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Temperature-dependent piezoelectric-field characteristics of GaInN/GaN blue quantum wells are experimentally investigated between 100 and 300 K. The results show that the magnitude of the piezoelectric field increases with decreasing temperature with a slope of 1.08 kV cm−1 K−1 due to the increase in a mismatch between thermal expansion coefficients. To understand the impact of temperature-dependent piezoelectric field on the device performance, the external quantum efficiencies (EQEs) of a blue light-emitting diode are measured in the same temperature range. More severe EQE droops are observed at lower temperatures, which can be attributed to the increase in carrier overflow/spill-over to the p-clad layer enhanced by the stronger piezoelectric fields. The larger blueshifts in mean photon energy are simultaneously observed at lower temperatures, which also confirms the stronger piezoelectric fields at lower temperatures. [ABSTRACT FROM AUTHOR]

GaInN/GaN multi-quantum-shell (MQS) nanowires (NWs) are gaining increasing attention as promising materials for developing highly efficient long-wavelength micro-light emitting diodes (LEDs). To improve the emission properties in GaInN/GaN MQS NWs, it is necessary to suppress the emission from the (0001) c-plane MQS at the apex region, which featured with low crystalline quality. In this study, we investigated the enhancement of optical properties and the realization of micro-LEDs by confirming the effect of the (0001) plane region. A 7.9-fold enhancement of the electroluminescence (EL) intensity was demonstrated by removal the (0001) plane region via inductively coupled plasma (ICP) dry etching, owing to the promoted current injection into the (1–101) semi-polar and (10–10) non-polar sidewall area. To investigate the effect of the emission area on the samples with and without truncated (0001) plane region, devices with three different mesa areas (50 × 50, 100 × 100, and 100 × 200 μm2) were fabricated. An increased EL intensity with the reduced mesa areas was observed in the samples without dry etching of the (0001)-plane area, because more current can be injected into the sidewall region with higher crystalline quality and luminous efficiency than the (0001)-plane MQS. Under the same injection current density, the truncated samples’ light output was increased for more than ten times as compared to the samples without (0001)-plane etching. Therefore, it confirms the possibility of realizing highly efficient GaInN/GaN MQS NWs LEDs by eliminating the (0001) plane MQS region. A precise etching and surface passivation of the apex region is expected to further reduce the reverse leakage current and improve the performance in NW-LEDs.

The superior crystalline quality of coaxial GaInN/GaN multiple-quantum shell (MQS) nanowires (NWs) was demonstrated by employing an AlGaN undershell during metal-organic chemical vapor deposition. Scanning transmission electron microscopy (STEM) results reveal that the NW structure consists of distinct GaInN/GaN regions on different positions of the NWs and the cores were dislocation-free. High-resolution atomic contrast STEM images verified the importance of AlGaN undershells in trapping the point defects diffused from n-core to MQSs (m-planes), as well as the improvement of the grown crystal quality on the apex region (c-planes). Time-integrated and time-resolved photoluminescence (PL) measurements were performed to clarify the mechanism of the emission within the coaxial GaInN/GaN MQS NWs. The improved internal quantum efficiency in the NW sample was attributed to the unique AlGaN undershell, which was able to suppress the point defects diffusion and reduce the dislocation densities on c-planes. Carrier lifetimes of 2.19 ns and 8.44 ns were derived from time-resolved PL decay curves for NW samples without and with the AlGaN undershell, respectively. Hence, the use of an AlGaN undershell exhibits promising improvement of optical properties for NW-based white and micro light-emitting diodes.

