Wall-plug Efficiency Research Articles

Efficient, high-CRI white LEDs by combining traditional phosphors with cadmium-free InP/ZnSe red quantum dots

Quantum dots (QDs) offer an interesting alternative for traditional phosphors in on-chip light-emitting diode (LED) configurations. Earlier studies showed that the spectral efficiency of white LEDs with high color rendering index (CRI) values could be considerably improved by replacing red-emitting nitride phosphors with narrowband QDs. However, the red QDs in these studies were cadmium-based, which is a restricted element in the EU and certain other countries. The use of InP-based QDs, the most promising Cd-free alternative, is often presented as an inferior solution because of the broader linewidth of these QDs. However, while narrow emission lines are the key to display applications that require a large color gamut, the spectral efficiency penalty of this broader emission is limited for lighting applications. Here, we report efficient, high-CRI white LEDs with an on-chip color converter coating based on red InP/ZnSe QDs and traditional green/yellow powder phosphors. Using InP/ZnSe QDs with a quantum yield of nearly 80% and a full width at half-maximum of 45 nm, we demonstrate high spectral efficiency for white LEDs with very high CRI values. One of the best experimental results in terms of both luminous efficacy and color rendering performance is a white LED with an efficacy of 132 lm/W, and color rendering indices of R a ≈ 90 , R 9 ≈ 50 for CCT ≈ 4000 K . These experimental results are critically compared with theoretical benchmark values for white LEDs with on-chip downconversion from both phosphors and red Cd-based QDs. The various loss mechanisms in the investigated white LEDs are quantified with an accurate simulation model, and the main impediments to an even higher efficacy are identified as the blue LED wall-plug efficiency and light recycling in the LED package.

Enhanced Light Extraction of DUV LEDs by Using a Quartz Lens With Three-Dimensional Structure

AlGaN based deep ultraviolet light-emitting diodes (DUV LEDs) have wide applications in many fields, such as air and water purification, disinfection, and biochemical inspection. However, the wall plug efficiency (WPE) of DUV LEDs is relatively low for these applications, which is due to the low light extraction efficiency (LEE) yielding limited light output from LED package. In this work, we proposed a quartz lens with three-dimensional (3D) structure to enhance the light extraction of DUV LEDs. The optical models of DUV LEDs were established and the optical performances of DUV LEDs were investigated by using optical simulation. The DUV LEDs with the 3D quartz lens exhibit higher light efficiency than those with traditional quartz lens structure. Compared with the traditional structure, the light output power of the 3D lens structure is enhanced by 25.93% at the current of 140 mA and the packaged DUV LED can achieve a wide light angle of 153.2°. It is attributed to the significant light extraction enhancement of the sidewall emission from DUV LED chip.

On the impact of a metal-insulator-semiconductor structured n-electrode for AlGaN-based DUV LEDs.

In this work, a 280 nm AlGaN-based deep ultraviolet light-emitting diode (DUV LED) with a metal-insulator-semiconductor (MIS) structured n-electrode is fabricated and studied. The SiO2 insulator layer is adopted to form the MIS structure by using an atomic layer deposition system. After adopting the MIS-structured n-electrode, the SiO2 intermediate layer enables electron affinity for the contact metal to be higher than the conduction band of the n-AlGaN layer, which favors the electrons to be injected into the n-AlGaN layer by intraband tunneling rather than thermionic emission. Moreover, the thin SiO2 insulator can share the applied bias, which makes the n-AlGaN layer surface less depleted and thus further facilitates the electron injection. The improved electron injection capability at the metal-semiconductor interface helps reduce the contact resistance and increase electron concentration in the active region, which then improves external quantum efficiency and wall-plug efficiency for the proposed DUV LED.

High-temperature electroluminescence properties of InGaN red 40 × 40 μ m2 micro-light-emitting diodes with a peak external quantum efficiency of 3.2%

We study the high-temperature electroluminescence properties of 600 nm InGaN red 40 × 40 μm2 micro-light-emitting diodes (μLEDs) with a peak external quantum efficiency (EQE) of 3.2%. Temperature-dependent peak wavelength measurements show a low redshift of 0.05 nm/K. The injection efficiency improves with increasing temperature. The hot/cold (HC) factor is used to quantify the thermal droop: at 400 K, the EQE and wall-plug efficiency HC factors at 50 A/cm2 reach high values of 0.72 and 0.85, respectively. This demonstrates the robustness of InGaN red μLEDs up to high temperature, with a much-improved stability over conventional AlInGaP red μLEDs.

