Investigation and direct observation of sidewall leakage current of InGaN-Based green micro-light-emitting diodes.

Electrical and optical characteristics of InGaN-based green micro-light-emitting diodes (µLEDs) with different active areas are investigated; results are as follows. Reverse and forward leakage currents of µLED increase as emission area is reduced owing to the non-radiative recombination process at the sidewall defects; this is more prominent in smaller µLED because of larger surface-to-volume ratio. Leakage currents of µLEDs deteriorate the carrier injection to light-emitting quantum wells, thereby degrading their external quantum efficiency. Reverse leakage current originate primarily from sidewall edges of the smallest device. Therefore, aggressive suppression of sidewall defects of µLEDs is essential for low-power and downscaled µLEDs.

Light extraction enhancement of AlGaN-based vertical type deep-ultraviolet light-emitting-diodes by using highly reflective ITO/Al electrode and surface roughening.

AlGaN-based vertical type high power ultraviolet-C light emitting diodes (UV-C LEDs), which have a Ga-face n-contact structure, were fabricated on a LED epilayer transferred to a carrier wafer through a laser lift-off (LLO) process. A significant light extraction enhancement of the vertical chip by using a highly reflective ITO/Al p-type electrode is demonstrated, along with surface roughening. A GaN-free LED epi structure is employed to prevent light absorption in the UV-C wavelength region. The vertical chip with the ITO/Al reflector and n-AlGaN surface roughening exhibited a high light output power of 104.4mW with a peak wavelength of 277.6nm at an injection current of 350mA. Comparing the device characteristics of the vertical chip and the flip chip showed that the light output power of the vertical chip was 1.31 times higher than that of the flip chip at 350mA. In particular, with the high power vertical type UV-C LED, a maximum light output power of 630mW could be achieved at a current of 3.5A, and this is mainly attributed to efficient heat dissipation through a metal substrate and the resulting relatively lower junction temperature of the vertical chip.

High efficiency ultraviolet GaN-based vertical light emitting diodes on 6-inch sapphire substrate using ex-situ sputtered AlN nucleation layer.

We demonstrated the growth of crack-free high-quality GaN-based UV vertical LEDs (VLEDs) (λ = 365 nm) on 6-inch sapphire substrates by using an ex-situ sputtered AlN nucleation layer (NL) and compared their performance with that of UV VLEDs with an in situ low temperature (LT) AlGaN NL. The X-ray diffraction (XRD) results showed that the ex-situ AlN sample contained lower densities of screw-type and edge-type threading dislocations than the in situ AlGaN NL sample. The micro-Raman results revealed that the ex-situ AlN sample was under more compressive stress than the in situ AlGaN sample. As the current was increased, the electroluminescence peaks of both of the samples blue-shifted, reached a minimum wavelength at 1000 mA, and then slightly red-shifted. Packaged VLEDs with the ex-situ AlN NL yielded 6.5% higher light output power at 500 mA than that with the in situ AlGaN NL. The maximum EQEs of the VLED with the in situ AlGaN and ex-situ AlN NLs were 43.7% and 48.2%, respectively. Based on the XRD and Raman results, the improved light output power of the ex-situ AlN sample is attributed to the lower density of TDs.

Light output performance of red AlGaInP-based light emitting diodes with different chip geometries and structures.

We investigated the optical and electrical properties of red AlGaInP light-emitting diodes (LEDs) as functions of chip size, p-cladding layer thickness, and the number of multi-quantum wells (MQWs). External quantum efficiency (EQE) decreased with decreasing chip size. The ideality factor gradually increased from 1.47 to 1.95 as the chip size decreased from 350 µm to 15 µm. This indicates that the smaller LEDs experienced larger carrier loss due to Shockley-Read-Hall nonradiative recombination at sidewall defects. S parameter, defined as ∂lnL/∂lnI, increased with decreasing chip size. Simulations and experimental results showed that smaller LEDs with 5 pairs of MQWs had over 30% higher IQE at 5 A/cm2 than the LED with 20 pairs of MQWs. These results show that the optimization of the number of QWs is needed to obtain maximum EQE of micro-LEDs.

