Characteristics of deep ultraviolet AlGaN-based light emitting diodes with p-hBN layer

The AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) with p-hBN layer are investigated numerically. In comparison with the conventional AlGaN DUV LEDs, the proposed LED can significantly improve the carrier injection, radiative efficiency, as well as the electroluminescence (EL) intensity under the same applied forward bias. Simultaneously, the light extraction efficiency in the LED using p-hBN instead of p-AlGaN exhibits a more than 250% increase at the applied voltage of 7.5 V due to the smaller loss of reflection and absorption of the emitted light.     

Hyperentanglement source by intersubband two-photon emission from semiconductor quantum wells

We propose an efficient hyperentanglement source emitting photon pairs entangled in both energy and polarization. The compact electrically driven room-temperature source, based on intersubband two-photon emission from semiconductor quantum wells (QWs) exhibits pair generation rates several orders of magnitude higher than alternative conventional schemes. A theoretical formalism is derived for the calculation of photon pair generation spectra and rates. The results are presented for superlattice structures similar to quantum cascade lasers of GaAs/AlGaAs QWs emitting in the mid-IR and far-IR and for InN/AlN QW structures suitable for telecommunication wavelengths.     

具有三角形InGaN/GaN多量子阱的高内量子效率的蓝光LED(英文)

对InGaN量子阱LED的内量子效率进行了优化研究。分别对发光光谱、量子阱中的载流子浓度、能带分布、静电场和内量子效应进行了理论分析。对具有不同量子阱数量的InGaN/GaN LED进行了理论数值比对研究。研究结果表明,对于传统结构的LED而言,2个量子阱的结构相对于5个和7个量子阱具有更好的光学性能。同时还研究了具有三角形量子阱结构的LED,研究结果显示,三角形多量子阱结构具有较高的电致发光强度、更高的内量子效率和更好的发光效率,所有的优点都归因于较高的电子-空穴波函数重叠率和低的Stark效应所产生的较高的载流子输入效率和复合发光效率。     

Optimization of a GaN-based irregular multiple quantum well structure for a dichromatic white LED

GaN-based irregular multiple quantum well(IMQW) structures assembled two different types of QWs emitting complementary wavelengths for dichromatic white light-emitting diodes(LEDs) are optimized in order to obtain near white light emissions.The hole distributions and spontaneous emission spectra of the IMQW structures are analysed in detail by fully considering the effects of strain,well-coupling,valence band-mixing and polarization effect through employing a newly developed theoretical model from the k · p theory.Several structure parameters such as well material component,well width,layout of the wells and the thickness of barrier between different types of QWs are employed to analyse how these parameters together with the polarization effect influence the electronic and the optical properties of IMQW structure.Numerical results show that uniform hole distributions in different types of QWs are obtained when the number of the QWs emitting blue light is two,the number of the QWs emitting yellow light is one and the barrier between different types of QWs is 8nm in thickness.The near white light emission is realized using GaN-based IMQW structure with appropriate design parameters and injection level.     

Effect of surface plasmon coupling with radiating dipole on the polarization characteristics of AlGaN-based light-emitting diodes

The optical polarization characteristics of surface plasmon (SP) coupled AlGaN-based light emitting diodes (LEDs) are investigated theoretically by analyzing the radiation recombination process and scattering process respectively. For the Al_0.5Ga_0.5N/Al/Al₂O₃ slab structure, the relative intensity of TE-polarized and TM-polarized spontaneous emission (SE) rate into the SP mode obviously depends on the thickness of the Al layer. The calculation results show that TM dominated emission will be transformed into TE dominated emission with the decrease of the Al thickness, while the emission intensities of both TE/TM polarizations will decrease significantly. In addition, compared with TM polarized emission, TE polarized emission is easier to be extracted by SP coupling. For the Al_0.5Ga_0.5N/Al nano-particle structure, the ratio of transmittance for TE/TM polarized emission can reach ～3.06, while for the Al free structure, it is only 1.2. Thus, the degree of polarization of SP coupled LED can be improved by the reasonable structural design.     