Femtosecond damage experiments and modeling of broadband mid-infrared dielectric diffraction gratings

High peak and average power lasers with high wall-plug efficiency, like the Big Aperture Thulium (BAT) laser, have garnered tremendous attention in laser technology. To meet the requirements of the BAT laser, we have developed low-dispersion reflection multilayer dielectric (MLD) gratings suitable for compression of high-energy pulses for operations at 2 micron wavelength. We carried out 10000-on-1 damage tests to investigate the fluence damage thresholds of the designed MLD gratings and mirrors, which were found between 100-230 mJ/cm2. An ultrashort pulsed laser (FWHM = 53 fs, λ = 1.9 μm) operating at 500 Hz was used in the serpentine raster scans. The atomic force microscope images of the damage sites show blister formation of the underlying layers at lower fluences but ablation of the grating pillars at higher fluences. We simulated the dynamic electronic excitation in the MLD optics with a finite-difference in the time domain approach in 2D. The simulation results agree well with the LIDT measurements and the observed blister formation. This model is able to evaluate the absolute LIDT of MLD gratings.

Effects of activation method and temperature to III-nitride micro-light-emitting diodes with tunnel junction contacts grown by metalorganic chemical vapor deposition

The optical and electrical characteristics of InGaN blue and green micro-light-emitting diodes (μLEDs) with GaN tunnel junction (TJ) contacts grown by metalorganic chemical vapor deposition (MOCVD) were compared at different activation temperatures among three activation methods from the literature, namely, sidewall activation, selective area growth (SAG), and chemical treatment before sidewall activation. The devices with chemical treatment before activation resulted in uniform electroluminescence and higher light output power, compared to the devices with sidewall activation and SAG. Moreover, the green μLEDs showed greater optical degradation at elevated activation temperatures, whereas the blue μLEDs yielded trivial difference with activation temperatures from 670 to 790 °C. The 5 × 5 μm2 devices with chemical treatment before activation and SAG yielded almost identical voltage at 20 A/cm2, and the voltage penalty significantly decreased with activation temperature in the case of devices with sidewall activation. The devices with chemical treatment before activation resulted in higher external quantum efficiency (EQE) and wall-plug efficiency (WPE) in low current density range compared to the devices with SAG. The enhancements in EQE and WPE were observed in different μLED sizes, suggesting that chemical treatment before sidewall activation enables the use of TJ contacts grown by MOCVD and is advantageous for applications that require high brightness and efficiency.

Investigation of the Optimum Mg Doping Concentration in p-Type-Doped Layers of InGaN Blue Laser Diode Structures

In GaN-based laser diode (LD) structures, Mg doping in p-type-doped layers has a significant influence on the device performance. As the doping concentration increases, the operation voltage decreases, whereas the output power decreases as a result of increased optical absorption, implying that optimization of the Mg doping concentration is required. In this study, we systematically investigated the effect of the Mg doping concentration in the AlGaN electron-blocking layer (EBL) and the AlGaN p-cladding layer on the output power, forward voltage, and wall-plug efficiency (WPE) of InGaN blue LD structures using numerical simulations. In the optimization of the EBL, an Al composition of 20% and an Mg doping concentration of 3 × 1019 cm−3 exhibited the best performance, with negligible electron leakage and a high WPE. The optimum Mg concentration of the p-AlGaN cladding layer was found to be ~1.5 × 1019 cm−3, where the maximum WPE of 38.6% was obtained for a blue LD with a threshold current density of 1 kA/cm2 and a slope efficiency of 2.1 W/A.

High power λ ~ 8.5 μm quantum cascade laser grown by MOCVD operating continuous-wave up to 408 K

Robust quantum cascade laser (QCL) enduring high temperature continuous-wave (CW) operation is of critical importance for some applications. We report on the realization of lattice-matched InGaAs/InAlAs/InP QCL materials grown by metal-organic chemical vapor deposition (MOCVD). High interface quality structures designed for light emission at 8.5 μm are achieved by optimizing and precise controlling of growth conditions. A CW output power of 1.04 W at 288 K was obtained from a 4 mm-long and 10 μm-wide coated laser. Corresponding maximum wall-plug efficiency and threshold current density were 7.1% and 1.18 kA/cm2, respectively. The device can operate in CW mode up to 408 K with an output power of 160 mW.

