Electron Blocking Layer Research Articles

Improving the hole injection efficiency in AlGaN DUV LEDs by minimizing the band offset at the p-EBL/hole supplier interface

In AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs), the large valence band offset between the Al-rich electron blocking layer (EBL) and p-AlGaN hole supplier weakens the chance of holes being injected into the active region. Only holes with kinetic energy larger than the barrier height at the EBL/p-AlGaN interface are allowed to climb over the EBL before entering the active region, limiting the hole injection efficiency and thus reducing the external quantum efficiency (EQE). In this work, we incorporate a thin AlGaN insertion layer between EBL and the p-AlGaN hole supplier to enhance the hole injection efficiency of DUV LEDs via regulating the energy band at the p-EBL/p-AlGaN interface. By systematically investigating and analyzing the effects of aluminum components in the insertion layers on the hole injection and the electron confinement, we found that the insertion layer with an Al composition of 45% can effectively enhance the EQE of DUV LEDs by 40.5% and suppress efficiency droop by 65.5%. The design strategy provides an effective approach to boost the hole injection efficiency for AlGaN-based DUV LEDs.

Optimization of AlGaN-based deep UV LED performance by p-AlInGaN/AlInGaN graded superlattice electron blocking layer.

In this paper, we significantly improved the internal quantum efficiency and output power of AlGaN-based deep UV (DUV) LEDs by replacing the conventional p-AlGaN electron blocking layer (EBL) with the p-AlInGaN/AlInGaN graded superlattice (SL) EBL. Simulation results show that the introduction of the p-AlInGaN graded SL EBL improved the carrier distribution while having the lower electric field, thus increasing the radiative recombination rate in multiple quantum wells (MQWs). The highest IQE obtained by p-AlInGaN/AlInGaN graded SL EBL is 96.6%, which is 44.9% higher than the conventional p-AlGaN EBL with no efficiency droop. At the same time, the output power is 4.6 times that of the conventional p-AlGaN EBL. It is believed that the proposed p-AlInGaN graded SL EBL will be helpful in the development of high-performance DUV LEDs.

Simultaneously improved hole injection and current uniformity in 293 nm AlGaN-based deep ultraviolet light-emitting diodes

Insufficient hole injection and current nonuniformity caused by the nature of the p-AlGaN hole injection layer (HIL) are two issues impeding the advances in the optical and electrical properties of AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs). To simultaneously improve the hole injection efficiency and current uniformity, an Al-linearly-decreasing AlGaN PN junction (ALD AlGaN PN junction) is proposed to replace the conventional p-AlGaN HIL with a constant Al component. The barrier height for holes at the interface between the p-type electron blocking layer (p-EBL) and HIL as well as the hole concentration and resistance in the p-AlGaN HIL can be modulated at the same time by the proposed ALD AlGaN PN junction. As a result, the hole injection efficiency is enhanced due to the reduced barrier height at the p-EBL/HIL interface and the polarization-induced three-dimensional hole gas (3DHG) induced in the proposed ALD AlGaN PN junction HIL. In addition, the lateral current uniformity is improved by the properly regulated resistance in the p-AlGaN HIL, which can be ascribed to the additional barrier for holes in the valence band because of the incorporated ALD AlGaN PN junction. The proposed DUV LED with ALD AlGaN PN junction exhibits enhanced EQE by a factor of 39.2% at 100 A/cm2 and simultaneously improved current uniformity in the active region. This designed ALD AlGaN PN junction structure provides a promising strategy for achieving high-performance DUV LEDs.

Resonant Tunneling Effects on the Double‐Barrier Electron Blocking Layer of a Nitride Deep‐UV Light‐Emitting Diode

The multi‐quantum‐barrier electron blocking layer (EBL) is reported to significantly improve efficiency by nearly 3 times over a single barrier in deep‐UV AlGaN light‐emitting diodes to deal with electron leakage. The improvement is usually attributed to the enhanced effective barrier height, and this article aims to explore the benefits of the tunneling effect by calculating the tunneling currents of electrons and holes through an Al0.6Ga0.4N/AlyGa1−yN double‐barrier EBL under external bias from opposite directions. The results show that the tunneling current for holes Jh is several orders of magnitude higher than that of the electrons Je as the barriers are with Al mole fraction y greater than 0.75 and thickness larger than 2 nm, which promises effective hole injection by tunneling without much electron leakage. Tunneling mechanism works better in EBL with higher and thicker barriers because the tunneling coefficients of light hole drop much slower than electrons due to its small effective mass. A proper distance between the barriers is needed to avoid electron leakage while holes tunnel through the EBL. Built‐in electric fields tilt the band to enlarge the peak‐to‐valley ratio. This work indicates that the tunneling effect substantially facilitates a multibarrier EBL to enhance carrier‐injection efficiency.

