Multiple Quantum Wells Light-emitting Diodes Research Articles

Effect of temperature on GaN-integrated optical transceiver chips

The gallium nitride (GaN) integrated optical transceiver chip based on multiple quantum wells (MQW) structure exhibits great promise in the fields of communication and sensing. In this Letter, the effect of ambient temperature on the performance of GaN-integrated optical transceiver chips including a blue MQW light-emitting diode (LED) and a MQW photodiode (PD) is comprehensively studied. Temperature-dependent light-emitting and current–voltage characteristics of the blue MQW LEDs are measured with the ambient temperature ranging from –70°C to 120°C. The experimental results reveal a decline in the electroluminescent (EL) intensity and an obvious redshift in the emission peak wavelength of the LED with increasing ambient temperature. The light detection performance of MQW PD under different temperatures is also measured with the illumination of an external blue MQW LED, indicating an enhancement in the PD sensitivity as the temperature rises. Finally, the temperature effect on the MQW PD under the illumination of the MQW LED on the GaN-integrated optical transceiver chip is characterized, and the PD photocurrent increases with higher ambient temperature. Furthermore, the measured temperature characteristics indicate that the GaN-integrated optical transceiver chip offers a promising application potential for optoelectronic temperature sensor.

Multiple-carrier-lifetime model for carrier dynamics in InGaN/GaN LEDs with a non-uniform carrier distribution

We introduce a multiple-carrier-lifetime model (MCLM) for light-emitting diodes (LEDs) with non-uniform carrier distribution, such as in multiple-quantum-well (MQW) structures. By employing the MCLM, we successfully explain the modulation response of V-pit engineered MQW LEDs, which exhibit an S21 roll-off slower than −20 dB/decade. Using the proposed model and employing a gradient descent method, we extract effective recombination and escape lifetimes by averaging the carrier behavior across the quantum wells. Our results reveal slower effective carrier recombination and escape in MQW LEDs compared with LEDs emitting from a single QW, indicating the advantages of lower carrier density achieved through V-pit engineering. Notably, the effective carrier recombination time is more than one order of magnitude lower than the effective escape lifetime, suggesting that most carriers in the quantum wells recombine, while the escape process remains weak. To ensure the reliability and robustness of the MCLM, we subject it to a comprehensive threefold validation process. This work confirms the positive impact of spreading carriers into several quantum wells through V-pit engineering. In addition, the MCLM is applicable to other LEDs with a non-uniform carrier distribution, such as micro-LEDs with significant surface recombination and non-uniform lateral carrier profiles.

Interplay between Auger recombination, carrier leakage, and polarization in InGaAlN multiple-quantum-well light-emitting diodes

In conventional hexagonal InGaAlN multiple-quantum-well (MQW) (h-) light-emitting diodes (LEDs), carrier leakage from QWs is the main source of internal quantum efficiency (IQE) degradation without contributing to the LED efficiency droop. Our analysis based on the newly developed Open Boundary Quantum LED Simulator indicates that radiative recombination is hampered by the poor electron–hole wavefunction overlap induced by strong internal polarization for which QW carriers mostly recombine via Auger scattering rather than by radiative processes. By contrast, in non-polar h-LEDs, the IQE peak doubles its value compared to conventional h-LEDs while quenching the efficiency droop by 70% at current density of 100 A/cm2. Those effects are further enhanced in cubic InGaAlN MQW (c-) LEDs for which the IQE peak increases by an additional 30%, and the efficiency droop is further reduced by 80% compared to non-polar h-LEDs, thanks to the larger optical transition matrix element and the strong electron–hole wavefunction overlap in c-LEDs. Overall, a c-LED with a low efficiency droop of 3% at 100 A/cm2 is anticipated, paving a clear pathway toward ultimate solid-state lighting.

