A polymer light-emitting diode as an optical communication light source

We demonstrate improvements in the electroluminescence switching rate of polymer light-emitting diodes by reducing their active area. We explore the frequency response of devices having a range of sizes and show that LED’s having an active area of 0.02 mm² can be modulated at 16 MHz. The emission from our devices peaks at around 540 nm and thus falls within a transmission window of plastic optical fibers, suggesting possible applications of polymer LED’s as low cost optical communication sources.     

Selective activation of failure mechanisms in packaged double-heterostructure light emitting diodes using controlled neutron energy irradiation

This paper demonstrates the feasibility of creating specific defects in double-heterostructure AlGaAsGaAs commercial light emitting diode by neutron irradiation. Using controlled neutron energy, only one failure mechanism can be activated. Defects are located in the side of the chip and increase the leakage current driven by the well-known Pool–Frenkel effect with E_c ? E_T = 130 meV electron trap energy level. The maximal amplitude of optical spectrum also reveals a drop about 20% associated to the rise of leakage current.     

Failure modes and effects analysis for high-power GaN-based light-emitting diodes package technology

In this study, nondestructive test is developed to analyze the structure failure of LED package. The relationship between thermal resistance analysis and LED package failure structure is build with T3Ster thermal transient tester and scanning electron microscope (SEM). The failure LED device with defect in the attaching layer and gap between LED chip and copper are designed advisedly. The failure factors of LED package have been measured with thermal resistance analysis and SEM cross-section images. The thermal dissipation performance of LED with defect in the attaching layer is indicated by thermal resistance analysis combined with SEM cross-section images. The blister in attaching layer results in 4.4 K/W additional thermal resistance. The gap between LED chip and copper also makes high additional thermal resistance with 8.6 K/W. Different failures of LED packages are indicated obviously using thermal transfer analysis. The LED package failure structure such as interface defect between solder and cup-shaped copper is able to forecast without destructive measurement.     

Thermal characteristics and fabrication of silicon sub-mount based LED package

In this paper, the cost of a light emitting diode (LED) package is lowered by using a silicon substrate as the base attached to the chip, in contrast to the conventional chip-on-board (COB) package. In addition we proposed an LED package with a new structure to promote reliability and lifespan by maximizing heat dissipation from the chip. We designed an LED package combining the advantages of COB based on conventional metal printed circuit board (PCB) and the merits of a silicon sub-mount as a substrate. When an input current 500–1000 mA was applied, the fabricated LED exhibited the light output of approximately 112 lm/W at 29 W. We also measured and compared the thermal resistance of the sub-mount package and conventional COB package. The measured thermal resistance of the sub-mount package with a reflective film of Ag and the COB package were 0.625 K/W and 1.352 K/W, respectively.     

Analysis of traps effect on AlGaN/GaN HEMT by luminescence techniques

The study is carried out on AlGaN/GaN HEMTs presenting current collapse effect at V_ds lower than 6 V. This effect is completely recovered by illuminating the component with light of 710 nm wavelength (1.75 eV). The spectral analysis of the light emission in the visible near infrared spectrum shows a bell-shape with superimposed distinct emission peaks. These features suggest that the electroluminescence (EL) signal is due to the direct intraband of electrons and inelastic intraband transition of electrons due to scattering by charged centres. Photoionisation experiments have been conducted to determine the light wavelengths/energies that separately change the drain current and the gate leakage current.     

