Structure of thin fluoride films deposited on amorphous substrates

In the thickness range 50-500 nm at substrate temperatures of 300, 375 and 575 K the structures of physical-vapour-deposited magnesium, lanthanum, calcium and lithium fluoride films were investigated using transmission electron microscopy microfractographical replication and diffraction techniques, secondary neutral mass spectrometry, depth profiles and packing density measurements. It was shown that the growth in the region next to the substrate can differ significantly from the subsequent growth. These differences were shown to be a function of the film material and the substrate temperature. The cross-sectional investigations show that the fluoride films examined can be divided into two groups: one group is characterized by strong columnar growth and the other by granular growth. Modelling the different growth structures using a hard disk model with anisotropic relaxation properties reveals that columnar growth occurs when the relaxation of the addisks is limited in at least one dir ection laterally or normal to the substrate plane, whereas high mobilities in both directions lead to a granular growth. A schematic structure pattern has been derived for each fluoride.