Gev-Scale Accelerators Driven by Plasma-Modulated Pulses from Kilohertz Lasers.

We describe a new approach for driving GeV-scale plasma accelerators with long laser pulses. We show that the temporal phase of a long, high-energy driving laser pulse can be modulated periodically by copropagating it with a low-amplitude plasma wave driven by a short, low-energy seed pulse. Compression of the modulated driver by a dispersive optic generates a train of short pulses suitable for resonantly driving a plasma accelerator. Modulation of the driver occurs via well-controlled linear processes, as confirmed by good agreement between particle-in-cell (PIC) simulations and an analytic model. PIC simulations demonstrate that a 1.7J, 1ps driver, and a 140mJ, 40fs seed pulse can accelerate electrons to energies of 0.65GeV in a plasma channel with an axial density of 2.5×10^{17} cm^{-3}. This work opens a route to high repetition-rate, GeV-scale plasma accelerators driven by thin-disk lasers, which can provide joule-scale, picosecond-duration laser pulses at multikilohertz repetition rates and high wall-plug efficiencies.

Improved Performance of GaN-Based Ultraviolet LEDs with the Stair-like Si-Doping n-GaN Structure

A method to improve the performance of ultraviolet light-emitting diodes (UV-LEDs) with stair-like Si-doping GaN layer is investigated. The high-resolution X-ray diffraction shows that the UV-LED with stair-like Si-doping GaN layer possesses better quality and a lower dislocation density. In addition, the experimental results demonstrate that light output power and wall plug efficiency of UV-LED with stair-like Si-doping GaN are significantly improved. Through the analysis of the experimental and simulation results, we can infer that there are two reasons for the improvement of photoelectric characteristics: reduction of dislocation density and alleviating of current crowding of UV-LEDs by introduced stair-like Si-doping GaN.

Comparison of Auger Coefficients in Type I and Type II Quantum Well Midwave Infrared Lasers

We perform a detailed comparison of 2D Auger coefficients in narrow-gap type-I and type-II quantum wells (T1QWs and T2QWs), and the relative effects of Auger non-radiative decay on midwave infrared lasers employing both types of gain media. Comparison is also made to 3D Auger coefficients in bulk mid-IR materials, by defining a “normalization length” that accounts for the spatial extent of the electron and hole wavefunctions in the quantum wells. The comparisons confirm that Auger recombination in both types of QW is substantially suppressed relative to bulk, due primarily to the effects of compressive strain on the valence subband dispersions. We find that the 2D Auger coefficients in T1QWs remain substantially lower than those in T2QWs out to wavelengths beyond <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.5~\mu \text{m}$ </tex-math></inline-formula> . However, this does not necessarily imply a lower lasing threshold because a substantial fraction of the holes injected into T1QWs occupy lower subbands that do not contribute gain, so more must be injected to reach the lasing threshold. When all of the relevant considerations are combined, the thresholds for the best T1QW and T2QW lasers cross over near <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda ~\approx ~3.0~\mu \text{m}$ </tex-math></inline-formula> . Other characteristics governing the relative T1QW and T2QW laser performances above threshold, such as maximum output power and wallplug efficiency, are also considered.

Temperature-dependent study on AlGaN-based deep ultraviolet light-emitting diode for the origin of high ideality factor

Deep ultraviolet light-emitting diodes (DUV LEDs) are promising light sources for disinfection, especially during the pandemic of novel coronavirus (COVID-19). Despite much effort in the development of DUV LEDs, the device temperature and ideality factor are key parameters of devices, which are often neglected. Here, we developed a simple and convenient method to study the behavior of a 280 nm AlGaN-based DUV LED, obtaining the electrical, optical, and thermal properties within one measurement. From the experimental results, we find that the light output power and wall-plug efficiency of the AlGaN-based DUV LED are strongly affected by device temperature, ideality factor (β), and series resistance (Rs). β decreases from 9.3 to 8.1 at 40 mA when the temperature increases from 302 to 317 K. We compared these results with simulations and found that the high potential barriers inside the device and the carrier concentration in n-type or p-type layers, especially the hole concentration in p-type layers, are the two key factors for the high value of the ideality factor from the LED structure. As the device temperature increases, carriers with higher energy would overcome some potential barriers and Mg acceptor activation would be more efficient, which are beneficial for carrier transportation. However, these also lead to the carrier overflow and weaken the radiative recombination rate. The trade-off role of device temperature in carriers between transportation and overflow is needed to be considered in the future development of DUV LEDs with higher efficiency and higher brightness.

