Higher Light Output Power Research Articles

Light extraction enhancement of sapphire‐free InGaN LEDs using single‐ and double‐side surface roughening techniques

AbstractWe compare the performance of three types of GaN‐based light‐emitting diodes (LEDs) epitaxially grown on patterned sapphire substrates (PSSs). For the devices with a single roughened p‐GaN surface, the light output power of the original LEDs with the PSS and Ag reflector (Sample A) is lower than that of thin‐film LEDs transferred onto Si carriers via glue bonding (Sample B). While the thin GaN LEDs with double‐side roughening (both p‐GaN and micropillar undoped‐GaN surfaces) and glue/Ag omnidirectional reflector (Sample C) shows much higher light output power than that of Sample B. As compared with the Sample A, the Sample C shows a performance enhancement of 50.3% in light output power and 46.3% in power efficiency at 350 mA. These results reflect that the contribution of the micropillar undoped‐GaN texturing for the light extraction is dominant. The employment of Si carrier decreases the junction temperature of the Sample C, a 46.6 °C lower than that of the conventional LEDs on PSS. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication of Mesa Shaped InGaN-Based Light-Emitting Diodes Through a Photoelectrochemical Process

InGaN-based light-emitting diodes (LEDs) were fabricated to have a higher light extraction through the photoelectrochemical (PEC) mesa shaping process. After the PEC selective wet oxidation and wet etching processes, stable and controllable crystallographic etching planes were formed as p-type GaN { $$ 10\overline{11} $$ } planes and n-type GaN { $$ 10\overline{1} 0 $$ } planes included at an angle of 27 deg. The ever-present cone-shaped structure of a PEC-treated LED has a larger light scattering area and higher light extraction cones on the mesa sidewall, as analyzed by microphotoluminescence and light output power measurement. This cone-shaped-sidewall LED has a higher light output power and a larger divergence angle compared with a conventional LED measured in an LED chip form.

수소저장합금을 이용한 p-GaN ITO 투명전극과 Au 전극과의 특성비교

In this work, the electrical and optical properties of the two different p-type GaN electrode schemes, ZnNi/ITO and ZnNi/Au, were compared each other, and applied to the top-emitting GaN/InGaN light-emitting diodes (LEDs). The ZnNi/ITO electrode showed much higher transmittance (90%) and slightly lower contact resistance <TEX>$(1.27{\times}10^{-4}{\Omega}cm^2)$</TEX> than those (77%, <TEX>$(2.26{\times}10^{-4}{\Omega}cm^2)$</TEX>) of the ZnNi/Au at a wavelength of 460 nm. In addition, GaN LEDs having ZnNi/ITO showed accordingly higher light output power and luminous intensity than those having ZnNI/Au did at the current levels up to 1 A.

Effect of electrode pattern on light emission distribution in InGaN/GaN light emitting diode

AbstractThe effect of geometrical electrode pattern on the blue InGaN/GaN multi‐quantum well (MQW) light emitting diode (LED) devices is investigated both theoretically and experimentally. A 3‐dimensional circuit model of an LED is constructed and then the corresponding circuit parameters are extracted. Circuit parameters of an LED consist of the resistances of the metallic film and epitaxial layer, and intrinsic diodes representing the light‐emitting active region. Both the transmission line model (TLM) and current‐voltage (I‐V) characteristics of LED are employed to extract circuit parameters experimentally. These circuit parameters are then applied to the circuit model to generate light emission patterns in a top‐surface emitting‐type LED. Experiment results verify that the emitting light distribution from fabricated LED coincides reasonably well with the theoretical results. This makes it possible to design an LED theoretically and predict its actual properties. The LED structure proposed using the model is shown to exhibit not only good light uniformity, but also higher light output power and lower voltage drop. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Enhanced light output from aligned micropit InGaN-based light emitting diodes using wet-etch sapphire patterning

The authors have demonstrated an effective method to obtain high light output power of GaN-based light-emitting diodes (LEDs) by simultaneous enhancement of internal quantum efficiency and light extraction efficiency. Micropit InGaN∕GaN LEDs were fabricated on hexagonal-shaped GaN template through wet-etched substrate patterning. The result indicated that under optimized growth condition of high temperature GaN template, micropits could be formed and distributed in an aligned manner by growing on wet-etch patterned sapphire substrate. The LED structures showed superior optical output power, which directly resulted from not only effective elimination of threading dislocation of the epitaxial layers but also significant increase in light extraction efficiency via the inclined facets of aligned micropits.

