InGaN Quantum Wells Research Articles

Study and optimising performance of enhancement‐mode monolithically integrated white‐light HEMT‐LED by inserting of InGaN quantum wells

AbstractIn this paper five enhancement‐mode monolithically integrated white‐light High electron mobility transistors‐light emitting diodes (HEMT‐LED) structures are proposed and simulated to obtain maximum light intensity, drain current Id and maximum trans‐conductance gm. In first four HEMT‐LED structures white light is generated by combining inbuilt yellow and blue lights and in fifth proposed structure the white light is generated with the combination of inbuilt red, green and blue lights. The InGaN quantum wells (QWs) are inserted in to e‐mode ITO/p‐GaN gate HEMT structures and the desired wavelength of light spectrums are generated by changing the in content (mole fraction), to obtain inbuilt white light. Among five proposed structures one shows Maximum Id‐max of 925 mA and maximum gm of 250 mS, which is significantly higher than any HEMT‐LED structures reported before. All the proposed structures are simulated in Silvaco TCAD software.

An Approach Toward Broader Emission Bands in Semipolar InGaN Quantum Wells on Convex Lens‐Shaped GaN Microstructures via Lower‐Temperature Growth

InGaN quantum wells (QWs) are grown at different temperatures on convex lens‐shaped GaN microstructures formed on the () plane. Microlens QWs grown at different temperatures have centrosymmetric convex lens shapes, and the wavelength spatial distributions within the structures exhibit similar tendencies. However, lowering the growth temperature broadens the wavelength range of room‐temperature cathodoluminescence spectra from the microlens QWs as ≈70 nm for ≈700 °C, ≈100 nm for ≈660 °C, and ≈150 nm for ≈650 °C. Peak wavelength profiles of the two orthogonal lines along [] and [] indicate that the broader emission bands are mainly due to the significant spatial distribution of the emission wavelength along the [] direction. Because the QW thickness variations are nearly the same along [] and [], the observed difference in the emission wavelength distributions along those directions is attributed to the difference in In incorporation, which is determined by the competition among the growth rate, crystal orientation, and growth temperature.

A novel InGaN-based bidirectional wavelength converter and optical amplifier based on hot electron-induced blue light-emitting device

In this work, the functioning of the Top-Hat Hot-Electron Light Emission and Lasing in a Semiconductor Heterostructure (TH-HELLISH) structure, which is based on InGaN/GaN multiple quantum wells, is investigated to employ it as an optical amplifier and wavelength converter. The heterojunction structure is formed by placing four InGaN quantum wells in the n-type GaN region between the n- and p-type GaN layers. The device with a TH-HELLISH structure was grown on a sapphire (Al2O3) substrate by Metal-Organic Chemical Vapor Deposition (MOCVD) with an indium ratio of 0.16. The TH-HELLISH device was fabricated in Top-Hat HELLISH (THH) geometry with four independent n- and p-channels contacts to create a non-isotropic potential distribution in the heterojunction. The electro-photoluminescence (E-PL) technique was used to obtain light amplification and the converted wavelength. A 370 nm excitation light is absorbed by the TH-HELLISH device, subsequently leading to the emission at 440 ± 1 nm, which fulfills the role of a wavelength converter. It was also observed that TH-HELLISH operates as an optical amplifier with a 1.6-times increase in emission at 370 nm excitation. When the device is illuminated with a wavelength of 370 nm, it is monitored that the threshold electric field decreased from 0.25 kV/cm to 0.18 kV/cm, accompanied by a corresponding decrease in the threshold voltage from 12.5 V to 9.0 V. In addition, the TH-HELLISH device is found to be operated independently of the polarity of the applied voltage, and the absorption and emission regions of the device can be switched by changing the polarity of the applied voltage, therefore behaving as a bi-directional wavelength converter and optical amplifier.

