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.