This study attempted to improve the internal quantum efficiency (IQE) of 580 nm emitting Ga0.70In0.30N/GaN quantum-wells (QWs) through the replacement of a conventional c-sapphire substrate and an in-situ low-temperature GaN (LT-GaN) buffer layer with the ScAlMgO4 (0001) (SCAM) substrate and an ex-situ sputtered-AlN (sp-AlN) buffer layer, simultaneously. To this end, we initially tried to optimize the thickness of the sp-AlN buffer layer by investigating the properties/qualities of an undoped-GaN (u-GaN) template layer grown on the SCAM substrate with the sp-AlN buffer layer in terms of surface morphology, crystallographic orientation, and dislocation type/density. The experimental results showed that the crystallinity of the u-GaN layer grown on the SCAM substrate with the 30 nm thick sp-AlN buffer layer [GaN/sp-AlN(30 nm)/SCAM] was superior to that of the conventional u-GaN template layer grown on the c-sapphire substrate with an LT-GaN buffer layer (GaN/LT-GaN/FSS). Notably, the experimental results showed that the structural properties and crystallinity of GaN/sp-AlN(30 nm)/SCAM were considerably different from those of GaN/LT-GaN/FSS. Specifically, the edge-type dislocation density was approximately two orders of magnitude higher than the screw-/mixed-type dislocation density, i.e., the generation of screw-/mixed-type dislocation was suppressed through the replacement, unlike that of the GaN/LT-GaN/FSS. Next, to investigate the effect of replacement on the subsequent QW active layers, 580 nm emitting Ga0.70In0.30N/GaN QWs were grown on the u-GaN template layers. The IQEs of the samples were measured by means of temperature-dependent photoluminescence efficiency, and the results showed that the replacement improved the IQE at 300 K by approximately 1.8 times. We believe that the samples fabricated and described in the present study can provide a greater insight into future research directions for III-nitride light-emitting devices operating in yellow–red spectral regions. [ABSTRACT FROM AUTHOR]

In this study, we aim to understand the thermodynamics inside the junctions of GaInN-based LEDs through optical operation of the device (the wavelength of the pumping laser is 405 nm for quasi-resonant optical excitation). First, to achieve this goal, the short-circuit current vs the open-circuit voltage curve and photoluminescence spectrum are carefully analyzed and compared with the current–voltage curve and electroluminescence spectrum. By this comparative study, we experimentally demonstrate the electrical–optical energy conversion efficiency (ECE) > 100% under optical excitation. Second, the intra-band phonon absorption and emission inside and/or outside the junction (PPeltier and PHC) are investigated through the power-loss analysis. In so doing, we ascertain that the carriers in the multiple quantum wells are energized by phonons, which introduces the ECE > 100%. Specifically, after the energization, the Boltzmann carrier distribution is no longer controlled by the ambient lattice temperature but by higher temperature, namely, that of the carriers themselves. Based on the analysis, we propose a thermally enhanced effective temperature for the Boltzmann distribution. Finally, we compare the junction efficiency by photoluminescence (PL), composed of the internal quantum efficiency by PL and the ECE by PL, of the sample at various temperatures to investigate the effect of PHC on the overall PL efficiency. [ABSTRACT FROM AUTHOR]

This study aims to improve the crystalline quality of 650-nm GaIn0.17N/GaIn0.38N red-emitting multiple quantum wells (MQWs) fabricated on a ScAlMgO4 (SCAM) substrate. When using the SCAM substrate, the diffusion and/or penetration of impurities, including Mg, Sc, O, and Al, from the SCAM substrate poses as a challenge. To address this issue, we introduced an Al0.74In0.26N layer between the SCAM substrate and MQWs, which was lattice-matched to the substrate. The Al0.74In0.26N layer effectively blocked the diffusion of impurities from the SCAM substrate into the adjacent layers during the metal-organic vapor epitaxy (MOVPE) growth. For further suppression, a thick AlN layer was deposited on the back of the SCAM substrate before the MOVPE growth, which effectively suppressed impurity penetration from the growth surface. The structure proposed in this study improved the crystallinity and the surface roughness of MQWs, resulting in the improvement of internal quantum efficiency by approximately three times compared to that of the conventional sample. [ABSTRACT FROM AUTHOR]

To investigate the mechanism of efficiency enhancement achieved by introducing underlying layers (ULs) in green light–emitting diodes (LEDs), this study compares green GaInN‐based LEDs with an identical epitaxial structure and chip structure comprising different types of ULs, i.e., without an UL or with a GaInN or AlInN UL. To this end, the samples are analyzed with respect to several characteristics, such as their electroluminescence spectra, internal quantum efficiency, recombination coefficients, joint density of states, localized state, and Stokes shift. Based on these analyses and considerations, a mechanism of efficiency enhancement achieved by introducing ULs is proposed, and the degradation mechanisms responsible for the green gap, efficiency droop, and electrical potential drop in green LEDs are also discussed. The results show that the piezoelectric field in the In‐clustering region and the nonradiative recombination center simultaneously play crucial roles in the degradation mechanisms for green LEDs. [ABSTRACT FROM AUTHOR]