Improvement of light extraction efficiency in AlGaInP-based vertical miniaturized-light-emitting diodes via surface texturing.

AlGaInP-based light-emitting diodes (LEDs) suffer from a low external quantum efficiency (EQE), which is mainly restrained by the poor light extraction efficiency. Here, we demonstrate AlGaInP-based vertical miniaturized-LEDs (mini-LEDs) with a porous n-AlGaInP surface using a wet etching process to boost light extraction. We investigated the effects of etching time on the surface morphology of the porous n-AlGaInP surface. We found that as the etching time is prolonged, the density of pores increases initially and decreases subsequently. In comparison with the vertical mini-LED with a smooth n-AlGaInP surface, the vertical mini-LEDs with the porous n-AlGaInP surface reveal improvement in light output power and EQE, meanwhile, without the deterioration of electrical performance. The highest improvement of 38.9% in EQE measured at 20 mA is observed from the vertical mini-LED with the maximum density of the pores. Utilizing a three-dimensional finite-difference time-domain method, we reveal the underlying mechanisms of improved performance, which are associated with suppressed total internal reflection and efficient light scattering effect of the pores.

Nanoimprinted patterned sapphire with silica array for efficient InGaN-based green mini-LEDs.

Here, we propose nanoimprinted patterned sapphire with a silica array (PSSA) with the aim to promote the efficiency of InGaN-based green (∼520 nm) mini-LEDs. According to x-ray diffraction measurements, the threading dislocation density of GaN epitaxial layers grown on nanoimprinted PSSA demonstrates a pronounced reduction compared with the epilayers on the conventional patterned sapphire substrate (PSS). Consequently, a mini-LED on PSSA exhibits a significantly boosted light output power (LOP) in comparison to a mini-LED on PSS. At 10 mA, the LOP of the mini-LED on PSS is 6.0 mW, and this is further improved to 6.8 mW for the mini-LED on PSSA. Moreover, the peak external quantum efficiencies of the mini-LEDs on PSS and PSSA are 41% and 47%, respectively. A three-dimensional (3D) finite-difference time-domain simulation demonstrates that the PSSA contributes enhanced light extraction for photons emitted from the active region. It is also highly feasible to use this nanoimprinted PSSA technology in red and blue mini-LEDs for the realization of full-color displays.

Unexpectedly Simultaneous Increase in Wavelength and Output Power of Yellow LEDs Based on Staggered Quantum Wells by TMIn Flux Modulation.

Pursuing efficient long-wavelength InGaN LED has been a troublesome issue to be solved, which forms interesting subjects for fundamental research, but finds also motivation in extensive applications. Here, we investigate the effect of TMIn (trimethylindium) flux variation for growing bandgap-engineered staggered quantum wells (QWs) on corresponding LED properties and demonstrate the unexpectedly simultaneous increase in light output power (LOP) and emission wavelength. At 20 mA, LEDs based on staggered QWs grown under low flux show an increase of 28% in LOP and longer wavelength compared to that under high flux. The experimental results reveal that TMIn flux affects crystalline quality and indium composition of epilayers. Under high TMIn flux, high in-plane strain exists between adjacent layers, accompanied by the composition pulling effect, which reduces indium incorporation for the following staggered QW growth and hinders realization of yellow light emission. According to simulation results, low-flux-grown staggered QWs contribute to increased carrier wavefunction overlap as well as enhanced electric field. Notably, the former enables high LOP, while the latter results in emissions towards long wavelength, promising to solve an ever-present concern that LED performance deteriorates with increasing emission wavelength. Therefore, this work shows great significance in thoroughly understanding growth conditions for bandgap-engineered staggered QW structures, which offers a facile solution to achieve efficient long-wavelength optoelectronics devices.