Light Output Power Research Articles

Temperature-dependent study on AlGaN-based deep ultraviolet light-emitting diode for the origin of high ideality factor

Deep ultraviolet light-emitting diodes (DUV LEDs) are promising light sources for disinfection, especially during the pandemic of novel coronavirus (COVID-19). Despite much effort in the development of DUV LEDs, the device temperature and ideality factor are key parameters of devices, which are often neglected. Here, we developed a simple and convenient method to study the behavior of a 280 nm AlGaN-based DUV LED, obtaining the electrical, optical, and thermal properties within one measurement. From the experimental results, we find that the light output power and wall-plug efficiency of the AlGaN-based DUV LED are strongly affected by device temperature, ideality factor (β), and series resistance (Rs). β decreases from 9.3 to 8.1 at 40 mA when the temperature increases from 302 to 317 K. We compared these results with simulations and found that the high potential barriers inside the device and the carrier concentration in n-type or p-type layers, especially the hole concentration in p-type layers, are the two key factors for the high value of the ideality factor from the LED structure. As the device temperature increases, carriers with higher energy would overcome some potential barriers and Mg acceptor activation would be more efficient, which are beneficial for carrier transportation. However, these also lead to the carrier overflow and weaken the radiative recombination rate. The trade-off role of device temperature in carriers between transportation and overflow is needed to be considered in the future development of DUV LEDs with higher efficiency and higher brightness.

Characteristics of Micro-Size Light-Emitting Diode With Pentagon-Type Structure

We systemically investigated the effect of chip structures on the optical characteristics of micro-size LEDs. The total internal reflection (TIR) can be improved by selecting a suitable chip structure, thereby increasing the light efficiency. The saturation light output power (LOP) of the micro-size LED using pentagon-type structure is 44.78 mW, an increase of 6.08% compared with that of micro-size LED using circular-type structure. Simulation and experimental results indicate that chip structure of pentagon-type can effectively enhance the light extraction from the top or side of the micro-size LED.

Polarization Selective Transparent Electrode With Patterned Metal for the 3D Display Pixel

We propose a polarization selective transparent electrode (PSTE) utilizing a rectangular-shaped metal nano-wire structure. The performance of the PSTE was evaluated with simulation based on finite element method (COMSOL Multiphysics). Parametric studies have been carried out to give a guide for geometric parameter selection to meet the requirement of the target application. According to the studies, 70 nm by 800 nm rectangular metal grid, having a thickness of 100 nm and linewidth of 30 nm were recommended to keep higher than 61% transmittance, 24 dB extinction ratio, and lower than 5 Ω/sq sheet resistance. As an application of PSTE, a design idea about crosstalk reduction of the passive stereoscopic 3D display is proposed by presenting its pixel structure employing the proposed PSTE. The light extraction efficiency of the proposed pixel employing PSTE structure was also evaluated and compared to the structure employing the lossless ITO (Indium Tin Oxide), by measuring extracted light output power from incoherently distributed dipole sources excited in the emission layer. Degradation of the extraction efficiency below 14 percent point was expected compared to the lossless ITO for all the primary color wavelength range.

Deep Ultraviolet AlGaN-Based Light-Emitting Diodes with p-AlGaN/AlGaN Superlattice Hole Injection Structures

The p-AlGaN/AlGaN superlattice (SL) hole injection structure was introduced into deep ultraviolet (DUV) light-emitting diodes (LEDs) to enhance their performances. The period thicknesses of the p-Al0.8Ga0.2N/Al0.48Ga0.52N SLs affected the performances of the DUV LEDs. The appropriate period thickness of the p-Al0.8Ga0.2N/Al0.48Ga0.52N SL may enhance the hole injection of DUV LEDs. Therefore, compared with the reference LEDs, the DUV LEDs with the 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) SL presented forward voltage reduction of 0.23 V and light output power improvement of 15% at a current of 350 mA. Furthermore, the 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) SL could slightly suppress the Auger recombination and current overflow of the DUV LEDs in a high-current operation region. In addition to improved carrier injection, the DUV LEDs with the p-Al0.8Ga0.2N/Al0.48Ga0.52N SL hole injection structure showed reduced light absorption at their emission wavelength compared with the reference LEDs. Therefore, the DUV LEDs with p-Al0.8Ga0.2N/Al0.48Ga0.52N SL may exhibit better light extraction efficiency than the reference LEDs. The enhancement of p-Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) SL may contribute to improvements in light extraction and hole injection.

