Light Output Characteristics Research Articles

Ultra-dense Green InGaN/GaN Nanoscale Pixels with High Luminescence Stability and Uniformity for Near-Eye Displays.

Ultra-dense (>4,000 pixels per inch) and highly stable full-color III-nitride nanoscale pixels are crucial for near-eye display technologies like virtual and augmented-reality glasses. In this context, InGaN-based long wavelength green microscale light-emitting diodes face major bottlenecks, such as low efficiency and inadequate wavelength stability. These challenges are associated with the presence of both nonradiative surface defects and the strain induced quantum-confined Stark effect. Herein, we report nanoscale pixelation of green InGaN/GaN LEDs incorporating strain-engineered ultra-dense nanowire (NW) arrays, corresponding to ∼36,000 pixels per inch. The NW pixel arrays exhibit a stable peak wavelength emission at ∼500 nm for over 3 orders of magnitude of injection current densities (from ∼4 A/cm2 to ∼1 kA/cm2). The observed wavelength stability enhancement (a reduced blue-shift of just ∼4 nm) directly results from the suppressed built-in electric field achieved by strain relaxation of the axial multi quantum wells in the NWs. Finite difference time domain simulations show that emission of the pixel array is significantly increased owing to the enhanced spontaneous emission rate (characterized by a high Purcell factor of ≈2) of the ultra-dense NWs. We have demonstrated top-down NWs, where each NW (diameter ranging down to 200 nm) shows excellent uniformity and light output characteristics in direct contrast to bottom-up grown NW heterostructures. The results of this study establish a viable route for realizing nanoscale pixels with high luminescence stability and wafer-scale uniformity with high (>20%) indium composition InGaN/GaN LED heterostructures, for next-generation near-eye displays.

Study on the performance of InGaN-based micro-LED by plasma etching combined with ion implantation process

This study utilized blue-light epitaxial wafers and employed semiconductor processes such as maskless laser writing, dry etching, wet etching, passivation layer deposition, electron beam evaporation, and ion implantation to fabricate micro-light emitting diode (μLED) arrays with different pixel sizes but the same emitting area (900 μm²). The μLED arrays with single pixel sizes of 5 μm, 10 μm, and 15 μm were fabricated, with array numbers of 6×6, 3×3, and 2×2, respectively. This study proposes etching the material in the channel region while retaining a certain width for implantation, known as the sidewall ion implantation process, aiming to achieve better insulation characteristics by using ion implantation technology to insulate the sidewall regions. It involves ion bombardment of the defect areas generated after plasma etching and the use of a passivation layer for protection. The isolation characteristics of μLED arrays processed by sidewall implantation exhibited better electrical isolation than those of μLED arrays processed only by plasma. The light output power, external quantum efficiency, and wall-plug efficiency were all superior for the sidewall implantation process when the device was miniaturized to 5 μm. Overall, the sidewall implantation process combined with plasma dry etching effectively improved the light output characteristics, with the enhancement ratio increasing as the device was miniaturized.

Fast Emitting Nanocomposites for High‐Resolution ToF‐PET Imaging Based on Multicomponent Scintillators

AbstractTime‐of‐Flight Positron Emission Tomography (ToF‐PET) is a medical imaging technique, based on the detection of two back‐to‐back γ‐photons generated from radiotracers injected into the body. Its limit is the ability of employed scintillation detectors to discriminate in time the arrival of γ‐pairs, that is, the coincidence time resolution (CTR). A CTR < 50 ps will enable fast imaging with ultralow radiotracer dose. Monolithic materials do not have simultaneously the required high light output and fast emission characteristics, thus the concept of scintillating heterostructure is proposed, where the device is made of a dense scintillator coupled to a fast‐emitting light material. Here a composite polymeric scintillator loaded with hafnium oxide nanoparticles is presented. This enhanced by +300% its scintillation yield, by surpassing commercial plastic scintillators. The nanocomposite is coupled to bismuth germanate oxide (BGO) realizing a multilayer metascintillator. The energy sharing between its components is observed, which activates the nanocomposite's fast emission enabling a net CTR improvement of 25% with respect to monolithic BGO. These results demonstrate that a controlled loading with dense nanomaterials is an excellent strategy to enhance the performance of polymeric scintillators for their use in advanced radiation detection and imaging technologies.