Operando ESR observation in thermally activated delayed fluorescent organic light-emitting diodes

Organic light-emitting diodes (OLEDs) using thermally activated delayed fluorescence (TADF) materials have advantages over OLEDs using conventional fluorescent materials or high-cost phosphorescent materials, including higher efficiency and lower cost. To attain further high device performance, clarifying internal charge states in OLEDs at a microscopic viewpoint is crucial; however, only a few such studies have been performed. Here, we report a microscopic investigation into internal charge states in OLEDs with a TADF material by electron spin resonance (ESR) at a molecular level. We observed operando ESR signals of the OLEDs and identified their origins due to a hole-transport material PEDOT:PSS, gap states at an electron-injection layer, and a host material CBP in the light-emitting layer by performing density functional theory calculation and studying thin films used in the OLEDs. The ESR intensity varied with increasing applied bias before and after the light emission. We find leakage electrons in the OLED at a molecular level, which is suppressed by a further electron-blocking layer MoO3 between the PEDOT:PSS and light-emitting layer, resulting in the enhancement of luminance with a low-voltage drive. Such microscopic information and applying our method to other OLEDs will further improve the OLED performance from the microscopic viewpoint.

Thin Quaternary Layer and Staggered Electron Blocking Layers for Improved Ultraviolet Light-Emitting Diodes

We have carried out numerical investigations of AlGaN-based ultraviolet light-emitting diodes (UV LEDs). We have shown that the unequal carrier distribution, which is typical in III-N LEDs, is mitigated in the active region. To improve the performance, we have used a thin AlInGaN quaternary layer (QL) between the final quantum barrier (FQB) and the electron blocking layer (EBL). We have also compared the results with a staggered or step-graded electron blocking layer which is known to show enhanced device output. The hole insertion has been found to greatly improve in all the multiquantum wells (MQWs) of our numerically designed LEDs. The radiative recombination rate has been found to be enhanced by ∼82% in comparison to reference LED.

Red Electroluminescence from Light Emitting Diodes Based on Eu-Doped ZnO Embedded in p-GaN/Al2O3/n-ZnO Heterostructures

Heterojunction p-GaN/n-ZnO light emitting diode (LED) structure using Eu-doped ZnO (ZnO:Eu) as an active component is demonstrated in order to realize low-cost and environmentally-friendly red LEDs with sharp linewidth and temperature stability against surrounding environment including operating temperature and injection current. Chemically stable Al2O3 is inserted as an electron blocking layer between p-GaN and ZnO:Eu/n-ZnO in order to facilitate the injection of carriers into the ZnO:Eu active layer. Al2O3 insertion with a moderate thickness (∼10 nm) facilitates the carrier recombination at the ZnO layer with comparatively low resistivity. Device characteristics of the p-GaN/Al2O3/ZnO:Eu/n-ZnO LED structures show red luminescence under current injection with reversed bias voltage originated from Eu3+ ions in the ZnO host. Detailed optical characteristics of the ZnO:Eu layer in the LED structures utilizing the combined excitation emission spectroscopy measurement enable the identification of the luminescence centers contributing to Eu luminescence under both indirect excitation and collisional excitation. The luminescence center contributing to Eu luminescence under indirect excitation via the ZnO host is different from that under collisional excitation, which would pave the way to understand the Eu luminescence mechanism in ZnO:Eu, and hence realize high-brightness LED structures based on rare-Earth doped ZnO as an active component.

