Nanotechnologies / Materials
Material evaluation, Sensing, Semiconductor device

Terahertz ellipsometry with high precision for 6G materials
and wide bandgap semiconductors
Institute of Laser Engineering
Associate Professor Makoto Nakajima
https://researchmap.jp/nakajimamakoto?lang=en

Abstract
We have developed a highly precision terahertz time-domain ellipsometry with a novel rotating analyzer. We succeeded in increasing the detection sensitivity by more than 10 times, making it possible to evaluate the dielectric constant, dielectric loss, and complex
refractive index in the terahertz region and Beyond 5G region withhigh accuracy. It is possible to evaluate the conduction characteristics of 6G materials and wide-gap semiconductors (GaN, SiC,Ga2O3) non-destructively and without contact.

Nanotechnologies / Materials▍ Background & Results
Beyond 5G and 6G are being developed at a rapid pace asnext-generation communication standards, and these communication bands correspond to the sub-terahertz frequency range. Widegap semiconductors such as GaN, SiC, and Ga2O3 are also expected to be used as power devices and high-frequency devices.
By terahertz ellipsometry and terahertz time domain spectroscopy,
it is possible to evaluate the conductivity properties (carrier concentration, mobility, scattering time, etc.) of these advanced materials and the dielectric constant and refractive index in the highfrequency region.

Image of high precision Terahertz time-domain ellipsometry▍ Significance of the research and Future perspective
Conductive property evaluation of wide-gap semiconductors has
been performed by electrical evaluation such as Hall measurement
and CV measurement up to now. Terahertz waves corresponds toelectromagnetic waves with a frequency range of 0.1 – 10 THz.
Optical evaluation using terahertz waves has made it possible
to evaluate electrical conduction characteristics such as carrier
concentration, mobility, and scattering time in a non-destructive,
non-contact method without the need to fabricate electrodes. By
increasing the measurement accuracy more than 10 times with the
newly introduced rotating analyzer method, we have succeededin evaluating the carrier concentration of 1015 -1020 cm-3. In themeasurement of mobility, it is possible to measure the anisotropyof the sample, and we plan to support the evaluation of in-planeanisotropy and out-of-plane anisotropy in the future. In 6G materialevaluation, it is possible to evaluate characteristics such as dielectric constant and dielectric loss in the sub-terahertz region, and it
is also possible to evaluate characteristics of artificial substancessuch as metamaterials as well as insulating substrates and absorbers. Accurate evaluation of conductivity and permittivity characteristics in the ultra-high frequency range, which is the next-generation communication band, is essential for new device development,
and we expect that the above technology we have developed will
be used.

Refractive index spectra for wide bandgap semiconductor of β-Ga2O3 in Terahertz range

P a t e n t Japanese Patent Application No. 2021-106147
V.C. Agulto, M. Nakajima, et al. Scientific Reports 11, 18129 (2021). doi: 10.1038/s41598-021-97253-z
T r e a t i s e V.C. Agulto, M. Nakajima, et al. Applied Physics Letters 118, 042101 (2021). doi: 10.1063/5.0031531
M. Ota, M. Nakajima, et al. Nature Physics 18, 1436 (2022). doi: 10.1038/s41567-022-01767-w
https://resou.osaka-u.ac.jp/ja/research/2021/20210915_2
UR
L https://resou.osaka-u.ac.jp/ja/research/2021/20210126_3
https://resou.osaka-u.ac.jp/ja/research/2022/20221021_1
K e y w o r d terahertz wave, non-destructive non-contact evaluation, semiconductor, 6G, beyond 5G