Investigation of InGaN green light-emitting diodes with chirped multiple quantum well structures

The effect of using chirped multiple quantum-well (MQW) structures in InGaN green light-emitting diodes (LEDs) is numerically investigated. An active structure, which is with both thick QWs with low indium composition on the p-side and thin QWs with high indium composition next to the n-region, is presented in this study. The thickness and indium composition in each single QW is specifically tuned to emit the same green emission spectrum. Comparing with conventional active structure design of green LEDs, which is using uniform MQWs, the output power is increased by 27% at 20 mA, and by 15% at 100 mA current injections. This improvement is mainly attributed to the enhanced efficiency of carrier injection into QWs and the improved capability of carrier transport.     

Theoretical analysis of semi/non-polar In GaN/GaN light-emitting diodes grown on silicon substrates

A theoretical study of polar and semi/non-polar In Ga N/Ga N light-emitting diodes(LEDs) with different internal surface polarization charges, which can be grown on Si substrates, is conducted by using APSYS software. In comparison with polar structure LEDs, the semi-polar structure exhibits a higher concentration of electrons and holes and radiative recombination rate, and its reduced built-in polarization field weakens the extent of band bending which causes the shift of peak emission wavelength. So the efficiency droop of semi-polar In Ga N/Ga N LEDs declines obviously and the optical power is significantly improved. In comparison with non-polar structure LEDs, although the concentration of holes and electrons as well as the radiative recombination rate of the semi-polar structure are better in the last two quantum wells(QWs) approaching the p-Ga N side, the uniformity of distribution of carriers and radiative recombination rate for the nonpolar structure is better. So the theoretical analysis indicates that the removal of the internal polarization field in the MQWs active regions for non-polar structure LEDs contributes to the uniform distribution of electrons and holes, and decreases the electron leakage. Thus it enhances the radiative recombination rate, and further improves the IQEs and optical powers, and shows the best photoelectric properties among these three structures.     

Enhanced carrier injection in InGaN/GaN multiple quantum wells LED with polarization-induced electron blocking barrier

In this report, we designed a light emitting diode (LED) structure in which an N-polar p-GaN layer is grown on top of Ga-polar In_0.1Ga_0.9N/GaN quantum wells (QWs) on an n-GaN layer. Numerical simulation reveals that the large polarization field at the polarity inversion interface induces a potential barrier in the conduction band, which can block electron overflow out of the QWs. Compared with a conventional LED structure with an Al_0.2Ga_0.8N electron blocking layer (EBL), the proposed LED structure shows much lower electron current leakage, higher hole injection, and a significant improvement in the internal quantum efficiency (IQE). These results suggest that the polarization induced barrier (PIB) is more effective than the AlGaN EBL in suppressing electron overflow and improving hole transport in GaN-based LEDs.     

Cascaded deep ultraviolet light-emitting diode via tunnel junction

The Al Ga N-based deep ultraviolet(DUV) light-emitting diode(LED) is an alternative DUV light source to replace traditional mercury-based lamps. However, the state-of-the-art DUV LEDs currently exhibit poor wall-plug efficiency and low light output power, which seriously hinder their commercialization. In this work, we design and report a tunnel-junctioncascaded(TJC) DUV LED, which enables multiple radiative recombinations within the active regions. Therefore, the light output power of the TJC-DUV LEDs is more than doubled compared to the conventional DUV LED. Correspondingly, the wall-plug efficiency of the TJC-DUV LED is also significantly boosted by 25% at 60 m A.     

量子阱层和垒层具有不同Al组分的270/290/330nm AlGaN基深紫外LED光电性能

为了研究AlGaN量子阱层和垒层中Al组分不同对AlGaN基深紫外发光二极管(LED)光电性能的影响,本文利用MOCVD生长、光刻和干法刻蚀工艺制备了AlGaN量子阱层和垒层具有不同Al组分的270/290/330nm深紫外LED,通过实验和数值模拟计算方法发现,量子阱层和垒层中具有低Al组分紫外LED的AlGaN材料具有较低的位错密度、较高的光输出功率和外量子效率。通过电流-电压(I-V)曲线拟合出的较大的理想因子(3.5)和能带结构图表明,AlGaN深紫外LED的电流产生是隧穿机制占据主导作用,这是因为高Al组分AlGaN量子阱中强极化场造成了有源层区域较大的能带弯曲和电势降。     

