Two light-emitting diode samples are grown with InGaN and GaN underlying layers beneath the multiple quantum wells (MQWs), respectively. By measuring the carrier lifetime as a function of photon energy, it is found that the MQW with InGaN underlying layer has a higher degree of carrier localization. Comparison between the external quantum efficiency and injection current of these two samples reveals that efficiency droop at small injection current is attributed to the delocalization of carriers, while further droop at a higher injection current is due mostly to the carrier leakage demonstrated through temperature-dependent electroluminescence measurements.

We demonstrate aluminum nitride (AlN) on sapphire as a novel platform for integrated optics. High-confinement AlN microring resonators are realized by adopting a partially etched (pedestal) waveguide to relax the required etching selectivity for exact pattern transfer. A wide taper is employed at the chip end facets to ensure a low fiber-to-chip coupling loss of ~2.8 dB/facet for both transverse-electric (TE) and transverse-magnetic (TM) modes. Furthermore, the intrinsic quality factors (Q_int) recorded with a high-resolution linewidth measurement are up to ~2.5 and 1.9 million at telecom band for fundamental TE₀₀ and TM₀₀ modes, corresponding to a low intracavity propagation loss of ~0.14 and 0.2 dB/cm as well as high resonant buildup of 473 and 327, respectively. Such high-Q AlN-on-sapphire microresonators are believed to be very promising for on-chip nonlinear optics.

Wurtzite aluminum nitride (AlN) is known to exhibit six Raman-active optic phonons, making it appealing for Raman lasing. Here, we demonstrate continuous-wave Raman lasers with a low threshold and a high slope efficiency in high quality factor AlN-on-sapphire microrings. Stokes radiations around 1.7–1.8 μm and cascaded operation into 1.9–2.0 μm are identified with a telecom pump. Two types of Stokes lights with distinct Raman shifts and polarizations are recorded via selective excitation of corresponding optic phonons, in accordance with the Raman selection rules in AlN. The observed lasing behavior is satisfactorily accounted for by a theoretical analysis. Our results indicate that AlN-on-sapphire should be promising for integrated nonlinear optics.

Ultra-small micro-LEDs are essential for next-generation display technology. However, micro-LEDs below 5 μm have been seldom reported. In this work, we demonstrate InGaN-based blue and green micro-LEDs from 1 to 20 μm by using laser direct writing lithography. The 1-μm blue micro-LEDs show a peak external quantum efficiency of 13.02%, which is 9.57% for green ones. By characterizing the size-dependent external quantum efficiency and simply assuming that this variety is dominantly determined by the dry-etching induced dead zone, we deduce that the dead zone sizes of carrier injection at the edge of chips are 0.18 and 0.15 μm in blue and green ones, respectively. A time-resolved photoluminescence measurement also shows that carrier lifetime reduction at the edge of blue ones is more serious than that of green ones, reflecting the easier carrier lateral diffusion in the former than the latter. These results exhibit the ability of laser direct writing lithography on micro-LED fabrication and also provide a reference for predicting the limit of their chip size scaling-down.