Emissions of the InGaN/GaN MQW LEDs with the InGaN well layer grown at different temperatures

Two light-emitting diodes (LEDs) enabled by InGaN/GaN multiple quantum wells (MQWs) with different well layer growth temperatures (WLGTs) were prepared. The dependences of electroluminescence (EL) spectra of these two structures on temperature at various fixed injection currents indicate that, a decreasing WLGT can result in a conversion of the well layer structure from a one-zone structure with better homogeneity in the localization depth into a two-zone structure with different average In contents and different localization depths, due to the increased In content-induced enhanced component fluctuation. The former is inferred from an “inverted-V-shaped” (increasing-decreasing) temperature-dependent behavior of peak energy at all fixed currents; the latter is mainly inferred from an “M-shaped” (increasing-decreasing-increasing-decreasing) temperature-dependent behavior of peak energy at intermediate fixed currents. These explanations also match those given for temperature-dependent behaviors in terms of external quantum efficiency (EQE) of these two LEDs, including “M-shaped” temperature-dependent behaviors of the EQE of LED B at the intermediate fixed currents. • EL properties of two different InGaN/GaN MQW-based LEDs A and B were studied. • LED B has a lower well layer growth temperature (WLGT) than LED A. • “M-shaped” temperature behavior of EL efficiency was observed only for LED B. • This is due to two-zone structural feature of well layer induced by its lower WLGT.

Luminous power improvement in InGaN V-Shaped Quantum Well LED using CSG on SiC Substrate

This paper presents the design and simulation of Silicon Carbide (SiC) based technology, Indium Gallium Nitride (InGaN) Multiple Quantum Well (MQW) Light-Emitting Diode (LED) with a Compositionally Step Graded (CSG) InGaN barrier and V-Shaped well in the active region. The simulations are obtained in Silvaco Computer Aided Design simulator and parameters such as Internal Quantum Efficiency (IQE) with respect to input current, spontaneous emission in regard to wavelength and power versus current in the device are theoretically studied. The CSG InGaN barrier LED with V-shaped quantum well shows substantial growth in output power when compared to the CSG GaN barrier structure with conventional MQW. The high carrier confinement in the V-shape well causes, transportation/injection of hole and change in band bending due to polarization effect. Moreover, lattice-matched SiC substrate over GaN material increases the InGaN V-shaped MQW LEDs radiative recombination rate which in turn leads to high output power. The optical luminous power of 160mW and 82% of peak IQE, emitting wavelength at 460 nm and 200mA of injection current is obtained for the proposed LED. The enactment of the V shape MQW CSG-InGaN device technology is a good alternative choice for commercial and industrial lighting applications.

A new model on recombination dynamics of polar InGaN/GaN MQW LED and IQE measurement

Understanding the recombination nature in a polar InGaN/GaN multiple quantum well (MQW) light-emitting diode (LED) or similar device is critical for their further performance enhancements. This paper reports a new theoretical model for investigating the recombination dynamics in MQW LEDs, which more comprehensively takes both localized exciton recombination (LER) and free carrier recombination (FCR) into account. The obtained rates for LER, FCR and nonradiative recombination show a clear picture of recombination paths in a commercial blue MQW LED wafer. They can be also used to calculate the internal quantum efficiency without involving any extra measurements or prerequisites. This model may provide a universal solution to express the complicated recombination dynamics in various kinds of MQW LEDs.

Improvement in efficiency of yellow Light Emitting Diode using InGaN barriers and modified electron injection layer

InGaN based MQW-LEDs (Multiple Quantum Well Light Emitting Diodes) are now the new generation of light sources for displays and general lightening applications. The efficiency of blue InGaN LEDs has been matured very much (>80%), but efficiency of yellow LEDs based on InGaN quantum wells still lags behind. In this paper, we have proposed a new structure for yellow LEDs based on InGaN MQW to increase the efficiency of these LEDs. A simulation approach has been used to model and simulate MQW yellow LED based on InGaN quantum wells by using k.p theory. The simulation results show that the proposed structure provides much better output characteristics than the conventional structure. Using the proposed structure, about 18% increment in internal quantum efficiency (IQE) and a decrease of about 36% in efficiency droop of the yellow LED has been successfully achieved.