Comparative study of III-phosphide- and III-nitride-based light-emitting diodes: understanding the factors limiting efficiency

Modern light-emitting diodes (LEDs) require further improvements in the wall-plug efficiency (WPE). Thus, understanding and elucidating the physical factors limiting the WPE is of crucial importance. This study aims to understand and elucidate such factors via a comparative study of two different LED material systems, i.e. III-phosphide (AlGaInP) red and III-nitride (AlGaInN) blue LEDs. The WPE was decoupled into its component elements, which indicated that the dominant contribution to WPE reduction depends on the sample. To further investigate, thermodynamic and current component analyses were conducted. These analyses clearly demonstrated that the carrier dynamics is dependent of the material systems, i.e. internal quantum efficiency and voltage efficiency are simultaneously degraded due to the carrier accumulation in the blue sample. We found that the WPE can be improved using strategies dependent on the samples, such as improvement of the light extraction in the red sample and suppression of the carrier accumulation in the blue sample.

Efficiency Boosting by Thermal Harvesting in InGaN/GaN Light-Emitting Diodes

On the same micro-LED display panel, LED pixels are always operated with high and low biased voltages simultaneously to show different brightness and colors. Thus, it is vitally important to understand the effect of the heat transmission between LEDs under high and low biased voltages. In this work, we design two different LED groups: Group A is two LEDs bonded together for heat transmission and Group B is two LEDs separated from each other. Then, the two LEDs are operated at one fixed and one tuned biased voltage respectively in each group in a vacuum chamber and the efficiency of the two groups is studied both experimentally and numerically. Here, our experimental results demonstrate that Group A exhibits a maximum improvement of 15.36% in optical output power compared with Group B. The underlying reason is that the wall-plug efficiency of the LED with a voltage lower than photon voltage (V &amp;lt; ℏω/q) is surprisingly enhanced by elevated temperature owing to the heat transmission by the LED under a high biased voltage in Group A. Our further study shows that in such a low voltage region the improvement in the efficiency is attributed to the enhanced carrier concentrations with elevated temperature. On the other hand, the LED in Group A under a high biased voltage further raises the overall efficiency by alleviating the thermal droop due to reduced temperature. Device temperature measurement and numerical calculation of radiative recombination under different temperatures further support the superior performance of Group A LEDs. Our research results can act as the research prototype to design the high-efficient LED arrays for better energy recycling and thermal control.

Narrow-Band AlGaN-Based UVB Light-Emitting Diodes

This paper reports an AlGaN-based narrow-band ultraviolet-B (NB-UVB) light-emitting diode (LED) exhibiting a single electroluminescence peak with a full-width at half-maximum (FWHM) of less than 10 nm at forward currents (If) from 10 to 200 mA, broadening to 11.6 nm when the forward current reached 350 mA. We attribute the narrow FWHM to effectively decreasing the degrees of piezoelectric polarization in the MQWs, and the excess electron overflow from the MQW to the p-layer was avoided. The maximum external quantum efficiency (EQE) of 2.16% and wall plug efficiency of 1.74% occurred when the forward current was 10 mA; the EQE dropped by 8.6% when the forward current increased from 10 to 60 mA. Furthermore, the light output power decreased to 85.4 and 82.5% of its initial value after 620 and 3500 h, respectively, under conditions of 60 mA dc aging. The characteristics of this NB-UVB LED suggest great potential for its application in phototherapy.

Improved forward voltage and external quantum efficiency scaling in multi-active region III-nitride LEDs

Ultra-low voltage drop tunnel junctions (TJs) were utilized to enable multi-active region blue light emitting diodes (LEDs) with up to three active regions in a single device. The multi-active region blue LEDs were grown monolithically by metal-organic chemical vapor deposition (MOCVD) without growth interruption. This is the first demonstration of a MOCVD grown triple-junction LED. Optimized TJ design enabled near-ideal voltage and EQE scaling close to the number of junctions. This work demonstrates that with proper TJ design, improvements in wall-plug efficiency at high output power operation are possible by cascading multiple III-nitride based LEDs.

