GaN Light-emitting Diodes Research Articles

An Electroless-Ag Reflector Developed for High-Brightness White LEDs

This study was conducted to investigate the reflectivity and the reflectivity stability of the electroless (Ag) metallization of reflectors used in high-brightness white GaN light-emitting diode packages. Two main reflector metallization schemes were studied: (1) electroless-Ag/electroless-pure-Pd/electroless-Ni plating and (2) electroless-Ag/electroless-Pd(P)/electroless-Ni plating. The reflectivity achieved using all reflector-metallization schemes was >85% in the visible range. However, in the electroless-Ag/electroless-pure-Pd/electroless-Ni reflector, reflectivity exhibited substantial thermal degradation; this was because of two principal factors: (1) the change in the surface morphology of the electroless-Ag surface grains; and (2) the alloying effect on the Ag layer exerted by the interdiffusion occurring with the underlying Pd layer. In this study, P was added to the Pd layer, and the thermal degradation of the annealed electroless-Ag/electroless-Pd(P)/electroless-Ni reflector was measured to be less than that of the electroless-Ag/electroless-pure-Pd/electroless-Ni reflector. The P content retarded the interdiffusion between the Ag and Pd(P) layers and preserved the faceted surface of the electroless-Ag layer, which enhanced the stability of the reflectivity of the electroless-Ag reflector. Furthermore, increasing the thickness of the electroless-Ag layer reduced the amount of Pd diffusing through the Ag layer, which helped retain the reflectivity of the Ag surface.

Transient voltage suppressor diode designed for the protection of high-brightness GaN-based LEDs from various electrostatic discharge shocks

Transient voltage suppressor (TVS) diodes were fabricated using low-temperature epitaxy technology and were employed to improve the electrostatic discharge (ESD) strength of GaN light emitting diodes (LEDs). The ESD performance and the protection capability of the TVS diodes were investigated using various ESD simulators of the human body model (HBM), the IEC (International Electrotechnical Commission) 61000-4-2 (IEC), and a transmission line pulse (TLP) analysis. According to the IEC, the manufactured TVS diode could withstand in excess of ±30 kV without any degradation in the I-V characteristics; meanwhile, the GaN LED itself exhibited catastrophic degradation caused by weak ESD power. The GaN LED assembled with the TVS diode had improved ESD robustness from ±3.8 kV to ±8 kV according to the HBM, from ±1.2 kV to > ±30 kV according to the IEC, and from 4.3 A to > ±30 A according to the TLP analysis. Furthermore, its performance was maintained perfect I–V manner with negligible changes in radiant power, leakage current and breakdown voltage up to the limit of the ESD simulators. Namely, the manufactured TVS diodes were effective in the protection of sensitive GaN LEDs from very strong ESD shocks.

Fabrication of wide-bandgap transparent electrodes by using conductive filaments: performance breakthrough in vertical-type GaN LED.

For realizing next-generation solid-state lighting devices, performance breakthroughs must be accomplished for nitride-based light-emitting diodes (LEDs). Highly transparent conductive electrodes (TCEs) may be key to achieving this goal, as they provide uniform current injection and distribution across a large device area, eventually increasing the light output power. However, the trade-off between electrical conductivity and optical transmittance of LEDs must be addressed. Herein, we introduce a novel strategy based on TCEs fabricated using wide-bandgap (WB) materials such as SiNx, incorporated beneath the n-type electrode of vertical-type LEDs, and show the feasibility of this strategy. We employ a novel electrical breakdown (EBD) technique to form conductive filaments (or current paths) between a TCE and n-GaN (GaN: gallium nitride). By employing the EBD process, we obtain both ohmic behavior for SiNx TCE/n-GaN and a current spreading effect across n-GaN. These results demonstrate the tremendous potential of WB-TCEs for use in high-performance optoelectronic devices.

GaN light-emitting diodes with an Al-coated graphene layer as a transparent electrode

We fabricated GaN light-emitting diodes with a layer of graphene as a transparent electrode. A 3-nm-thick Al layer was deposited on the graphene layer by electron-beam evaporation. This Al layer plays an important role in protecting the graphene layer during the device fabrication process. Moreover, this Al layer can also enhance the light emission of GaN light-emitting diodes through the investigation of electroluminescence spectra. The significantly improved light emission is attributed to the current expansion, the enhanced plasmonic density of states, and the decreased non-radiative recombination rate.

