Polaron formation in the hydrogenated amorphous silicon nitride Si₃N₄ : H

Abstract

Silicon nitride (Si₃N₄) is commonly used as the charge storage layer of nonvolatile charge trap flash (CTF) memory devices. Previous theoretical investigations have shown a large variety of possible trapping centers in Si3N4 which might contribute to the memory effect, including vacancies, dangling bonds, and overcoordinated atoms. So far, the high H concentration detected in silicon nitride thin films, resulting in a large number of passivated Si-H and N-H bonds, has not been considered in depth in theoretical studies regarding the charge trapping properties. In this work, we show that this passivation of dangling bonds with H gives rise to the formation of hole and electron (bi)polarons at fully coordinated sites and hole traps at Si-H bonds. We employ density functional theory combined with a hybrid functional to demonstrate that both holes and electrons can localize at intrinsic, fully coordinated sites in the amorphous Si₃N₄:H network. We classify precursor configurations of the polaron states by analyzing bond lengths, partial charges, maximally localized Wannier functions, bond orders, and electronic states introduced in the band gap upon charge trapping. All trapping sites are characterized according to their thermodynamic properties, such as charge transition level (CTL) and relaxation energy on a statistical level. The CTLs of the (bi)polarons are in the energetic vicinity of the band edges of a Si substrate, showing that these intrinsic sites can trap charges from Si under applied voltages and contribute to the memory effect.