Deep Ultraviolet Light-emitting Diodes Research Articles

Reduction in operating voltage of AlGaN homojunction tunnel junction deep-UV light-emitting diodes by controlling impurity concentrations

We reduced the operating voltage of AlGaN homojunction tunnel junction (TJ) deep-ultraviolet (UV) light-emitting diodes (LEDs) by two approaches: the suppression of carbon incorporation and the doping of a high concentration of silicon in an n+-AlGaN layer. The AlGaN homojunction TJ deep-UV LEDs had a significantly reduced forward voltage upon suppressing the incorporation of carbon in the n+-AlGaN layer. The suppression of electron compensation by carbon in nitrogen sites and the doping of a high concentration of silicon in an n+-AlGaN layer are important for reducing the operating voltage of AlGaN homojunction TJ deep-UV LEDs.

Extraction efficiency simulation in deep ultraviolet AlGaN light emitting diodes

A ray tracing model is developed to study the light extraction efficiency (LEE) of the nitride wide band gap semiconductor deep ultraviolet light emitting diodes (DUV-LEDs). The basic device structure is flip-chip LED device with dome shape encapsulation. Various device parameters such as reflecting p-metal, absorption coefficient of the AlN layer, sapphire surface roughness etc have been examined. We have found that the transparency of the AlGaN/AlN epitaxial layers as well as the encapsulation material play key roles in improving the LEE and spatial intensity distribution. To verify the transparency of the epi-layers, highly doped n-type Al\(_{0.61}\)Ga\(_{0.39}\)N on AlN template are grown on sapphire substrate by high-temperature metalorganic chemical vapor deposition (HT-MOCVD), then the crystal and optical properties are measured and analyzed. The calculated the absorption coefficient of the AlN is below 10\(^{3}\)cm\(^{-1}\), which paves the way to achieve high extraction efficiency DUV LEDs.

Improved Reliability of AlGaN-Based Deep Ultraviolet LED With Modified Reflective N-Type Electrode

Cr/Al/Ti/Au stacks with two thicknesses of Al layers were employed as reflective n-type electrodes for 274 nm AlGaN-based flip-chip deep ultraviolet light-emitting diodes (DUV LEDs). Large bulges arose in the annealed n-type electrode with 120-nm-thick Al layer, resulting in cracks within the upper rugged SiO2 passivation layer after burn-in test. Sn atoms from solder paste migrated along the cracks and served as leakage current channels, which accelerated device degradation. In contrast, the annealed n-type electrode with 60-nm-thick Al layer retained uniform morphology. And the DUV LEDs with such n-type electrode exhibited good reliability and normalized optical power of 86.8% after burn-in test of 1000 h. This study provides insight into electrode defects related degradation behavior and helps to improve the reliability of AlGaN-based flip-chip DUV LEDs with reflective electrode.

Some Measures for COVID-19 Including Deep Ultraviolet Light-Emitting Diode (DUV-LED), Gc protein-derived Macrophage-Activating Factor (Gcmaf), and 5-Aminolevulinic Acid (5-ALA)

Several trials for the COVID-19 pandemic are found. Prof. Shunji Nakamura, Tokushima University was awarded the 2014 Nobel prize in Physics for light-emitting diodes (LEDs). Deep ultraviolet LED (DUV-LED) causes SARS-CoV-2 inactivation as 99.9% by 265 nm. Gc protein-derived macrophage-activating factor (GcMAF)-based immunotherapy has a wider application for serious infection, chronic fatigue syndrome, and cancers. Covid center in Naples submitted a phase II report concerning GcMAF and COVID-19 in Feb 2021. 5-aminolevulinic acid (5-ALA) and its metabolite protoporphyrin IX (PpIX) show anti-inflammatory and antiviral effects for Zika, Dengue, influenza A viruses, and SARS-CoV-2. These results hopefully contribute to the research development for COVID-19.

AlGaN-based deep ultraviolet micro-LED emitting at 275 nm.

