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Deep-UV LEDs attracting attention about water disinfection

Leading UV light field with high-output products that hold the key to market expansion

Stanley Electric's high-power Deep-UV LEDs have attracted a great deal of attention, especially in the hygiene field.
Stanley achieved the highest output power in the field (50 mW) emitting at 265 nm, optimal for sterilizing water.
This is because the possibilities of expanding Deep-UV light applications to equipment such as water purification devices have further increased.
The company is pursuing an even higher light output by making full use of technologies, materials and processes that have been cultivated with visible light LED and IREDs for many years. Their idea is to accelerate the development of Deep-UV LEDs' new market.

Two countermeasures to the problems that hinder high output

The main problems to Deep-UV LED are caused by its structure. In general, a Deep-UV LED possesses a structure in which a substrate, an n-AlGaN (aluminum gallium nitride) layer, an AlGaN active layer, and a p-AlGaN layer are stacked starting from the light-extraction side. Furthermore, in the n-AlGaN layer there is an n-type electrode, while in the p-AlGaN layer there is a p-type electrode; a p-GaN (gallium nitride) layer called "contact layer" is sandwiched between p-AlGaN layer and p-type electrode (Fig. 3).

Here, one of the main factors that inhibits high power output and high efficiency is dislocation (a type of crystal defect) generated in the AlGaN layer due to the difference in lattice constant and thermal expansion coefficient between the AlGaN layer and the substrate. As this dislocation increases, the output and luminous efficiency decrease. Also, due to the large refractive index difference between the substrate and the air, light is totally reflected at the interface of the substrate; light generated in the AlGaN active layer cannot be taken out to the outside and stay inside the device. In addition, since the p-GaN of the contact layer absorbs light emitting at about 360 nm or less, most of the light which cannot be extracted to the outside is taken by the p-GaN layer and eventually converted into heat.

Fig. 3: Structure of Deep-UV LEDs

Illustration of layer structure
											Air
											Substrate
											n-AlGaN layer
											AlGaN active layer
											p-AlGaN layer
											p-GaN layer

AlN substrate with low dislocation and high transparency

One of the two technologies adopted by Stanley Electric as a countermeasure against these problems is the adoption of an AlN laminated substrate produced with the hydride vapor phase epitaxy (HVPE) method. It is a substrate with a laminated structure in which an AlN layer is formed via HVPE method on an AlN seed substrate with low dislocation density, fabricated by sublimation method. As compared with the AlN substrate fabricated by the conventional sublimation method, the transmittance at a wavelength of 300 nm or less is remarkably high (Fig. 4). That is, it has advantageous characteristics for extracting 265 nm light. Moreover, the density of dislocations generated in the AlGaN layer is remarkably small. The dislocation density dramatically decreases to about 1/10,000 compared to the sapphire substrate. "Reduction in efficiency caused by increasing the current flowing in the die ― the so-called droop ― is also extremely small, which is also advantageous for achieving high output" (Mr. Kinoshita).

Fig. 4: Transmittance characteristics of AlN laminated substrate fabricated via HVPE method

Illustration of layer structure
											Linear transmittance
											AlN substrate fabricated via HVPE method
											AlN substrate fabricated via sublimation method
											Substrate thickness
											Wavelength

Another countermeasure technique is to provide minute protrusions (nanophotonic crystals) on the entire surface of the AlN substrate to suppress reflection (total reflection) generated at the interface between substrate and air (Fig. 5): increasing the amount of light extracted from the substrate while suppressing reflection, thereby improving the output. "Although technology to suppress the reflection inside the substrate by roughening the surface of the substrate is commonly used, in our case we control the shape and density of the protrusions with high precision, thereby enhancing the effect of reflection suppression" (Mr. Kinoshita). In addition, emphasis is placed on mass productivity, and lithography by nanoimprint is adopted as a process for forming protrusions. In other words, it is a highly productive process to form minute protrusions efficiently by pressing a mold against the substrate surface. "At an early development stage, we adopted the method of drawing using electron beams, but we switched to nanoimprinting with emphasis on productivity," Kinoshita said.

Fig. 5: Minute protrusions are formed on the substrate surface to suppress reflection inside the substrate

Illustration of layer structure
												Air
												Substrate
												n-AlGaN layer
												AlGaN active layer
												p-AlGaN layer
												p-GaN layer
Electron microscope screen

Forming unique nanophotonic crystal by nanoimprint lithography

In Stanley Electric, we are planning to further improve the output by pursuing conventional approaches such as improving light extraction efficiency and substrate characteristics. High-power output Deep-UV LEDs applications are spreading, resulting in a new market. Deep-UV LEDs also contribute to improving the global environment by bringing innovation in the field of water treatment. Stanley Electric's future efforts to strongly promote the development of Deep-UV LEDs will surely attract more attention.