GaN-based Blue Light-emitting Diodes Research Articles

Investigation into the Effects of Cross-Sectional Shape and Size on the Light-Extraction Efficiency of GaN-Based Blue Nanorod Light-Emitting Diode Structures

We investigated the effect of cross-sectional shape and size on the light-extraction efficiency (LEE) of GaN-based blue nanorod light-emitting diode (LED) structures using numerical simulations based on finite-difference time-domain methods. For accurate determination, the LEE and far-field pattern (FFP) were evaluated by averaging them over emission spectra, polarization, and source positions inside the nanorod. The LEE decreased as rod size increased, owing to the nanorods’ increased ratio of cross-sectional area to sidewall area. We compared circular, square, triangular, and hexagonal cross-sectional shapes in this study. To date, nanorod LEDs with circular cross sections have been mainly demonstrated experimentally. However, circular shapes were found to show the lowest LEE, which is attributed to the coupling with whispering-gallery modes. For the total emission of the nanorod, the triangular cross section exhibited the highest LEE. When the angular dependence of the LEE was calculated using the FFP simulation results, the triangular and hexagonal shapes showed relatively high LEEs for direction emission. The simulation results presented in this study are expected to be useful in designing high-efficiency nanorod LED structures with optimum nanorod shape and dimensions.

Optimizing the design of highly linearly polarized white LED with multilayer metal-dielectric grating structure

In order to emit a linearly polarized light and reduce energy loss for a white light emitting diode (LED), a multilayer metal-dielectric grating structure using GaN-based blue LED is proposed. This multilayer structure is composed of three major functional layers: a fluorescent ceramics layer, a dielectric anti-reflection layer and a linear polarization grating layer. Fluorescent ceramics layer is integrated on the top of GaN-based blue LED, to achieve white light emission. In turn, a dielectric anti-reflection layer and a linear polarization grating layer are also integrated to obtain a higher transmission of white light and improve the conversion efficiency of linear polarization. The structural parameters of each functional layer, including period (150 nm), duty cycle (0.5), height of metal-dielectric grating (55 nm and 80 nm) and thickness of dielectric layer (15 nm), are simulated and optimized by finite-difference time-domain method. Then the multilayer structure is fabricated by UV nanoimprint and its transmission and polarization characteristics are tested. The results show that the transmittance and extinction ratio are higher than 50% and 33.8 dB in the wavelength range of 400–800 nm, and the maximum value can reach 91.3% and 41.3 dB, respectively. It is well verified by experiments that the proposed structure has a steady polarization performance of TM mode in the range of ±20°, which provides a fundamental guarantee for developing the potential applications of polarized white LED.

Enhanced performance of GaN-based long-wavelength blue light-emitting diodes with graphene-ITO composite film as transparent conductive electrode

Abstract We present enhanced performance of GaN-based long-wavelength blue light emitting diodes (LEDs) with hybrid transparent conductive electrodes (TCE) made of graphene and indium tin oxide (ITO) composite. The nearly 100% transmittance TCE were gained when graphene grown by chemical-vapor-deposition was annealed with ITO on the top of it. Compared to conventional LEDs, the work voltage was reduced to 3.5 V at 5 mA forward current. The light emission power was improved about 190%. The good peformance can be attributed to the improved lateral current spreading over the composite graphene-ITO TCE before p-GaN layer injected by carriers.

Color-tunable quantum-dot light emitting diode and its integration with GaN-based blue LED for smart white-light emission

White-light emission with tunable color temperature have potential applications in the fields of smart lighting and backlight. In this study, we introduced an electron blocking layer (EBL) between red and green emission layers (EMLs) in a quantum-dot light-emitting diode (QLED). The emitting color of QLED can be tuned from red to green as the carrier distribution and recombination region are regulated by the applied voltage. In addition, the color tunable QLED device was integrated with a GaN-based blue light emitting diode (LED), achieving a hybrid QLED/LED device for white-light emission. Under the alternate-current (AC) driving method, the QLED was forwardly driven while the LED was reversely biased, and thus, both devices can be independently addressed by the polarity of the AC driving voltage. The results showed that the color temperature of white-light emission can be tuned from 3102K (warm white) to 12853K (cold white) and the highest color rendering index (CRI) of 88.6 can be also achieved, exhibiting great prospects in the application of lighting.

