mA Injection Current Research Articles

Optical Simulation and Fabrication of Nitride-Based LEDs With the Inverted Pyramid Sidewalls

In this study, the numerical and experimental demonstrations for the enhancement of light-extraction efficiency in nitride-based LEDs with randomly inverted pyramid sidewalls (IPSs) by chemical etching of the chip edge are presented. With 20 mA injection current, it was found that forward voltages were 3.69 and 3.75 V while output powers were 7.07 and 8.95 mW for the conventional LED and inverted pyramid sidewall LED, respectively. The larger LED output power is attributed to the increased light-extraction efficiency by IPSs.

GaN-Based LEDs Output Power Improved by Textured GaN/Sapphire Interface Using <emphasis emphasistype="italic">In Situ</emphasis> <formula formulatype="inline"><tex Notation="TeX">$\hbox{SiH}_{\bf 4}$</tex> </formula> Treatment Process During Epitaxial Growth

In this study, we demonstrate an in situ roughening technique at the GaN/sapphire interface in GaN-based LEDs using a silane treatment (SiH4 treatment) process that forms a thin SiNx layer with nanometer-sized holes on the sapphire surface that behave like a patterned sapphire substrate. A plurality of voids at the GaN/sapphire interface is observed according to the transmission electron microscopy analysis. With a 20 mA current injection, the results indicate that the typical output power of LEDs grown with and without the SiH4 treatment process are approximately 18.0 and 15.6 mW, respectively. In other words, the output power can be enhanced by 15% with the use of the SiH4 treatment process. The enhancement of output power is mainly due to light scattering at the naturally textured GaN/sapphire interface, which can lead to a higher escape probability for the photons emitted from the active layer in an LED.

A modelling study to inform specification and optimal electrode placement for imaging of neuronal depolarization during visual evoked responses by electrical and magnetic detection impedance tomography

Electrical impedance tomography (EIT) has the potential to achieve non-invasive functional imaging of fast neuronal activity in the human brain due to opening of ion channels during neuronal depolarization. Local changes of resistance in the cerebral cortex are about 1%, but the size and location of changes recorded on the scalp are unknown. The purpose of this work was to develop an anatomically realistic finite element model of the adult human head and use it to predict the amplitude and topography of changes on the scalp, and so inform specification for an in vivo measuring system. A detailed anatomically realistic finite element (FE) model of the head was produced from high resolution MRI. Simulations were performed for impedance changes in the visual cortex during evoked activity with recording of scalp potentials by electrodes or magnetic flux density by magnetoencephalography (MEG) in response to current injected with electrodes. The predicted changes were validated by recordings in saline filled tanks and with boundary voltages measured on the human scalp. Peak changes were 1.03 ± 0.75 µV (0.0039 ± 0.0034%) and 27 ± 13 fT (0.2 ± 0.5%) respectively, which yielded an estimated peak signal-to-noise ratio of about 4 for in vivo averaging over 10 min and 1 mA current injection. The largest scalp changes were over the occipital cortex. This modelling suggests, for the first time, that reproducible changes could be recorded on the scalp in vivo in single channels, although a higher SNR would be desirable for accurate image production. The findings suggest that an in vivo study is warranted in order to determine signal size but methods to improve SNR, such as prolonged averaging or other signal processing may be needed for accurate image production.

Power Enhancement of GaN-Based Flip-Chip Light-Emitting Diodes with Triple Roughened Surfaces

The flip-chip light emitting diodes (FC-LEDs) with triple roughened surfaces were fabricated comprising top surface sapphire textured layer, interface patterned sapphire layer, and bottom naturally textured p-GaN layer. Light extraction efficiency was enhanced by such triple textured layers. The light output power of FC-LEDs was increased 60% (at 350 mA current injection) compared to that of conventional FC-LEDs by implementing the triple roughened surfaces. The enhancement efficiency can be simulated and the simulated results showed the same trend as the results of experiment.

Enhanced light extraction efficiency of GaN-based light-emitting diodes with ZnO nanorod arrays grown using aqueous solution

We report a dramatic increase in the light extraction efficiency of GaN-based blue light-emitting diodes (LEDs) by ZnO nanorod arrays on a planar indium tin oxide (ITO) transparent electrode. ZnO nanorods were grown into aqueous solution at the low temperature of 90 °C. With 20 mA current injection, the light output efficiency of the LED with ZnO nanorod arrays on ITO was increased by about 57% with no increase in a forward voltage over the conventional LEDs with planar ITO. The increased light extraction by the ZnO nanorod arrays is due to the formation of sidewalls and a rough surface, resulting in a multiple photon scattering at the LED surface.