High-Efficiency InGaN Photo Cell Irradiated by 532 nm Laser with AlGaN Electron Blocking Layer

In this paper, a high-efficiency InGaN photo cell irradiated by 532 nm laser (at green wavelength) with AlGaN electron blocking layer (EBL) is proposed based on the blue-green light window effect of seawater. Firstly, the InGaN/GaN multiple quantum wells (MQWs) structured material intercalated with AlGaN EBL was designed and grown for InGaN photo cells. Then, by measuring the AFM, XRD and PL of the material, it is found that the insertion of AlGaN EBL can effectively reduce the defect density and improve the steepness of the interface in the active region. Further, based on the above material characteristics, the performance of the InGaN photo cells with AlGaN EBL is evaluated in Silvaco software under 532 nm laser irradiation. The results show that the introduction of AlGaN EBL in InGaN photo cell can not only decrease the non-radiative recombination rates, but also reduce the piezoelectric polarization effect, which contribute to the transport of effective photo-generated carriers and eventually improve the conversion efficiency by 13.325% compared to that with conventional structure. These findings provide critical new insights on high-efficiency GaN-based Photo Cell irradiated by 532 nm laser in the application of underwater communications.

Impedance spectroscopic analysis of long-lifetime blue fluorescent organic light emitting diode

Hole and electron carrier transport dynamics of blue fluorescent organic light emitting diodes (OLEDs) were analyzed by impedance spectroscopic technique, and the relation between lifetimes and the dynamics was investigated. The OLEDs with different hole carrier transport property were fabricated by optimizing thickness of electron-blocking layer (EBL), while the electron transport level was fixed. The EBL was arranged between emission and hole transport layers (EML and HTL), and highest occupied molecular orbital level of the EBL was between those of EML and HTL. Both Cole-Cole plot and Bode-plot of imaginary part indicated that relaxation rate of hole carrier transport for the OLED with thin EBL was larger than that for the OLED with thick EBL. The relaxation rates of electron carrier transport were the same level with each other. The lifetime of the OLED with the thin EBL was longer than that of the OLED with the thick EBL. The results give us estimation that the hole carrier injection into the EML is increased for the OLED with relatively thin EBL, inducing long lifetime due to high level of hole injection into the EML even the occurrence of electron leakage from the EML to EBL.

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.

Demonstrating the electron blocking effect of AlGaN/GaN superlattice cladding layers in GaN-based laser diodes

Electron leakage currents seriously limit the power conversion efficiencies (PCEs) of gallium nitride (GaN)-based laser diodes (LDs). To minimize the leakage currents, electron blocking layers are generally applied in the p-type region. However, few works have discussed the electron blocking effect of a p-cladding layer, which is found to be critical in suppressing the leakage currents of an LD. In this work, we compare the blocking performance of uniform AlGaN p-cladding layers and AlGaN/GaN superlattice (SL) p-cladding layers with the same average Al component respectively. Both light-emitting diodes (LEDs) and LDs with the same epitaxy structures are characterized by light–current (L–I) and current–voltage (I–V) measurements. The latest analytical model of leakage currents is applied to fit the L–I curves of LEDs, where smaller leakage coefficients are observed in the SL structures compared with the uniform-layer structures. Eighty LDs with varying ridge widths are studied by comparing the threshold current densities, slope efficiencies, and PCEs. The SL-based p-cladding layer shows statistically significant advantages over a uniform AlGaN layer. The blocking effects of both scattering- and bound-state electrons in SLs are investigated theoretically. Repetitive reflection and thermal relaxation are responsible for the blocking effect of scattering-state electrons. Simulation results indicate that the tunneling effect of bound-state electrons through a miniband mechanism is insignificant at a large injection level due to a negative differential conductivity by the Esaki–Tsu effect. We demonstrate a better electron blocking performance of p-cladding layers based on SLs than uniform AlGaN layers in GaN-based LDs.

High-Performance Graphene Nanowalls/Si Self-Powered Photodetectors with HfO2 as an Interfacial Layer.

Graphene/silicon (Si) heterojunction photodetectors are widely studied in detecting of optical signals from near-infrared to visible light. However, the performance of graphene/Si photodetectors is limited by defects created in the growth process and surface recombination at the interface. Herein, a remote plasma-enhanced chemical vapor deposition is introduced to directly grow graphene nanowalls (GNWs) at a low power of 300 W, which can effectively improve the growth rate and reduce defects. Moreover, hafnium oxide (HfO2) with thicknesses ranging from 1 to 5 nm grown by atomic layer deposition has been employed as an interfacial layer for the GNWs/Si heterojunction photodetector. It is shown that the high-k dielectric layer of HfO2 acts as an electron-blocking and hole transport layer, which minimizes the recombination and reduces the dark current. At an optimized thickness of 3 nm HfO2, a low dark current of 3.85 × 10-10, with a responsivity of 0.19 AW-1, a specific detectivity of 1.38 × 1012 as well as an external quantum efficiency of 47.1% at zero bias, can be obtained for the fabricated GNWs/HfO2/Si photodetector. This work demonstrates a universal strategy to fabricate high-performance graphene/Si photodetectors.

