Interface trap density in amorphous La₂Hf₂O₇/SiO₂ high-κ gate stacks on Si

Abstract

The present paper investigates the interface trap density of a new high-κ gate dielectric stack, La₂Hf₂O₇/SiO₂ on Silicon. Amorphous La₂Hf₂O₇ thin films are deposited by metal evaporation in the presence of atomic oxygen beams on an ultra-thin SiO₂ layer (1.5 nm) grown by rapid thermal oxidation on a p-type Si substrate. A combination of electrical (C–V) and cross sectional TEM measurements indicates a value of the dielectric constant κ of about 19±2.2. The interface states density (D_it) was measured using the conductance method for different La₂Hf₂O₇ thicknesses ranging from 3 nm to 14 nm. D_it ranges from 3.4×10¹⁰ eV^-1  cm^-2 to 4.8×10¹² eV^-1  cm^-2 and shows a quasi-linear dependence on the La₂Hf₂O₇ layer thickness. The interface state density increase with film thickness could have different explanations, such as oxygen de-passivation and/or defect generation at the Si-SiO₂ interface, formation of a new Si-SiO₂ interface by Si oxidation or alternatively, the SiO₂ interfacial layer reduction by oxygen vacancies.