Atomic layer deposition of epitaxial HfO2 thin films on r-cut sapphire

Textured epitaxial HfO2 thin films of monoclinic structure were grown on r-cut Al2O3 by atomic layer deposition from HfCl4 and H2O at temperatures 450-750 °C. The film-to-substrate out-of-plane orientation was determined to have a single (001)HfO2//(1102)α-Al2O3 relationship. The in-plane orientation showed the existence of two possible relationships: [100]HfO2//[110]α-Al2O3 and [100]HfO2//[110]α-Al2O3. In films deposited at 400 °C traces of (010) growth plane were observed in addition to the preferential (001) growth. The lattice of HfO2 was compressed in the surface plane and expanded in the surface normal direction. The strain was highest in the films grown at 450-550 °C. With the increase of deposition temperature to 750 °C, the strain decreased. The strain relaxation in films deposited at 750 °C was in correlation with marked surface roughening in the initial stage of deposition at this temperature. The roughness of the epitaxial films was lower than that of polycrystalline films with comparable thickness deposited on Si(100) and SiO2 substrates.