Operation-induced degradation mechanisms of 275-nm-band AlGaN-based deep-ultraviolet light-emitting diodes fabricated on a sapphire substrate

Degradation mechanisms of 275-nm-band AlxGa1-xN multiple quantum well deep-ultraviolet light-emitting diodes fabricated on a (0001) sapphire substrate were investigated under hard operation conditions with the current of 350 mA and the junction temperature of 105 °C. The optical output power (Po) initially decreased by about 20% within the operating time less than 102 h and then gradually decreased to about 60% by 484 h. For elucidating the causes for the initial and subsequent degradations, complementary electrical, time-resolved photoluminescence (TRPL), and impurity characterizations were carried out making a connection with the energy band profiles. Because the degradation of the wells was less significant than the Po reduction, the initial degradation is attributed essentially to the decrease in carrier injection efficiency (ηinjection), not in internal quantum efficiency of the wells, most likely due to depassivation of initially H-passivated preexisting nonradiative recombination centers (NRCs) in a Mg-doped p-type Al0.85Ga0.15N electron blocking layer. The principal cause for the subsequent Po reduction until 484 h is attributed to further decrease in ηinjection due to the appearance of certain current bypasses in addition to continuous depassivation of the NRCs in p-type AlxGa1-xN layers. According to our database on the species of vacancy-type defects acting as NRCs in GaN and AlN, which have been identified using the combination of positron annihilation and TRPL measurements, vacancy clusters comprised of a cation vacancy (VIII) and nitrogen vacancies (VN), such as VIIIVN2∼4, are the most suspicious origins of the NRCs in Mg-doped p-type AlxGa1-xN layers.

Evolution of Frenkel defect pairs in β-Ga2O3 thin film with the introduction of oxygen and its application in GaN-based ultraviolet light-emitting diode

In this work, β-Ga2O3 thin films were deposited by radio frequency (RF) magnetron sputtering technique. The effect of oxygen-argon flow ratio (OAFR) on the formation of Frenkel defect pairs was systematically analyzed. The Frenkel defect pairs would become abundant when the OAFR was 25:25 sccm, resulting in a severe damage in crystalline quality and an increase in the density of disorder. With the further increase of OAFR, Frenkel defect pairs would dramatically decrease or even vanish due to the significant reduction of oxygen vacancies (V0). Immediately after that, on the basis of avoiding the influence of Frenkel defect pairs inside the β-Ga2O3 films to the devices, the p-NiO/i-Ga2O3/n-GaN diode was fabricated from the as-prepared high-quality β-Ga2O3 film after post-deposition annealing as the electron-blocking layer. Subsequently, its electrical characteristics were investigated in detail which exhibited excellent rectification characteristics under all test temperatures. Correspondingly, the device achieved electroluminescence (EL) with a dominant sharp emission peak in the ultraviolet range (∼375 nm). In the end, the energy band diagram was used for in-depth analysis of the EL mechanism.

Synchronous Modulation of Energy Level Gradient and Defects for High-Efficiency HTL-Free Carbon-Based All-Inorganic Perovskite Solar Cells.

In order to improve the thermal stability of perovskite solar cells (PSCs) and reduce production costs, hole transport layer (HTL)-free carbon-based CsPbI3 PSCs (C-PSCs) have attracted the attention of researchers. However, the power conversion efficiency (PCE) of HTL-free CsPbI3 C-PSCs is still lower than that of PSCs with HTL/ metal electrodes. This is because the direct contact between the carbon electrode and the perovskite layer has a higher requirement on the crystal quality of perovskite layer and matched energy level at perovskite/carbon interface. Herein, the acyl chloride group and its derivative trichloroacetyl chloride are used to passivate CsPbI3 C-PSCs for the first time. The results show that the carbonyl group of trichloroacetyl chloride can effectively passivate the uncoordinated Pb2+ ions in perovskite. At the same time, leaving group Cl- ions can increase the grain size of perovskite and improve the crystallization quality of perovskite layer. In addition, the trichloroacetyl chloride tends to generate cesium chloride acetate, which acts as an electron blocking layer, reduces charge recombination, promotes gradient energy level arrangement, and effectively improves the separation and extraction ability of carriers. The PCE of CsPbI3 HTL-free C-PSCs is successfully increased from 13.40% to 14.82%.