Divergent evolution of multiple virus-resistance genes from a progenitor in Capsicum spp.

Plants have evolved hundreds of nucleotide-binding and leucine-rich domain proteins (NLRs) as potential intracellular immune receptors, but the evolutionary mechanism leading to the ability to recognize specific pathogen effectors is elusive. Here, we cloned Pvr4 (a Potyvirus resistance gene in Capsicum annuum) and Tsw (a Tomato spotted wilt virus resistance gene in Capsicum chinense) via a genome-based approach using independent segregating populations. The genes both encode typical NLRs and are located at the same locus on pepper chromosome 10. Despite the fact that these two genes recognize completely different viral effectors, the genomic structures and coding sequences of the two genes are strikingly similar. Phylogenetic studies revealed that these two immune receptors diverged from a progenitor gene of a common ancestor. Our results suggest that sequence variations caused by gene duplication and neofunctionalization may underlie the evolution of the ability to specifically recognize different effectors. These findings thereby provide insight into the divergent evolution of plant immune receptors.

Light-output enhancement of GaN-based vertical light-emitting diodes using periodic and conical nanopillar structures.

We investigated GaN-based vertical light-emitting diodes (VLEDs) with periodic and conical nanopillar arrays (CNAs) to improve the light-output efficiency. We found that a 470 nm diameter and 0.8-0.9 µm height increased the light output, and the devices suffered no significant electrical property degradations. The light-output power was 272% and 5.1% greater than flat- and rough-surface VLEDs at 350 mA, respectively. These improved optical properties are attributed to the optimized CNAs, which increase the effective photon escape cone and reduce the total internal reflection at the n-GaN-air interface. We also investigated the emission characteristics and mechanisms with finite-difference time-domain simulations.

Enhanced luminous efficacy in phosphor-converted white vertical light-emitting diodes using low index layer.

We demonstrated improved luminous efficacy for GaN-based vertical light emitting diodes (VLEDs) employing a low index layer composed of silicon dioxide (SiO(2)) on the top surface. Three-dimensional ðnite-difference time-domain simulations for the fabricated VLED chip show that the penetration ratio of the emitted/reflected light into the VLED chip decreased by approximately 20% compared to a normal VLED chip. This result is in good agreement with an empirical study stating that white VLEDs having a SiO(2) layer exhibit an 8.1% higher luminous efficacy than white VLEDs with no layer at an injection current of 350 mA. Photons penetrating into the VLED chip, which become extinct in the VLED chip, are reflected from the SiO(2) layer due to the index contrast between the SiO(2) layer and epoxy resin containing phosphor, with no degradation of the light-extraction efficiency of the VLED chip. As such, this structure can contribute to the enhancement of the luminous efficacy of VLEDs.

Microstructures and electrochemical properties of Si-Ni-xTi alloys for anode materials.

The phase change due to varying content of titanium in Si-Ni-xTi alloys and its effect on the electrochemical behavior has been investigated. Specimens were prepared by melt-spinning to reduce the microstructure scale. Results showed that silicon particles of 50-100 nm diameter and dendrites of somewhat larger scale were formed in the Si-Ni-Ti alloys ribbons. The microstructure of Si70Ni15Ti15 alloy ribbons was composed of silicon particles finely dispersed in Si7Ni4Ti4 phase. The cycle performance was improved by the formation of TiSi2 or NiSi2 phase at the presence of Si7Ni4Ti4 phase, either of which combined with Si7Ni4Ti4 phase effectively accommodated the volume change of silicon particles during cycling. The reduced scale of silicon particles contributed to the enhanced cycle efficiency as well.

Microstructures and electrochemical properties of Si-xTiNi alloys for lithium secondary batteries.

