Nanotechnologies / Materials
Development support tools for nano/advanced electronic materials, Semiconductor process monitoring and inspection technology

Terahertz emission spectroscopy for advanced electronic materials
and devices ~from phenomenology to quantitative analysis~
Institute of Laser Engineering
Professor Masayoshi Tonouchi
https://researchmap.jp/read0051752?lang=en

Abstract
Photocarriers excited with an femtosecond laser generates terahertz radiation. The terahertz emission spectroscopy has been
attracting much attention to unveil the intrinsic ultrafast carrier dynamics. Now we are extending the method from the phenomenological discussion tool of the space-time photocarrier dynamics toa quantitative analysis tool that can extract specific physical properties of semiconductors in a non-destructive, non-contact manner.
Here, we introduce two examples of our work.

Nanotechnologies / Materials▍ Background & Results
Breakthroughs in the development of advanced materials anddevices are always waiting novel analytical methods. The terahertz
wave emission spectroscopic imaging technique is characterized
by its ability to visualize the spatio-temporal transfer of photo-excited high-speed carriers. Extension of this technique to various fields
will lead to breakthroughs in the developments of the advanced
materials, devices, and semiconductor industries. In the future, it
is expected not only to bring important innovations in the development of advanced electronic materials and devices, but also toexpand into a large market by ensuring its use in semiconductor
R & D.

▍ Significance of the research and Future perspective
Terahertz radiation spectroscopy has been used mainly for semiconductor characterization since the 1990s, and we have beenplaying a leading role in its application to other novel materials.
Recently, it has also been used for the evaluation of spin materials. However, its application as a spectroscopic method has been
mainly limited to supporting phenomenological discussions, andwe have been working on extracting various case studies in order
to apply it to more practical fields. Here, we introduce two examples.
One is an example in which we have clarified the dynamics ofexcitons and post-dissociated free-carriers in carbon nanotubes
with the theoretical model in the phenomenological discussion. Inparticular, by comparing an array of nanotubes with a random system, the former shows ballistic conduction within the same tube,
while the latter reveals a picture in which the contacts between thetubes act as defects and drift like conventional free electrons. Thisapproach is a new spectroscopic technique that will greatly contribute to the development of advanced materials and devices.
Another example is the application to the evaluation of surface
treatment of silicon wafers. Surface treatment is an important partof the semiconductor process. As an example, a surface is covered with oxides, fluorine, and hydrogen before, during, and aftertreatment with a buffered hydrogen fluoride (BHF) solution. The
surface potentials during these changes can be easily analyzed
non-destructively, non-contact, quantitatively, and over a large
area. We are proud of the fact that we have completed this method
to the point where it can be used in semiconductor R&D, together
with our previous work on MOS structures, through-silicon vias, andsolar cell evaluations.

Fig. 1 One-dimensional confinement of quasiparticles within aligned and randomly oriented carbon nanotubes (CNTs) leads to highly anisotropic electronic andoptical properties. In macroscopic aggregates, hot carriers move in ballistic andrelaxed manners, respectively.

Fig.2 Si surface chemistry during BHF etching and the surface dipole moment
resulting from the different electronegativity of the surface atoms. The native oxide film is removed, F-terminated Si is formed as an intermediate step, and finallyH-terminated surface is formed as the stable state.

P a t e n t
Tonouchi, Masayoshi. Terahertz time domain spectroscopy. Uchidarokakuho 2021. ISBN978-4-753-2318-1. (in Japanese)
Michael Wais et al. Transition from diffusive to superdiffusive transport in carbon nanotube networks via nematic order control. Nano Lett. 2023, 23(10),T r e a t i s e 4448–4455. doi: 10.1021/acs.nanolett.3c00765
Dongxun Yang et al. Rapid, noncontact, sensitive, and semiquantitative characterization of buffered hydrogen-fluoride-treated silicon wafer surfaces byterahertz emission spectroscopy. Light Sci Appl. 2022, 11, 334. doi: 10.1038/s41377-022-01033-x
URL https://www.ile.osaka-u.ac.jp/research/thp/HPE/index.html
K e y w o r d terahertz emission spectroscopy, support tool for the development of advanced materials and devices, support tool for semiconductor R&D