Stacked GaN/AlN last quantum barrier for high-efficiency InGaN-based green light-emitting diodes.

Realization of efficient InGaN-based green light-emitting diodes (LEDs) is highly desirable in solid-state lighting industry. Here, we propose a stacked last quantum barrier (SLQB) GaN/AlN layer for green (∼550nm) LEDs grown on patterned sapphire substrate to improve the device performance. A green LED with a SLQB achieves a light output power (LOP) of 13 mW at 15 mA, which is 35% higher than that of LEDs with a conventional last quantum barrier (CLQB) GaN layer. In addition, the forward voltage of the green LED with the SLQB is reduced by 0.16 V compared with green LEDs with the CLQB. Simulation results demonstrate that the AlN layer in the SLQB increases the effective barrier height for electrons and alleviates the electron leakage effect. It also enables an intraband tunneling process for holes to inject into the active region, promoting the hole injection efficiency. As a result, we successfully obtain InGaN-based green LEDs with remarkably improved carrier injection efficiency by adopting the SLQB structure.

Improving the External Quantum Efficiency of High-Power GaN-Based Flip-Chip LEDs by Using Sidewall Composite Reflective Micro Structure.

For high-power applications, it is important to improve the light extraction efficiency and light output of the vertical direction of LEDs. Flip-chip LEDs (FCLEDs) with an Ag/SiO2/distributed Bragg reflector/SiO2 composite reflection micro structure (CRS) were fabricated. Compared with the normal Ag-based FCLEDs, the light output power of the CRS-FCLEDs was increased by 6.3% at an operational current of 1500 mA, with the corresponding external quantum efficiency improved by 6.0%. Further investigation proved that the CRS structure exhibited higher reflectance compared with the commonly used Ag-mirror reflective structure, which originates from the increased reflective area in the sidewall and partial area of the n-GaN contact orifices. It exhibited markedly smaller optical degradation and thus higher device reliability as compared to normal Ag-based FCLED. Moreover, the light emission intensity distributions and far-field angular light emission measurements show that the CRS-FCLED has a strengthened light output in the vertical direction, which shows great potential for applications in high-power fields, such as headlamps for automobiles.

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.

Improving the Current Spreading by Fe Doping in n-GaN Layer for GaN-Based Ultraviolet Light-Emitting Diodes

GaN-based light-emitting diodes (LEDs) grown on the insulating sapphire substrate suffer from a severe current crowding effect. To alleviate the current crowding effect, we demonstrate a current spreading structure for 365-nm ultraviolet LED using Fe doping in n-GaN layer. The surface morphology was studied by atomic force microscope, and the crystal quality was evaluated by high-resolution X-ray diffraction, which shows that a thin insertion layer doped with Fe has less influence on the crystalline quality. Additionally, inserting a Fe-doped layer with appropriate doping concentration enables to induce a barrier in the n-type layer, which contributes to the uniform current distribution. As a result, compared with conventional LED, the light output power obtains a 170.6% enhancement at 100 mA. Furthermore, the uniform optical emission distribution is also achieved by inserting a Fe-doped layer. Finally, the conduction band structures and horizontal hole concentration distributions are calculated by Advanced Physical Models of Semiconductor Devices, which illustrates the mechanism of current spreading.

Fluorescence Microscopy Light Source Review.

