Electrical conduction mechanism in silicon nitride and oxy-nitride-sputtered thin films

We have studied the effect of reactive and nonreactive sputtering preparations on the composition and properties of silicon nitride thin films. Films were prepared from both silicon nitride ceramic and pure silicon targets under different Ar∕N2 gas mixtures. For the different resulting samples, we have performed optical, x-ray photoemission spectroscopy (XPS), and transport measurements. The preparation conditions change the sample atomic composition and the effect of oxygen in the films, which in turn determines the dominant conduction mechanism. It becomes important to determine both the nonstoichiometry of the film and the phase where the oxygen is incorporated. Oxygen may appear as silicon oxide, forming a secondary phase inside silicon nitride; or it may consist of silicon oxy-nitride phases. The presence of these different phases, as revealed by XPS, determines the electrical properties and conduction mechanisms. Samples presenting space-charge-limited current as the dominant conduction mechanism correspond to those where a silicon oxy-nitride phase is formed (that becomes Ohmic for overstoichiometric Si content samples), while a Poole–Frenkel conduction behavior is characteristic of the silicon nitride phase.