InGaN-based Green Light-emitting Diodes Research Articles

Ultraviolet, blue, and green InGaN-based light-emitting diodes functionalized with ZnO nanorods

Abstract We investigated the augmentation in the light-output power of InGaN light-emitting diodes (LEDs) functionalized with ZnO nanorods (NRs) according to the emission wavelength of the LEDs and the volume fraction of the ZnO NRs. The ZnO NRs were fabricated on top of the ultraviolet, blue, and green InGaN LEDs using the hydrothermal method. The optical properties of the LEDs and ZnO NRs were examined by electroluminescence, photoluminescence, and Raman scattering at room temperature. The measurements of light-output power versus current and current as a function of voltage showed that there were considerably different enhancements in the light-output power of InGaN LEDs functionalized with ZnO NRs at different emission wavelengths of the LEDs, while no changes were observed in the electrical properties. Three-dimensional finite-difference time-domain simulations were conducted to support the experimental results. Both experimental and theoretical results indicated that the light-extraction efficiency of InGaN LEDs functionalized with ZnO NRs was significantly affected by the differences between the refractive indices of the GaN layers, indium–tin oxide layer, and ZnO NRs, which could be changed by the emission wavelength of the LEDs and volume fraction of the ZnO NRs.

Effect of Back Diffusion of Mg Dopants on Optoelectronic Properties of InGaN-Based Green Light-Emitting Diodes**Supported by the National Natural Science Foundation of China under Grant Nos 61505197 and 61334009, and the National High-Technology Research and Development Program of China under Grant No 2014AA032604.

The effect of back-diffusion of Mg dopants on optoelectronic characteristics of InGaN-based green light-emitting diodes (LEDs) is investigated. The LEDs with less Mg back-diffusion show blue shifts of longer wavelengths and larger wavelengths with the increasing current, which results from the Mg-dopant-related polarization screening. The LEDs show enhanced efficiency with the decreasing Mg back-diffusion in the lower current region. Light outputs follow the power law L ∝ Im, with smaller parameter m in the LEDs with less Mg back-diffusion, indicating a lower density of trap states. The trap-assisted tunneling current is also suppressed by reducing Mg-defect-related nonradiative centers in the active region. Furthermore, the forward current-voltage characteristics are improved.

High-performance InGaN-based green light-emitting diodes with quaternary InAlGaN/GaN superlattice electron blocking layer.

In this study, high-performance InGaN-based green light-emitting diodes (LEDs) with a quaternary InAlGaN/GaN superlattice electron blocking layer (QSL-EBL) have been demonstrated. The band structural simulation was employed to investigate the electrostatic field and carriers distribution, show that the efficiency and droop behavior can be intensively improved by using a QSL-EBL in LEDs. The QSL-EBL structure can reduce the polarization-related electrostatic fields in the multiple quantum wells (MQWs), leading to a smoother band diagram and a more uniform carriers distribution among the quantum wells under forward bias. In comparison with green LEDs with conventional bulk-EBL structure, the light output power of LEDs with QSL-EBL was greatly enhanced by 53%. The efficiency droop shows only 30% at 100 A/cm2 comparing to its peak value, suggesting that the QSL-EBL LED is promising for future white lighting with high performance.

Confocal electroluminescence investigations of highly efficient green InGaN LED via ZnO nanorods

The light emission mechanism of green InGaN-based light-emitting diodes (LEDs) coated with ZnO nanorods (ZnO-nanorods/InGaN LEDs) was investigated by confocal scanning electroluminescence (EL) spectroscopy (CSEM). The ZnO nanorods were grown by a hydrothermal method on the surface of fully fabricated InGaN LEDs. The vertical alignment and high crystallinity of the ZnO nanorods were confirmed by scanning electron microscopy, X-ray diffraction, photoluminescence spectroscopy, micro-Raman spectroscopy, and high-resolution transmission electron microscopy. Measurement of the light output as a function of the injection current, far-field radiation pattern, and current versus voltage curves revealed that the light output power of the green InGaN LEDs with ZnO nanorods was enhanced by approximately 60% compared to that of conventional green InGaN LEDs without ZnO nanorods. However, the electrical properties of the ZnO-nanorods-coated device were not appreciably affected by the nanorods. To analyse the light emission mechanism of the ZnO-nanorods/InGaN LEDs, CSEM was used to observe the EL intensity distributions at different focal planes.