The investigation of electrical and optical properties of micro-scale AlGaN deep ultraviolet (DUV) light-emitting diodes (LEDs) emitting at ∼275nm was carried out, with an emphasis on fabricated devices having a diameter of 300, 200, 100, 50, and 20 µm, respectively. It was revealed that the LED chips with smaller mesa areas deliver considerably higher light output power density; meanwhile, they can sustain a higher current density, which is mainly attributed to the enhanced current spreading uniformity in micro-scale chips. Importantly, when the diameter of LED chips decreases from 300 µm to 20 µm, the peak external quantum efficiency (EQE) increases by 20%, and the EQE peak current density can be boosted from 8.85A/cm2 and 99.52A/cm2. Moreover, we observed a longer wavelength emission with enlarged full-width at half-maximum (FWHM) in the LEDs with smaller chip sizes because of the self-heating effect at high current injection. These experimental observations provide insights into the design and fabrication of high-efficiency micro-LEDs emitting in the DUV regime with different device geometries for various future applications.

Significantly boosted external quantum efficiency of AlGaN-based DUV-LED utilizing thermal annealed Ni/Al reflective electrodes

Improvement of light extraction efficiency of deep ultraviolet light-emitting-diodes (DUV-LEDs) was obtained by adopting a Ni/Al reflective electrode. Optical reflectivity up to 78.1% at 277 nm was achieved. The enhanced reflectivity after thermal annealing was ascribed to the formation of Ni clusters embedded inside the Al matrix as revealed by the correlation between the surface morphology and chemical stoichiometry. The peak external quantum efficiency of a 277 nm DUV-LED incorporating annealed Ni/Al reflective electrodes reaches 3.03%, which is 44% higher than that with conventional Ni/Au p-electrodes, revealing itself as a promising candidate in the realization of high-efficiency DUV emitters.

Enhanced light extraction efficiency via double nano-pattern arrays for high-efficiency deep UV LEDs

This work reports on the packaging structure of double-layer nano-pattern arrays (NPAs) for AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs), which can significantly enhance the light extraction efficiency (LEE), a major device performance bottleneck. The double-layer NPAs were fabricated on the surface of the flip-chip DUV LED by etching sapphire and coating the fluoropolymer resin, which can alleviate the total internal reflection (TIR) at the top interfaces and enhance light extraction on sidewalls, leading to the great improvement of both TE and TM-polarized light. As a result, the 273 nm DUV LED with double-layer NPAs realized 28.3% enhancement in light output power (LOP) compared with the reference device without the NPA at large current of 100 mA. High peak external quantum efficiency (EQE) and wall-plug efficiency (WPE) of 5.19% and 4.38% were demonstrated, respectively. Combined with the finite element analysis (FEA), it is further confirmed that the double-layer NPAs have higher external coupling efficiency to enhance LEE. The proposed LEE enhancement strategy comes from the combination of nanostructured materials and packaging technology, which is cost-effective and meets mass production, providing efficient value in practical applications of UV devices.

On the exciton-assisted radiative recombination via impurity trap levels in AlGaN deep ultraviolet light-emitting diodes

For decades, problems of parasitic emissions have been ubiquitously encountered in nearly all deep ultraviolet light-emitting diodes (DUV-LEDs). In this work, 450 nm parasitic peaks in 275 nm AlGaN DUV-LEDs have been studied in details. Upon careful comparisons and analyses on the electroluminescence and photoluminescence spectra at various injection levels and different temperatures, we have discovered a mechanism of exciton-assisted radiative recombination, namely, the reinforcement on radiative recombination via other impurity-trap levels (ITLs) by excitons that are generated in the midst of the band gap. For DUV-LED samples under investigation herein, a system of radiative ITLs within the band gap cannot be neglected. It includes two types of impurities located at two different energy levels, 3.80 eV and 2.75 eV, respectively. The former, establishing a sub-band edge, which behaves like an energy entrance to this system, contains a series of hydrogen-like excitons at a temperature lower than 100 K, which behaves like an energy entrance to this system. On the one hand, these excitons absorb carriers from band-edge and reduce the band-edge recombination. On the other hand they transfer the energy to lower impurity levels, enhancing the radiative recombination and giving rise to the 450 nm parasitic peak.

