STM, LEED, and DFT characterization of epitaxial ZrO2 films on Pt(1 1 1)

Thin epitaxial ZrO2 insulator films on a Pt(1 1 1) substrate have been studied by means of scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and density functional theory (DFT) calculations. The films have been prepared by Zr vapor deposition in an O2 atmosphere followed by post-annealing also in an O2 atmosphere. The process of film formation was investigated from the island state up to film thicknesses where the films completely cover the substrate. Continuous ZrO2(1 1 1) films of high quality with smooth surfaces and high structural order are obtained for film thicknesses above 3 monolayers (ML) and temperatures of deposition and post-annealing of 470 and 950 K, respectively. A carpet like growth mode is observed which corresponds to the relatively weak film/substrate interaction found in DFT calculations. The investigations reveal a clear p(1 × 1) surface structure, which rotates during annealing with respect to the substrate. Based on DFT calculations, the possibility of different surface terminations is discussed. Annealing the samples at temperatures >950 K induces a three-dimensional restructuring of the ZrO2 films which destroys the film continuity. Imaging discontinuous ZrO2 films at small tunneling voltages yield a characteristic contrast reversal for local film thicknesses >2 ML indicating the retarded formation of the insulator gap of ZrO2 during film growth which has been proven also by DFT calculations.