Study of GaN LED ITO Nano-Gratings With Standing Wave Analysis

This study reveals the effect of nanoscale ITO transmission gratings on light emission from the top, sides, and bottom of a GaN light-emitting diode (LED), based on the substrate standing wave analysis. First, we show that sapphire substrate thickness affects the standing wave pattern in the LED and find the best- and worst-case sapphire thicknesses. Second, we find that adding nanoscale ITO transmission gratings can improve light extraction by 222% or 253%, depending on the reference chosen. Third, we observe that maximizing top light emission with the nano-grating can significantly reduce bottom and side light emissions. Finally, we study grating performance over different wavelengths and generate the LED spectrum.

Al2O3 Passivation Layer for InGaN/GaN LED Deposited by Ultrasonic Spray Pyrolysis

This letter utilizes ultrasonic spray pyrolysis (USP) to deposit Al2O3 as the passivation layer of GaN light-emitting diodes (LEDs). The composition of Al2O3 is analyzed by X-ray photoelectron spectroscopy. The refractive index and transmittance of USP-grown Al2O3 are investigated. The thickness of Al2O3 is determined to be 70 nm, which is close to the optimum from antireflection theorem calculation for the wavelength of 455 nm. The coverage quality and thickness are confirmed by transmission electron microscopy. The light output power (LOP) of USP-grown-Al2O3-passivated InGaN/GaN LED is improved from 321 mW to 347 mW compared with an LED without passivation. The forward voltage ( \(V_{\mathrm {\mathbf {F}}}\) ) and series resistance ( \(R_{\mathrm {\mathbf {s}}}\) ) are not subject to significant variations but the shunt resistance ( \(R_{\mathrm {\mathbf {SH}}}\) ) is increased after Al2O3 passivation. LED chips with Al2O3 and SiO2 passivation are packaged as LED lamps. Devices passivated by USP-grown Al2O3 show slightly better LOP performance than those passivated by plasma-enhanced chemical vapor deposition-grown SiO2.

Polarization ratio enhancement of a-plane GaN light emitting diodes by asymmetric two-dimensional photonic crystals

Fabricating photonic crystals (PhCs) on GaN based non-polar light emitting diodes (LEDs) is an effective way to increase light extraction and meanwhile to preserve or improve polarization ratio. In this work, a-plane GaN LEDs with two-dimensional PhCs were demonstrated. With the E // m polarized modes (which mean the optical polarization with the electric field parallel to m-axis) as the target of diffraction, we matched E//m modes to the photonic bands and aligned E//c modes to fall within the photonic band gap. The results show stronger E//m but weaker E//c mode diffractions on both c- and m-axes. At the vertical direction, the polarization ratio is enhanced from 45.8% for the planar device to 52.3% for the LEDs with PhCs.

Comparative study of field-dependent carrier dynamics and emission kinetics of InGaN/GaN light-emitting diodes grown on (112¯2) semipolar versus (0001) polar planes

The characteristics of electroluminescence (EL) and photoluminescence (PL) emission from GaN light-emitting diodes (LEDs) grown on (112¯2) semipolar plane and (0001) polar plane have been comparatively investigated. Through different bias-dependent shifting trends observed from the PL and time-resolved PL spectra (TRPL) for the two types of LEDs, the carrier dynamics within the multiple quantum wells (MQWs) region is systematically analyzed and the distinct field-dependent emission kinetics are revealed. Moreover, the polarization induced internal electric field has been deduced for each of the LEDs. The relatively stable emission behavior observed in the semipolar LED is attributed to the smaller polarization induced internal electric field. The study provides meaningful insight for the design of quantum well (QW) structures with high radiative recombination rates.

Applying the selective Cu electroplating technique to light-emitting diodes

We successfully fabricated a predefined patterned copper (Cu) substrate for thin GaN light-emitting diodes without barriers by the selective electroplating technique. The contours of Cu bumps fabricated using different electroplating modes and parameters were measured. We observed that the average thickness diminished with increasing current density. The current density conditions to obtain the best upright structure in the process were 40 and 80 mA/cm2.

