Orientation-dependent chemistry and band-bending of Ti on polar ZnO surfaces
Orientation-dependent reactivity and band-bending are evidenced upon Ti deposition (1–10 Å) on polar ZnO(0001)–Zn and ZnO(000)–O surfaces. At the onset of the Ti deposition, a downward band-bending was observed on ZnO(000)–O while no change occurred on ZnO(0001)–Zn. Combining this with the photoemission analysis of the Ti 2p core level and Zn L3(L2)M45M45 Auger transition, it is established that the Ti/ZnO reaction is of the form Ti + 2ZnO → TiO2 + 2Zn on ZnO(0001)–Zn and Ti + yZnO → TiZnxOy + (y − x)Zn on ZnO(000)–O. Consistently, upon annealing thicker Ti adlayers, the metallic zinc is removed to leave ZnO(0001)–Zn surfaces covered with a TiO2-like phase and ZnO(000)–O surfaces covered with a defined (Ti, Zn, O) compound. Finally, a difference in the activation temperature between the O-terminated (500 K) and Zn-terminated (700 K) surfaces is observed, which is tentatively explained by different electric fields in the space charge layer at ZnO surfaces.