Tetrahedron-model analysis of silicon nitride thin films and the effect of hydrogen and temperature on their optical properties

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Abstract
A series of silicon nitride thin films prepared by different techniques are investigated with respect to their optical properties with spectroscopic ellipsometry in the energy region 1.5–9.5 eV, using conventional and synchrotron-radiation light sources. The dielectric-function spectra of the films are analyzed with the microscopic Si-centered tetrahedron model and thus the applied growth techniques are compared, with respect to the resulting tetrahedron types and volume fractions, and the superiority of the chemical-vapor deposition techique is distinctly denoted. Moreover, a model is developed for the deduction of the films’ stoichiometry from the above analysis and the results are compared with corresponding ones from Rutherford backscattering spectroscopy and elastic recoil detection. The effect of stoichiometry on the film quality and optical parameters is discussed. In addition, the effect of hydrogen on the fundamental and mean optical gaps of silicon nitride is investigated and it is concluded that in both cases hydrogen causes a shift to the red, which is interpreted as due to the existence of a significant number of Si-H bonds. Temperature-dependent optical studies on SiN bulk materials and thin films are performed in order to examine the temperature shifts of the fundamental and mean optical gaps, which are found to be redshifted and blueshifted, respectively. The effect of stoichiometry on the observed temperature coefficients is discussed.