Performance Of GaN-based Light-emitting Diodes Research Articles

Enhancing the Performance of GaN-Based Light-Emitting Diodes by Incorporating a Junction-Type Last Quantum Barrier

In this paper, an n-i-p-type GaN barrier for the final quantum well, which is closest to the p-type GaN cap layer, is proposed for nitride light-emitting diodes (LEDs) to enhance the confinement of electrons and to improve the efficiency of hole injection. The performances of GaN-based LEDs with a traditional GaN barrier and with our proposed n-i-p GaN barrier were simulated and analyzed in detail. It was observed that, with our newly designed n-i-p GaN barrier, the performances of the LEDs were improved, including a higher light output power, a lower threshold voltage, and a stronger electroluminescence emission intensity. The light output power can be remarkably boosted by 105% at an injection current density of 100 A/cm2 in comparison with a traditional LED. These improvements originated from the proposed n-i-p GaN barrier, which induces a strong reverse electrostatic field in the n-i-p GaN barrier. This field not only enhances the confinement of electrons but also improves the efficiency of hole injection.

Enhanced luminescence property of GaN-based LEDs with p-InGaN cap layer grown on p-GaN surface

The influence of a p-In0.06Ga0.94N cap layer grown on p-GaN on the device performance of GaN-based light-emitting diodes (LEDs) was investigated. Compared to conventional p-GaN LED structure, the light output power of p-InGaN/GaN structure LED was enhanced by 20% and the operating voltage decreased from 3.81 to 3.29 V under 20 mA injection current. Based on surface morphology and electrical analysis, these improvements were mainly attributed to the higher light extraction efficiency and lower contact resistence. It was observed that V-shaped pits appearing on the surface of p-InGaN increased surface roughness, resulting in higher extraction efficiency. After molten KOH etching to p-InGaN layer, the V-shaped pits became bigger, further increasing surface roughness. The larger surface roughness further increased light output power from 20% to 44%. In addition, the forward voltage under 20 mA increased from 3.29 V to 3.41 V which indicated etching process decreased the surface ohmic contact.

Effect of Strains and V-Shaped Pit Structures on the Performance of GaN-Based Light-Emitting Diodes

Strains and V-shaped pits are essential factors for determining the efficiency of GaN-based light-emitting diodes (LEDs). In this study, we systematically analyzed GaN LED structures on patterned sapphire substrates (PSSs) with two types of growth temperature employed for prestrained layers and three different thickness of n-type GaN layers by using cathodoluminescence (CL), microphotoluminescence (PL), and depth-resolved confocal Raman spectroscopy. The results indicated that V-pits formation situation can be analyzed using CL. From the emission peak intensity ratio of prestrained layers and multiple quantum wells (MQWs) in the CL spectrum, information regarding strain relaxation between prestrained layers and MQWs was determined. Furthermore, micro-PL and depth-resolved confocal Raman spectroscopy were employed to validate the results obtained from CL measurements. The growth conditions of prestrained layers played a dominant role in the determination of LED performance. The benefit of the thick layer of n-GaN was the strain reduction, which was counteracted by an increase in light absorption in thick n-type doped layers. Consequently, the most satisfactory LED performance was observed in a structure with relatively lower growth temperature of prestrained layers that exhibited larger V-pits, leading to higher strain relaxation and thinner n-type GaN layers, which prevent light absorption caused by n-type GaN layers.

Suppressing the Initial Growth of Sidewall GaN by Modifying AlN-Coated Patterned Sapphire with KOH-Based Etchant

Micron-sized patterned sapphire substrates (PSSs) with an ex situ sputtered AlN nucleation layer (NL) have been used to improve the performance of GaN-based light-emitting diodes (LEDs). The growth of GaN was enhanced not only from bottom c-plane, but also from the sidewall of the micron-sized patterns. In this study, KOH solution was used to etch AlN (especially sidewall AlN) for the first time. The additional etching process is very simple. It was found that KOH etching 1 min did enhance the light output power (LOP) of LED. However, with the increase of etching time to 4 min, the LOP decreased. Besides, the effect of remained AlN on GaN growth mechanism was investigated in detail.