Optimizing n-type contact design and chip size for high-performance indium gallium nitride/gallium nitride-based thin-film vertical light-emitting diode

The effects of the n-contact design and chip size on the electrical, optical and thermal characteristics of thin-film vertical light-emitting diodes (VLEDs) were investigated to optimize GaN-based LED performance for solid-state lighting applications. For the small (chip size: 1000×1000 µm²) and large (1450×1450 µm²) VLEDs, the forward bias voltages are decreased from 3.22 to 3.12 V at 350 mA and from 3.44 to 3.16 V at 700  mA, respectively, as the number of n-contact via holes is increased. The small LEDs give maximum output powers of 651.0–675.4 mW at a drive current of 350 mA, while the large VLEDs show the light output powers in the range 1356.7–1380.2 mW, 700 mA, With increasing drive current, the small chips go through more severe degradation in the wall-plug efficiency than the large chips. The small chips give the junction temperatures in the range 51.1–57.2 °C at 350  mA, while the large chips show the junction temperatures of 83.1–93.0 °C at 700  mA, The small LED chips exhibit lower junction temperatures when equipped with more n-contact via holes.     

Junction temperature and reliability of high-power flip-chip light emitting diodes

In this work, infrared micro-imaging, emission microscope measurements are performed on the chip surface of flip-chip light emitting diodes (FCLEDs). The temperature deviation on the chip surface increases from 19 to 146 °C when the injection current changes from 20 to 2000 mA. When the structure of FCLED is optimized, the temperature deviation becomes smaller. And the thermal resistance is achieved to as low as 10.4 °C/W. The finite element method calculation based on the model of steady-state current field and temperature field is carried out to investigate the effects of current spreading on thermal performance of FCLED.     

Strength determination of high-power LED die using point-load and line-load tests

The strength of high-power light emitting diode (LED) dies, cut from wafers with a laser, has to be determined for the need of design and quality control in order to assure the good reliability of packages in manufacturing and service. The objective of this study is to determine the strength of high-power LED die with a size of 1 × 1 × 0.1 mm³ by point-load test (PLT) and line-load test (LLT) associated with a plate-on-elastic-foundation configuration. ANSYS (one of commercial finite element codes) analysis is used to calculate the stress distributions of the die under both PLT and LLT. The ANSYS models of the PLT and LLT are validated by comparing with experimental force–displacement curves, and the results are further used to convert the die failure force from the tests into the die strength. The mechanism of tensile-stress dominated die strength has been discussed and validated in detail via these test results and analyses. The results of the PLT and LLT also indicate that for the die failure on chip surface, the average die strengths are about 1.44 GPa and 1.52 GPa from the PLTs with two different-radius pins, and about 1.2 GPa from the LLT. On the other hand, for failures on sapphire surface, the average die strengths are reasonably about 1.49 GPa and 1.26 GPa from the two PLTs, but the average one from the LLT is about 0.64 GPa (with less than 50% of the values from the PLT). The inconsistent data between two PLT and LLT for failure on sapphire surfaces were found to result from the edge chipping of the die specimen observed by scanning electron microscopy. It was also observed that the thin-layer GaN material has to be taken into account in the ANSYS analyses with a bi-material model of the LED die for precisely determining the die strength for failure on the chip surface. Otherwise, these strength data would be overestimated by a few tens of percent with a uni-material model of the LED die. All in all, this study has successfully demonstrated that the LED die strength can be determined by these feasible, easy-to-use and reliable test methods.     

Deep blue/ultraviolet microcavity OLEDs based on solution-processed PVK:CBP blends

