X-ray photoemission spectroscopy and scanning tunneling spectroscopy study on the thermal stability of WO3 thin films

Abstract

In this work the surface electronic and structural properties of about 150 nm thick WO3 films, deposited in high vacuum by thermal evaporation onto Si substrates, have been studied in ultrahigh vacuum (UHV) by means of x-ray photoemission spectroscopy (XPS) and scanning tunneling microscopy (STM)/spectroscopy. After deposition these films have been annealed in atmospheric oven for 24 h at different temperatures (300 and 500 °C) to stabilize the film morphology. XPS measurements to follow W 4f, O 1s peaks and the valence band, have been performed on these samples both as prepared and after a re-annealing in UHV at temperatures ranging from 50 to 350 °C. The UHV re-annealing procedure strongly modifies the W 4f peak of both the as deposited and the 300 °C/24 h treated samples, and produces an increase of metallic states at the Fermi edge. Instead, the 500 °C/24 h sample, after heating in UHV shows substantial stability of the nearly stoichiometric WO3 phase. Using STM in UHV we have investigated the morphology of the samples at room temperature and after the annealing at elevated temperatures up to 350 °C. In particular, we have taken I–V curves on typical grains of the polycrystalline sample. Our findings on the electronic structure of samples close to the Fermi level are in agreement and allow a clearer understanding of the findings from the parallel XPS study.