Hole transport improvement in CdZnO/ZnO light emitting diodes with wedge shaped electron blocking layers

Hole transport improvement in CdZnO/ZnO light emitting diodes with wedge shaped electron blocking layers

In-depth insights into polarization-dependent light extraction mechanisms of AlGaN-based deep ultraviolet light-emitting diodes.

The AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) dominated by transverse-magnetic (TM) polarized emission suffer from extremely poor light extraction efficiency (LEE) from their top surface, which severely limits the device performance. In this study, the underlying physics of polarization-dependent light extraction mechanisms of AlGaN-based DUV LEDs has been explored in depth via simple Monte Carlo ray-tracing simulations with Snell's law. It is especially worth noting that the structures of the p-type electron blocking layer (p-EBL) and multi-quantum wells (MQWs) have a significant impact on light extraction behavior, especially for TM-polarized emission. Thus, an artificial vertical escape channel (named GLRV) has been constructed to efficiently extract the TM-polarized light through the top surface, by adjusting the structures of the p-EBL, MQWs, sidewalls, and using the adverse total internal reflection in a positive manner. The results show that the enhancement times of the top-surface LEE is up to 18 for TM-polarized emission in the 300 × 300 µm2 chip comprising a single GLRV structure, and further increases to 25 by dividing this single GLRV structure into a 4 × 4 micro-GLRV array structure. This study provides a new perspective for understanding and modulating the extraction mechanisms of polarized light to overcome the inherently poor LEE for the TM-polarized light.

Research on the Electron-blocking Layers in the GaN-based LEDs

The electron overflow happening in the active region of GaN-based light-emitting diodes (LEDs) will decrease the rate of radiative recombination, leading to a reduction of light output power. To deal with the problem, the electron blocking layer(EBL) is designed to weaken electrons leakage. Various designs and structures of the electron blocking layers embedded in the multiple quantum layers of GaN-based LEDs are reviewed in this paper. Designs such as trapezoidal EBL, step-graded superlattice EBL, and hybrid EBL can all effectively limit the amount of electron overflow. With many designs of EBL, LEDs are expected to have a wider range of applications in the opto-electric field.

High-power, multi-junction, 905 nm vertical-cavity surface-emitting laser with an AlGaAsSb electron-blocking layer.

We report high-power multi-junction vertical-cavity surface-emitting lasers (VCSELs) with a significantly suppressed carrier leakage issue under high injection current and temperature. By carefully optimizing the energy band structure of quaternary AlGaAsSb, we obtained a 12-nm-thick AlGaAsSb electron-blocking layer (EBL) with a high effective barrier height (∼122 meV), a low compressive strain (∼0.99%), and a reduced electronic leakage current. The resulting three-junction (3J) 905 nm VCSEL with the proposed EBL exhibits an improved maximum output power (∼46.4 mW) and power conversion efficiency (PCE; ∼55.4%) during room-temperature operation. Also, it was found from thermal simulation that the optimized device shows more advantages over the original device during high-temperature operation. The type-II AlGaAsSb EBL provided an excellent electron-blocking effect and would be a promising strategy for multi-junction VCSELs to realize high-power applications.

Design tradeoffs between traditional hexagonal and emerging cubic InXGa(1–X)N/GaN-based green light-emitting diodes

Here, we report on the design tradeoffs between traditional hexagonal and emerging cubic InXGa(1−X)N/GaN-based green (520nm≤λ≤550nm) light-emitting diodes with special emphasis on the electron blocking layer, number of quantum wells, and thicknesses of quantum wells and barriers. We identified three crucial design rules for cubic green light-emitting diodes: (1) no need for an electron blocking layer; (2) use of a wide quantum well; and (3) choice of thin quantum barriers in multi-quantum well light-emitting diode designs. These design rules increase the internal quantum efficiency of cubic green light-emitting diodes by ∼30.5% under 100A/cm2 injection with respect to traditional designs. Overall, the design rules of cubic light-emitting diodes and their differentiating nature from the traditional, hexagonal ones are crucial for the advent of next-generation cubic light-emitting diodes.