Blue InGaN Light-emitting Diodes Research Articles

Highly efficient blue-light-excitable ultra-broadband orange-emitting Mg2.5Y1.5Al1.5Si2.5O12:Ce3+ garnet phosphors for solid-state lighting

Lighting and display technologies are revolutionized by the invention of YAG:Ce3+ phosphors driven by InGaN blue light-emitting diode (LED). Yet, the faint emission components in the red spectral region from typical LED devices can cause circadian rhythm disorders and aesthetic fatigue of the human eye. Developing broadband orange-emitting phosphor with abundant red coverage is one of the approaches for addressing this challenging problem. Herein, an innovative ultra-broadband orange-emitting Mg2.5Y1.5Al1.5Si2.5O12:Ce3+ (MYASO:Ce3+) garnet phosphor is successfully reported to bridge the above gap. The representative MYASO:4%Ce3+ sample exhibits an unexpectedly luminescence band with the full width at half maximum (FWHM) of up to 155 nm, the high thermal robustness (58%@423 K), and satisfactory internal quantum efficiency of 67% upon 465 nm blue light excitation. Such a large FWHM value results from the occupation of a single crystallographic site of Ce3+ in the dodecahedral [YO8] position of MYASO garnet host. Red-shift emission with respect to commercial YAG:Ce3+ is mainly attributed to the cooperative effect of the enhanced crystal field strength and increased Stokes shift. The emission intensity reduction at the high-temperature environment of MYASO:4%Ce3+ is derived from the enhancement of the thermal cross-relaxation associated with the larger Stokes shift (4839 cm−1). The resulting high-quality warm white LED device fabricated from the optimal MYASO:4%Ce3+ sample shows a satisfactory color rendering index (CRI = 84), comfortable correlated color temperature (CCT = 4032 K) and an excellent luminous efficacy (LE = 58.7 lm W−1) under a 60 mA drive current. These findings stimulate the exploration of ultra-broadband orange-emitting phosphors with abundant red emission to fabricate high-CRI warm-white LEDs for healthy solid-state lighting.

Recent Research on Indium-Gallium-Nitride-Based Light-Emitting Diodes: Growth Conditions and External Quantum Efficiency

The optimization of the synthesis of III-V compounds is a crucial subject in enhancing the external quantum efficiency of blue LEDs, laser diodes, quantum-dot solar cells, and other devices. There are several challenges in growing high-quality InGaN materials, including the lattice mismatch between GaN and InGaN causing stress and piezoelectric polarization, the relatively high vapor pressure of InN compared to GaN, and the low level of incorporation of indium in InGaN materials. Furthermore, carrier delocalization, Shockley–Read–Hall recombination, auger recombination, and electron leakage in InGaN light-emitting diodes (LEDs) are the main contributors to efficiency droop. The synthesis of high-quality III-V compounds can be achieved by optimizing growth parameters such as temperature, V/III ratios, growth rate, and pressure. By reducing the ammonia flow from 200 sccm to 50 sccm, increasing the growth rate from 0.1 to 1 m/h, and lowering the growth pressure from 250 to 150 Torr, the external quantum efficiency of III-V compounds can be improved at growth temperatures ranging from 800 °C to 500 °C. It is crucial to optimize the growth conditions to achieve high-quality materials. In addition, novel approaches such as adopting a microrod crystal structure, utilizing the piezo-phototronic effect, and depositing AlN/Al2O3 on top of the P-GaN and the electron-blocking layer can also contribute to improving the external quantum efficiency. The deposition of a multifunctional ultrathin layers of AlN/Al2O3 on top of the P-GaN can enhance the peak external quantum efficiency of InGaN blue LEDs by 29%, while the piezo-phototronic effect induced by a tensile strain of 2.04% results in a 183% increase in the relative electroluminescence intensity of the LEDs. This paper also discusses conventional and inverted p-i-n junction structures of LEDs.

Ultra-low-current driven InGaN blue micro light-emitting diodes for electrically efficient and self-heating relaxed microdisplay

InGaN-based micro-light-emitting diodes have a strong potential as a crucial building block for next-generation displays. However, small-size pixels suffer from efficiency degradations, which increase the power consumption of the display. We demonstrate strategies for epitaxial structure engineering carefully considering the quantum barrier layer and electron blocking layer to alleviate efficiency degradations in low current injection regime by reducing the lateral diffusion of injected carriers via reducing the tunneling rate of electrons through the barrier layer and balanced carrier injection. As a result, the fabricated micro-light-emitting diodes show a high external quantum efficiency of 3.00% at 0.1 A/cm2 for the pixel size of 10 × 10 μm2 and a negligible Jmax EQE shift during size reduction, which is challenging due to the non-radiative recombination at the sidewall. Furthermore, we verify that our epitaxy strategies can result in the relaxation of self-heating of the micro-light-emitting diodes, where the average pixel temperature was effectively reduced.

