The Future of DUV LEDs

−Development of light-extraction technology
by nanophotonic structures
for practical application of DUV-LEDs−
Shin-ichiro INOUE

Senior Researcher, Nano ICT Laboratory,Advanced ICT Institute
After completing doctoral course and working as Special Postdoctoral Researcher of the Institute ofPhysical and Chemical Research, RIKEN, Assistant Professor of Institute for Materials Chemistry andEngineering, Kyushu University, he joined NICT in 2010. He has been engaged in research ofnano-optoelectronics. He is also Associate Professor, Graduate School of Engineering Faculty andEngineering, Kobe University and Researcher of PRESTO, Japan Science and Technology Agency.
Ph.D. (Engineering).

Background
The importance of deep-ultraviolet (DUV) light with 200–350 nm
wavelength has been increasing in various fields from information,
electronic devices to safety and sanitary environment, medicalapplication, such as high-density optical data storage, disinfection ofbacteria and viruses, purification of water and air, biosensing,biological and materials analysis, photolithography, prevention ofnosocomial infection, and photodynamic therapy. It's becoming animportant infrastructure that supports our society. For deepultraviolet light sources, gas light sources such as mercury lamps
and excimer lasers has been conventionally used. However, the gas
light source is short-lived, large in size and requires large power
consumption. For such reasons, replacing it with semiconductor
solid state light source, which is small in size and power consumption, has been much needed. In addition, as typified by the adoptionof Minamata Convention on Mercury in last year, global efforts
have been accelerated to reduce or eradicate harmful substances tothe human body or environments such as mercury and fluorine.
Therefore, the application of deep-ultraviolet light-emitting diodes(DUV-LEDs) with low environmental impacts, high efficiency andlong life is strongly desired (Figure 1).

Figure 1

5

The impact and the applications of DUV-LEDs

NICT NEWS APR. 2014

Under these circumstances, the R&D of DUV-LEDs has beenvery active all over the world. Although the recent performance has
been greatly improved, the further improvement in efficiency is necessary for the practical use. The R&D of DUV-LEDs by NICT wasadopted as A-STEP (Adaptable and Seamless Technology Transfer
Program through Target-driven R&D), the stage for promotion ofindustry-academia collaborative R&D (feasibility study stage),Seeds Validation. NICT has been engaged in the full-fledged R&D
with the aim to apply DUV-LEDs in cooperation with a company,
which has been conducting joint research with NICT since December 2013. In this article, we will introduce our efforts towards theefficiency improvement of DUV-LEDs and its application such as
the development of significant enhancement technologies in lightextraction efficiency by making use of nanophotonic structures.

The progress and challenges of DUV-LEDs
DUV-LEDs consist of AlGaN-based semiconductors of the directtransition type. Varying the mole fraction of AlN and GaN enables
AlGaN-based DUV-LEDs to emit the light with almost full DUV
range (210–365 nm). In semiconductor LEDs, if there are highdensity defects in the crystal, the electron-hole pairs generated byinjection current can easily bechanged into the heat throughdefects without light emission. InAlGaN-based DUV-LEDs, since
10 8 cm -2 or more of high-density
defects (dislocations) occur on theactive layer (Figure 2 (a)) due to thelarge lattice mismatch in commonly used sapphire (Al2O3) substrates, there has been issues of thelow internal quantum efficiency
and the short lifetime. However,
the recent progress of R&D of
buffer layer technology and AlN
substrates in several groups hasbrought the significant improvement on these issues. NICT has
been conducting R&D of DUVLED using AlN single crystal
substrates in cooperation with
Tokuyama Corporation. Since it ispossible to reduce the defects(reduction of dislocations) such asless than 106 cm-2 with DUV-LEDs

Figure 2

The device structure of AIGaN-based DUV-LEDs(a) Schematic diagram showing problems of conventional DUV-LEDs (high-density dislocation defects, very low light-extraction eﬃciency)(b) Schematic diagram of light-extraction structure with nanophotonic structure and its picture by Scanning Electron Microscopeon AlN substrates, AlN substrates has
high advantages in the lifetime and thereliability.
The current biggest factor that inhibits the efficiency improvement inDUV-LEDs is the problem of the very
low light-extraction efficiency. This is aparticular problem of DUV-LEDs. It isdifficult to extract the light due to theafter processinternal absorption in the p-type GaN
before processcontact layer and the total reflection atthe substrate interface. Therefore, mostof the light emitted in the active layerchange into thermal energy (Figure 2(a)). Figure 3 The emission characteristics and the performance improvement results of DUV-LEDs
Especially, with AlN single crystal(a) The improvement of the output power of DUV-LEDs with nanophotonic structure
substrates, since the refractive-index is(b) Emission spectra for each driving current
large (n = 2.29 @265 nm) compared tosapphire's, the critical angle get small (25.9 degrees). Thus, you canwere as high as 1.7 or more (Figure 3). Also, the standard deviaextract the only small amount of light. The results of theoreticaltions of enhancement between devices were less than 0.03, andcalculation by 3D finite-difference time-domain (3D-FDTD)
we highly achieved the reduction of the dispersion of output
method, with absorption in p-type GaN layer considered, showed
power between devices, which is essential for practical use. Inthat the efficiency of light extraction from the flat surface (lightfact, we fabricated a prototype of 265 nm DUV-LEDs inextraction surface) of AlN substrates was just only about 4%. In
collaboration with Tokuyama Corporation and achieved theother words, this problem is the main cause of very low external
output power of more than 30 mW. And furthermore, for the reliquantum efficiency in DUV-LEDs. With that being said, theability measurements, the lifetime for the 50% power reduction
improvement of DUV-LEDs depends on how to improve the lightat 150 mA was estimated to be over 6,000 hours in 265 nm
extraction efficiency.
DUV-LEDs.

Performance improvementof DUV-LEDs by nanophotonic structure
The biggest issue preventing performance improvement and practical application of DUV-LEDs is their limited light-extraction efficiency. Currently, we have achieved the highest light-extraction
enhancement of DUV-LEDs on AlN substrates in the world. We
achieved the high light-extraction efficiency with the reduction oftotal internal reflection on AlN substrates by proposing and developing new light-extraction structure. The remarkable enhancementof light-extraction is caused by the multiplier light enhancement
effects of periodic convex structures at the wavelength scale andsmaller fine round convex structures (Figure 2(b)). Not only
improvement of the light-extraction efficiency but light output
uniformity between devices, manufacture cost, and yield rate arealso considered in this unique structure. We achieved the fabricationof light-extraction nanophotonic structures with high precision andhigh uniformity for DUV-LEDs on AlN substrates. When comparedto the devices without the nanophotonic structures, the relativeemission intensities of DUV-LEDs recorded with the structures

Future prospects
These results are expected to bring significant progress toward
the improvement of efficiency and reliability, and the practical useof DUV-LEDs. In the future, using nanophotonic structure, we willfurther improve the light-extraction efficiency and develop the mass
production and low cost technology such as nano-imprint technology. By these technologies, we will develop DUV-LEDs that has ahigh competitive advantage in both performance and cost.
The industrial uses of DUV-LEDs in various fields are expected,
and there is a chance that the LED grows into a huge market.
Ultimately, by the realization of mercury-free, ultra-compact,
highly efficient, and long lifetime DUV solid light sources opening the possibility of new DUV applications, we would like tocontribute to build lively, safe and secure and environmentallyfriendly society.

NICT NEWS APR. 20146