Terahertz Emission Spectroscopy and Imaging as a Quantitative Analytical Tool

Masayoshi Tonouchi

Osaka University, Japan

Terahertz emission spectroscopy (TES) has been used for many years to explore photocarrier dynamics in various electronic materials [1]. Although TES utilizes the same system for THz time-domain spectroscopy (THz-TDS), they provide completely different information on material properties. THz-TDS has established itself as a tool to estimate the refractive index of materials, whereas TES discusses the THz excitation mechanism upon femtosecond laser illumination from various materials phenomenologically, which is now recognized as a powerful tool to explain the ultrafast photocarrier dynamics [2-6]. However, this “phenomenological” feature has conversely hindered the widespread use of TES. For the market development of TES, it should evolve into a versatile tool by providing the quantitative parameters of the materials. Thus, recently, we have focused on this point. We have proven that one can extract the various parameters of semiconductors from the TES data, which extends the market into semiconductor R&D. Examples are the parameter extraction for Si-wafer, p-n junctions, MOS, wide bandgap semiconductors, and so on [7-12]. Figure 1 shows the schematics for photoresponse near the surface of a semiconductor. By simplifying the emission mechanism [7], one can estimate its surface potential from the emission data in a wafer scale noncontactly and non-destructively. This analysis is sensitive enough to assess the potential change due to the surface dipole formation during surface treatment with BHF. In the presentation, we introduce such performance as a quantitative analytical tool.

Figure 1: (a) Photo excitation followed by the carrier displacement to generate THz waves. (b) BHF etching modifies the strength of the surface dipoles at the Si surface bonded with oxygen, fluorine, and hydrogen.

M.T. acknowledges support in part by JSPS KAKENHI Grant No. JP23H00184.

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