Light-emitting Area Research Articles

Influence of quantum well thickness on the Cathodoluminescent properties of AlGaN-based UVC light source tube with CNT field emitters

Far ultraviolet C (UVC) light sources work well for a variety of applications, such as water purification, bacteria and virus sterilization, and sensing, because of their short wavelength. Herein, the recognized problems with a conventional UVC-LED are solved by creating a triode structure UVC light source tube (UVC-LST) operating at ∼268 nm employing a carbon nanotube (CNT) electron emitter as an excitation source. The UVC light emission of an aluminum reflector-capped AlGaN multiple quantum well (MQW) heterostructures synthesized on a nanopatterned sapphire substrate based on a cathodoluminescence (CL) layer is assessed. For luminescence results comparison, the heterostructure of AlGaN-based MQWs was fabricated with various QW layer thicknesses of 1, 1.5, and 2 nm. The utilization of a thinner QW layer with a thickness of 1 nm significantly improved the UVC light emission efficiency compared to the UVC-LSTs fabricated with thicker QW layers of 1.5 nm and 2 nm. The UVC-LST with the 1 nm QW layer exhibited approximately a 30 % improvement in light output power compared to the UVC-LST with a 1.5 nm QW layer, and closely a 33 % improvement compared to the UVC-LST with a 2 nm QW layer. Moreover, the UVC-LST with the thinner QW layer demonstrated a high power efficiency of 5.24 % when operating on a 2-inch light-emitting area, while consuming a low power of 7 W. This high power efficiency indicates that a significant portion of the input electrical power is effectively converted into UVC light output, highlighting the superior performance of the UVC-LST based on the 1 nm QW layer design.

A scattered volume emitter micropixel architecture for ultra efficient light extraction from DUV LEDs

Deep ultraviolet light-emitting diodes (DUV LEDs) typically suffer from strong parasitic absorption in the p-epitaxial layer and rear metal contact/mirror. This problem is exacerbated by a substantial portion of the multiple quantum well (MQW) emissions having a strong out-of-plane dipole component, contributing to emission in widely oblique directions outside the exit cone of the front semiconductor emitting surface. To address this, we propose an architecture that leverages such a heavily oblique angular emission profile by utilizing spaced-apart or scattered volume emitter micropixels that are embedded in a low-index dielectric buffer film with a patterned top surface. This approach achieves high light extraction efficiency at the expense of enlarging the effective emission area, however, it does not require a high-index (e.g., sapphire) substrate or a lens or a nanotextured epi for outcoupling purposes. Hybrid wave and ray optical simulations demonstrated a remarkable larger than three to sixfold increase in light extraction efficiency as compared to that of a conventional planar LED design with a sapphire substrate depending on the assumed epi layer absorption, pixel size, and ratio of light emission area to the MQW active area. An extraction efficiency three times greater than that of a recent nanotextured DUV LED design was also demonstrated. This architecture paves the way for DUV LEDs to have a plug efficiency comparable to that of mercury lamps while being significantly smaller.

Nonlinear and Negative Effective Diffusivity of Interlayer Excitons in Moiré-Free Heterobilayers.

Interlayer exciton diffusion is studied in atomically reconstructed MoSe_{2}/WSe_{2} heterobilayers with suppressed disorder. Local atomic registry is confirmed by characteristic optical absorption, circularly polarized photoluminescence, and g-factor measurements. Using transient microscopy we observe propagation properties of interlayer excitons that are independent from trapping at moiré- or disorder-induced local potentials. Confirmed by characteristic temperature dependence for free particles, linear diffusion coefficients of interlayer excitons at liquid helium temperature and low excitation densities are almost 1000 times higher than in previous observations. We further show that exciton-exciton repulsion and annihilation contribute nearly equally to nonlinear propagation by disentangling the two processes in the experiment and simulations. Finally, we demonstrate effective shrinking of the light emission area over time across several hundreds of picoseconds at the transition from exciton- to the plasma-dominated regimes. Supported by microscopic calculations for band gap renormalization to identify the Mott threshold, this indicates transient crossing between rapidly expanding, short-lived electron-hole plasma and slower, long-lived exciton populations.