Broadband Near-Infrared Luminescence in Garnet Y3Ga3MgSiO12: Cr3+ Phosphors.

Broadband near-infrared (NIR) phosphors are the critical component of phosphor converted NIR light-emitting diode (LED) light sources. However, there are still a lack of NIR phosphors with excellent external quantum efficiency (EQE) and thermal stability. Here, we report a highly efficient broadband NIR phosphor Y3Ga3MgSiO12: Cr3+. The optimized phosphor yields an internal quantum efficiency (IQE) and an EQE of 79.9 and 33.7%, respectively. The integrated emission intensity still remains at 84.4% of that at room temperature when heated to 423 K. A broadband NIR LED lamp was made by combining as-prepared phosphor and a blue InGaN LED chip, which shows an output power of 89.8 mW with a photoelectric conversion efficiency of 17.1% driven at 525 mW input power. Our research provides a promising NIR phosphor with high efficiency broadband for the NIR light source.

(Invited) Characterization of Semiconductor Crystals Based on Omnidirectional Photoluminescence (ODPL) Spectroscopy

The performance of optical devices based on nitride semiconductors operating in the ultraviolet and visible wavelength regimes has been remarkably improved. For example, high-efficiency blue InGaN light-emitting diodes (LEDs) with an external quantum efficiency (EQE) of over 80% was achieved [Y. Narukawa, M. Ichikawa1, D. Sanga, M. Sano, and T. Mukai, J. Phys. D: Appl. Phys. 43, 354002 (2010)], and green laser diodes (LDs) was realized [Y. Enya, Y. Yoshizumi, T. Kyono, K. Akita, M. Ueno, M. Adachi, T. Sumitomo, S. Tokuyama, T. Ikegami, K. Katayama and T. Nakamura, Appl. Phys. Express 2, 082101 (2009)]. Recently, deep-ultraviolet AlGaN LEDs and LDs have also become hot topics. The nitride semiconductors have been actively applied not only to optical applications but also to electronic devices such as high-voltage transistors and high-electron mobility transistors based on their high breakdown voltage and saturation electron velocity. The performance of these optical and electronic devices relies on the crystal growth technology, where quality of GaN freestanding substrate is one of the most important issues.In recent years, it has become common for the "high-quality" GaN freestanding crystals to have a threading dislocation (TD) density of 106 cm-2 or less. TD density of 106 cm-2 corresponds to an average distance between TDs of 10 μm, which means that the TDs are sparser than 1 μm, the order of the carrier diffusion length. In other words, the TDs are no longer dominant in recombination processes of minority carriers (although this is a rather broad argument). In this case, the recombination processes, especially nonradiative recombination process, are likely to be dominated by point defects and impurities rather than by the sparse TDs.In fact, a negative correlation between room-temperature (RT) photoluminescence (PL) lifetime for the near-band-edge (NBE) emission and the concentration of VGaVN (a divacancy consisting of Ga and N vacancies) has been pointed out by a combined study of time-resolved PL measurements, positron annihilation spectroscopy, and theoretical calculations [S. F. Chichibu, A. Uedono, K. Kojima, H. Ikeda, K. Fujito, S. Takashima, M. Edo, K. Ueno, and S. Ishibashi, J. Appl. Phys. 123, 161413 (2018)]. Since PL lifetime measured at RT indicates nonradiative recombination lifetime when the internal quantum efficiency (IQE) of the crystal is low, this negative correlation means that the lower concentration of VGaVN induces larger intensity of the NBE emission.Actually, we have observed PL lifetime exceeding 2 ns at RT in a GaN crystal grown by halide vapor phase epitaxy (HVPE) on m-plane GaN grown by the acidic ammonothermal method as a seed crystal [K. Kojima, Y. Tsukada, E. Furukawa, M. Saito, Y. Mikawa, S. Kubo, H. Ikeda, K. Fujito, A. Uedono, and S. F. Chichibu, Appl. Phys. Express 8, 095501 (2015)]. This is an extremely long value for a GaN crystal, which means that the nonradiative recombination lifetime is long, i.e., the IQE of the NBE emission is large. In this case, the validity of the approximation where the PL lifetime at RT is directly read into the nonradiative recombination lifetime becomes an issue.Therefore, in order to compare the quality of GaN freestanding crystals, it is important to measure the IQE as well as the PL lifetime. The IQE value for the NBE emission in semiconductor crystals is a quantity that represents the balance between radiative and nonradiative recombination processes and is directly related to the concentration of point defects and impurities that form deep levels. Therefore, the IQE can be used as an indicator of not only the energy-saving performance of optical devices but also the performance of GaN crystals as a substrate material in electronic devices.In this presentation, the characterization of semiconductors with direct bandgap by using omnidirectional photoluminescence (ODPL) spectroscopy, a technique that can determine the IQE from the EQE without the need for model calculations, will be reviewed. Using GaN, ZnO, and metal halide perovskites as examples, specific procedures and applications of IQE determination will be discussed. In particular, the ability to measure large sample crystals or wafers is one of the major features of the ODPL spectroscopy, and it is expected to be combined with mapping measurements of the entire surface of semiconductor wafers and various types of nonlinear and microscopic spectroscopy.

