Reference LEDs Research Articles

Passive Internet of Events Enabled by Broadly Compatible Self-Powered Visualized Platform Toward Real-Time Surveillance.

Surveillance is an intricate challenge worldwide especially in those complicated environments such as nuclear plants, banks, crowded areas, barns, etc. Deploying self-powered wireless sensor nodes can increase the system's event detection capabilities by collecting environmental changes, while the incompatibility among components (energy harvesters, sensors, and wireless modules) limits their application. Here, a broadly compatible self-powered visualized platform (SPVP) is reported to construct a passive internet of events (IoE) network for surveillance systems. By encoding electric signals into reference and working LEDs, SPVP can visualize resistance change generated by commercial resistive sensors with a broad working range (<107 Ω) and the transmission distance is up to 30 meters. Visible light signals are captured by surveillance cameras and processed by the cloud to achieve real-time event monitoring and identification, which forms the passive IoE network. It is demonstrated that the passive-IoE-based surveillance system can detect intrusion, theft, fire alarm, and distress signals quickly (30ms) for 106 cycles. Moreover, the confidential information can be encrypted by SPVPs and accessed through a phone application. This universal scheme may have huge potential for the construction of safe and smart cities.

Bit Depth of Drivers for Micro-LED Displays Adopting Low-Temperature Polysilicon Oxide Thin-Film Transistors

In this paper, the minimum bit depth of drivers for active-matrix light emitting diode (AMLED) displays has been specified to overcome luminance nonuniformity of LEDs for the first time. The pixel circuit comprises seven transistors and one capacitor (7T1C) based on low-temperature polysilicon oxide (LTPO) process. For the experiments, thirty green and blue LEDs were used. When the same data voltage was applied, there was luminance nonuniformity of up to 19 % and 41 % for green and blue LEDs compared to the reference LEDs. Data voltages were measured to get the same luminance as the target one. It was found that the maximum voltage intervals of 0.9 mV and 1.1 mV were necessary to overcome the luminance nonuniformity for green and blue LEDs, respectively. 1.62 V and 2.80 V were the data voltage ranges of green and blue LEDs, respectively. The bit depth of 11-bit (2,048) is the minimum one because 1.62 V / 0.9 mV for green LEDs is smaller than 2,047. In this way, at least 12-bit is necessary for the bit depth of drivers for blue LEDs.

Goniometric complex for investigation the optical characteristics of radiation of luminescent structures

In this paper we describe an automated goniometric complex for investigation the optical characteristics and them spatial distribution of luminescent structures.On the market most instruments for measuring of the spatial distribution of radiation are designed for commercial light sources, such as LEDs, incandescent lamps, fluorescent lamps. Often scientific research requires measurements of the spatial distribution of emitted or scattered radiation upon excitation by external laser radiation. Therefore, the device for these measurements was designed and manufactured. In the article the design and principle of operation of the complex are described and its main technical characteristics are presented. During metrological certification specially developed five reference LEDs with different spectral characteristics were used to ensure traceability to the national measurement standards of the Republic of Belarus. The automated goniometric complex allows one to measure the spatial distribution of luminescence and scattering radiation with external laser excitation in the units of the spectral irradiance, luminous flux, chromaticity coordinates and correlated color temperature.The main metrological characteristics of the complex: spectral range — from 396 nm to 753 nm with a spectral resolution of 0.5 nm; power spectral density measurement range — from 3.5∙102 W/m3 to 1∙106 W/m3 in the spectral range from 400 nm to 750 nm with relative combined standard uncertainty of 7.8 %; chromaticity coordinate measurement range for coordinate x from 0.088 to 0.676, for coordinate y — from 0.160 to 0.715, with relative combined standard uncertainty of 5.5 % for coordinate x and 8.4 % for coordinate y; measurement range of correlated color temperature — from 2000 K to 7000 К with relative combined standard uncertainty of 9.3 %; luminous flux measurement range — from 0.6 lm to 35.0 lm with relative combined standard uncertainty of 9.3 %; rotation angle — from minus 90° to plus 90° in tangential direction and from minus 180° to plus 180° in radial direction with angular resolution of 0.5°.Measurement results are processed by specially developed software. Results can be presented in the form of three-dimensional diagrams for selected units of measurement or in form of a text file.

Lithographic technologies suitable for PhC patterning and optical properties of patterned LED surfaces

There are several promising methods for photonic crystal (PhC) patterning of LED surfaces for improving the LED efficiency, i.e. the light out coupling. 2D patterns with different periods and various symmetries were fabricated using interference lithography (IL), non-contact near-field scanning optical microscopy (NSOM) lithography, and e-beam direct write (EBDW) lithography, respectively. The periodicity and quality of fabricated structures have been investigated using AFM and SEM imaging. Light extraction enhancement (LEE) was studied by measuring of the light-current (L-I) characteristics of untreated reference LEDs and patterned LEDs. The highest achieved LEE was measured for LEDs patterned by interference lithography.

