We fabricated high power ultraviolet (UV) light emitting diodes (LEDs), whose emission wavelength is around 365 nm. We found that, in order to improve the external quantum efficiency (ηex) of UV LEDs, it is very important to reduce the optical self-absorption and the threading dislocation density (TDD) of epi-layers. Therefore, at first, UV LEDs epi-layers were grown on high-quality GaN templates (TDD = 2x 108/cm2) with sapphire substrates, and then the GaN templates and the sapphire substrates were removed by using laser-induced lift-off and polishing techniques. As a result, we obtained the low self-absorption and low TDD UV LEDs. When this UV LED was operated at a forward-bias pulsed current of 500 mA at room temperature (RT), the peak wavelength, the output power (Po), the forward voltage (Vf) and the ηex were 365 nm, 410 mW, 5.3 V, 24%, respectively. Moreover, at a forward-bias direct current of 500 mA at RT, Po, Vf and ηex were 360 mW, 5.0 V, 21%, respectively.
We show recent results of GaN based LEDs, the high efficiency blue LED, the high efficacy white LED, the high color rendering LED, the warm white LED, and the high output power 365-nm UV LED. The output power and the external efficiency of the high efficiency blue LED at 20 mA were 19.3 mW and 35.8%, respectively. Moreover, the lifetime of the molded this lamp was improved. For the high efficacy white LED using this high efficiency blue LED, the correlated color temperature (CCT) and the luminous efficacy (ηL) at 20 mA were 5470 K and 61.4 lm/W, respectively. This luminous efficacy is close to that of a fluorescent lamp. When the high color rendering LED using new red phosphor and shorter-YAG was operated at 20 mA, the CCT, ηL and the general color rendering index (Ra) were 4670 K, 28.3 lm/W and 87.7, respectively. In particular, the red color reproduction was improved. For the warm white, the CCT, ηL and Ra at 20 mA were 2830 K, 18.9 lm/W and 87.5, respectively. It can be used as a replacement for incandescent bulbs. When the high output power 365-nm UV LED was operated at a forward bias direct current (DC) of 500 mA, Po and ηex were 210 mW and 12.4%, respectively.
GaN-based laser diodes (LDs), which emit from UV to blue-green, are reviewed. For fabricating the UV LDs, we used the AlInGaN active layer instead of InGaN one. We demonstrated the UV LDs with a lasing wavelength 368nm under continuous-wave (cw) operation. Moreover, we fabricated the blue-green LDs whose lasing wavelength was 480 nm. It was investigated that the relationship between the threshold current density and the lasing wavelength. From our experiments, we successfully expanded the range of GaN-based LDs lasing wavelength, from UV (368 nm) to blue-green (480 nm). Regarding high power 405 nm-LDs, we could demonstrate the cw operated LD array devices with an output power of 1W by decreasing the thermal resistance of the LD chips.