DUV LEDs Research Articles

Enhanced Emission of Deep Ultraviolet Light-Emitting Diodes through Using Work Function Tunable Cu Nanowires as the Top Transparent Electrode.

Deep ultraviolet light-emitting diodes (DUV LEDs) (<280 nm) have been important light sources for broad applications in, e.g., sterilization, purification, and high-density storage. However, the lack of excellent transparent electrodes in the DUV region remains a challenging issue. Here, we demonstrate an architectural engineering scheme to flexibly tune the work function of Cu@shell nanowires (NWs) as top transparent electrodes in DUV LEDs. By fast encapsulation of shell metals on Cu NWs and a shift of electron binding energy, the electronic work function could be widely tailored down to 4.37 eV and up to 5.73 eV. It is revealed that the high work function of Cu@Ni and Cu@Pt NWs could overcome the interfacial barrier to p-AlGaN and achieve direct ohmic contact with high transparency (91%) in 200-400 nm. Completely transparent DUV LED chips are fabricated and successfully lighted with sharp top emission (wall-plug efficiency reaches 3%) under a turn-on voltage of 6.4 V. This architectural strategy is of importance in providing highly transparent ohmic electrodes for optoelectronic devices in broad wavelength regions.

Metal-insulator-semiconductor structure for deep-ultraviolet light-emitting diodes to increase the electron injection in the cathode region

Abstract In this work, we propose using metal-insulator-semiconductor (MIS) structure on the n-AlGaN layer to reduce the contact resistance and improve the wall-plug efficiency (WPE) for AlGaN based deep ultraviolet light-emitting diodes (DUV LEDs). The thin insulator between the cathode metal and the n-AlGaN layer can share the bias, which makes the n-AlGaN layer surface less depleted and thus the MIS-type cathode structure favors the intraband tunneling effect for electrons. Moreover, the adoption of the MIS-structure aligns the electron affinity for the metal above the conduction band for the n-AlGaN layer, and this avoids the electron transport by thermionic emission. Thanks to the MIS-type cathode, we find that excellent electron transport can be obtained even if the metal with big electron affinity is used. This work also conducts an in-depth investigation into the impact of the relative dielectric constant, the bandgap, the affinity, the length and the thickness for the insulator on the electron transport and the WPE of DUV LEDs. We find that an insulator with a small dielectric constant is required for promoting the electron tunneling in the MIS-type cathode region.

Inactivation of Listeria and E. coli by Deep-UV LED: effect of substrate conditions on inactivation kinetics

Irradiation with deep-ultraviolet light-emitting diodes (DUV LEDs) is emerging as a low energy, chemical-free approach to mitigate microbial contamination, but the effect of surface conditions on treatment effectiveness is not well understood. Here, inactivation of L. innocua and E. coli ATCC25922, as examples of Gram-positive and Gram-negative bacteria, respectively, by DUV LED of 280 nm wavelength was studied. Surface scenarios commonly encountered in environmental, clinical or food processing environments were used: nutrient rich surfaces, thin liquid films (TLF), and stainless steel surfaces (SS). DUV LED exposure achieved 5-log reduction for both strains within 10 min in most scenarios, except for TLF thicker than 0.6 mm. Inactivation kinetics in TLF and on dry SS followed the Weibull model (0.96 ≤ R2 ≤ 0.99), but the model overestimated inactivation by small-dose DUV on wet SS. Confocal microscopy revealed in situ that bacteria formed a dense outer layer at the liquid-air interface of the liquid droplet, protecting the cells inside the droplet from the bactericidal DUV. This resulted in lower than anticipated inactivation on wet SS at small DUV doses, and deviation from the Weibull model. These findings can be used to design effective DUV LED disinfection strategies for various surface conditions and applications.

