Hole Injection Efficiency Research Articles

Enhanced Performance of N-Polar AlGaN-Based Ultraviolet Light-Emitting Diodes With Lattice- Matched AlInGaN Insertion in n-AlGaN Layer

AlGaN-based ultraviolet-A light-emitting diodes (UVA LEDs) inevitably suffer from current crowding effects at high injection levels due to their lateral device structure, resulting in non-uniform light emission and device overheating. In N-polar UV LEDs, the problem is further exacerbated by increased hole injection efficiency, leading to current crowding and aggravated hole leakage, which limits the device performance. An n-AlGaN/AlInGaN/AlGaN structure is adopted in this study, through modulation of the Al and In compositions in the AlInGaN quaternary alloy, lattice matching and greater bandgap of AlInGaN to AlGaN template is designed. The numerical results prove that the n-AlGaN/AlInGaN/AlGaN structure can promote current spreading and thus mitigate hole leakage, resulting in the significantly enhanced performance of N-polar UVA LEDs. Furthermore, the use of lattice-matched AlInGaN layers in practical epitaxy is feasible, which can avoid the defect introduction resulting from the lattice mismatch.

발광층 호스트 구조에 따른 용액공정 기반 저분자 유기 발광 다이오드의 소자 거동

A method for achieving efficient and stable solution-processed small-molecule organic light-emitting diodes (OLEDs) is presented by utilizing a combination of a multifunctional hole injection layer (HIL) and a mixed-host emitting layer (EML). The polymeric HIL facilitates efficient hole injection into the solution-processed EML and blocks electrons from the EML through self-organization of polymer chains in the HIL. In addition to the multifunctional HIL, the optimized mixed-host EML, composed of electron and hole transporting host materials, along with phosphorescent dopant, enables efficient energy transfer, balanced charge transport, and efficient charge carrier recombination in the device. As a result, it improves luminance (~14,000 cd/m2), luminous efficiency (~55 cd/A), and operational lifetime (~180 minutes under constant current emitting initial luminance of 1,000 cd/m2, equivalent to approximately 150 hours at an initial luminance of 100 cd/m2). Notably, this device architecture does not include an additional hole transporting/electron blocking layer. This is because the introduction of a mixed-host composition widens the recombination zone in the EML, effectively preventing triplet-triplet excitons/triplet-polaron annihilation caused by charge carriers and excitons accumulated at the narrow heterointerfaces in OLEDs.

Effect of AlGaN quantum barrier thickness on electron-hole overlapping in deep-ultraviolet laser diode

AlGaN deep ultraviolet (DUV) with the high induced polarization emitters effect degrades the performance of the laser diode (LD) and brings a higher injection current density, resulting in reduced laser gain. Quantum barriers (QB) play a vital role in carrier flow (electrons, holes) in multiple quantum well (MQW). This work investigates the quantum barrier thickness effect on the performance of DUV LDs. Furthermore, the performance of DUV LD is enhanced by working on overlapping electrons and holes in the active region. The theoretically analyzed results illustrate that by minimizing the thickness of the AlGaN quantum barrier (QB), the electron-hole overlapping can be improved. Higher overlapping increases the rate of stimulated recombination. Compared with the reference structure AlGaN QB with 3 nm thickness, the emitted power can be increased by 11%, the optical confinement factor (OCF) by 71%, the electron-hole overlapping by 55%, and the laser gain by 149%. But selecting 12 nm QBs, power starts decreasing along with OCF, electron-hole overlapping, and laser gain. When the AlGaN QB is thicker, the height of the conduction band barrier diminishes, which boosts electron leakage coming from MQM, and when the valance band barrier's height is elevated, which reduces the efficiency of hole injection toward the MQW. Thus, it is concluded the below 12 nm QB thickness, the thinner the quantum barriers, the more carrier flows toward MQW, improving performance.

