Research
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・Time-resolved two-photon photoemission spectroscopy to evaluate surface recombination in next-generation semiconductor materials
・Time-resolved two-photon photoemission spectroscopy to evaluate surface recombination in next-generation semiconductor materials
The recent development of spectroscopic techniques using ultrashort laser pulses has made it possible to elucidate high-speed physical phenomena in materials. In solid state materials, electron interaction or relaxation processes are known to occur on a time scale from a few femtoseconds to hundreds of femtoseconds, and in order to elucidate the dynamic behavior of electrons injected into semiconductor devices until their energy dissipation, high-precision time-resolved characterization on the order of femtoseconds to nanoseconds is required. By combining a femtosecond pulsed laser generator and a photoelectron spectrometer, we have developed a time-resolved two-photon photoemission spectroscopy (TPS) measurement system, and are working on the dynamic characterization of electrons in next-generation semiconductor materials such as gallium nitride (GaN) semiconductors. In particular, we are promoting research on the effects of crystal defects in semiconductors upon device operation from the viewpoints of both device development and physical property characterization.
References
References
S. Ichikawa, Y. Matsuda, M. Funato, Y. Kawakami, and K. Kojima,
[invited] "Surface carrier dynamics of nitride semiconductors evaluated by time-resolved photoemission spectroscopy'',
The 12th International Workshop on Nitride Semiconductors (IWN2024), 386, Hawai'i, USA (Nov. 2024).
[Link]S. Ichikawa and K. Kojima,
"Ultrafast Surface Carrier Recombination in Semiconductors Evaluated by Two-photon Photoemission Spectroscopy'',
2024 Materials Research Society Fall Meeting (2024 MRS Fall Meeting), CH06.07.04, Boston, USA (Dec. 2024).
[Link]S. Ichikawa, Y. Fujiwara, and K. Kojima,
[invited] "Combinational integration of Eu-doped GaN and InGaN LEDs and their prospects for miniaturization'',
10th International Conference on Light-Emitting Devices and Their Industrial Applications (LEDIA2024), LEDIA5-01, Yokohama, Japan (Apr. 2024).
[Link]
・Fabrication of integrable optical-polarization-controlled devices
・Fabrication of integrable optical-polarization-controlled devices
In addition to brightness and color, light have another factor known as "optical-polarization," which describes the orientation in which the light's electric field oscillates over time. Although we rarely notice polarization in our daily lives compared to the brightness and the color, it has already been utilized in various practical applications. Notably, "circularly polarized light"—unlike linearly polarized light—possesses the significant advantage that its properties are independent of the choice of coordinate axes and are less susceptible to the effects of scattering; consequently, it is employed in technologies such as GPS that require accurate transmission of positional information. While circularly polarized light holds great promise for future applications in fields ranging from next-generation 3D displays and plant factories to security tags, optical communications, and chemical sensors, most of these potential applications have yet to be realized in practice. To overcome this challenge, our laboratory promotes a research aimed at developing semiconductor devices capable of efficiently generation and detection of circularly-polarized light using a single chip device that is suitable for integration. We are pursuing this goal through an approach that combines device design based on optical phase control with the actual fabrication of the devices.
References
References
Yuki Murata, Shintaro Toda, Yasufumi Fujiwara, Kazunobu Kojima, and Shuhei Ichikawa,
"Metasurface-integrated semipolar (20-21) InxGa1-xN quantum wells towards efficient circularly-polarized LEDs",
Optical Materials Express 16(4), 924 (2026).
DOI: 10.1364/OME.588632
Yohei Taguchi, Yuki Murata, Kyohei Suzuki, Shintaro Toda, Hiroshi Tabata, Kazunobu Kojima, and Shuhei Ichikawa,
"Circularly polarized (0001) InGaN-based LED integrated with GaN metasurface",
Optics Letters 51(5), 1287 (2026).
