Wall-plug Efficiency Research Articles

Demonstration of high wall-plug efficiency III-nitride micro-light-emitting diodes with MOCVD-grown tunnel junction contacts using chemical treatments

High wall-plug efficiency (WPE) micro-light-emitting diodes with metalorganic chemical vapor deposition-grown tunnel junction (TJ) contacts are demonstrated. By employing chemical treatments before sidewall activation, the 20 × 20 μm2 TJ devices resulted in a voltage penalty of 0.2 V at 20 A cm−2, compared to devices with indium-tin oxide (ITO) contacts. Moreover, the enhancement in light output power was more than 40% higher than ITO devices. Hence, the TJ devices yielded the peak external quantum efficiency (EQE) and WPE of 56% and 55%, respectively, indicating the improvements of 64% and 77% in peak EQE and WPE compared to ITO devices.

Development of Metal Bonding for Passive Matrix Micro-LED Display Applications

Recently, there are many breakthroughs of display in the high resolution and vivid colors for applications in virtual/augmented reality. In this study, we have successfully developed a $64 \times 32$ array inline structure of a micro-LED ( $\mu $ LED) display by metal bonding an LED array with a glass circuit for connecting a passive matrix IC driver. The dimension of each $\mu $ LED was $100\,\,\mu \text{m}\,\,\times 100\,\,\mu \text{m}$ . After bonding, the optoelectronic characteristics were measured to evaluate the best process. By the circuit fabricated on glass and by using a bonding design, we were able to increase the usage ratio of the epitaxy layer and reduce the cost of the $\mu $ LED display. Au–In metal bonding demonstrated an excellent bonding result that can withstand a shear force up to 12.58 kg after being reinforced with UV glue. The output power of the display was 61 mW under 120 mA (@1.9 V) current injection to the whole display and the wall plug efficiency is 26.75%.

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.

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.

On the role of dissociative recombination on the effectiveness of a plasma neutralizer in DEMO fusion plant

In order to reduce the recirculating electrical power of DEMO the wall-plug efficiency of the additional heating systems plays a crucial role. The efficiency of the heating neutral beams (HNBs) of ITER is mostly limited by the gas neutralizer, whose efficiency is theoretically limited to about 55%. The use of a plasma neutralizer is a valuable alternative to increase the neutralization efficiency to about 80%, with a consequent improvement of the overall efficiency. The effectiveness of a plasma neutralizer increases with the ionization degree of the plasma, which is thus the key parameter of the system. The concept of a so-called “beam driven” plasma neutralizer (BDPN), in which the power to sustain the plasma is provided by the ion beam itself, is very appealing, and is explored in several works. Up to now though, a proof of principle for a plasma neutralizer was demonstrated only in experiments with an external plasma source and the scaling to an ITER-relevant plasma neutralizer suggests that a power in the order of 0.5 MW should be injected from outside to sustain a plasma with a 30% ionization degree. This work tries to bridge the gap between the models predicting the feasibility of a high efficiency BDPN and the experimental scaling. In particular, the role of the volume losses is highlighted and a much better agreement with experiment based scaling is obtained.

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.

Design and construction of holmium:yttrium aluminum garnet laser for lithotripsy applications

A holmium:yttrium aluminum garnet (Ho:YAG) laser is mainly used in medical surgeries such as tissue ablation and removal of urinary stones. In the present study, a 20 W solid-state 2.08 μm Ho:YAG laser was designed and implemented for lithotripsy applications. Effects of the temporal profile beam of the Ho:YAG laser on stone fragmentation were demonstrated. Moreover, the power supply driver of the xenon flashlamp was implemented with a distinctive trigger circuit at the start of the laser operation. The home-built power driver was customized to deliver a 0.2–3 ms rectangular pulse duration between 1 and 20 Hz frequencies. Laser pulse energies at 5 Hz in response to pumping energy were measured with 3.55% slope efficiency. Similarly, laser pulse energies at 10 Hz were recorded with a slope efficiency of 2.96%. Overall, 1.7% wall-plug (electrical to optical) efficiency was obtained at 5 and 10 Hz laser operations.

Improving Emission Uniformity of InGaN/GaN-Based Vertical LEDs by Using Reflective ITO/Ag n-Contact

We investigated the effect of Ti/Al and ITO/Ag n-type contacts on the emission uniformity and light output of different chip-size vertical-geometry light-emitting diodes (VLEDs) for vehicle headlamp application. The forward voltage of the Ti/Al-based reference VLEDs decreased from 3.38 to 3.20 V at 1500 mA with increasing chip size from (1280 × 1000 µm2) to (1700 × 1700 µm2), whereas that of the ITO/Ag-based samples changed from 3.37 to 3.15 V. Regardless of chip size, the ITO/Ag-based samples revealed higher light output power than the reference samples. For example, the ITO/Ag-based samples (chip size of 1700 × 1700 µm2) exhibited 3.4% higher light output power at 1500 mA than the reference samples. The ITO/Ag samples underwent less degradation in the Wall-plug efficiency (WPE) than the reference sample. For instance, the ITO/Ag-based samples (1700 × 1700 µm2) gave 4.8% higher WPE at 1500 mA than the reference samples. The ITO/Ag-based samples illustrated more uniform emission than the Ti/Al-based sample. Both the reference and ITO/Ag-based samples underwent no degradation when operated at 1500 mA for 1000 h.

