Sidewall Emission Research Articles

Numerical simulation of deep ultraviolet LED, micro-LED, and nano-LED with different emission wavelengths based on FDTD.

The current low external quantum efficiency (EQE) of deep ultraviolet (DUV) LEDs and micro-LEDs is largely attributed to their low light extraction efficiency (LEE). To address this issue and increase the LEE of DUV devices, various strategies such as reducing size, modifying surface with nanostructures and roughening substrates have been proposed. While some studies have investigated the effects of nanopillar and size on DUV LED, there remains a lack of systematic research on the LEE enhancement mechanism across different wavelengths and sizes of DUV LEDs, micro-LEDs, and nano-LEDs. Therefore, in this study, we employed the numerical simulation method to explore the LEE, near-field intensity distribution, and far-field light intensity distribution from various angles for DUV LEDs, micro-LEDs, and nano-LEDs with wavelengths of 255 nm, 260 nm, and 275 nm, respectively. Our findings reveal a significant improvement in the LEE of DUV nano-LEDs and micro-LEDs, accompanied by reduced divergence angles. Moreover, we observe that longer wavelengths correspond to higher LEE values for devices with similar size. This enhancement in LEE is attributed to factors such as increased sidewall emission and reduced p-GaN absorption. Our investigation indicates that as the size of the DUV device decreases, the sidewall LEE for both transverse electric (TE) and transverse magnetic (TM) modes increases, with TM mode exhibiting a larger enhancement. This enhancement is mainly attributed to the reduction of total reflection within the DUV LEDs and micro-LEDs resulting from size reduction. Despite this, TE mode remains the main contributor to overall LEE. Additionally, our study reveals a reduction in p-GaN absorption of DUV light with decreasing device size, further contributing to the enhancement of LEE in DUV micro-LEDs and nano-LEDs. The increased LEE and reduced divergence angles of small-size DUV micro-LEDs and nano-LEDs not only promote lower power consumption but also enable easier optical system coupling. Consequently, these advancements have significant potential in optical wireless communication, charge management and high-precision lithography.

3.1: Invited Paper: Improving the front emission of Micro‐LED by using a sidewall light guide structure

Micro‐LED is one of the most promising display devices, which has Micron‐sized inorganic structure. The miniaturized size of Micro‐LED leads to a large amount of sidewall emission, which causes crosstalk for display devices. To solve the problem, this paper proposes a novel Micro‐LED structure by designing a sidewall light guide structure where SiO2 and TiO2 layers are successively added to the Micro‐LED's sidewall to form a total reflection layer. Thereby, large part of the light emitting from sidewall can be guided to the front of this structure. The simulation results show that RGB Micro‐LEDs with the sidewall light guide structure can improve the efficiency of front emission in the range of sidewall inclination degree from 0° to 50°. Further, the ratio of front emission to the total emission can be improved to 68%.

Superlattice-Induced Variations in Morphological and Emission Properties of GaInN/GaN Multiquantum Nanowire-Based Micro-LEDs.

Core-shell GaInN/GaN multiquantum shell (MQS) nanowires (NWs) are gaining great attention for high-efficiency micro-light-emitting diodes (micro-LEDs) owing to the minimized etching region on their sidewall, nonpolar or semipolar emission planes, and ultralow density of dislocations. In this study, we evaluated the changes in NW morphologies and the corresponding device properties induced by GaInN/GaN superlattice (SL) structures. The cathodoluminescence intensities of the samples with 20 and 40 pairs of SLs were about 2.2 and 3.4 times higher, respectively, than that of the sample without SLs. The high-resolution scanning transmission electron microscopy (STEM) inspection confirmed that the high growth temperature of SLs prevented growth in the semipolar plane region close to the n-GaN core. A similar phenomenon was also observed for the GaN quantum barriers of the semipolar MQS region under a high growth temperature of 810 °C. This phenomenon was ascribed to the passivation of the semipolar plane surface by hydrogen atoms and the high probability of decomposition through NH3 or N-H-related bonds. Although no clear SL grew on the semipolar plane near the n-core region, the top area of the nonpolar plane SL was expected to adequately suppress the point defects propagating from the n-GaN core to the semipolar plane MQS. The electroluminescence (EL) spectra and light output curves demonstrated a clear enhancement of more than 3-folds compared to the fabricated micro-LEDs without SL structures, which was associated with the improved crystalline quality of the MQS and enlarged area of the semipolar planes. Moreover, by increasing the growth time of GaN quantum barriers, the EL emission intensity of the micro-LED devices exhibited a 4-fold improvement owing to the reduced carrier overflow in the thickened GaN barriers on the semipolar (11̅01) planes. Thus, the results verified the possibility of realizing highly efficient NW-based micro-LEDs by optimizing the NW morphology using SL structures.