Influences of screw dislocations on electroluminescence of AlGaN/AlN-based UVC LEDs

We investigated two types of threading dislocations in high Al-composition Al0.55Ga0.45N/AlN multiple quantum well (MQW) structures grown on AlN substrate for electrically injected deep ultraviolet light-emitting diodes (LEDs). The surface morphology and defects electrical characteristics of the MQW LED structures were examined via conductive atomic force microscopy (CAFM). We found that the disparity between photoluminescence (PL) and electroluminescence (EL) spectra in terms of light emission output and wavelength shift are attributed to the existence of the surface hillocks, especially to the ones that have open-core dislocations. The open-core dislocations form current leakage paths through their defect states and the composition inhomogeneity (i.e., Ga rich) at the dislocation sites are responsible for the light emission at longer wavelengths.

Enhanced performance of InGaN/GaN MQW LED with strain-relaxing Ga-doped ZnO transparent conducting layer.

We report the enhanced optical and electrical properties of InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) with strain-relaxing Ga-doped ZnO transparent conducting layers (TCLs). Ga-doped ZnO was epitaxially grown on p-GaN by metal-organic chemical vapor deposition. The optical output power of a LED with a 500-nm- thick-Ga-doped ZnO TCL increased by 30.9% at 100 mA, compared with that of an LED with an indium tin oxide (ITO) TCL. Raman spectroscopy measurement and the simulation of wavefunction overlap of electron and hole in MQWs revealed that the enhanced optical output power was attributed to the increased internal quantum efficiency due to the decreased compressive strain in the active region. The increase of optical output was also attributed to the increased optical transmittance of the Ga-doped ZnO TCL owing to its higher refractive index compared to that of ITO TCL. Furthermore, the forward voltage of LED with a Ga-doped ZnO TCL was lower than that of LED with an ITO TCL because of the increased carrier concentration and mobility in the Ga-doped ZnO TCL.

Efficiency enhancement of III-nitride light-emitting diodes with strain-compensated thin-barrier InGaN/AlN/GaN multiple quantum wells

We introduced strain-compensated thin-barrier indium gallium nitride (InGaN)/aluminum nitride (AlN)/gallium nitride (GaN) multiple quantum wells (MQWs) to replace thin-barrier InGaN/GaN MQWs. The AlN insert layers would effectively compensate the strain of the thin-barrier InGaN/GaN MQWs to improve the opto-electrical properties of light-emitting diodes (LEDs). The 120-mA light output power of thin-barrier InGaN/GaN MQW LEDs could be improved from 31.9 mW to 35.3 mW by introducing 20-s-growth AlN insert layers, possibly reaching almost the same 120-mA light output power of traditional thick-barrier InGaN/GaN MQWs. Moreover, the current dependent external quantum efficiency (EQE) of the thin-barrier InGaN/AlN/GaN MQW LEDs with 20-s-growth AlN insert layers also indicated the largest peak EQE, showing high efficiency in low current injection. The severe carrier overflow effect that degrades the light output efficiency of the thin-barrier InGaN/GaN MQW LED in high current injection can be suppressed by introducing thin-barrier InGaN/AlN/GaN MQW with 20-s-growth AlN insert layers.