Traditional arc-based light sources and Light Emitting Diodes (LEDs) are described, and the pros and cons of these sources with respect to fluorescence microscopy are discussed. For multi-color applications, arc-based light sources offer white light ranging from the ultraviolet (UV) to the infrared (IR), while LEDs come in a range of colors spanning the same wavelengths. The power of traditional arc-based sources is controlled with neutral density (ND) filters, reducing power across the entire range of wavelengths, while LED-based sources can be controlled directly by modulating current passing through the electronics. Similarly, arc-based sources use physical shutters to control sample exposure to light in a range of tens to hundreds of milliseconds (ms), while LEDs can be turned ON/OFF electronically in <1 ms. The complexity of comparing and measuring light power on the sample, due to normalization of available light source spectra and complex power measurements, is discussed. The superiority of LEDs for stability of light power output is covered. Direct coupling of light sources to the microscope is more cost effective and leads to higher available light power. Various options for setting up multi-color imaging, including high-speed imaging with multiple LEDs and a triple cube, are described. A brief introduction to lasers, with suggested further reading, is included in this article. Finally, the smaller environmental footprint of LEDs relative to arc-based light sources is highlighted. © 2021 Wiley Periodicals LLC.

Improved performance of InGaN-based red light-emitting diodes by micro-hole arrays.

This study demonstrates the performance improvements of InGaN-based red light-emitting diodes (LEDs) by fabricating micro-holes in the planar mesa. The peak wavelengths of the micro-hole LEDs (MHLEDs) exhibited a blue-shift of around 3 nm compared to the planar LEDs (PLEDs) at the same current density. The lowest full width at half maximum of MHLEDs was 59 nm, which is slightly less than that of the PLEDs. The light output power and external quantum efficiency of the MHLED with a wavelength of 634 nm at 20 mA were 0.6 mW and 1.5%, which are 8.5% higher than those of the PLED.

Narrow-Band AlGaN-Based UVB Light-Emitting Diodes

This paper reports an AlGaN-based narrow-band ultraviolet-B (NB-UVB) light-emitting diode (LED) exhibiting a single electroluminescence peak with a full-width at half-maximum (FWHM) of less than 10 nm at forward currents (If) from 10 to 200 mA, broadening to 11.6 nm when the forward current reached 350 mA. We attribute the narrow FWHM to effectively decreasing the degrees of piezoelectric polarization in the MQWs, and the excess electron overflow from the MQW to the p-layer was avoided. The maximum external quantum efficiency (EQE) of 2.16% and wall plug efficiency of 1.74% occurred when the forward current was 10 mA; the EQE dropped by 8.6% when the forward current increased from 10 to 60 mA. Furthermore, the light output power decreased to 85.4 and 82.5% of its initial value after 620 and 3500 h, respectively, under conditions of 60 mA dc aging. The characteristics of this NB-UVB LED suggest great potential for its application in phototherapy.

A Realization of Stabilizing the Output Light Power from a Laser Diode: A Practical Approach

Semiconductor Laser Diodes (LDs) are known for their sensitivity to variation in ambient temperature. With the rise in case temperature the threshold current of the LD increases, causing the output light power to deteriorate drastically. Therefore, it is necessary to stabilize the temperature of the diode. Various approaches could be adopted in this regard. In this paper, an active cooling approach using the temperature compensation technique has been followed and presented in the form of a full design of the circuit according to the various datasheet parameters of the LD and other components. As a result of temperature stabilization, a significant improvement in the output light power stabilization was observed and the results are presented.

Toward efficient long-wavelength III-nitride emitters using a hybrid nucleation layer.

The realization of efficient III-nitride emitters in the green-to-amber region is fundamental to the monolithic integration of multicolor emitters and the development of III-nitride-based full-color high-resolution displays. A hybrid nucleation layer, which includes sputtered AlN and mid-temperature GaN components, was proposed for the development of efficient III-nitride emitters in the green-to-amber region. The mid-temperature GaN component in the hybrid nucleation layer induced the formation of a stacking fault band structure, which effectively relaxed the misfit stress at the GaN/sapphire interface. A reduced dislocation density and in-plane compressive stress in InGaN/GaN multiple quantum wells were obtained on the hybrid nucleation layer in comparison with the conventional sputtered AlN nucleation layer. Consequently, a significantly enhanced internal quantum efficiency and improved light output power were achieved for the LEDs grown on the hybrid nucleation layer. This gain is attributed to the increased localization depth and spatial overlapping of the electron and hole wave functions. In the present study, the hybrid nucleation layer provides a promising approach for the pursuit of efficient III-nitride emitters in the green-to-amber region.

