1, the extension of technical difficulties.

Sapphire substrate is still the mainstream. InGaN material is the mainstream of blue and green LED. We can get the wavelength range from red to ultraviolet light by using the different components of indium (In). The maximum photoelectric conversion efficiency is 430 ~ 450 nm. The band gap width of GaN is 3.4 electron volts (eV), which just falls at 365 nm. This is also the limit of InGaN material. But the difficulty of UV short wavelength LED is here, at 365 nm. There are a lot of problems to be overcome for UVA LEDs below metres, and I think there are two key technical difficulties that are the most deadly.

The first problem is the optical absorption of layers other than the luminous layer. When the wavelength is shorter than 370 nm, P-type GaN will absorb light, leading to a large number of absorption of light from quantum wells. Another problem is that the shorter the wavelength, the lower the indium component is needed, and the lower the indium component, the lower the nonuniformity of the indium-gallium-nitrogen luminous layer will be broken. In order to get shorter wavelength, the introduction of Quaternary AlInGaN and aluminum nitride AlN (6.2eV, 197 nm) in the luminous layer is an urgent technology for shorter wavelength UV LED technology. GaN band gap wavelength is 365 nm, to shorter wavelength, aluminum (Al) content must be increased. The structure produces tensile strain, and the structure will produce compressive strain when the in content is increased to the long wavelength. Compared with the compressive stress of the traditional blue and green light, the tensile stress with the increase of aluminum content will make the epitaxy more difficult.

At present, this problem has been plaguing the epitaxial engineers of UV LEDs, resulting in the internal quantum efficiency of UVC has always been less than 50%. Of course, the shorter the luminous wavelength, other P-layer and N-layer materials more need to add Al components, so that the absorbance ratio is reduced, so aluminum nitride (AlN) and aluminum gallium nitrogen (AlGaN) material growth is more important, which requires a higher temperature MOCVD system design, the mainstream blue MOCVD system does not have such conditions.

Therefore, because of the accumulation of these problems, the current UVA 365 nm wavelength and UVC band epitaxy technology is limited, resulting in high cost.

2, chip technical problems

Chip problems are not less than epitaxy, the main is that the orthogonal chip technology has been unable to meet the requirements of UVLED, especially the 380 nm below the UVLED chip, the most mainstream UVA technology is vertical structure chip, because the vertical structure of the chip luminous surface in N-type materials, can effectively reduce the problem of light absorption, in addition to vertical structure. The structure has stable light type, most of which are axial light, almost no lateral light, high radiation efficiency, and relatively stable and uniform light distribution in the curing process. At present, silicon substrate chemical peeling technology and sapphire substrate laser peeling technology are used to fabricate vertical structure chips. The cost of these two technologies is high because of their low yield and complex technology. The unit price is 3 to 5 times the current price of the original chip.

For 280nm and 265nm UVC structures, flip-flop structure is the mainstream technology at present. The key problem is how to reduce the absorption of GaN to UVC and good ohmic reflective electrode, and the appropriate ohmic contact electrode with N-type Al-Ga-N is also very important.

Figure 8 is a comparative diagram of the three structures of UVLEDs. From the performance and cost point of view, the wavelength above 385 nm has advantages in using inexpensive formal structure and excellent vertical structure. The wavelength below 375 nm is suitable for vertical structure. Because of better heat dissipation path, the band of UVC is suitable for flip-chip structure. This is also the current market. One of the reasons why devices over 385 nm are cheap is that UVs with shorter wavelengths are getting more expensive.

3, the technical difficulties of packaging

Although relatively easy, but the difficulty compared with traditional LED packaging, many difficulties, mainly the current LED packaging materials can not meet the requirements of UV band, usually in response to UV LED packaging requirements, the use of inorganic air-tight glass packaging UV LED, UV LED to deal with high-energy radiation. Therefore, reduce the use of organic materials, or even completely do not use organic materials for UV LED packaging, thereby reducing or avoiding the attenuation caused by organic materials and wet thermal stress caused by failure problems. Inorganic materials with high penetration in UV band. Current glasses, quartz and NOVAXIL glasses are essential for UV packaging. Fig. 9 is a schematic comparison of their penetration in UVA and UVC with other characteristics.

In addition to packaging materials, another challenge is the thermal management of UV LEDs, especially UVC LEDs with extremely low external quantum efficiency (EQE), which converts only about 2-3% of the power input into light. The remaining 97% of the power is converted to heat and the heat must be removed quickly, so the thermal conductivity of the substrate must be very high. In the past, PCB, ceramic and aluminum substrates were difficult to meet this requirement unless active cooling technology was added. Aluminum nitride (AIN), the most popular packaging substrate, has excellent thermal conductivity (140 W/mK-170W/mK), but it is very expensive. In addition, DPC ceramic substrate is formed in 3D to form metal sealing cavity on the surface of ceramic substrate. Forming ceramic-metal 3D sealing structure can also meet the development needs of existing UV packaging technology. Fig. 10 is an illustration. I think the cost of UVC packaging is very high, and besides the chips that are so expensive, it's mostly needed.