Surface Of Light-emitting Diodes Research Articles

Enhanced efficiency of GaN-based light-emitting diodes with periodic textured Ga-doped ZnO transparent contact layer

GaN-based light-emitting diodes (LEDs) with indium tin oxide (ITO)/Ga-doped ZnO (GZO) composite oxide films serving as a transparent contact layer (TCL) were demonstrated. In this study, the wall-plug efficiency of LEDs (LED-III) with textured ITO/GZO composite TCL can be markedly improved by 200% and 45% of magnitude as compared to conventional LEDs with Ni∕Au TCL(LED-II) and planar ITO/GZO TCL(LED-I), respectively. Compared to LED-II, this enhancement is due to the enhanced light extraction efficiency of ITO/GZO composite TCL with high transparency. Compared to LED-I, ZnO-based TCL with a higher refractive index (n∼2.0) allows further enhancement of light extraction through the creation of a textured structure on transparent conductive oxide TCL deposited on the top surface of LEDs. In addition, the ITO/GZO composite TCL with a thickness of 550nm is far larger than that of Ni∕Au TCL with a thickness of approximately 15nm. Therefore, in addition to the effect of high transparency, the thicker ITO/GZO TCL with low lateral resistance would also act as a current-spreading layer leading to an enhancement of light extraction.

GaN-based Indium‐tin-oxide light emitting diodes with nanostructured silicon upper contacts

GaN-based indium-tin-oxide (ITO) light emitting diodes (LEDs) with p-GaN, n + -short period superlattice (SPS) and nanostructured silicon contact layers were fabricated. It was found that surface of the ITO LED with nanostructured silicon layer was very rough. It was also found that 20 mA forward voltages measured from ITO LEDs with p-GaN, n + -SPS and nanostructured silicon contact layers were 6.01, 3.25 and 3.26 V, respectively. Compared with ITO LED with n + -SPS, it was found that output power of ITO LED with nanostructured silicon contact was 17% larger. Furthermore, it was found that ITO LED with nanostructured silicon contact was more reliable.

High Illumination Efficiency Linear Light Source Using Light Emitting Diodes

We propose a linear light source using light emitting diode (LED) and a light diffusion bar for linear illumination. In order to get high illumination efficiency, the following factors of the illumination system have been investigated: (1) the effect of the diffusion area, (2) the effect of the relative size of LED and the light diffusion bar. In conclusion, a bigger diffusion size will produce higher emitting efficiency, and a small relative size of emitting surface of LED and cross section of the light diffusion bar will also achieve higher emitting.

Hybrid inorganic/organic microstructured light-emitting diodes produced using photocurable polymer blends

Light-emitting diodes (LEDs) in the form of a one-dimensional array of microstripes emitting at 370nm were fabricated from AlInGaN inorganic semiconductor. These microlight sources were then used to “directly write” microstructures in photocurable blends of organic light-emitting polymers (LEPs) spin coated onto the LED surface. In this way, thin microstripes of LEP as narrow as 50μm have been fabricated and integrated with the micro-LEDs. These “self-aligned” polymer microstripes serve as wavelength downconverters under further excitation by the UV micro-LEDs, producing hybrid inorganic/organic microstructured LEDs.

APPLYING DISCRETE COSINE TRANSFORM AND GREY RELATIONAL ANALYSIS TO SURFACE DEFECT DETECTION OF LEDS

ABSTRACT Light-emitting diodes (LEDs), owing to their lower power requirements, higher efficiency, and longer lifetimes, are widely used in modern electronic devices. Nevertheless, tiny defects that often appear in the surface of LEDs impair not only their appearances but also their functions. This paper proposes a global approach for the automated visual inspection of tiny surface defects in non-diffused LED encapsulations. The proposed method, taking advantage of the discrete cosine transform (DCT) and grey relational analysis techniques, overcomes the difficulties of inspecting tiny defects on uneven illumination images of LEDs. We apply the grey relational analysis to the frequency components in the DCT domain, and select the large-magnitude frequency components that represent the background texture of the surface. Then, by reconstructing the frequency matrix without the selected frequency values, we eliminate not only random texture but also uneven illumination patterns and retain anomalies in the restored image. Experimental results demonstrate the effectiveness of the proposed method in inspecting tiny defects in non-diffused LED surfaces.