External stress effects on the optical and electrical properties of flexible InGaN-based green light-emitting diodes.

Flexible InGaN-based green light emitting diodes (LEDs) were fabricated by transferring epilayer to a flexible polyimide substrate with laser lift-off (LLO) and double-transfer technologies. We present a method of increasing light output power in flexible LEDs without modifying their epitaxial layers. These improvements are achieved by reducing the quantum-confined Stark effect by reducing piezoelectric polarization that results from compressive stress in the GaN epilayer. The compressive stress is relaxed due to the external stress induced by increasing bending displacement of flexible substrate. The light output power of the flexible LED at an injection current of 150 mA is increased by approximately 42.2%, as the external bending went to the case of effective length of 15 mm. The experimental results demonstrated that applying external tensile stress effectively compensates for the compressive strain and changes the piezoelectric field in the InGaN/GaN MQWs region, thereby increases the probability of radiative recombination.

Temperature and injection current dependent optical and thermal characteristics of InGaN-based green large-area light-emitting diodes

Optical, spectral, and thermal characteristics of InGaN/GaN multiple quantum well green light-emitting diodes (LEDs) with a large chip size of 0.8 × 1 mm2, operating at λ ∼ 525 nm, on patterned sapphire substrate (PSS) were studied. The temperature-dependent optical and spectral properties of LEDs were measured and analyzed. The junction temperature (Tj) was also determined by the forward voltage method, in comparison with the simulation results which were theoretically calculated by a three-dimensional anisotropic heat dissipation model based on the finite element method. Under an injection current of 350 mA at 298 K, the optical output power and forward voltage were obtained to be 63.5 mW and 3.98 V, respectively. The characteristic temperature was found to be ∼438 K at an injection current of 350 mA. The Tj was increased with increasing the injection current. From the measured Tj values, the thermal resistance (Rth) value was experimentally obtained to be ∼6.31 K W−1, which was well consistent with the simulated result (Rth ∼ 6.24 K W−1).

Reduction of Efficiency Droop and Modification of Polarization Fields of InGaN-Based Green Light-Emitting Diodes via Mg-Doping in the Barriers

We demonstrate that the Mg-doping in barriers can partially screen the polarization fields of InGaN-based green light-emitting diodes. The photocurrent spectra show that the Mg-doping samples have smaller polarization fields and the blue shift of the peak with increasing current is observed. The reduction of polarization fields can be attributed to the screening of the impurity holes generated by the Mg atoms in the barriers. The efficiency droop is sensitive to the Mg-doping concentration in barriers, while the sample with Mg concentration of 5 × 1019 cm−3 exhibits the lowest efficiency degradation of 12.4% at a high injection current.

Study of droop phenomena in InGaN-based blue and green light-emitting diodes by temperature-dependent electroluminescence

InGaN-based blue and green light-emitting diodes are studied by temperature-dependent electroluminescence (EL) from 300 to 50 K to elucidate the effects of carrier overflow and the saturation in radiative recombination rate on the efficiency droop. Severe efficiency droop at cryogenic temperatures is attributed to the carrier overflow, which is confirmed by the EL spectra. The degree of overflow is thought to be related to the reduced effective active volume and the subsequent saturation in radiative recombination rate. Carrier transport and indium clustering in the active region are discussed in relation to the reduced effective active volume.

Improvement of quantum efficiency in green light-emitting diodes with pre-TMIn flow treatment

The effects of pre-trimethlyindium (TMIn) flow on the improved electrical characteristics and highly stable temperature properties of InGaN green light-emitting diodes (LEDs) are discussed. For the LED sample with a pre-TMIn flow treatment, the tunnelling of injected carriers associated with threading defects is significantly reduced, which promotes the diffusion–recombination of injected carriers, as well as the overall emission efficiency of the LED. In addition, the pre-TMIn flow treatment evidently reduces the dependence of external quantum efficiency on temperature and efficiency droop of green LEDs. As a result, we conclude that the pre-TMIn flow treatment is a promising scheme for the improvement of output performance of InGaN-based green LEDs.