Semipolar ( $$ 1\bar{1}01 $$ 1 1 ¯ 01 ) InGaN/GaN red–amber–yellow light-emitting diodes on triangular-striped Si (100) substrate

The epitaxial growth of InGaN/GaN light-emitting diodes (LEDs) with high-indium (In) content on Si (100) substrate faces significant challenges. The study described in this paper focuses on semipolar yellow InGaN/GaN LEDs formed on a triangular-striped Si (100) substrate by metal organic chemical vapor deposition. By controlling the growth temperature of InGaN to modulate the In content of InGaN/GaN multiple quantum wells (MQWs), high-In-content InGaN/GaN MQWs were grown on semipolar ( $$ 1\bar{1}01 $$ ) planes at relatively low temperatures. Consequently, InGaN/GaN MQW LEDs grown on triangular-striped substrates produce emissions ranging from red to yellow under different injection currents. In particular, when the injection current exceeds 160 mA, the LEDs achieve stable yellow emission. This is the first time such a long waveband emission has been achieved in semipolar InGaN/GaN LEDs formed on Si (100) substrate.

Trade-off between bandwidth and efficiency in semipolar (202¯1¯) InGaN/GaN single- and multiple-quantum-well light-emitting diodes

InGaN/GaN light-emitting diodes (LEDs) with large modulation bandwidths are desirable for visible-light communication. Along with modulation speed, the consideration of the internal quantum efficiency (IQE) under operating conditions is also important. Here, we report the modulation characteristics of semipolar (202¯1¯) InGaN/GaN (LEDs) with single-quantum well (SQW) and multiple-quantum-well (MQW) active regions grown on free-standing semipolar GaN substrates with peak internal quantum efficiencies (IQEs) of 0.93 and 0.73, respectively. The MQW LEDs exhibit on average about 40–80% higher modulation bandwidth, reaching 1.5 GHz at 13 kA/cm2, but about 27% lower peak IQE than the SQW LEDs. We extract the differential carrier lifetimes (DLTs), RC parasitics, and carrier escape lifetimes and discuss their role in the bandwidth and IQE characteristics. A coulomb-enhanced capture process is shown to rapidly reduce the DLT of the MQW LED at high current densities. Auger recombination is also shown to play little role in increasing the speed of the LEDs. Finally, we investigate the trade-offs between the bandwidth and efficiency and introduce the bandwidth-IQE product as a potential figure of merit for optimizing speed and efficiency in InGaN/GaN LEDs.

Investigation of efficiency enhancement in InGaN MQW LED with compositionally step graded GaN/InAlN/GaN multi-layer barrier

The advantage of InGaN multiple Quantum well (MQW) Light emitting diode (LED) on a SiC substrate with compositionally step graded GaN/InAlN/GaN multi-layer barrier (MLB) is studied. The Internal quantum efficiency, Optical power, current-voltage characteristics, spontaneous emission rate and carrier distribution profile in the active region are investigated using Sentaurus TCAD simulation. An analytical model is also developed to describe the QW carrier injection efficiency, by including carrier leakage mechanisms like carrier overflow, thermionic emission and tunnelling. The enhanced electron confinement, reduced carrier asymmetry, and suppressed carrier overflow in the active region of the MLB MQW LED leads to render a superior performance than the conventional GaN barrier MQW LED. The simulation result also elucidates the efficiency droop behaviour in the MLB MQW LED, it suggests that the efficiency droop effect is remarkably improved when the GaN barrier is replaced with GaN/InAlN/GaN MLB barrier. The analysis shows a dominating behaviour of carrier escape mechanism due to tunnelling. Moreover, the lower lattice mismatching of SiC substrate with GaN epitaxial layer is attributed with good crystal quality and reduced polarization effect, ultimately enhances the optical performance of the LEDs.

Suspended light-emitting diode featuring a bottom dielectric distributed Bragg reflector

Here, we propose, fabricate and characterize the light manipulation of a suspended-membrane InGaN/GaN multiple-quantum-well light-emitting diode (MQW-LED) with a dielectric distributed Bragg reflector (DBR) positioned at the bottom, implemented on a GaN-on-silicon platform. Silicon removal is conducted to obtain the suspended MQW-LED architecture, and back wafer thinning of the epitaxial film is performed to improve the device performance. A 6-pair SiO2/Ta2O5 DBR is deposited on the backside to manipulate the emitted light. The experimental results demonstrate that the bottom dielectric DBR exhibits high reflectivity and distinctly changes the light emission, which are consistent with the performed simulation results. This work represents a significant step towards the realization of inexpensive, electrically driven and simply fabricated GaN VCSELs for potential use in number of applications.

