Thermally induced changes in the optical properties of SiNx : H films deposited by the electron cyclotron resonance plasma method

We analyze the effect of thermal processes on the optical properties (refra ctive index, optical gap, Tauc coefficient, and Urbach energy) of SiNx:H fi lms. Films with three different nitrogen to silicon ratios (x = 0.97, x = 1 .43, and x = 1.55, respectively) were deposited by a chemical vapor deposit ion technique assisted by an electron cyclotron resonance generated plasma. After deposition they were subjected to rapid thermal annealing at tempera tures ranging from 300 degrees C to 1050 degrees C. We found that the perco lation threshold for Si-Si bonds (at x = 1.1) separates films with differen t response to thermal treatments. The changes of the Tauc coefficient and t he Urbach energy at moderate annealing temperatures indicate a structural r elaxation of the network for the films with x above the percolation thresho ld, while at higher temperatures the trends are inverted. In the case of x below the percolation limit the inversion point is not observed. These tren ds are well correlated with the width of the Si-N infrared stretching absor ption band. Additionally the samples with as-grown x = 1.43 show a good cor relation between the Urbach energy and the density of unpaired spins in sil icon dangling bonds. (C) 1999 American Institute of Physics. [S0021-8979(99 )08016-0].