Closing the yellow gap with Eu- and Tb-doped GaN: one luminescent host resulting in three colours

Gallium nitride (GaN) is a key material when it comes to light-emitting diodes (LEDs) and has pushed the LED revolution in lighting and displays. The concept of down-conversion of a GaN-based blue LED offers the possibility to provide efficient generation of monochromatic, high-color purity light resulting in a highly efficient warm-white all-nitride phosphor-converted light emitting diode (pc-LED). Although the down conversion of blue light from InGaN LEDs has become a dominant technique for producing white light, there are still some technical challenges, e.g. the immiscibility of GaN and InN and the lattice mismatch between the substrate and InGaN, that have to be overcome. Here we demonstrate the doping of bulk GaN with europium, terbium and the combination of both resulting in intriguing luminescence properties, pushing the role of GaN:Eu,Tb as a chief component in future light emitting diodes. This colour tuning proves that one luminescence host can provide three colours (red, green and orange) and that even the so called “yellow gap” could be closed with a III-nitride. By using one material for all colours, it will be possible to overcome the technical challenges in building up LED devices, which will open up new capabilities for modern highly efficient phosphors.

Accurate determination of junction temperature in a GaN-based blue light-emitting diode using nonlinear voltage-temperature relation

We investigate the junction temperature measurements for GaN-based blue light emitting diodes (LEDs) using nonlinear dependence of the forward voltage (Vf) on temperature. Unlike the conventional linear model of the dependence of Vf on temperature, the modeling of the temperature dependent Vf with a quadratic function showed good agreements with measured data in the temperature range between 20 and 100 °C. Using the proposed quadratic model, the junction temperature and thermal resistance of the measured LED could be accurately determined as the ambient temperature varied. It was observed that the junction temperature increment remained almost unchanged as the ambient temperature increased from 20 to 80 °C, which could be attributed to the interplay between the decrease in series resistance and the increase in non-radiative recombination with increasing temperature. The presented method for accurate determination of the junction temperature is expected to be advantageously employed for the thermal management of high-power LEDs.

Numerical Investigation of Light Extraction Efficiency of GaN-based Vertical Blue Micron-Scale Light-Emitting Diode Structures.

The light extraction efficiency (LEE) of GaN-based vertical blue micron-scale light-emitting diode (μ-LED) structures was investigated numerically using three-dimensional finite-difference timedomain (FDTD) methods. The entire μ-LED chip was included in the FDTD computational domain to determine the LEE accurately. As the lateral dimensions of μ-LEDs increased from 5 to 30 μm, the LEE decreased gradually because of the increased portion of light trapped inside the LED chip and the increased light absorption in the GaN layers with increasing chip size. The LEE varied strongly with the p-GaN thickness for the μ-LED with a flattop surface, which could be explained by the strong dependence of the spatial distribution of the emission patterns on the p-GaN thickness. This dependence on the p-GaN thickness decreased when the surface of the μ-LED chip was patterned. A high LEE of >80% could be achieved in LEDs with properly chosen parameters. The FDTD simulation results presented in this study are expected to be employed advantageously in designing μ-LED structures with a high LEE.

Influence of multiple quantum well number on modulation bandwidth of InGaN/GaN light-emitting diodes

In this work, we investigated the relationship between the modulation bandwidths and the number of quantum wells of GaN-based blue light-emitting diodes (LEDs). The number of quantum wells is designed to be 1, 3, 6, 9 and 12, respectively. Our results show that when the number of quantum wells of the LED is set to be 3 at the same injection current, not only the −3 dB bandwidth of the LED gets its largest, but also the modulation characteristics obtain best responses. Combining results from time-resolved photoluminescence spectroscopy (TRPL), alternating current (AC) impedance spectroscopy and optical output power of the LEDs further concluded that setting the quantum well number of LEDs 3 optimizes visible light communication.

Optical Characterization of GaN-Based Vertical Blue Light-Emitting Diodes on P-Type Silicon Substrate

Fabricating GaN-based light-emitting diodes (LEDs) on a silicon (Si) substrate, which is compatible with the widely employed complementary metal–oxide–semiconductor (CMOS) circuits, is extremely important for next-generation high-performance electroluminescence devices. We conducted a systematic investigation of the optical properties of vertical LEDs, to reveal the impacts of the manufacturing process on their optical characteristics. Here, we fabricated and characterized high-efficiency GaN-based LEDs with integrated surface textures including micro-scale periodic hemispherical dimples and nano-scale random hexagonal pyramids on a 4 inch p-type Si substrate. The highly reflective Ag/TiW metallization scheme was performed to decrease downward-absorbing light. We demonstrated the influence of transferring LED epilayers from a sapphire substrate onto the Si substrate on the emission characteristics of the vertical LEDs. The removal of the sapphire substrate reduced the adverse impacts of the quantum-confined Stark effect (QCSE). The influence of integrated surface textures on the light extraction efficiency (LEE) of the vertical LEDs was studied. With the injection current of 350 mA, vertical LEDs with integrated surface textures demonstrated an excellent light output power of 468.9 mW with an emission peak wavelength of 456 nm. This work contributes to the integration of GaN-based vertical LEDs into Si-based integrated circuits.

