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"The wearable monitor that used to be dead in 1 hour battery can now be used continuously for 8 hours." The technology developed by the R&D team of Tatsuki Tsuyoshi, one of the professors of the Graduate School of Engineering of the University of Tokyo and the Center for Nanoelectronics Photonics International Center, was well received by the wearable equipment technicians (Figure 1). In order to achieve 24-hour drive in the near future, the R&D team will continue to promote development.
Display light source to save energy
The Otsu R&D team developed the "polarized SiC-LED" technology. In 2013, the R&D team successfully made its basic technology SiC-LED achieve high luminous efficiency. Unlike conventional LEDs, SiC-LEDs emit multiple colors of light without the use of fluorescent materials. Therefore, there is no need to use fluorescent materials that will produce energy loss when wavelength conversion (Note 1).
Figure 1: The battery-powered display's battery-powered drive has been extended to 8 times
With the development of polarized SiC-LEDs, wearable displays with battery-driven time of just one hour using previous LEDs can now be used for 8 hours. In the future, we plan to implement 24-hour drive.
Note 1) In addition, SiC-LEDs have the following features compared to conventional LEDs. "Since crystal growth equipment is simple, it consumes less energy when it is manufactured; it does not require the use of rare metals and toxic elements, and materials can easily be recycled. The use of energy is also less ("Otsu"). SiC-LEDs were mass-produced by the machinery manufacturer Shadick. The Otsu R&D team disclosed samples at the International Display Workshops, an international display conference held in December 2014.
In addition, the R&D team successfully developed polarized SiC-LEDs. Using this as a display light source can reduce energy loss when passing through the polarizing plate. Since no fluorescent material is required, the loss of the polarizing plate can also be reduced, and the power consumption of the display light source can be significantly reduced as compared with before. In general, the power consumption of the display is basically consumed by the light source, so reducing the power consumption of the display light source can substantially extend the battery life.
SiC-LEDs that can control light
Semiconductors such as SiC and Si, which have indirect transition energy band structures, generally do not emit light even when a pn junction diode is fabricated and applied with a voltage. However, the SiC-LEDs developed by the Otsu R&D team can emit light. Here's a brief introduction to the reasons.
LEDs generally use direct transition semiconductors such as GaN. The electrons excited to the conduction band emit light when they return to the valence band. Indirect transition semiconductors do not undergo electronic transitions.
Therefore, the R&D team thought about using the “bridge†method to make electrons from indirect transition semiconductors also migrate from the conduction band to the valence band. Specifically, an intermediate band (electron orbital energy band exists) is formed between the conduction band and the valence band. Electrons can make transitions via the intermediate band.
The intermediate band is formed using the following method. A pn junction element doped with Al in n-type SiC is loaded with a forward current, and annealing is performed using the Joule heat generated. At the same time, light is irradiated from outside the element. In this way, phonons* will be generated, and the phonons will form an intermediate energy note2).
* Phonon = Particles that quantize the vibration. It is related to the lattice vibration in the crystal.
Note 2) "Dressed photons" are collected around the nanosized Al particles irradiated with light. The Dressed photons combine with the electrons of Al nanoparticles to generate phonons. Phonons have discrete levels (phonon levels) that form intermediate bands.
The SiC-LED manufactured by the above method does not need to irradiate light and emits light when injected with current. The light emitting wavelength (color) is the same as the light irradiated at the time of manufacture. That is, the luminescent color of the SiC-LED can be controlled by the wavelength of the irradiated light at the time of manufacture.
Confirm polarized light
Therefore, Otsu's R&D team considers that when SiC-LEDs are manufactured with polarized light, they should be able to realize polarized light LEDs. As a result, the pn junction element of SiC was irradiated with a polarized laser light to produce an LED.
The R&D team investigated the luminescence spectra of SiC-LEDs fabricated using this method and confirmed that the light emitted was polarized. The R&D team found that the luminous intensity in the same direction as the direction of the irradiated light at the time of manufacture was higher than that in the vertical direction, and the emitted light was polarized (Fig. 2(a)). The degree of polarization at annealing time of 19,800 seconds was 12% (Fig. 2(b)). In addition, observation of the light emission by the polarizing plate confirmed that the light intensity in the same direction as the vibration direction and the irradiation light was higher than the light intensity in the vibration direction perpendicular to the irradiation light ( FIG. 3 ).
Figure 2: Emission spectra and polarization of SiC-LEDs with polarized light at the time of manufacture
The degree of polarization at annealing time of 19,800 seconds is 12%. The degree of polarization of the illumination light and the light emission is different in a region where the annealing time is less than 19,800 seconds. This is because the original light emission is weak, and the weak light emission caused by indirect transitions and impurities is significant. (Photo courtesy of University of Tokyo Otsu Research Laboratory)
Fig. 3: When SiC-LEDs with polarized light are illuminated at the time of manufacture
Both (a) and (b) are SiC-LEDs that have been annealed for 1 hour. Polarizers were placed on the front of the camera, and the polarizers were rotated 90 degrees and photographed. (Photo courtesy of University of Tokyo Otsu Research Laboratory)
There is room for further improvement in the degree of polarization. From Fig. 2(b), it can be seen that extending the annealing time can increase the degree of polarization. By further optimizing the manufacturing conditions, "polarization can be increased to 30% or 50%" (Otsu R&D team). (Reporter: Tanaka Naoki)
Simple installation, different lamps can be equipped, and the number of lamps can be increased or decreased freely. When there is no limit to the number of lamps installed on the track, it can theoretically not exceed the wattage of the transformer power supply.
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