IVb Transition Metal Oxides and Silicates: An Ab Initio Study

Abstract

Using density-functional theory, we investigate the structural, vibrational and dielectric properties of group IVb transition metals (M=Hf, Zr, Ti) oxides and silicates which have drawn considerable attention as alternative high-k materials. For the oxides, three crystalline phases of dioxide are considered. The first two are the cubic and tetragonal structures which exist for hafnia HfO2 or zirconia ZrO2, while it is hypothetical for titania TiO2. The third one is the rutile structure, which on the contrary is the naturally occurring phase of titania, while it is hypothetical for hafnia and zirconia. For the silicates, we analyze first the crystalline phases: hafnon HfSiO4, zircon ZrSiO4 and an hypothetical TiSiO4 structure. Finally, we consider the amorphous silicates. We introduce a scheme which relates the dielectric constants to the local bonding of Si and metal atoms, based on the definition of parameters characteristic of the basic structural units centered on Si and metals atoms and including their nearest O neighbors. This scheme which considerably reduces the computational cost of the calculations allows one to treat much larger systems. Applied to amorphous Zr silicates, it provides a good description of the measured dielectric constants, both of the optical and the static ones.