Improvement in the Output Power of Near-Ultraviolet LEDs of p-GaN Nanorods through SiO2 Nanosphere Mask Lithography with the Dip-Coating Method.

In this paper, the conditions of the dip-coating method of SiO2 nanospheres are optimized, and a neatly arranged single-layer SiO2 array is obtained. On this basis, a “top-down” inductively coupled plasma (ICP) technique is used to etch the p-GaN layer to prepare a periodic triangular nanopore array. After the etching is completed, the compressive stress in the epitaxial wafer sample is released to a certain extent. Then, die processing is performed on the etched LED epitaxial wafer samples. The LED chip with an etching depth of 150 nm has the highest overall luminous efficiency. Under a 100 mA injection current, the light output power (LOP) of the etched 150 nm sample is 23.61% higher than that of the original unetched sample.

Effects of three-step magnesium doping in p-GaN layer on the properties of InGaN-based light-emitting diode

Purpose The purpose of this paper is to enhance the efficiency of the LED by introducing three-step magnesium (Mg) doping profile. Attention was paid to the effects of the Mg doping concentration of the first p-GaN layer (i.e. layer close to the active region). Attention was paid to the effects of the Mg doping concentration of the first p-GaN layer (i.e. layer close to the active region). Design/methodology/approach Indium gallium nitride (InGaN)–based light-emitting diode (LED) was grown on a 4-inch c-plane patterned sapphire substrate using metal organic chemical vapor deposition. The Cp2Mg flow rates for the second and third p-GaN layers were set at 50 sccm and 325 sccm, respectively. For the first p-GaN layer, the Cp2Mg flow rate varied from 150 sccm to 300 sccm to achieve different Mg dopant concentrations. Findings The full width at half maximum (FWHM) for the GaN (102) plane increases with increasing Cp2Mg flow rate. FWHM for the sample with 150, 250 and 300 sccm Cp2Mg flow rates was 233 arcsec, 236 arcsec and 245 arcsec, respectively. This result indicates that the edge and mixed dislocations in the p-GaN layer were increased with increasing Cp2Mg flow rate. Atomic force microscopy (AFM) results reveal that the sample grown with 300 sccm exhibits the highest surface roughness, followed by 150 sccm and 250 sccm. The surface roughness of these samples is 2.40 nm, 2.12 nm and 2.08 nm, respectively. Simultaneously, the photoluminescence (PL) spectrum of the 250 sccm sample shows the highest band edge intensity over the yellow band ratio compared to that of other samples. The light output power measurements found that the sample with 250 sccm exhibits high output power because of sufficient hole injection toward the active region. Originality/value Through this study, the three steps of the Mg profile on the p-GaN layer were proposed to show high-efficiency InGaN-based LED. The optimal Mg concentration was studied on the first p-GaN layer (i.e. layer close to active region) to improve the LED performance by varying the Cp2Mg flow rate. This finding was in line with the result of PL and AFM results when the samples with 250 sccm have the highest Mg acceptor and good surface quality of the p-GaN layer. It can be deduced that the first p-GaN layer doping has a significant effect on the crystalline quality, surface roughness and light emission properties of the LED epi structure.

Quantitative correlation of hot electron emission to Auger recombination in the active region of c-plane blue III-N LEDs

Using electron emission spectroscopy, measurement and analysis were conducted on the energy distribution of vacuum emitted electrons from an electrically driven InGaN/GaN commercial blue c-plane (peak wavelengths λ≈465 nm) light emitting diode (LED) with 60 nm of p-GaN on top of the active region. The signal-to-noise ratio of semiconductor peaks is improved on the thin p-GaN LED compared to previously published data on thicker p-GaN samples and is attributed to reduced loss of electrons en route to emission into vacuum during transit through the p-GaN. This further proves that hot electrons are generated in the bulk region and not by light or other hot electron generation mechanisms at the surface. Using square root of the light output power as a proxy for the active region carrier density, n, the hot electron integrated peak intensity is shown to be proportional to n3 and, thus, is directly attributed to a 3-body Auger process. Since there are significant Auger recombination currents even at low injection current densities, it is expected that Auger recombination current will dominate over radiation recombination and Shockley–Read–Hall (SRH) currents at higher current densities. This identifies Auger recombination as the dominant cause of efficiency droop.

