Size-dependent photon scattering effects in AlGaN-based deep ultraviolet LEDs with varied length-to-width ratios.

In this work, we investigate systematically the effect of the mesa L/W ratio on the optical performance of DUV LEDs. The results indicate that as the L/W ratio increases, the large-sized DUV LEDs achieve a higher LEE, whereas the small-sized DUV LEDs exhibit a lower LEE. The abnormal phenomenon can be attributed to the fact that the different scattering mechanisms occur for large and small chips at varying L/W ratios. For large-sized chips, when the L/W ratio increases, the amount of light propagating to the incline sidewall and directly reflected into the escape cone of the bottom out-light plane increases, thereby enhancing the bottom LEE. In addition, the increased sapphire sidewall area leads to more light escaping through the sapphire side, which increases the LEE from the chip side. For small-sized chips, some light escapes from the side of the sapphire at a low L/W ratio. However, when the L/W ratio increases, this portion of the light is directly reflected by the inclined sidewall of the mesa and eventually absorbed by GaN and the top metal, resulting in decreased LEE. These findings can provide essential geometry design principles for DUV LEDs of various sizes.