Broadband Short-Wave Infrared Light-Emitting Diodes Based on Cr3+-Doped LiScGeO4 Phosphor

Short-wave infrared (SWIR) spectroscopy has recently emerged as an important technology across a wide range of areas, whether industrial, biomedical, or environmental. Nevertheless, it is still a long-standing challenge to develop robust SWIR light sources. The SWIR phosphor-convert light emitting diodes (LEDs) by coating blue LED chips with desirable SWIR-emitting phosphors are becoming an ideal alternative for solid-state SWIR light sources due to its compactness, low-cost, and long operating lifetime, as does the commercial white LEDs. Herein, we report a blue-pumped Cr3+-doped LiScGeO4 SWIR phosphor as a luminescent converter for phosphor-convert SWIR LEDs. This phosphor shows an intense SWIR emission band with a peak wavelength at ∼1120 nm owing to the 4T2 → 4A2 electron transition of Cr3+ when exciting with blue light. The full width at half-maximum (fwhm) of the phosphor is ∼300 nm and the absolute quantum efficiency is determined to be ∼26%. SWIR LED prototypes are constructed by combining the optimized phosphor materials with commercial blue InGaN LED chips, which can generate a commendable emission band in the SWIR region over 800-1600 nm and achieve a maximum output power of ∼4.78 mW at 60 mA with the photoconversion efficiency of 4.4%. The current exploration of Cr3+-doped SWIR-emitting phosphors will lay the foundation to engineer phosphor-convert SWIR LEDs for applications in night-vision surveillance and SWIR spectroscopy technology. These blue-light-excitable SWIR-emitting phosphors can serve as an important complement to the spectral gap of the current Cr3+-doped phosphors in the SWIR region and will pave the way toward cost-effective phosphor-converted solid-state SWIR light sources.

Rapid synthesis of Ce3+:YAG via CO2 laser irradiation combustion method: Influence of Ce doping and thickness of phosphor ceramic on the performance of a white LED device

A new, rapid method was reported to produce Y3Al5O12 phosphors doped with different concentrations of Ce3+ (YAG:Ce3+) within 1.5 ​min via continuous-wave CO2 laser irradiation, in which a CO2 laser is utilised as the combustion source. X-ray powder diffraction confirmed the formation of pure cubic phase in the as-synthesised powder. Thus, laser irradiation plays an important role in optimising reaction kinetics to obtain high-quality crystal with high luminescence intensity. Furthermore, the incorporation of Ce3+ ions into the YAG matrix can be enhanced through sintering powders at 1050 ​°C for 5 ​h. Optical properties were examined comparatively using photoluminescence emission. Additionally, the performance of white light-emitting diode (LED) can be regulated easily by altering the Ce3+ doping concentration and YAG:Ce3+ layer thickness on blue InGaN LED chip. Excellent white light, which has a CIE coordinate of (0.31, 0.36) that approached the standard WLED values, a colour rendering index of about 69.4, a correlated colour temperature of about 6299 ​K and a luminous radiation efficiency exceeding 169 lm/W, was obtained for YAG doped with 2 ​mol% Ce concentration with a coating layer thickness of 1 ​mm.

