An asymmetry of conduction mechanisms and charge trapping in thin high-k HfxTiySizO films

Abstract

The electrical behavior of high-permittivity (high-k) hafnium titanium silicate (HfxTiySizO) layers with different Hf:Ti ratios in the films is investigated. The films were deposited by metal-organic chemical-vapor deposition using a mixture of two single-source precursors for the deposition of Hf silicate and Ti silicate. The attention is focused on the gate bias asymmetry of leakage currents, conduction mechanisms, and trapping behavior dependent on film composition. The pure Hf silicate, the Ti silicate, and the sample with the lowest Hf content reveal a gate bias symmetry of the conduction process, implying the formation of a single-layer structure, whereas samples containing both Hf and Ti in significant amounts exhibit an asymmetrical behavior to the gate polarity. A continuous change from Poole-Frenkel conduction in pure Hf silicates to phonon-assisted tunneling in pure Ti silicates is established. It is found that a strong negative charge trapping occurs in the samples containing significant amounts of both Hf and Ti. With an increase of Ti content in the films, the density of the trapped charge increases and its centroid moves farther away from the dielectric/Si interface. Two possible processes are considered to explain the observed asymmetry of the conduction process—negative charge trapping and formation of a double-layer structure. It is suggested that both phenomena occur and are consequences of one and the same structural process, phase separation, i.e. the formation of TiO2, HfO2, and SiO2 islands in the film. The degree of phase separation depends strongly on the film composition and defines the process, which has the dominating role in the gate bias asymmetry of the conduction mechanism.