The rapidly solidified Si-xTiNi (x = 0.2-0.45) alloy ribbons were fabricated via melt spinning process. The thickness of the melt-spun ribbons was about 12.5 microm, and the sound section was selected for the experiment. The microstructures of the ribbons were analyzed using XRD, FE-SEM, and HR-TEM: The primary silicon particles of 30 nm-100 n min diameter were finely dispersed in the inactive buffering matrix of Si7Ni4Ti4 phase. The charge/discharge energy capacity and electrochemical properties were significantly influenced by the relative ratio of NiTi to silicon. With increasing the total amount of Ni and Ti content up to 45 at%, the amount of Si7Ni4Ti4 phase increased and the cycle performance was improved. The Si7Ni4Ti4 phase acted as a buffer for the volume expansion/contraction of Si occurring during the alloying and dealloying, and it could prevent a significant deterioration in cycle performance of the battery.

The effect of Ni on the microstructures and electrochemical properties of Si-Ti base alloys for lithium secondary batteries.

This paper presents the microstructures and electrochemical properties of Si-Ti-Ni alloys of various compositions prepared by a rapid solidification process. Si-15Ti-(0-25 at%)Ni alloy ingots prepared by arc-melting was melt-spun to produce thin strip of -15 Om thickness. The Si-Ni-Ti alloy electrode were fabricated by mixing the active powdered materials (88 wt%) with ketjen black (4 wt%) as a conductive material and polyamide-imide binder (PAI, 8 wt.%) dissolved in N-methyl-2-pyrrolidinone (NMP). Results showed that the microstructures of melt-spun Si-Ti-Ni ribbons consist of silicon, TiSi2, Si7Ni4Ti4, and NiSi2 phases depending on the composition. As the content of nickel increased in silicon matrix, TiSi2 phase disappeared while Si7Ni4Ti4 and NiSi2 phases are generated. The cycle efficiency of Si65Ti15Ni20 and Si60Ti15Ni25 alloys was significantly improved because of the increased volume fraction of Si7Ni4Ti4 and NiSi2 phases and fine particulated silicon phase.

Influence of HCl Concentration on Corrosion Behavior between Au or Cu Bonding Wires and the Bond Pad for Semiconductor Packaging.

Wire bonding, one of the methods for electrically connecting a semiconductor chip with a substrate, involves attaching thin metal wires to pads. It is the oldest electrical connection method that exhibits high compatibility with other processes. The metal wires used for electrical connection in wire bonding are mainly made of Au, Cu, and Ag. After the wire bonding, molding is performed using the epoxy molding compound (EMC). However, EMC inevitably contains ions such as halogen elements. In addition, it absorbs moisture due to its hydrophilicity, creating a corrosive environment with electrolytes. In this study, we evaluated the influence of hydrochloric acid concentration on corrosion behavior between Au or Cu bonding wires and sputtered Al bond pads. The electrochemical factors such as corrosion potential difference (ΔE), galvanic corrosion current density (ig), and anodic and cathodic Tafel slopes were found to influence galvanic corrosion behavior. Galvanic corrosion tendency in first bond and second bond areas of PCB unit specimen was confirmed.

Carbazole/triarylamine based polymers as a hole injection/transport layer in organic light emitting devices.

This study examined the influence of the charge injection barriers on the performance of organic light emitting diodes (OLEDs) using polymers with a stepwise tuned ionization potential (I(p) approximately -5.01 - -5.29 eV) between the indium tin oxide (ITO) (phi approximately -4.8 eV) anode and tris(8-hydroxyquinolinato) aluminium (Alq3) (I(p) approximately -5.7 eV) layer. The energy levels of the polymers were tuned by structural modification. Double layer devices were fabricated with a configuration of ITO/polymer/Alq3/LiF/Al, where the polymers, Alq3, and LiF/Al were used as the hole injection/transport layer, emissive electron transport layer, and electron injection/cathode, respectively. Using the current density-voltage (J-V), luminescence-voltage (L-V) and efficiencies in these double layer devices, the device performance was evaluated in terms of the energy level alignments at the interfaces, such as the hole injection barriers (phi(h)(iTO/polymer) and phi(h)(polymer/Alq3)) from ITO through the polymers into the Alq3 layer, and the electron injection barrier (phi(e)(polymer/Alq3) or electron/exciton blocking barrier) at the polymer/Alq3 interface.