Influence of pillar-and hole-patterned sapphire substrates on MOVPE grown GaN bulk and LED structures

Bulk GaN material and LED structures on pillar-patterned sapphire substrates （PSS-p） and hole-patterned sapphire substrates （PSS-h） were grown by MOCVD and the characteristic was compared in detail. X-ray diffraction and atomic force microscope measurements show a better crystal quality and surface morphology of GaN on PSS-h than that of GaN on PSS-p, which is due to the lateral growth of GaN on PSS-h observed from cross-sectional scanning electron microscopy. Furthermore, the output power of LED on PSS-p and PSS-h with 20 mA injection current are 46% and 33% higher than LED on conventional sapphire substrate, respectively. The temperature-dependent photoluminesence measurements indicate that the internal quantum efficiencies of all samples are quite close. Therefore, the airgaps between GaN and PSS-h act against the improvement of light extraction efficiency.

Characterization and Fabrication of InGaN-based Blue LED with Underlying AlGaN/GaN SLS Cladding Layer Grown on Si(111) Substrate

AbstractThis paper reports improved optical characteristics of InGaN-based light-emitting-diode (LED) grown on Si(111) substrate by the insertion of an Al0.06Ga0.94N/GaN strained-layer-superlattices (SLS) cladding layer after AlN/GaN multilayer (ML) growth, under the multi-quantum-well (MQW) active layer. The insertion of underlying Al0.06Ga0.94N/GaN SLS cladding layer has shown to improve epitaxial layer quality in x-ray diffraction (XRD) analysis, reduce wavelength peak fluctuations in photoluminescence (PL) surface mapping, and improve optical and electrical characteristics of the LED sample. A 34% increase of light intensity at 50 mA current injection and a narrower wavelength peak have been achieved by the insertion of Al0.06Ga0.94N/GaN SLS cladding layer. LED with underlying Al0.06Ga0.94N/GaN also shows superior current-voltage (I-V) characteristics with operation voltage of 3.2 V at 20 mA and series resistance of 16 Ω.

InGaN/AlGaN Ultraviolet Light-Emitting Diode with a Ti3O5/Al2O3 Distributed Bragg Reflector

Here, we report InGaN/AlGaN ultraviolet (UV) light-emitting diodes (LEDs) using distributed Bragg reflectors (DBRs). The DBRs consist of 11 layers of alternating quarter-wave thickness Ti3O5 and Al2O3 deposited onto the indium–tin-oxide ohmic contact layer of a 385 nm UV LED by ion-assisted electron beam evaporation. Numerical calculations showed that the angle averaged reflectivity of the GaN/DBR structure at 385 nm was 12% higher than that of a conventional GaN/Ag structure. The measured light output–current–voltage of the DBR UV LEDs at 20 mA current injection showed an increase of 15% in output power in comparison to a conventional Ag reflector with no degradation in electrical properties. We attributed this light output enhancement to the increased light extraction from the higher DBR reflectivity.

Light Output Power Enhancement of GaN-based Vertical Light Emitting Diodes via Fabricating Surface Nanorod Arrays

In this research, we demonstrated two approaches to fabricate nanorod arrays on the surface of GaN-based vertical light emitting diodes (VLEDs). The first approach was via synthesizing single-crystal ZnO nanorod arrays in aqueous solution at room temperature. The light output intensity and wall-plug efficiency of the GaN-based VLEDs with the ZnO nanorod arrays shows 38.9 and 41.2% increases, respectively, at 200 mA current injections compared to that of a VLED without ZnO nanorod arrays. The second approach was formed by spinning nano silica spheres and inductively coupled plasma-reactive ion etching. The light output power of the VLEDs with the dry etched nanorod arrays showed about 40% enhancement than that without the nanorod arrays. The large enhancement of VLED with both ZnO or GaN nanorod arrays was mainly due to the total internal reflection destruction, and the gradually changed refractive index from the nanorod arrays layer.