Effect of Interface Nanotexture on Light Extraction of InGaN-Based Green Light Emitting Diodes

We report the enhancement of the light extraction of InGaN-based green light emitting diodes (LEDs) via the interface nanotexturing. The texture consists of high-density nanocraters on the surface of a sapphire substrate with an in situ etching. The width of nanocraters is about 0.5 μm and the depth is around 0.1 μm. It is demonstrated that the LEDs with interface texture exhibit about a 27% improvement in luminance intensity, compared with standard LEDs. High power InGaN-based green LEDs are obtained by using the interface nanotexture. An optical ray-tracing simulation is performed to investigate the effect of interface nanotexture on light extraction.

Study on Hydrogen Ion-Implanted Characteristic of Thin-Film Green Resonant-Cavity Light-Emitting Diodes

This letter investigates the effects of hydrogen ion implantation on the characteristics of InGaN-based green resonant-cavity light-emitting diodes (RCLEDs). RCLEDs with ion implantation were fabricated by implanting hydrogen ion (H+) in a selective area. The implanted region was used to form current-confinement layers due to an existing deep-level ( ~ 512 nm, 2.4 eV). Superior directionality was also obtained because the selective area of p-GaN layer of RCLEDs with ion implantation provided a low refractive index. The light emission enhancement was due to the high current density increasing and the total reflection of the emission ray. The electroluminescence spectrum exhibited narrow full-width at half-maximum of 45 nm for the RCLEDs with ion implantation. The fiber-coupled power of RCLEDs with H+ implantation was 2.2 times greater than that of a similar structure without H+ implantation at an injection current of 20 mA.

Green Light-Emitting Diodes with a Photoelectrochemically Treated Microhole-Array Pattern

Green InGaN-based light-emitting diodes (LEDs) with roughened microhole-array (MHA) structures were fabricated through a dry etching process and a photoelectrochemical (PEC) process. The PEC process consisted of a bandgap-selective lateral etching process at the InGaN active layer, an N-face bottom-up crystallographic etching process at the bottom p-type GaN:Mg layer, and a PEC oxidation process at the n-type GaN:Si surface. The light output power of the MHA-LED and the photoelectrochemically treated microhole-array light-emitting diode (PMHA-LED) had 7 and 65% enhancement, respectively, compared to a conventional LED at a 20 mA operation current.

High-Performance InGaN-Based Green Resonant-Cavity Light-Emitting Diodes for Plastic Optical Fiber Applications

High-performance InGaN-based green resonant-cavity light-emitting diodes (RCLEDs) with a plating Cu substrate for plastic optical fiber communication applications are reported. Good stability of emission wavelength was obtained at 0.016 nm/mA . The RCLEDs presents low temperature dependence, showing only a 3% drop in light output power as the temperature increasing from 25 to 85degC. The superior performance can be attributed to the decreased dynamic series resistance and the enhanced thermal dissipation of the heat sink substrate.

Experimental technique to correlate optical excitation intensities with electrical excitation intensities for semiconductor optoelectronic device characterization

We discuss a proposed experimental technique for correlating optical excitation intensities in photoluminescence measurements with electrical excitation intensities in electroluminescence measurements for optoelectronic device characterization purposes. The technique utilizes the reverse saturation current under optical excitation as a measure of the optical excitation intensities and translates it to electrical excitation intensities as the forward current of the same amount. InGaN-based blue and green light-emitting diodes were examined to demonstrate the validity of the proposed technique. 4 W cm−2 of optical excitation intensity was determined to be approximately the same as a sub-mA forward current under the present experimental conditions. Photoluminescence and electroluminescence characteristics were compared by applying the determined excitation intensity; spatial uniformity of luminescence was found to differ from each other in green-emitting devices.

Enhanced light extraction of InGaN-based green LEDs by nano-imprinted 2D photonic crystal pattern

In this paper, we propose a simple, low cost and mass producible nanoimprint lithography (NIL) method to texture the surface of GaN-based light emitting diodes (LEDs) with a two-dimensional photonic crystal (2DPC). Such a 2DPC structure not only enhanced the light output power but also changed the far-field pattern simultaneously. Also, a TiO2/SiO2 omnidirectional reflector (ODR) was deposited on the backside of the LEDs to further increase the light output power. Under 350 mA current injection, it was found that forward voltages were 3.35, 3.34 and 3.75 V while the light output powers of the LEDs were 59.5, 92.5 and 112.1 mW for the conventional LED, the PCLED with 20 nm depth, and the PCLED with 120 nm depth all with chip size of 1 mm × 1 mm, respectively. A 88.4% enhancement in light output power of PCLED with a 120 nm depth and ODR on the backside could be achieved when compared to the conventional LED under the driving current of 350 mA. From the measurement results, it was also found that the NIL process does not degrade the electrical properties of the fabricated LEDs.