Influences of Microhole Depth and SiO2 Nanoparticle/Microsphere Passivation Layer on the Performance of GaN-Based Light-Emitting Diodes

A hybrid structure, including 45° sidewalls, a microhole array, and a thin SiO2 nanoparticle (NP)/microsphere (MS) passivation layer, is used to produce GaN-based light-emitting diodes (LEDs). The influences of the microhole depth and SiO2 NP/MS passivation layer on the LED performance are studied. A shallower depth of a microhole array shows better optical properties due to the complete preservation of the GaN/InGaN multiple quantum well (MQW) region. In addition, the use of a thin SiO2 NP/MS passivation layer gives a remarkably reduced reverse-biased leakage current and improved optical performance. Experimentally, under an injection current density of 110 A/cm2, the studied device, with a proper hybrid structure, shows enhancements of 25.9%, 29.2%, and 29.4% in light output power (LOP), external quantum efficiency (EQE), and wall-plug efficiency, respectively, as compared to a LED device with a deeper depth of microhole array. These improvements are mainly caused by the reduced total internal reflection (TIR) and the Fresnel reflection, which increase scattering probability and the opportunity to find photon escape cones. Hence, the studied hybrid structure in this work is a promising route to fabricate high-performance GaN-based LEDs.

Effects of three-dimensional strain distribution on the performance of GaN-based light-emitting diodes on patterned sapphire substrates

Depth-resolved confocal Raman and micro photoluminescence (PL) are adopted to analyze GaN-based light-emitting diodes (LEDs) with an ex situ sputtered AlN nucleation layer on patterned sapphire substrates (PSSs). The strain on PSSs is measured by Raman spectral mapping, and the results demonstrate that the strain distribution within the LED structures strongly correlates to the pattern array of the PSS. Cone regions with less dislocations have lower strain than plane regions. The array pattern of the PSS’s surface can directly affect strain existing in the later crystal growth even at the position of the layer before the p-GaN growth. Moreover, the two-dimensional micro-PL mapping of the GaN-based LED samples on a PSS reveals a close relationship with the strain distribution on the sample surface. It is found that the strain as well as the PL intensity difference can be mitigated by proper pre-strained layers in order to realize high efficiency LEDs.

Suppressing the Initial Growth of Sidewall GaN by Modifying Micron-Sized Patterned Sapphire Substrate with H₃PO₄-Based Etchant.

Micron-sized patterned sapphire substrates (PSS) are used to improve the performance of GaN-based light-emitting diodes (LEDs). However, the growth of GaN is initiated not only from the bottom c-plane but also from the sidewall of the micron-sized patterns. Therefore, the coalescence of these GaN crystals creates irregular voids. In this study, two kinds of nucleation layers (NL)—ex-situ AlN NL and in-situ GaN NL—were used, and the growth of sidewall GaN was successfully suppressed in both systems by modifying the micron-sized PSS surface.

Improvement of the Optical and Electrical Performance of GaN-Based Light-Emitting Diodes (LEDs) Using Transferrable ZnSnO3 (ZTO) Microsphere Monolayers

We report on the formation of transferable microspheres monolayers consisting of ZnSnO3 (ZTO) synthesized via fast ethanol precipitation and their application to GaN-based light-emitting diodes (LEDs) to improve device performance. ZTO microspheres with diameters of 820 ± 20 nm (ZTO-1), 910 ± 10 nm (ZTO-2), and 1200 ± 10 nm (ZTO-3) were synthesized and arrayed to form a monolayer using polydimethylsiloxane (PDMS) and a unidirectional rubbing method. ZTO-1, ZTO-2, and ZTO-3 monolayers exhibited optical transmittance percentages of 94.4%, 92.98%, and 87.09% at 450 nm, respectively. Effects of ZTO monolayers on the light extraction efficiency of LEDs were evaluated using LED chips sized 800 μm × 800 μm. LEDs with a ZTO-3 monolayer as a top layer had a light extraction efficiency of ∼144%, relative to regular LEDs, as well as improved electrical properties, as evidenced by a decrease in Vturn on of ∼0.6 V and a decrease in Ron by 0.036 kΩ. XPS analysis indicated that the improvement in the electrical properti...

Numerical Analysis of the Temperature Impact on Performance of GaN-Based 460-nm Light-Emitting Diode.

The influence of temperature on the characteristics of a GaN-based 460-nm light-emitting diode (LED) prepared on sapphire substrate was simulated using the SiLENSe and SpeCLED software programs. High temperatures impose negative effects on the performance of GaN-based LEDs. As the temperature increases, electrons acquire higher thermal energies, and therefore LEDs may suffer more from high-current loss mechanisms, which in turn causes a reduction in the radiative recombination rate in the active region. The internal quantum efficiency was reduced by about 24% at a current density of 35 A/cm2, and the electroluminescence spectral peak wavelength was redshifted. The LED operated at 260 K and exhibited its highest light output power of ~317.5 mW at a maximum injection current of 350 mA, compared to 212.2 mW for an LED operated at 400 K. However, increasing temperature does not cause a droop in efficiency under high injection conditions. The peak efficiency at 1 mA of injection current decreases more rapidly by ~15% with increasing temperature from 260 to 400 K than the efficiency at high injection current of 350 mA by ~11%.

