V-pit-induced electric field redistribution enabling efficient hole injection in InGaN-based red light-emitting diodes grown on silicon

This study investigates optical properties and carrier dynamics of InGaN-based light-emitting diodes grown on cone-shaped patterned sapphire (CSPS) and planar sapphire substrates. Edge-type threading dislocations were dramatically reduced in InGaN multiple quantum wells (MQWs) on CSPS substrates compared to the case of planar substrates. We observed a smaller Stokes shift and enhanced quantum efficiency for CSPS substrates. From time-resolved optical analysis, we found that the non-radiative (radiative) recombination rate of MQWs on CSPS is lower (higher) than that of MQWs on planar substrates, which is consistent with improved crystal quality (strain relaxation) of the MQWs on CSPS.

Modeling and analysis of the effects of inhomogeneous carrier distributions in InGaN multiple quantum wells

Using subpicosecond optical pump-probe techniques, coherent zone-folded longitudinal acoustic phonons (ZFLAPs) were investigated in an InGaN multiple quantum well structure. A two-pump differential transmission technique was used to generate and control coherent ZFLAP oscillations through the relative timing and amplitude of the two pump pulses. Enhancement and suppression of ZFLAP oscillations were demonstrated, including complete cancellation of generated acoustic phonons for the first time in any material system. Coherent control was used to demonstrate that ZFLAPs are generated differently in InGaN multiple quantum wells than in GaAs/AlAs superlattices.

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text Ümit Özgür, Chang-Won Lee, and Henry O. Everitt, "Control of Coherent Acoustic Phonons," Optics & Photonics News 12(12), 66-66 (2001) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

We have demonstrated nitrogen-polar (0001̅) (N-polar) InGaN multiple quantum wells (MQWs) with significantly improved luminescence properties prepared by pulsed metalorganic chemical vapor deposition. During the growth of InGaN quantum wells, Ga and N sources are alternately injected into the reactor to alter the surface stoichiometry. The influence of flow duration in pulsed growth mode on the luminescence properties has been studied. We find that use of pulsed-mode creates a high density of hexagonal mounds with an increased InGaN growth rate and enhanced In composition around screw-type dislocations, resulting in remarkably improved luminescence properties. The mechanism of enhanced luminescence caused by the hexagonal mounds is discussed. Luminescence properties of N-polar InGaN MQWs grown with short pulse durations have been significantly improved in comparison with a sample grown by a conventional continuous growth method.

Phosphor-free monolithic white light emitting diodes (LEDs) based on InGaN/ InGaN multiple quantum wells (MQWs) on ternary InGaN substrates are proposed and analyzed in this study. Simulation studies show that LED devices composed of multi-color-emitting InGaN/ InGaN quantum wells (QWs) employing ternary InGaN substrate with engineered active region exhibit stable white color illumination with large output power (∼ 170 mW) and high external quantum efficiency (EQE) (∼ 50%). The chromaticity coordinate for the investigated monolithic white LED devices are located at (0.30, 0.28) with correlated color temperature (CCT) of ∼ 8200 K at J = 50 A/cm2. A reference LED device without any nanostructure engineering exhibits green color emission shows that proper engineered structure is essential to achieve white color illumination. This proof-of-concept study demonstrates that high-efficiency and cost-effective phosphor-free monolithic white LED is feasible by the use of InGaN/ InGaN MQWs on ternary InGaN substrate combined with nanostructure engineering, which would be of great impact for solid state lighting.

Stimulated emission (SE) was measured from two InGaN multiple quantum well (MQW) laser structures with different QW In compositions x. SE threshold energy densities (Ith) increased with increasing x-dependent QW depth. Time-resolved differential transmission measurements mapped the carrier relaxation mechanisms and explained the dependence of Ith on x. Carriers are captured from the barriers to the QWs in <1 ps, while carrier recombination rates increased with increasing x. For excitation above Ith, an additional, fast relaxation mechanism appears due to the loss of carriers in the barriers through a cascaded refilling of the QW state undergoing SE. The increased material inhomogeneity with increasing x provides additional relaxation channels outside the cascaded refilling process, removing carriers from the SE process and increasing Ith.