Enhanced light output power from AlGaN-based deep ultraviolet LEDs achieved by a four-in-one mesa structure

This study aimed to investigate the impact of mesa geometry on the light output characteristics of AlGaN-based 275 nm deep ultraviolet light-emitting diodes (DUV-LEDs). By dividing the original single-junction mesa into four parts and connecting them serially (four-in-one), high-voltage (HV) DUV-LEDs with rectangular, hexagonal, circular, triangular, and square submesas were realized, achieving significant enhancement of the light output power (LOP) and wall-plug efficiency (WPE). The LOP of HV DUV-LEDs with hexagonal submesas has been promoted substantially compared to that of the original DUV-LEDs. Among the investigated five different types of submesas, hexagonal-type HV DUV-LEDs can achieve the highest LOP and WPE due to the higher sidewall light extraction. Furthermore, it is also demonstrated that pulse current driving can reduce the self-heating effect of HV DUV-LEDs.

Heavily Ge-doped GaN as transparent current spreading layer for blue tunnel junction light emitting diodes

We report on metalorganic vapor phase epitaxy of highly conductive germanium-doped GaN layers and their application for blue tunnel-junction light emitting diodes (TJ-LEDs). Using Ge as donor, carrier densities of up to 2 × 1020 cm−3 and low bulk resistivities down to 3 × 10−4 Ωcm are achieved. Under optimum growth conditions, no degradation of the crystalline quality is observed and layers exhibit high transparency making GaN:Ge very attractive as current spreading layer in light emitting devices. We have realized GaN-based TJ-LEDs by capping conventional InGaN LED structures with highly doped GaN:Ge. Such TJ-LEDs withstand operation currents up to 20 kA/cm2 in continuous and up to 60 kA/cm2 in pulsed operation conditions. Moreover, TJ-LEDs exhibit homogeneous electroluminescence and light output through the top surface that is increased by more than 60% as compared to conventional LEDs with transparent indium tin oxide contacts. The impact of the doping profile, carrier gas conditions, and acceptor activation by annealing for low-resistive TJ characteristics is discussed. Light output and current voltage characteristics of blue-light emitting devices with GaN-TJ are presented at low and high-current densities.

Heterogeneously integrated InGaN-based green microdisk light-emitters on Si (100).

Heterogeneous integration of nitrides on Si (100) is expected to open the door to the new possibilities for this material system in the fields of high-speed integrated photonics and information processing. In this work, GaN epitaxial layer grown on the patterned sapphire substrate is transferred onto Si (100) by a combination of wafer bonding, laser lift-off and chemical mechanical polishing (CMP) processes. The GaN epilayer transferred is uniformly thinned down to 800 nm with a root mean square surface roughness as low as 2.33 Å. The residual stress within the InGaN quantum wells transferred is mitigated by 79.4% after the CMP process. Accordingly, its emission wavelength exhibits a blue shift of 8.8 nm, revealing an alleviated quantum-confined Stark effect. Based on this platform, an array of microcavities with diverse geometrics and sizes are fabricated, by which optically-pumped green lasing at ∼505.8 nm is achieved with a linewidth of ∼0.48 nm from ∼12 µm microdisks. A spontaneous emission coupling factor of around 10-4 is roughly estimated based on the light output characteristics with increasing the pumping densities. Lasing behaviors beyond the threshold suggest that the microdisk suffers less thermal effects as compared to its undercut counterparts. The electrically-injected microdisks are also fabricated, with a turn-on voltage of ∼2.0 V and a leakage current as low as ∼2.4 pA at -5 V. Being compatible with traditional semiconductor processing techniques, this work provides a feasible solution to fabricate large-area heterogeneously integrated optoelectronic devices based on nitrides.

(INVITED) Ultraviolet cross-luminescence in ternary chlorides of alkali and alkaline-earth metals

After the discovery of a cross-luminescence (CL) in BaF2 in 1982, a large number of CL scintillators were investigated. However, no CL scintillator superior to BaF2 has been discovered, and the research of CL scintillators has subsided. Recent technological development in medical imaging and high-energy physics created a new demand for ultra-fast scintillators further supported by the development of UV-sensitive semiconductor photodetectors. As a consequence, renewed interest in CL scintillators appeared. To satisfy the requirements of fast timing applications high photo-detection efficiency, e. i. a good spectral match between the scintillator and photodetector must be achieved. Cesium-based ternary chlorides could provide a red-shift (∼1.5 eV) of CL towards the sensitive region of the photodetector (PMT or SiPM) while keeping light output and timing characteristics comparable to BaF2.