Design of inclined omni-directional reflector for sidewall-emission-free micro-scale light-emitting diodes

Micro-scale light emitting diode (µLED) reduces the front self-luminous display area by orders of magnitude, which brings the increase of sidewall emission and the resulting crosstalk between adjacent sub-pixels of displays. Reflector coating on the µLED’s sidewall is effective for the collection and utilization of side emission light. However, the existing multilayer periodic dielectric mirrors are undesirable for all angular reflection. In this paper, an omni-directional reflector (ODR) is designed for the µLED display pixel to effectively utilize light leakage from the sidewall emission and improve the total light extraction efficiency (LEE). The designed ODR is insensitive to the angular distribution of the incident light, and has the capability to reflect the light reaching the µLED’s sidewall into the escape cone for front emission. By introducing the two-dimensional finite-difference time-domain method, the µLED laminated structure is established to analyze the sidewall emission with an optimal inclination angle. By introducing the ODR for the inclined µLED pixel, the overall LEE can be increased by 2.24 times and the sidewall emission is reduced by 99.6% compared with conventional vertical µLEDs. Simulation and experimental results are in good agreement. The proposed ODR is expected to achieve the increase of LEE and sidewall-emission-free µLED displays.

Enhanced Light Extraction of DUV LEDs by Using a Quartz Lens With Three-Dimensional Structure

AlGaN based deep ultraviolet light-emitting diodes (DUV LEDs) have wide applications in many fields, such as air and water purification, disinfection, and biochemical inspection. However, the wall plug efficiency (WPE) of DUV LEDs is relatively low for these applications, which is due to the low light extraction efficiency (LEE) yielding limited light output from LED package. In this work, we proposed a quartz lens with three-dimensional (3D) structure to enhance the light extraction of DUV LEDs. The optical models of DUV LEDs were established and the optical performances of DUV LEDs were investigated by using optical simulation. The DUV LEDs with the 3D quartz lens exhibit higher light efficiency than those with traditional quartz lens structure. Compared with the traditional structure, the light output power of the 3D lens structure is enhanced by 25.93% at the current of 140 mA and the packaged DUV LED can achieve a wide light angle of 153.2°. It is attributed to the significant light extraction enhancement of the sidewall emission from DUV LED chip.

Morphological effect of patterned sapphire substrate on efficiency of white-light phosphor LED package