Carrier transport and emission efficiency in InGaN quantum-dot based light-emitting diodes

We present a study of blue III-nitride light-emitting diodes (LEDs) with multiple quantum well (MQW) and quantum dot (QD) active regions (ARs), comparing experimental and theoretical results. The LED samples were grown by metalorganic vapor phase epitaxy, utilizing growth interruption in the hydrogen/nitrogen atmosphere and variable reactor pressure to control the AR microstructure. Realistic configuration of the QD AR implied in simulations was directly extracted from HRTEM characterization of the grown QD-based structures. Multi-scale 2D simulations of the carrier transport inside the multiple QD AR have revealed a non-trivial pathway for carrier injection into the dots. Electrons and holes are found to penetrate deep into the multi-layer AR through the gaps between individual QDs and get into the dots via their side edges rather than via top and bottom interfaces. This enables a more homogeneous carrier distribution among the dots situated in different layers than among the laterally uniform quantum well (QWs) in the MQW AR. As a result, a lower turn-on voltage is predicted for QD-based LEDs, as compared to MQW ones. Simulations did not show any remarkable difference in the efficiencies of the MQW and QD-based LEDs, if the same recombination coefficients are utilized, i.e. a similar crystal quality of both types of LED structures is assumed. Measurements of the current–voltage characteristics of LEDs with both kinds of the AR have shown their close similarity, in contrast to theoretical predictions. This implies the conventional assumption of laterally uniform QWs not to be likely an adequate approximation for the carrier transport in MQW LED structures. Optical characterization of MQW and QD-based LEDs has demonstrated that the later ones exhibit a higher efficiency, which could be attributed to better crystal quality of the grown QD-based structures. The difference in the crystal quality explains the recently observed correlation between the growth pressure of LED structures and their efficiency and should be taken into account while further comparing performances of MQW and QD-based LEDs. In contrast to experimental results, our simulations did not reveal any advantages of using QD-based ARs over the MQW ones, if the same recombination constants are assumed for both cases. This fact demonstrates importance of accounting for growth-dependent factors, like crystal quality, which may limit the device performance. Nevertheless, a more uniform carrier injection into multi-layer QD ARs predicted by modeling may serve as the basis for further improvement of LED efficiency by lowering carrier density in individual QDs and, hence, suppressing the Auger recombination losses.

A 30 Mbps in-plane full-duplex light communication using a monolithic GaN photonic circuit

We propose, fabricate and characterize photonic integration of a InGaN/GaN multiple-quantum-well light-emitting diode (MQW-LED), waveguide, ring resonator and InGaN/GaN MQW-photodiode on a single chip, in which the photonic circuit is suspended by the support beams. Both experimental observations and simulation results illustrate the manipulation of in-plane light coupling and propagation by the waveguide and the ring resonator. The monolithic photonic circuit forms an in-plane data communication system using visible light. When the two suspended InGaN/GaN MQW-diodes simultaneously serve as the transmitter and the receiver, an in-plane full-duplex light communication is experimentally demonstrated with a transmission rate of 30 Mbps, and the superimposed signals are extracted using the self-interference cancellation method. The suspended photonic circuit creates new possibilities for exploring the in-plane full-duplex light communication and manufacturing complex GaN-based monolithic photonic integrations.

