Metal/Oxide Interfacial Reactions:  Oxidation of Metals on SrTiO₃ (100) and TiO₂ (110)

Abstract

We studied chemical reactions between ultrathin metal films (Al, Cr, Fe, Mo) and single-crystal oxides (SrTiO3 (100), TiO2 (110)) with X-ray photoelectron spectroscopy (XPS). The work function of the metal and the electron density in the oxide strongly influence the reaction onset temperature (T(RO)), where metal oxidation is first observed, and the rate of metal oxidation at the metal/oxide interfaces. The Fermi levels of the two contacting phases affect both the space charges formed at the interfaces and the diffusion of ionic defects across the interfaces. These processes, which determine metal oxidation kinetics at relatively low temperatures, can be understood in the framework of the Cabrera-Mott theory. The results suggest that the interfacial reactivity is tunable by modifying the Fermi level (E(F)) of both contacting phases. This effect is of great technological importance for a variety of devices with heterophase boundaries.

Metallicity of the SrTiO3 surface induced by room temperature evaporation of alumina

“…But out-diffusion of oxygen at room temperature has been much rarely reported. Metal oxidation and corresponding SrT iO 3 reduction was indeed observed at room temperature when reactive metals are deposited on top of SrT iO 3 surfaces under ultra high vacuum [24,25], the results being strongly dependent on the metal element [26]. The heat of formation of the corresponding metal oxide determines if the metal oxidation is energetically favorable and the metal work function influences the onset temperature of metal oxidation.…”

“…The role of the metal work function was attributed to the existence of local electric fields at the metal-SrT iO 3 interface that favor or prevent the out-diffusion of the negatively charged oxygen ions. Metal oxidation was indeed observed under ultra high vacuum and at room temperature for Al [25], Y , T i and Ba [24], metals which combine large oxide heat of formations and low metal work functions [26,27]. For Y , T i and Ba, the metal oxidation was found to be diffusion limited, with a gradual decrease of the oxygen content in the first nanometers of the metal overlayer [24].…”

Reconstructions at complex oxide interfaces

“…the relative Fermi level of the metal and that of the TiO 2 or SrTiO 3 before contact. [90,91] An interfacial redox reaction 4 with TiO 2 can occur at room temperature when the heat of metal oxide formation per mole of oxygen, ∆H f O , is lower than -250 kJ/(mol O) and the work function of the metals, ϕ, is in the range of 3.75 eV < ϕ < 5.0 eV. [90] The heat of metal oxide formation and the work function of several metals are shown in Figure 3.12.…”

“…For example, Cr reacts with SrTiO 3 above 640 • C, while Al oxidizes already at room temperature. [91] For metals with an relatively high ϕ or less negative ∆H f O , the temperature has to be increased above room temperature to result in an interfacial redox reaction between metal and SrTiO 3 . [91,93] So far, the focus was on the B-site atoms.…”

“…[91] For metals with an relatively high ϕ or less negative ∆H f O , the temperature has to be increased above room temperature to result in an interfacial redox reaction between metal and SrTiO 3 . [91,93] So far, the focus was on the B-site atoms. However, the influence of, e.g., La is expected to be minimal as LaB O 3 -SrTiO 3 interfaces could become conducting or remained insulating.…”