Six-facets patterned sapphire substrates (A-PSS) are manufactured by wet-etching regular semispherical patterned sapphire substrates (r-PSS). The possible crystallographic planes of the etched planes on the semi-sphere pattern is the family of (1102) plane. Metalorganic Chemical Vapor Deposition (MOCVD) GaN epitaxial LED structure was grown on the c-plane sapphire wafer, r-PSS, and A-PSS, which were further processed as horizontal LED chips. Compared to LEDs on the flat c-plane sapphire, the light output power of LEDs on r-PSS and A-PSS is enhanced by 71% and 67%, respectively. The output power of the bare LEDs decreases after the white-light LED package. Remarkably, the output power of the white-light packaged LED on the A-PSS is larger than the white-light packaged LED on the r-PSS. The Monte Carlo ray tracing simulation shows that, before and after silicone-lens encapsulating, the LEDs on r-PSS and A-PSS emit more on the top surface. However, the sidewall emission power ratio of the LED on r-PSS decreases by 3.44% after silicone encapsulating. On the other hand, the sidewall emission power ratio of the LED on A-PSS increases by 1.06% after silicone encapsulating. Note that the emission light of LEDs out of the top surface travels a longer path in the silicone lens than the emission light out of the sidewalls. Hence, more re-extracted light out of the sidewall of LED on A-PSS would be the reason for the LED on A-PSS having a less package loss and the white-light package efficiency than the LED on r-PSS. In conclusion, the light radiation distribution pattern defined by the pattern morphology of A-PSS is the key for the better white-light package efficiency of the LED on A-PSS than that of the LED on r-PSS. • The patterned sapphire substrate morphology can alter the propagation direction of the light emission. • Monte Carlo ray tracing simulation shows the emission power of top surface and sidewall vary before and after lens package. • The emission light of LEDs out of the top surface travels a longer path in the phosphor layer than the sidewalls. • The light radiation distribution defined by the pattern morphology is the key for white-light package efficiency of LED.

Subwavelength structure enabled ultra-long waveguide grating antenna.

Because of the high index contrast, current silicon photonics based optical phased arrays cannot achieve small beam divergence and large field-of-view simultaneously without increasing fabrication complexity. To resolve the dilemma, we propose an ultra-long waveguide grating antenna formed by placing subwavelength segments within the evanescent field of a conventional strip waveguide. Bound state in the continuum effect is leveraged to suppress the sidewall emission. As a proof of concept, we theoretically demonstrated a millimeter-long through-etched waveguide grating antenna with a divergence angle of 0.081° and a feature size compatible with current silicon photonics foundries.

Diffraction engineering for silicon waveguide grating antenna by harnessing bound state in the continuum

Abstract Silicon waveguide grating antennas (SWGAs) have been widely employed to interface the guided and radiation modes in various integrated photonic systems. However, ultrasmall feature sizes or heteromaterial integrations are usually required to obtain long propagation length and small far-field divergence. Moreover, for conventional SWGAs, the diffraction strength is wavelength sensitive, so the output power and far-field divergence will deviate in the beam steering process. In this paper, we propose and demonstrate a novel approach to engineer the diffraction in SWGA by harnessing the bound state in the continuum (BIC). A new degree of freedom is attained in diffraction engineering by introducing the “modified” diffraction formula. The side-wall emission can be dramatically depressed by building the quasi-BIC at critical waveguide width, leading to ultrauniform diffraction. The extremely weak diffraction strength (~3.3 × 10−3 dB/μm) has been experimentally realized for the fabricated device with a large feature size (~60 nm). From the measurement results, one can predict a centimeter-scale propagation length and an ultrasmall divergence (~0.027°). Moreover, the diffraction strength dispersion can be flattened for SWGA with critical waveguide width. Such effect has also been experimentally verified. Our proposed design is the first one that introduces the BIC effect into SWGA optimization, paving the way for precise diffraction engineering and high-performance integrated optical antennas.

Light Extraction Analysis of AlGaInP Based Red and GaN Based Blue/Green Flip-Chip Micro-LEDs Using the Monte Carlo Ray Tracing Method.

Micro-scale light emitting diodes (micro-LEDs) commonly employ a thin-film flip-chip (TFFC) structure whose substrate is lifted off by an excimer laser. However, flip-chip (FC) micro-LEDs with a substrate can provide a sharp rise on sidewall emission by increasing the sidewall area. Here, we investigate the influence of substrate thickness, encapsulation, surface texture, microstructures between the substrate and epilayer, as well as the size, cutting shape, and angle of the chip on the light extraction efficiencies (LEEs) of FC micro-LEDs by using the Monte Carlo ray tracing method. We find that the LEE of the blue FC micro-LED chip increases by 46.5% over that of the blue TFFC micro-LED chip. After the encapsulation with the epoxy lens is applied, the LEEs of the blue TFFC micro-LED and blue FC micro-LED increase by 129% and 110.5%, respectively. The underlying mechanisms for the use of surface texture, patterned sapphire substrate, air-void array, and chip shaping technologies to improve the LEEs of FC micro-LEDs are also investigated in detail. We find that the LEEs AlGaInP based red FC micro-LED and GaN based blue/green FC micro-LEDs exhibit a sharp rise when the chip size drops from 30 to 10 µm. The inverted trapezoid FC micro-LED with patterned sapphire substrate (PSS) and encapsulation shows extraordinarily strong top emission and high collimation. We believe that our study offers a promising and practical route for obtaining high efficiency micro-LEDs.