Research on 1/f noise degradation of GaN LED caused by γ radiation under high bias

GaN-based blue light-emitting diodes (LEDs) were radiated with 60Co γ-rays for an accumulated dose of up to 2.5 Mrad (SiO2). The radiation-induced current and 1/f noise degradations were studied when the devices operate at the high bias voltage. The current of GaN blue LED is determined by series resistance at the high bias voltage. Defects created by γ radiation in the LED lead to an increase in the series resistance, which causes the decrease of current. Based on the theory of Hooge mobility fluctuations, it suggests that the degradation of 1/f noise might also be attributed to these defects, which caused a decrease in the carrier mobility and the carrier number.

A new strategy to the NaY(MoO4)2:Eu3+ phosphors modified by tetraethyl orthosilicate for LEDs: Photoludminescence properties and morphology

NaY(MoO4)2:Eu3+ microcrystals with different morphologies have been successfully synthesized via a hydrothermal method by varying the molar ratios of tetraethyl orthosilicate (TEOS). The structure and photoluminescence properties of phosphors are investigated by the X-ray powder diffraction (XRD), transmission electron microscope (TEM), the ultraviolet-visible diffuse reflectance spectra (UV–vis DRS), photoluminescence excitation (PLE) and emission (PL) spectra as well as the decay curves. There are two intense PLE peaks at 395 and 465 nm in the PLE spectra, which well matched with the near ultraviolet (NUV) and blue GaN-based light-emitting diodes (LEDs). In the PL spectra, the strongest PL peak is caused by 5D0 → 7F2 electric dipole transition. In addition, the amount of TEOS has an important impact on the morphologies and PL intensity of samples. With the increase of the amount of TEOS, the morphologies of the samples vary from double shuttle-like architectures, truncated rhombic polyhedrons, to quasi-cubes. It is found that the International Commission on Illumination (CIE) chromaticity coordinates of phosphors are located in red region, and the color purity is 99.70 % for S2 sample. Besides, the correlated color temperature (CCT) values of S1−3 samples are in the range of “warm appearance”. Therefore, the above obtained results suggest that the samples possess potential application value in the field of optical display systems, photoelectric devices and LEDs.

Research on Degradation of GaN-Based Blue LED Caused by γ Radiation under Low Bias

GaN multiquantum-well blue light-emitting diodes (LEDs) were radiated with 60Co γ-rays for accumulated doses up to 2.5 Mrad (SiO2). The radiation-induced current and 1/f noise degradations were studied when the devices operate at the low bias voltage. The current increased by 2.31 times, and the 1/f noise increased by 275.69 times after a dose of 2.5 Mrad (SiO2). Based on Hurkx’s trap-assisted tunneling model, the degradation of current was explained. γ radiation created defects in the space-charge region of LEDs. These defects as generation-recombination centers lead to the increase in the current. In addition, based on the quantum l/f noise theory, the degradation of 1/f noise might be also attributed to these defects, which caused an increase in the Hooge constant and a decrease in the carrier lifetimes. The current and 1/f noise degradations can be attributed to the same physical origin. Compared to the current, the 1/f noise parameter is more sensitive, so it may be used to evaluate the radiation resistance capability of GaN blue LEDs.

Investigation of the Electroluminescence Mechanism of GaN-Based Blue and Green Light-Emitting Diodes with Junction Temperature Range of 120–373 K

Junction temperature (Tj) and current have important effects on light-emitting diode (LED) properties. Therefore, the electroluminescence (EL) spectra of blue and green LEDs were investigated in a Tj range of 120–373 K and in a current range of 80–240 mA based on accurate real-time measurements of Tj using an LED with a built-in sensor unit. Two maxima of the emission peak energy with changing Tj were observed for the green LED, while the blue LED showed one maximum. This was explained by the transition between the donor-bound excitons (DX) and free excitons A (FXA) in the green LED. At low temperatures, the emission peak energy, full width at half maximum (FWHM), and radiation power of the green LED increase rapidly with increasing current, while those of the blue LED increase slightly. This is because when the strong spatial potential fluctuation and low exciton mobility in the green LED is exhibited, with the current increasing, more bonded excitons are found in different potential valleys. With a shallower potential valley and higher exciton mobility, excitons are mostly bound around the potential minima. The higher threshold voltage of the LEDs at low temperatures may be due to the combined effects of the band gap, dynamic resistance, piezoelectric polarization, and electron-blocking layer (EBL).

