InGaN-based Light-emitting Diodes Research Articles

Size-dependent capacitance study on InGaN-based micro-light-emitting diodes

We report a detailed study on size-dependent capacitance, especially the negative capacitance (NC), in InGaN-based micro-pixelated light-emitting diodes (μLEDs). Similar to conventional broad-area LEDs, μLEDs show NC under large forward bias. In the conventional depletion and diffusion capacitance regimes, a good linear relationship of capacitance with device size is observed. However, the NC under high forward bias shows slight deviation from above-mentioned linear relationship with device size. This behaviour can be understood if the effects of current density and junction temperature on NC are considered. The measured temperature dependence and frequency dispersion of the capacitance underpin this point of view. The NCs of two reference broad-area LEDs were also measured and compared with that of μLED clusters with the same total size. A stronger NC effect is observed in the μLED clusters, which is attributed to the increased number of sidewall defects during fabrication process.

Color Tunability of Blue Phosphors by Changing Cations for Near-UV InGaN-Based Light-Emitting Diode

The serials of blue phosphors, CaAl2Si2O8: Eu2+ doped with different content Sr2+ and Mg2+ ions, were prepared by solid-state reaction at high temperature. And their structure and photo-luminescent properties were investigated. In Ca0.96-ySryAl2Si2O8: 0.04Eu2+ (y = 0.10, 0.30, 0.50, 0.70, 0.90) system, for y ≦ 0.03, all the compositions crystallize in triclinic structure of CaAl2Si2O8, and on further increase of y, the system undergoes a compositionally induced phase transition from triclinic to monoclinic structure. For Ca0.96-yMgyAl2Si2O8: 0.04Eu2+ (y = 0.10, 0.30, 0.50, 0.70, 0.90) system, for z ≦ 0.03, all the compositions are of triclinic structure of CaAl2Si2O8. With the further increase of z, other phase appears. The emission spectra of these phosphors show blue shift with the introduction of Sr2+ ions, and red shift with Mg2+ ions. The reason may be due to the difference ionic radii of Mg2+, Ca2+, Sr2+. These phosphors show excellent blue emission and broad excitation band in near-UV (ultraviolet) range. They may be potential phosphors for near-UV light-emitting diodes (LEDs).

Design strategies for enhancing carrier localization in InGaN-based light-emitting diodes

By designing the quantum well structure with the introduction of GaN and InN interlayers into the InGaN wells, the carrier localization is enhanced by demonstrating temperature-dependent photoluminescence (PL) measurements. Two emission peaks corresponding to In-rich localized state and quantum well ground state emissions are observed from the electroluminescence (EL) spectra, which demonstrates that the phase separation takes place in our designed structures and the enhanced phase separation is most likely the dominated mechanism for the formation of In-rich localized states, accounting for the stronger localization and hence improved light-emission characteristics. Therefore, it is suggested that enhancing localization is possible, to some extent, by modulating the QW structures with the introduction of the GaN and InN interlayers for superior light-emission performances in InGaN-based light-emitting diodes (LEDs). Moreover, the nonradiative channel probably associated with InN droplet is proved to be existent from the Arrhenius plots in our InGaN–delta-InN MQW structure.

Fabrication of full-color InGaN-based light-emitting diodes on amorphous substrates by pulsed sputtering.

InGaN-based light-emitting diodes (LEDs) have been widely accepted as highly efficient light sources capable of replacing incandescent bulbs. However, applications of InGaN LEDs are limited to small devices because their fabrication process involves expensive epitaxial growth of InGaN by metalorganic vapor phase epitaxy on single-crystal wafers. If we can utilize a low-cost epitaxial growth process, such as sputtering on large-area substrates, we can fabricate large-area InGaN light-emitting displays. Here, we report the growth of GaN (0001) and InGaN (0001) films on amorphous SiO2 by pulsed sputtering deposition. We found that using multilayer graphene buffer layers allows the growth of highly c-axis-oriented GaN films even on amorphous substrates. We fabricated red, green, and blue InGaN LEDs and confirmed their successful operation. This successful fabrication of full-color InGaN LEDs on amorphous substrates by sputtering indicates that the technique is quite promising for future large-area light-emitting displays on amorphous substrates.