Efficacy of 265-nm ultraviolet light in inactivating infectious SARS-CoV-2

Although, Low-pressure (LP) mercury lamps that emit wavelengths of around 254 nm have been widely applied as ultraviolet (UV) light devices for decontamination of microorganisms, they have raised environmental concerns due to their mercury content. Therefore, UV-LED lamps have high potential for practical use as a replacement for LP mercury lamps. In this study, we evaluated the efficacy of 265-nm UV irradiation in comparison to 254-nm and 280-nm UV irradiation for inactivating infectious severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Irradiation from a 265-nm deep UV light-emitting diode (DUV-LED) lamp efficiently inactivated SARS-CoV-2 at a similar level as a 254-nm UV cold cathode lamp, and at a higher level than a 280-nm DUV-LED lamp.

Rapid Inactivation of SARS-CoV-2 Variants by Continuous and Intermittent Irradiation with a Deep-Ultraviolet Light-Emitting Diode (DUV-LED) Device.

More than 1 year has passed since social activities have been restricted due to the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). More recently, novel SARS-CoV-2 variants have been spreading around the world, and there is growing concern that they may have higher transmissibility and that the protective efficacy of vaccines may be weaker against them. Immediate measures are needed to reduce human exposure to the virus. In this study, the antiviral efficacy of deep-ultraviolet light-emitting diode (DUV-LED) irradiation (280 ± 5 nm, 3.75 mW/cm2) against three SARS-CoV-2 variants was evaluated. For the B.1.1.7, B.1.351, and P.1 variant strains, irradiation of the virus stocks for 1 s resulted in infectious titer reduction rates of 96.3%, 94.6%, and 91.9%, respectively, and with irradiation for 5 s, the rates increased to 99.9%, 99.9%, and 99.8%, respectively. We also tested the effect of pulsed DUV-LED irradiation (7.5 mW/cm2, duty rate: 50%, frequency: 1 kHz) under the same output conditions as for continuous irradiation and found that the antiviral efficacy of pulsed and continuous irradiation was the same. These findings suggest that by further developing and optimizing the DUV-LED device to increase its output, it may be possible to instantly inactivate SARS-CoV-2 with DUV-LED irradiation.

Impact of composite last quantum barrier on the performance of AlGaN-based deep ultraviolet light-emitting diode

Impact of composite last quantum barrier on the performance of AlGaN-based deep ultraviolet light-emitting diode

Electron overflow of AlGaN deep ultraviolet light emitting diodes

We have studied the design, epitaxy, and performance characteristics of deep ultraviolet (UV) AlGaN light emitting diodes (LEDs). By combining the tunnel junction and polarization-engineered AlGaN electron blocking layer, a maximum external quantum efficiency and wall-plug efficiency of 0.35% and 0.21%, respectively, were measured for devices operating at ∼245 nm, which are over one order of magnitude higher than previously reported tunnel junction devices at this wavelength. Severe efficiency droop, however, was measured at very low current densities (∼0.25 A/cm2), which, together with the transverse magnetic (TM) polarized emission, is identified to be the primary limiting factors for the device performance. Detailed electrical and optical analysis further shows that the observed efficiency droop is largely due to an electrical effect instead of an optical phenomenon. Our studies suggest that AlGaN deep UV LEDs with efficiency comparable to InGaN blue-emitting quantum wells can be potentially achieved if issues related to electron overflow and TM polarized emission are effectively addressed.