Light Extraction Efficiency Optimization of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes

Using finite-difference time-domain method, the light extraction efficiency (LEE) of AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) is investigated. Simulation results show that compared to flat sapphire substrate, the nano-patterned sapphire substrate (NPSS) expands the extraction angles of top surface and sidewalls. As a result, the LEE of transverse-magnetic (TM) polarized light is improved significantly. Roughening on the backside of n-AlGaN surface significantly enhances the LEE of top surface of thin-film flip-chip DUV LEDs. However, the LEE of sidewalls of thin-film flip-chip DUV LEDs is greatly weakened. For bare DUV LED, the LEE of flip-chip LED on NPSS is estimated to be about 15%, which is around 50% higher than that of thin-film flip-chip DUV LED with roughening on the backside of n-AlGaN surface.

Enhanced wall plug efficiency of AlGaN-based deep-UV LEDs using Mo/Al as p-contact

P-type contacts with a high reflectivity in the ultraviolet spectral region made of molybdenum/aluminum (Mo/Al) on AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) emitting at 265 nm have been investigated. Optimized Mo/Al contacts are shown to have a high optical reflectivity above 75% at 265 nm. DUV LEDs with an absorbing p-AlGaN heterostructure operated at 20 mA show a 15% higher light output power and a 1 V lower voltage when Mo/Al instead of Pt is used as p-contact. The effect on the voltage of DUV LEDs with a UV-transparent p-side heterostructure is similar. Moreover, DUV LEDs with a Mo/Al contact show a lower operation voltage compared to LEDs with an indium tin oxide/aluminum (ITO/Al) p-contact where the ITO is intended to form a semitransparent low-resistance contact and the Al serves as a reflector.

High-reflection Al-plated DPC ceramic substrate for AlGaN-based DUV LED packaging

High-reflection Al-plated DPC ceramic substrate for AlGaN-based DUV LED packaging

Interplay between various active regions and the interband transition for AlGaN-based deep-ultraviolet light-emitting diodes to enable a reduced TM-polarized emission

Al-rich AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) have a low light extraction efficiency, especially when the emission wavelength is shorter than 280 nm, and this is partially because of the dominant transverse-magnetic polarized light. Our results show that the transverse-electric (TE) polarized light can be obtained even if the emission wavelength becomes even shorter by reducing the quantum well thickness. The ultrathin quantum well enables the enhanced TE-polarized emission that arises from the redistributed subbands for holes. On the contrary to the common belief, we observe a blueshift for the emission wavelength when the AlN composition in the quantum barrier increases. The internal quantum efficiency (IQE) for DUV LEDs with ultrathin quantum wells is no longer determined by the quantum-confined Stark effect, while quantum barrier with high AlN composition is vitally important to improve the electron injection efficiency and thus enhance the IQE.

Performance Improvement of AlGaN-Based Deep Ultraviolet Light-Emitting Diodes With Step-Like Quantum Barriers

AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) still confront many challenges, which is partially limited by the poor carrier injection in the active region. Although incorporating a high Al-composition quantum barrier (QB) may boost carrier confinement capability, it will aggravate the quantum confined Stark effect (QCSE) and thus deteriorate the optical performance. In this article, a DUV LED structure with step-like QBs has been proposed and carefully investigated. This unique QB structure suppresses the electron overflow into the p-side of the device and benefits the hole injection efficiency simultaneously, thereby promoting the radiative recombination rate in the active region. As a result, the internal quantum efficiency (IQE) and light output power (LOP) of the DUV LED with step-like QBs are significantly improved with an enhancement factor of 40% under 60 mA current injection. Therefore, our step-like QB design provides a feasible approach to the enhancement of the optical performance of DUV LEDs.

Enhanced Optical Performance of AlGaN-Based Deep Ultraviolet Light-Emitting Diodes by Electrode Patterns Design

Low external quantum efficiency of deep ultraviolet light-emitting diodes (DUV LEDs) and current crowding can result in considerable heat generation, which has a great negative impact on device performance. In this paper, we investigate the influence of different electrode patterns on the photoelectric and thermal performance of DUV LEDs. We find that different electrode designs can achieve drastically different optical powers, with the superior design being the n-type electrode surrounding the active region. Moreover, compared with the counterpart, the superior design does not affect the electrical performance. The main reason is that the N-surrounding electrode pattern can provide enough current paths for carrier transport, thus realizing a more uniform current injection and can further improve the external quantum efficiency for DUV LEDs.