Fabrication of InGaN light-emitting diodes with embedded air-gap disks using laser-drilling and electrochemical processes

InGaN-based light-emitting diodes (LED) with an embedded air-gap disk structure were fabricated using a laser drilling process and an electrochemical (EC) wet etching process. The disk-shaped air-gap structure was formed surrounding the laser-drilled hole as a reflective structure to enhance the light extraction efficiency. The light output power of the treated LED structure was enhanced by 22% compared with a conventional LED at 20 mA. High light emission intensity of the treated LED structure was observed at the 40-µm-diameter embedded air-disk patterns in the mesa region. The InGaN LED structures with the laser-drilling holes and embedded air-disks can reduce the light trapping effect and increase the light extraction efficiency in the device’s central mesa region.

Analysis of InGaN light-emitting diodes with GaN-AlGaN and AlGaN-GaN composition-graded barriers

The effects of InGaN-based light-emitting diodes (LEDs) with Al composition increasing and decreasing GaN-AlGaN barriers along the growth direction are studied numerically. Simulation results suggest that the LEDs with GaN-AlGaN composition-decreased barriers show more significant enhancement of light-output power and internal quantum efficiency than LEDs with composition-increasing GaN-AlGaN barriers when compared with the conventional LED with GaN barriers, due to the improvement in hole injection efficiency and electron blocking capability. Moreover, the optical performance is further improved by replacing GaN-AlGaN barriers with AlGaN-GaN barriers of the same Al composition-decreasing range, which are mainly attributed to the modified band diagrams. In addition, the major causes of the different efficiency droop behaviors for all the designed structures are explained by the electron leakage current and the different increase rates of hole concentration with injection current.

Performance enhancement by different sidewall structures for InGaN-based light-emitting diodes

We demonstrate high-performance GaN-based light-emitting diodes (LEDs) using different sidewall structure. The patterned pyramidal sidewall (PPS) structure in LED is fabricated by crystallographic etching process, which is applying hot phosphoric acid wet etching at the GaN/Al2O3 interface after normal front-side laser scribing. To obtain continuous, regular and bigger pyramidal sidewalls we applied higher temperature and longer time on the LEDs. However, there might be negative effect on the reliability of the PPS-LED by the higher experimental conditions. On the other hand, we applied the new stealth dicing process to form the clean and non-destructive sidewalls. The three times stealth dicing process can get a better well-distributed and shipshape sidewall than one time stealth dicing. The light output power of the PPS-LED had a 39.6 % enhancement compared to the standard LED owing to the larger light-scattering when measured in LED chip form. But the LED with stealth dicing is more reliable than PPS-LED regardless of the crystal quality of the LED.

Performance of Blue LEDs With N-AlGaN/N-GaN Superlattice as Electron-Blocking Layer

The characteristics of InGaN-based blue light-emitting diodes (LEDs) with an AlGaN/GaN superlattice (SL) electron-blocking layer (EBL) of gradual Al molar fraction below the active region are analyzed numerically. The output power, internal quantum efficiency, electrostatic fields, energy band diagrams, carrier concentrations, radiative recombination, and electron leakage current are investigated. The results indicate that the LED with n-type AlGaN/GaN SL EBL of gradual Al molar fraction has smaller electrostatic fields and lower electron leakage in its active region than the LED with a rectangular p-AlGaN EBL or n-AlGaN EBL. These result in a markedly reduced efficiency droop.

Utilizing Two-Dimensional Photonic Crystals in Different Arrangement to Investigate the Correlation Between the Air Duty Cycle and the Light Extraction Enhancement of InGaN-Based Light-Emitting Diodes

This paper demonstrates that the optimal light extraction enhancement of the 2-D photonic crystals (PhCs) light-emitting diodes (LEDs) among different air duty cycles (ADADCsCs) is independent of the geometry and the shape of the 2-D PhCs. Moreover, it also discusses that the side-directional emission property of the 2-D PhCs LEDs in hexagonal lattice arrangement (HLA) is better than that in square lattice arrangement (SLA). Finally, the light output power of the 2-D PhCs LEDs in SLA and in HLA with the ADC of 51% are significantly improved by 60.4% and 81.9%, respectively, as compared with the reference LED at an injection current of 350 mA.

Analysis of trap and its impact on InGaN-based blue light-emitting diodes using current-transient methodology

A reversible increase in the current of InGaN-based blue LEDs is observed when constant forward voltage is applied. This characteristic is assumed to be the result of trapping process, and a trap activation energy of 0.30 eV is extracted. Through a numerical simulation, it is confirmed that the multi-quantum well (MQW) barrier height is reduced by the hole trapping process and that the current is increased by lowering this barrier. We also confirmed the effect of this trap on the optical characteristics of InGaN-based blue LEDs by a numerical simulation and measurement.