Effects of remote sediment phosphor plates on high power laser-based white light sources

Phosphor-converted blue laser diodes are regarded as the next-generation high-brightness solid-state lighting sources. However, it is difficult to obtain white light with high angular color uniformity due to the Gaussian distribution of the laser light sources. Meanwhile, laser excitation power density of the light source is high, which would bring serious heating effects to the phosphor layers. In this study, a strategy has been proposed to solve the problem by using remote sediment phosphor plates. In detail, we have compared the effects of remote sediment/non-sediment phosphor plates to the phosphor-converted blue laser diodes on the overall light output characteristics, angular optical distribution properties, as well as their thermal performance. The emission from sediment phosphor samples has been found more divergent, and angular deviation in the correlated color temperature of the emitted light could be greatly reduced from 1486 to 294 K, yet with only 5% luminous flux loss, as compared to non-sediment phosphor samples. Most importantly, the sediment phosphor sample pushes the power damage threshold up to 588.1 W/cm2 (non-sediment sample: 512.3 W/cm2). Our work has demonstrated the sediment phosphor plates would ameliorate the angular color uniformity for the laser-based lighting source, while extending its lifespan with improved thermal stability.

Development of a Constant Temperature Control Light Emitting Diode Light Cold Mask Reflecting Real-Time Skin Temperature Change.

To treat and improve skin condition, photo masks using LEDs have been developed and widely marketed to consumers. However, since the skin condition of the user varies, it is difficult to simulate suitable conditions, because various variables such as light output intensity and irradiation time must be applied to the mask development. Currently, photo masks on the market were developed considering only the parameters related to light irradiation. Existing products do not consider the burning sensation after skin treatment and skin temperature increase from the use of masks, which has been causing user inconvenience. In this paper, the LED light cold mask was designed, fabricated, and analyzed for light output characteristics to control wavelength selection and output to maximize the therapeutic effect of various LED lights. Additionally, circulating water was used to eliminate the burning sensation caused by the use of LED masks. The temperature of the circulating water was lowered below 10 °C using a thermoelectric element and PT100 Ω temperature sensor. An infrared temperature sensor was used to measure the skin temperature in real time. When the skin temperature increased by 0.5 °C, the water was cooled and circulated. As a result, it was confirmed that the temperature of the skin could be maintained uniformly within the set temperature range.

Validity of Bench Test Method for Infrared Flame Detector

In this study, the validity of an indoor bench test to replace and overcome the limitations of the real-range test, an infrared flame detector sensitivity test, was analyzed. A micro burner (diameter 6 mm) was used to substitute for the infrared flame detector. In order to measure the amount of light by distance, the voltage at the output end of the IR sensor was acquired. In addition, normal heptane (n-heptane) was used as a fuel for the actual distance experiment, and the amount of light at each distance was measured using an IR sensor. A bench tester was developed to measure the light output characteristics of infrared flame detectors by distance inside, and the actual distance optical characteristics from outside were measured and compared. As a result, both methods were able to confirm the same optical characteristics by distance. This study confirmed the validity of the bench test method conducted inside, which can replace the experiment at the actual distance conducted to confirm the performance of the infrared flame detector. If this test method is used in the future, it is believed the flame detector will improve product quality and development.

I2PL—What is it and what does it mean?

Abstract Intense pulsed light (IPL) is today a standard tool in most cosmetic and dermatological clinics providing a variety of optical treatment options. The range of available IPL systems keeps growing and today's physicians have a wide selection to choose from. IPL can be separated into broadband IPL and narrowband IPL. Broadband IPL heats the ubiquitous tissue water in the skin leading to unspecific heating of the tissue and increasing risk of side effects. To reduce this, contact cooling is needed, which can negatively influence treatment efficiency. Narrowband IPL limits the spectra of emitted light, thus reducing the energy reaching the skin. Square‐shaped pulses provide the best light output characteristics and square pulses are obtained with large electrical capacitor banks and advanced pulse shutter mechanisms. Longer wavelengths reaching the skin surface lead to absorption in tissue water and doubles energy requirements as compared to systems filtering away wavelengths above 950 nm, known as narrowband I2PL. Having a long wave cut‐off filter at 950 nm halves the energy requirement and reduces the risk of side effects. It also eliminates the need for active cooling of the surplus energy and thus enhances treatment results and comfort. Ideally, systems should be able to provide pulses with a duration shorter than 1 ms with sufficient energy to create purpura if needed. Individual adjustment of pulse duration and fluency provides for more efficient treatments and allows treating a wider variety of patients. Systems allowing parameters to be adjusted based on “patient parameters” reduces the risk that improper settings are used.

