Development of 230 nm Wavelength Far-Ultraviolet
C Light-Emitting Diodes
波長 230nm far-UVC LED の開発

Hirokazu KAWASHIMA

Hiroyuki OOGAMI

Kengo MOURI

河島 宏和

大神 裕之

毛利 健吾

Mitsuhiro MUTA

Yasushi IWAISAKO

牟田 実広

祝迫 恭

Keywords； far-ultraviolet C light-emitting diode, wavelength spectrum, light output power, RIKEN,
metal organic chemical vapor deposition, luminous intensity distribution
キーワード；far-UVC LED，波長スペクトル，発光出力，理化学研究所，MOCVD 法，
配光分布
Abstract
Although deep ultraviolet (UV) light with a wavelength of 265 to 280 nm has a high virusinactivation effect, there are concerns about its effect on the human body. UV light with a wavelengthof 220 to 230 nm (far-UV C (UVC) light) is considered to be harmless to the human body, but it has
not been put into practical use because its luminous efficiency is extremely inferior to that of lightwith a wavelength of 265 to 280 nm. In this research, we developed a 230 nm far-UVC light-emitting
diode (LED) through joint research with RIKEN. In the fabricated 230 nm far-UVC LED, a single
chip achieved a maximum light output power of 2.7 mW in pulse operation, a maximum light output
power of 1.1 mW in continuous operation, and a maximum external quantum efficiency of 0.25%. We
also created a panel equipped with 25 LEDs and confirmed that there were no problems with its
operation.

Nippon Tungsten Review Vol.42 (2024)- 26 -波長 265nm～280nm の深紫外光は高いウイルス不活化効果がある反面，人体への影響が懸念される．波長 220nm～230nm の紫外光(far-UVC)は人体に無害とされているが，発光効率が波長 265nm～280nm の LED に比べて極端に劣ることから，実用化には至っていな
い．本研究では，理化学研究所との共同研究により，230nm far-UVC LED の開発を行った．
作製した 230nm far-UVC LED において，チップ単体でパルス動作では最大 2.7mW，連続動作では最大 1.1mW，最高 EQE0.25%を達成した．また，LED を 25 個搭載したパネルを作製し，動作に問題が無い事が確認できた．

1. Introduction
Ultraviolet (UV) light is used for air purification and water purification because of its virus
inactivation effect. However, the wavelength of the deep UV light that is generally used is 265 to 280
nm, which is considered harmful to the human body, and its use is limited to spaces in which nohumans are present. Therefore, in recent years, UV light of 220 to 230 nm (far-UV C (UVC) light),
which is considered harmless to the human body, has attracted attention.
The reason why UV light of 265 to 280 nm is considered harmful to the human body is that light ofthis wavelength is close to the absorption spectrum of DNA and can penetrate the skin and cornea.
Conversely, 230 nm far-UVC light is mostly absorbed at the surface of the skin and cornea, so it isless likely to damage human cells. Additionally, because viruses are much smaller than human cells,
230 nm far-UVC light has an inactivating effect on viruses.
The excimer lamp produced by Ushio Inc., which takes advantage of the characteristics of thesewavelengths, uses 222 nm far-UVC light. It is considered to be harmless to the human body andeffective in inactivating viruses, has a track record of being used in public institutions, and is widely
accepted around the world.
We wondered if it would be possible to develop a light source device that is even more compact anddurable than excimer lamps by using light-emitting diodes (LEDs) as the light source. However, UVC
LEDs have the problem that the luminous efficiency decreases as the wavelength decreases [1].
In this study, we developed a 230 nm LED in collaboration with the Hirayama Quantum Light
Device Laboratory of RIKEN. LEDs with this wavelength were fabricated and their performance wasinvestigated. In addition, we developed a device that can mount multiple LEDs to achieve high output
power.

2. Experimental
2.1 Chip and panel fabrication
LED wafers were fabricated by forming AlGaN-based LED layers on sapphire substrates using themetal organic chemical vapor deposition method. Sapphire was chosen as the substrate because it isinexpensive and has better light out-coupling efficiency than AlN.
After forming p- and n-electrodes on the fabricated wafers, the wafers were stealth diced into 1.2mm × 1.3 mm pieces. The p-electrodes were composed of Ni/Au, and the n-electrodes were composed- 27 -

Nippon Tungsten Review Vol.42 (2024)

of V/Al/Ti/Au. The individualized chips were mounted on a circuit board (3.45 mm × 3.45 mm pieces)with Cu electrodes and then sealed with microlenses (NGK).
Twenty-five LEDs (single LEDs) sealed with microlenses were mounted on a copper base and fixedon a heat sink with a cooling fan. The panel consisted of five LEDs connected in series and five rowsof LEDs in parallel.
2.2 Performance check
The light output, spectrum, and radiation pattern of the single LED were measured, and theoperation of the panel was verified. The light output was measured in pulsed (1 ms, 5% duty ratio)
and continuous (CW) operation, and the light emitted from the LEDs was measured using a measuring
instrument equipped with an integrating sphere. Both the instruments and power supplies wereprovided by RIKEN.

