Optical and electrical properties of a-Si xN y:H films prepared by rf plasma using N 2+SiH 4 gas mixtures

Abstract

Hydrogenated amorphous silicon nitride (a-SixNy:H) films, prepared from SiH4 and N2 gas mixture by a low temperature rf plasma process, create new perspectives to obtain highly transparent films with low electrical conductivity. Films were deposited on c-Si substrates at a temperature of 350°C using different gas concentrations and several rf power densities. The films were optically characterized by ellipsometry during the process. The results show dependences of the refractive index and the extinction coefficient on silane concentration and rf power. Ellipsometric data point to an abrupt interface film-substrate and an evolution of the surface roughness associated with the enhancement of the ion bombardment. Electrical measurements on Al/a-SixNy:H/c-Si MIS structures show conductivity based on the Poole-Frenkel transport mechanism for Si/N ratios lower than 1.2. The Poole-Frenkel constant and the trap energy values agree with those previously reported taking into account the concentration of Si-H bonds and the temperature dependence of the conductivity. The electrical parameters, such as electrical conductivity and density of states, show critical dependence on the gas mixture and on the rf power density, which corroborates the ellipsometric results.