Low Switching Loss Built-In Diode of High-Voltage RC-IGBT with Shortened P+ Emitter

In this paper, a low switching loss built-in diode of a high-voltage reverse-conducting insulated gate bipolar transistor (RC-IGBT) is proposed without deteriorating IGBT characteristics. It features a particular shortened P+ emitter (SE) in the diode part of RC-IGBT. Firstly, the shortened P+ emitter in the diode part can suppress the hole injection efficiency resulting in the reduced carriers extracted during the reverse recovery process. The peak of the reverse recovery current and switching loss of the built-in diode during reverse recovery is therefore lowered. Simulation results indicate that the diode’s reverse recovery loss of the proposed RC-IGBT is lowered by 20% compared with that of the conventional RC-IGBT. Secondly, the separate design of the P+ emitter prevents the performance of IGBT from deteriorating. Finally, the wafer process of the proposed RC-IGBT is almost the same as that of conventional RC-IGBT, which makes it a promising candidate for manufacturing.

Reconfigurable Field‐Effect Transistor Technology via Heterogeneous Integration of SiGe with Crystalline Al Contacts

AbstractReconfigurable field‐effect transistors, capable of being dynamically programmed during run‐time, overcome the static nature of conventional complementary metal‐oxide semiconductors by reducing the transistor count and the circuit path delay. Thereby, SiGe and Ge are predicted to boost drive currents, switching speed and to reduce power consumption. Nevertheless, Ge‐based reconfigurable field‐effect transistor prototypes have so far fallen short in reaching both the promised performance due to interface instability to their contacts and gate oxides, as well as in reaching the current–voltage symmetry necessary for circuit applicability. Here, a top‐down fabricated SiGe‐based reconfigurable transistor technology is reported that is comprised of a vertical Si‐Si0.67Ge0.33 heterostructure enabling the envisioned high and symmetric on‐currents of both n‐ and p‐type operation. Monolithic integration with single‐elementary crystalline Al contacts alleviates process variability compared to conventional Ni‐silicide/Ni‐germanide contacts and introduces an ultra‐thin Si interlayer providing stability and equal injection efficiency of holes and electrons. The implementation of a three top‐gate transistor in combination with a hysteresis‐free Si/SiO2/HfO2 gate stack enhances polarity control and leakage current suppression to limit static power dissipation. Importantly, the obtained Al‐Si‐SiGe multi‐heterojunction and advanced reconfigurable transistor design is the first Ge‐based technology showing the envisioned stability and performance enhancements.

Impact of Mg-doping on the performance and degradation of AlGaN-based UV-C LEDs

We investigate the impact of Mg-doping on the performance and degradation kinetics of AlGaN-based UV-C light-emitting diodes (LEDs). By comparing LEDs from three wafers with different nominal doping levels [Mg/(Al+Ga) ratio: 0.15%, 0.5%, and 1% in the gas phase during epitaxy] in the AlGaN:Mg electron-blocking layer (EBL), we demonstrate the following results: (i) A higher Mg-doping in the EBL results in a higher optical power at low current levels, which is ascribed to an increased hole injection efficiency. (ii) The reduction of the optical power follows a non-exponential trend, which can be reproduced by using the Hill's formula and is ascribed to the generation/activation of defects within the quantum wells. (iii) A higher Mg-doping in the EBL mitigates the degradation rate. An interpretation of the experimental data is proposed, assuming that hydrogen, which is present in and moving from the EBL, can reduce the rate of de-hydrogenation of point defects in the active region, which is responsible for degradation.

First‐Principles Modeling of the Adsorption Mechanism of Carboxylic and Phosphonic Acids onto Pristine and Defective Delafossite CuAlO2 Surfaces

In order to provide atomistic insights on the mechanism and stability of dye grafting to delafossite surfaces, the authors study, using density‐functional theory calculations in vacuo and in implicit solvent, the adsorption of carboxylic and phosphonic anchoring groups onto stoichiometric and defective slabs exposing the most stable (012) termination. A remarkable increase of the adsorption energies is found on the reduced surfaces when the vacancy is on the top surface layer, in proximity to the molecule. While on the pristine substrate, the monodentate anchoring is the most stable one, and on the reduced interface, a strong bidentate bridging anchoring mode is favored, after the transfer of the hydroxyl proton to the oxygen surface atom adjacent to the copper vacancy. Identification of a largely stable bidentate coordination suggests a possible long‐term device stability, as well as a larger efficiency of the interfacial hole injection, indicating that may be a good alternative to NiO in p‐type photocathodes.