Heteroepitaxial Growth of High-Quality and Crack-Free AlN Film on Sapphire Substrate with Nanometer-Scale-Thick AlN Nucleation Layer for AlGaN-Based Deep Ultraviolet Light-Emitting Diodes.

High-quality and crack-free aluminum nitride (AlN) film on sapphire substrate is the foundation for high-efficiency aluminum gallium nitride (AlGaN)-based deep ultraviolet light-emitting diodes (DUV LEDs). We reported the growth of high-quality and crack-free AlN film on sapphire substrate with a nanometer-scale-thick AlN nucleation layer (NL). Three kinds of nanometer-scale-thick AlN NLs, including in situ low-temperature AlN (LT-AlN) NL, oxygen-undoped ex situ sputtered AlN NL, and oxygen-doped ex situ sputtered AlN NL, were prepared for epitaxial growth of AlN films on sapphire substrates. The influence of nanoscale AlN NL thickness on the optical transmittance, strain state, surface morphology, and threading dislocation (TD) density of the grown AlN film on sapphire substrate were carefully investigated. The average optical transmittance of AlN film on sapphire substrate with oxygen-doped sputtered AlN NL was higher than that of AlN films on sapphire substrates with LT-AlN NL and oxygen-undoped sputtered AlN NL in the 200–270 nm wavelength region. However, the AlN film on sapphire substrate with oxygen-undoped sputtered AlN NL had the lowest TD density among AlN films on sapphire substrates. The AlN film on sapphire substrate with the optimum thickness of sputtered AlN NL showed weak tensile stress, a crack-free surface, and low TD density. Furthermore, a 270-nm AlGaN-based DUV LED was grown on the high-quality and crack-free AlN film. We believe that our results offer a promising and practical route for obtaining high-quality and crack-free AlN film for DUV LED.

Deep-Ultraviolet LEDs Fabricated by Nanoimprinting

In this study, deep-ultraviolet 280-nm-wavelength light-emitting diodes (DUV LEDs) with a nanopatterned sapphire substrate (NPSS) were fabricated using a nanoimprint lithography process. The output power of these flip-chip (FC) DUV LEDs was measured and analyzed in a ray-tracing simulation using NPSSs: cones with diameters of 2.5 μm (LED1), 400 (LED2) and 200 nm (LED3) as well as holes with diameters of 350 (LED4) and 750 nm (LED5). For comparison, a mirror for the c-plane FC DUV LEDs was introduced by coating Ni/Au and Al thin films. With the injection current at 350 mA, the output powers of the c-plane, LED1, LED2, LED3, LED4, and LED5 were 13.38, 14.68, 20.05, 16.79, 15.74, and 18.93 mW, respectively. The LED2 efficiency improved 49.9% compared with mirror LED and the result was consistent with ray-tracing simulation.

Monolithic integration of deep ultraviolet LED with a multiplicative photoelectric converter

Vertically monolithic integration of multiple devices on a single chip has emerged as a promising approach to overcome the fundamental limits of material and physical properties, providing unique opportunities to harness their complementary physics through integrated solutions to significantly enhance device performance. Herein, we demonstrate a deep ultraviolet light emitting diode (DUV LED) integrated with a multiplicative photoelectric converter (MPC) that is composed of p-GaN/intrinsic GaN/n-GaN (p-i-n GaN) structure to induce the electric-optic conversion, thus considerably improve the hole injection efficiency. This p-i-n GaN structure acts as hole-multiplier via firstly DUV light absorption and then electron-hole pair generation. The newly generated electron-hole pairs are firstly separated by the electric field in the p-i-n GaN structure so that multiple holes are driven into multiple quantum wells (MQWs), and finally contribute to the radiative recombination, thus achieving a high wall plug efficiency (WPE) of 21.6%, which exhibits a 60-fold WPE enhancement compared to the conventional DUV LEDs. The monolithic integration strategy demonstrated here sheds light on developing highly efficient light emitters.