DOI: 10.1364/OL.587468
・Characterization of localized emission in optical devices via spatiotemporally-resolved spectroscopy
・Characterization of localized emission in optical devices via spatiotemporally-resolved spectroscopy
Deep-ultraviolet (DUV) light with wavelengths from 200 to 300 nm is known to interact strongly with DNA, proteins, and biological organisms; consequently, it is applied in various fields such as medical care, sterilization and water purification, high-density optical recording, and semiconductor processing light sources. AlGaN-based semiconductors have attracted much attention as novel materials for DUV light-emitting devices to replace conventional DUV lamps, which contain toxic substances such as fluorine and mercury and suffer from low efficiency and short lifespans. In recent years, there has been vigorous development of UV-LEDs operating in the short-wavelength range of 220–230 nm, driven by the potential for non-invasive applications on the human body; however, it has been reported that the quantum efficiency of the LEDs drops significantly when using AlGaN with high Al content to achieve such short-wavelength emissions, making efficiency improvement an urgent necessity.
Our research group has revealed—through spatio-temporally resolved luminescence measurements—that "localized emission associated with specific surface morphology" arising during crystal growth can influence luminous efficiency; we are currently conducting research aimed at proposing new device structures that effectively utilize these phenomena.
References
References
Kenjiro Uesugi, Ryota Akaike, Shuhei Ichikawa, Takao Nakamura, Kazunobu Kojima, Masahiko Tsuchiya, and Hideto Miyake,
"230 nm wavelength range far-UVC LED with low Al-composition differentiation between well and barrier layers of MQWs'',
Applied Physics Express 17(4), 042008 (2024).
DOI: 10.35848/1882-0786/ad3e48
M. Hayakawa, S. Ichikawa, M. Funato, and Y. Kawakami,
"AlxGa1-xN-Based Quantum Wells Fabricated on Macrosteps Effectively Suppressing Nonradiative Recombination'',
Advanced Optical Materials 7(2), 1801106 (2019).
DOI: 10.1002/adom.201801106H. Kobayashi, S. Ichikawa, M. Funato, and Y. Kawakami,
"Self-Limiting Growth of Ultrathin GaN/AlN Quantum Wells for Highly Efficient Deep Ultraviolet Emitters'',
Advanced Optical Materials 7(21), 1900860 (2019).
DOI: 10.1002/adom.201900860S. Ichikawa, M. Funato, and Y. Kawakami,
"Dominant Nonradiative Recombination Paths and Their Activation Processes in AlxGa1-xN-related Materials'',
Physical Review Applied 10(6), 064027 (2018).
DOI: 10.1103/PhysRevApplied.10.064027
・Development of novel optical devices using host-guest semiconductor materials
・Development of novel optical devices using host-guest semiconductor materials
Some semiconductor devices derive their high performance from "guest" materials embedded within a "host" matrix. Our research group is advancing the development of devices such as novel light emitters using rare-earth-doped semiconductors which shows ultra-stable emission wavelengths under current injections, and optical cooling devices utilizing "dot-in-crystal" metal-halide perovskites, in which quantum dots are embedded within a host material.
References
References
S. Yamazaki, S. Ichikawa, T. Iwaya, J. Tatebayashi, and Y. Fujiwara,
"Ultra-stable multiple emission wavelengths produced by Tb-doped AlxGa1-xN-based light-emitting diodes",
Applied Physics Letters 128(26), 261112 (2026).
DOI: 10.1063/5.0331734
A. Takeo, S. Ichikawa, D. Timmerman, J. Tatebayashi, and Y. Fujiwara,
"Preferential formation of highly efficient Eu luminescent centers in Eu-doped GaN grown on semipolar (20-21) GaN",
Applied Physics Letters 128(11), 113304 (2026).
DOI: 10.1063/5.0308400S. Ichikawa, K. Shiomi, T. Morikawa, D. Timmerman, Y. Sasaki, J. Tatebayashi, and Y. Fujiwara,
"Eu-doped GaN and InGaN monolithically stacked full-color LEDs with a wide color gamut'',
Applied Physics Express 14(3), 031008 (2021).
DOI: 10.35848/1882-0786/abe603Yasuhiro Yamada, Takeru Oki, Takeshi Morita, Takumi Yamada, Mitsuki Fukuda, Shuhei Ichikawa, Kazunobu Kojima, and Yoshihiko Kanemitsu,
"Optical Cooling of Dot-in-Crystal Halide Perovskites: Challenges of Nonlinear Exciton Recombination'',
Nano Letters 24(36), 11255 (2024).
DOI: 10.1021/acs.nanolett.4c02885
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