Light extraction efficiency and internal quantum efficiency of fully UVC-transparent AlGaN based LEDs

The light extraction efficiency (LEE), external quantum efficiency (EQE), and current–voltage characteristics of deep ultraviolet light emitting diodes (DUV-LEDs) with different aluminum mole fractions in the p-AlGaN layers have been investigated. Optimizing the p-AlGaN layer composition requires a tradeoff between reducing the absorption losses and limiting the increases in the p-contact resistance and operation voltage. AlGaN multiple quantum well LEDs emitting around 263 nm with different AlGaN:Mg short period super lattices (p-SPSL) ranging from x = 33% (UV-absorbing) to x = 68% (UV-transparent) average aluminum mole fraction have been explored. DUV-LEDs with different p-contact metals and UV-reflectivities have been characterized by electroluminescence measurements and analyzed by ray-tracing simulations. The comparison shows an increased operating voltage and a five-fold increase of the on-wafer EQE with a maximum value of 3.0% for DUV-LEDs with UV-transparent p-SPSL (x = 68%) and UV-reflective indium contacts in comparison to LEDs with a UV-absorbing p-SPSL (x = 33%). Ray-tracing simulations show that the increase in EQE can be partially ascribed to a 2.5-fold improved LEE in combination with a two-fold increase in internal quantum efficiency.

Greatly Enhanced Wall-Plug Efficiency of N-polar AlGaN-Based Deep Ultraviolet Light-Emitting Diodes

In this work, we numerically investigate the N-polar AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) over the Ga-polar DUV LEDs. The light output power has increased from 7.1 mW of Ga-polar DUV LED to 18.8 mW of N-polar DUV LED at 60 mA, and the wall-plug efficiency (WPE) of N-polar DUV LED is boosted by 104% at 60 mA with the same structure of Ga-polar conventional DUV LED. Furthermore, the higher operation voltage of N-polar DUV LED induced by the large energy difference between the p-type interlayer and the p-GaN hole supplier is noted. To reduce the operation voltage of N-polar DUV LED, the structure with a staircase-like p-type interlayer is proposed. The incorporation of staircase-like aluminum composition p-type interlayer mitigates the large potential barrier for holes injection, and the operation voltage is comparable to the Ga-polar DUV LED. The reduced operation voltage further promotes the WPE of N-polar DUV LEDs. Thus, combined with the promoted electron blocking ability and the mitigated potential barrier for holes of N-polar DUV LED, the greatly enhanced WPE is as high as 4.9% at 60 mA, which is 2.88 times higher than the Ga-polar DUV LED.

Effect of HfO2 Passivation Layer on Light Extraction Efficiency of AlInN Nanowire Ultraviolet Light-Emitting Diodes

Diverse applications of ultraviolet (UV) light in the field of purification, sterilization, detection, surface disinfection, sensing, and many others have been studied mostly on AlGaN light-emitting-diodes (LEDs). However, the external quantum efficiency (EQE) of AlGaN deep UV LEDs is limited due to strong induced polarization fields, high dislocation density, severe electron leakage, difficulty in p-type doping, low internal quantum efficiency (IQE), IQE droop, and low light extraction efficiency (LEE). Alternatively, AlInN semiconductor has been recently gained tremendous attention for high power UV LEDs [1-3]. Our group has successfully demonstrated the first AlInN core-shell nanowire UV LEDs with a relatively high IQE of ~52%[1]. However, the light extraction study of such nanowire LEDs is unexplored. In this context, we have studied the LEE of a single AlInN nanowire UV LED at 220 nm - 400 nm wavelength regime. Moreover, we report that the LEE of a single AlInN nanowire UV LED could be increased by ~60% using HfO2 passivation compared to conventional nanowire LED at 282 nm.The schematic structure of AlInN nanowire UV LED is shown in Figure 1(a). The nanowire is composed of 200 nm n-GaN, 100 nm n-AlxIn1-xN quantum barrier (QB)/40 nm i-AlyIn1-yN quantum well (QW)/100 nm p-AlxIn1-xN QB, and 20 nm p-GaN layer. We have estimated the LEE of nanowire LEDs using the finite difference time domain (FDTD) simulation method. Silicon (Si) is used as a substrate, and the n-GaN layer is tapered with a diameter from 70 nm (bottom) to 100 nm (top). The diameter is set to be 100 nm for the rest of the regions. We have placed a single TM-polarized dipole source at the QW center to calculate LEE for a nanowire LED in the simulations. The estimated LEE of 282 nm wavelength single nanowire AlInN UV LED with various HfO2 passivation layers thickness is provided in Figure 1(b). The LEE of such LED without any passivation is ~25.2%, whereas the maximum recorded LEE is ~40.6% with the utilization of 35nm thickness HfO2 passivation layer. This study provides a promising approach for enhancing the light extraction efficiency of nanowire UV LEDs. Reference R. T. Velpula, B. Jain, M. R. Philip, et al., Sci. Rep., 10(1) 2547-2556 (2020).R. T. Velpula, B. Jain, H.Q.T. Bui, et al., Opt. Mater. Express, 10(2), 472-483 (2020).B. Jain, R. T. Velpula, M. Tumuna, et al., Opt. Express, 28(15), 22908-22918 (2020). Figure 1