Advantages of AlGaN-based deep-ultraviolet light-emitting diodes with an Al-composition graded quantum barrier.

AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) still suffer from poor quantum efficiency and low optical power. In this work, we proposed a DUV LED structure that includes five unique AlxGa1-xN quantum barriers (QBs); Each QB has a linear-increment of Al composition by 0.03 along the growth direction, unlike those commonly used flat QBs in conventional LEDs. As a result, the electron and hole concentration in the active region was considerably increased, attributing to the success of the electron blocking effect and enhanced hole injection efficiency. Importantly, the optical power was remarkably improved by 65.83% at the injection current of 60 mA. After in-depth device optimization, we found that a relatively thinner graded QB layer could further boost the LED performance because of the increased carrier concentrations and enhanced electron and hole wave function overlap in the QW, triggering a much higher radiative recombination efficiency. Hence, the proposed graded QBs, which have a continuous increment of Al composition along the growth direction, provide us with an effective solution to boost light output power in the pursuit of high-performance DUV emitters.

Deep ultraviolet light-emitting diodes based on a well-ordered AlGaN nanorod array

The nanorod structure is an alternative scheme to develop high-efficiency deep ultraviolet light-emitting diodes (DUV LEDs). In this paper, we first report the electrically injected 274-nm AlGaN nanorod array DUV LEDs fabricated by the nanosphere lithography and dry-etching technique. Nanorod DUV LED devices with good electrical properties are successfully realized. Compared to planar DUV LEDs, nanorod DUV LEDs present >2.5 times improvement in light output power and external quantum efficiency. The internal quantum efficiency of nanorod LEDs increases by 1.2 times due to the transformation of carriers from the exciton to the free electron–hole, possibly driven by the interface state effect of the nanorod sidewall surface. In addition, the nanorod array significantly facilitates photons escaping from the interior of LEDs along the vertical direction, contributing to improved light extraction efficiency. A three-dimensional finite-different time-domain simulation is performed to analyze further in detail the TE- and TM-polarized photon extraction mechanisms of the nanostructure. Our results demonstrate the nanorod structure is a good candidate for high-efficiency DUV emitters.

Modulating the Layer Resistivity by Band-Engineering to Improve the Current Spreading for DUV LEDs

In this letter, we propose to enhance the hole injection efficiency by modulating the layer resistivity in the n-AlGaN layer for 280 nm AlGaN based deep ultraviolet light-emitting diodes (DUV LEDs). The layer resistivity for the n-AlGaN layer is controlled by generating the energy barriers, which is enabled by locally engineering the energy band gap for the n-AlGaN layer, such that a thin n-AlGaN layer with high Al composition is inserted before growing the subsequent multiple quantum wells (MQWs). As a result, such inserted n-AlGaN layer is able to tune the current flow path, i.e., improving the current spreading effect in the p-type hole injection layer. The improved current spreading effect favors the promoted hole injection into the active region. We numerically and experimentally obtain the improved external quantum efficiency, the optical power, and the wall-plug efficiency, thanks to the better current spreading and the correspondingly enhanced hole injection capability.