Threading dislocation reduction in epitaxial GaN using V-groove patterned sapphire substrate with embedded silica nanospheres

A dramatic reduction in threading dislocation density and stress-relaxation was simultaneously achieved in GaN epilayer using a silica nanosphere embedded structure on V-groove patterned sapphire substrate by metal-organic chemical vapor deposition. By depositing silica nanospheres at two different instances during a growth process, a two-step growth that included selective area growth and lateral overgrowth was initiated. This approach led to GaN template of high crystal quality, which was confirmed from x-ray diffraction rocking curve and micro-Raman measurements and further corroborated by transmission electron microscopy. GaN light-emitting diode fabricated by this strategy showed a significant enhancement in the light output power.

Improving Light Extraction From GaN Light-Emitting Diodes by Buried Nano-Gratings

Recent experimental work has demonstrated that the light extraction enhancement due to scattering by a metallic nano-grating in an InGaN/GaN quantum well (QW) structure can be improved significantly by burying the grating in a dielectric, such as polyvinyl alcohol (PVA). In this paper, we employ the fluctuational electrodynamics method to investigate the origin of this improvement and to provide guidelines on how to optimize emission efficiency in these structures. Our results show that metallic grating diffracts efficiently the high-intensity PVA resonances trapped in the structure, because of the large permittivity contrast between the metal and semiconductor, providing the reported exceptional enhancement for s-polarization. We also study the effect of two important physical factors in the enhancement: 1) the thickness of the GaN barrier separating the QW from the grating and 2) the thickness of the InGaN QW. Results reveal that the enhancement efficiency can be maintained even when the QW is not in the near field of the grating, for a QW-grating separation of up to 1 μm. This is in contrast to plasmonic structures, where enhancement strongly decreases as the separation is increased. However, the enhancement factor can also vary strongly at the local level in smaller spatial intervals. Results also show that the enhancement significantly decreases with the QW thickness due to the losses in the QW.

Planar micro- and nano-patterning of GaN light-emitting diodes: Guidelines and limitations

The emission area of GaN light-emitting diodes can be patterned by etch-free current aperturing methods which exploit the thin and highly resistive nature of the p-doped layer in these devices. Here, the fundamental underlying electrical and optical aspects of high-resolution current aperturing are investigated theoretically. The most critical parameter for the possible resolution is the thickness d of the p-GaN layer, but the interplay of p-GaN resistivity and electrical junction characteristics is also important. A spatial resolution of 1.59d can in principle be achieved, corresponding to about 300 nm in typical epitaxial structures. Furthermore, the emission from such a small emitter will spread by about 600 nm while propagating through the p-GaN. Both values can be reduced by reducing d.

Performance evaluation of GaN light-emitting diodes using transferred graphene as current spreading layer

This study elucidates the correlation among conductivity of graphene and interface aspects in GaN light-emitting diodes (LEDs). Using a multilayer graphene of low sheet resistance, it is demonstrated that graphene alone can make ohmic contact with p-GaN without necessitating additional interlayer. Large-area blue LED with relatively low contact resistance in the order of 10−2 ohm-cm2 and improved forward voltage of 3.2 ± 0.1 V was realized irrespective of the use of the interlayer. The results from parallel evaluation experiments performed by varying the layer numbers of graphene with ultrathin NiOx interlayer revealed that the poor lateral conductivity of monolayer or few layer graphene can be well compensated by the interlayer. A combination of three layer graphene and NiOx offered device with enhanced electro-optical performance. But the Schottky barrier associated with the inadequate adhesion of transferred graphene dominates all the benefits and becomes a major bottleneck preventing the formation of low resistance stable ohmic contact.

Design principles of external and internal patterns for efficient light extraction

We studied external and internal patterns for boosting light extraction from GaN light-emitting diodes (LEDs). With a transparent conductive layer (TCL) inserted between an external pattern and an upper GaN plane, light extraction increased in a thin, high-refractive-index TCL. A periodic array of air voids embedded in a GaN medium yielded an extraction efficiency similar to the external pattern’s one. For double-layer patterns, as the internal pattern was close to multiple quantum wells within a wavelength, the extraction efficiency was enhanced ∼30% over that of a single-layer pattern. This work will be useful for designing blue-to-ultraviolet-emitting GaN-based LEDs with multi-layer patterns.

