Probing the Atomic-Scale Structure of Thin Films Grown By Atomic Layer Deposition

Abstract

The ability to grow conformal films with Ångstrom resolution makes atomic layer deposition (ALD) an attractive means of protecting and/or chemically functionalizing surfaces for catalysis, energy storage, photoelectrochemistry, and other applications. However, the performance of ALD coatings frequently falls short of expectations, largely because the atomic-scale structure (and therefore the chemical and physical properties) of ALD films deviate from idealized preconceptions. Single-crystal materials are only rarely achieved by ALD. More often, ALD films form amorphous, polycrystalline, or defective structures. ALD process conditions such as precursor selection(s), precursor exposure(s), and reactor temperature are known to impact the properties of resulting films. The substrate also affects the properties of ALD films, especially for ultrathin film thicknesses. To date, the ALD community has had difficulty connecting the atomic-scale structure of ALD films with their performance, largely because of limited availability of techniques to determine the atomic-scale structure of the ALD films. In this work, we demonstrate the ability to probe the structure of ALD films (e.g. Al2O3) using various advanced characterization techniques including both ex situ and in situ high-energy X-ray diffraction (HE-XRD) coupled with pair distribution function (PDF) analysis, electron diffraction (ED), and neutron scattering (NS). We evaluate the effect of ALD process variables (e.g. growth temperature and substrate) on atomic-scale structure, and connect the measured ALD structural features to observed electrochemical properties. The techniques we demonstrate can be applied broadly to probe the local structure of ALD films, while the fundamental understanding gained from this work will help tailor ALD for electrochemical applications. Figure 1