Performance Enhancement of InGaN Light-Emitting Diodes with InGaN/GaN/InGaN Triangular Barriers

InGaN light-emitting diodes (LEDs) with InGaN/GaN/InGaN triangular (IGIT) barriers were designed and investigated on a theoretical basis. The carrier concentration and radiative recombination rate distribution in multiple quantum wells, energy band diagrams, light output power–current–voltage performance curves, and internal quantum efficiency of the LEDs with IGIT barriers were studied. The simulations showed that the InGaN LEDs with IGIT barriers have higher output light power, lower turn-on voltage, and less efficiency droop than that of LEDs with conventional GaN and InGaN barriers. These improvements originate from the appropriately designed energy band diagram of the LEDs with IGIT barriers, which improves injection efficiency of holes and confinement of electrons.

Improving light output power of AlGaN-based deep-ultraviolet light-emitting diodes by optimizing the optical thickness of p-layers

In this study, we investigated the relationship of light output power with the optical thickness of the p-layers in AlGaN-based deep ultraviolet light-emitting diodes with a transparent high-Al-composition p-AlGaN clad layer, a thin p-GaN contact layer, and a reflective p-type electrode. By adjusting the thickness of the transparent high-Al-composition p-AlGaN clad layer, we observed a marked change in light output power. A maximum light output power of 385 mW at 1500 mA, a maximum external quantum efficiency of 15.7% at 10 mA, and a maximum wall-plug efficiency of 15.3% at 10 mA were obtained at an emission wavelength of 275 nm.

Effects of V/III ratio of InGaN quantum well at high growth temperature for near ultraviolet light emitting diodes

Purpose The aims of this paper is to study the effects of the V/III ratio of indium gallium nitride (InGaN) quantum wells (QWs) on the structural, optical and electrical properties of near-ultraviolet light-emitting diode (NUV-LED). Design/methodology/approach InGaN-based NUV-LED is successfully grown on the c-plane patterned sapphire substrate at atmospheric pressure using metal organic chemical vapor deposition. Findings The indium composition and thickness of InGaN QWs increased as the V/III ratio increased from 20871 to 11824, according to high-resolution X-ray diffraction. The V/III ratio was also found to have an important effect on the surface morphology of the InGaN QWs and thus the surface morphology of the subsequent layers. Apart from that, the electroluminescence measurement revealed that the V/III ratio had a major impact on the light output power (LOP) and the emission peak wavelength of the NUV-LED. The LOP increased by up to 53% at 100 mA, and the emission peak wavelength of the NUV-LED changed to a longer wavelength as the V/III ratio decreased from 20871 to 11824. Originality/value This study discovered a relation between the V/III ratio and the properties of QWs, which resulted in the LOP enhancement of the NUV-LED. High TMIn flow rates, which produced a low V/III ratio, contribute to the increased LOP of NUV-LED.

Demonstration of high wall-plug efficiency III-nitride micro-light-emitting diodes with MOCVD-grown tunnel junction contacts using chemical treatments

High wall-plug efficiency (WPE) micro-light-emitting diodes with metalorganic chemical vapor deposition-grown tunnel junction (TJ) contacts are demonstrated. By employing chemical treatments before sidewall activation, the 20 × 20 μm2 TJ devices resulted in a voltage penalty of 0.2 V at 20 A cm−2, compared to devices with indium-tin oxide (ITO) contacts. Moreover, the enhancement in light output power was more than 40% higher than ITO devices. Hence, the TJ devices yielded the peak external quantum efficiency (EQE) and WPE of 56% and 55%, respectively, indicating the improvements of 64% and 77% in peak EQE and WPE compared to ITO devices.