Wall-plug Efficiency Research Articles

Cascaded deep ultraviolet light-emitting diode via tunnel junction

The AlGaN-based deep ultraviolet (DUV) light-emitting diode (LED) is an alternative DUV light source to replace traditional mercury-based lamps. However, the state-of-the-art DUV LEDs currently exhibit poor wall-plug efficiency and low light output power, which seriously hinder their commercialization. In this work, we design and report a tunnel-junction-cascaded (TJC) DUV LED, which enables multiple radiative recombinations within the active regions. Therefore, the light output power of the TJC-DUV LEDs is more than doubled compared to the conventional DUV LED. Correspondingly, the wall-plug efficiency of the TJC-DUV LED is also significantly boosted by 25% at 60 mA.

Watt-level, high wall plug efficiency, continuous-wave room temperature quantum cascade laser emitting at 7.7µm.

In this article, a InP based strain-balanced In0.58Ga0.42As/In0.47Al0.53As quantum cascade laser emitting at 7.7µm is reported. The active region is based on a slightly-diagonal bound to continuum design with 50 cascade stages and a low voltage defect Δinj of 96 meV. By optimizing the active region and waveguide structure, the waveguide loss αw of 1.18cm-1 are obtained, which contribute to a high wall-plug efficiency (WPE) of 9.08% and low threshold current of only 1.09 kA/cm2 in continuous-wave(CW) operation at 293K. The maximum single facet output power of 1.17W in CW operation and 2.3W in pulsed operation are measured at 293K. The narrow ridge and buried ridge structure epi-side-down-mounted on the diamond heatsink improved the heat dissipation of the device. A beam of pure zero order mode and a broad external-cavity tuning range from 7.16µm to 8.16µm are also achieved.

Development of a neural probe integrated with high-efficiency MicroLEDs for in vivo application

A neural probe with six micro-light-emitting diodes (MicroLEDs) and 15 neural electrodes was fabricated for optogenetic application. Local field potentials, which provide information about the neural activity, were successfully recorded using the neural probe, indicating the effectiveness of the neural electrodes. The MicroLEDs on the probe exhibited highly consistent current–voltage characteristics and sufficient light output of 20 mW mm−2 at 1 mA to manipulate neural activity. The light distribution in brain tissue was simulated to estimate the optical stimulation area and a number of optically stimulated neurons. The increase in LED temperature, i.e. ΔT, was investigated because high temperatures can damage brain tissue. A curve illustrating the relationship between ΔT and the wall-plug efficiency was derived. The wall-plug efficiency was increased 1.8 times by installing an Ag mirror on the back of a MicroLED. These results suggest that the MicroLED neural probe would significantly contribute to the development of neuroscience research-purposed optogenetic technology.

Virus and Bacteria Inactivation Using Ultraviolet Light-Emitting Diodes

The control of infectious diseases is a critical challenge in developing and developed nations alike. Humans can be exposed to pathogenic microorganisms such as viruses and bacteria through numerous routes of transmission, including waterborne transmission. Accordingly, effective water disinfection processes are paramount for public health protection. In this study, the inactivation efficacy of P22 virus and Escherichia coli bacteria was tested using ultraviolet light-emitting diodes (UV LEDs), which provided effective disinfection of both waterborne microorganisms. Using a peak wavelength emission of 255, 265, or 285 nm, the batch-test, UV LED collimated beam system demonstrated high inactivation potential for both microorganisms at all wavelengths, suggesting that it could be an effective alternative to typical, low-pressure (LP) or medium-pressure (MP), mercury-based UV systems. Increased inactivation kinetics for both microorganisms was observed with decreasing wavelength (255 > 265 > 285 nm). However, the trend reversed when accounting for energy efficiency of the systems (255 < 265 < 285 nm) due to the currently lower wall plug efficiency for lower UV LED wavelength emissions. Substantial improvements in LED efficiency are needed to make them energetically competitive with current LP and MP UV technologies.