Enhancing the blue GaN laser diodes by modifying the first barrier structure and adjusting the growth conditions of quantum wells

The structure of the first barrier (FB) of quantum wells was modified from the single-layer GaN to the GaN-InGaN-GaN composite structure. Experiments have shown that the composite first barrier can improve the crystal quality of the quantum wells, which increase the luminous power and reduce the stress accumulation before the InGaN quantum well. Furthermore, the composite first barrier can reduce the polarized electric fields and suppress the impact of quantum confinement Stark effect (QCSE) on the luminescence properties of the quantum wells. The growth conditions of the InGaN/GaN quantum wells were studied. Both the growth rate and the ammonia flow are related to the incorporation of In in InGaN quantum wells and the formation of In clusters. These two parameters have optimal values when the other one is fixed. After optimizing the growth rate and the ammonia flow, the internal quantum efficiency of the quantum wells was finally improved from 24.14 % to a commercial level of 77.2 %.

Dynamical Behavior of a Stacked Blue Laser Diode Consisting of Two Active Regions with Wide InGaN Quantum Wells and Two Tunnel Junctions

In nitride‐based laser diodes, the poor conductivity of the p‐type region, higher light absorption, and the difficult processing of an ohmic p‐contact with low resistance are major challenges. Recently interband tunnel junctions (TJs) have been used to replace much of the hole transport layers by electron transport layers, and also to allow for two n‐type contacts. In this work, GaN‐based laser diode stacks with TJs are analyzed. The devices are grown with plasma‐assisted molecular beam epitaxy and include two waveguides with active regions consisting of 25 nm wide InGaN quantum well (QW), each, which are connected through a TJ. A second TJ is placed on top to eliminate the p‐type contact. In this study, the time‐dependent spectral behavior of such structures is analyzed by streak camera measurements. The two QWs have different lasing thresholds and emission wavelength. Using the temporal resolution of the streak camera, a lasing onset of the second QW a few nanoseconds after the first one is observed. Single shot measurements show unusual intensity oscillations from both QWs, however, one is stable and the other laser diode (LD) has irregular oscillations.

High-efficiency InGaN red light-emitting diodes with external quantum efficiency of 10.5% using extended quantum well structure with AlGaN interlayers

In this study, we have demonstrated a high-efficiency InGaN red (625 nm) light-emitting diode (LED) with an external quantum efficiency (EQE) of 10.5% at a current density of 10 A/cm2. To achieve this, we introduced GaN cap layers on InGaN quantum wells and AlGaN interlayers. The introduction of these layers resulted in a red shift of the wavelength. The AlGaN interlayer caused band bending, while the GaN cap layer modulated the electron wavefunction, thus helping to achieve the wavelength red shift of the InGaN red LED with high EQE. This technology is crucial for the realization of discrete or monolithic full-color micro-LED displays.

Evaluation of radiative and non-radiative recombination lifetimes in InGaN quantum wells with different ion-implantation damage

In this study, we have separately evaluated the radiative and non-radiative recombination lifetimes for InGaN quantum well (QW) samples with different amounts of ion-implantation damage, and have investigated their temperature dependence. The radiative and non-radiative recombination lifetimes were calculated from photoluminescence (PL) decay time measured by time-resolved PL measurements, combined with the absolute internal quantum efficiency values estimated by the simultaneous photoacoustic and PL measurements. As a result, the experimentally observed radiative recombination lifetimes are almost the same for all samples, while the non-radiative recombination lifetimes are shorter for samples with larger ion-implantation damage. These findings will lead to a comprehensive understanding of carrier dynamics in InGaN-QW optical devices.

Anisotropic emission wavelength distribution of semipolar InGaN quantum wells on symmetric convex lens-shaped GaN microstructures

We fabricated InGaN quantum wells (QWs) on convex lens-shaped GaN microstructures formed on semipolar (1¯1¯22¯) GaN substrates. Despite the centrosymmetric lens-like shapes, the overgrown InGaN QWs exhibit an anisotropic emission wavelength distribution. This behavior is distinct from the concentric wavelength distribution found in microlens QWs on the polar (0001) plane. The anisotropic distribution is mainly attributed to the In composition variations, which are influenced by the asymmetry of the (1¯1¯22¯) crystallographic plane. The obtained results demonstrate that the (1¯1¯22¯) microlens QWs serve as multi-wavelength light emitters and allow us to investigate unstable planes around the (1¯1¯22¯) plane, which have remained unexplored.