Enhanced performance of GaN-based light-emitting diodes with composite electron blocking layer

The performance for GaN-based light-emitting diodes (LEDs) with specially designed composite electron blocking layers (CEBLs) that are inserted between the InGaN/GaN multiple quantum wells (MQWs) and the top p-GaN layer was studied intensively. The CEBLs are consisted of an AlGaN layer and a p-InAlGaN/GaN superlattice (SL). The simulation results indicated that the light output power and internal quantum efficiency for the GaN-based LED with CEBLs could be enhanced by 35.6% and 36% at an injection current of 350mA, respectively, as compared to the previous GaN-based LED only with p-InAlGaN/GaN SL EBL. The improved uniformity of carrier distribution, the enhanced hole concentration and injection efficiency, as well as the increase in effective potential height for electrons and the decrease in the strain-induced polarization electric field are identified to be responsible for this significant improvement.

Effect of Sputtered AlN Location on the Growth Mechanism of GaN

Patterned sapphire substrates (PSSs) have been used to enhance the performance of GaN-based light-emitting diodes (LEDs). This performance can be further improved by using an ex situ sputtered AlN nucleation layer. This improvement has been attributed to the reduction of the GaN formation on the sidewall of the cone shape pattern. In this study, four kinds of PSS samples were fabricated to investigate the effect of sputtered AlN location on the growth mechanism of GaN in detail.

Performance of GaN-Based LEDs with Nanopatterned Indium Tin Oxide Electrode

The indium tin oxide (ITO) has been widely applied in light emitting diodes (LEDs) as the transparent current spreading layer. In this work, the performance of GaN-based blue light LEDs with nanopatterned ITO electrode is investigated. Periodic nanopillar ITO arrays are fabricated by inductive coupled plasma etching with the mask of polystyrene nanosphere. The light extraction efficiency (LEE) of LEDs can be improved by nanopatterned ITO ohmic contacts. The light output intensity of the fabricated LEDs with nanopatterned ITO electrode is 17% higher than that of the conventional LEDs at an injection current of 100 mA. Three-dimensional finite difference time domain simulation matches well with the experimental result. This method may serve as a practical approach to improving the LEE of the LEDs.

Effect of Top-Region Area of Flat-Top Pyramid Patterned Sapphire Substrate on the Optoelectronic Performance of GaN-Based Light-Emitting Diodes

The flat-top pyramid patterned sapphire substrates (FTP-PSSs) have been prepared for the growth of GaN epilayers and the fabrication of lateral-type light-emitting diodes (LEDs) with an emission wavelength of approximately 470 nm. Three kinds of FTP-PSSs, which were denoted as FTP-PSS-A, FTP-PSS-B, and FTP-PSS-C, respectively, were formed through the sequential wet etching processes. The diameters of circle areas on the top regions of these three FTP-PSSs were 1, 2, and 3 μm, respectively. Based on the X-ray diffraction results, the full-width at half-maximum values of rocking curves at (002) plane for the GaN epilayers grown on conventional sapphire substrate (CSS), FTP-PSS-A, FTP-PSS-B, and FTP-PSS-C were 412, 238, 346, and 357 arcsec, while these values at (102) plane were 593, 327, 352, and 372 arcsec, respectively. The SpeCLED-Ratro simulation results reveal that the LED prepared on FTP-PSS-A has the highest light extraction efficiency than that of the other devices. At an injection current of 350 mA, the output powers of LEDs fabricated on CSS, FTP-PSS-A, FTP-PSS-B, and FTP-PSS-C were 157, 254, 241, and 233 mW, respectively. The results indicate that both the crystal quality of GaN epilayer and the light extraction of LED can be improved via the use of FTP-PSS, especially for the FTP-PSS-A.

High-speed GaN/GaInN nanowire array light-emitting diode on silicon(111).

The high speed on-off performance of GaN-based light-emitting diodes (LEDs) grown in c-plane direction is limited by long carrier lifetimes caused by spontaneous and piezoelectric polarization. This work demonstrates that this limitation can be overcome by m-planar core-shell InGaN/GaN nanowire LEDs grown on Si(111). Time-resolved electroluminescence studies exhibit 90-10% rise- and fall-times of about 220 ps under GHz electrical excitation. The data underline the potential of these devices for optical data communication in polymer fibers and free space.