A Mixed Organic-Inorganic Interlayer With Tunable Electrical Properties Enabling Stable and Efficient Perovskite Light-Emitting Diodes

The operational stability of perovskite light-emitting diodes (PeLEDs) is subjected to undesired Joule heating induced by the charge imbalance and the current leakage. The hole injection efficiency is inferior to that of electrons in the electron-dominated PeLEDs, owing to the low hole mobility of hole transport materials and a relatively large hole injection barrier. Herein, we inserted a mixed interlayer of 4,7-diphenyl-1,10-phenanthroline and cesium carbonate (Bphen:Cs2CO3) (~10 nm) between the electron transport layer (ETL) TPBi and the metal cathode to improve the charge imbalance by optimizing the electron injection and suppressing the current leakage. Compared to TPBi, the Bphen interlayer builds a larger electron injection barrier from the cathode to perovskite layer owing to its shallower lowest unoccupied molecular orbital (LUMO), leading to inefficient electron injection. The Cs2CO3, an n-type dopant, was co-evaporated with Bphen to increase the conductivity and thus balance the electron and hole transport. The resultant quasi-2D PeLED achieves an external quantum efficiency of 17.62% and an operational lifetime of over 1900 min at an initial luminance of 100 cd <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{-{2}}$ </tex-math></inline-formula> .

A Conformal Carbon Nanolayer Coated Fe2 O3 Cocatalyst for the Promoted Activity of Plasma-Sputtered BiVO4 Photoanode.

To simultaneously improve the hole extraction ability of the BiVO4 photoanode and accelerate the surface reaction kinetics, herein, a carbon nanolayer conformally coated Fe2 O3 (C-Fe2 O3 ) as oxygen evolution catalyst (OEC) is loaded on the H2 plasma treated nanoporous BiVO4 (BVO(H2 )) surface by a hydrothermal reaction. It is found that the H2 plasma induced vacancies in BVO remarkably increases the conductivity, and the C-Fe2 O3 enables hole extraction from the bulk to the surface as well as efficient hole injection to the electrolyte. As a result, the C-Fe2 O3 /BVO(H2 ) photoanode achieves a photocurrent density of 4.4 mA/cm2 at 1.23 V vs. reversible hydrogen electrode (RHE) and an ABPE value of 1.5 % at 0.68 V vs. RHE, which are 4.8-fold and 13-fold higher than that of BVO photoanode, respectively.

Unlocking the Full Potential of Electron-Acceptor Molecules for Efficient and Stable Hole Injection.

Understanding the hole-injection mechanism and improving the hole-injection property are of pivotal importance in the future development of organic optoelectronic devices. Electron-acceptor molecules with high electron affinity (EA) are widely used in electronic applications, such as hole injection and p-doping. Although p-doping has generally been studied in terms of matching the ionization energy (IE) of organic semiconductors with the EA of acceptor molecules, little is known about the effect of the EA of acceptor molecules on the hole-injection property. In this work, the hole-injection mechanism in devices is completely clarified, and a strategy to optimize the hole-injection property of the acceptor molecule is developed. Efficient and stable hole injection is found to be possible even into materials with IEs as high as 5.8 eV by controlling the charged state of an acceptor molecule with an EA of about 5.0eV. This excellent hole-injection property enables direct hole injection into an emitting layer, realizing a pure blue organic light-emitting diode with an extraordinarily low turn-on voltage of 2.67V, a power efficiency of 29lm W-1 , an external quantum efficiency of 28% and a Commission Internationale de l'Eclairage y coordinate of less than 0.10.