Enhancement in output power of blue gallium nitride-based light-emitting diodes with omnidirectional metal reflector under electrode pads

In this study, we demonstrate a GaN-based light-emitting diode (LED) with nonalloyed metal contacts onto the n+-GaN surface and transparent contact layer (indium tin oxide) to serve as the n-type electrode (cathode) and the p-type electrode pad (anode), respectively. Comparing with the conventional LEDs, which the electrode pads and/or Ohmic contacts form through conventional Cr∕Au metal contacts, the nonalloyed metal contacts (Ag∕Cr∕Au or Al∕Cr∕Au) used in the present experimental blue LEDs also play the role of reflector to prevent the emitted light from absorption by the opaque electrode pads with low reflectivity (Cr∕Au). With an injection current of 20mA, the enhancement in the light output power has approximately a 14% magnitude compared to the GaN-based LEDs without Ag or Al reflectors under the Cr∕Au electrode pads.

Effect of p–n Junction Location on Characteristics of InGaN/GaN Multiple-Quantum-Well Light-Emitting Diodes

InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with different p–n junction locations have been investigated by current–voltage (I–V), electroluminescence (EL), and capacitance–voltage (C–V) measurements. At 20 mA injection current, the operating voltage of the LEDs decreases by 0.15 V, the EL intensity increases by 14% and the EL wavelength shifts from 465 to 455 nm owing to the shortening of the distance between n-InGaN/GaN MQW and p-type materials. C–V measurements indicate that only the quantum well immediately adjacent to the p-type region still remains inside the depletion region when LEDs are forward biased. Analysis reveals that the characteristics of the LEDs can be improved by optimizing the p–n junction location through the improvement of the holes' tunneling injection and weakening of the piezoelectric field in InGaN quantum wells.

GaN-Based Light-Emitting Diodes Grown on Photonic Crystal-Patterned Sapphire Substrates by Nanosphere Lithography

GaN-based light-emitting diodes (LEDs) grown on photonic crystal-patterned sapphire substrates (PCPSS) have been demonstrated. PCPSS was fabricated by nanosphere lithography, and the photonic crystal structure was the hexagonal-lattice pattern. The forward voltages of PCPSS and patterned sapphire substrates (PSS) LEDs were smaller than that of conventional sapphire substrates (CSS) LED, and it infers the epitaxial film quality of PCPSS and PSS LEDs has been slightly improved. The luminance intensity of PCPSS LED was 1.63 and 1.51 times higher than those of CSS and PSS LED at 20 mA injection current. The enhancement in the luminance intensity of PCPSS LED is attributed to the photonic crystal structure.

Effect of InGaN underneath layer on MOVPE-grown InGaN/GaN blue LEDs

This paper describes a study on the effect of InGaN underneath layer (UL) on InGaN/GaN LEDs operating in the spectral range from 425 to 460 nm. Samples were grown by metalorganic vapor phase epitaxy (MOVPE). The In content of the UL is studied in the range of 0–2.7%. Multiple quantum well (MQW) surface morphology and crystal structure were characterized by atomic force microscopy (AFM), high resolution X-ray diffraction (XRD) and secondary ion mass spectroscopy (SIMS). Electroluminescence (EL) intensity was found to increase almost 50% at 20 mA injection current by introducing an optimized UL under the MQW stack in the LED structure. Also a decrease in the full width of half maximum (FWHM) and the blue-shift of the EL spectrum compared to the reference sample were observed in the all UL consisting samples.

High light output intensity of titanium dioxide textured light-emitting diodes

Higher light output intensity and wider polar radiation pattern of InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) with a different nanoscale titanium dioxide (TiO2) textured densities film have been observed. The light output power values and external quantum efficiency of the conventional LEDs at an injection current of 20mA are 6.34mW and 11.7%, respectively. The light output power values and external quantum efficiency of the nanoscaled TiO2 textured LEDs at an injection current of 20mA are 7.55mW and 14%, respectively. The light output intensity and power values of the nanoscaled TiO2 textured LEDs is approximately 65% and 20% higher than that of the conventional LEDs, respectively.

Reliable nitride‐based near‐ultraviolet light‐emitting diodes with meshed p‐GaN

AbstractThis investigation presents nitride‐based near ultraviolet (n‐UV) light emitting diodes (LEDs) with a meshed p‐GaN layer. With 20 mA injection current, it was found that forward voltages were 3.29, 3.31 and 3.39 V while output powers were 7.5, 9.0 and 10.6 mW for the planar indium‐tinoxide (ITO) LED, mesh ITO LED and meshed p‐GaN LED, respectively. The larger LED output power is attributed to increased light extraction efficiency. It was also found that we could use such a meshed p‐GaN layer to achieve reliable nitride‐based n‐UV LEDs. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhancement of (AlGaIn)N near‐UV LED efficiency using freestanding GaN substrate

