Last Quantum Barrier Research Articles

Study of ultraviolet light emitting diodes with InGaN quantum dots and lattice matched superlattice electron blocking layers.

Ultraviolet light emitting diodes (UV-LEDs) face the challenges including insufficient hole injection and severe electron leakage. Quantum dots (QDs) have been proven to provide three-dimensionally localized states for carriers, thereby enhancing carrier confinement. Therefore, UV-LEDs employing InGaN QDs are designed and studied in this paper. The APSYs software is used to simulate UV-LEDs. Simulation results indicate that the QDs effectively improve the electron and hole concentration in the active region. However, UV-LEDs with QDs experience efficiency droop due to serious electron leakage. What's more, the lattice mismatch between last quantum barrier (LQB) and electron blocking layer (EBL) leads to the polarization field, which induces the downward band bending at the LQB/EBL interface and reduces effective barrier height of EBL for electrons. The AlInGaN/AlInGaN lattice matched superlattice (LMSL) EBL is designed to suppress electron leakage while mitigating lattice mismatch between LQB and EBL. The results indicate that the utilization of QDs and LMSL EBL contributes to increasing the electron and hole concentration in the active region, reducing electron leakage, enhancing radiative recombination rate, and reducing turn-on voltage. The efficiency droop caused by electron leakage is mitigated. When the injection current is 120 mA, the external quantum efficiency is increased to 9.3% and the output power is increased to 38.3 mW. This paper provides a valuable reference for addressing the challenges of insufficient hole injection and severe electron leakage.

Improved external quantum efficiency of deep UV LEDs with an ultra-thin AlGaN last quantum barrier by controlling the desorption-kinetics process

An ultra-thin AlGaN last quantum barrier (LQB) with a high Al content is effective in enhancing the hole injection in AlGaN-based deep UV LEDs (DUV-LEDs). However, it is very challenging to realize the ultra-thin LQB by the metalorganic chemical vapor deposition technique. In this work, a well-defined 1.0 nm thick AlGaN LQB with unintentionally high Al content was achieved by controlling the surface desorption kinetics for the growth of multiple quantum wells. The light output power of the DUV-LEDs with 1 nm thick LQB was increased by 44% to 7.16 mW at 40 mA, and the peak external quantum efficiency reached 4.04%.

Simulation of performance enhancement of GaN-based VCSELs by composition gradient InGaN last-quantum barrier

Electron leakage in the active region decreases the internal quantum efficiency and impedes the performance of gallium nitride (GaN)-based vertical-cavity surface-emitting lasers (VCSELs). In this study, we propose a novel InGaN last-quantum barrier (LQB) structure with gradient indium (In) composition, and the device performance was simulated by the commercial software PICS3D. Compared with the device with conventional GaN LQB, the electron leakage is greatly reduced and the hole injection efficiency is also improved by the graded LQB structure. Consequently, the threshold current is reduced by 44%, and output power is increased by 392% in GaN-based VCSEL based on composition gradient InGaN LQB. The composition gradient InGaN can also allow us to increase the thickness of the LQB in epitaxy without degrading the carrier injection efficiency due to the reduced polarization in the LQB. The results of this study suggest that the composition gradient InGaN LQB is promising for the realization of high-performance GaN-based VCSELs.

Effect of graded Al content in the last quantum barrier on the luminescence properties of ultraviolet InGaN/AlGaN multiple quantum wells

In this paper, the influence of graded Al content in the AlGaN last quantum barrier (LQB) on the luminescence properties of ultraviolet light-emitting InGaN/AlGaN multiple quantum wells (MQWs) is investigated. The average Al content in the AlGaN LQB is the same for all samples, but the distributions of Al content are different from each other. It is found that in the Al-content-graded LQB where the Al content decreases along the growth direction, a potential well for holes and a potential barrier for electrons can be formed simultaneously, causing an enhanced injection of holes and a better confine of electrons, respectively. As a result, the luminescence intensity can be improved. However, on the contrary, if the Al content in LQB varies in an inverse way, the luminescence efficiency is reduced. This can be ascribed to the inhibited hole injection as well as the increased electron leakage, as the hole barrier and electron well may be formed in LQB, where the Al content increases along the growth direction.

