Growth-induced mechanical gradients and directional stiffness in ultrathin low-temperature ALD alumina coatings

Low-temperature atomic layer deposition (ALD) produces hydroxyl-rich alumina films whose mechanical response at nanometer thickness remains poorly understood. Here, alumina coatings deposited at 50 °C on native-oxide Si(100) were studied over the 1–112 nm thickness range by combining spherical nanoindentation, laser-induced surface acoustic wave spectroscopy (LiSAWS), thickness metrology, and atomistic simulations. Spherical nanoindentation gave out-of-plane moduli of about 137–126 GPa from 7 to 112 nm, consistent with the intrinsic bulk-like response predicted for hydroxylated amorphous alumina. By contrast, LiSAWS yielded lower in-plane moduli and dropped to 60 ± 26 GPa in the ultrathin regime, revealing reduced lateral network connectivity during the earliest ALD stages. The divergence between both methods indicates a growth-induced mechanical gradient that is not directly resolved by TEM or ellipsometry alone. Thicker films remained mechanically compliant, with yield strength around 4.5–5.2 GPa. These findings explain why nanolaminate architectures can provide mechanical advantages over a single thicker monolithic alumina layer in barrier applications for flexible electronics, optoelectronics, and related devices.