Angular color shift of micro-LED displays.

Sidewall emission of a micro-scale light emitting diode (micro-LED) improves the light extraction efficiency, but it causes mismatched angular distributions between AlGaInP-based red micro-LED and InGaN-based blue/green counterparts due to material difference. As a result, color shift of RGB micro-LED displays may become visually noticeable. To address this issue, we first analyze the angular distributions of RGB micro-LEDs and obtain good agreement between simulation and experiment. Next, we propose a device structure with top black matrix and taper angle in micro-LEDs, which greatly suppresses the color shift while keeping a reasonably high light extraction efficiency.

Enhanced light extraction of deep ultraviolet light-emitting diodes by using optimized aluminum reflector

Deep ultraviolet light-emitting diodes (DUV-LEDs) have become a promising UV light source for sterilization, disinfection, and purification. However, the challenge in practical application of DUV-LEDs still remains in their low light efficiency. In this paper, we propose an optimized aluminum (Al) reflector for the light extraction enhancement of DUV-LEDs. The optical model of DUV-LEDs was established, and the optical simulations were performed to achieve the optimized reflector. The DUV-LEDs exhibit the highest light efficiency when the reflector has the optimized feature sizes with an angle of 60°, a height of 2.0mm, and an internal radius of 2.5mm. Furthermore, the optimized reflector with different reflectance was fabricated and applied for the packaging of DUV-LEDs. Consequently, the light output powers of DUV-LEDs are enhanced by 28.8%, 37.0%, and 43.8%, respectively, by using the different reflectors at the driving current of 100mA. It is attributed to the remarkable reflection effect of the Al reflector, which increases the light extraction of the sidewall emission from the DUV-LED chip.

Performance analysis of solar thermophotovoltaic conversion enhanced by selective metamaterial absorbers and emitters

By converting broadband sunlight into narrowband thermal radiation matched to the bandgap of thermophotovoltaic (TPV) cells, solar thermophotovoltaic (STPV) systems could potentially reach a high conversion efficiency far exceeding the Shockley–Queisser limit. However, actual STPV systems exhibit much lower efficiency due to non-idealities in solar absorbers, thermal emitters and TPV cells. In this work, the STPV system with selective metamaterial solar absorber and emitter is investigated, whose conversion efficiency is between 8% and 10% with concentration factor varying between 20 and 200. This conversion efficiency is remarkably enhanced compared with the conversion efficiency of less than 2.5% for the STPV system employing black absorbers and emitters. The sidewall emission losses from the absorber–emitter module and the non-unity view factor between the thermal emitter and TPV cell will diminish the performance of the STPV system, whose effects are also quantitatively discussed in this work. Furthermore, the non-planar STPV systems with larger emitter–absorber area ratios are investigated, whose conversion efficiency can reach up to 12.6% under 200 suns when the emitter is four times as large as the absorber.