Detecting glucose in a cell culture medium by surface-enhanced Raman scattering on InGaN quantum wells

Cell cultivation is a multi-billion dollar industry. The industry is currently in great need of a glucose-monitoring tool to maximize the yield of biological products. However, detecting glucose in a cell culture medium is no easy task. This is because the medium contains complex cell nutrients, from which the interfering noises make it extremely difficult to extract reliable glucose signals. We address the issue by surface-enhanced Raman spectroscopy (SERS) built with InGaN quantum wells, delivering concentration-dependent glucose signals from the noisy medium. The breakthrough is made by the quantum-confined charges whose oscillating frequency matches the plasmonic resonance desired for SERS.

Strain control and its effect on the optical properties of InGaN/GaN multiple quantum wells

The strain modulation mechanism of green InGaN quantum wells (QWs) was investigated by designing and growing additional InGaN/GaN strain modulation layers with different Indium (In) content and structures. Electroluminescence (EL) measurements were employed to analyze the strain in InGaN QWs by checking the change of EL peak energy with injection current. The experimental results suggest that proper In content in the InGaN strain modulation layer is required to reduce the compression strain in multiple quantum wells (MQWs). Meanwhile, the material quality and flatness of the pre-grown modulation layer may significantly affect the homogeneity and the strain state of MQWs. Temperature-dependent photoluminescence (TDPL) and micro-luminescence were measured to characterize the optical properties of MQWs prepared with strain modulation layers. By carefully modulating the strain in QWs, the InGaN/GaN green multiple quantum wells (MQWs) exhibit higher In incorporation and uniform luminescence.

Effect of varying threading dislocation densities on the optical properties of InGaN/GaN quantum wells with intentionally created V-shaped pits

The effect of varying threading dislocation densities on the internal quantum efficiencies (IQEs) of InGaN quantum wells (QWs), with and without intentionally created “V-pits,” is reported here. InGaN QW samples grown on GaN-on-sapphire templates with threading dislocation densities of <1 × 108 and <1 × 109 cm−2 are compared, with and without GaN/InGaN superlattice (SL) layers incorporated to intentionally open up the threading dislocation cores and form large-size “V-pits.” The formation of “V-pits” is confirmed by cross-sectional transmission electron microscopy to initiate from threading dislocations in the SL layers. The densities of the pits are confirmed by plan-view SEM to agree with the substrate threading dislocation densities. The experimental room temperature IQEs of the “V-pit” QW samples are enhanced to 15% ± 1% compared to 6% ± 2% for conventional QW samples. Both conventional and “V-pit” samples show insensitivity to the magnitude of the dislocation densities with respect to IQE performance, while the “V-pit” samples show shifts in the peak emission wavelengths compared to the conventional samples, attributed to strain modulation. This study provides additional understanding of the causes of the observed insensitivity of the IQEs to different threading dislocation densities.

Evaluation of radiative and nonradiative recombination lifetimes in c-plane InGaN quantum wells with emission colors ranging from blue to red

In this study, we investigate In composition and the carrier density dependences of radiative and nonradiative recombination lifetimes for a series of c-plane InGaN quantum well (QW) samples with different emission wavelengths (450 nm to 620 nm). The two lifetimes can be separately evaluated using photoluminescence (PL) decay time, obtained by time-resolved PL measurement, combined with the value of internal quantum efficiency (IQE) obtained by simultaneous photoacoustic and PL measurements. It is found that the decrease in IQE with increasing In composition is caused by the reduction in radiative recombination lifetime, not by the enhancement of nonradiative lifetime, which shows little dependence on In composition. In addition, it is found that the carrier density dependence of IQE is also mainly determined by the change in radiative recombination lifetime. These findings will lead to a comprehensive understanding of carrier dynamics in InGaN-QW optical devices.

Investigation of the Impact of Point Defects in InGaN/GaN Quantum Wells with High Dislocation Densities.

In this work, we report on the efficiency of single InGaN/GaN quantum wells (QWs) grown on thin (<1 µm) GaN buffer layers on silicon (111) substrates exhibiting very high threading dislocation (TD) densities. Despite this high defect density, we show that QW emission efficiency significantly increases upon the insertion of an In-containing underlayer, whose role is to prevent the introduction of point defects during the growth of InGaN QWs. Hence, we demonstrate that point defects play a key role in limiting InGaN QW efficiency, even in samples where their density (2-3 × 109 cm-2) is much lower than that of TD (2-3 × 1010 cm-2). Time-resolved photoluminescence and cathodoluminescence studies confirm the prevalence of point defects over TDs in QW efficiency. Interestingly, TD terminations lead to the formation of independent domains for carriers, thanks to V-pits and step bunching phenomena.

