In this work, the size-dependent effect for InGaN/GaN-based blue micro-light emitting diodes (µLEDs) is numerically investigated. Our results show that the external quantum efficiency (EQE) and the optical power density drop drastically as the device size decreases when sidewall defects are induced. The observations are owing to the higher surface-to-volume ratio for small µLEDs, which makes the Shockley-Read-Hall (SRH) non-radiative recombination at the sidewall defects not negligible. The sidewall defects also severely affect the injection capability for electrons and holes, such that the electrons and holes are captured by sidewall defects for the SRH recombination. Thus, the poor carrier injection shall be deemed as a challenge for achieving high-brightness µLEDs. Our studies also indicate that the sidewall defects form current leakage channels, and this is reflected by the current density-voltage characteristics. However, the improved current spreading effect can be obtained when the chip size decreases. The better current spreading effect takes account for the reduced forward voltage.

For the [0001] oriented AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs), the holes in the p-type electron blocking layer (p-EBL) are depleted due to the polarization induced positive sheet charges at the last quantum barrier (LQB)/p-EBL interface. The hole depletion effect significantly reduces the hole injection capability across the p-EBL. In this work, we propose inserting a thin AlN layer between the LQB and the p-EBL, which can generate the hole accumulation at the AlN/p-EBL interface. Meanwhile, the holes can obtain the energy when traveling from the p-EBL into the multiple quantum wells (MQWs) by intraband tunneling through the thin AlN layer. As a result, the hole injection and the external quantum efficiency (EQE) have been remarkably enhanced. Moreover, we point out that the thick AlN insertion layer can further generate the hole accumulation in the p-EBL and increase the hole energy which helps to increase the hole injection. We also prove that the intraband tunneling for holes across the thick AlN insertion layer is facilitated by using the optimized structure.

GaN-based micro-size light-emitting diode ( μ LED) have emerged as a promising light sources for a wide range of applications in displays, visible light communication etc. In parallel with the two key technological bottlenecks: full-color scheme and mass transfer technique that need overcoming, it is known that the low external quantum efficiency (EQE) is also another challenge for μ LEDs from the perspective of manufacturing technology and device physics. The low EQE for GaN based μ LEDs is opposite to the common belief for GaN-based LEDs, such that GaN based LEDs are featured with high quantum efficiency, mechanically robust and energy saving. Therefore, in this work, we have reviewed the origin for the low EQE for μ LEDs. More importantly, we have also reported the underlying devices physics and proposed optimization strategies to boost the EQE for μ LEDs. Our work is targeted to provide a guideline for the community to develop high-performance GaN-based μ LEDs.