The 83rd JSAP Autumn Meeting 2022 Highlighted Presentations
October 24, 2022

Electron irradiation examination of GaPN alloys for controlling bonding states in dilute nitride alloys

An accidental discovery that led to the development ofsolar cells whose performance is improved byirradiation
Kento Hirai (1), Keisuke Yamane (1), Akihiro Wakahara (1), Takeshi Oshima (2), Tetsuya Nakamura (3), Tsutomu Imaizumi (3)Toyohashi University of Technology (1), National Institutes for Quantum Science and Technology (2), Japan Aerospace Exploration
Agency (3)summary• Discovered a phenomenon in which electron-beam irradiation improves the electrical andoptical properties and restores the crystallinity of III-V dilute nitride alloys used in solar
cells, which normally deteriorate.
• The combination of electron-beam irradiation and heat treatment was investigated fromthe viewpoint of luminescence properties, and the optical properties were confirmed to beimproved by electron-beam irradiation alone.
• This research indicates the possibility of creating completely new radiation-resistantelements that could find various applications, including those in the space industry.
A research group led by Kento Hirai, Keisuke Yamane, and Akihiro Wakahara of the Toyohashi University ofTechnology; Takeshi Oshima of the Japan Quantum Science and Technology Agency; and Tetsuya
Nakamura and Mitsuru Imaizumi of the National Institutes for Quantum Science and Technology discoveredthat electron and proton beam irradiation can improve the electrical and optical properties of III-V dilutenitride mixtures used in solar cells, which would normally deteriorate, and restore their crystallinity. In theanalysis, the combination of electron-beam irradiation and heat treatment was repeatedly examined from theviewpoint of luminescence properties. As a result, it was confirmed that the optical properties were improvedby electron-beam irradiation alone. This research indicates the possibility of creating completely new
radiation-resistant elements that could find various applications, including those in the space industry.

detail"For some reason, I think its properties are improving."
In recent years, the space industry has been experiencing unprecedented growth. Private
spaceflight companies such as US entrepreneur Elon Musk’s SpaceX are being founded, and they
are reducing launch costs. Private spaceflight companies are also becoming more active in thedevelopment of satellites and services. As the space industry expands, the development ofradiation-resistant devices that can operate in outer space environments where they will beexposed to severe radiation will be in high demand.
A research group led by Kento Hirai of the Toyohashi University of Technology has discovered thatelectron and proton beam irradiation improves the electrical and optical properties of III-V dilutenitride mixtures used in solar cells, which normally deteriorate, and restores their crystallinity.
Hirai noted that “We were not originally developing this material for space applications. The
phenomenon was discovered by chance when we measured the radiation resistance of gallium
arsenide-phosphorus-nitride solar cells as a low-cost III-V dilute nitride mixture for use in solar
cells. The research began when a student in the laboratory at the time of the phenomenon's
discovery commented, “For some reason, I think its properties are improving.”
Hirai took on the challenge of analyzing this phenomenon. When he examined the photovoltaic
characteristics (*1), which is a common method for evaluating solar cells, he found that thecharacteristics improved after electron and proton beam radiation. Furthermore, in terms of shortcircuit current density (*2), the performance of the solar cells improved after electron and proton
beam irradiation. An evaluation of the acceleration voltage dependence (Figure) confirmed that theluminescence intensity improved (i.e., crystallinity improved) even with electron-beam irradiation
alone (i.e., without heat treatment).
Hirai said that "Normally, electron irradiation would cause degradation in conventional III-V
compound alloys, but irradiation alone improves crystallinity of our diluted nitride alloys. Based on
first-principles calculations, we are now predicting that the improvement in crystallinity may be dueto the disappearance of defects caused by nitrogen (nitrogen is taken in during the crystal growth
process and becomes unstable in the crystal), which are eliminated by irradiation.”
Currently, the group is continuing research to experimentally verify the results of first-principles
calculations to elucidate the mechanism. Details on the discovery of the phenomenon can bereferred from the paper published by the group (*3).
Widely applicable as radiation-resistant devices
One application of the above mentioned discovery is solar cells for use in satellites. Current solarcells for space applications always deteriorate over time owing to radiation. The development of asolar cell that overcomes this drawback should represent a major breakthrough in this area. The
research group has been conducting joint research on solar cells with the Japan Aerospace
Exploration Agency (JAXA) for more than 10 years and will continue to do so in the future.
Furthermore, there is potential for applications beyond solar cells. Hirai said that “We can use it insolar cells themselves, but since it is essentially a semiconductor, we think it could also be used inelectronic devices and as a light-emitting material. If we look at it as a radiation-resistant electronic
device material, I think the range of applications will expand even further.”
The group hopes to further advance this research in anticipation of continual growth in the spaceindustry.
Annotations!1 Photovoltaic characteristics evaluation: Characteristics of how much current is generated when the voltage ischanged while the solar cell is exposed to light.
!2 Short-circuit current density: In the evaluation of photovoltaic characteristics, the current density at zero voltage iscalled the short-circuit current density.
!3 https://doi.org/10.35848/1347-4065/ac4a06
https://doi.org/10.1063/5.0096345"

figures

PL spectra of electron-beam irradiated GaPN: (a) accelerating voltage dependence and (b) results showing
enhanced luminescence intensity with electron-beam irradiation alone.
Higher PL emission intensity indicates better crystallinity. In both cases, we compare the irradiation of samples with rapid
thermal annealing (RTA) (heat treatment/short-time annealing at high temperature) with the irradiation of samples
without RTA. (a) At 240 K, the luminescence intensity of the samples subjected to RTA and electron-beam irradiation ishigher than that of the samples not subjected to RTA. (b) At 20 K, the luminescence intensity of the sample subjected toonly electron-beam irradiation and not RTA is approximately five times higher than that of the sample not subjected toboth electron-beam irradiation and RTA (i.e., as-grown). This indicates that electron-beam irradiation alone is effective inrestoring the crystallinity.