GaN-Based Multiple-Quantum-Well Light-Emitting Diodes Employing Nanotechnology for Photon Management

Nanostructures have been proved to be an efficient way of modifying/improving the performance of GaN-based light-emitting diodes (LEDs). The achievements in photon management include strain relaxation, light extraction enhancement, radiation pattern control, and white-light devices. In this paper, we discuss the impact and the underlying physics of applying nanotechnology on LEDs. A variety of nanostructures are introduced, as well as the fabrication techniques.

Silver nanowires for transparent conductive electrode to GaN-based light-emitting diodes

Transparent, conductive, and uniform Ag nanowires (NWs) were introduced to improve the optical performance of GaN-based light-emitting diodes (LEDs) by a spin-coating technique. The Ag NWs acted as a current spreading layer, exhibiting high transmittance and low sheet resistance, and ultimately leading to high performance GaN-based LEDs with an ultra large size of 5 × 5 mm2. Compared to the transmittance of conventional LEDs without Ag NWs, the relative transmittance of LEDs with Ag NWs was approximately 90% of the overall wavelength region. However, the electroluminescence (EL) intensity of LED with Ag NWs was much higher than that of conventional LEDs without Ag NWs for injection current above 45 mA. In addition, the EL full width at half maximum of LEDs with Ag NWs was much lower than that of conventional LEDs without Ag NWs. Based on these results, we believe that the enhanced optical performance of ultra large LEDs was due to an increase in the current spreading effect.

Enhanced performance of GaN-based light-emitting diodes with InGaN/GaN superlattice barriers

GaN-based multiple quantum well light-emitting diodes (LEDs) with conventional and superlattice barriers have been investigated numerically. Simulation results demonstrate using InGaN/GaN superlattices as barriers can effectively enhance performances of the GaN-Based LEDs, mainly owing to the improvement of hole injection and transport among the MQW active region. Meanwhile, the improved electron capture decreases the electron leakage and alleviates the efficiency droop. The weak polarization field induced by the superlattice structure strengthens the intensity of the emission spectrum and leads to a blue-shift relative to the conventional one.

Enhancement of performance of GaN-based light-emitting diodes by insertion and optimization of n-InGaN/GaN composite current-spreading layers

Si-doped n-InGaN/GaN composite current-spreading layers (CCSLs) inserted between the n-GaN layer and the InGaN/GaN multiple-quantum-well (MQW) active region were developed and optimized for the GaN-based light-emitting diodes (LEDs). It was found that with the optimized n-InGaN/GaN CCSLs lightly doped with Si, the crystalline quality as well as the electric and optical properties of the GaN-based LEDs, such as forward voltage Vf, photoluminescence (PL) intensity, and electrostatic discharge (ESD) endurance could be significantly improved. In particular, an ESD pass yield as high as 99.49% at a 2000 V reverse bias has been achieved for the GaN-based LEDs with the 70 nm/70 nm n-InGaN/GaN CCSLs. This means that the n-InGaN/GaN CCSLs with carefully optimized layer thickness could play a key role in improving the crystalline quality and enhancing the device performance of the GaN-based LEDs.

Performance of GaN-based light-emitting diodes fabricated using GaN epilayers grown on silicon substrates.

Light extraction of GaN-based light-emitting diodes grown on Si(111) substrate (GaN-on-Si based LEDs) is presented in this study. Three different designs of GaN-on-Si based LEDs with the lateral structure, lateral structure on mirror/Si(100) substrate, and vertical structure on mirror/Si(100) substrate were epitaxially grown by metalorganic chemical vapor deposition and fabricated using chemical lift-off and double-transfer techniques. Current-voltage, light output power, far-field radiation patterns, and electroluminescence characteristics of these three LEDs were discussed. At an injection current of 700 mA, the output powers of LEDs with the lateral structure on mirror/Si(100) substrate and vertical structure on mirror/Si(100) substrate were measured to be 155.07 and 261.07 mW, respectively. The output powers of these two LEDs had 70.63% and 187.26% enhancement compared to that of LED with the lateral structure, respectively. The result indicated this vertical structure LED was useful in improving the light extraction due to an enhancement in light scattering efficiency while the high-reflection mirror and diffuse surfaces were employed.

GaN-Based LEDs With Rough Surface and Selective KOH Etching

The authors propose a simple method to enhance the performances of GaN-based light-emitting diodes (LEDs) with rough surface. By using KOH to selectively smooth the area beneath the p-contact pad, it was found that we could reduce the forward voltage and enhance output power of the GaN-based LEDs. It was also found that wall-plug efficiency (WPE) of the proposed LEDs was 10.3% larger than that of the conventional LEDs with rough surface and 33.9% larger than that of the conventional LEDs with flat surface. Furthermore, it was found that the use of selective KOH etching will not degrade drooping effect of the LEDs.