Performance Improvement of AlGaN‐Based Deep‐Ultraviolet Light‐Emitting Diodes with Multigradient Electron Blocking Layer and Triangular Last Quantum Barrier

The performance improvements of AlGaN‐based deep ultraviolet light‐emitting diodes (DUV‐LEDs) with multigradient electron blocking layer (EBL) and triangular last quantum barrier (LQB) are investigated. The results show that the maximum internal quantum efficiency (IQE) is improved by 44.9% and light output power (LOP) is improved by 58.1% at 200 A cm−2, exhibiting a tremendous improvement compared to the conventional structure. These improvements are mainly attributed to the fact that both multigradient EBL and triangular LQB can generate negative polarization charges in the graded composition layer, which leads to a significant increase in hole concentration compared to the conventional EBL and LQB structures. Meanwhile, the multigradient EBL can transform the conventional triangular barrier into an arc‐shaped barrier, increasing the electron potential barrier height and decreasing the hole potential barrier height. This unique design suppresses electron leakage and enhances hole injection, leading to a significant enhancement of the IQE and alleviation of the efficiency droop.

Theoretical Optical Output Power Improvement of InGaN-Based Violet Laser Diode Using AlGaN/GaN Composite Last Quantum Barrier.

Electron leakage has an adverse influence on the optical output power for laser diodes (LDs), especially where the conventional last quantum barrier (LQB) in the multiple quantum well (MQW) active region may cause severe leakage problems. In this article, a composite last quantum barrier (CLQB) composed of p-type doped AlGaN (p-AlGaN) and unintentionally doped GaN (u-GaN) layers is designed to replace the conventional one, for overcoming the problem of electron overflow. Theoretical calculations with LASTIP software demonstrate that CLQB with optimized parameters of Al composition, thickness and p-type doping concentration of the p-AlGaN layer in the CLQB can have a 50% improvement in slope efficiency (SE) compared with the conventional structure LD. This will help to realize a higher optical output power in InGaN-based violet LDs.

Carrier confinement in 232 nm emission AlGaN-based ultraviolet light-emitting diodes with p-AlN layer

The influence of the p-AlN layer on the electron leakage, in 232 nm ultraviolet wavelength light-emitting diodes, has been numerically investigated. We sandwiched last quantum barrier (LQB) with p-AlN layer to lower effective barrier heights in the p-region of the device. The simulation results demonstrate that employing p-AlN not only enhances hole concentration but also suppresses electron leakage notably. Employing p-AlN layer, after LQB, works as an efficient p-EBL due to its higher effective conduction band offset. LED with p-AlN layer exhibits almost no droop. Based on these results, we believe that this study may provide a feasible approach for the development of efficient 232 nm deep ultraviolet (DUV) LEDs, which is a crucial wavelength in the disinfection processes.

High radiative recombination in GaN-based yellow light-emitting diodes

The radiative recombination rate is greatly enhanced by the conversion of GaN last quantum barrier (LQB) into aluminium gallium nitride (AlGaN) LQB and further into graded AlGaN electron blocking layer (EBL) in GaN-based yellow light-emitting diode (LED) with emission between 510 nm and 580 nm. Our simulation results reveal an increase in carrier concentration and, therefore, reduction in electron-hole asymmetry in multiple quantum wells (MQWs). The proposed structure demonstrates that in MQWs, the chance of the electron-hole (e-h) overlapping increases. When compared to conventional LED, the radiative recombination increases by 70%, while the hole concentration increases by 20%. In our proposed structure, the turn-on voltage is also decreased from 3.2 V to 2.9 V

Enhanced Carrier Injection in AlGaN-Based Deep Ultraviolet Light-Emitting Diodes by Polarization Engineering at the LQB/p-EBL Interface