P‐10.11: Pixelation of Nano‐LED arrays with high effective light‐emitting area ratio by ion injection: A Simulation study

The etch damage caused by the conventional bench-top etching process makes it difficult to produce Nano-LED devices. Recently, some scholars have studied the use of ion injection to achieve electrical isolation between pixels for the purpose of pixelation. In this paper, we focus on the distribution characteristics of ions in GaN targets to improve the effective light-emitting area ratio of devices by reducing the lateral diffusion effect of ions.

High color rendering and high-luminance laser lighting using all inorganic nitride phosphor films.

Despite the huge advances that have been made in the development of ultra-high luminance laser lighting, achieving high color rendering properties in such systems at the same time remains a challenge. Recent studies show that in most cases, the luminous efficacy (LE) of laser lighting is compromised to improve the color rendering index (CRI). In this study, a possible solution to this problem has been proposed by preparing phosphor-in-glass (PiG) films comprised of the yellow-emitting phosphor (LSN:Ce3+) and the red-emitting phosphor (CASN:Eu2+). The composite material synthesized in this study exhibited outstanding optical and thermal properties. A uniform white light with a high CRI of 80.0 and a high LE of 185.9 lm W-1 was achieved by optimizing the yellow/red ratio and the emission peak position of the blue laser. Furthermore, it was found that this design enabled the phosphor to restrict the light emission area effectively, thus attaining a high luminous exitance of 1302 lm mm-2. With their superior optical performance, the PiG films can be regarded as promising color converter candidates for future high-quality laser-based white light sources.

Investigation of Pressure Dependence in Photoresist Ashing Process using Microwave Excited Water Vapor Plasma

Microwave-excited water plasma asher (WPA) is our laboratory's new technology for the photoresist (PR) ashing process in semiconductor manufacturing which uses water vapor as process gas. It has the potential to solve many problems of conventional PR ashing technologies including high temperature, oxidation of metal structure, low ashing rate for ion-implanted PR, and high costs of chemicals. In this study, the pressure dependence of various aspects of plasma in water plasma asher, including light emission area, emission intensity, and substrate temperature was investigated. The ashing rate of the novolak PR film on the silicon wafer at the various condition of process pressure was also investigated. Average emission intensity and emission area of plasma significantly increased as process pressure in the chamber were decreased, and they were the highest when the process pressure was 0.3 kPa. The average ashing rate of PR film was only 0.42 μm/min when process pressure was 2.14 kPa, and it gradually increased as process pressure was decreased. It is also found that the ashing rate was improved after reducing the temperature of the water. The ashing rate when using air-rich condition of plasma was observed to be higher than when only water vapor is used as process gas.

Effects of ITO Contact Sizes on Performance of Blue Light MicroLEDs

In this study, the effect of ITO contact ratio for blue light micro-light-emitting diode (µLED) with dimensions 40 μm × 40 μm was assessed. The contact ratio from 0.2 to 0.8 was designed for the ratio of electrode area to light-emitting area. As the contact ratio increased from 0.2 to 0.8, the turn-on voltage of µLED decreased. It could be due to the short lateral diffusion length in multiple quantum wells (MQW) and lower parallel resistance for the µLED with a large contact ratio. The leakage currents of single µLED were below 5.1 × 10–9 A, no matter the contact ratio. It means that the contact ratio does not affect the leakage current as measured on single chip. Moreover, µLED array with a 0.8 contact ratio presented the highest output power than other samples (5.25 mW as the current density of 1875 A/cm2). It could attribute to the MQWs usage, the metal contact reflective behavior and less current crowding, which generated more carriers and extracted more lighting from the µLED. The simulation data using SpeCLED software agreed well with these experiments, and µLED with a 0.8 contact ratio showed the best optoelectronic properties.