Fabrication of 380 nm ultra violet light emitting diodes on nano-patterned n-type GaN substrate.

380 nm ultraviolet (UV) light emitting diodes (LEDs) were grown on patterned n-type GaN substrate (PNS) with silicon dioxide (SiO2) nano pattern to improve the light output efficiency. Wet etched self assembled indium tin oxide (ITO) nano clusters serves as dry etching mask for converting the SiO2 layer grown on n-GaN template into SiO2 nano patterns by inductively coupled plasma etching. Three different diameter of ITO such as 200, 250 and 300 nm were used for SiO2 nano pattern fabrication. PNS is obtained by n-GaN regrowth on SiO2 nano patterns and UV LEDs were grown on PNS template by MOCVD. Enhanced light output intensity was observed by employing SiO2 nano patterns on n-GaN. Among different PNS UV LEDs, LED grown on PNS with 300 nm ITO diameter showed enhancement in light output intensity by 2.1 times compared to the reference LED without PNS.

Realization of Broad-Spectrum Using Chirped Multi-Quantum Well Structures in AlGaInP-Based Light-Emitting Diodes.

We studied broad-spectrum light emitting diodes appropriate for special lighting applications in terms of their optical behaviors and device performances according to the chirped multi-quantum well structures. As the well thickness from 1 st to 3rd well was changed from 6 nm to 15 nm and repeated three times, the electroluminescent spectrum was broadened by 65% and the light output power was increased by 8% in comparison to light emitting diodes having conventional multi-quantum well structures. In the case of the chirped multi-quantum well structures having sequentially decreasing the well thickness from 15 nm to 6 nm and repeating three times, the optical output power was decreased by 5% due to the carrier leakage out of the active region.

A discrete core-shell-like micro-light-emitting diode array grown on sapphire nano-membranes.

A discrete core-shell-like micro-light-emitting diode (micro-LED) array was grown on a 100 nm-thick sapphire nano-membrane array without harmful plasma etching for chip singulation. Due to proper design for the sapphire nano-membrane array, an array of multi-faceted micro-LEDs with size of 4 µm × 16 µm was grown. Threading dislocation density in the micro-LED formed on sapphire nano-membrane was reduced by 59.6% due to the sapphire nano-membranes, which serve as compliant substrates, compared to GaN formed on a planar substrate. Enhancements in internal quantum efficiency by 44% and 3.3 times higher photoluminescence intensity were also observed from it. Cathodoluminescence emission at 435 nm was measured from c-plane multiple quantum wells (MQWs), whereas negligible emissions were detected from semi-polar sidewall facets. A core-shell-like MQWs were formed on all facets, hopefully lowering concentration of non-radiative surface recombination centers and reducing leakage current paths. This study provides an attractive platform for micro-LEDs by using sapphire nano-membrane.

Synthesis of carbazole/triarylamine containing polyfluorene and its application to electroluminescent devices.

Solution-processable carbazole/triarylamine-based polyfluorene (CT-PF) was designed and synthesized by Suzuki coupling reaction. CT-PF was originally designed to have multiple functions at the same polymer backbone; blue emission and large band gap energy by fluorine group as well as good hole transporting property and tunable ionization potential by excellent electron donating triarylamine and carbazole moieties. It was confirmed from TGA measurement that the synthesized CT-PF had thermal stability up to 379 degrees C. The UV-Visible absorption spectra of CT-PF had absorption maxima at 292 nm and 351 nm in dilute solution, and 332 nm and 400 nm in film state, which might be originated from fluorene and arylamine moieties. PL emission maxima were also measured in the blue range at 425 nm in dilute solution and 468 nm in film state. Turn-on voltage of double-layered OLED with CT-PF as HTL was reduced to 7 V, and the luminescence were enhanced up to 1,397 cd/m2 with maximum current efficiency of 0.32 lm/W and power efficiency of 1.1 cd/A, which were over 3 times higher than those of single-layered device without HTL. PhOLED with CT-PF as a host material for Ir(ppy)3 phosphorescent dopant showed bright green emission with CIE color coordinates as (x = 0.31, y = 0.57).