There is an increasing need to develop stable, high-intensity, efficient OLEDs in the deep blue and UV. Applications include blue pixels for displays and tunable narrow solid-state UV sources for sensing, diagnostics, and development of a wide band spectrometer-on-a-chip. With the aim of developing such OLEDs we demonstrate an array of deep blue to near UV tunable microcavity (μc) OLEDs (λ ∼373–469 nm) using, in a unique approach, a mixed emitting layer (EML) of poly(N-vinyl carbazole) (PVK) and 4,4′-bis(9-carbazolyl)-biphenyl (CBP), whose ITO-based devices show a broad electroluminescence (EL) in the wavelength range of interest. This 373–469 nm band expands the 493–640 nm range previously attained with μcOLEDs into the desired deep blue-to-near UV range. Moreover, the current work highlights interesting characteristics of the complexity of mixed EML emission in combinatorial 2-d μcOLED arrays of the structure 40 nm Ag/x  nm MoO_x/∼30 nm PVK:CBP (3:1 weight ratio)/y  nm 4,7-diphenyl-1,10-phenanthroline (BPhen)/1 nm LiF/100 nm Al, where x = 5, 10, 15, and 20 nm and y = 10, 15, 20, and 30 nm. In the short wavelength μc devices, only CBP emission was observed, while in the long wavelength μc devices the emission from both PVK and CBP was evident. To understand this behavior simulations based on the scattering matrix method, were performed. The source profile of the EML was extracted from the measured EL of ITO-based devices. The calculated μc spectra indeed indicated that in the thinner, short wavelength devices the emission is primarily from CBP; in the thicker devices both CBP and PVK contribute to the EL. This situation is due to the effect of the optical cavity length on the relative contributions of PVK and CBP EL through a change in the wavelength-dependent emission rate, which was not suggested previously. Structural analysis of the EML and the preceding MoO_x layer complemented the data analysis.     

Effect of 805 nm on reliability of 735/805/850-nm LED involved near-infrared spectroscopy biomedical device

Near-infrared spectroscopy (NIRS) has been widely used in biomedicine due to its capability of noninvasively detecting hemodynamic variations in relative deep tissue. Most NIRS devices utilized multiple-wavelengths integrated LED as the sources, of which the 735/805/850-nm LED was mostly employed. As we known, the 735/850-nm combination is enough for quantifying the changes of oxy-hemoglobin (∆ HbO₂) and deoxy-hemoglobin (∆ Hb). Then how is the effect of the wavelength 805 nm of 735/805/850-nm LED on the measurement reliability? Here we performed blood model experiments with 57 human blood samples and recorded optical density variations at above 3 wavelengths. Both of the least squares method and multi-variable linear regression analysis were used to quantify ∆ HbO₂ and ∆ Hb with three-wavelength combination (735/805/850-nm) and two-wavelength combination (735/850-nm) respectively. By comparing the quantified values with the real values, we found that the results obtained from 735/850-nm combination are more close to reality than the 735/805/850-nm combination. This study reported, for the first time, that 805 nm actually took a negative effect on measurement reliability of NIRS. It indicates to get rid of 805 nm from such LED design to reduce the LED cost and get higher reliability for NIRS instrumentation.     

Improving performance of high-power indium gallium nitride/gallium nitride-based vertical light-emitting diodes by employing simple n-type electrode pattern

In this work, simple n-type electrode structures were used to enhance the electrical and optical performances of fully packaged commercially mass-produced vertical-geometry light-emitting diodes (VLEDs). The forward bias voltage of the VLED with a Y-pattern electrode increased less rapidly than that of VLEDs with a reference electrode. The VLEDs with the reference and Y-pattern electrodes exhibited forward voltages of 2.93±0.015 and 2.89±0.015 V at 350 mA and 3.77±0.015 and 3.53±0.015 V at 2000 mA, respectively. The VLEDs with the Y-pattern electrode resulted in a higher light output than the VLEDs with the reference electrode with increase in the drive current to 2000 mA. The emission images showed that the VLEDs with the Y-pattern electrode exhibited better current spreading behavior and lower junction temperatures than the VLEDs with the reference electrode. With increase in the current from 350 to 2000 mA, the VLEDs with the Y-pattern electrode experienced a 39.4% reduction in the wall plug efficiency, whereas the VLEDs with the reference electrode suffered from a 43.3% reduction.     

Organic field-effect transistors based on low-temperature processable transparent polymer dielectrics with low leakage current

A solution-based transparent polymer was investigated as the gate dielectric for organic field-effect transistors (OFETs). Organic thin films (400 nm) are readily fabricated by spin-coating a polyhydrazide solution under ambient conditions on the ITO substrates, followed by annealing at a low temperature (120 °C). The smooth transparent dielectrics exhibited excellent insulating properties with very low leakage current densities of ～10^?8 A/cm². High performance OFETs with evaporated pentacene as organic semiconductor function at a low operate voltage (?15 V). The mobility could reach as high as 0.7 cm²/Vs and on/off current ratio up to 10⁴. Solution-processed TIPS-pentacene OFETs also work well with this polymer dielectric.     