3. Results and DiscussionThe measured optical output of the fabricated chip versus the current is shown in Fig. 1. The
maximum external quantum efficiency (EQE) was 2.7 mW in pulsed operation (1 ms, 5% duty ratio)
and 1.1 mW in continuous operation, and the highest EQE was 0.25%. There are few reports of LEDs
that emit 230 nm light using inexpensive sapphire as the substrate, and this can be considered to be
one of the leading-edge research results in this field.

pulse condition

CW condition

Fig. 1 Light output power versus current of the LED
The emission spectrum of the LED is shown in Fig. 2. Although the peak position varied dependingon the current value, the main peak was at 232–234 nm, which was as expected. Although there wasa sub-peak near 270 nm, the intensity was approximately 1/100 to 1/200 of that of the main peak. Inaddition, the main peak had a tail at approximately 260 nm. To suppress light on the long wavelengthside (wavelengths of 240 to 280 nm), future challenges include shortening the wavelength, suppressing
the half-value (steepening the peak), and developing filters that cut specific wavelengths.

Nippon Tungsten Review Vol.42 (2024)- 28 -Fig. 2 Emission spectra of the LED at different currents
The radiation patterns of the chip with and without a lens are shown in Fig. 3. Without a lens, thedirectivity angle was approximately 140° to 160°. However, with a lens attached, the light was focusedin front of the chip.
It was confirmed that the electrode pattern of the fabricated LED chip caused a tilt of the opticalaxis (although it is not shown in Fig. 3), and the issue is to improve the electrode pattern to improve
the luminous efficiency.
With lensWith lensWithout lensWithout lensFig. 3 Radiation patterns of the LED with and without a lensPhotographs of the panel equipped with 25 LEDs and the light-emitting condition are shown in
Fig.4. By installing 25 LEDs in this panel, a maximum output of 2.7 mW × 25p = 68mW during pulse- 29 -

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operation and 1.1 mW × 25p = 28 mW during CW operation can be expected. Although the accurate
light output cannot be measured at present, it was confirmed that all of the installed LEDs emitted
light, indicating that the device worked without any problems.

Single LED(25p)
PanelHeat sink
FanFig. 4 Photographs of the panel (left) and light-emitting condition (right)
There are many issues to overcome before the reported 230 nm LED can be put into practical use.
Among these issues, improving the external quantum efficiency is an important issue. It is thoughtthat the decrease in the p-type and n-type concentration as the Al composition of the AlGaN lightemitting layer improves affects the deterioration of the characteristics.
To improve the hole conductivity on the p-side, we introduced a polarization-doped structure andattempted to improve the efficiency of the 230 nm LEDs. The polarization-doping effect generates ahigh concentration of holes simply by tilting the Al mixed crystal of AlGaN from a larger direction toa smaller direction. This feature means that by using an undoped compositionally graded layer, thesame carrier generation as that in the case of impurity doping can be obtained.
By fabricating 230nm LEDs using this polarization-doping method and comparing the results
obtained, it was confirmed that the EQE can be improved by approximately 10 times. We filed a patentapplication for these results, and the patent number 7291357 is registered in Japan [2].

4. Conclusions
In this study, we developed a LED that produces 230 nm far-UVC light, which has less effect on
the human body than higher wavelength light. We fabricated a LED and evaluated its performance.
The conclusions are as follows.
(1) The maximum optical output of the fabricated chip was 2.7 mW in pulsed operation and 1.1mW
in continuous operation.
(2) The main peak of the fabricated LED was at 232–234 nm.
(3) A good light focusing effect was obtained by implementing microlenses.
(4) We created a panel equipped with 25 LEDs and confirmed that there were no problems with its
operation.

Nippon Tungsten Review Vol.42 (2024)- 30 -

References[1] Hideki Hirayama et al. (2023). "Recent Progress of 230 nm AlGaN far-UVC LED on sapphire
substrate" OPTRONICS , No.5, 1-8[2] Hideki Hirayama et al. Ultraviolet light emitting device and electrical equipment with theultraviolet light emitting device. JP patent 7291357. 2023-6-15.

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