Enhanced Performance of an AlGaN-Based Deep-Ultraviolet LED Having Graded Quantum Well Structure

AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) suffer from severe quantum confined Stark effect (QCSE) due to the strong polarization field in the quantum wells (QWs) grown on c -plane substrates. In this paper, we propose a novel DUV LED structure embedded with graded QWs in which the Al composition was linearly changed to screen the QCSE. A significant increase of the internal quantum efficiency and thus an enhancement of the light output power by nearly 67% can be achieved, attributing to the improvement of the electron-hole wave function overlap (Гe-hh) to 58.6% in the Increased-Al-composition graded QWs, as compared to the QW without grading (Гe-hh = 40.4%) and reverse grading (Гe-hh = 33.6%). Further investigations show that the grading profile of the Al composition in the QWs, including either linearly increases or decreases along the growth direction and the thickness of graded QWs, determine the polarization electrical field in the QWs and as a result, significantly affecting the performance of the devices. In the end, a careful optimization of the graded QWs is called. The proposed structure with such unique graded QWs provides us an effective solution to suppress the QCSE effect in the pursuit of high-performance DUV emitters.

Enhanced optical performance of AlGaN-based deep-ultraviolet light-emitting diode with m-shaped hole blocking layer and w-shaped electron blocking layer

Abstract In this study, five AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) with different electron blocking layer (EBL) and hole blocking layer (HBL) have been investigated numerically. The simulation results demonstrate that the DUV LED with m-shaped HBL and w-shaped EBL has a better light output power and IQE compared with other DUV LEDs. The improvements are chiefly benefit to the high carrier concentration and high radiation recombination rate, which are caused by the lower carrier leakage and higher carrier injection in the active region of the DUV LED with m-shaped HBL and w-shaped EBL. In addition, smaller electrostatic fields in the active region of the DUV LED with m-shaped HBL and w-shaped EBL are conducive to raising the overlap of electrons and holes, thus enhance the radiation recombination rate.

Marked enhancement in the efficiency of deep ultraviolet light-emitting diodes by using a AlxGa1-xN carrier reservoir layer

The advantages of using an AlxGa1-xN carrier reservoir layer (CRL) instead of the traditional last quantum barrier for deep-ultraviolet light-emitting diodes (DUV LEDs) were investigated. The results indicate that the internal quantum efficiency is markedly enhanced and the efficiency droop phenomenon is alleviated. These improvements are mainly attributed to the significantly enhanced hole-injection and radiative recombination rate. Additionally, when the Al contents of the CRL gradually decrease to that of the quantum wells, a large number of electrons and holes are reserved in the CRL region and recombined to emit DUV light, leading to marked enhancement of the optical performance.

Degradation processes of 280 nm high power DUV LEDs: impact on parasitic luminescence

With this work we report on characterization and reliability analysis of 280 nm high power commercial LEDs emitting 18 mW at 200 mA, with a peak wavelength of 280 nm. Spectral characterization reports the presence of a main emission peak and a sideband peak separated by 0.8 eV. DLTS analysis has also been carried out identifying two different defect traps, a deep level for electrons with Ea = 0.73–0.92 eV and a shallow level for holes with Ea = 0.18–0.28 eV. Results from constant current degradation tests report two different degradation mechanisms, a first degradation kinetic more noticeable at high current and a second more intense kinetic correlated to the increase of a third peak at higher wavelength. The work then suggests a reasoned interpretation by comparing the results with literature.

On the origin of enhanced hole injection for AlGaN-based deep ultraviolet light-emitting diodes with AlN insertion layer in p-electron blocking layer.

For the [0001] oriented AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs), the holes in the p-type electron blocking layer (p-EBL) are depleted due to the polarization induced positive sheet charges at the last quantum barrier (LQB)/p-EBL interface. The hole depletion effect significantly reduces the hole injection capability across the p-EBL. In this work, we propose inserting a thin AlN layer between the LQB and the p-EBL, which can generate the hole accumulation at the AlN/p-EBL interface. Meanwhile, the holes can obtain the energy when traveling from the p-EBL into the multiple quantum wells (MQWs) by intraband tunneling through the thin AlN layer. As a result, the hole injection and the external quantum efficiency (EQE) have been remarkably enhanced. Moreover, we point out that the thick AlN insertion layer can further generate the hole accumulation in the p-EBL and increase the hole energy which helps to increase the hole injection. We also prove that the intraband tunneling for holes across the thick AlN insertion layer is facilitated by using the optimized structure.