Mechanism of growth and structure of titanium oxide ultrathin films deposited on Cu(0 0 1)

Abstract

The growth mechanism, composition and structure of ultrathin films of titanium oxide deposited on the Cu(001) surface were investigated by means of XPS, LEIS, LEED and STM. Titanium oxide films were deposited on the Cu(001) surface previously saturated with a (√2×2√2)R45° structure of chemisorbed oxygen. The oxide films were prepared by evaporation of titanium in O2 atmosphere (pO2 in the 10−6mbar range) while the substrate temperature was kept at 573K. The Cu LEIS signal versus the amount of deposited titanium (as determined by means of XPS) indicates the growth of 2D islands in the early stages of deposition. Upon increasing the amount of deposited titanium multilayer islands begin to grow. The XPS results indicate that the oxide phases formed for Ti coverages above 1ML have a TiO2 stoichiometry. At very low coverages, a LEED pattern with a centred rectangular unit cell is observed. STM measurements show that at this stage of the growth the oxide islands are incorporated in the outermost layer of the substrate and the removed copper atoms form islands around the oxide regions. The very early stages of titanium oxide growth corresponding to the formation of this rectangular phase were also investigated by Ti deposition on the oxygen chemisorbed phase under UHV conditions at 573K. In this way it is possible to study the reaction of Ti with chemisorbed oxygen. Upon increasing the Ti coverage above 0.5ML, an oxide film with a slightly distorted hexagonal unit cell begins to grow. The quasi-hexagonal phase of titanium oxide can also be formed by annealing at 773K the rectangular phase. At higher coverages, when the substrate surface is completely covered by the oxide, the film exhibits a LEED pattern with a regular hexagonal unit cell.