The external quantum efficiency (EQE) of AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) is still far from satisfactory due to the main issues of electron leakage and insufficient hole injection. The positive sheet charges generated by polarization at the interface between the last quantum barrier (LQB) and the p-type electron blocking layer (p-EBL) can induce electron accumulation and hole depletion in the vicinity of this interface, leading to electron leakage and hindering the hole injection. In this paper, we propose an Al-composition-increasing AlGaN layer (ACI-AlGaN) inserted between the LQB and p-EBL to enhance the carrier injection ability of DUV LEDs via modulating the sheet charges generated by polarization at the LQB/EBL interface and the underlying mechanism was analyzed by numerical calculation. The inserted structure can eliminate the positive sheet charges at the p-side interface of LQB and induce hole accumulation in the vicinity of the n-side interface of p-EBL, which can subsequently reduce electron leakage and favor hole injection respectively. The proposed DUV LED structure with an ACI-AlGaN layer exhibited enhanced EQE by 45.7% and its forward voltage maintained undegraded. This design scheme can provide an alternative way to promote the performance of DUV LEDs with a variety of applications.

Polarization Modulation at Last Quantum Barrier for High Efficiency AlGaN-Based UV LED

The performance of AlGaN-based light-emitting diodes (LEDs) emitting at UVA–UVC regions can be severely compromised due to the polarization difference (ΔP) between the last quantum barrier (LQB) and the electron blocking layer (EBL). In this work, the different situations of the bandgap difference (ΔE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> ) and ΔP of InAlN/AlGaN and AlGaN/AlGaN heterojunctions fully strained on GaN and AlN substrates are discussed. It shows that the InAlN/AlGaN heterojunctions could produce positive or negative sheet charges at the heterointerface under ΔE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> >0, which could not be realized by the conventional AlGaN/AlGaN heterojunctions. To demonstrate and utilize the feature, the polarization-modulated InAlN LQBs with 0.14–0.16 indium compositions of 320 nm UVB LEDs are designed and investigated. It is observed that the InAlN LQBs could replace the conventional AlGaN LQB to improve electron confinement and hole injection by affecting effective barrier heights. By modulating the LQB/EBL polarization using InAlN, the proposed UV LED has a 32% enhancement in internal quantum efficiency and lower efficiency droop (from 16.9% to 0.7%) compared with the conventional one without modulation. The operation voltage at the same current also significantly decreases. The improvement of optical output power and wall plug efficiency at 60 mA in proposed structures are near 90% and 100%, respectively. This study provides a novel and highly effective methodology for development of high efficiency UV LEDs.

Performance improvement of InGaN LEDs by using strain compensated last quantum barrier and electron blocking layer

The influence of u-AlGaN/InGaN superlattices (SLs) last quantum barrier (LQB) and p-AlGaN/InGaN SLs electron blocking layer (EBL) on the performance of InGaN light-emitting diodes (LEDs) are analyzed experimentally and theoretically. Research results indicated that LED C with u-AlGaN/InGaN SLs LQB and p-AlGaN/InGaN SLs EBL exhibit the highest output power among the three groups of LED samples. At 200 mA, the output power of LED C is enhanced by 27.5% as compared with that of reference LED A with u-GaN LQB and p-AlGaN/GaN SLs EBL. The efficiency droop of LED C has also been suppressed effectively. At 200 mA, the efficiency droop of LED C was 28.5%, which was smaller than that of LED A 34%. By analyzing the distribution of electron/hole concentration in multiple quantum wells, it is indicated that the leakage of electrons from the active layer can be effectively reduced and the injection efficiency of holes into the active layer can be dramatically enhanced by using strain compensated AlGaN/InGaN SLs LQB and EBL. The reason is that the potential energy barriers for electrons and holes could be effectively regulated by using the strain compensated LQB and EBL, which is consistent with simulation results of energy band diagram.

Remarkable efficiency improvement in AlGaN-based ultraviolet light-emitting diodes using graded last quantum barrier

The optical properties of AlGaN-based deep ultraviolet light-emitting diodes (DUV LED) with step-graded AlInGaN last quantum barrier (LQB) are numerically studied. In contrast to conventional LQB, step-quaternary-graded last quantum barrier (QGLQB) exhibits enhancement in the internal quantum efficiency (IQE). This is attributed to the reduction in lattice mismatch, leading to suppressed leakage of electrons and enhanced hole transportation into the multiquantum well (MQW). Electrons are enhanced by ~124% whereas holes are enhanced by ~22% in the QGLQB structure. Moreover, the efficiency droop is also reduced from ~77% (conventional structure) to ~8% (proposed structure).