Continuum correlations in accreting X-ray pulsars

Two-dimensional structure of accretion columns in the radiation-diffusion limit is calculated for two possible geometries (filled and hollow cylinder) for mass accretion rates Ṁ ranging from 1017 to 1.2 x 1018 g s-1. The observed spectral hardening in the transient X-ray pulsars with increasing Ṁ can be reproduced by a Compton-saturated sidewall emission from optically thick magnetized accretion columns with taking into account the emission reflected from the neutron star atmosphere. At Ṁ above some critical value Ṁcr ∼ (6 — 8) x 1017 g s-1, the height of the column becomes such that the contribution of the reflected component to the total emission starts decreasing, which leads to the saturation and even slight decrease of the spectral hardness. Hollow-cylinder columns have a smaller height than the filled-cylinder ones, and the contribution of the reflected component in the total emission does not virtually change with Ṁ (and hence the hardness of the continuum monotonically increases) up to higher mass accretion rates than Ṁcr for the filled columns.

Regularly patterned multi-section GaN nanorod arrays grown with a pulsed growth technique

The growth of regularly patterned multi-section GaN nanorod (NR) arrays based on a pulsed growth technique with metalorganic chemical vapor deposition is demonstrated. Such an NR with multiple sections of different cross-sectional sizes is formed by tapering a uniform cross section to another through stepwise decreasing of the Ga supply duration to reduce the size of the catalytic Ga droplet. Contrast line structures are observed in either a scanning electron microscopy or transmission electron microscopy image of an NR. Such a contrast line-marker corresponds to a thin Ga-rich layer formed at the beginning of GaN precipitation of a pulsed growth cycle and illustrates the boundary between two successive growth cycles in pulsed growth. By analyzing the geometry variation of the contrast line-markers, the morphology evolution in the growth of a multi-section NR, including a tapering process, can be traced. Such a morphology variation is controlled by the size of the catalytic Ga droplet and its coverage range on the slant facets at the top of an NR. The comparison of emission spectra between single-, two-, and three-section GaN NRs with sidewall InGaN/GaN quantum wells indicates that a multi-section NR can lead to a significantly broader sidewall emission spectrum.

On the dependence of the X-ray continuum variations with luminosity in accreting X-ray pulsars

Using RXTE/ASM archival data, we investigate the behaviour of the spectral hardness ratio as a function of X-ray luminosity in a sample of six transient X-ray pulsars (EXO 2030+375, GX 304-1, 4U 0115+63, V 0332+63, A 0535+26 and MXB 0656-072). In all sources we find that the spectral hardness ratio defined as $F_{5-12\mathrm{keV}}/ F_{1.33-3\mathrm{keV}}$ increases with the ASM flux (1.33--12 keV) at low luminosities and then saturates or even slightly decreases above some critical X-ray luminosity falling into the range $\sim(3-7)\times10^{37}$~erg~s$^{-1}$. Two-dimensional structure of accretion columns in the radiation-diffusion limit is calculated for two possible geometries (filled and hollow cylinder) for mass accretion rates $\dot M$ ranging from $10^{17}$ to 1.2$\times 10^{18}$~g s$^{-1}$. The observed spectral behaviour in the transient X-ray pulsars with increasing $\dot M$ can be reproduced by a Compton saturated sidewall emission from optically thick magnetized accretion columns with taking into account the emission reflected from the neutron star atmosphere. At $\dot M$ above some critical value $\dot M_{cr}\sim (6-8)\times 10^{17}$~g~s$^{-1}$, the hight of the column becomes such that the contribution of the reflected component to the total emission starts decreasing, which leads to the saturation and even slight decrease of the spectral hardness. Hollow-cylinder columns have a smaller height than the filled-cylinder ones, and the contribution of the reflected component in the total emission does not virtually change with $\dot M$ (and hence the hardness of the continuum monotonically increases) up to higher mass accretion rates than $\dot M_{cr}$ for the filled columns.

M-Plane Core–Shell InGaN/GaN Multiple-Quantum-Wells on GaN Wires for Electroluminescent Devices

Nonpolar InGaN/GaN multiple quantum wells (MQWs) grown on the {11-00} sidewalls of c-axis GaN wires have been grown by organometallic vapor phase epitaxy on c-sapphire substrates. The structural properties of single wires are studied in detail by scanning transmission electron microscopy and in a more original way by secondary ion mass spectroscopy to quantify defects, thickness (1-8 nm) and In-composition in the wells (∼16%). The core-shell MQW light emission characteristics (390-420 nm at 5 K) were investigated by cathodo- and photoluminescence demonstrating the absence of the quantum Stark effect as expected due to the nonpolar orientation. Finally, these radial nonpolar quantum wells were used in room-temperature single-wire electroluminescent devices emitting at 392 nm by exploiting sidewall emission.