InGaN directional coupler made with a one-step etching technique

We propose, fabricate and characterize an on-chip integration of light source, InGaN waveguide, directional coupler and photodiode, in which AlGaN layers are used as top and bottom optical claddings to form an InGaN waveguide for guiding the in-plane emitted light from the InGaN/GaN multiple-quantum-well light-emitting diode (MQW-LED). The difference in etch rate caused by different exposure windows leads to an etching depth discrepancy using the one-step etching technique, which forms the InGaN directional coupler with the overlapped underlying slab. Light propagation results directly confirm effective light coupling in the InGaN directional coupler, which is achieved through high-order guided modes. The InGaN waveguide couples the modulated light from the InGaN/GaN MQW-LED and transfers part of light to the coupled waveguide via the InGaN directional coupler. The in-plane InGaN/GaN MQW-photodiode absorbs the guided light by the coupled InGaN waveguide and induces the photocurrent. The on-chip InGaN photonic integration experimentally demonstrates an in-plane light communication with a data transmission of 50 Mbps.

Efficient Red Perovskite Light-Emitting Diodes Based on Solution-Processed Multiple Quantum Wells.

This paper reports a facile and scalable process to achieve high performance red perovskite light-emitting diodes (LEDs) by introducing inorganic Cs into multiple quantum well (MQW) perovskites. The MQW structure facilitates the formation of cubic CsPbI3 perovskites at low temperature, enabling the Cs-based QWs to provide pure and stable red electroluminescence. The versatile synthesis of MQW perovskites provides freedom to control the crystallinity and morphology of the emission layer. It is demonstrated that the inclusion of chloride can further improve the crystallization and consequently the optical properties of the Cs-based MQW perovskites, inducing a low turn-on voltage of 2.0 V, a maximum external quantum efficiency of 3.7%, a luminance of ≈440 cd m-2 at 4.0 V. These results suggest that the Cs-based MQW LED is among the best performing red perovskite LEDs. Moreover, the LED device demonstrates a record lifetime of over 5 h under a constant current density of 10 mA cm-2 . This work suggests that the MQW perovskites is a promising platform for achieving high performance visible-range electroluminescence emission through high-throughput processing methods, which is attractive for low-cost lighting and display applications.

Multi-dimensional spatial light communication made with on-chip InGaN photonic integration

Here, we propose, fabricate and characterize suspended photonic integration of InGaN multiple-quantum-well light-emitting diode (MQW-LED), waveguide and InGaN MQW-photodetector on a single chip. The unique light emission property of InGaN MQW-LED makes it feasible to establish multi-dimensional spatial data transmission using visible light. The in-plane light communication system is comprised of InGaN MQW-LED, waveguide and InGaN MQW-photodetector, and the out-of-plane data transmission is realized by detecting the free-space light emission via a commercial photodiode module. Moreover, a full-duplex light communication is experimentally demonstrated at a data transmission rate of 50 Mbps when both InGaN MQW-diodes operate under simultaneous light emission and detection mode. The in-plane superimposed signals are able to be extracted through the self-interference cancellation method, and the out-of-plane superimposed signals are in good agreement with the calculated signals according to the extracted transmitted signals. These results are promising for the development of on-chip InGaN photonic integration for diverse applications.

Effect of SiN Treatment on Optical Properties of In x Ga1−x N/GaN MQW Blue LEDs

In this paper, Si-doped GaN template layer and nitride-based multiple quantum well (MQW) light-emitting diodes (LEDs) with conventional GaN buffer layer and GaN buffer layer using SiN treatment were elaborated by metalorganic vapor phase epitaxy (MOVPE). On both kinds of structures, five InxGa1−xN/GaN quantum wells were deposited simultaneously in identical growth conditions. GaN template layer defects that influence on the growth of InxGa1−xN/GaN MQW LEDs were systematically studied by means of scanning electron microscopy, high-resolution x-ray diffraction, temperature-dependant photoluminescence measurement, and electroluminescence. It is shown that optical properties of InxGa1−xN/GaN MQWs depend on the defect density of elaborated templates. Thereafter, we report an enhancement of the emission of blue MQW LEDs using SiN treatment, compared to the MQW LED emissions deposited on a conventional GaN buffer layer.