Investigation of the Effect of ITO Size and Mesa Shape on the Optoelectronic Properties of GaN-Based Micro LEDs

In this paper, in order to explore the influence of indium tin oxide (ITO) size and mesa shape on the performance of GaN-based micro light emitting diodes (Micro LEDs) on sapphire substrates, Micro LEDs of different sizes with ITO area smaller than or equal to the light-emitting area were designed and fabricated. The experiment results show that when the ITO area of the Micro LED is equal to the area of the light-emitting area, its optoelectronic performance is significantly better than that of the Micro LEDs whose ITO area is smaller than the area of the light-emitting area. When the light-emitting area size is 40 μm, the wall-plug efficiency (WPE) of the two structures of Micro LEDs can differ by more than 50%. Based on above experiment results, this paper designed and fabricated Micro LEDs with different sizes of square and circular mesa with the same ITO area as the light-emitting area. The experimental results show that the WPE of the circular mesa Micro LED is slightly higher than that of the square mesa Micro LED at low current density. However, as the current density and chip size increase, the performance of the Micro LED with a square mesa is better.

High-Brightness Perovskite Microcrystalline Light-Emitting Diodes.

Here a high-brightness perovskite microcrystalline light-emitting diode (LED) is reported, in which the perovskite microcrystals were grown directly on the conductive substrate and a simple metal-insulator-semiconductor structure was adopted. A peak external quantum efficiency of 0.46% was obtained, which is high for perovskite microcrystalline LEDs. Importantly, the maximum luminance of the device reaches 8848.4 cd m-2, indicating an ultrahigh brightness of >1.2 × 106 cd m-2 for the microcrystals (corresponding to an ultrahigh current density of 80.9 A cm-2), because the light-emitting area of the microcrystals accounts for only ∼0.7% of the device area. In addition, we have studied the degradation of the device at a high current density by in situ microscopic observation and found that a severe Joule heating effect at large injection is the primary problem to be solved to realize electrically pumped perovskite microcrystal lasing.

Progress and Challenges of InGaN/GaN-Based Core-Shell Microrod LEDs.

LEDs based on planar InGaN/GaN heterostructures define an important standard for solid-state lighting. However, one drawback is the polarization field of the wurtzite heterostructure impacting both electron–hole overlap and emission energy. Three-dimensional core–shell microrods offer field-free sidewalls, thus improving radiative recombination rates while simultaneously increasing the light-emitting area per substrate size. Despite those promises, microrods have still not replaced planar devices. In this review, we discuss the progress in device processing and analysis of microrod LEDs and emphasize the perspectives related to the 3D device architecture from an applications point of view.

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.

Sapphire Wafer for 226 nm Far UVC Generation with Carbon Nanotube-Based Cold Cathode Electron Beam (C-Beam) Irradiation.

Far ultraviolet C (UVC) light sources have the potential for numerous applications ranging from sterilization, purification, sensing, deodorization, surface modification, and so on. In particular, a short wavelength of far UVC is effective at sterilizing viruses and bacteria by minimizing damage to mammalian skin. Recently, many researchers are devoting materials and alternative light sources to overcome low efficiency, small light-emitting area, UV absorption, and complicated manufacturing processes of far UVC generation. Here, the sapphire wafer is evaluated for far UVC light generation using electron beam irradiation with carbon nanotube (CNT) emitters. A CNT-based cold cathode electron beam (C-beam) that emits electrons and accelerated onto κ-Al2O3 of the sapphire wafer was used as an excitation source to demonstrate high-power far UVC light generation. High-efficiency 226 nm far UVC is made with a power conversion efficiency of 0.87% and a light-emitting area of 960 mm2. Far UVC generation depends on the input power and the crystallinity of sapphire wafers.

Light Fidelity (Li - Fi) Access Point Selection for Heterogeneous Network with Non Overlapping Li - Fi Access Points

Data transmission is a part of our life in today’s scenario. Data communication can be carried out by wired technology and wireless technology. Radio frequency is a part of the electromagnetic spectrum, which is used for wireless communication. Many technologies are used for wireless communication such as Wi-Fi, Blue Tooth, Ad-hoc, and etc. These are all transmitting data through radio spectrum for short distance. New technology called as Li-Fi (Light-Fidelity) is going to be a major wireless communication technology in future. In the proposed system a heterogeneous network has been consider as a system model. Li-Fi technology is having the capacity of low coverage area. So, non movable wireless devices are getting better usage of Li-Fi technology in an indoor environment. For mobile devices need more number of Li-Fi access points. When the user moves from one access point to another access point covering range, the service should be continued without break. System handover is a major issue in the Li-Fi technology. If the user goes out of the range of light emission area, then the user can get the service through Wi-Fi access point. Proposed system focusing on the handover process based on the measured handover efficiency value. Data transmission can be disturbed due to some power failure or LED fault. In this situation immediately the entire load should be handled by the Wi-Fi access point. In such a situation data rate is coming down than the threshold value. So handover process will be handled by the common controller. Data rate is observed when the number of user increased and for variation in channel gain and the simulation results are analysed.

