Growth of ultra-thin amorphous Al2O3 films on CoAl(1 0 0)

The oxidation of a CoAl(1 0 0) surface at 300 K was studied by means of Auger electron spectroscopy, high resolution electron energy loss spectroscopy (EELS), low energy electron diffraction and scanning tunneling microscopy (STM). For an exposure ⩽0.3 L, the oxygen atoms are chemisorbed on the CoAl(1 0 0) surface, while for a larger O2 exposure the oxidation of the surface sets in. For an exposure ⩾300 L the surface is entirely covered with amorphous Al2O3 (a-Al2O3) whereas the Co atoms seem to be unaffected. The EEL spectra of a-Al2O3 exhibit Fuchs–Kliewer modes at around 640 and 890 cm−1. The thickness of the a-Al2O3 film is estimated to be 7.1 ± 0.7 Å. The STM images show that the oxide grows as large islands which cover the whole surface. The band gap of the ultra-thin a-Al2O3 film on CoAl(1 0 0) is found to be 3.2 eV and thus it is strongly diminished with respect to the bulk value.