Light Extraction Simulation of Surface-Textured Light-Emitting Diodes by Finite-Difference Time-Domain Method and Ray-Tracing Method

The light extraction efficiency (ηl) of surface-textured light-emitting diodes (LEDs) is calculated using a simulation based on the combination of the finite-difference time-domain (FDTD) method and the ray-tracing method. Considering the transmittance and light-scattering effects, which depend on the size of the surface-textured structure (STS), we investigated the dependence of ηl on the size of the STS including the period and aspect ratio. As a result, a clear dependence on the size of the STS has been shown. Furthermore, by optimizing the size of the STS, we found ηl to be about 4 times larger than that of the surface flat structure (SFS). In addition, we demonstrated that the absorption coefficient of the GaN layer (αGaN) affected ηl of the STS more strongly than that of the SFS. We suggest that both wide light-scattering and a high GaN crystalline quality are important points for improving ηl when texturing the top surface of LEDs.

Light-polarization characteristics of electroluminescence from InGaN∕GaN light-emitting diodes prepared on (112¯2)-plane GaN

Light polarization and emission spectra from InGaN∕GaN quantum-well light-emitting diodes (LEDs) were investigated. The LEDs were prepared on the (112¯2) plane of wurtzite GaN. Polarization and spectrum measurement was performed at different observation angles with respect to the LED surface. Partially polarized electroluminescence was confirmed at any angle of observation, where the emission intensity tended to be greater when a polarizer was aligned along the c axis of the InGaN∕GaN LED structure. The results clearly indicated the inclination of the c axis relative to the LED surface. As a result, two light polarizations were identified and they were assigned to two different electronic transitions in relation to emission peak energies. Possible alteration of the valence-band structure was suggested due to the induced strain.

Enhance the Luminance Intensity of InGaN-GaN Light-Emitting Diode by Roughening both the p-GaN Surface and the Undoped-GaN Surface Using Wafer Bonding Methods

An InGaN-GaN light emitting diode (LED) with double roughened surfaces was fabricated by surface-roughening, wafer-bonding and laser lift-off technologies. It was found that the frontside luminance intensity of double roughened LED was 2.77 times higher than that of the conventional LED at an injection current of 20 mA. The backside luminance intensity was 2.37 times higher than that of the conventional LED. This is because the double roughened surfaces can provide photons multiple chances to escape from the LED surface, and redirect photons, which were originally emitted out of the escape cone, back into the escape cone.

Light-Output Enhancement of GaN-Based Light-Emitting Diodes by Photoelectrochemical Oxidation in H2O

The light output of GaN-based light-emitting diodes (LEDs) was improved by an oxide film being directly grown on the p-GaN surface utilizing photoelectrochemical (PEC) oxidation via H2O. The light outputs of the LEDs were enhanced by approximately 16 and 37% after 30 and 45 min PEC oxidation, respectively, compared to those of a conventional LED at 20 mA, and the PEC oxidized LEDs exhibited almost the same dynamic resistance (R=dV/dI) as conventional LEDs without PEC oxidation. Atomic force microscopy (AFM) data show that the roughness of the interface between oxide film and p-GaN increases with oxidation time. In addition, the oxide film on the top surface of the GaN-based LED caused the antireflection effect and the changes of the surface effective refractive indices consequently increased the critical angle and in caused more light to be emitted from the GaN-based LED.

Detection of high-speed voltage waveforms in GaN devices using electric-field-induced second-harmonic generation

We use electric-field-induced second-harmonic generation (EFISHG) to measure high-speed voltage waveforms in commercial GaN light-emitting diodes (LEDs). The output of an ultrafast passively mode-locked Ti : sapphire laser is reflected from the surface of a GaN UV LED. Copropagating with the specularly reflected Ti : sapphire fundamental is second-harmonic light that is proportional to and synchronous with a high-speed voltage pulse that reverse biases the LED. The measured amplitude of the detected pulse was found to vary with dc bias across the p-n junction of the LED indicating that the detected voltage waveform was localized to the p-n junction. Because of synchronous detection, the detection bandwidth (<10 THz) is limited only by the pulsewidth of the probe laser pulse (<100 fs)