Performance of Cu-Plating Vertical LEDs in Heat Dissipation Using Diamond-Like Carbon

Performance in heat dissipation of Cu-plating type vertical GaN light-emitting diodes (CVLEDs) with a diamond like carbon (DLC) layer was investigated at various injection current levels. Through the incorporation of DLC, the CVLED with DLC exhibits a high heat dissipating ability, where the DLC-CVLEDs can be handled at an ultrahigh injection current of 2000 mA and reach an output power of 620 mW. In addition, the thermal resistance of the CVLED with DLC calculated by surface temperature data at 1400 mA injection current was 34% lower than that of CVLED without DLC, which clearly indicated that the benefit of using DLC layer on improvement of heat dissipation will be more significant as a higher current is injected.

Tailoring evanescent wave coupling in GaN/AlGaN core/cladding light-emitting diodes

We investigated GaN/AlGaN core/cladding light-emitting diodes (LEDs) that surpass the limit of light extraction that is typically exhibited by conventional GaN LED structures. Full-vectorial electromagnetic calculations confirmed that with an upper two-dimensional dielectric pattern, the extraction efficiency of the subwavelength-thick GaN/AlGaN waveguide increased by ∼50% relative to that of the homogeneous GaN structure. Electric-field intensity profiles demonstrated that this significantly enhanced light extraction was a result of the thin waveguide effect in which the evanescent field of guided modes penetrates further into the upper dielectric pattern. The waveguide effect was investigated at various refractive indices and thicknesses of the cladding.

Laser lift-off transfer printing of patterned GaN light-emitting diodes from sapphire to flexible substrates using a Cr/Au laser blocking layer

We develop a method to directly transfer the array of GaN-based light-emitting diodes (LEDs) from sapphire onto flexible substrates by a laser lift-off (LLO) process. Cr/Au layers are employed as a laser blocking layer to protect the supporting polymer layers from the laser beam and sharply separate the LEDs from the sapphire during the LLO process. This method dramatically increases the transfer yield of patterned LEDs up to 95% by decreasing the laser-induced damage in the supporting polymer layers and LEDs.

Facile Fabrication of Free-Standing Light Emitting Diode by Combination of Wet Chemical Etchings

Free-standing GaN light-emitting diode (LED) structure with high crystalline quality was fabricated by combining electrochemical and photoelectrochemical etching followed by regrowth of LED structure and subsequent mechanical detachment from a substrate. The structural quality and composition of the regrown LED film thus produced was similar to standard LED, but the photoluminescence and electroluminescence intensity of the LED structures on the etched template were several times higher than for standard LED. The performance enhancement was attributable to additional light scattering and improved crystalline quality as a result of the combined etching scheme.

Large-scale SiO2 photonic crystal for high efficiency GaN LEDs by nanospherical-lens lithography

Wafer-scale SiO2 photonic crystal (PhC) patterns (SiO2 air-hole PhC, SiO2-pillar PhC) on indium tin oxide (ITO) layer of GaN-based light-emitting diode (LED) are fabricated via novel nanospherical-lens lithography. Nanoscale polystyrene spheres are self-assembled into a hexagonal closed-packed monolayer array acting as convex lens for exposure using conventional lithography instrument. The light output power is enhanced by as great as 40.5% and 61% over those of as-grown LEDs, for SiO2-hole PhC and SiO2-pillar PhC LEDs, respectively. No degradation to LED electrical properties is found due to the fact that SiO2 PhC structures are fabricated on ITO current spreading electrode. For SiO2-pillar PhC LEDs, which have the largest light output power in all LEDs, no dry etching, which would introduce etching damage, was involved. Our method is demonstrated to be a simple, low cost, and high-yield technique for fabricating the PhC LEDs. Furthermore, the finite difference time domain simulation is also performed to further reveal the emission characteristics of LEDs with PhC structures.

Micro-structured light emission from planar InGaN light-emitting diodes

Investigation of the surface modification of the p-type layer in GaN light-emitting diodes (LEDs) by exposure to a trifluoromethane plasma is reported. It is found that the plasma treatment reduces the conductivity of the p-GaN by several orders of magnitude, and when applied at room-temperature through a patterned mask, localized current channels into the active region of a p–i–n device are created. This provides a novel approach to laterally modulate the light emission from an LED over essentially planar areas. This technique allows the projection of high-resolution images from non-pixelated devices, and an example application of maskless pattern transfer with sub-micron features into photoresist is demonstrated.