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 (refractive index, optical gap, Tauc coefficient, and Urbach energy) of SiNx:H films. 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 deposition technique assisted by an electron cyclotron resonance generated plasma. After deposition they were subjected to rapid thermal annealing at temperatures ranging from 300 degrees C to 1050 degrees C. We found that the percolation threshold for Si-Si bonds (at x = 1.1) separates films with different response to thermal treatments. The changes of the Tauc coefficient and the Urbach energy at moderate annealing temperatures indicate a structural relaxation of the network for the films with x above the percolation threshold, while at higher temperatures the trends are inverted. In the case of x below the percolation limit the inversion point is not observed. These trends are well correlated with the width of the Si-N infrared stretching absorption band. Additionally the samples with as-grown x = 1.43 show a good correlation between the Urbach energy and the density of unpaired spins in silicon dangling bonds. (C) 1999 American Institute of Physics. [S0021-8979(99)08016-0].