High-speed visible light communication systems based on Si-substrate LEDs with multiple superlattice interlayers

High-speed visible light communication (VLC), as a cutting-edge supplementary solution in 6G to traditional radio-frequency communication, is expected to address the tension between continuously increased demand of capacity and currently limited supply of radio-frequency spectrum resource. The main driver behind the high-speed VLC is the presence of light emitting diode (LED) which not only offers energy-efficient lighting, but also provides a cost-efficient alternative to the VLC transmitter with superior modulation potential. Particularly, the InGaN/GaN LED grown on Si substrate is a promising VLC transmitter to simultaneously realize effective communication and illumination by virtue of beyond 10-Gbps communication capacity and Watt-level output optical power. In previous parameter optimization of Si-substrate LED, the superlattice interlayer (SL), especially its period number, is reported to be the key factor to improve the lighting performance by enhancing the wall-plug efficiency, but few efforts were made to investigate the influence of SLs on VLC performance. Therefore, to optimize the VLC performance of Si-substrate LEDs, we for the first time investigated the impact of the SL period number on VLC system through experiments and theoretical derivation. The results show that more SL period number is related to higher signal-to-noise ratio (SNR) via improving the wall-plug efficiency. In addition, by using Levin-Campello bit and power loading technology, we achieved a record-breaking data rate of 3.37 Gbps over 1.2-m free-space VLC link under given optimal SL period number, which, to the best of our knowledge, is the highest data rate for a Si-substrate LED-based VLC system.

Exploring superlattice DBR effect on a micro-LED as an electron blocking layer.

The role of a superlattice distributed Bragg reflector (SL DBR) as the p-type electron blocking layer (EBL) in a GaN micro-light-emitting diode (micro-LED) is numerically investigated to improve wall-plug efficiency (WPE). The DBR consists of AlGaN/GaN superlattice (high refractive index layer) and GaN (low refractive index layer). It is observed that the reflectivity of the p-region and light extraction efficiency (LEE) increase with the number of DBR pairs. The AlGaN/GaN superlattice EBL is well known to reduce the polarization effect and to promote hole injection. Thus, the superlattice DBR structure shows a balanced carrier injection and results in a higher internal quantum efficiency (IQE). In addition, due to the high refractive-index layer replaced by the superlattice, the conductive DBR results in a lower operation voltage. As a result, WPE is improved by 22.9% compared to the identical device with the incorporation of a conventional p-type EBL.

Structural design and performance improvement of flip-chip AlGaInP mini light-emitting diodes

Flip-chip (FC) aluminum gallium indium phosphide (AlGaInP) mini light-emitting diodes (mLEDs) with p and n lateral electrode structures and a larger light-emitting area have significant potential for integrating red, green, and blue mLEDs of locally dimmable zones for developing new-generation LED-backlit liquid crystal displays. Herein, an FC AlGaInP mLED having 9 × 6 mil dimensions is fabricated using wafer bonding and transfer technology methods. We propose a patterned n-gallium-arsenide (GaAs) epilayer by etching for ohmic contact and optimizing the specific contact resistance of an FC mLED. The light-output intensity images demonstrated that this etching process improved the uniformity of current distribution and matched the simulation results. Compared with conventional FC mLEDs, the output powers of FC mLEDs with a patterned n-GaAs layer increased from 3.4% to 4.5% and the wall-plug efficiency improved by 1.04%. Moreover, Ag was applied as a reflector on patterned n-GaAs rather than AuGe. The output power of FC mLEDs with an Ag reflector showed a 3.5%–8.2% improvement over FC mLEDs with an AuGe reflector. Finally, FC mLEDs that were encapsulated with epoxy and far-field radiation patterns indicated that packaged FC mLEDs had a smaller divergence angle and narrower viewing angle.

Improving light output power of AlGaN-based deep-ultraviolet light-emitting diodes by optimizing the optical thickness of p-layers

In this study, we investigated the relationship of light output power with the optical thickness of the p-layers in AlGaN-based deep ultraviolet light-emitting diodes with a transparent high-Al-composition p-AlGaN clad layer, a thin p-GaN contact layer, and a reflective p-type electrode. By adjusting the thickness of the transparent high-Al-composition p-AlGaN clad layer, we observed a marked change in light output power. A maximum light output power of 385 mW at 1500 mA, a maximum external quantum efficiency of 15.7% at 10 mA, and a maximum wall-plug efficiency of 15.3% at 10 mA were obtained at an emission wavelength of 275 nm.