Morphology of RuO2(110) oxide films on Ru(0001) studied by scanning tunneling microscopy

Using scanning tunneling microscopy we have investigated the morphology of RuO2(110) films grown on Ru(0001). The films were prepared by dosing 2 x 10(5) to 1 x 10(6) L of O2 at temperatures between 650 and 850 K. It was found that the films are between 2 and 5 RuO2(110) layers thick, largely independent of the growth conditions. Within the temperature range investigated the perfectness of the films varied significantly. The morphology can be understood according to the habit of RuO2 single crystals, namely, the formation of extended (110), (100), and (101) surfaces. Upon annealing the films decompose by the formation of holes that go down to the metal substrate rather than in a layer-by-layer fashion. The Ru atoms released during decomposition form terraces with shapes similar to the terraces obtained by homoepitaxial metal-on-metal growth. Under the oxidation conditions used, we have not observed suboxides RuOx or the O-Ru-O trilayer that had been postulated to play a role for the RuO2 formation. The partially covered surfaces exclusively showed RuO2 and the known phases of adsorbed O atoms on the metallic Ru.