Polymer–perovskite blend light-emitting diodes using a self-compensated heavily doped polymeric anode

Perovskite-based light-emitting diodes (PeLEDs) are drawing great attention due to their remarkable performance and ease of processing. Nevertheless, a critical aspect is the perovskite film formation on top of solution-processed anodes such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). Use of conventional PEDOT:PSS anodes gives rise to high leakage currents that mask the hole transport properties of the perovskite semiconductor. Here, we show a feasible approach to overcome this constraint by implementing a solution-processed, self-compensated, hole-doped triarylamine-fluorene copolymer (p-pTFF-C2F5SIS) with a work function of 5.85 eV as the anode for polymer–perovskite blend LED devices. Highly efficient hole injection was obtained, near that of evaporated MoOx. Hole-only devices reveal that the hole transport in the polymer–perovskite blend is trap-limited. PeLEDs with the ultrahigh-workfunction p-pTFF-C2F5SIS anode show much lower leakage and much better stability in current-voltage and light output characteristics than those with the PEDOT:PSSH anode.

Light Extraction Enhancement of InGaN Based Micro Light-Emitting Diodes with Concave-Convex Circular Composite Structure Sidewall

We demonstrate that the concave-convex circular composite structure sidewall prepared by inductively coupled plasma (ICP) etching is an effective approach to increase the light efficiency without deteriorating the electrical characteristics for micro light-emitting diodes (LEDs). The saturated light output power of the device using the concave-convex circular composite structure sidewalls with a radius of 2 μm is 39.75 mW, an improvement of 7.2% compared with that of the device using flat sidewalls. The enhanced light output characteristics are primarily attributed to the increased photon emitting due by decreasing the total internal reflection without losing the active region area.

Multi-Domain Modelling of LEDs for Supporting Virtual Prototyping of Luminaires

This paper presents our approaches to chip level multi-domain LED (light emitting diode) modelling, targeting luminaire design in the Industry 4.0 era, to support virtual prototyping of LED luminaires through luminaire level multi-domain simulations. The primary goal of such virtual prototypes is to predict the light output characteristics of LED luminaires under different operating conditions. The key component in such digital twins of a luminaire is an appropriate multi-domain model for packaged LED devices that captures the electrical, thermal, and light output characteristics and their mutual dependence simultaneously and consistently. We developed two such models with this goal in mind that are presented in detail in this paper. The first model is a semi analytical, quasi black-box model that can be implemented on the basis of the built-in diode models of spice-like circuit simulators and a few added controlled sources. Our second presented model is derived from the physics of the operation of today’s power LEDs realized with multiple quantum well heterojunction structures. Both models have been implemented in the form of visual basic macros as well as circuit models suitable for usual spice circuit simulators. The primary test bench for the two circuit models was an LTspice simulation environment. Then, to support the design of different demonstrator luminaires of the Delphi4LED project, a spreadsheet application was developed, which ensured seamless integration of the two models with additional models representing the LED chips’ thermal environment in a luminaire. The usability of our proposed models is demonstrated by real design case studies during which simulated light output characteristics (such as hot lumens) were confirmed by luminaire level physical tests.

Characterization of nanowire light-emitting diodes grown by selective-area metal-organic vapor-phase epitaxy

We report a systematic study on the current injection and radiative carrier recombination in InP nanowire (NW) light-emitting diodes (LEDs). The InP NWs with axial p–n structures, grown by selective-area metal organic vapor-phase epitaxy, had mixed crystal structures between those of zincblende and wurtzite, mainly in the p-regions. The temperature dependence of the current–voltage (I–V), electroluminescence (EL), and current–light output (I–L) characteristics was investigated. The temperature dependence of the I–V characteristics revealed that tunneling was the main mechanism of carrier transport through the p–n junction in the present NW-LEDs. The temperature and bias voltage dependences of EL showed a complex but systematic behavior, where peaks exhibiting bias-dependent and independent energy positions coexisted and the relative intensity showed a transition with increasing temperature. The external quantum efficiency showed a droop at low temperatures, indicating a reduced injection efficiency at low temperatures. These observations were explained by the radiative and nonradiative tunneling, and suggested a strong effect of the nonradiative tunneling at low temperatures.

Fine control of optical scattering characteristics of porous polymer light-extraction layer for organic light-emitting diodes

Abstract The optical scattering properties of porous polyimide (PI) films were investigated for the light-extraction layer in organic light-emitting diodes (OLEDs). By adjusting mixing ratios of different non-solvents during the immersion precipitation process, the light scattering at the generated porous PIs can be precisely tuned. Analyzed from the final OLED devices based on prepared scattering substrates, we found that the light-output characteristics of fabricated devices are correlated with the optical scattering properties, which depends on both the size of generated pores inside porous PIs and the surface roughness at the air-interface. From the help of Mie scattering and Surface scattering simulations, a systematic approach has been performed to explain the influence of the generated micro-structure on both the optical scattering and OLED-performance.