Linear GRINSCH 1.55-μm InGaAsP ∕ InP Strained Multiple Quantum Well Laser Diodes Grown by Substrate Temperature Control

We demonstrated a novel method to grow InGaAsP linear graded-index separate confinement heterostructure (GRINSCH) by ramping the growth temperature in the metallorganic chemical vapor deposition system. From secondary ion mass spectroscopy and photoluminescence analysis, the composition of linearly graded layer is well in control. By introducing this GRINSCH InGaAsP structure, SMQW ridge waveguide laser diodes with a back-facet high-reflection coating exhibit a low threshold current of 6.5 mA at 20°C, a high light output power of 20 mW at 80 mA, and a high slope efficiency of , also showing potential for high-temperature, continuous-wave operation up to 95°C. The longitudinal mode oscillates at at 20°C and the of red-shift rate in wavelength.

Effect of Annealing on Low-Threshold-Current Large-Wavelength InGaAs Quantum Well Vertical-Cavity Laser

An emission wavelength record emission wavelength of 1245 nm in double InGaAs/GaAs quantum wells without a strain-compensated barrier is obtained by metalorganic chemical vapor deposition (MOCVD). The effect of annealing on highly strained InGaAs quantum wells is analyzed using phtoluminanescence spectra. By comparing devices p-type distributed Bragg reflector (p-DBR) growth temperatures, the preliminary results indicate that a device with a low p-DBR growth temperature of 670°C exhibits a low threshold current and a high light output power. A threshold current of 6.7 mA in an InGaAs vertical-cavity surface emitting laser (VCSEL) in CW operation is realized with an emission spectrum up to 1.26 µm.

Light-output enhancement of GaN-based light-emitting diodes by using hole-patterned transparent indium tin oxide electrodes

We have demonstrated the improvement of the light-output power of GaN-based light-emitting diodes (LEDs) using hole-patterned indium tin oxide (ITO) p-type electrodes. Hole patterns were defined by a laser holographic lithography combined with a postlithography deposition process. It is shown that near-UV LEDs made with the patterned ITO with a hole period of 710nm and a size of 320nm give 23% and 67% higher light-output power (at 20mA) than those of LEDs with unpatterned ITO and Ni∕Au contacts, respectively. It is further shown that the reduction of the hole period results in an additional improvement of light-output power.

Fabrication of p‐side down GaN vertical light emitting diodes on copper substrates by laser lift‐off

The fabrication process and performance characteristics of the laser lift-off (LLO) GaN light emitting diodes (LEDs) were investigated. The LLO-GaN LEDs with 300 × 300 μm2 on Cu show a nearly 4-fold increase in the light output power over the regular LLO-LEDs on the sapphire substrate. High operation current for the LLO-LEDs on Cu was also demonstrated. The large area-emission of 1000 × 1000 μm2 of p-side down GaN LLO-LEDs on Cu were fabricated. Further improvement of max light output are achieved. The LLO process should be applicable to other GaN-based light emitting devices in particular for those high light output power and high operation current devices. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of AlGaInP multiquantum-well/double heterostructure light-emitting diodes with In-added GaP window layer regrown by antimony-based liquid phase epitaxy

The characteristics of surface-emitting AlGaInP multiquantum-well (MQW) and double heterostructure (DH) light-emitting diodes (LEDs) with a GaP window layer regrown by a GaP source melt with the addition of indium in the liquid phase epitaxy (LPE) system are studied. Using antimony in the source melt instead of gallium eliminates the phenomenon of melt back at the top layer of the LED during the LPE regrowth process. Meanwhile, adding In and Zn to the source melt significantly increases the GaP growth rate and effectively reduces residual melt during the regrowth process. Thus, the AlGaInP LED with a Zn-doped GaP window layer regrown by In being added, Sb-based LPE exhibits a lower turn-on voltage and higher light output power than that without the regrown GaP window layer. However, the emission peak wavelength of the MQW LED shifts a little after the LPE regrowth process. The DH LED does not shift. Thus, the DH structure is more suitable than the MQW structure for the LPE regrowth process of GaP.

