Elliptically polarized light-induced second harmonic generation in SiNxOy

Elliptically polarized photoinduced second harmonic generation (PISHG) in SiNxOy films was studied for specimens with different nitrogen to oxygen (N/O) ratios. As a source for the photoinducing light, we used a nitrogen Q-switched pulse laser at a wavelength of 337 nm. YAG:Nd pulse laser (λ=1.06 μm; W=30 MW; τ=10–50 ps) was used to measure the PISHG. All measurements were made in a reflected light regime. We found that the output PISHG signal was dependent on the N/O ratio and the film thickness. The SiNxOy films were synthesized using a technique of chemical evaporation at low pressures. Films with thickness varying between 10 and 30 nm and with an N/O (nitrogen/oxygen) ratio between 0 and 1 were obtained. The stochiometry of these films was measured after their deposition on Si[1 1 1] substrates by using an extended X-ray absorption fine structure (EXAFS) method. Distances between O–Si and N–Si atoms for different N/O ratio and film thickness were determined by data fitting analysis. Electrostatic potential distribution at the Si[1 1 1]–SiNxOy interfaces was calculated. Comparison of the experimentally obtained and quantum chemically calculated PISHG data are presented. High sensitivity of eliptically-polarized PISHG to the N/O ratio and film thickness is demonstrated. The role of the electron–phonon interactions in the dependencies observed is discussed. We have shown that the PISHG method has higher sensitivity than the traditional EXAFS spectroscopic method for films with an N/O ratio higher than 0.50.