Effects of Asymmetric Quantum Wells on the Structural and Optical Properties of InGaN-Based Light-Emitting Diodes.

A metalorganic vapor phase epitaxy-grown InGaN/GaN multiple-quantum-well (MQW) with three graded-thickness wells (the first-grown well had the greatest width) near the n-GaN was used as the active layer of an LED. For LEDs with an asymmetric quantum well (AQW), high-resolution X-ray diffraction and transmission electron microscopic reveal that the modified MQWs with a reasonable crystalline quality were coherently strained on the underlying GaN epilayers without any relaxation. In addition, the slight increase of indium segregation in the LED with an AQW may be attributed to variations in indium contents experienced during epitaxial growth of the wide well-containing MQWs. By preventing the energetic electrons from accumulating at the topmost quantum well nearest the p-GaN, the presence of light intensity roll-off in the LED with an AQW is shifted to higher currents and the corresponding maximum light output power is increased with a ratio 7.9% higher than that of normal LEDs. Finally, similar emission wavelengths were observed in the electroluminescence spectra of both LEDs, suggesting that light emitted mostly from the top quantum wells (near the p-GaN) while the emissions from the AQW region were insignificant.

High-Efficiency and Crack-Free InGaN-Based LEDs on a 6-inch Si (111) Substrate With a Composite Buffer Layer Structure and Quaternary Superlattices Electron-Blocking Layers

In this paper, a composite buffer layer structure (CBLS) with multiple AlGaN layers and grading of Al composition/u-GaN1/(AlN/GaN) superlattices/u-GaN2 and InAlGaN/AlGaN quaternary superlattices electron-blocking layers (QSLs-EBLs) are introduced into the epitaxial growth of InGaN-based light-emitting diodes (LEDs) on 6-inch Si (111) substrates to suppress cracking and improve the crystalline quality and emission efficiency. The effect of CBLS and QSLs-EBL on the crystalline quality and emission efficiency of InGaN-based LEDs on Si substrates was studied in detail. Optical microscopic images revealed the absence of cracks and Ga melt-back etching. The atomic force microscopy images exhibited that the root-mean-square value of the surface morphology was only 0.82 nm. The full widths at half maxima of the (0002) and (1012) reflections in the double crystal X-ray rocking curve were ~330 and 450 respectively. The total threading dislocation density, revealed by transmission electron microscopy, was <; 6× 108 cm-2. From the material characterizations, described above, blue and white LEDs emitters were fabricated using the epiwafers of InGaN-based LEDs on 6-inch Si substrates. The blue LEDs emitter that comprised blue LEDs chip and clear lenses had an emission power of 490 mW at 350 mA, a wall-plug efficiency of 45% at 350 mA, and an efficiency droop of 80%. The white LEDs emitter that comprised blue LEDs chip and yellow phosphor had an emission efficiency of ~110 lm/W at 350 mA and an efficiency droop of 78%. These results imply that the use of a CBLS and QSLs-EBL was found to be very simple and effective in fabricating high-efficiency InGaN-based LEDs on Si for solid-state lighting applications.

Design of an LED chip structure with an integrated two-dimensional photonic crystal to enhance the light-extraction efficiency

We numerically simulated the light-extraction efficiency of light-emitting diodes (LEDs) with an integrated two-dimensional photonic crystal (PC) structure on the top surface in order to enhance light extraction. We considered InGaN-based LED chips with a typical emission wavelength of λo = 460 nm and an emission wavelength inside the LED chip of λ = λo/nGaN, where nGaN is the refractive index of GaN. We used positive (relief) and negative (intaglio) patterns for the PC structures with square arrangements. The pattern period (Λ), width (d), and height (h) of the PC structure were varied systematically in the PC-LEDs; then the light-extraction efficiency of each PC-LED was simulated numerically using a three-dimensional finite-difference time-domain method to optimize the PC structure in terms of light extraction. The PC LED with a square pilla pattern with Λ ≃ 1.4λ, d ≃ 0.75Λ, and h ≃ 0.75Λ had the maximum light-extraction efficiency for positive patterns while the cylindrical hole pattern with Λ ≃ 1.2λ, d ≃ 0.5Λ, and h ≃ 0.5Λ had the maximum light-extraction efficiency for negative patterns.

