Combined experimental and theoretical study of thin hafnia films

We present a joint experimental and theoretical study of ultrathin hafnia films grown on Si (001) by atomic layer deposition for applications as a gate dielectric of a field-effect transistor. The structural analysis by means of high-resolution transmission electron microscopy, electron diffraction, and x-ray diffractometry indicates films with thickness of 4 nm or less to be polycrystalline, predominantly monoclinic and textured, with the texture axis being the normal to the (211), (112), and their equivalent planes. Films with thickness around 10 nm consist of a mixture of monoclinic and tetragonal phases more or less randomly oriented. Films thicker than 25 nm are purely monoclinic with (111) and (111) textures. Using density-functional theory we investigate surface energies of monoclinic and tetragonal hafnia films in search for thermodynamic means of controlling the film microstructure. We report the atomic and electronic structures of these films including the surface energy, work function, and electron affinity.