Doping-Free Complementary Metal-Oxide-Semiconductor Inverter Based on N-Type and P-Type Tungsten Diselenide Field-Effect Transistors With Aluminum-Scandium Alloy and Tungsten Oxide for Source/Drain Contact

In this study, we experimentally demonstrated concepts for realizing doping-free Tungsten Diselenide (WSe2) complementary metal-oxide-semiconductor (CMOS) inverter by developing alloys and compound metals used as source/drain (S/D) contacts. Aluminum – scandium alloy (AlSc) and tungsten oxide (WOx)-based S/D contacts enable efficient electron and hole injection into WSe2 for n-type and p-type FET operation because the work function (WF) of AlSc and WOx are aligned to neighboring the conduction and valence band edge of WSe2, respectively. A dual-gate bias architecture is used to improve electrical characteristics of FETs and enhance CMOS inverter performance after device fabrication. By utilizing AlSc and WOx-based S/D contacts in conjunction with the dual-gate bias architecture, our fabricated WSe2 CMOS inverter realized a higher gain at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}_{\mathrm{ dd}}$ </tex-math></inline-formula> of 1 V or higher than those in the literatures. Furthermore, the fabricated WSe2 CMOS inverter is operated at a power supply voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}_{\mathrm{ dd}}$ </tex-math></inline-formula> ) of as low as 0.5 V. This study paves the way towards research and development of transition metal dichalcogenides-based devices and circuits.

Efficient Carrier Confinement in AlGaN‐Based Deep‐Ultraviolet Light‐Emitting Diodes with a Composition‐Graded Electron‐Blocking Layer

The serious electron leakage and poor transport of hole injection layers in deep‐ultraviolet (DUV) light‐emitting diodes (LEDs) lead to an imbalance between the hole and electron currents, which reduces the device's performance. Herein, a new DUV LED structure is designed. The aluminum composition of the p‐type electron‐blocking layer (p‐EBL) gradually decreases from 0.95 to 0.75 along the growth direction, replacing the traditional bulk p‐EBL. When the injection current is 100 mA, the optical power of this structure is about 32% higher than that of the traditional structure. In addition, when the p‐EBL adopts the opposite gradient (the Al composition gradually increases from 0.75 to 0.95), the device performance suddenly drops dramatically. The related devices are simulated by Silvaco TCAD software. The results show that the performance outputs of the two opposite aluminum gradient growth modes are completely opposite. The proposed p‐EBL with a gradual step‐down Al composition along the growth direction helps to improve the efficiency of hole injection into multiple quantum wells (MQWs) and reduce the level of electron leakage. This work provides an effective solution for increasing the light output power of high‐performance DUV LEDs.

Mechanically robust stretchable semiconductor metallization for skin-inspired organic transistors.

Despite recent remarkable advances in stretchable organic thin-film field-effect transistors (OTFTs), the development of stretchable metallization remains a challenge. Here, we report a highly stretchable and robust metallization on an elastomeric semiconductor film based on metal-elastic semiconductor intermixing. We found that vaporized silver (Ag) atom with higher diffusivity than other noble metals (Au and Cu) forms a continuous intermixing layer during thermal evaporation, enabling highly stretchable metallization. The Ag metallization maintains a high conductivity (>104 S/cm) even under 100% strain and successfully preserves its conductivity without delamination even after 10,000 stretching cycles at 100% strain and several adhesive tape tests. Moreover, a native silver oxide layer formed on the intermixed Ag clusters facilitates efficient hole injection into the elastomeric semiconductor, which transcends previously reported stretchable source and drain electrodes for OTFTs.

Improving the Performance of Solution-Processed Quantum Dot Light-Emitting Diodes via a HfOx Interfacial Layer.

