Si3N4 is not an ideal insulator, it contain Si-H, Si-N bonds, and Silicon dangling bonds which are all contribute in the electrical conduction via Poole-Frenkle conduction effect. the amount of the above bonds is temperature dependent, increasing the deposition temperature leads to decrease of these bonds, this may serve ..try to study the following article:

Vik J. Kapoor, et. al. , IEEE transaction on components, Packaging and manufacturing Technology, part A , No. 3 (1994), pp. 367-372