Role of Intrinsic Electron Trapping in Negative Charging of Amorphous Alumina

Abstract

Negative charging of amorphous alumina (a- (Formula presented.)) films is important for many applications, but its nature is not yet fully understood. We investigate whether intrinsic electron traps in a- (Formula presented.) can contribute to film charging before and after high-temperature anneal. Amorphous and partially crystallized alumina models are created using the melt-quench approach. Analysis of a large set of models covering a wide range of film densities using hybrid density functional theory shows both spontaneous and activated trapping of electrons in a- (Formula presented.). Spontaneous trapping is found to be more prominent in high-density, partly crystallized, models. Two types of traps are found: single-center traps and two-center traps. In the single-center case, electrons localize on five-coordinated Al ions in tetragonal pyramid coordination. Two-center traps have electron states shared between close Al-Al pairs. The optical ionization energies of these traps are in the range of 1–4 eV. We discuss these findings in the context of exhaustive photo-depopulation spectroscopy (EPDS) measurements of trapped electrons in as-deposited and partially crystallized a- (Formula presented.) films.