Sheet Charge Engineering Towards an Efficient Hole Injection in 290 nm Deep Ultraviolet Light-Emitting Diodes

The hole injection efficiency is one of the bottlenecks that restrict the external quantum efficiency (EQE) and optical power of AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs). The polarization-induced positive sheet charges at the last quantum barrier (LQB)&#x002F;electron blocking layer (EBL) interface reflect the holes back to the p-type layer and weaken the hole injection capability into the active region. In this work, we designed and incorporated a polarization-engineered Al_xGa_1-xN&#x002F;Al_yGa_1-yN superlattice layer at the LQB&#x002F;EBL interface. The positive sheet charges at the LQB&#x002F;EBL interface can be inverted into negative charges with optimal Al compositions in the Al_xGa_1-xN&#x002F;Al_yGa_1-yN superlattice layer. The electron confinement and hole injection efficiency can also be improved through increasing the effective barrier height for electrons and decreasing the effective barrier height for holes, resulting in an enhanced optical power by 29.4&#x0025; and alleviated efficiency droop by 78.4&#x0025; for the proposed device with an Al_0.67Ga_0.33N&#x002F;Al_0.7Ga_0.3N superlattice insertion layer. The sheet charge engineering method by polarization provides an alternative approach to boost the hole injection efficiency towards an enhanced device performance for DUV LEDs.

On the enhancement of carrier injection efficiency by employing AlInN last quantum barrier in 277 nm ultraviolet light-emitting diodes

The AlInN last quantum barrier (LQB) has been employed instead of conventional AlGaN in ultraviolet light-emitting diodes (UV LEDs). The simulation results reveal that p-doped AlInN and un-doped AlInN LQB effectively suppressed electron leakage and facilitated the movement of holes to the active zone, which is greatly attributed to the higher conduction band offset and lower valence band offset of the AlInN alloy. Moreover, AlInN has reduced the lattice mismatching, leading to a decrease in the induced polarization field. Hence, the internal quantum efficiency (IQE) and the radiative recombination rate, with the AlInN LQB, has been increased by 54% and 260%, respectively, as compared with conventional AlGaN-based LQB LEDs. Importantly, the AlInN LQB not only increased the IQE but also alleviated the efficiency droop dramatically. Additionally, the emission spectrum at 277 nm has been remarkably enhanced as compared with conventional LEDs. Thus, based on these results, it was found that the un-doped AlInN LQB is a feasible route for achieving efficient UV LEDs.

Reducing the polarization mismatch between the last quantum barrier and p-EBL to enhance the carrier injection for AlGaN-based DUV LEDs

In this work, we report an AlGaN-based ∼275 nm deep ultraviolet light-emitting diode (DUV LED) that has AlGaN based quantum barriers with a properly large Al composition. It is known that the increased conduction band barrier height helps to enhance the electron concentration in the active region. However, we find that the promoted hole injection efficiency is also enabled for the proposed DUV LED when the Al composition increases. This is attributed to the reduced positive polarization charge density at the last quantum barrier (LQB) and p-type electron blocking layer (p-EBL) interface, which can suppress the hole depletion effect in the p-EBL. Thus, the hole concentration in the p-EBL gets promoted, which is very helpful to reduce the hole blocking effect caused by the p-EBL. Therefore, thanks to the improved carrier injection, the proposed DUV LED increases the optical power and reduces the forward voltage when compared with the conventional DUV LED.

The marvelous optical performance of AlGaN-based deep ultraviolet light-emitting diodes with AlInGaN-based last quantum barrier and step electron blocking layer.

The optoelectronic characteristics of AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) with quaternary last quantum barrier (QLQB) and step-graded electron blocking layer (EBL) are investigated numerically. The results show that the internal quantum efficiency (IQE) and radiative recombination rate are remarkably improved with AlInGaN step-graded EBL and QLQB as compared to conventional or ternary AlGaN EBL and last quantum barrier (LQB). This significant improvement is assigned to the optimal recombination of electron–hole pairs in the multiple quantum wells (MQWs). It is due to the decrease in strain and lattice mismatch between the epi-layers which alleviates the effective potential barrier height of the conduction band and suppressed the electron leakage without affecting the holes transportation to the active region. Moreover, to figure out quantitatively, the electron and hole quantity increased by ~ 25% and ~ 15%, respectively. Additionally, the IQE and radiative recombination rate are enhanced by 48% and 55%, respectively, as compared to conventional LED. So, we believe that our proposed structure is not only a feasible approach for achieving highly efficient DUV LEDs, but the device physics presented in this study establishes a fruitful understanding of III nitride-based optoelectronic devices.

