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

Inspired by nature, we investigate the short-range order effect on the physical properties of amorphous materials. Amorphous Al2O3 thin films exhibit a higher proportion of their 4-coordinated Al sites close to the surface, causing variations in the average short-range order of the film. Below some thickness, the density of these films changes with size. In this work, we address the short-range order effect, through the thickness, on the electronic and optical properties of atomic layer deposited Al2O3 thin films. Both the refractive index and the permittivity were found to vary with size. The refractive index increased with thickness, and for thick films (∼50 nm), it was comparable to that of bulk amorphous Al2O3. The permittivity values increased with thickness as well, but did not reach those of the bulk material. Our experimental design circumvents the unpredictable Al2O3–Si interface, allowing new insights into the permittivity–thickness relations. By combining this design with accurate thickness and density measurements, we systematically correlate the refractive index and permittivity with the density and short-range order. These results shed light on the size effects in thin amorphous oxides and may guide the design of electronic and optical components and devices.