Improving output power performance of InGaN-based light-emitting diodes by employing step-down indium contents

We investigated the effect of step-down indium content in InGaN quantum wells (QWs) on the output efficiency of fully packaged InGaN-based light-emitting diodes (LEDs). Both the reference and step-down LED chips give maximum external quantum efficiencies (EQE) of 54.65 and 54.99% at a current density of 4.17 A/cm2, respectively. Step-down LEDs show a lower efficiency droop than reference LEDs. The step-down LEDs exhibit a 5.3% higher EQE at 83.3 A/cm2 than the reference LEDs. As the current density increases from 1.39 to 9.03 A/cm2, the electroluminescence (EL) intensity peaks of the step-down LEDs are slightly more shifted towards the larger energy side than those of the reference LEDs. The polarization field is estimated to be 1.34 and 1.41 MV/cm for the reference and step-down LEDS, respectively. The simulated internal quantum efficiency results of the reference and step-down LEDs are in agreement with the experimental results. The simulation results show that the step-down LEDs have higher hole injection efficiency than the reference LEDs. On the basis of the simulation results, the blue-shift behavior of the reference and step-down LEDs is described and discussed.

Improved performance of semi-polar (11-22) GaN-based light-emitting diodes grown on SiNx interlayer

We report on the effectiveness of the in-situ SiNx nanomask in reducing defects in semipolar (11-22) GaN films grown on m-plane sapphire. The properties of the semipolar InGaN/GaN double quantum well (DQW) LEDs were improved with a high-quality (11-22) GaN epilayer grown on the SiNx interlayer. High resolution X-ray diffraction analysis revealed that there was a great reduction in the full width at half maximum of both on-axis and off-axis planes on SiNx interlayer. The room temperature cathodoluminescence (CL) band-edge emission intensity of (11-22) GaN grown on the SiNx interlayer was approximately 4 times higher than that of GaN without the SiNx interlayer, which suggests reduction in the nonradiative recombination centers. The optical power of LEDs with the SiNx interlayer was 200% and 270% higher at injection currents of 20mA and 100mA, respectively, compared to the reference LEDs.

A novel randomly textured phosphor structure for highly efficient white light-emitting diodes

We have successfully demonstrated the enhanced luminous flux and lumen efficiency in white light-emitting diodes by the randomly textured phosphor structure. The textured phosphor structure was fabricated by a simple imprinting technique, which does not need an expensive dry-etching machine or a complex patterned definition. The textured phosphor structure increases luminous flux by 5.4% and 2.5% at a driving current of 120 mA, compared with the flat phosphor and half-spherical lens structures, respectively. The increment was due to the scattering of textured surface and also the phosphor particles, leading to the enhancement of utilization efficiency of blue light. Furthermore, the textured phosphor structure has a larger view angle at the full width at half maximum (87°) than the reference LEDs.

Electrical, spectral and optical performance of yellow–green and amber micro-pixelated InGaN light-emitting diodes

Micro-pixelated InGaN LED arrays operating at 560 and 600 nm, respectively, are demonstrated for what the authors believe to be the first time. Such devices offer applications in areas including bioinstrumentation, visible light communications and optoelectronic tweezers. The devices reported are based on new epitaxial structures, retaining conventional (0 0 0 1) orientation, but incorporating electron reservoir layers which enhance the efficiency of radiative combination in the active regions. A measured output optical power density up to 8 W cm−2 (4.4 W cm−2) has been achieved from a representative pixel of the yellow–green (amber) LED array, substantially higher than that from conventional broad-area reference LEDs fabricated from the same wafer material. Furthermore, these micro-LEDs can sustain a high current density, up to 4.5 kA cm−2, before thermal rollover. A significant blueshift of the emission wavelength with increasing injection current is observed, however. This blueshift saturates at 45 nm (50 nm) for the yellow–green (amber) LED array, and numerical simulations have been used to gain insight into the responsible mechanisms in this microstructured format of device. In the relatively low-current-density regime (<3.5 kA cm−2) the blueshift is attributable to both the screening of the piezoelectric field by the injected carriers and the band-filling effect, whereas in the high-current regime, it is mainly due to band-filling. Further development of the epitaxial wafer material is expected to improve the current-dependent spectral stability.

