Microscale light-emitting diodes (µLEDs) have become an emerging technology for use in practical applications including visible light communication for advanced wireless communication, optogenetics for neuron networks, and self-emissive micro-displays for high-pixel-per-inch images. However, because it has a smaller pixel size (<100 µm) compared to that of conventional LEDs, the light extraction from the active layer of µLEDs is limited by the opaque p-type metal electrode; therefore, the ratio of the emission area to the pixel size of µLEDs is low, which inhibits light extraction. To extract more light from µLEDs, the emission area should be expanded by reducing the area of the p-type metal electrode. However, this decreases the current density of µLEDs because the contact area covered by the metal electrode is reduced. Several groups have reported improved electrical and optical properties of µLEDs by changing the pixel shape to alter the path of light extraction and/or preventing re-absorption in the device, or by removing defect states causing a non-recombination process by passivating the sidewall of the µLED. In addition to these efforts, the effect of the p-type contact structure with a lateral oxide-confined scheme was investigated to achieve a high current density with the µLED. Although all of these techniques were effective in improving either light extraction or electrical characteristics, there have been few reports to date on the simultaneous enhancement of the optical and electrical properties.
UV LED is a diode that emits light in the ultraviolet region (<280 nm) as well as the near ultraviolet (280-400 nm) region. The LED market makes a giant leaf with expanding its application to BLU, lighting and automotive etc. With the UV LED power increasing (increment), the technology is also used in a wider range of applications. However, ITO based LEDs have some issues such as ohmic contact problem and degradation due to light absorption. To solve this problem, graphene, metal nanowire, thin metal, metal oxide based electrodes are being studied and technologies such as PhCs, PSS, mesh-type, and surface roughening are also being studied; however, the electrode structure (or material) having the high transmittance in the UV region and good electrical performances (ex. Current density) in the device has not been proposed yet.
In our study, we are developing micro- & deep UV-LEDs with high efficiency by demonstrating the use of highly transparent (>98%) and conductive wide-bandgap materials (ex. SiO2, AlN) as p-type contact electrodes, instead of conventional ITO films. The proposed p-type contact electrodes can form the direct ohmic contact with high-resistance semiconductors such as p-AlxGa1-xN by engineering the material’s band-gap and be applied to top-emitting lateral LED and bottom emitting flip-chip LED; therefore it is impossible to achieve high performance and high efficiency DUV LEDs. In addition, we are also studying the advanced array-based µLED display technology with high resolution and high density by utilizing the proposed electrodes.