Ion implantation isolation based micro-light-emitting diode device array properties

Compared with conventional light-emitting diode (LED), micro-LED has excellent photo-electric properties such as high current density, light output power density, light response frequency. It has widespread application prospects in the field of light display, optical tweezers, and visible light communication. However, dry etching inevitably leads the sidewall to be damaged, which results in the degradation of device properties. In this letter, a micro-LED array device based on F ions implantation isolation technology is presented to avoid damaging the sidewall. We systemically investigate the influence of fluorine ion implantation energy and light-emitting apertures on the photoelectric properties of the micro-LED array device by testing the current-voltage characteristic and light output power. The investigation results show that comparing with F ion 50 keV single implantation device, the reverse leakage of 50/100 keV double implantation device decreases by 8.4 times and the optical output density increases by 1.3 times. When the light-emitting apertures are different (6, 8, 10 μm respectively), the reverse leakage current remains constant, and the forward operating voltage decreasesfrom 3.3 V to 3.1 V and to 2.9 V with the increase of the aperture. Besides, the available area ratio, i.e. the ratio of actual light-emitting area to device area of single micro-LED with different light-emitting apertures are 85%, 87%, and 92%, respectively. The electrical isolation of the micro-LED array is realized by ion implantation isolation technology, and the micro-LED has some advantages over the conventional mesa etching micro-LED device, such as low reverse leakage current density, high optical output power density, and high effective light-emitting area ratio.

Planarization of slot-die coated multilayer stripes via pre-drying by a heating roll for organic light-emitting diodes

Micro-patterning of single-layer thin films in the cross-web (transverse) direction is feasible using a slot-die coating system. For its practical applications in organic light-emitting diodes (OLEDs), however, the fabrication and planarization of multilayer thin-film stripes are indispensable. We have coated multilayer stripes using an aqueous poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) and non-aqueous poly(N-vinylcarbazole) (PVK) and planarized them via pre-drying by a heating roll in a roll-to-roll slot-die coating system. It is demonstrated that the PEDOT:PSS stripe can be planarized at a pre-drying (heating roll) temperature of 40 °C. At higher temperatures, it is found that there appear the evaporation-induced inward and/or outward capillary flows depending on the wet film thickness. Using a low-boiling-point solvent, the PVK stripe coated atop the planarized PEDOT:PSS stripe is shown to be planarized at a pre-drying temperature of 65 °C, the step coverage and thickness uniformity by which are substantially enhanced. Such a planarization scheme also brings in the positive effect on the surface morphology of those stripes. To verify the planarization effect at the device level, we have fabricated phosphorescent OLED stripes. Without planarization, the light-emitting area shrinks and a spot with strong light emission appears, increasing the probability of a short circuit in OLEDs. With planarization, however, we can achieve brighter light emission from OLED stripes with high inter-stripe emission uniformity.

752-MHz Modulation Bandwidth of High-Speed Blue Micro Light-Emitting Diodes

In this paper, the optical frequency response of gallium nitride-based blue micro-LEDs with different quantum well numbers in the active region is presented. Single quantum well (SQW) blue micro-LEDs have about 1.6 times higher modulation bandwidth ( $f_{-3\,\text {dB}}$ ) by comparison with triple quantum well blue micro-LEDs. The larger bandwidth enhancement of the SQW micro-LEDs can be attributed to more carriers stored in a quantum well leading to higher recombination rate and lower recombination lifetime. The high $f_{-3\,\text {dB}}$ up to 752 MHz is demonstrated in the SQW micro-LED with a light-emitting area of $10\,\,\mu \text {m} \times 10\,\,\mu \text{m}$ . It suggests that micro-LEDs designed with fewer quantum well numbers may have the potential for the application of visible light communication (VLC) technology.

