Density of Nanometrically Thin Amorphous Films Varies by Thickness

Abstract

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 2 newly deposited layer is a new surface layer that 'remembers' its structure, resulting in thin films of substantially lower density.

Thickness dependence of the physical properties of atomic-layer deposited Al2O3

“…Given its very wide ALD window this procedure serves as an ALD model process, and the resulting thin films are of high quality, smooth and pin-hole free 9,10 . Al2O3 films were grown in a plasma-enhanced ALD reactor (PEALD, ALD R-200 Advanced, Picosun, Finland) at 200°C using trimethyl aluminum and water, as described elsewhere 5 . The thickness of the films was measured with x-ray reflectivity (XRR, SmartLab, Rigaku, Japan) and spectroscopic ellipsometry (VASE, Woollam, USA).…”

“…In our previous study we examined the relationship between the density of an amorphous Al2O3 film and its critical angle (θc) 5 . According to Snell's law, the ratio between the refractive indexes (n) and the refraction angles (θ) when a ray passes from one medium to another is defined by Eq.…”

“…To gain this control, the organism manipulates the short-range order within the amorphous phase 2 . Whereas in nature such control is achieved through different additives 1,2 , in amorphous thin films the short-range order was found to vary with size 4,5 . Amorphous Al2O3 thin films deposited by atomic layer deposition (ALD) have shown size-dependent structural variations, where thinner films exhibited a higher proportion of their 4coordinated Al (Al4) sites than thicker ones.…”

“…It was also shown that the density of the films increases with size by more than 15%. 5 Since several physical properties of a material are density-dependent, 6,7,8 we expect that technologically important electronic and optical properties would exhibit significant variations with thickness. We test this hypothesis on films grown by ALD, which allows fabrication of amorphous oxides of excellent quality, with precise and conformal morphology 9,10,11,12 .…”

On Simulating the Formation of Structured, Crystalline Systems via Non-classical Pathways

“…These intermediate states, like their biological equivalents, are templated by an organic component and their utility is to aid in the pre-programed construction of larger, complex assemblies required to build the hierarchical architectures observed in biological materials. By manipulating the unit assemblies and local parameters, the same mechanistic behaviors can be utilized and lead to drastic divergence in morphology (Etinger-Geller et al, 2017;Zhao et al, 2018;Kartha et al, 2019) and subsequently materials properties. Biological control of substrate chemistry is vital and determines the energetic barriers of the following layers of assemblages.…”