One of the major obstacles in the way of high-performance quantum dot light-emitting diodes (QLEDs) is the charge imbalance arising from more efficient electron injection into the emission layer than the hole injection. In previous studies, a balanced charge injection was often achieved by lowering the electron injection efficiency; however, high performance next-generation QLEDs require the hole injection efficiency to be enhanced to the level of electron injection efficiency. Here, we introduce a solution-processed HfOx layer for the enhanced hole injection efficiency. A large amount of oxygen vacancies in the HfOx films creates gap states that lower the hole injection barrier between the anode and the emission layer, resulting in enhanced light-emitting characteristics. The insertion of the HfOx layer increased the luminance of the device to 166,600 cd/m2, and the current efficiency and external quantum efficiency to 16.6 cd/A and 3.68%, respectively, compared with the values of 63,673 cd/m2, 7.37 cd/A, and 1.64% for the device without HfOx layer. The enhanced light-emitting characteristics of the device were elucidated by X-ray photoelectron, ultra-violet photoelectron, and UV-visible spectroscopy. Our results suggest that the insertion of the HfOx layer is a useful method for improving the light-emitting properties of QLEDs.

Numerical study on light triggering characteristics of NiO/SiC heterojunction thyristor

In this paper, the performance of silicon carbide (SiC) light-triggered thyristor (LTT) with a p-type NiO emitter region is analyzed through numerical simulation. The conductivity modulation in SiC LTT is significantly enhanced with the help of high injection efficiency of holes in NiO/SiC heterojunction. The injected hole density at the surface of the p− long base is increased by ∼21.2 times and the corresponding specific on-state resistance (Ron,sp) is only 36.7 mΩ cm2, which is reduced by about 29%. Moreover, hole-injection enhancement by NiO/SiC heterojunction also exhibits excellent potential in improving the dynamic characteristics of SiC LTTs. The simulation results indicate that the turn-on time of SiC LTT can be reduced by ∼57.76% when triggered by 1.0 W/cm2 ultraviolet light. Furthermore, energy dissipations of SiC LTT during the turn-on and turn-off processes can be reduced by 91.4% and 21.9%, respectively.

Enhancement of the light output efficiency and thermal stability of AlGaN-based deep-ultraviolet light-emitting diodes with Ag-nanodot-based p-contacts and an 8-nm p-GaN cap layer.

The efficiency of AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) is limited by the high absorption issue of the p-GaN contact layer or poor contact properties of the transparent p-AlGaN contact layer. Enhancement of the light output efficiency and thermal stability of DUV LEDs with an emission wavelength of 272 nm was investigated in this work. Ag nanodots on an 8-nm p-GaN cap layer were used to form ohmic contact, and Al and Mg reflective mirrors were employed to enhance the light output power (LOP) of DUV LEDs. However, serious deterioration of LOP occurred after the high-temperature process for the LEDs with Al and Mg reflective mirrors, which can be attributed to the damage to the ohmic contact properties. A Ti barrier layer was inserted between the Ag/p-GaN and Al layers to prevent the degeneration of ohmic contact. The wall-plug efficiency (WPE) of DUV LEDs fabricated by the Ag-nanodot/Ti/Al electrode is 1.38 times that of LEDs fabricated by adopting a thick Ag layer/Ti/Al at 10 mA after a high-temperature process. The Ag-nanodot/Ti/Al electrode on thin p-GaN is a reliable technology to improve the WPE of DUV LEDs. The experimental and simulated results show that the ohmic contact is important for the hole-injection efficiency of the DUV LEDs when p-GaN is thin, and a slight increase in the contact barrier height will decrease the WPE drastically. The results highlighted the importance of thermally stable ohmic contacts to achieve high-efficiency DUV LEDs and demonstrated a feasible route for improving the LOP of DUV LEDs with a thin p-GaN cap layer and stable reflective electrodes.

Very Low-Efficiency Droop in 293 nm AlGaN-Based Light-Emitting Diodes Featuring a Subtly Designed p-Type Layer.

