AVS 46th International Symposium
    Applied Surface Science Division Tuesday Sessions
       Session AS-TuP

Paper AS-TuP19
Scanning Tunneling Microscopy and X-ray Photolectron Spectroscopy Studies of WO3 Thin Films Submitted to Various Thermal Treatments in Air and Ultra High Vacuum

Tuesday, October 26, 1999, 5:30 pm, Room 4C

Session: Poster Session
Presenter: S. Santucci, University of L'Aquila and INFM, Italy
Authors: S. Santucci, University of L'Aquila and INFM, Italy
L. Lozzi, University of L'Aquila and INFM, Italy
M. Passacantando, University of L'Aquila and INFM, Italy
L. Ottaviano, INFM, Italy
C. Cantalini, University of L'Aquila, Italy
L. Odorisio, University of L'Aquila, Italy
Correspondent: Click to Email
The gas sensitive properties of WO3 thin films seem to be due to a loss of oxygen at the surface, which determines the presence of active sites for the reaction with gases (Co,NOx,ozone). The reactivity of the WO3 films is strongly influenced by the morphology and surface properties. 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 UHV by means of XPS and Scanning Tunneling Microscopy/Spectroscopy (STM/STS). After the deposition these films have been annealed in atmospheric owen for 24 h at different temperatures ( 300 and 500° C) to stabilise the film morphology. The XPS measurements, by using a monochromatic Al source 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 600 ° C, . The UHV re-annealing procedure strongly modifies the W 4f peak of both the as deposited and 300 ° C samples, showing the loss of oxygen. Moreover, at the Fermi edge, before the UHV annealing there is a small signal due to the presence of metallic states. This signal after the UHV annealing strongly increases, indicating the presence of an high density of states at the Fermi edge. Instead the 500 ° C sample, after the heating in UHV has shown a substantial stability of the nearly stoichiometric WO3 phase. Using STM in UHV we have investigated the morphology of the samples at room temperature and while scanning at elevated temperatures up to 600 °C. In particular we have simoultaneously taken I-V curves at the boundaries and centers of the typical grains of the polycrystalline sample. Our findings on the electronic structure of the samples close to the Fermi level are in agreement and allow clearer understanding of the findings from a parallel XPS study.