A Simulation Study of Carrier Capture Ability of the Last InGaN Quantum Well with Different Indium Content for Yellow-Light-Emitting InGaN/GaN Multiple Quantum Wells.

Currently, GaN-based blue- and green-light-emitting devices have achieved successful applications in practice, while the luminescence efficiency of devices with longer wavelengths (such as yellow light) is still very low. Therefore, in this paper, the electroluminescence characterization of yellow-light-emitting InGaN/GaN multiple quantum wells (MQWs) with different In content in the last InGaN quantum well, which is next to the p-type GaN electrode layer, are investigated numerically to reveal a possible physical mechanism by which the different distribution of In content in the active region impacts the carrier capture and the light emission process in yellow InGaN/GaN MQWs. The simulation results show that at low injection currents, the luminescence efficiency of high-In-content yellow MQWs is enhanced, which can be ascribed to the enhanced radiative recombination process induced by the increased carrier concentration in the last InGaN quantum wells with promoted carrier capture ability. However, in the case of high injection condition, the luminescence efficiency of yellow MQWs deteriorates with increasing In content, i.e., the droop effect becomes remarkable. This can be ascribed to both significantly enhanced Auger recombination and electron leakage in the last InGaN quantum well, induced also by the promoted capture ability of charge carriers.

Investigation of the Indium migration mechanism in the growth of InGaN quantum wells by MOCVD

Improvement of InGaN quantum wells (QWs) with high In content is crucial for realizing well GaN-based optoelectronic devices, such as laser diodes (LDs) and light-emitting diodes (LEDs). In this paper, the migration mechanism of In atoms and its impact on the luminescence properties of green QWs are studied, and the material and optical quality of high-In-content InGaN QWs can be improved by increasing the Indium migration ability through low-pressure growth method. The experimental results show that the In incorporation into InGaN QWs decreases while the luminescence property increases with the decrease of growth pressure when the growth process is under the surface-reaction-controlled mode. The analysis of the evolution behavior of InGaN/GaN interfaces through TEM measurements reveals that the migration ability of In atoms is a key factor affecting the continuity of InGaN QWs and the formation of In aggregation. Further temperature-dependent photoluminescence (TDPL) analysis and theoretical investigation on the LSE model confirm that the improvement in the luminescence efficiency of QWs is attributed to the enhancement of interface quality and the weakening of localized states. These results provide a new technical direction for the development of high-performance LEDs and LDs.

Optimized InGaN/GaN Quantum Structure for High-Efficiency Micro-LEDs Displays With Low Current Injection

As the field of state-of-the-art displays continues to evolve, micro light-emitting diodes (Micro-LEDs) with their diminutive pixel size and exceptional efficiency are emerging as a pivotal player. It is essential for displays to achieve the highest possible efficiency under low current injection. Through the investigation of Micro-LEDs incorporating various quantum structures, we have exhibited remarkable enhancement of their optoelectronic properties at low current injection. The optimized InGaN/GaN Micro-LED structure, incorporating a 2-nm-thick quantum well (QW) and a 6-nm-thick quantum barrier (QB), demonstrated superior photonic performance at current densities ranging from 0.1 to 10 A/cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\text{2}}$</tex-math> </inline-formula> . It is discovered that the electron–hole wave function overlap can be enhanced by thinning the InGaN QW, thereby increasing radiation efficiency. Meanwhile, uniform carrier distribution and reduced quantum confinement stark effect (QCSE) in QWs can be achieved by a thinner GaN barrier, resulting in more uniform luminescence at each QW in multiple QWs (MQWs). The optimized Micro-LEDs devices exhibit uniform emission and remarkably high brightness, making them a suitable emission source to be combined with quantum dots (QDs) for the realization of full-color displays. Convincing results proved that optimized Micro-LEDs array can serve as the solution for high-resolution and brightness displays.