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

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

Sandwiching electron blocking layer with p-AlInN layer to enhance hole injection in AlGaN-based deep ultraviolet light-emitting diodes

This work reports the enhanced optoelectronic performance of AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) by sandwiching p-EBL with thin p-AlInN layers. The simulated results show that the internal quantum efficiency (IQE) and radiative recombination rate are remarkably improved as compared to conventional AlGaN-based p-EBL. The primary cause of this enhancement is the reduction of lattice mismatch between the electron blocking layer (EBL) and p-AlGaN due to the insertion of thin p-AlInN layers, which ultimately decreases the polarization effect. Moreover, p-AlInN layers also improved the hole injection efficiency via intra-band tunneling while hindered the electron leakage to the p-type layer. Interestingly, the proposed structure not only increased the IQE but also suppressed the efficiency droop dramatically.

Effect of HfO2 Passivation Layer on Light Extraction Efficiency of AlInN Nanowire Ultraviolet Light-Emitting Diodes

One of the probable reasons behind the limitation of light extraction efficiency (LEE) in III-nitride nanowire (NW) deep ultraviolet (UV) light-emitting diodes (LEDs) is the presence of the high surface density states that significantly contribute to the non-radiative recombination near the surface. Herein, we investigate the LEE of a single AlInN NW UV LED in the entire UV wavelength regime using finite-difference time-domain simulations. It is found that these LEDs favor transverse-magnetic (TM) polarized LEE over transverse-electric (TE) polarized emission. Further, we examine the role of the HfO2 surface passivation layer in the improvement of LEE of AlInN NW LED at ~282 nm wavelength. Our results show that the TM-polarized LEE of such LED without any passivation is only ~25.2%, whereas the maximum recorded LEE is ~40.6% with the utilization of 40 nm HfO2 passivation layer. This study provides a promising approach for enhancing the LEE of NW UV LEDs.

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

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

Control of dislocations in heteroepitaxial AlN films by extrinsic supersaturated vacancies introduced through thermal desorption of heteroatoms

Efficient reduction of dislocations is a key topic in heteroepitaxial AlN films for optoelectronic or electronic applications. For this purpose, we explore a strategy of supersaturated vacancy engineering to promote the climb and meeting of dislocations. Through the intentional incorporation and subsequent thermal desorption of heteroatoms, the thermodynamic limitation for vacancy concentration under equilibrium conditions is broken, i.e., the concentration depends directly on the desorption number of heteroatoms, instead of formation energy. As such, extrinsic supersaturated vacancies can be introduced, significantly magnifying the climb of dislocations and thus enhancing the probability of dislocation meeting and annihilating. Specifically, the supersaturated vacancy engineering is applied to AlN on sapphire with a lattice mismatch as high as 13.3%, which demonstrates a threading dislocation density of 1.56 × 108 cm−2, one order of magnitude lower than that by conventional methods. Furthermore, 280-nm deep-ultraviolet light-emitting diodes are fabricated on such AlN template, and the light output power reaches 39.1 mW at 200 mA, 56% better than that without this approach. This study sheds light on the effective control of vacancies and dislocations and then paves the way for heteroepitaxial films of high quality, as well as consequent optoelectronic or electronic devices of high performance.