Characterization of semi-polar (20overline{2}1) InGaN microLEDs

In this paper, semi-polar (20overline{2}1) InGaN blue light-emitting diodes (LEDs) were fabricated and compared the performance with those of LEDs grown on c-plane sapphire substrate. LEDs with different chip sizes of 100 μm × 100 μm, 75 μm × 75 μm, 25 μm × 25 μm, and 10 μm × 10 μm were used to study the influence of chip size on the device performance. It was found that the contact behavior between the n electrode and the n-GaN layer for the semi-polar (20overline{2}1) LEDs was different from that for the LEDs grown on the c-plane device. Concerning the device performance, the smaller LEDs provided a larger current density under the same voltage and presented a smaller forward voltage. However, the sidewall’s larger surface to volume ratio could affect the IQE. Therefore, the output power density reached the maximum with the 25 μm × 25 μm chip case. In addition, the low blue-shift phenomenon of semi-polar (20overline{2}1) LEDs was obtained. The larger devices exhibited the maximum IQE at a lower current density than the smaller devices, and the IQE had a larger droop as the current density increased for the LEDs grown on c-plane sapphire substrate.

Bottom tunnel junction blue light-emitting field-effect transistors

A recent thrust toward efficient modulated light emitters for use in Li-Fi communications has sparked renewed interest in visible III-N InGaN light-emitting diodes (LEDs). With their high external quantum efficiencies, blue InGaN LEDs are ideal components for such devices. We report a method for achieving voltage-controlled gate-modulated light emission using monolithic integration of fin- and nanowire-n–i–n vertical FETs with bottom-tunnel junction planar blue InGaN LEDs. This method takes advantage of the improved performance of bottom-tunnel junction LEDs over their top-tunnel junction counterparts, while allowing for strong gate control on a low-cross-sectional area fin or wire without sacrificing the LED active area as in lateral integration designs. Electrical modulation of five orders and an order of magnitude of optical modulation are achieved in the device.

Effect of Interface State Density of the AlGaN Electron Blocking Layer/GaN Barrier Layer in InGaN Blue Light-Emitting Diodes

Herein, two kinds of blue InGaN light-emitting diodes (LEDs) were compared in order to understand the effect of the strain originating from two different sapphire substrates. One of the LED types was grown on a patterned sapphire substrate (PSS), and the other was grown on a conventional sapphire substrate (CSS). The internal strains biased on InGaN quantum wells were compared by studying the time-resolved photoluminescence and the electroreflectance. Through analysis of the dynamic capacitance dispersion measurements, the relation between the trap states and trap density of the AlGaN electron blocking layer/GaN barrier interface above the InGaN quantum wells can be understood in great detail. The trapped charges of the CSSs have faster response times than those of the PSSs; this is attributed to the stronger piezo-field, which induces the AlGaN EBL/GaN barrier interface trap states to be effectively located at shallower levels from the conduction band.

Temperature dependence of current–voltage and carrier lifetime characteristics in InGaN blue light-emitting diode

In this report, we have measured current–voltage (I–V) and carrier lifetime characteristics of an InGaN-based commercial blue light-emitting diode in a temperature range from 350 to 110 K. Two distinctly different current domains, namely, region I and region II have been identified in I–V dependence. I–V data for region I have been used for the evaluation of ideality factor which varies from 1.478 to 3.932 when the temperature changes from 350 to 110 K. We have also investigated how far the existing models account for our experimental results. From an analysis of the Arrhenius plot of I–V–T curves in terms of the model due to Hirsch and Barriere it is found that the domains are distinguished by current transport mechanisms governed by hole thermionic emission and hole tunneling which are discussed in detail. Next, estimating the data on voltage gradient obtained from the open circuit voltage decay method, as well as those of ideality factor from the I–V measurement, the temperature dependence of carrier lifetime has been evaluated. A logarithmic plot of carrier lifetime as a function of inverse temperature shows a weak temperature-dependence below 228 K and this region gives important information regarding the dominant energy level of traps assisting tunneling of current carriers in the device.

Cryostat setup for measuring spectral and electrical properties of light-emitting diodes at junction temperatures from 81 K to 297 K.