Effects of etching depth for n-contact and current spreading layer in InGaN/GaN light emitting diodes

InGaN/GaN multiple quantum well (MQW) light-emitting diode (LED) structures possessing current spreading layers of various carrier concentrations were grown by a metal organic chemical vapour deposition method, and LEDs were fabricated with various etching depths for the n-contact. The current–voltage and light output power characteristics of these LEDs were investigated. Device properties of LEDs possessing a current spreading layer showed a lower operation voltage and a higher light output power than those without, although the reverse breakdown voltage decreased when the carrier concentration in the current spreading layer was too high. The best LED characteristics are shown in a LED with a carrier concentration of 6 × 1018 cm−3 in the current spreading layer. Operating voltages of LEDs at 20 mA show the best value at an etching depth for the n-electrode of 800 nm from the top layer. The depth of etching for the n-contact in a LED also affects the light output power. On the basis of our results, it is suggested that the etching depth for the n-contact and the carrier concentration of the spreading layer in InGaN/GaN LED should be controlled properly to obtain the best performance from LEDs.

Study of GaN light-emitting diodes fabricated by laser lift-off technique

The fabrication process and performance characteristics of the laser lift-off (LLO) GaN light-emitting diodes (LEDs) were investigated. The LLO-GaN LEDs were fabricated by lifting off the GaN LED wafer structure grown on the original sapphire substrate by a KrF excimer laser at 248 nm wavelength with the laser fluence of 0.6 J/cm2 and transferring it onto a Cu substrate. The LLO-GaN LEDs on Cu show a nearly four-fold increase in the light output power over the regular LLO-LEDs on the sapphire substrate. High operation current up to 400 mA for the LLO-LEDs on Cu was also demonstrated. Based on the emission wavelength shift with the operating current data, the LLO-LEDs on Cu show an estimated improvement of heat dissipation capacities by nearly four times over the light-emitting devices on sapphire substrate. The LLO process should be applicable to other GaN-based LEDs in particular for those high light output power and high operation current devices.

Improvement of electrical and optical properties of ingan/ganbased lightemitting diodes with triangular quantum well structure

Substantial improvement of electrical and optical properties of InGaN/GaN multiple quantum wells (MQWs) was obtained with a triangular band structure. Transmission electron microscopy (TEM) images from the–50 nm. The light-emitting diodes (LEDs) with the triangular QWs showed a lower operation voltage, a higher light output power, and higher intensities and narrower line widths of electroluminescence spectra than those with the rectangular QWs. Very bright and uniform light emission from the triangular MQW LEDs was also observed at a low injection current, but spatially non-uniform emission from the rectangular ones.

Effect of well profile on optical and structural properties in InGaN/GaN quantum wells and light emitting diodes

Effects of the quantum well (QW) shape on structural and optical properties of QWs fabricated by gradating In composition during the well layer growth and characteristics of light emitting diodes (LEDs) composed of those QW structures are investigated. Photoluminescence (PL) spectra of trapezoidal-shape QWs show the best light emission efficiency. PL spectra exhibit a main peak associated with transition in In-rich quantum dot-like clusters at 468 nm and a relatively small peak originated from QWs at 430 nm, and their phonon replicas. Temperature dependent PL spectra indicated that excitons in trapezoidal QW structure are more strongly localized than in rectangular QWs. Under external electric fields, the magnitude of the variation in PL peak and intensity of trapezoidal QWs at room temperature is almost constant in contrast to those of rectangular ones. In a plan-view transmission electron microscopic image, it was observed that quantum dots (QDs) formed inside the dislocation loop in trapezoidal QWs. The size and density of QDs depend on the QW shape. Furthermore, the interactions of QDs with dislocations such as dislocation pinning by QDs and the preferential formation of QDs on dislocations lying on the growth plane are observed. LEDs fabricated using trapezoidal QWs have superior device properties such as high light output power, low forward operation voltage, good thermal stability, and stability under external electric fields. It suggests that the shape of QWs plays an important role in dislocation density control and QD engineering, and, in turn, the light emission efficiency. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High-Power 660-nm-Band AlGaInP Laser Diodes with a Small Aspect Ratio for Beam Divergence