Conventional and inverted organic light emitting diodes based on bright green emmisive polyfluorene derivatives

Electro-active green light-emitting polymers based on polyfluorene derivatives (SF5 and SF6) were synthesized by Suzuki coupling reaction. Effect of fluorenone moiety on the main chain of the polyfluorene with allyl subunit was investigated against to performance of both inverted bottom emission and conventional organic light emitting diodes (IBOLEDs and OLEDs, respectively). 2,2′,2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBI) was then utilized as an electron transport layer (ETL) to analyze the change in transport characteristics of the devices with a structure of ITO/PEDOT:PSS/polyfluorene/TPBI/Ca/Al. The results showed that efficiency enhanced nearly four times while the light output values were lowered with the insertion of TPBI as an ETL. In addition, IBOLEDs were also fabricated with a device architecture of ITO/ZnO/polyfluorene/V2O5/Al being keen on backwards charge injection behavior of the polymers. IBOLEDs were fabricated by using synthesized zinc oxide (ZnO) as the electron-injecting layer. Luminance and luminous efficiency were significantly low while turn on voltages were too low with respect to the conventional devices fabricated. Further, annealing treatment were optimized for light output characteristics of the fabricated OLEDs. This study revealed the importance of charge transport feature of the synthesized polyfluorene derivatives in electroluminescent devices with variant architecture.

Optically-Pumped Single-Mode Deep-Ultraviolet Microdisk Lasers With AlGaN-Based Multiple Quantum Wells on Si Substrate

In this work, we report demonstration of optically-pumped single-mode deep-ultraviolet lasing actions operating at room temperature from ∼1- μ m 150-nm-thick undercut microdisks with AlN/Al0.35Ga 0.65N (5.5 nm/2.5 nm) multiple quantum wells. These AlGaN-based microdisks are grown on Si substrate by metal-organic chemical vapor deposition. The lasing wavelength centers at ∼300.1 nm with the linewidth of ∼1.0 nm as the excitation exceeds the lasing threshold of ∼24.2 mJ/cm2. An emission coupling factor ( β ) of 9.2 × 10−2 is estimated based on the light output characteristics of the AlN/AlGaN microdisks with increasing the pumping densities. Concurrently, a 100 meV blue-shift in the mode energy has also been observed. The lasing spectral peak is attributed to fundamental-order transverse-electric whispering-gallery modes, confirmed by three-dimensional finite-difference time-domain simulations.

The role of ITO resistivity on current spreading and leakage in InGaN/GaN light emitting diodes

The effect of a transparent ITO current spreading layer on electrical and light output properties of blue InGaN/GaN light emitting diodes (LEDs) is discussed. When finite conductivity of ITO is taken into account, unlike in previous models, the topology of LED die and contacts are shown to significantly affect current spreading and light output characteristics in top emitting devices. We propose an approach for calculating the current transfer length describing current spreading. We show that an inter-digitated electrode configuration with distance between the contact pad and the edge of p-n junction equal to transfer length in the current spreading ITO layer allows one to increase the optical area of LED chip, as compared to the physical area of the die, light output power, and therefore, the LED efficiency for a given current density. A detailed study of unpassivated LEDs also shows that current transfer lengths longer than the distance between the contact pad and the edge of p-n junction leads to increasing surface leakage that can only be remedied with proper passivation.

Pulse-shape discrimination with Cs2HfCl6 crystal scintillator

The results of investigation into cesium hafnium chloride (Cs2HfCl6) scintillating crystals as a promising detector to search for rare nuclear processes occurring in Hf isotopes is reported. The light output, quenching factor, and pulse-shape characteristics have been investigated at room temperature. The scintillation response of the crystal induced by α-particles and γ-quanta were studied to determine possibility of particle discrimination. Using the optimal filter method we obtained clear separation between signals with a factor of merit (FOM) = 9.3. This indicates that we are able to fully separate signals originating from α-particles and γ-quanta. Similar fruitful discrimination power was obtained by applying the mean time method (FOM = 7) and charge integration method (FOM = 7.5). The quenching factor for collimated 4 MeV α-particles is found to be 0.36, showing that α-particles generate more than a third of the light compared to γ-quanta at the same energy.