Nonradiative recombination mechanisms in InGaN/GaN-based light-emitting diodes investigated by temperature-dependent measurements

Two kinds of InGaN-based light-emitting diodes (LEDs) are investigated to understand the nonradiative carrier recombination processes. Various temperature-dependent measurements such as external quantum efficiency, current-voltage, and electroluminescence spectra are utilized from 50 to 300 K. Based on these experimental results, we analyze the dominant nonradiative recombination mechanism for each LED device. We also analyze the effect of the dominant nonradiative recombination mechanism on the efficiency droop. On the basis of correlation between the efficiency droop and nonradiative recombination mechanisms, we discuss an approach to reducing the efficiency droop for each LED device.

Enhancement of hole injection with Mg–Si-codoped barriers in InGaN-based light-emitting diodes

The advantages of an InGaN light-emitting diode (LED) structure with Mg–Si-codoped barriers are studied numerically in this paper. Energy band diagrams, carrier concentrations in the active region, radiative recombination rates in the quantum wells (QWs), light-current performances, and internal quantum efficiency (IQE) are investigated. Simulation results suggest that by employing the Mg–Si-codoped barriers, the light-output power and IQE are more significantly improved than LEDs with step-Si-doped barriers when compared with the conventional LEDs with wholly Si-doped barriers, due to modified band diagrams, which are favorable for improvement of hole injection efficiency. Moreover, simulation also indicates that the optical performance can be further improved if the Si doping concentration of quantum barriers (QBs) is decreased gradually along the growth direction for the LED with Mg–Si-codoped barriers.

High Efficiency InGaN Blue Light-Emitting Diode With ${&gt;}{\rm 4}\hbox{-}{\rm W}$ Output Power at 3 A

This letter reports high-power and high-efficiency characteristics of the InGaN-based blue light-emitting diode (LED) operating at > 10-W electrical input power in a single-chip package. The LED chip is fabricated as a vertical-injection structure with chip dimensions of 1.8 mm × 1.8 mm. InGaN/GaN short-period superlattice (SL) structures are employed below multiple-quantum-well active region as current spreading layers. It is found, by simulation, that SL layers are quite effective in improving current spreading and uniformity in carrier distribution. When the characteristics of the fabricated LED package are measured under pulsed operation conditions, efficiency droop is found to be greatly reduced in the LED structure with SL layers. A record high light output power of 4.18 W and external quantum efficiency of 51% are demonstrated at 3-A injection current.

(Invited) Point Defect Characterization of Group-III Nitrides by Using Monoenergetic Positron Beams

The nitride semiconductor material system has played a dominant role in the current success of the solid-state lighting (SSL) industry. Because the recent progress of the SSL technology has been mainly driven by light-emitting diodes (LEDs) that use GaN and Ga-rich InGaN, many studies have been carried out on those alloys. The potential of InGaN as an SSL material comes from the ability to control band gaps from 0.65 eV to 3.43 eV, which correspond to entire wavelengths from infrared to ultraviolet regions. Because those band gap energies can cover the solar spectrum, InGaN is also recognized as having a high potential as a photovoltaic device material. The external quantum efficiency (EQE) of an InGaN-based LED, however, is known to drop rapidly with increasing InN composition. The EQE decrease is attributed to (i) piezoelectric fields in multiplayer structures which cause a separation between electron and hole wavefunctions, and (ii) the degradation of crystal quality. For InGaN-based tandem solar cells, the degradation of conversion efficiency occurs when InN composition increases. During growth of InGaN layers, the large difference in the lattice constants of InN and GaN cause unusually high strain to be introduced into the layer, and this causes dislocations, stacking faults, misorientations, phase separations, etc. Native point-defects are also known to act as non-radiative recombination centers in group III nitrides. Thus, further knowledge regarding such defects is needed to improve the optical and electrical properties of InGaN-based LEDs and solar cells. Positron annihilation is a powerful technique for evaluating vacancy-type defects in semiconductors [1], and the defects in group-III nitrides have been investigated using this method [2-6]. In the present study, we have used monoenergetic positron beams to probe native vacancies and deformation-induced defects in GaN and InGaN grown by MOCVD, MBE, and HVPE. For InGaN, it was found that vacancy-type defects were introduced with increasing InN composition, and the major defect species was identified as complexes between a cation vacancy and a nitrogen vacancy. The presence of the defects correlated with lattice relaxation of the InGaN layer and the increase in photon emissions from donor-acceptor-pair recombination. The concentration of the defects, however, was found to be suppressed by Mg doping, suggesting that Mg is an excellent suppressor of cation vacancies in InGaN. The introduction mechanism of vacancy-type defects and their interactions with impurities will be discussed. 1. R. Krause-Rehberg and H. S. Leipner, Positron Annihilation in Semiconductors, Solid-State Sciences (Springer-Verlag, Berlin, 1999) vol. 127.2. A. Uedono, S. Ishibashi, T. Ohdaira, and R. Suzuki, J. Crystal Growth 311, 3075 (2009).3. A. Uedono, S. Ishibashi, N. Oshima, and R. Suzuki, Jpn. J. Appl. Phys. 52, 08JJ02 (2013).4. A. Uedono, S. Ishibashi, K. Tenjinbayashi, T. Tsutsui, K. Nakahara, D. Takamizu, and S. F. Chichibu, J. Appl. Phys. 111, 014508 (2012).5. A. Uedono, T. Tsutsui, T. Watanabe, S. Kimura, Y. Zhang, M. Lozac’h, L. W. Sang, S. Ishibashi, and M. Sumiya, J. Appl. Phys. 113, 123502 (2013).6. A. Uedono, I. Yonenaga, T. Watanabe, S. Kimura, N. Oshima, R. Suzuki, S. Ishibashi, and Y. Ohno, J. Appl. Phys. 114, 084506(1-6) (2013).

