Induced work function changes at Mg-doped MgO/Ag(001) interfaces: Combined Auger electron diffraction and density functional study

Abstract

The properties of MgO/Ag(001) ultrathin films with substitutional Mg atoms in the interface metal layer have been investigated by means of Auger electron diffraction experiments, ultraviolet photoemission spectroscopy, and density functional theory (DFT) calculations. Exploiting the layer-by-layer resolution of the Mg KL23L23 Auger spectra and using multiple scattering calculations we first determine the inter-layer distances as well as the morphological parameters of the MgO/Ag(001) system with and without Mg atoms incorporated at the interface. We find that the Mg atom incorporation drives a strong distortion of the interface layers and that its impact on the metal/oxide electronic structure is an important reduction of the work function (0.5 eV) related to band-offset variations at the interface. These experimental observations are in very good agreement with our DFT calculations which reproduce the induced lattice distortion and which reveal (through a Bader analysis) that the increase of the interface Mg concentration results in an electron transfer from Mg to Ag atoms of the metallic interface layer. Although the local lattice distortion appears as a consequence of the attractive (repulsive) Coulomb interaction between O 2− ions of the MgO interface layer and the nearest positively (negatively) charged Mg (Ag) neighbors of the metallic interface layer, its effect on the work function reduction is only limited. Finally, an analysis of the induced work function changes in terms of charge transfer, rumpling, and electrostatic compression contributions is attempted and reveals that the metal/oxide work function changes induced by interface Mg atoms incorporation are essentially driven by the increase of the electrostatic compression effect.

Reactive metal–oxide interfaces: A microscopic view

“…Depending on the oxygen partial pressure and the substrate temperature employed during the reactive deposition of ultra-thin Mg oxide films, the p-type Schottky barrier has been tuned over 0.7 eV [125]. These remarkable variations of the Schottky barrier were ascribed to the decrease of the silver work function induced by the Mg enrichment of the substrate surface region (associated to the formation of a Mg-Ag alloy) when Mg is evaporated in oxygen-poor conditions or at high substrate temperatures [125,126]…”

Controlling the electronic and physical coupling on dielectric thin films

“…In addition, PT confirms an integer charge transfer, which would be expected to result from tunneling [15]. However, using the ability to tune the MgO(100)/Ag(100) work function after MgO film growth (Φ MgO ) by changing the composition at the dielectric-metal interface without changing its surface [16][17][18][19], it has been recently shown that two distinct adsorption regimes exist [20]. On films with a high Φ MgO , all molecules in the ML remain neutral, whereas on a low-Φ MgO substrate, charge transfer is observed.…”

Excited states at interfaces of a metal-supported ultrathin oxide film

“…The metal-induced gap states at the metal/oxide interface, the oxide band gap, and a surface core exciton involving an image-potential state of the vacuum are revealed through resonant Auger spectroscopy of the Mg KL 23 (001) is a model system of the metal/oxide interface at the ultrathin limit. Although the structure and the growth mechanism [4][5][6][7][8][9], as well as changes in electronic properties associated with depositing ultrathin films of MgO on Ag(001), have been investigated [10][11][12][13][14][15][16][17], capturing the physical nature of such a mixed system, and in particular of the interfaces, remains challenging.In resonant auger spectroscopy (RAS), the Auger process can be very different from that occurring with photon energies far above absorption thresholds [18]. Sub-lifetime narrowing effects [19], as well as strong modulations in Auger signals [20], can occur.…”

“…In this context, MgO/Ag(001) is a model system of the metal/oxide interface at the ultrathin limit. Although the structure and the growth mechanism [4][5][6][7][8][9], as well as changes in electronic properties associated with depositing ultrathin films of MgO on Ag(001), have been investigated [10][11][12][13][14][15][16][17], capturing the physical nature of such a mixed system, and in particular of the interfaces, remains challenging.…”

“…The MgO films were grown in situ on Ag(001) (for details see Ref. [5]). X-ray absorption was measured by recording the total electron yield (TEY) and RAS was obtained at room temperature using a VG Scienta EW4000 with 60…”