The invention of blue LED (light emitting diode), which has been considered difficult to achieve for many years, gave birth to energy-saving and long-lived lighting fixtures and displays, which greatly changed the world. One of the latest frontier research is that it can produce UV LEDs with shorter wavelengths than blue LEDs.
Among ultraviolet rays, deep ultraviolet rays with a particularly short wavelength have high sterilization capabilities and are expected to be used in factories and water purification plants (Figure 1). Most of the germicidal lamps currently in use use mercury, but with the entry into force of the Minamata Treaty on Mercury in 2017, the international community has begun to work to reduce the use of mercury. In this context, deep ultraviolet UVC LEDs are expected. Products using deep-ultraviolet LEDs have begun to go on the market, but the current luminous efficiency and output power are insufficient.
Hirayama, who started researching UV LEDs in 1996, said with confidence: “Although the development competition is fierce, the deep UV UVC LED we developed has achieved the world’s highest luminous efficiency of 20.3%. However, if we want to achieve widespread use, the luminous efficiency will be It needs to be further improved to exceed the low-pressure mercury lamp used as a germicidal lamp, and the current goal is to exceed 30%."
The basic structure of an LED is a pn junction formed by joining an n-type semiconductor with more electrons and a p-type semiconductor with insufficient electrons (with holes). After a voltage is applied, electrons and holes combine to emit light, but the color (wavelength) of light and the voltage required to emit light are different depending on the type of semiconductor. In order to develop semiconductors that can generate light of the desired wavelength, a large number of researchers have explored various materials. Hirayama said: "If it can only emit light in the ultraviolet region, it is not practical. Because it also needs to emit light more efficiently than previous light sources, and can be mass-produced at a lower cost." Aluminum gallium nitride (AlGaN) is expected as a relatively promising material, but there are many issues.
A new technology that can generate neat crystals, LEDs form pn junctions by growing crystals with ordered atoms on a basic substance (substrate). The semiconductor substrate uses cheap sapphire (Al2O3), but due to the difference in the distance (lattice constant) between the atoms that make up the crystal, the AlGaN crystal is deformed when it grows, causing defects called lattice defects. Cracks that expand along the defect line are called crystal defects. If the defect density (threading dislocation density) increases, the luminous efficiency decreases.
Blue LEDs need to form a gallium nitride (GaN) crystal film with fewer defects on the substrate. The technology to realize this film was developed by the Nobel Prize-winning professor at Meijo University, Isamu Akasaki. For deep ultraviolet LEDs, an aluminum nitride (AlN) crystal film is formed on the substrate and AlGaN crystal is grown on it. Established a high-quality AlN film on the substrate to reduce defects. He recalled: "This method has made a breakthrough in improving luminous efficiency, surpassing the competitor's American research team."
AlN crystals are produced by metal organic chemical vapor deposition (MOCVD). The gaseous material is supplied to the sapphire substrate at a high temperature of about 1400 degrees to make it grow as a crystal. The method developed by Hirayama first grows AlN nitride as the core on the substrate, and blows ammonia gas in a pulse to make it grow laterally to fill the gap between the core. Then the gas is continuously supplied to stack them vertically. By repeating this crystal growth process, a high-quality AlN layer without cracks can be formed (Figure 2). Said: "To make neat crystals, you need to finely control the gas concentration, flow rate and reaction temperature, etc.. The gas flow is easy to be turbulent at high temperatures and requires a wealth of experience. Therefore, the equipment is semi-self-made and modified as needed." .
Improve the luminous efficiency by working on the structure
The luminous efficiency is related to 3 factors. The first is "internal quantum efficiency", the second is "electron injection efficiency", and the third is "light extraction efficiency". Hirayama is working hard to improve these three efficiencies.
The internal quantum efficiency is a value indicating the ratio of electron and hole pairs generated by current to emit light, and indicates the degree to which the light-emitting layer emits light smoothly. By making the crystal grow neatly and reducing defects, the internal quantum efficiency has been successfully improved.
The electron injection efficiency refers to the proportion of electrons that enter the light-emitting layer in the injected current. The conventional deep ultraviolet UV LED has a problem that the injected electrons do not enter the light-emitting layer, but leak from the p-layer side.
The introduction said: "The reason is that the number of holes in the p-type semiconductor is not balanced with the number of electrons in the n-type semiconductor. Because it is difficult to increase the number of holes, an electron blocking layer (multi-quantum barrier) is formed to reflect the unbound electrons that pass directly. , Effectively combined" (Figure 3). As a result, the electron injection efficiency is greatly improved.
Dream is to be applied to laser light source
The deep ultraviolet UV LED developed by AlGaN also has advantages in the application range. He expressed expectantly: "By changing the composition of the crystal, the wavelength of deep ultraviolet can be adjusted. This is also a feature. Currently, deep ultraviolet UVC LEDs have been implemented in the 222-351 nm band. You can freely generate the desired wavelength according to the application. Deep ultraviolet light, such as light of about 310 nanometers used to treat atopic dermatitis and psoriasis, etc."
This is a technology under development. The output power needs to be increased from the current tens of milliwatts to a few watts. It is expected to be used in sterilization, water purification, air purification, medical treatment, biochemical industry, resin hardening and processing, and printing in the future. And painting and other fields.
Looking to the future, he said: "In the future, we plan to develop a deep ultraviolet laser diode (LD) that can achieve greater output power. If it can be achieved, it should also be able to decompose large-capacity storage media and harmful substances that exceed the capacity of Blu-ray Discs."
The development space of deep ultraviolet UVC LED is still very large.