This paper reports an AlGaN-based ultraviolet-B light-emitting diode (UVB-LED) with a peak wavelength at 293 nm that was almost free of efficiency droop in the temperature range from 298 to 358 K. Its maximum external quantum efficiencies (EQEs), which were measured at a current density of 88.6 A cm-2, when operated at 298, 318, and 338 K were 2.93, 2.84, and 2.76%, respectively; notably, however, the current droop (J-droop) in each of these cases was less than 1%. When the temperature was 358 K, the maximum EQE of 2.61% occurred at a current density of 63.3 A cm-2, and the J-droop was 1.52%. We believe that the main mechanism responsible for overcoming the J-droop was the uniform distribution of the concentrations of injected electrons and holes within the multiple quantum wells. Through the subtle design of the p-type AlGaN layer, with the optimization of the composition and doping level, the hole injection efficiency was enhanced, and the Auger recombination mechanism was inhibited in an experimental setting.

Enhancing hole-injection efficiency of top-emitting organic light-emitting diodes based on Al/MoNx anodes

High reflectivity bottom anodes are essential for high luminance efficiency top-emitting organic light-emitting diodes (TOLEDs). Although the element Al has high optical reflectivity, its poor hole-injection property severely hinders the realization of high efficient TOLED due to its low work function. In this letter, a high work function MoNx layer with different growth time was deposited on Al layers for enhancing the hole-injection efficiency of TOLEDs. Compared with the bottom-emitting OLED (BOLED) using indium-tin oxide (ITO) anode, the TOLED with Al/MoNx (8 s) anode shows a lower driving voltage of 3.1 eV than that (3.4 eV) of the BOLED at 20 mA/cm2, which may attribute to higher work function of MoNx leading to a stronger hole-injection. Moreover, the TOLED with Al/MoNx (8 s) anode exhibits higher brightness (11800 cd/m2) at 20 mA/cm2, higher maximum current efficiency (64.6 cd/A) and narrower electroluminescence spectra. These results indicate that the Al/MoNx layers could be effective hole-injection anodes for achieving high-performance TOLEDs.

Plasmonic hot-hole injection combined with patterned substrate for performance improvement in trapezoidal PIN GaN microwire self-powered ultraviolet photodetector

Harvesting nonequilibrium carriers from plasmonic-metal nanostructures offers unique strategies for improving the performance of various photodetectors. Most studies have focused on the capture and conversion of plasmonic hot-electron. Here, we report a hot-hole injection enhanced self-powered UV photodetector based on trapezoidal PIN GaN microwire modified with Au nanoparticles (NPs) on patterned sapphire substrate. Efficient injection of hot holes and enhanced light scattering of plasmonic Au NPs combined with patterned substrate that increases the light absorption of the device considerably improve the performance of the photodetector. The responsivity at 325 nm reaches up to 1.9 A W−1, with an ideal detectivity at 5.25 × 1011 Jones without a power supply, which is well beyond the photodetectors without Au NPs modification on flat substrate. Moreover, our device also has a high on/off ratio (∼3 ×105) and fast response speed (rise/fall time of 0.25/0.38 ms). This work demonstrates potential applications, such as energy-saving communication, environmental monitoring and self-powered integrated systems and provides guidance for subsequent photodetectors that operate by hot-hole collection.

A polarization mismatched p-GaN/p-Al0.25Ga0.75N/p-GaN structure to improve the hole injection for GaN based micro-LED with secondary etched mesa

A low hole injection efficiency for InGaN/GaN micro-light-emitting diodes (μLEDs) has become one of the main bottlenecks affecting the improvement of the external quantum efficiency (EQE) and the optical power. In this work, we propose and fabricate a polarization mismatched p-GaN/p-Al0.25Ga0.75N/p-GaN structure for 445 nm GaN-based μLEDs with the size of 40 × 40 μm2, which serves as the hole injection layer. The polarization-induced electric field in the p-GaN/p-Al0.25Ga0.75N/p-GaN structure provides holes with more energy and can facilitate the non-equilibrium holes to transport into the active region for radiative recombination. Meanwhile, a secondary etched mesa for μLEDs is also designed, which can effectively keep the holes apart from the defected region of the mesa sidewalls, and the surface nonradiative recombination can be suppressed. Therefore, the proposed μLED with the secondary etched mesa and the p-GaN/p-Al0.25Ga0.75N/p-GaN structure has the enhanced EQE and the improved optical power density when compared with the μLED without such designs.