We introduce a cryostat setup for measuring fundamental optical and electrical properties of light-emitting diodes (LEDs). With the setup, the cryostat pressure and the LED properties of the forward voltage, junction temperature, and electroluminescence spectrum are monitored with temperature steps less than 1.5 K, over the junction temperature range of 81-297 K. We applied the setup to commercial yellow AlGaInP and blue InGaN LEDs. At cryogenic temperatures, the fine structure of the electroluminescence spectra became resolved. For the yellow LED, we observed the phonon replica at 2.094 eV that was located 87 meV below the peak energy at the junction temperature of 81 K. For the blue LED, we observed the cascade phonon replicas at 2.599 eV, 2.510 eV, and 2.422 eV with the energy interval of 89 meV. For both LED types, the forward voltage increased sharply toward the lower temperatures due to the increased resistivity of materials in the LED components. We found significant differences between the temperature dependent behaviors of the forward voltages, spectral peak energies, and bandgap energies of LEDs obtained from the Varshni formula. We also noted a sharp pressure peak at 180-185 K arising from the solid-vapor phase transition of water when the base level of the cryostat pressure was approximately 0.4 mPa.

Analytical analysis of internal quantum efficiency with polarization fields in GaN-based light-emitting diodes

Abstract This paper presents the analytical model for the analysis of internal quantum efficiency as well as light output power of GaN-based light-emitting diodes by introducing the polarization factor, which accounts for the polarization fields in the active region, in the standard ABC model. With the increase of the polarization, effective volume decreases which causes strong localization of carriers at lower potentials causing the carrier density to increase. This effect leads to loss mechanisms such as carrier leakage and Auger loss. Our theoretical analysis for internal quantum efficiency and light output power shows good agreement with the experimental results for both the blue and green InGaN light-emitting diodes.

Toward Ultra-Low Efficiency Droop in C-Plane Polar InGaN Light-Emitting Diodes by Reducing Carrier Density with a Wide InGaN Last Quantum Well

We demonstrate an ultra-low efficiency droop in c-plane polar InGaN blue light-emitting diodes (LEDs) by reducing the carrier density using a wide InGaN last quantum well (LQW). It is found that the LEDs with a 5.2 nm thick LQW show a negligible efficiency droop, with an external quantum efficiency (EQE) reducing from a peak value of 38.8% to 36.4% at 100 A/cm2 and the onset-droop current density is raised from 3 A/cm2 to 40 A/cm2 as the LQW thickness increases from 3.0 nm to 5.2 nm. The analysis based on the ABC model indicates that small efficiency droop is caused by the reduced carrier density using a wide LQW. The peak efficiency is reduced with a wide LQW, which is caused by the reduction of the electron-hole wavefunction overlap and the deterioration of the crystal quality of the InGaN layer. This study suggests that the application of the InGaN LEDs with a wide LQW can be a promising and simple remedy for achieving high efficiency at a high current density.

InGaN RGB Light-Emitting Diodes With Monolithically Integrated Photodetectors for Stabilizing Color Chromaticity

In this paper, a solution toward realizing color chromaticity stabilized InGaN red, green, and blue (RGB) light-emitting diode (LED) is proposed and demonstrated. The InGaN/GaN multiple quantum wells (play a key role in light emission from the LEDs and photodetection from the photodetectors (PDs). The spectral overlaps between the emission and absorption spectra are measured and the photocurrents of the PDs exhibit linear behavior with increasing LED driving currents. The solution involves the use of RGB chips with monolithically integrated PDs that detect the levels of light output from an individual chip in real time, whose photocurrent signals are fed to LED driver circuits that make use of the signal to provide a driving current that stabilizes the light output. Adoption of this feedback strategy results in CIE coordinates drifts of Δ(0.003, 0.005) over the 400 h duration of testing, proving to be an effective way of stabilizing color chromaticity from RGB LEDs.

Quantum Dot-Acrylic Acrylate Oligomer Hybrid Films for Stable White Light-Emitting Diodes.

White light-emitting diodes (LEDs) have been achieved using photopolymerization. Red and green CdSe/ZnS core–shell quantum dots (QDs) are dispersed in photopolymerized aliphatic acrylic acrylate resins, cured by using 36 W UV light for 1.5 min producing QD–acrylate nanocomposites, and then a hybrid LED device is achieved using the QD–acrylate nanocomposite with two emission wavelengths excited by using a blue InGaN LED chip. The cured QD–acrylate nanocomposite films are characterized by using ultraviolet–visible, fluorescence, scanning electron microscopy, atomic force microscopy, and thermogravimetric analysis measurements. Photopolymerization is conveniently employed to adjust several parameters of the CIE-1931 coordinate (x, y), color temperature, and color rending index (CRI). Good white balance of the red-green hybrid device achieves a luminance of 7976 lm/m2 at a 30 mA working current. The light emission efficiency, CRI, and color temperature of 14%, 47, and 11 204 K, respectively, are attained at this working current.