High-power 660-nm-band AlGaInP laser diodes with a small aspect ratio have been successfully fabricated with a window-mirror structure. The relationship between optical confinement in the perpendicular direction and internal loss was investigated, and a real index-guided structure with an AlInP current blocking layer was applied on the basis of this investigation. A high-power laser diode with a small aspect ratio of 1.65 for beam divergences of 16.5° and 10° in the perpendicular and parallel directions, respectively, has shown a high kink level of 160 mW and a high maximum light output power of 180 mW under pulsed condition. These laser diodes have operated stably for more than 1500 h with a light output power of 90 mW at 60°C under pulsed condition. Stable pulsed operation at 60°C with a high power of 90 mW and small aspect ratio of 1.65 have been simultaneously achieved for the first time.

Improvement of diodes performance with a multiple-pair buffer layer by MOCVD

The GaN homo-junction light-emitting diodes (LEDs) with a multiple-pair buffer layer (MBL) were grown by metalorganic vapor phase epitaxy on sapphire substrates. Each pair of the buffer layer consists of a 300 Å thick GaN nucleation layer grown at a low temperature of 525°C and a 4 m thick GaN epitaxial layer grown at a high temperature of 1025°C. It is found that the GaN LEDs with a three-pair buffer layer will exhibit a low turn-on voltage, stronger electroluminescence intensity and higher light-output power as compared to the conventional growth without a buffer layer. This is attributed to the effective reduction in the propagation of defects and dislocations near the p– n junction for the LEDs with MBL.

Electrical and optical characteristics of the GaN light-emitting diodes with multiple-pair buffer layer

The GaN homo-junction light-emitting diodes (LEDs) with multiple-pair buffer layer (MBL) were grown by metalorganic vapor phase epitaxy on sapphire substrates. Each pair of the buffer layer consists of a 300 Å thick GaN nucleation layer grown at a low temperature of 525°C and a 4 μm thick GaN epitaxial layer grown at a high temperature of 1025°C. As compared to the conventional growth without buffer layer, the GaN LEDs with MBL will exhibit a low turn-on voltage, stronger electroluminescence intensity, and higher light output power. It is attributed to the effective reduction in the propagation of defects and dislocations near the p–n junction for the LEDs with MBL.

The influence of window layers on the performance of 650 nm AlGaInP/GaInP multi-quantum-well light-emitting diodes

In this article, we investigate the influence of AlGaAs and GaP window layers on the device performance of 650 nm AlGaInP/GaInP multi-quantum-well (MQW) light-emitting diodes (LEDs) grown by metalorganic chemical-vapor deposition. The AlGaInP/GaInP MQW structure with different window layers are characterized by double-crystal X-ray diffraction, secondary ion mass spectrometry and photoluminescence. By using the AlGaAs window layer, the LEDs have a lower cut-in voltage, a smaller dynamic series resistance and a higher breakdown voltage, while the LEDs with a GaP window layer show a stronger electroluminescence intensity, a higher light output power, a higher external quantum efficiency and a slower degradation of light output with increasing bias current. These results indicate that the GaP material is more adequate to be used as a window layer for the AlGaInP optical devices.

Metal-clad ridge waveguide structure InGaP/InGaAIP visible laser diodes

InGaP/InGaAIP visible-light laser diodes with a metal-clad ridge waveguide structure have been fabricated. Compressively strained SCH-MQW structures were grown by low-pressure MOCVD and the lasing wavelength was 690 nm. The threshold current was 28 mA at 25°C for a 400 μm cavity and 5 μm stripe width. CW operation was obtained as the cleaved device by employing thick gold layer as a heat spreader. The light-output power versus CW current was linear up to a maximum light output power of 38 mW. Stable operation of the fundamental transverse optical mode was maintained up to 30 mW. These results show that this metal-clad ridge waveguide structure provides sufficient current confinement even under high light output power operation.

Liquid crystal light valves for schlieren optical projection

The advantage of schlieren optical large-screen projection is its ability to control a high light output power and to create bright images. The imaging element is a phase modulator which can be implemented using liquid crystal (LC) technology. In this contribution the operating principle and optical properties of three different types of phase modulators, i.e. a tandem cell, a modulator with a twisted LC structure and a (LC cell-phase retarder-mirror) compound cell are investigated.