Enhanced performance of GaN-based light-emitting diodes with InGaN/GaN superlattice barriers

GaN-based multiple quantum well light-emitting diodes （LEDs） with conventional and superlattice barriers have been investigated numerically. Simulation results demonstrate using InGaN/GaN superlattices as barriers can effectively enhance performances of the GaN-Based LEDs, mainly owing to the improvement of hole injection and transport among the MQW active region. Meanwhile, the improved electron capture decreases the electron leakage and alleviates the efficiency droop. The weak polarization field induced by the superlattice structure strengthens the intensity of the emission spectrum and leads to a blue-shift relative to the conventional one.     

Performance improvement of AlGaN-based deep ultraviolet light-emitting diodes with double electron blocking layers

The AlGaN-based deep ultraviolet light-emitting diodes(LED) with double electron blocking layers(d-EBLs) on both sides of the active region are investigated theoretically. They possess many excellent performances compared with the conventional structure with only a single electron blocking layer, such as a higher recombination rate, improved light output power and internal quantum efficiency(IQE). The reasons can be concluded as follows. On the one hand, the weakened electrostatic field within the quantum wells(QWs) enhances the electron–hole spatial overlap in QWs, and therefore increases the probability of radioactive recombination. On the other hand, the added n-AlGaN layer can not only prevent holes from overflowing into the n-side region but also act as another electron source, providing more electrons.     

ZnO-based deep-ultraviolet light-emitting devices

Deep-ultraviolet(DUV) light-emitting devices(LEDs) have a variety of potential applications.Zinc-oxide-based materials,which have wide bandgap and large exciton binding energy,have potential applications in high-performance DUV LEDs.To realize such optoelectronic devices,the modulation of the bandgap is required.This has been demonstrated by the developments of Mg_xZn_(1-x)O and Be_xZn_(1-x)O alloys for the larger bandgap materials.Many efforts have been made to obtain DUV LEDs,and promising successes have been achieved continuously.In this article,we review the recent progress of and problems encountered in the research of ZnO-based DUV LEDs.     

A dual-blue light-emitting diode based on strain-compensated InGaN-AlGaN/GaN quantum wells

A strain-compensated InGaN quantum well(QW) active region employing a tensile AlGaN barrier is analyzed.Its spectral stability and efficiency droop for a dual-blue light-emitting diode(LED) are improved compared with those of the conventional InGaN/GaN QW dual-blue LEDs based on a stacking structure of two In0.18Ga0.82N/GaN QWs and two In0.12Ga0.88N/GaN QWs on the same sapphire substrate.It is found that the optimal performance is achieved when the Al composition of the strain-compensated AlGaN layer is 0.12 in blue QW and 0.21 in blue-violet QW.The improvement performance can be attributed to the strain-compensated InGaN-AlGaN/GaN QW,which can provide a better carrier confinement and effectively reduce leakage current.     

Optical polarization characteristics for AlGaN-based light-emitting diodes with AlGaN multilayer structure as well layer

The optical properties of AlGaN-based quantum well(QW) structure with two coupled thin well layers are investigated by the six-by-six K-P method.Compared with the conventional structure,the new structure,especially the one with lower Al-content in the barrier layer,can enhance the TE-/TM-polarized total spontaneous emission rate due to the strong quantum confinement and wide recombination region.For the conventional QW structure,the reduction of well thickness can lead the degree of polarization(DOP) to decrease and the internal quantum efficiency(IQE) to increase.By using the coupled thin well layers,the DOP for the structure with high Al-content in the barrier layer can be improved,while the DOP will further decrease with low Al-content in the barrier layer.It can be attributed to the band adjustment induced by the combination of barrier height and well layer coupling.The IQE can also be further enhanced to 14.8%-20.5% for various Al-content of barrier layer at J=100 A/cm~2.In addition,the efficiency droop effect can be expected to be suppressed compared with the conventional structure.     