Red Luminescent Eu(III) Coordination Bricks Excited on Blue LED Chip.

Three types of red luminescent Eu(III) complexes with Schiff base and hfa ligands (hfa: hexafluoroacetylacetonate), mononuclear [Eu(hfa)2(OAc)(salen)2] (OAc: acetate anion, salen: N,N'-bis(salicylidene)ethylenediamine), brick-type [Eu2(hfa)4(OAc)2(salbn)2] (salbn: N,N'-bis(salicylidene)-1,4-butanediamine), and polynuclear [Eu(hfa)2(OAc)(salhen)] n (salhen: N,N'-bis(salicylidene)-1,6-hexanediamine) are reported for white light-emitting diode (LED) devices. Among these complexes, brick-type [Eu2(hfa)4(OAc)2(salbn)2] excited by blue light (460 nm) exhibits the photosensitized quantum yield (Φπ-π* = 47%) and remarkably high efficiency of sensitization (ηsens = 96%). The efficiency of sensitization is caused by the excited state based on ligand-ligand interaction between the Schiff base and hfa ligands in Eu(III) complexes. To fabricate LED devices, the red luminescent [Eu2(hfa)4(OAc)2(salbn)2] was mounted on an InGaN blue LED chip.

Measuring the Internal Quantum Efficiency of Light-Emitting Diodes at an Arbitrary Temperature

We present a method of measuring the internal quantum efficiency (IQE) of light-emitting diodes (LEDs) as a function of driving current at an arbitrary temperature only with experimentally measurable quantities of optical power and current. The measurement procedure starts to find an exact value of the IQE at the reference point where the injection efficiency is considered as 100% and the radiative efficiency is calculated from the infinitesimal change of the relative external quantum efficiency (EQE). Once the IQE at the reference point is exactly known, the IQE at any other point is calculated by the relative ratio of the EQE values to the reference one. We show the theoretical formalism of the proposed method and demonstrate its validity using a commercial InGaN blue LED as an example. The comparison with the IQE obtained by the conventional temperature-dependent electroluminescence is also given for various temperatures.

Relationships between junction temperature, electroluminescence spectrum and ageing of light-emitting diodes

We have developed spectral models describing the electroluminescence spectra of AlGaInP and InGaN light-emitting diodes (LEDs) consisting of the Maxwell–Boltzmann distribution and the effective joint density of states. One spectrum at a known temperature for one LED specimen is needed for calibrating the model parameters of each LED type. Then, the model can be used for determining the junction temperature optically from the spectral measurement, because the junction temperature is one of the free parameters. We validated the models using, in total, 53 spectra of three red AlGaInP LED specimens and 72 spectra of three blue InGaN LED specimens measured at various current levels and temperatures between 303 K and 398 K. For all the spectra of red LEDs, the standard deviation between the modelled and measured junction temperatures was only 2.4 K. InGaN LEDs have a more complex effective joint density of states. For the blue LEDs, the corresponding standard deviation was 11.2 K, but it decreased to 3.5 K when each LED specimen was calibrated separately. The method of determining junction temperature was further tested on white InGaN LEDs with luminophore coating and LED lamps. The average standard deviation was 8 K for white InGaN LED types. We have six years of ageing data available for a set of LED lamps and we estimated the junction temperatures of these lamps with respect to their ageing times. It was found that the LEDs operating at higher junction temperatures were frequently more damaged.

III-nitride ultraviolet, blue and green LEDs with SiO2 photonic crystals fabricated by UV-nanoimprint lithography

In this paper, we report the effect of the SiO2 photonic crystals (PhCs) on the enhancement of light-output power (LOP) in the UV, blue and green InGaN light-emitting diodes (LEDs). The SiO2 PhCs were fabricated by an UV-nanoimprint lithography technique. It is found that the LOP of the LEDs is effectively enhanced by the utilization of the SiO2 PhCs, and the enhancement of the LOP is strongly dependent on the emission wavelength of the active region. As compared to the LEDs without PhC structure, the LOP of the UV, blue and green LEDs with the SiO2 PhCs were enhanced by 27.7%, 14.6% and 40.5%, respectively. Moreover, the dependence of the light extraction efficiency on the emission wavelength was conducted through three-dimensional finite-difference time-domain (FDTD) simulation. The experimental observations were qualitatively consistent with the tendency of the FDTD simulation results.