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

Abstract

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.

Optically modified second harmonic generation in silicon oxynitride thin films via local layer heating

“…We have previously observed that for the studied Si 3 N 4 layers deposited via IBS, the SHG efficiency scales with the layer thickness, and, also, its angular dependence suggests that the bulk-like SHG dominates for layers with a thickness exceeding 1 μm 6 . If the enhancement originated from the SHG on the layer-Si interface, we would expect to observe a lower SHG enhancement for higher Si 3 N 4 layer thickness because the dominating bulk-like SHG would remain unchanged.…”

“…As the first step, we studied laser-induced SHG enhancement in Si 3 N 4 thin films deposited via dual ion beam sputtering on the Si substrate 22 . Throughout the study, we used p-polarized IR pulses at 1028 nm both for the sample irradiation and SHG measurement; the incident angle of the IR beam was 70 deg and we measured p-polarized SHG—see “ Methods ” for details 6 .…”

“…In particular, we propose measurements of SHG under various laser repetition rates and varying laser pulse length as a simple way to verify the presence of heat-induced modification. In general, for studying Si 3 N 4 layers, it is highly beneficial to use a reduced laser repetition rate as a simple means of minimizing laser-induced layer modification 6 .…”

“…Sample preparation was carried out using the dual ion beam sputtering described in detail in Ref. 6 . A beam of Ar + ions (beam current 108 mA, beam voltage 600 V) sputtered Si atoms from a target onto substrates (Silicon or BK7), where the deposited atoms interacted with nitrogen and oxygen ions generated by an assistance ion beam (emission current 0.6 A, discharge voltage 70 V).…”

“…Many studies have investigated the nonlinear optical characteristics of Si 3 N 4 , focusing in particular on the source of effective SHG 4 – 13 . The studies reveal two possible sources of SHG: (i) SHG generation on the Si 3 N 4 –Si interface 6 , 11 , and (ii) bulk-like SHG, which features dipolar character 4 , 6 , 13 . The bulk SHG with dipolar character has been assigned to breaking the material symmetry via strain or local inhomogeneities in these articles 11 , 14 .…”