Bulk and interface second harmonic generation in the Si₃N₄ thin films deposited via ion beam sputtering

The nonlinear optical second harmonic generation (SHG) in Si₃N₄ has attracted considerable attention due to a variety of promising applications in optoelectronics. However, reports on SHG in Si₃N₄ thin films and microstructures lead to diverse conclusions about the SHG origin, pointing towards the Si₃N₄ bulk, as well as to the Si₃N₄–Si interface. Here we report on the measurement of polarization-resolved angle-dependent SHG in Si₃N₄ thin films in the reflective mode. This mode allowed us to measure the nonlinear response of Si₃N₄ thin films on the Si single crystal substrate. By measuring three samples deposited via ion beam sputtering, we were able to analyze the bulk and interface contributions. We have demonstrated that apart from the bulk SHG, the Si₃N₄–Si interface contributes with a significant amount of SHG for the thin sample (600 nm). Our result provides a link between the previous measurements in the Si₃N₄ thin films and on the microstructures. (paper)