Mitsuhiro Saito (MSc)

The Pd/ZnO interface was produced by growing 30 nm thick epitaxial Pd overlayers on the ZnO(0001) surface. The local atomistic structure of the interface was analyzed with UHRTEM (JEM ARM-1250 at 1250kV). Prior to deposition the ZnO (0001) surface was annealed in different atmospheres and at different temperatures. The surface structure was examined by surface x-ray diffraction [3] with synchrotron radiation at ANKA (Angstrom Quelle Karlsruhe, Germany). A quantitative analysis of the crystal truncation rod (CTR) measurements (Fig. 1) revealed that the ZnO(0001) surface was unreconstructed and had steps with a height of c/2 (c: ZnO lattice constant) indicating that the oxide surface was Zn terminated.

Figure 1: Crystal truncation rod (950 °C, 1atm oxygen). Measured and simulated CTRs were compared quantitatively.

Depending on the substrate surface processing the occupancy of the topmost Zn layer was 25% (950 °C, 1 atm oxygen ), 40% (600 °C, UHV), and 75% (600 °C, 0.1 atm oxygen). We used the defect model of Kröger [4] to calculate the point defect concentration in ZnO. An increase of the Zn interstitial concentration is related to a decrease of the Zn concentration on the ZnO surface. A treatment of ZnO in an oxidizing environment (600 °C, 0.1 atm oxygen) decreases the Zn interstitial concentration resulting in a segregation of Zn to the ZnO surface. Our experimentally determined final Zn concentration of 75% agrees well with that calculated by Noguera [5]. On UHV annealed surfaces we deposit Pd at 600 °C by MBE. ARM images (Fig. 2) showed atomically flat Pd/ZnO interfaces with a largely increased Zn occupancy (Fig. 3) compared to the UHV annealed substrate 40% (Zn occupancy).

Figure 2: ARM image of the Pd/ZnO interface. The interface is atomically abrupt. The following well defined orientation relationship was extracted: Pd(111)//ZnO(0001) and Pd[110]//ZnO[11-20]

Figure 3: Simulated and experimentally recorded HRTEM images of the Pd/ZnO interface. The image simulations were performed for interfaces with different Zn occupancies, i.e. 40 % and 100 %

ELNES spectra (O-K edge) recorded in bulk ZnO and in the interface indicated Zn termination of the Pd/ZnO interface. This is consistent with first principle calculations of the Pd/ZnO interface [6]. The present results suggest that the interaction between Pd and ZnO is fairly weak and of mixed ionic and covalent character.