Blue light‐emitting diodes for disinfection

Disinfection processes are currently of particular interest due to growing concerns over illnesses caused by human pathogenic germs. Blue light‐emitting diodes (LEDs) can reduce the microbial load on surfaces, in liquids and in air. The high wall‐plug efficiency of the diodes, their additive‐free antimicrobial effect and the good tolerability of the light enable affordable and safe disinfection systems.

Increasing the Carrier Injection Efficiency of GaN-Based Ultraviolet Light-Emitting Diodes by Double Al Composition Gradient Last Quantum Barrier and p-Type Hole Supply Layer

A 365 nm AlxGa1-xN-based ultraviolet light-emitting diodes (LEDs) with double Al composition gradient last quantum barrier and hole supply layer structure has been studied. Experimental results show that the proposed structure enhances the carrier injection efficiency and suppresses the overflow of electrons. The introduction of three-dimensional hole gas further enhances the hole injection efficiency. As a result, the wall plug efficiency and electroluminescence intensity are significantly improved. In addition, the carrier concentration and the radiative recombination rate in the active region of the quantum well increases. Looking at the energy band diagram, one sees that the double Al composition gradient structure leads to higher electron blocking and alleviates the hole blocking effect. Overall, we conclude that the double Al composition gradient structure provides a pathway to achieve high-efficiency ultraviolet LEDs.

Improved Reliability of 278 nm Deep Ultraviolet AlGaN-Based Flip-Chip Light Emitting Diodes by Using ITO/Al Contact

We investigated the electrical and optical performance and reliability of 278 nm deep ultraviolet (DUV) AlGaN-based flip-chip light-emitting diodes (FCLEDs) fabricated with ITO/Al and reference Ni/Au contacts. The DUV FCLEDs with the Ni/Au and ITO/Al contacts yielded forward voltages of 6.52 and 6.65 V at 50 A cm−2 and light output of 6.36 and 10.06 mW at 50 A cm−2, respectively. The ITO/Al-based FCLEDs produced higher Wall plug efficiency (WPE) (3.04% at 50 A cm−2) than the Ni/Au-based samples (1.96%). The ITO/Al-based FCLEDs revealed 55% higher WPE at 50 A cm−2 than the Ni/Au-based sample. For both of the samples, the output power decreased with increasing operation time at 100 A cm−2. For example, after 2000 h, the Ni/Au and ITO/Al-based FCLEDs showed a reduction in the output power by 37% and 22%, respectively. Despite the good output characteristics, the ITO/Al contact-based FCLEDs exhibited higher forward bias voltages than the Ni/Au-based sample. Based on the energy dispersive X-ray spectroscopy (EDS) depth profiles, high-angle annular dark field (HAADF), and electron energy loss spectroscopy (EELS) results, reason for the increase in the forward voltage of ITO/Al-based FCLEDs is described and discussed.

Study of a GaN-Based Light-Emitting Diode with a Specific Hybrid Structure

A specific hybrid structure, including 45° sidewalls, a shallower (0.25 μm in depth) microhole array, and a 3-D like backside reflector, is used to fabricate GaN-based light-emitting diodes (LEDs). The 3-D like backside reflector is formed by the deposition of 100 nm-diameter SiO2 nanospheres (NSs) and an Al layer on the sapphire substrate. Based on the use of 45° sidewall and microhole array, the reduced total internal reflection (TIR) and Fresnel reflection and the increased scattering probability and the opportunity of photons to find escape cones can be expected. In addition, the use of 3-D like backside reflector can effectively reflect scatter, and redirect the downward photons emitted from multiple quantum well (MQW) region into arbitrary directions. Thus, the light extraction performance can be substantially improved. In experiment, under an injection current of 400 mA, the studied Device D with this hybrid structure shows enhancements of 47.4%, 47.7%, 33.1%, and 47.8% in light output power (LOP), external quantum efficiency (EQE), luminous flux, and wall-plug efficiency (WPE), respectively, as compared to a conventional LED. Moreover, the related electrical properties are not degraded for the studied Device D. Therefore, this hybrid structure shows a promise for high-performance GaN-based LED applications.

