Thickness-dependent surface morphology and crystallization of HfO2 coatings prepared with ion-assisted deposition

The thickness-dependent surface morphologies and crystallization of HfO2 coatings were investigated. Five HfO2 single layers with thicknesses ranging from 4-20 nm were deposited on fused silica substrates with ion-assisted deposition. Atomic force microscopy was first used to characterize the surface morphologies of the HfO2 coatings. The surface morphologies of the thinner HfO2 coatings duplicated the surface morphologies of the substrates. When the thickness was equal to or greater than 12 nm, grains appeared. The grain size and density increased with an increase in the thickness and resulted in a larger surface roughness. X-ray diffractometry can provide information about the grain size but the sensitivity of the method was not sufficient to determine whether the 12 nm HfO2 coating was crystalline. Transmission electron microscopy exposed the cross-sectional microstructure of the HfO2 coatings and electron diffraction verified that the HfO2 coatings with a thickness equal to or greater than 12 nm were crystalline. Finally, the physical mechanism behind the observed thickness-dependent crystallization is proposed and discussed. When the coating thickness decreases to a critical value, the surface energy contributes considerably to the total energy, which suppresses the structural transformation from the amorphous to the polycrystalline state.