ZnO nanorods have been prepared by electrodeposition under identical conditions on various p-GaN-based thin film structures. The devices exhibited lighting up under both forward and reverse biases, but the turn-on voltage and the emission color were strongly dependent on the p-GaN-based structure used. The origin of different luminescence peaks under forward and reverse bias has been studied by comparing the devices with and without ZnO and by photoluminescence and cathodoluminescence spectroscopy. We found that both yellow-orange emission under reverse bias and violet emission under forward bias, which are commonly attributed to ZnO, actually originate from the p-GaN substrate and/or surface/interface defects. While the absolute brightness of devices without InGaN multiple quantum wells was low, high brightness with luminance exceeding 10 000 cd/m2 and tunable emission (from orange at 2.1 V to blue at 2.7 V, with nearly white emission with Commission internationale de l’éclairage (CIE) coordinates (0.30, 0.31) achieved at 2.5 V) was obtained for different devices containing InGaN multiple quantum wells.

Localized exciton and its stimulated emission in InGaN multiple quantum wells

Optical characteristics of multiple layered InGaN quantum wells on GaN nanowalls grown on Si (111) substrate

InGaN multiple quantum wells were grown on InGaN underlying layers 50 nm thick by metalorganic vapor phase epitaxy. Photoluminescence (PL) measurements were performed by selective excitation of the quantum wells under a weak excitation condition. The PL intensity was almost constant at temperatures ranging from 17 to 150 K. Assuming that the internal quantum efficiency (ηint) equals unity at 17 K, we obtained ηint as high as 0.71 even at room temperature. The reason for the high ηint is the reduction of nonradiative recombination centers by the incorporation of indium atoms into the underlying layer.

To clarify the influence of diffused Mg impurities on the carrier recombination in InGaN multiple quantum wells (MQWs), we have performed a systematic study using time-resolved photoluminescence (PL) measurements. It was found that the MQWs with a p-contact layer and the MQWs with a nondoped GaN layer had almost the same carrier lifetime and PL intensity below 200 K. However, the MQWs with the p-contact layer had shorter carrier lifetime and lower PL intensity than the MQWs with the nondoped GaN layer above 200 K. This degradation of the PL for the MQWs with the p-contact layer can be attributed to nonradiative recombination caused by the diffusion of Mg impurities from the p-contact layer into MQWs.

Growth of InGaN multiple quantum wells and GaN eplilayer on GaN substrate

We investigated the potential of low-temperature pulsed sputtering deposition (PSD) for the fabrication of high-In-composition thick InGaN multiple quantum wells (MQWs). Low-temperature PSD growth allowed the growth of a 100-period 1.2-nm-thick In0.3Ga0.7N MQW on GaN bulk crystals without apparent lattice relaxation. We fabricated a nitride-based photovoltaic device using 100-period In0.3Ga0.7N MQW absorption layers and obtained a clear photovoltaic response with an open-circuit voltage of 1.24 V, a short-circuit current density of 1.76 mA·cm−2, and a maximum output power density of 1.10 mW·cm−2 under 1 sun with air mass 1.5 illumination.

InGaN multiple quantum wells (MQWs) with green light emission have been grown on GaN stripes oriented along the [11-20] direction by selective metal-organic vapor phase epitaxy (MOVPE). Several different window widths were designed in the SiO₂ mask. Completed pyramidal InGaN stripes with flat and smooth {1-101} sidewall were produced on 2-μm windows while trapezoidal stripes with both {1-101} sidewall and (0001) top surface were obtained on the 5-μm windows. The former has uniform CL emissions at 500 nm on the {1-101} sidewall and at 550 nm on the narrow ridge. The latter exhibits similar CL emissions at 500 nm on the sidewall and at 570 nm on the top surface. These wavelength shifts relative to the CL spectrum peak (450 nm) from the reference region are attributed to thickness enhancement and indium enrichment in selective MOVPE. The short-wavelength shoulder near 500 nm in the spectrum from the ridge of the completed pyramidal strip is attributed to overlapping excitation of the sidewall by the SEM incident beam.