Improving Optical Performance of NUV-LEDs by Highly Reflective BN/Silicone Structures

Near-ultraviolet light-emitting diodes (NUV-LEDs) are more likely to be commercially fabricated. The improvement of their wall-plug efficiency (WPE) by facial and effective methods has made a great impact. We propose a highly reflective boron nitride silicone (BN/silicone) layer to improve the light output power for NUV-LEDs. Our results show that increasing the BN concentration contributes to a higher UV-reflectivity of the BN/silicone layers. When the BN concentration increases from 0.5 to 15 wt%, the light output power enhancement increases gradually, thus achieving a maximum light output power enhancement of 46.2% for NUV-LEDs at a driving current of 350 mA compared with pure silicone encapsulation devices. This significant improvement is attributed to the increased UV-reflectance by the polymer-based lead-frame and packaging materials. Furthermore, the emission angle is decreased by 7.8° at a BN concentration of 15 wt% at 350 mA owing to the strong scattering ability of the BN/silicone layer. This provides more concentrated energy in lighting applications.

State-of-the-art and current challenges for TiO2/UV-LED photocatalytic degradation of emerging organic micropollutants.

The development of ultraviolet light-emitting diodes (UV-LED) opens new possibilities for water treatment and photoreactor design. TiO2 photocatalysis, a technology that has been continuously drawing attention, can potentially benefit from LEDs to become a sustainable alternative for the abatement of organic micropollutants (OMPs). Recently reported data on photocatalytic degradation of OMPs and their parameters of influence are here critically evaluated. The literature on OMP degradation in real water matrices, and at environmentally relevant concentrations, is largely missing, as well as the investigations of the impact of photoreactor design in pollutant degradation kinetics. The key factors for reducing UV-LED treatment technology costs are pointed out, like the increase in external quantum and wall-plug efficiencies of UV-LEDs compared to other technologies, as well as the need for an appropriate design optimizing light homogeneity in the reactor. Controlled periodic illumination, wavelength coupling and H2O2 addition are presented as efficiency enhancement options. Although electrical energy per order (EEO) values for UV-LED photocatalysis have decreased to the range of traditional mercury lamps, values are still not low enough for practical employment. Moreover, due to the adoption of high initial OMP concentration in most experiments, it is likely that most literature EEO values are overestimated. Given the process characteristics, which are favoured by translucent matrices and small diameters for more homogenous light distribution and better transportation of radicals, innovative reactor designs should explore the potential of point-of-use applications to increase photocatalysis applicability at large scale.

Efficient emission of InGaN-based light-emitting diodes: toward orange and red

Indium gallium nitride (InGaN)-based light-emitting diodes (LEDs) are considered a promising candidate for red-green-blue (RGB) micro displays. Currently, the blue and green LEDs are efficient, while the red ones are inefficient for such applications. This paper reports our work of creating efficient InGaN-based orange and red LEDs on silicon(111) substrates at low current density. Based on the structure of InGaN yellow LEDs, by simply reducing the growth temperature of all the yellow quantum wells (QWs), we obtained 599 nm orange LEDs with peak wall-plug efficiency (WPE) of 18.1% at 2 A / cm 2 . An optimized QW structure was proposed that changed two of the nine yellow QWs to orange ones. Compared with the sample containing nine orange QWs, the sample with two orange QWs and seven yellow QWs showed similar emission spectra but a much higher peak WPE up to 24.0% at 0.8 A / cm 2 with a wavelength of 608 nm. The improvement of peak WPE can be attributed to the improved QW quality and the reduced active recombination volume. Subsequently, a series of efficient InGaN-based orange and red LEDs was demonstrated. With further development, the InGaN-based red LEDs are believed to be attainable and can be used in micro LED displays.

Study of GaN-Based Light-Emitting Diode (LED) With a Hybrid Surface Structure

A hybrid surface structure, incorporating an Ag-grid-based aluminum-doped zinc oxide (AZO) transparent conductive layer (TCL), a microhole array, 45°-sawtooth sidewalls, and an SiO2 nanoparticle (NP)/microsphere (MS) passivation layer, is proposed for the characteristic improvement of GaN-based light-emitting diodes (LEDs). In order to optimize the contact behavior between the AZO and p-GaN layers, a 2-D Ag grid is applied on the contact interface. Because series resistance ${R}_{s}$ is reduced by the conductive Ag-grid pattern, a lower forward voltage and a better current spreading ability are obtained. Compared with a conventional GaN LED with an injection current of 200 mA, the proposed device exhibits a forward voltage of 4.01 V, reduced from 4.34 V, and presents 33%, 34.4%, 45.4%, 33.1%, and 45.3% enhancements in the light output power (LOP), luminous flux, luminous efficacy, external quantum efficiency (EQE), and wall-plug efficiency (WPE), respectively. Moreover, a more effective current spreading in the light emission mapping image and a higher intensity in the far-field pattern are achieved. Improvements of both electrical and optical properties verify that the proposed device is promising for practical applications in solid-state lighting.