A Comparative Study of GaN‐Based Direct Current and Alternating Current High Voltage Light‐Emitting Diodes

In this work, one type of direct current high voltage light‐emitting diode (DC‐HV LED) and six types of alternating current high voltage LEDs (AC‐HV LEDs) are demonstrated. Comparative current‐voltage (I–V) and light output power (LOP)‐current (L‐I) characteristics are performed between 24 V DC‐HV LED and 24 V AC‐HV LEDs with eight working cells. The AC‐HV LED has relatively larger wall‐plug efficiency (WPE) than DC‐HV LED over a current density range from 0 to 152 A cm−2. The effect of the layouts on the optical and electrical properties of AC‐HV LEDs is further investigated. Owing to larger heat dissipation area and fewer number of chips, our combined numerical and experimental results demonstrate that the AC‐HV LED I has a more favorable current spreading uniformity compared to other AC‐HV LEDs. In addition, it is found that the LOP of AC‐HV LEDs is dependent on both the number of working cells and the ratio of radiation area to total chip area. Larger ratio of light emission area to total chip area can be obtained by decreasing the area of rectifier cells. Therefore, in order to achieve a much higher LOP, AC‐HV LEDs have to be designed with smaller area of rectifier cells and with more working cells simultaneously.

Evaluation of organic light-emitting diodes as light sources for a compact optoelectronic integrated neural coprocessor

Organic light-emitting diodes (OLEDs) are a most promising candidate for light sources of the compact optoelectronic integrated neural (COIN) coprocessor because of their easy integration with the silicon electronics. However, the optical properties of OLEDs are different from those of vertical-cavity surface-emitting lasers, which were previously used in the COIN system. The interconnect behavior of the COIN coprocessor is unknown if OLEDs are employed. In this paper, we build a mathematical model of the interconnect scheme of the COIN coprocessor and investigate the influence of the spectral bandwidth, light-emitting area, and angular emission profile of OLEDs by numerical simulations. The simulation results show that OLEDs with properly selected properties can be used as the light sources for the COIN coprocessor.

Formation properties of an InGaN active layer for high-efficiency InGaN/GaN multi-quantum-well-nanowire light-emitting diodes

Nitride-based nanowires (NWs) have several advantages, such as flexibility in choosing a substrate, easy fabrication, large light-emitting area, no internal electric field, enhanced light extraction, and reduced defects by strain relief, that are useful for enhancing the efficiency of light-emitting diodes (LEDs). Here, we report how crucial the formation properties of the InGaN active layer are for enhancing the efficiency of core-shell InGaN/GaN multi-quantum-well (MQW)-NW LEDs that are selectively grown on oxide templates with perfectly-circular hole patterns. The nanostructures are analyzed for two types of LEDs, one containing defect-free MQW active layer and the other containing MQW layer with defects by using high-resolution transmission electron microscopy. The I-V curve of the defect-free LED shows a rectifying behavior with an on/off ratio of ~109, typical of a diode, and the off-state leakage current of the LED with defects is much larger than that of the defect-free LED, resulting in brighter electroluminescence from the latter device. These results suggest that well-defined nonpolar InGaN/GaN MQW-NWs can be utilized for the realization of high-performance LEDs.

An Organic Device with Thin Film Transistor Merged with Light Emitting Diode through Use of an Accumulation Layer in TFT As an Electrode

A light-emitting device structure is proposed which includes an organic thin-film structure merged with an organic light-emitting diode structure by utilizing a part of the electron accumulation layer in the organic thin-film transistor as a common electrode for each structure. The organic thin-film structure and the organic light-emitting diode structure each include an organic semiconductor material in which hole mobility is greater in bulk region of the material than electron mobility in the bulk region but less in the channel region. The advantages of such a light-emitting device include less complex processing and a simpler pixel circuit structure in comparison to separately fabricating OTFT and OLED structures and subsequently interconnecting them to form a pixel. Furthermore, relative to a light-emitting transistor, the proposed structure offers the advantage of a broader light emission area which is more suitable for use in display devices.