(Invited) Second Harmonic Generation: Non-Linear Optics for Characterization of Electrical Properties of Dielectric-on-Semiconductor Interfaces

Dielectric layers are deposited on semiconductor materials for many applications, either in the active stacks of the devices (e.g. the gate dielectric in a metal-oxide-semiconductor transistor) or as passivation solutions (e.g. in image sensors [ 1 ], solar cells [ 2 ] etc.). The progress at the material and technology levels must be accompanied by the development of new characterization tools. One of the challenges in terms of characterization is to obtain the information on the electrical quality of the dielectric-on-semiconductor interface during the fabrication flow, non-destructively, just after the corresponding deposition step (if possible) and without needing to fully fabricate a particular test structure. At wafer level, a typical electrical evaluation method is the Corona characterization of semiconductors [ 3 ] that provides the interface state density (D it ) and the “total” charge responding in the structure. Its main drawback is the charging of the surface during the measurement. Optical methods (like the photoconductance decay or the photoluminescence) are non-destructive, but they are directly related to material and interface quality through the carrier lifetime [ 4 ] and they don’t allow a simple separation between fixed charge in the oxide (Q ox ) and D it . However optical-based methods are still a recommended strategy, since they ensure non-destructive measurements. In this context, the second harmonic generation (SHG) is a good option, because it can be sensitive to the “static” electric field induced between layers. In the SHG, the surface of the sample is irradiated with a femtosecond laser and a second harmonic wave is then generated and detected. In general, the SHG contains both bulk and surface contributions, but for centrosymmetric materials (such as silicon, silicon dioxide, alumina, etc.) in the dipolar approximation, the interface signal is dominant and it is related to the symmetry breaking due to both the interface itself and to the “static” electric field [ 5 ]. The SHG has already been used for dielectric characterization using various modalities: SHG versus power, time, wavelength, etc. [ 6 ], [ 7 ], [ 8 ]. In this paper, we focus on the analysis of the interface electric field, which is related to Q ox and D it . If trapping/detrapping phenomena occur during the illumination of the sample, this field can actually depend on time. The value of the SHG (proportional to the square of the field) will then be related to Q ox