Density of Nanometrically Thin Amorphous Films Varies by Thickness

Organisms in nature can alter the short-range order of an amorphous precursor phase, thereby controlling the resulting crystalline structure. This phenomenon inspired an investigation of the effect of modifying the short-range order within the amorphous phase of a selected material. Amorphous thin films of aluminum oxide deposited by atomic layer deposition method were found to vary structurally as a function of size. Thinner films, as predicted and also confirmed by atomistic simulations, exhibited more 4-coordinated Al sites. These atomistic alterations were expected to change the amorphous thin film's average density. The density indeed varied with the alumina layer thickness, and the measured effect was even stronger than predicted theoretically. This effect is explained in terms of the deposition process, where each newly deposited layer is a new surface layer that "remembers" its structure, which results in thin films of substantially lower density.