Ultra-thin polymer gate dielectrics for top-gate polymer field-effect transistors

We have demonstrated top-gate polymer field-effect transistors (FETs) with ultra-thin (30–50 nm), room-temperature crosslinkable polymer gate dielectrics based on blending an insulating base polymer such as poly(methyl methacrylate) with an organosilane crosslinking agent, 1,6-bis(trichlorosilyl)hexane. The top-gate polymer transistors with thin gate dielectrics were operated at gate voltages less than ?8 V with a relatively high dielectric breakdown strength (>3 MV/cm) and a low leakage current (10–100 nA/mm² at 2 MV/cm). The yield of thin gate dielectrics in top-gate polymer FETs is correlated with the roughness of underlying semiconducting polymer film. High mobilities of 0.1–0.2 cm²/V s and on and off state current ratios of 10⁴ were achieved with the high performance semiconducting polymer, poly(2,5-bis(3-alkylthiophen-2yl)thieno[3,2-b]thiophene.     

Laser induced impact ionization effect in MOSFET during 1064 nm laser stimulation

Laser stimulation with 1300 nm as thermal (TLS) and with 1064 nm as photoelectric (PLS) laser stimulation techniques are now widely used in failure analysis of Integrated Circuits. The stimulation signatures when using a 1064 nm laser are often a combination of PLS and TLS along with laser induced impact ionization. The results show the existence of laser induce impact ionization current component when high laser power is applied. This work presents a quantitative investigation of 1064 nm laser stimulation effects on single NMOSFET devices. For high laser power applications the impact ionization current becomes the dominant component for 1064 nm laser stimulation.     

Study on thermal performance of high power LED employing aluminum filled epoxy composite as thermal interface material

This paper elucidates the thermal behavior of an LED employing metal filled polymer matrix as thermal interface material (TIM) for an enhanced heat dissipation characteristic. Highly thermal conductive aluminum (Al) particles were incorporated in bisphenol A diglycidylether (DGEBA) epoxy matrix to study the effect of filler to polymer ratio on the thermal performance of high power LEDs. The curing behavior of DGEBA was studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The dispersion nature of the Al fillers in polymer matrix was verified with Field Emission Scanning Electron Microscope (FESEM). The thermal performance of synthesized Al filled polymer composite as TIM was tested with an LED employing thermal transient measurement technique. Comparing the filler to polymer ratio, the rise in junction temperature for 60 wt% Al filled composite was higher by 11.1 °C than 50 wt% Al filled composite at cured state. Observed also from the structure function analysis that the total thermal resistance was 10.96 K/W higher for 60 wt% Al filled composite compared to 50 wt% Al filled composite. On the other hand, a significant rise of 9.5 °C in the junction temperature between cured and uncured samples of 50 wt% Al filled polymer TIM was observed and hence the importance of curing process of metal filled polymer composite for effective heat dissipation is discussed extensively in this work.     

Fluorosilicate and fluorophosphate superfluorescent multicore optical fibers co-doped with Nd3+/Yb3+

In the paper spectroscopic properties of two fluorosilicate and fluorophosphate glass systems co-doped with Nd³⁺/Yb³⁺ ions are investigated. As a result of optical excitation at the wavelength of 808 nm strong and wide emission in the 1 μm region corresponding to the superposition of optical transitions ⁴F_3/2 → ⁴I_11/2 (Nd³⁺) and ²F_5/2 → ²F_7/2 (Yb³⁺) can be observed. The optimization of Nd³⁺ → Yb³⁺ energy transfer in both glasses allows to manufacture multicore optical fibers with narrowing and red-shifting of amplified spontaneous emission (ASE) at 1.1 μm.     

