Technical progress and application of deep UV LED

With the continuous development of LED technology, the emission wavelength of visible light band is extended to the deep ultraviolet region, its technology is gradually mature and the cost decline will make UV LED more widely used, and may even go beyond the current blue LED. In this paper, the current research progress and industrial application of deep UV LED are introduced from the aspects of the luminescence characteristics and fabrication process of deep UV LED.

In 1997, Nichia successfully developed the world's first light emitting wavelength of GaN based UV 371 nm light LED. 2003, the United States SETi company developed a wavelength of 280 nm based A1GaN deep UV LED. October 24, 2014, Nobel prize winner, one of the days of the wild ho told reporters at the meeting to introduce their ongoing research, including the wavelength of about 250~350 nm deep UV LED.

UV LED as the 1 branch of the LED, although it can not be illuminated with all the advantages of LED, in theory, can replace all the traditional UV light source, greatly expanding the application of LED. The most common ultraviolet is mainly derived from the sun radiation, according to the UV wavelength can be divided into ultraviolet (ultraviolet A, UVA), ultraviolet (ultraviolet B, UVB) and ultraviolet (ultravioletC, UVC), nm 280~320nm wavelength were 320~400, 100~280, nm. To reach the surface of the earth's ultraviolet rays, including ultraviolet UVA and ultraviolet UVB, while the ultraviolet UVC is basically absorbed by the atmosphere ozone layer (so UVC belongs to the blind area).

Introduction of UV LED manufacturing technology

1.1 preparation of luminescent materials

Epitaxial growth process, through the MOCVD device on the sapphire substrate in turn to grow A1N template layer, N type A1GaN layer, a lot of the well layer, the electric barrier layer and P type GaN contact layer, the epitaxial structure diagram see figure 1.

1.2 electrode fabrication process

Chip process, lithography, etching by the leakage of N type contact layer, through evaporation and alloy, N type, P type electrode and ohmic contact (Figure 1), then by thinning, lobes, separation of small core particles and flip chip insulation. Figure 2 is the flip chip structure sketch map.

1.3 encapsulation

The encapsulation mode affects the optical efficiency, and the lens package and the non lens package have great influence on the ultraviolet LED light output. In addition, different base caused by heat dissipation and antistatic ability, will also affect the life of the device, figure 3 is a commonly used form of packaging.

1.4 manufacturing process

The process flow of deep UV LED is basically the same as that of blue light. The main difference is that the blue light can pass through the top layer P, while the deep UV LED can only be removed from the back surface due to the absorption of p-GaN. The process flow of deep UV LED is shown in figure 4.

Current research progress and existing problems

After more than 10 years of research and development, the external quantum efficiency of deep UV LED below 280 nm has been more than 5%, corresponding to the luminous power of more than 5 mW, life of up to 5000 H. To enhance the power to promote the development of applications, use of deep UV LED include: optical sensors and instruments (230~400 nm), ultraviolet authentication, barcode (230~280 nm), drinking water disinfection, sterilization and portable (240~280 nm).

2.1 low power

The external quantum efficiency of LED is more than 5%, but it is still very low compared with the blue light of 60%.

(1) the template material quality defects, the dislocation density of A1N materials grown on sapphire by up to 1X109cm2, and the dislocation density of patterned substrate growth of GaN material is about 1 X10 7cm2, so the use of patterned substrate improve template material quality.

(2) the total internal reflection loss of deep ultraviolet multilayer structure, and the P type electrode due to absorption of light extraction efficiency, the light extraction efficiency is only 6%. To obtain P type ohmic contact on the breakthrough, reduce the high absorption of light on p-GaN; optimization of multilayer heterostructure of refraction rate difference; using the patterned substrate; coarse smooth out.

(3) the A1GaN of the high aluminum component has a significant amount of polarization effect, which causes the polarization field in the quantum well and the barrier, resulting in the increase of the working voltage and the decrease of the quantum efficiency. The solution is to use non polar surfaces (such as the a surface) of sapphire as a substrate, or to use the component gradient method to cancel.

2.2 poor heat dissipation

The external quantum efficiency is low and most of the energy is converted into heat energy, so the heat dissipation is very important. From the aspect of chip and package, the film inversion deep UV LED and flip chip deep UV LED can improve the heat dissipation, and can produce high power deep UV LED.

2.3 low life

Compared with the blue 100000h, the lifetime of the deep UV LED is only 5000h, and the low lifetime is mainly attributed to the defects of the material and the heat dissipation, and the packaging material is easy to be aged by ultraviolet irradiation.

Application of deep UV LED

The comparison of the performance of the deep UV LED and the traditional UV mercury lamp is shown in the following table. The advantages of UV LED are as follows.

(1) high efficiency: the light intensity per unit area is more than 1000 times that of mercury lamp. For example, the luminous area of a single LED is only 0.3 mm X 0.3mm but the optical power is greater than 1mW, while the mercury lamp light power in such a small area of less than 1 mW.

(2) environmental protection: semiconductor material non-toxic harmless, ultraviolet LED also does not contain any toxic substances.

(3) energy saving: the same optical output power, energy consumption is only 1/10.

(4) reliability: embodied in the switching characteristics and service life can be intelligent control. Because of the luminescence characteristics of LED, the pulse switch has no effect on the life of LED