Polarization-engineered AlGaN last quantum barrier for efficient deep-ultraviolet light-emitting diodes

AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) have been identified as a prospective mercury-free UV source. However, the observation of severe electron overflow and low hole injection efficiency in the conventional DUV LED deteriorates the device performance, attributing to the downward band bending as a result of the strong polarization-induced electric fields between the last quantum barrier (LQB) and the electron blocking layer (EBL). In this study, a composition-graded AlGaN layer with linearly increasing of Al composition from 0.5 to 0.65 is proposed to act as the LQB, replacing the conventional flat LQB to reduce the effective barrier height for hole injection while improving the electron blocking ability. Hence, a considerable enhancement of the output power can be obtained. Moreover, further investigations show that the thickness of graded LQB determine the band bending in the LQB and thus significantly suppress the electron leakage, eventually leading to a boosted output power. The thorough investigation on the LQB could pave the way towards the realization of efficient DUV LEDs of the future.

Improving carrier transport in AlGaN deep-ultraviolet light-emitting diodes using a strip-in-a-barrier structure.

This paper reports the illustration of electron blocking layer (EBL)-free AlGaN light-emitting diodes (LEDs) operating in the deep-ultraviolet (DUV) wavelength at ∼270nm. In this work, we demonstrated that the integration of an optimized thin undoped AlGaN strip layer in the middle of the last quantum barrier (LQB) could generate enough conduction band barrier height for the effectively reduced electron overflow into the p-GaN region. Moreover, the hole injection into the multi-quantum-well active region is significantly increased due to a large hole accumulation at the interface of the AlGaN strip and the LQB. As a result, the internal quantum efficiency and output power of the proposed LED structure has been enhanced tremendously compared to that of the conventional p-type EBL-based LED structure.

Systematical and Numerical Investigations on InGaN-Based Green Light-Emitting Diodes: Si-Doped Quantum Barriers, Engineered p-Electron Blocking Layer and AlGaN/GaN Structured p-Type Region

Abstract Here, we numerically prove that the severe polarization-induced electric field in the active region for [0001]-oriented InGaN-based green light-emitting diodes (LEDs) is reduced when heavily Si-doped GaN quantum barriers are adopted. However, the electron injection is accordingly sacrificed for the insufficient confinement capability of the p-type electron blocking layer (p-EBL). Hence, p-EBL structures with/without gradient AlN alloys are discussed to reduce the electron leakage, and the importance of the positive sheet polarization charges at the interface between the last quantum barrier (LQB) and the p-EBL on affecting the blocking barrier height for electrons is especially highlighted. Moreover, we also suggest utilizing specially designed p-AlGaN/p-GaN structure instead of p-GaN structure to serve as the p-type hole supplier, which is able to increase the kinetic energy of holes, thus assisting holes to overcome the energy band barrier height in p-EBL and increasing the hole concentration within the quantum wells.

Improved Output Power of GaN-based VCSEL with Band-Engineered Electron Blocking Layer.

The vertical-cavity surface-emitting laser (VCSEL) has unique advantages over the conventional edge-emitting laser and has recently attracted a lot of attention. However, the output power of GaN-based VCSEL is still low due to the large electron leakage caused by the built-in polarization at the heterointerface within the device. In this paper, in order to improve the output power, a new structure of p-type composition-graded AlxGa1−xN electron blocking layer (EBL) is proposed in the VCSEL, by replacing the last quantum barrier (LQB) and EBL in the conventional structure. The simulation results show that the proposed EBL in the VCSEL suppresses the leaking electrons remarkably and contributes to a 70.6% increase of the output power, compared with the conventional GaN-based VCSEL.