrm{TiO}}_x/\mathrm{STFO}$ interface with a thin Ti interlayer, respectively. The work function of Pt is ${\Phi }_{Pt}=5.65\mathrm{eV}$, and the electron affinity of STO is ${\chi }_{STO}=4.1\mathrm{eV}$.