Enhanced oxidation of Ge(1 1 1) surface precovered with Na: Scanning tunneling microscopy and X-ray photoemission spectroscopy study

We have studied the enhanced oxidation of the Ge(1 1 1) surface predeposited with Na. The surface morphology during the oxidation was studied using scanning tunneling microscopy (STM) and the surface chemical change at the early stage of oxidation was investigated using X-ray photoemission spectroscopy (XPS). Our microscopic data revealed that, on the clean surface, the oxidation was initiated at the grain boundaries and defects. When Na was deposited, tunneling images obtained as a function of oxygen dosage showed that the initial oxidation occurred also at the Na-cluster adsorption sites and then expanded laterally. The observed surface morphology agreed with XPS measurements, which showed the formation of Na-O, compound at the early stage of oxygen dosing. Our results suggested that the role of Na adsorbates in the enhanced oxidation was to provide additional oxidation sites. Upon annealing the oxidized surface, the oxygen photoemission peak associated with the Na-O compound faded away with the drastic change in the surface morphology.