Terahertz Optical Properties of HRFZ Silicon Wafer

Silicon (Si) is one of the most advanced technological materials, which can be used to design various optical components for the rapidly developing THz electronics. Compared with other optical materials, Si has lower crystal growth and processing costs, and has a larger size range, which can increase the diversity of optical components manufactured. High resistance floating zone (HRFZ) monocrystalline silicon provides a transmittance of 50-54% in the wavelength range of 50 to 1000um (or even longer, up to 8000um), and is commonly used in the far-infrared and terahertz regions, shown as in Fig. 1&2. PAM-XIAMEN can provide various types of FZ monocrystalline silicon wafers, please refer to website for the specific parameters:

https://www.powerwaywafer.com/silicon-wafer/float-zone-mono-crystalline-silicon.html

Fig. 1 Transmission (Red) and reflection (Green) of HR-Si (1mm thick) in the THz range

Fig. 2 Transmission of Si wafer (5mm thick) in 16-1000um

High resistance silicon exhibits relatively low losses in the THz range. From Fig. 3, it can be seen that the shape of THz signals transmitted through air and through high resistance silicon is the same. This indicates that there is no significant absorption in silicon. The main transmission loss is mainly caused by Fresnel reflection. The absorption coefficient of high resistance silicon in the terahertz range of 0.25-2 THz is less than 0.5 cm^-1.

Fig. 3 THz signal transmitted through air and HRFZ-Si

The absorption capacity of silicon for terahertz waves is mainly reflected by the loss tangent (tan δ), with higher values indicating stronger absorption. Research has found that the loss tangent of phosphorus doped silicon wafers is generally higher than that of boron doped silicon wafers. In addition, for boron doped silicon wafers, the lower the resistivity, the higher the loss tangent value. Correspondingly, the absorption rate of silicon towards terahertz waves is closely related to its loss tangent, showing a positive correlation, that is, the higher the loss tangent, the greater the absorption rate. Therefore, phosphorus doped silicon wafers have higher terahertz wave absorption than boron doped silicon wafers, and under the same type of doping, the lower the resistivity, the higher the absorption rate.

Based on these unique terahertz characteristics, silicon materials have shown significant potential for applications in terahertz imaging, terahertz spectroscopy, and terahertz devices such as modulators and filters.

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