Slow degradation of a multiemitter 638-nm high-power broad-area laser diode during long-term aging

Laser-based displays have attracted much attention because they are the only displays that can express the full color gamut of Ultra-High Definition Television (U-HDTV), International Telecommunication Union Radio Communication Sector Broadcasting Service (ITU-R BT).2020. We have developed 638-nm broad-area laser diode (LD) with 75-μm-wide dual emitters and achieved wall plug efficiency of over 40.5% at 25°C. But slow degradation is still a factor limiting the lifetime due to the requirement for a high temperature and high-power operation. We performed the long-term aging test of 638-nm LDs under the different conditions, including a high-power operation such as 1.3 to 2.5 W, and investigated the behavior of output power characteristics, especially the dependence of its output characteristics on temperature. After long-term aging, the threshold current of the LD increased and its slope efficiency (SE) decreased. The measured dependence of the output characteristics of the LD on temperature was converted into that on the junction temperature using the thermal resistance between the junction and package. The latter dependence showed an increase of threshold current and no change of SE. This result indicates that the slow degradation of the red broad-area LD was caused by the increase of the nonradiative recombination rate in the active layer during aging.

19-element vertical cavity surface emitting laser arrays with inter-vertical cavity surface emitting laser ridge connectors

We achieve record concurrent combinations of bandwidth (18 GHz), optical output power (150 mW), and wall plug efficiency (30%) with a unique arrangement of 19-element, electrically parallel 980 nm vertical cavity surface emitting laser (VCSEL) arrays. We use a new two-dimensional, quasi honeycomb geometry with inter-VCSEL ridge connectors—made nonconducting by selective thermal oxidation—to improve heat dissipation and facilitate a single top surface anode contact. Via on-wafer probing we perform static and dynamic measurements over the wide temperature range of 23 °C to 85 °C and extract, report, and discuss key array figures-of-merit.

Experimental and computational evaluation of a flow-through UV-LED reactor for MS2 and adenovirus inactivation

The design of efficient ultraviolet light emitting diode (UV-LED) reactors is of paramount importance for delivering a high UV dose (fluence). This is particularly critical to the disinfection of UV-resistant organisms, such as adenovirus, in continuous UV systems, given the low wall plug efficiency of UV-LEDs. The UV-LED offers reactor design flexibility, allowing to conform the reactor fluence rate to its hydrodynamics to a high degree, leading to improved design efficiency. In this study, we designed and fabricated a flow-through multi-baffle UV-LED reactor containing 18 UV-LEDs for delivering high UV fluence to a flow of water. Further, we developed a computational model for simulating the reactor performance, to fine-tune the fluence rate and hydrodynamics conformity through exploring several design concepts. The model was evaluated by experimental studies using male-specific-2 (MS2) bacteriophage and adenovirus as the model UV resistance microorganisms. The radiant field was adjusted through the LED arrangement and radiation profile modification. The reactor performance was studied under six different LED arrangements, three irradiation modes, and flow rates of 0.75, 1, and 2 L min−1. Adenovirus inactivation at 1 L min−1 from 1.6 logs increased to 4.1 logs through radiant field modification and enhancing the design efficiency. It was observed that the flexibility offered by UV-LEDs in adjusting the radiation pattern and hydrodynamics could lead to the design of highly-efficient reactors through positively correlating the UV fluence rate profile with the velocity filed. This study presents a detailed discussion concerning irradiation methods and provides insights applicable to UV-LED reactor design.

Improvement of p-electrode structures for 280 nm AlGaN LED applications

An improvement of Ni/Au/p+-GaN p-electrode for AlGaN deep-ultraviolet light-emitting diodes (DUV LEDs) with the emission wavelength of 280 nm is proposed for both p-side-up and flip-chip structures. An interdigitated multi-finger Ni/Au was employed in p-side-up DUV LED, where the p-GaN contact layer was partially removed to improve the light extraction efficiency without a serious current-crowding effect. The 9- and 12-finger LEDs were determined to have higher thermal dissipation and lower surface temperatures and correlated well with the theoretical simulation. For the comparison of p-side-up emission LEDs, the output power of 9-finger LED is 172% higher than that of conventional LED at the current injection of 350 mA. The optimum p-electrode pattern was further applied to the flip-chip LED structure. It is determined that the output power of 9-finger flip-chip LED at 350 mA is still 14.6% higher than that of a conventional flip-chip LED. The higher output power of 9-finger flip-chip LED with a wall-plug efficiency of 1.05% is attributed to the combination of the improved current-spreading path and the higher reflection through the moderate removal of partial p+-GaN absorbing layer.