Size-Dependent Quantum Efficiency of Flip-Chip Light-Emitting Diodes at High Current Injection Conditions

Versatile applications call for InGaN-based light-emitting diodes (LEDs) to operate at ultra-high current densities with high quantum efficiency. In this work, we investigated the size-dependent effects of the electrical and optical performance of LEDs as increasing the current density up to 100 A/cm2, which demonstrated that mini-strip flip-chip LEDs were superior option to achieve better performance. In detail, at a current density of 100 A/cm2, the light output power density of these mini-strip LEDs was improved by about 6.1 W/cm2, leading to an improvement in the wall-plug efficiency by 4.23%, while the operating temperature was reduced by 11.3 °C, as compared with the large-sized LEDs. This could be attributed to the increase in the sidewall light extraction, alleviated current crowding effect, and improved heat dissipation. This work suggests an array of mini-strip LEDs would provide an option in achieving higher luminescent efficiency at ultrahigh current injection conditions for various applications.

Band-Engineered Structural Design of High-Performance Deep-Ultraviolet Light-Emitting Diodes

In this study, systematic structural design was investigated numerically to probe into the cross-relating influences of n-AlGaN layer, quantum barrier (QB), and electron-blocking layer (EBL) on the output performance of AlGaN deep-ultraviolet (DUV) light-emitting diodes (LEDs) with various Al compositions in quantum wells. Simulation results show that high-Al-composition QB and high-Al-composition EBL utilized separately are beneficial for the enhancement of carrier confinement, while the wall-plug efficiency (WPE) degrades dramatically if both high-Al-composition QB and EBL are existing in a DUV LED structure simultaneously. DUV LEDs may be of great optical performance with appropriate structural design by fine-tuning the material parameters in n-AlGaN layer, QB, and EBL. The design curves provided in this paper can be very useful for the researchers in developing the DUV LEDs with a peak emission wavelength ranging from 255 nm to 285 nm.

Towards a maintainable and high efficiency neutral beam system for future fusion reactors

Achievement of an economic fusion reactor imposes a high level of unprecedented requirements for neutral beam systems; the first one is the nuclear safety constraints which impose a prerequisite on the ease of access of all injector components to provide a preventive (and curative) maintenance by remote handling while maintaining reactor operation. In addition, the challenge is to develop high-power injectors with very high wall-plug efficiency (above 60%) that are able to operate in stable conditions over several months. There is a significant gap to bridge with respect to the present neutral beam systems, which are characterized by a low efficiency and by complex and long maintenance operations. Evidence that this injector concept does not offer adaptations to cope with the reactor requirements makes it clear that a new concept has to be addressed. An injector concept with modular sources at ground voltage is proposed. The concept makes remote maintenance of the injector components easier, where each source module can be replaced by a new one without breaking the vacuum and affecting injector conditioning. With the grounded and modular ion source, photoneutralization associated with energy recovery appears to be the best route capable of attaining the reactor requirements. This concept of a maintainable neutral beam system would provide a high heating power with a wall-plug efficiency above 70% and unprecedented features such as the capacity of producing temporal and spatial modulation of the beam power for a better control of the plasma stability. Up to now, photoneutralization feasibility studies already carried out on reduced-scale prototypes have not highlighted any showstoppers. Continuation of research and development in the years to come can pave the way towards the achievement of a first full-scale high-power cavity in the 1 MW range, and the realization of a multi-amperes (∼10 A) thin blade-like D− beam.

High-power solid-state amplifier for superconducting radio frequency cavity test facility.

A horizontal test facility is set up at the Raja Ramanna Centre for Advanced Technology to test the superconducting radio frequency dressed cavities. Along with the cryomodule, control instrumentation, and the power coupler, this facility incorporates a high-power solid-state amplifier for establishing the desired cavity voltage gradient during the testing. This article describes the design, construction, rigorous testing, and measured results of this high-power solid-state radio frequency amplifier and its constituent components. Its maximum output power is 36 kW (average) at the operating frequency of 650 MHz. Its main features are its modular and scalable design with in-house developed constituent components. These components include 500 W, 20 dB gain modules, novel two-tier radial dividers, combiners, power sensors, and aperture-coupled directional couplers. Their excellent reprise performance for the multiple quantities confirms the design methodology presented here. The measured wall plug efficiency of this 36 kW amplifier is 43.6%, and its power gain is 86 dB. The designed radial combiner is highly efficient (power-combining efficiency of 98.4%), and the directional coupler exhibits a very low loss (insertion loss of 0.05 dB).