One-Shot Exposure for Patterning Two-Dimensional Photonic Crystals to Enhance Light Extraction of InGaN-Based Green LEDs

To promote the feasibility of using photonic crystals to enhance light extraction of light-emitting diodes (LEDs) for industrial applications, the request for the high-yield and simple exposure process to pattern two-dimensional photonic crystals (2DPCs) becomes rather urgent. In this work, we developed a one-shot three-beam laser interference system to fabricate a 2DPC on an InGaN-based green LED to address its performance for backlight display applications. Using the current exposure system we develop here, a submicro-patterned area of 6 mm in diameter can be achieved. The experimental results show that the operating voltage of a 1 times 1 mm2 in-area LED incorporated with the 2DPC is around 4 V, and the light extraction efficiency is enhanced by a factor of 2.1.

Fabrication and Characterization of InGaN-Based Green Resonant-Cavity LEDs Using Hydrogen Ion-Implantation Techniques

The InGaN-based green resonant-cavity light-emitting diodes (RCLEDs) have been fabricated using hydrogen ion-implantation and laser liftoff techniques. The RCLEDs structure consisted of an multiple-quantum-well active layer between the top (5 pairs) and bottom (7.5 pairs) dielectric distributed Bragg reflectors with optical reflectance of 85% and 99.9%, respectively. The insulation layers of the RCLEDs with and without implantation were formed by the hydrogen ion-implantation layers of concentration and film, respectively. The corresponding forward turn-on voltage at dc current density injection were about and for the RCLEDs with and without implantation. The light output intensity of the RCLEDs with implantation is higher by a factor of 1.4 as compared to that of the similar structure without implantation at a current density of . The directionality of RCLEDs with implantation is superior to that of RCLEDs without implantation.

Effects of Transparent Conductive Layers on Characteristics of InGaN-Based Green Resonant-Cavity Light-Emitting Diodes

InGaN-based green resonant-cavity light-emitting diodes (RCLEDs) with indium–tin oxide (ITO) and Ni/Au transparent conductive layers (TCLs) have been fabricated on Si substrates by laser lift-off and wafer bonding techniques. The RCLED structure consisted of an InGaN/GaN multiple-quantum-well active layer between the top (5 pairs) and bottom (7.5 pairs) dielectric TiO2/SiO2 distributed Bragg reflectors. It was found that the cavity mode of the RCLED with an ITO TCL shows a linewidth of 4 nm at the main emission peak at 494 nm. The electroluminescence intensity of the ITO-RCLED sample is 1.73 times higher in magnitude than that of the Ni/Au-RCLED one. It was found that the quality factor of the InGaN RCLED structure increased from 84 to 120 when the Ni/Au TCL was replaced by ITO. The improvements in both the optical output and the quality factor could be attributed to the higher optical transmittance of the ITO TCL enhancing the spontaneous emission at its resonant wavelength.

Fabrication and Characterization of InGaN-Based Green Resonant Cavity Light-Emitting Diodes Using H+ Ion-Implantation

InGaN-based green resonant-cavity light-emitting diodes (RCLEDs) have been fabricated using hydrogen ion-implantation and laser lift-off techniques. The RCLED structure consisted of an InGaN/GaN multiple-quantum-well active layer between top (5-pairs) and bottom (7.5-pairs) dielectric TiO2/SiO2 distributed Bragg reflectors with optical reflectances of 85% and 99.9%, respectively. The insulating layers of the RCLEDs were formed by either hydrogen ion implanted layers of 1x1014 ions/cm2 or SiO2 films, respectively. The corresponding forward turn-on voltages for a 0.6 KA/cm2 dc injection current density were about 4.58 and 4.55 V, respectively. The light output intensity of the RCLEDs with H+ implantation is higher by a factor of 1.4 compared with the SiO2 structure at a current density of 0.6 KA/cm2. The RCLEDs with H+ implantation have superior directionality compared to the RCLEDs without H+ implantation.

Fabrication and Characterization of InGaN-Based Green Resonant Cavity Light-Emitting Diodes by Using H+ Ion-Implantation

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