Ring-shaped field emission patterns from carbon nanotube films

Highly symmetric ring-shaped field emission patterns were observed from broad-area flat cathodes prepared by growing a film of vertically aligned carbon nanotubes (CNTs) on TiN coated Si substrates. The images were obtained utilizing a luminescent screen of a specially designed triode cell composed of parallel electrodes. The emission rings sporadically appeared during voltage scans in which the emission patterns and cathode currents were recorded. The fine structure and stability of the rings suggests that their formation is due to an emission state of an individual CNT. The observed patterns are consistent with models that predict the formation of emission rings produced by the inhomogeneous electron emission from CNTs. The macroscopic value of the electric field when the rings were observed was between 0.7 and 2.5 V/μm, and the emission current corresponding to individual rings was estimated to be in the range of 2–4 μA. Numerical simulation of electron trajectories for sidewall emission from similar shaped metallic structures is in qualitative and quantitative agreement with the experimentally observed ring-shaped field emission patterns. The results also appear consistent with a recent model [Marchand M, Journet C, Adessi C, Purcell ST. Phys Rev B 2009;80:245425] based on thermal-field emission due to Joule heating.

MnSb‐based spin LED with side‐wall emission

AbstractCombination of the spin‐dependent optical transition and the spin current would lead us to the opportunity of developing photo‐ and light‐emitting diodes that respectively senses and emits circularly polarized light. Here, we describe preparation of spin LED consisting of an epitaxial MnSb spin injection layer and GaAs‐AlGaAs double hetero‐structures, and show characteristics of EL from a side wall emission. Taking advantage of remnant state of ferromagnet, circular polarization emission has been realized without application of an external field. Demonstration of electrical switching of circular polarization has been carried out with alternative switching between two spin‐LEDs having antiparallel magnetization configuration. While the technique used in the present demonstration was a crude one, it certainly show the feasibility of building a compact and robust light source whose circular polarization is controlled electrically. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of Residual Stress and Sidewall Emission of InGaN-Based LED by Varying Sapphire Substrate Thickness

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The effect of residual stress and the sidewall emission in InGaN–GaN films with different thickness of sapphire substrate were investigated. The peak wavelength of electroluminescence was blue-shifted as thinning the sapphire substrate, because the wafer bowing-induced mechanical stress alters the piezoelectric field in the InGaN–GaN multiple quantum-well active region of the light-emitting diode (LED). A sideview LED with 170-<formula formulatype="inline"><tex Notation="TeX">$\mu\hbox{m}$</tex> </formula>-thick sapphire exhibited the highest output power of 14.04 mW at a forward current of 20 mA, improved by 7% compared to that with 80-<formula formulatype="inline"><tex Notation="TeX">$\mu\hbox{m}$</tex></formula>-thick sapphire. The maximum output power can be obtained by considering both the photon escaping probability from the edges of the sapphire and the photon absorption probability in the sapphire as well as the residual mechanical stress induced by the wafer bowing. </para>

Enhanced light collection of GaN light emitting devices by redirecting the lateral emissionusing nanorod reflectors

We demonstrate a method of utilizing self-assembled nanorod array reflectorsto collect the laterally propagating guided modes from a light emitting diode(LED). We measure an enhancement factor of 12.2% and 18.4%, respectively,from the sidewall emission of GaN-based LEDs encompassed with 10 and20 µm thick nanorod array reflectors. Such enhancement is found to be omnidirectional due to abroken symmetry from a randomized distribution of the nanorod array placed along theperiphery of the LED’s mesa. These observations indicate that the use of nanorod reflectorscan efficiently redirect the propagation of the laterally guided modes to the surface normaldirection.