The sidewall condition is a key factor determining the performance of micro-light emitting diodes (μLEDs). In this study, we prepared equilateral triangular III-nitride blue μLEDs with exclusively m-plane sidewall surfaces to confirm the impact of sidewall conditions. It was found that inductively coupled plasma-reactive ion etching (ICP-RIE) caused surface damages to the sidewall and resulted in rough surface morphology. As confirmed by time-resolved photoluminescence (TRPL) and X-ray photoemission spectroscopy (XPS), tetramethylammonium hydroxide (TMAH) eliminated the etching damage and flattened the sidewall surface. After ICP-RIE, 100 µm 2 -micro-LEDs (µLEDs) yielded higher external quantum efficiency (EQE) than 400 µm 2 -µLEDs. However, after TMAH treatment, the peak EQE of 400 µm 2 -µLEDs increased by around 10% in the low current regime, whereas that of 100 µm 2 -µLEDs slightly decreased by around 3%. The EQE of the 100 µm 2 -µLED decreased after TMAH treatment although the internal quantum efficiency (IQE) increased. Further, the IQE of the 100 µm 2 -µLEDs before and after TMAH treatment was insignificant at temperatures below 150 K, above which it became considerable. Based on PL, XPS, scanning transmission electron microscopy, and scanning electron microscopy results, mechanisms for the size dependence of the EQE of µLEDs are explained in terms of non-radiative recombination rate and light extraction.

We demonstrate high, up to 30% In content InGaN sub-micrometer platelets on GaN by metalorganic vapor phase epitaxy. These InGaN platelets were selectively grown on flat GaN seeds formed in sub-micrometer-scale openings in a SiNx mask. The platelets were highly uniform without any dislocations or pits, with an atomically flat (0001) surface. The typical height was ∼120 nm, which significantly exceeded the normal critical layer thickness of a c-plane InGaN film. The strain state was comprehensively characterized by microbeam x-ray diffraction and transmission electron microscopy. Due to a gradual elastic relaxation of strain, the In content increased almost linearly from bottom to top because of the strong strain-dependent In incorporation. These platelets can serve as high-quality strain-relaxed templates for long wavelength micro-light-emitting diodes.

Photo-electrochemical (PEC) etching is a promising technique for fabricating GaN microelectromechanical systems devices. In this study, we demonstrate the fabrication of GaN cantilevers by the bandgap-selective PEC etching of an InGaN superlattice sacrificial layer. By using an InGaN superlattice as a sacrifice layer, we found the PEC etching rate became higher than using a normal InGaN layer. As a result, the InGaN superlattice was completely etched and we fabricated GaN-based cantilevers whose resonance characteristics were measured. The Young’s modulus of GaN was determined from the resonance characteristics of GaN cantilevers to be the same as the highest value reported previously.

The sidewall condition is a key factor determining the performance of micro-light emitting diodes (μLEDs). In this study, we prepared equilateral triangular III-nitride blue μLEDs with exclusively m-plane sidewall surfaces to confirm the impact of sidewall conditions. It was found that inductively coupled plasma-reactive ion etching (ICP-RIE) caused surface damages to the sidewall and resulted in rough surface morphology. As confirmed by time-resolved photoluminescence (TRPL) and X-ray photoemission spectroscopy (XPS), tetramethylammonium hydroxide (TMAH) eliminated the etching damage and flattened the sidewall surface. After ICP-RIE, 100 µm2-micro-LEDs (µLEDs) yielded higher external quantum efficiency (EQE) than 400 µm2-µLEDs. However, after TMAH treatment, the peak EQE of 400 µm2-µLEDs increased by around 10% in the low current regime, whereas that of 100 µm2-µLEDs slightly decreased by around 3%. The EQE of the 100 µm2-µLED decreased after TMAH treatment although the internal quantum efficiency (IQE) increased. Further, the IQE of the 100 µm2-µLEDs before and after TMAH treatment was insignificant at temperatures below 150 K, above which it became considerable. Based on PL, XPS, scanning transmission electron microscopy, and scanning electron microscopy results, mechanisms for the size dependence of the EQE of µLEDs are explained in terms of non-radiative recombination rate and light extraction.

As the size of micro light-emitting diodes ( μLEDs) decreases, μLEDs encounter etching damage especially at the sidewalls that critically affects their properties. In this study, we investigated the influence of etching bias power ( Pbias) on the performance of μLEDs and found that the current–voltage and light output–current characteristics of μLEDs were enhanced when Pbias was reduced. It was shown that at low Pbias, the chemical reaction between etching gas and gallium nitride, rather than ion sputtering, dominated the etching process, leading to low plasma damage and rough surface morphology. Additionally, to understand the etching-induced surface roughening behaviors, various substrates with different threading dislocation densities were treated at low Pbias. It was found that for the sample (with p-contact size of 10 × 10  μm2), the efficiency droop was approximately 20%, although the current reached 10 mA due most probably to the suppressed polarization effect in the quantum well. It was further observed that the external quantum efficiency (EQE) was dependent on Pbias, where the lowest Pbias yielded the highest maximum EQE, indicating that the plasma damage was mitigated by reducing Pbias. Optimization of dry etching and polarization-suppression conditions could pave the way for realizing high-performance and brightness μLEDs for next-generation displays.