Improvement in quantum efficiency of green GaN-based micro-LED by trapezoidal quantum well

The main challenges in the application of GaN-based full-color micro-light-emitting diodes (micro-LEDs) are size-dependent effects and low quantum efficiency, especially for green micro-LEDs. In this study, we propose to use a trapezoidal quantum well (QW) structure in GaN-based green micro-LEDs. The trapezoidal QW structure is able to improve the electron hole wave function overlap, thus increasing the external quantum efficiency (EQE). Green micro-LEDs with active region size of 75 × 64 μm2 are fabricated by metal organic chemical vapor deposition. The peak EQE of the green trapezoidal QW micro-LED reaches 15.8% at a low injection current density of 0.6A/cm2, which is 1.96 times higher than that of conventional rectangular QW. The test results indicate that the trapezoidal QW structure greatly improves the quantum efficiency of green micro-LED, inhibits the blue shift of peak wavelength and improves color purity, which paves the way to single substrate full-color micro-LED display with high-performance. The technology proposed in this paper does not change the production line equipment, but improves the traditional process, so it is expected to significantly improve the external quantum efficiency of the existing green micro-LED under the existing equipment. The production technology of existing InGaN QW micro-LED is directly adopted, and the EQE can be further improved if some new dry etching and passivation methods are applied.

Bright Emission at Reverse Bias After Trailing Edge of Driving Pulse in Wide InGaN Quantum Wells

In group‐III‐nitride quantum wells (QWs), a strong piezoelectric field is formed. A built‐in potential from the p–n junction is working in the opposite direction depending on an externally applied voltage. Furthermore, the electric field in the QW can be partially screened by a high charge carrier density. Herein, the influence of these effects on recombination and spectrum for over 10 nm wide InGaN QWs is investigated. The very low overlap of electrons and holes would suggest inefficient devices. However, it is shown that thick QWs can be more effective and reach high optical gain. This can be explained by the screening of the electric field, resulting in a high overlap of excited electron and hole states that enable lasing. Herein, a pulsed electrical excitation scheme is used, where carrier injection at forward bias and predominant recombination at zero or reverse bias are separated in time. The interplay between the piezoelectric field and the built‐in potential on carrier recombination in dependence on an external bias voltage is observed. In particular, a strong increase of the radiative recombination rate after the trailing edge of the driving pulse is observed.

Optical and structural properties of InGaN/GaN quantum well green laser diodes with AlGaN interlayer

Scanning transmission electron microscope and temperature-dependent photoluminescence were used to study the effects of AlGaN interlayer (IL) following InGaN quantum wells (QWs) on the structural and optical properties of InGaN green laser diodes (LDs) grown on c-plane GaN substrate. It is found that AlGaN IL improves InGaN QW interface sharpness, whereas it does not suppress the localization effect, and moreover, it leads to increasing nonradiative recombination centers in the active region of green InGaN LD. We believe AlGaN IL has advantages and disadvantages to green InGaN LD structures. The advantage is to suppress indium (In) surface segregation, but the disadvantage is that it covers In-rich clusters, which evolve into dark spots during the growth of p-type layers of green LD structures.

Strain-Induced Band Gap Variation in InGaN/GaN Short Period Superlattices

The use of strained substrates may overcome indium incorporation limits without inducing plastic relaxation in InGaN quantum wells, and this is particularly important for short-period InGaN/GaN superlattices. By incorporating elastic strain into these heterostructures, their optoelectronic behavior is modified. Our study employed density functional theory calculations to investigate the variation in the band-gap energy of short-period InGaN/GaN superlattices that comprise pseudomorphic quantum wells with a thickness of just one monolayer. Heterostructures with equibiaxially strained GaN barriers were compared with respective ones with relaxed barriers. The findings reveal a reduction of the band gap for lower indium contents, which is attributed to the influence of the highly strained nitrogen sublattice. However, above mid-range indium compositions, the situation is reversed, and the band gap increases with the indium content. This phenomenon is attributed to the reduction of the compressive strain in the quantum wells caused by the tensile strain of the barriers. Our study also considered local indium clustering induced by phase separation as another possible modifier of the band gap. However, unlike the substrate-controlled strain, this was not found to exert a significant influence on the band gap. Overall, this study provides important insights into the behavior of the band-gap energy of strained superlattices toward optimizing the performance of optoelectronic devices based on InGaN/GaN heterostructures.