Improved luminance intensity of InGaN–GaN light-emitting diode by roughening both the p-GaN surface and the undoped-GaN surface

The InGaN–GaN epitaxial films were grown by low-pressure metal-organic chemical vapor deposition on a sapphire substrate, and then the light-emitting diode (LED) with double roughened (p-GaN and undoped-GaN) surfaces was fabricated by surface-roughening, wafer-bonding, and laser lift-off technologies. It was found that the front side luminance intensity of double roughened LED was 2.77 times higher than that of the conventional LED at an injection current of 20mA. The backside luminance intensity was 2.37 times higher than that of the conventional LED. This is because the double roughened surfaces can provide photons multiple chances to escape from the LED surface, and redirect photons, which were originally emitted out of the escape cone, back into the escape cone.

InGaN light-emitting diodes with naturally formed truncated micropyramids on top surface

GaN-based light-emitting diodes (LEDs) with truncated micropyramids surfaces performed by metalorganic chemical vapor deposition were demonstrated. In this study, a growth-interruption step and an Mg-treatment process were simultaneously performed to create multiple truncated micropyramids on LED surface. Experimental results indicated that GaN-based LEDs with the truncated micropyramids on the top surface demonstrate improved external efficiency of around 60% at 20mA. It is worth noting that the typical 20mA driven forward voltage is only 0.15V higher than that of conventional LEDs (LEDs with specular surface).

GaN-based LEDs with Al-deposited V-shaped sapphire facet mirror

A GaN-based light-emitting diodes (LEDs) with V-shaped sapphire facet reflector was fabricated using the double transferred scheme and sapphire chemical wet etching. The {1-102} R-plane V-shaped facet reflector with a 57/spl deg/ against {0001} C-axis has the superior capability for enhancing the light extraction efficiency. The light output power of the V-shaped sapphire facet reflector LED was 1.4 times higher than that of a flat reflector LED at an injection current of 20 mA. The significant improvement is attributable to the geometrical shape of sapphire facet reflector that efficiently redirects the guided light inside the chip toward to the top escape-cone of the LED surface.

AlGaInP Light-Emitting Diodes with Stripe Patterned Omni-Directional Reflector

An n-side-up AlGaInP-based light-emitting diode (LED) with a stripe-patterned omni-directional reflector (ODR) was fabricated by adopting the adhesive-layer bonding scheme. Comparing to the conventional ODR LED, the stripe-patterned ODR LED significantly enhanced the output power with an only slight higher forward voltage. This improvement was analyzed by the scanning near-field optical microscope (SNOM) and could be attributable to the geometrical shape of stripe patterns that redirects the guided light towards to the escaping cone of the LED surface. The optimized dimension of stripe patterns was also calculated by simply modeling a LED dice with a stripe-patterned ODR structure, according to optical ray-tracing method.

Calculation of the external quantum efficiency of light emitting diodes with different chip designs

External quantum efficiency of GaN light emitting diode (LED) with SiO2-coated grating patterned substrate and/or surface-textured structure were calculated theoretically. Our results show that the enhancement of the reflectance coefficient was obtained by a factor of about 30% in LED chip with SiO2-coated grating patterned substrate, compared to conventional LED chip. The grating parameters such as height and width should be optimized above h=w/4. Moreover, by employing surface-textured structure, the loss factor at top surface of LED was reduced dramatically by decreasing of total internal reflectance at GaN-epoxy interface as the incident angle is higher than the critical angle. Therefore, external quantum efficiency can reach up to about 60% if internal quantum efficiency corresponds to about 90% because of reducing the total loss parameter at top and bottom surface of LED. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

TiN Enhancement of Output Performance for Light-Emitting Diodes under High Injection Current

Thin titanium nitride (TiN) films with low sheet resistance and high transparency are deposited on AlGaInP light-emitting diodes (LEDs) to improve light extraction from the LED surface. Comparison test devices are fabricated both with and without TiN spreading layers. At a high injection current, the TiN film spreads the current over a large area of the device and improves the distribution of light emission. Therefore, the current crowding effect is reduced and light-output power is increased. Background theory is discussed and experimental results are presented. It is shown by theory and experiment that the transparent conducting TiN films can be used as inexpensive, easily implemented current-spreading layers for improved global reliability and efficiency of AlGaInP LEDs.