High-power mid-wave infrared LED using W-superlattices and textured surfaces

Efficient mid-infrared light output has been obtained by incorporating a W-superlattice into a cascaded mid-infrared LED structure and by thinning and roughening of the emission side of the structure. At cryogenic temperatures, a radiance of ∼13.4 W/cm2-sr is achieved. Compared to the best published InAs/GaSb mid-IR LED, the maximum radiance is improved by ∼2.0×, while the wallplug efficiency improvement at the maximum radiance is improved >10×. For room temperature measurements on an un-thinned 400 μm diameter diode, the radiance (light output power) for a quasi-continuous wave and 1% duty cycle were ∼ 0.48 W/cm2-sr (2.4 mW) and ∼1.35 W/cm2-sr (6.8 mW), respectively. When compared to previous room temperature 4.2 μm LEDs, at a 1% duty cycle, this LED has optical powers that are 3× brighter. When compared to thermal emitters used in gas sensors, in the quasi-continuous wave, this LED uses ∼100× less energy per measurement.

Highly efficient ∼8 μm-emitting, step-taper active-region quantum cascade lasers

Recently, it was shown for 4.5 μm–5.0 μm-emitting, state-of-the-art quantum cascade lasers (QCLs) that the internal efficiency can be fully accounted for when considering interface-roughness (IFR)-triggered carrier leakage from both the upper-laser (ul) level and key injector states. By applying the same formalism to ∼8.0 μm-emitting QCLs of a step-taper active-region (STA) design, we find that the devices’ internal efficiency reaches a high value of ∼76%. That is partly due to a record-high injection-efficiency value (89%), as a result of strong carrier-leakage suppression, and partly due to an IFR-scattering enhanced laser-transition efficiency value of 85.2%. By comparison, when the same analysis is applied to conventional ∼8.0 μm-emitting QCLs, grown by the same crystal-growth technique: metal–organic chemical vapor deposition (MOCVD), the internal efficiency is found to be only ∼59%, typical of values extracted from experimental data of mid-infrared-emitting conventional QCLs. When further comparing the ∼8.0 μm-emitting STA QCLs with conventional QCLs, the ul-level lifetime is found to be controlled by both LO-phonon and alloy-disorder scattering, similar to what we recently found for 4.5 μm–5.0 μm-emitting QCLs. However, unlike 4.5 μm–5.0 μm-emitting QCLs, the lower-laser level lifetime is found to be controlled by both LO-phonon and IFR scattering. In addition to the high internal-efficiency value, the use of excited-state injection and a low voltage defect result in the STA QCL reaching a front-facet wall-plug efficiency value of 10.6%, a record-high, front-facet value for 8 μm–11 μm-emitting QCLs grown by MOCVD and holding potential for continuous-wave operation.

Conditioning nano-LEDs in arrays by laser-micro-annealing: The key to their performance improvement

A local so-called laser-micro-annealing (LMA) conditioning technology, which is suitable for the fabrication of a large range of hybrid nano-optoelectronic devices, was applied to III-nitride-based nano-light emitting diodes (LEDs). The LEDs with a diameter of ∼100 nm were fabricated in large area arrays and designed for hybrid optoelectronic applications. The LMA process was developed for the precise local conditioning of LED nano-structures. Photoluminescence measurements reveal the enhancement of nano-LED properties, which is in very good agreement with a simple model introduced based on the reduction of the defect layer depth by the LMA process. The experimental data confirm the reduction of the defect layer depth from ∼17 nm to ∼5 nm determined. In consequence, an increase in work currents up to 40 nA at 5 V bias after the LMA procedure as well as high electroluminescence (EL) and output optical power up to 150 nW in the ∼440–445 nm emission wavelength range corresponding to ∼75% wall-plug efficiency were achieved. Additionally, the LEDs' electroluminescence intensities reach the desired values by conditioning the contact/annealed regions of individual LEDs accordingly. Furthermore, the LMA process affects the long-term stability of the electroluminescence (EL) intensity of single nano-LED devices. A study of the EL during 5000 h in the continuous wave operation testing mode revealed a moderate ∼15% decrease in the intensity in comparison to ∼50% for their non-LMA counterparts. Finally, Raman measurements indicate that the “work” temperature for nano-LED conditioned structures decreases.

395 nm Light-Emitting Diode with 647 mW Output Power Realized Using a Double p-Type Aluminum Composition Gradient with Polarization-Induced Hole Doping

Three p-type layer structures, i.e. a p-GaN layer, a short-period superlattice (SL) insertion layer, and a polarization-induced hole-doped double-aluminum composition gradient layer, were compared in terms of their performance in 395 nm high-power light-emitting diodes (LEDs). The device with the polarization-induced hole-doped double-aluminum composition gradient layer has an operating voltage of 3.34 V and a wall-plug efficiency of 55.3% under a 350 mA injection current. Moreover, devices with the polarization-induced hole-doped double-aluminum composition gradient layer structure have better optical output power than devices with a short-period SL inserted in p-GaN.