The Interband Cascade Laser

We review the history, development, design principles, experimental operating characteristics, and specialized architectures of interband cascade lasers for the mid-wave infrared spectral region. We discuss the present understanding of the mechanisms limiting the ICL performance and provide a perspective on the potential for future improvements. Such device properties as the threshold current and power densities, continuous-wave output power, and wall-plug efficiency are compared with those of the quantum cascade laser. Newer device classes such as ICL frequency combs, interband cascade vertical-cavity surface-emitting lasers, interband cascade LEDs, interband cascade detectors, and integrated ICLs are reviewed for the first time.

19-Element 2D Top-Emitting VCSEL Arrays

We achieve 200 mW optical output power, 18 GHz bandwidth, and 35% wall plug efficiency with 19-element (novemdecuple) top-surface-emitting 980 nm oxide-confined vertical-cavity surface-emitting laser (VCSEL) arrays - a record combination of these three attributes. We grow wafers by metal-organic vapor phase epitaxy and process arrays into a unique two-dimensional honeycomb geometry with top mesa inter-VCSEL ridge connectors and p-metal anode interconnections. We study the impact on array performance of inter-VCSEL spacing and bottom n-doped mesa area. The top emitting VCSELs are optically uncoupled but electrically connected in parallel via anode ring-to-ring p-metal along the ridges and a common horseshoe ohmic cathode n-metal. We demonstrate the potential of our arrays as sources for optical wireless communication via 25 Gb/s data transmission tests.

Efficiency enhancement in InGaN-based laser diodes using an optimized Al0.12Ga0.88N electron blocking layer

In this paper, a design for an electron blocking layer (EBL) is proposed to enhance the wall plug efficiency of InGaN-based laser diodes. Using calibrated 3D simulations (including thermal models), a comprehensive analysis of various design aspects (composition, thickness and p-doping) of EBLs is conducted, including their impact on carrier (electron and hole) wavefunction overlap and stimulated recombination rate in the quantum wells (QWs) along with the space charge density, electric field and free carrier absorption (FCA) at the interface of the p-side waveguide/EBL. The results indicate that Poole–Frenkel emission is vital for consideration of FCA in the p-doped layers of the epitaxial structure. Consequently, the proposed EBL design reduces electron overflow, improves hole injection, decreases the internal absorption losses and thus, enhances the internal quantum efficiency of the device. The threshold current is reduced from 230 mA to 205 mA as compared to the reference structure. The hole barrier is reduced by 23.93%. Hence, the output power is increased from 1.746 W to 1.95 W, and the voltage drop as well as the device temperature is reduced. These improvements enhance the efficiency from 37.4% of the reference structure to 42.1% in the proposed structure (corresponding to a bias current of 1 A).

Extended L-band monolithically tunable InAs/InP quantum-dash multimode laser with integrated amplifier

A two-sectioned InAs/InP quantum dash laser structure is proposed and investigated as a monolithic broadband multimodal tunable laser with an integrated semiconductor optical amplifier. The optical power-injection current and spectral characteristics of the device at different operating conditions demonstrated a total wavelength tunability of ∼15.8 nm in the extended-L-band (∼1615 to ∼1630.8 nm) window with ∼2.0 times −3 dB bandwidth enhancement. Furthermore, due to the unique tunability mechanism of forward biasing the amplifier section, the device exhibits simultaneous wavelength tuning as well as optical amplification features, with an estimated gain of ∼8.5 dB affirmed by an increase in the wall-plug efficiency up to 6.8% from 3.9%, shown by its single-section counterpart. This demonstration paves a potential platform for the deployment of broadband quantum-dash laser-amplifiers as unified light sources in next-generation optical access networks.