Preparation and photoluminescence properties of a new orange–red Ba3P4O13:Eu3+ phosphor

A new orange–red Ba3P4O13:Eu3+ phosphor has been synthesized by solid-state technique, and its photoluminescence properties were investigated. X-ray powder diffraction (XRD) analysis indicates that doping Eu3+ does not change the lattice of Ba3P4O13. Field-emission scanning electron microscope (FE-SEM) images illustrate that microstructure of the phosphor consists of oval grains with average diameter of 1μm and heavy agglomerate phenomenon. The excitation spectra indicate the phosphor can be effectively excited by near ultraviolet (NUV) light, making it attractive as conversion phosphor for LED applications. The phosphor exhibits a bright orange–red emission excited by 394nm light. The CIE chromaticity can be varied slightly by adjusting the content of Eu3+, which is attributed to the different lattice sites occupied by Eu3+ in Ba3P4O13 host. The photoluminescence studies indicate that Ba3P4O13:Eu3+ is a potential orange–red phosphor for near-ultraviolet InGaN-based white light-emitting diodes (WLEDs).

Synthesis and luminescent properties of CaTiO3:Eu3+, Al3+ phosphors

CaTiO3:Eu3+, Al3+ phosphors were synthesized by a sol–gel method at a relatively low calcination temperature of 1000°C. Upon excitation under UV light of wavelength 397nm, all the CaTiO3:Eu3+ phosphors show bright red emission due to the 4f–4f transitions in Eu3+ ions. With the co-doping of Al3+ ions, we observed enhancement in the emission intensity at 616nm. Furthermore, the addition of Al3+ was found to improve the color purity of phosphor. With the addition of 3mol% Al3+ ions, the color purity of Ca0.95Ti0.97O3:Eu0.053+,Al0.033+ was found to improve from 83.1% to 84.9%, with the emission intensity almost equal to that of the Ca0.90TiO3:Eu3+0.10 phosphor. These results suggest that the Ca0.95Ti0.97O3:Eu0.053+,Al0.033+ red phosphor prepared in this study is a potential candidate for the fabrication of near-UV InGaN-based light-emitting diodes.

InGaN light-emitting diodes with band-pass-filter-like GaN : Si nanoporous structures

InGaN-based light-emitting diodes (LEDs) with both GaN : Si nanoporous and air-gap structures were fabricated through a wet lateral etching (LE) process. Light output power of the LE-LED structure was enhanced by 58% compared with a non-treated LED structure, due to the increased light extraction from the GaN : Si nanoporous and air-gap structure. Optical transmittance of the structure was analysed using photoluminescence from the LED epitaxial layers. The transmittance of the LE-LED was measured to be 2.56 times for the blue emission and 0.43 times for the yellow emission, compared with the non-treated LED structure at a detection angle of 35° from the lateral direction. The optical properties of the GaN : Si nanoporous structure were similar to a band-pass filter with a 460 nm centre wavelength and a 70 nm bandwidth, which effectively enhanced the light-extraction efficiency in InGaN LEDs.