Titanium nitride as spreading layers for AlGaInP visible LEDs

Thin titanium nitride (TiN) films with low sheet resistance and high transparency were deposited on AlGaInP light-emitting diodes (LEDs) to improve light extraction from the LED surface. Comparison test devices were fabricated both with and without TiN spreading layers. Results show LED current crowding at high current is reduced for devices with TiN current spreading film, improving external efficiency. It is confirmed that TiN films are feasible as current spreading layers of AlGaInP LEDs.

Characteristics of polythiophene surface light emitting diodes

Surface light emitting diodes (SLEDs), in which previously microfabricated electrodes were coated with a conjugated polymer, were made with greatly different electrode spacings (250 nm and 10 or 20 μm) and with different electrode material combinations. The fabrication process allowed us to compare several electrode materials. The SLED structures also enabled imaging of the light emission zone with fluorescence video microscopy. Conventional sandwich structures were also made for comparison (electrode separation 50 nm). In this study, the emitting layer was poly[3-(2′,5′-bis(1″,4″,7″trioxaoctyl)phenyl)-2,2′-bithiophene] (EO–PT), a conjugated polymer based on polythiophene with oligo(ethyleneoxide) side chains. The current–voltage ( I( V)) and light–voltage ( L( V)) characteristics of the SLEDs were largely insensitive to electrode separation except at high voltages, at which the current in the devices with the largest separations was limited. Sandwich structures had the same light output at a given current. Light could be obtained in forward and reverse bias from indium tin oxide (ITO)–aluminum, gold silicide–aluminum, and gold silicide–gold SLEDs, but the turn-on voltages were lowest with the ITO–aluminum devices, and these were also the brightest and most reliable. Adding salt to the EO–PT increased the current and brightness, decreased the turn-on voltages, and made the I( V) characteristics symmetric; thus, a device with an electrode separation of 10 μm had the extraordinarily low turn-on voltage of 6 V. The location of the light emission was at the electron-injecting contact.

Sensitivity of Polythiophene Planar Light-Emitting Diodes to Oxygen

Surface light emitting diodes (SLEDs) with a polymer-on-top geometry were used to study the sensitivity of light emission to oxygen. In these devices, pre-fabricated electrodes were coated with a conjugated polymer, which was thus directly exposed to the environment. Oxygen caused an immediate ten-to hundred fold decrease in electroluminescence efficiency relative to that in nitrogen or argon. Above the voltage for light emission, there was a sharp increase in current. Removing the oxygen led to recovery of the light intensity over a period of minutes, but the current returned immediately to its lower, original level. The electroluminescence and photoluminescence spectra were identical and were unaltered in shape by oxygen exposure (only decreasing in size). However, photoluminescence was unaffected by oxygen alone. This result indicates that oxygen does not affect excitons directly, but rather influences an intermediate species on the path to exciton formation, one that is significant only in electroluminescence and not in photoluminescence. Under simultaneous exposure to oxygen and UV light, the photoluminescence irreversibly decreased, presumably due to photo-oxidation

Planar microfabricated polymer light-emitting diodes

Conjugated polymers are organic semiconducting materials that can emit light. These polymers have the advantages of being light, cheap and easy to process, and in addition the band gap can be tailored. We report the microfabrication of surface light-emitting diodes (SLEDs) on silicon substrates in which the electrodes are underneath the organic electroluminescent layer. Patterned electrodes are separated by a 2500 Å thick insulating layer of silicon oxide or are interdigitated with a separation of 10 or 20 m; the luminescent polymer is spin coated or solvent cast on top of the electrodes. This fabrication method is completely compatible with conventional silicon processing because the polymer is deposited last and the light is emitted from the upper surface of the diodes. Despite the large spacing between electrodes, and despite the absence of an evaporated top contact, the voltages required for light emission were not much greater than those used in conventional sandwich-type structures.