Stable and efficient transfer-printing including repair using a GaN-based microscale light-emitting diode array for deformable displays.

GaN-based microscale light-emitting diodes (µLEDs) are reported for assembly into deformable displays and repair systems. A stamp-imprinting method that enables large area assembly without spatial limitation is involved in the system, and a selective pick-up method is presented that includes a method for removing detected defective chips through micro-pulsed laser scanning. The photosensitive functional material, which is an accepted layer for the stable imprinting of chips, is determined by controlling the adhesion. In addition, selective pick-up and adhesion-controlled functional materials allow the implementation of defect-free displays through two pick-and-place cycles. Displays and related systems fabricated with this method can offer interesting optical and electrical properties.

Synthesis of carbazole-based polymers with 1,3,4-oxadiazole pendant group and their application in organic light-emitting diodes.

Structurally well-defined copolymers with high solubility were prepared via palladium-catalyzed polycondensation of N-(2-ethylhexyl)-3,6-dibromocarbazole with 2-aryl-5-(4-aminophenyl)-1,3,4-oxadiazole (aryl = phenyl, p-methylphenyl, p-methoxyphenyl). Copolymers consisted of alkylcarbazole groups in the main chain and 1,3,4-oxadiazole pendants in the side chain. The influence of their aryl substituents on physical, optical, band gaps and electroluminescent characteristics of the copolymers was investigated. Both UV-Visible absorption and photoluminescence emission peaks of the copolymers were similar to each other. The band gap energy of the polymers was measured in the range of 2.84 to approximately 2.88 eV, and HOMO energy in the range of -5.13 to approximately -5.18 eV. These copolymers were used as hole-transporting layers (HTL) in the light-emitting diodes with Alq3 as an emitting layer. Compared to devices with P-H and P-OCH3 used as HTL, it should be noted that device with P-CH3 used as HTL showed higher luminescence. Maximum luminescence of devices was measured to be 276 cd/m2 at 14 V with P-H, 625 cd/m2 at 15 V with P-CH3, and 471 cd/m2 at 14 V with P-OCH3. This might be due to effect of subtle changes in HOMO energy level of polymers with changing substituent groups. Phosphorescent polymer light emitting diodes were also fabricated with an emitting layer consisting of P-CH3 matrix and a red phosphorescent dopant (lr-PIQCH).

Air-Hybrid Distributed Bragg Reflector Structure for Improving the Light Output Power in AlGalnP-Based LEDs.

We investigated air gap-induced hybrid distributed Bragg reflectors (AH-DBRs) for use in high brightness and reliable AlGalnP-based light emitting diodes (LEDs). An air gap was inserted into the side of DBRs by selectively etching the Al(x),Ga1-xAs DBR structures. With the AH-DBR structures, the optical output power of LEDs was enhanced by 15% compared to LEDs having conventional DBRs, due to the effective reflection of obliquely incident light by the air gap structures. In addition, the electrical characteristics showed that the AH-DBR LED is a desirable structure for reducing the leakage current, as it suppresses unwanted surface recombinations.

Three-Dimensional Porous Copper-Graphene Heterostructures with Durability and High Heat Dissipation Performance.

Porous materials have historically been of interest for a wide range of applications in thermal management, for example, in heat exchangers and thermal barriers. Rapid progress in electronic and optoelectronic technology necessitates more efficient spreading and dissipation of the heat generated in these devices, calling for the development of new thermal management materials. Here, we report an effective technique for the synthesis of porous Cu-graphene heterostructures with pores of about 30 µm and a porosity of 35%. Graphene layers were grown on the surfaces of porous Cu, which was formed via the coalescence of molten Cu microparticles. The surface passivation with graphene layers resulted in a thermal conductivity higher than that of porous Cu, especially at high temperatures (approximately 40% at 1173 K). The improved heat dissipation properties of the porous structures were demonstrated by analysis of the thermal resistance and temperature distribution of LED chips mounted on the structures. The effective combination of the structural and material properties of porous Cu-graphene heterostructures provides a new material for effective thermal management of high-power electronic and optoelectronic devices.