Stable and wavelength-tunable RSOA- and SOA-based fiber ring laser

In this work, we propose and experimentally investigate a wavelength-tunable fiber ring laser architecture by using the reflective semiconductor optical amplifier (RSOA) and semiconductor optical amplifier (SOA). Here, the wavelength tuning range from 1538.03 to 1561.91 nm can be obtained. The measured output power and optical signal to noise ratio (OSNRs) of the proposed fiber laser are between -0.8 and -2.5 dBm and 59.1 and 61.0 dB/0.06 nm, respectively. The power and wavelength stabilities of the proposed laser are also studied. In addition, the proposed laser can be directly modulated at 2.5 Gbit/s quadrature phase shift keying-orthogonal frequency-division multiplexing (QPSK-OFDM) signal and 20–50 km single-mode fiber (SMF) transmissions are achieved within the forward error correction (FEC) limit without dispersion compensation. It could be a cost-effective and promising candidate for the standard-reach and extended-reach wavelength division multiplexed passive optical network (WDM-PON).     

Highly efficient green top-emitting organic light-emitting devices with metal electrode structure

In this work, the electrical and optical characteristics of top-emitting organic light-emitting device (TEOLED) using metal Ag as anode with different thicknesses have been investigated. The emission peak of fabricated TEOLED is 512 nm for a full-width at half-maximum (FWHM) of 48 nm in forward direction. The TEOLED turns on at 3 V with luminance of 2.38 cd/m² and reaches 16,300 cd/m² at 9 V. The maximum of current efficiency is 5.2 cd/A at 7 V, corresponding to the external quantum efficiency of 1.72%.     

Modeling of Ga1−xInxAs1−y−zNySbz/GaAs quantum well properties for near-infrared lasers

The aim of this work is to model the properties of GaInAsNSb/GaAs compressively strained structures. Indeed, Ga_1?xIn_xAs_1?y?zN_ySb_z has been found to be a potentially superior material to GaInAsN for long wavelength laser dedicated to optical fiber communications. Furthermore, this material can be grown on GaAs substrate while having a bandgap smaller than that of GaInNAs. The influence of nitrogen and antimony on the bandgap and the transition energy is explored. Also, the effect of these two elements on the optical gain and threshold current density is investigated. For example, a structure composed of one 7.5 nm thick quantum well of material with In=30%, N=3.5%, Sb=1% composition exhibits a threshold current density of 339.8 A/cm² and an emission wavelength of 1.5365 μm (at T=300 K). It can be shown that increasing the concentration of indium to 35% with a concentration of nitrogen and antimony, of 2.5% and 1%, respectively, results in a decrease of the threshold current density down to 253.7 A/cm² for a two well structure. Same structure incorporating five wells shows a threshold current density as low as 221.4 A/cm² for T=300 K, which agrees well with the reported experimental results.     

Tailoring chromatic dispersion in chalcogenide–tellurite microstructured optical fiber

We report fabrication of a highly nonlinear hybrid microstructured optical fiber composed of chalcogenide glass core and tellurite glass cladding. The flattened chromatic dispersion can be achieved in such an optical fiber with near zero dispersion wavelength at telecommunication wavelengths λ = 1.35–1.7 μm, which cannot be achieved in chalcogenide glass optical fibers due to their high refractive index, i.e. n > 2.1. We demonstrate a hybrid 4-air hole chalcogenide–tellurite optical fiber (Δn = 0.25) with flattened chromatic dispersion around λ = 1.55 μm. In optimized 12-air hole optical fiber composed of the same glasses, the chromatic dispersion values were achieved between −20 and 32 ps/nm/km in a broad wavelength range of 1.5–3.8 μm providing the fiber with extremely high nonlinear coefficient 86,000 km⁻¹W⁻¹. Hybrid chalcogenide/tellurite fibers pumped with the near infrared lasers give good promise for broadband optical amplification, wavelength conversion, and supercontinuum generation in the near- to mid-infrared region.