Effects of Mg doping in the quantum barriers on the efficiency droop of GaN based light emitting diodes

The effects of Mg doping in the quantum barriers(QBs) on the efficiency droop of GaN based light emitting diodes(LEDs) were investigated through a duel wavelength method. Barrier Mg doping would lead to the enhanced hole transportation and reduced polarization field in the quantum wells(QWs), both may reduce the efficiency droop. However,heavy Mg doping in the QBs would strongly deteriorate the crystal quality of the QWs grown after the doped QB. When increasing the injection current, the carriers would escape from the QWs between n-GaN and the doped QB and recombine non-radiatively in the QWs grown after the doped QB, leading to a serious efficiency droop.     

Efficiency droop suppression in GaN-based light-emitting diodes by chirped multiple quantum well structure at high current injection

Gallium nitride(Ga N) based light-emitting diodes(LEDs) with chirped multiple quantum well(MQW) structures have been investigated experimentally and numerically in this paper. Compared to conventional LEDs with uniform quantum wells(QWs), LEDs with chirped MQW structures have better internal quantum efficiency(IQE) and carrier injection efficiency. The droop ratios of LEDs with chirped MQW structures show a remarkable improvement at 600 m A/mm2,reduced down from 28.6%(conventional uniform LEDs) to 23.7%(chirped MQWs-a) and 18.6%(chirped MQWs-b),respectively. Meanwhile, the peak IQE increases from 76.9%(uniform LEDs) to 83.7%(chirped MQWs-a) and 88.6%(chirped MQWs-b). The reservoir effect of chirped MQW structures is the significant reason as it could increase hole injection efficiency and radiative recombination. The leakage current and Auger recombination of chirped MQW structures can also be suppressed. Furthermore, the chirped MQWs-b structure with lower potential barriers can enhance the reservoir effect and obtain further improvement of the carrier injection efficiency and radiative recombination, as well as further suppressing efficiency droop.     

Electroluminescence from the InGaN/GaN Superlattices Interlayer of Yellow LEDs with Large V-Pits Grown on Si(111)

A blue emission originated from In GaN/GaN superlattice(SL) interlayer is observed in the yellow LEDs with V-pits embedded in the quantum wells(QWs), revealing that sufficient holes have penetrated through the QWs into SLs far away from the p-type layer. In the V-pits embedded LEDs, hole transport has two paths: via the flat c-plane region or via the sidewalls of V-pits. It is proved that the holes in SLs are injected from the sidewalls of V-pits, and the transportation process is significantly affected by working temperature, current density, and the size of V-pits. Four motion possibilities are discussed when the holes flow via the sidewalls. All these may contribute to a better understanding of hole transport and device design.     

Numerical study on the wavelength-dependence of light extraction efficiency in AlGaN-based ultraviolet light-emitting diodes

Wavelength-dependence of light extraction efficiency (LEE) in AlGaN-based ultraviolet (UV) light-emitting diode (LED) structures is numerically studied based on three-dimensional finite-difference time-domain methods. Due to strong UV light absorption in the p-GaN contact layer, LEE of the UV vertical LED structures remains to be only 6–7 % for the transverse-electric mode and 2–3 % for the transverse-magnetic mode, respectively. The effective LEE of UV LEDs is calculated by considering the optical polarization-dependent LEE, and is found to increase from 4 to 5.5 % as the wavelength increases from 260 to 360 nm. It is shown that the wavelength-dependence of LEE can partially explain the decrease in external quantum efficiency with decreasing wavelengths in AlGaN-based UV LEDs.     

Modeling of color-coded III-nitride LED structures with deep quantum wells

We present, to the best of our knowledge, the first successful simulation of color-coded III-nitride light-emitting diodes (LEDs) incorporating in their active regions shallow and deep InGaN quantum wells (QWs). Dichromatic violet–aquamarine semipolar LEDs grown in Ga-polar and N-polar crystallographic orientations (Kawaguchi et al. in Appl Phys Lett 100:231110–231114, 2012) were used as an experimental benchmark. Opposite interface polarization charges in Ga-polar and N-polar LEDs provide different conditions for carrier transport and account for different shape of color-coded emission spectra. To reproduce experimentally observed trends, several effects specific for deep III-nitride QWs were essential in our modeling including \((1)\) strongly non-equilibrium character of active QW populations, \((2)\) dynamic carrier overshoot of narrow QW layers, and \((3)\) Auger-assisted QW depopulation.