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 \AA) on the polar ZnO(0001)-Zn and ZnO(000$\bar{1}$)-O surfaces. At the onset of the Ti deposition, a downward band-bending was observed on ZnO(000$\bar{1}$)-O while no change occurred on ZnO(0001)-Zn. Combining this with the photoemission analysis of the Ti 2p core level and Zn L$_3$(L$_2$)M$_{45}$M$_{45}$ Auger transition, it is established that the Ti/ZnO reaction is of the form Ti + 2 ZnO $\rightarrow$ TiO$_2$ + 2 Zn on ZnO(0001)-Zn and Ti + y ZnO $\rightarrow$ TiZn$_x$O$_y$ + (y-x) Zn on ZnO(000$\bar{1}$)-O. Consistently, upon annealing thicker Ti adlayers, the metallic zinc is removed to leave ZnO(0001)-Zn surfaces covered with TiO$_2$-like phase and ZnO(000$\bar{1}$)-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.