AbstractThe effect of freestanding GaN‐substrates with low defect density (DD) on the electroluminescence (EL) characteristics of near UV‐LEDs has been investigated. Three series of LED‐structures with wavelengths between 377 nm and 435 nm are compared, one grown on freestanding HVPE GaN‐substrates, one grown on low dislocation GaN templates on sapphire (ULD), and one grown directly on sapphire substrates with conventional low temperature nucleation. The reduction in DD results in an enhancement of the EL intensity over the whole wavelength range, which is particularly pronounced for wavelengths shorter than 390 nm. Comparing the output power of LEDs on these three different substrates special attention has to be paid to differences in light extraction efficiency. Though band‐to‐band absorption limits the use of freestanding GaN to wavelengths exceeding 380 nm, it offers the possibility to enhance the light extraction via simple backside surface texturing by a factor of at least 2. This way we fabricated a LED‐structure on freestanding GaN with an output power measured on‐wafer of 13 mW at 395 nm and 40 mA injection current, i.e. an enhancement in EL intensity of 270% over the same LED structure grown on sapphire. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhanced light extraction of InGaN-based green LEDs by nano-imprinted 2D photonic crystal pattern

In this paper, we propose a simple, low cost and mass producible nanoimprint lithography (NIL) method to texture the surface of GaN-based light emitting diodes (LEDs) with a two-dimensional photonic crystal (2DPC). Such a 2DPC structure not only enhanced the light output power but also changed the far-field pattern simultaneously. Also, a TiO2/SiO2 omnidirectional reflector (ODR) was deposited on the backside of the LEDs to further increase the light output power. Under 350 mA current injection, it was found that forward voltages were 3.35, 3.34 and 3.75 V while the light output powers of the LEDs were 59.5, 92.5 and 112.1 mW for the conventional LED, the PCLED with 20 nm depth, and the PCLED with 120 nm depth all with chip size of 1 mm × 1 mm, respectively. A 88.4% enhancement in light output power of PCLED with a 120 nm depth and ODR on the backside could be achieved when compared to the conventional LED under the driving current of 350 mA. From the measurement results, it was also found that the NIL process does not degrade the electrical properties of the fabricated LEDs.

Improvement of the performance of GaN-based LEDs grown on sapphire substrates patterned by wet and ICP etching

Sapphire substrates patterned by a selective chemical wet and an inductively coupled plasma (ICP) etching technique was proposed to improve the performance of GaN-based light-emitting diodes (LEDs). GaN-based LEDs were fabricated on sapphire substrates through metal organic chemical vapor deposition (MOCVD). The LEDs fabricated on the patterned substrates exhibit improved device performance compared with the conventional LED fabricated on planar substrates when growth and device fabricating conditions were the same. The light output powers of the LEDs fabricated on wet-patterned and ICP-patterned substrates were about 37% and 17% higher than that of LEDs on planar substrates at an injection current of 20mA, respectively. The enhancement is attributable to the combination of the improvement of GaN-based epilayers quality and the improvement of the light extraction efficiency.

Enhancement of LED light extraction via diffraction of hexagonal lattice fabricated in ITO layer with holographic lithography and wet etching

A novel method for enhancing light extraction efficiency of LEDs via diffraction of the lattice fabricated in ITO layers of LEDs is proposed. The lattice fabrication process includes holographic lithography and wet etching. 3-beam interference holographic approach was used to fabricate large-area hexagonal lattice mask which can cover 2-inch semiconductor wafer, and acid etching was used to transfer the lattice structure into p-contact ITO layer. 1.4 fold enhancement of light output at 20 mA injection current was obtained from GaN-based LEDs in the primary experiment. The lattice fabrication process is rapid and cost-effective thus enabling industrial mass production of high brightness LEDs.

Phosphor-conversion white light using InGaN ultraviolet laser diode

We fabricated a phosphor-conversion white light using an InGaN laser diode that emits 405nm near-ultraviolet (n-UV) light and phosphors that emit in the blue and yellow regions when excited by the n-UV and blue light, respectively.The relationship of the luminous flux and the luminous efficacy of the white light with injection current was discussed. The luminous flux increased linearly with increasing current above the threshold of the laser diode, and at 80mA injection current, the luminous flux and luminous efficacy were estimated to be 5.7lm and 13lm∕w, respectively. The shift of the Commission International de I’Eclairage coordinates, color temperature, and color rendering index with current are very slight and negligible, which indicates that the blue and the yellow phosphors have an excellent stability and a highly stable white light can be obtained by this way.