Mechanism and efficacy of virus inactivation by a microplasma UV lamp generating monochromatic UV irradiation at 222 nm

This study evaluated the potential of a microplasma UV lamp as an alternative UV source to the current mercury-based (Hg-based) UV lamp for water disinfection. We developed a set of PCR-based molecular assays (long-range qPCR, DNase, and binding assay) to quantify the adenovirus genome, capsid, and fiber damage with a wide detection range (100.5-106.5 PFU/mL). We used these molecular assays to characterize adenovirus (AdV) inactivation kinetics by microplasma UV that produced monochromatic UV at 222 nm. We found that the inactivation rate constant (0.142 cm2/mJ) due to microplasma UV was 4.4 times higher than that of low-pressure Hg UV (0.032 cm2/mJ). This high efficacy was attributed to monochromatic UV wavelength at 222 nm damaging the AdV capsid protein. The results of these molecular assays also proved that microplasma UV and medium-pressure Hg UV with a bandpass filter at 223 nm (MPUV223nm) have a similar influence on AdV (p>0.05). We then estimated the relative energy efficiency of MPUV and microplasma UV to LPUV for 4 log reduction of the viruses. We found that the microplasma UV resulted in higher inactivation rate constants for viruses than the current Hg-based UV. Consequently, microplasma UV could be more energy efficient than low-pressure Hg UV for water disinfection if the wall-plug efficiency of the microplasma UV lamp improved to 8.4% (currently 1.5%). Therefore, the microplasma UV lamp is a promising option for water disinfection.

Theoretical Analysis of Tunnel-Injected Sub-300 nm AlGaN Laser Diodes

Electrically-pumped AlGaN-based edge-emitting laser diodes with a buried tunnel junction (TJ) for sub-300 nm emission are designed in this paper. Hole injection is one of the major concerns for the design of ultraviolet (UV) lasers based on this material system. The use of a low-resistive TJ as an intracavity contact within the devices will offer an opportunity to replace highly resistive p-type AlGaN-based cladding and contact layers by their n-doped counterparts. This advanced polarization-engineered interband TJs will lead to improved hole injection and a significantly reduced threshold voltage. The thermal properties of the tunnel-injected devices are thoroughly studied theoretically. For the demonstration of continuous-wave operating lasers, possible improvements in terms of better thermal management of the device are also discussed. Our improved design allows CW-operating AlGaN lasers with a threshold current density of <; 8 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and maximum optical output power of > 220 mW, yielding a wall-plug efficiency of > 2.8%.

High power density and temperature stable vertical-cavity surface-emitting laser with a ring close packing structure

We report on the design and fabrication of high power density vertical-cavity surface-emitting laser (VCSEL) with ring close packing structure (RCP) emitting at 808 nm. The RCP-VCSELs with a broad area of 100 μm in diameter contains seven-small-mesas sharing the same super-cavity, and the oxide-aperture diameters of each small mesa was 20 μm. A detailed theoretical analysis is conducted with the help of a thermoelectric model and far-field model. The current density distribution and temperature characteristics were improved. The RCP-VCSELs with maximum power density of 3.73 kW/cm2, slope efficiency of 1.1 W/A, and overall wall-plug efficiency (WPE) of 35% have been realized from the broad area of 100 μm in diameter. An improved transverse-mode emission performance featuring a low beam divergence of FWHM (3.3°) and a stable temperature tolerance of the studied device shows the potential of our approach. It is anticipated that this work will provide an effective strategy for the fabrication of high performance large-area VCSELs with good beam quality under ambient and harsh conditions.

Light-emitting diodes with AlN polarization-induced buried tunnel junctions: A second look

Interband Zener tunneling of electrons has been recently used in III-nitride semiconductor based light emitters to efficiently inject holes into p-cladding layers. Zener tunneling probabilities can be significantly enhanced if crystal symmetry-induced internal polarization fields assist the dopant-induced built-in electric fields of tunnel junctions because of the large reduction of the tunneling distance. In a metal-polar buried tunnel junction geometry, such electric field alignment needs an AlN interlayer at the tunnel junction. Because AlN is a larger bandgap semiconductor than GaN, it is not clear a priori if the net tunneling probability is reduced or enhanced compared to a homojunction. By combining theoretical modeling with experimental blue light emitting diodes, we find that the large tunneling enhancement due to the polarization field and band realignment overcome the reduction in tunneling due to the larger bandgap of AlN. Compared to a homojunction tunnel-junction, the inclusion of AlN in the tunnel junction is found to lower the turn-on and operating voltages and increase the wall-plug efficiency. This proves that polarization-induced AlN tunnel junctions are superior to homojunctions at low injection currents, resulting in higher optical emission intensity and superior uniformity.