Improving current spreading of GaN‐based LEDs by N ‐pad current surrounding design

AbstractIn this report, the motivation of why we attempt to design current spreading, or current surrounding, is try to provide other current paths, expect to improve the current spreading of LEDs and get lower forward voltage at the same time. Attempt to avoid the trade‐off between p ‐electrode area for uniform current spreading and emitting area for light output of LEDs.Compared to reference LEDs, we can find that the light output intensity of chips with n ‐pad current surrounding design will decay at higher current injection. This can be attributed to the metal on pitch we design, with better current spreading and much metal area connected to n ‐pad, so less current crowding effect and less heat‐accumulation in chips (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Photometry and colorimetry of reference LEDs by using a compact goniophotometer

In the development, production control, quality assurance and during the selection of appropriate LEDs for very different applications, it is essential to know their photometric and colorimetric quantities very precisely. The calibration of the measuring devices required for this purpose is performed via LED transfer standards. These LED standards transfer photometric, spectroradiometric and colorimetric units and characteristics, from the National Metrology Institute (NMI) to the user. Finally, LED standards serve as references for the user. Usually a set of LED reference standards consists of at least four different LED colours (blue, green, red and white), sometimes even more. Further more it is very often necessary to have more than only one set of reference standards as a backup. This generally causes a large number of calibrations. To handle this work load, a special reference standard and a compact goniophotometer prove to be a considerable advantage. PTB developed a special LED transfer standard which operates under controlled conditions: a controlled LED chip temperature and a controlled chip current help to keep the photometric and colorimetric properties constant. On the other hand, a compact goniophotometer was designed to measure luminous flux, luminous intensity and colorimetric quantities of LED transfer standards in a relatively short time with low measurement uncertainties.

Fabrication and performance of parallel-addressed InGaN micro-LED arrays

High-performance, two-dimensional arrays of parallel-addressed InGaN blue micro-light-emitting diodes (LEDs) with individual element diameters of 8, 12, and 20 /spl mu/m, respectively, and overall dimensions 490 /spl times/490 /spl mu/m, have been fabricated. In order to overcome the difficulty of interconnecting multiple device elements with sufficient step-height coverage for contact metallization, a novel scheme involving the etching of sloped-sidewalls has been developed. The devices have current-voltage (I-V) characteristics approaching those of broad-area reference LEDs fabricated from the same wafer, and give comparable (3-mW) light output in the forward direction to the reference LEDs, despite much lower active area. The external efficiencies of the micro-LED arrays improve as the dimensions of the individual elements are scaled down. This is attributed to scattering at the etched sidewalls of in-plane propagating photons into the forward direction.

Efficient GaN-based Micro-LED Arrays

ABSTRACTHighly efficient, two-dimensional arrays of parallel-addressed InGaN blue microLEDs with individual element diameters of 8, 12 and 20μm have been fabricated. In order to overcome the difficulty of interconnecting multiple device elements with sufficient step-height coverage for contact metallisation, a novel scheme involving the etching of sloped-sidewalls has been developed. The devices have I-V characteristics similar to those of broad-area reference LEDs fabricated from the same wafer, and give superior (3mW) light output in the forward direction to the reference LEDs, despite much lower active area. The external efficiencies of the micro-LED arrays improve as the dimensions of the individual elements are scaled down. This is attributed to scattering at the etched sidewalls of in-plane propagating photons into the forward direction.

Monolithic Integration of III-V Microcavity Leds on Silicon Drivers using Conformal Epitaxy

AbstractConformal epitaxy is an epitaxial growth technique capable of yielding low dislocation density III-V films on Silicon. In this technique, the growth of the III-V material occurs parallel to the silicon substrate, from the edge of a previously deposited III-V seed, the vertical growth being stopped by an overhanging capping layer. As an example, conformal GaAs layers on Silicon, presenting dislocation densities below 105cm−2, have been obtained using selective vapor phase epitaxy. These layers have then been used as high quality GaAs on Si substrates for subsequent vertical MBE regrowth of active structures. In this paper, we report on the integration of surface-emitting microcavity LEDs with their silicon drivers using this conformal growth technique. The global technology concept and the design of the active structures are first presented. The compatibility of the conformal growth technique with CMOS technology is then checked: the impact of the integration process on the performances of the drivers is for example quantified. Characterisations of the high crystalline quality of the conformal layers and of the LEDs structures grown on it are then shown. The electro-optical characteristics of the LEDs on Si are finally compared to those of reference LEDs on GaAs substrates in order to prove the efficiency of the integration procedure.

Interocclusal distance measurement comparing chin and tooth reference points

In a group of nine dentate students with a mean age of 23.1 years, a comparison of their interocclusal distances was determined using tooth and chin reference points. This determination was made with two reference LEDs, one attached to a lower tooth and the other attached to a point on the chin. These two LEDs were registered simultaneously using the Selspot system and analyzed with the use of a computer. Three different methods were used to establish the interocclusal distance: a phonetic method, a relaxation method, and a swallowing method. Independent of the method selected, there was a greater interocclusal distance recorded by the tooth-attached reference LED. This study indicates that, in the clinical determination of interocclusal distance, where a chin reference is used the dentist should be aware of this difference—especially when lip activity is involved.