Growth and Morphology of Calcium-Oxide Films Grown on Mo(001)

Crystalline calcium-oxide films of various thicknesses have been grown on a Mo(001) support. The growth behavior and resulting film morphology have been characterized with low-energy electron diffraction, Auger spectroscopy, and scanning tunneling microscopy. At low film thickness, a considerable interdiffusion of Mo ions from the support into the ad-layer is revealed, giving rise to the development of a sharp (2 x 2) superstructure with respect to the Mo(001). The Ca/Mo mixed oxide has a similar lattice parameter as the metal substrate and therefore grows pseudomorphically with a very low defect concentration. At 4-5 ML thickness, the supply of Mo from the support is interrupted and pristine CaO patches start to grow on the surface. The reemerging lattice mismatch drives the system into the Stranski-Krastanov mode, as reflected by the formation of three-dimensional oxide islands. Above 15 ML nominal thickness, the islands merge into a closed film with high surface quality and low defect concentration. The bulk character of the oxide is deduced from its band gap and optical properties, which are in good agreement with literature data reported for bulk CaO.