A simulation study on the enhancement of the efficiency of GaN-based blue light-emitting diodes at low current density for micro-LED applications

We numerically investigated the optical/electrical characteristics of blue light-emitting-diodes (LEDs) and developed some optimization strategies. Our aim is to increase the optical performance of LEDs at low current density (0.02–2 A cm−2) for micro-LEDs used in display applications. First, we investigated the effects of quantum well (QW) number in blue In0.225Ga0.775N/In0.10Ga0.90N LEDs. We found that at low current density LEDs with lower QW number have much higher internal quantum efficiency (IQE) than those with higher QW number. Second, we also numerically investigated the effects of the electron blocking layer, n-side quantum barrier (QB) doping concentration, QB material and QW width with single quantum well (SQW) structure. For SQW LEDs at low current density, the electron injection efficiency was nearly one. In terms of the QB doping concentration, we found that an appropriate n-side QB doping can ensure Shockley-Read-Hall recombination appropriately equal Auger recombination, which ultimately results in high IQE. For QB material, we choose In0.05Ga0.95N as QB material to achieve both high IQE and low forward voltage. Regarding the QW width, we found that although LEDs with wide QW had high IQE, there were also two peaks in the electroluminescence (EL) spectrum, thus 3 nm was used as an optimal QW width. The original 5-QW structure showed a lower IQE range from 50 to 80% at the current density range from 0.01 to 2 A cm−2 and a forward voltage of 3.06 V at 1 A cm−2. In contrast, our optimized LED structure achieved an IQE of 92.3% at the current density range from 0.01 to 2 A cm−2 and a forward voltage of 2.72 V at 1 A/cm2.

Emission dynamics of GaN-based blue resonant-cavity light-emitting diodes

We fabricated GaN-based blue resonant-cavity light-emitting diodes (RCLEDs) by inserting InGaN quantum well (QW) active region between two dielectric distributed Bragg reflectors (DBRs). Due to the different gain enhancement factors in a single device, multi-longitudinal modes were observed and tuned with changing the injection current density: pure-blue (PB) at low current density, violet-blue (VB) at intermediate current density, and PB again at high current density. The variation of emission spectra is explained by the competition between band-filling effect and self-heating effect.

Over 700 MHz –3 dB Bandwidth UOWC System Based on Blue HV-LED With T-Bridge Pre-Equalizer

To achieve a good balance among the raw modulation bandwidth, the output optical power, and the dynamic swing range, three small GaN-based blue light-emitting diode (LED) cells are cascade interconnected on chip to form a 9 V HV-LED. A single-stage T-bridge equalizer is proposed to upgrade the -3 dB bandwidth and -6 dB bandwidth of a visible light communication/underwater optical wireless communication (VLC/UOWC) system based on the HV-LED to be over 700 and 900 MHz, respectively. To the best of our knowledge, it is the widest bandwidth of a VLC/UOWC system at the similar LED optical power level (120 mW). Under the forward error correction (FEC) limit of 3.8 × 10 -3 , data rates of 3.4 Gb/s over 1.8-m free-space transmission and 3 Gb/s over 1.2-m underwater channel are obtained using 16QAM-OFDM modulation scheme without complex algorithm. The T-bridge equalizer also improves the error free modulation bandwidths up to 450 MHz under 16QAM-OFDM, and up to 600 MHz under 8QAM-OFDM, respectively. Furthermore, the conjunction of the developed HV-LED and T-bridge equalizer endows the merit of a wide linear region of the modulation response, which will make the communication system be more tolerance to LED's degradation and system fluctuations.

High-stability reflective bonding pads for GaN-based flip-chip light-emitting diodes packaged by reflow soldering

We provide a detailed insight into the effects of metal bonding pads on the performance of GaN-based flip-chip blue light-emitting diodes. The flip-chip blue light-emitting diodes with four kinds of reflective bonding pads were fabricated and compared. During the chip package process by reflow soldering, different bonding pads exhibited distinct voidage and bonding force with the solder layers on Cu-coated AlN ceramic substrates. Compared with other samples in this work, the packaged chips with Ti/Al/Ti/Pt/Au bonding pads showed the lowest voidage of 15.48% and thermal resistance, resulting in the highest bonding shear force and light output power. Meanwhile, the optical power of these samples with Ti/Al/Ti/Pt/Au bonding pads only exhibited a degradation rate of 1.8% after accelerated ageing at 100 °C for more than 1000 h. It is concluded that GaN-based flip-chip light-emitting diodes with such Ti/Al/Ti/Pt/Au bonding pads bear good mechanical stability, excellent thermal performance and long-term reliability.

Temperature dependence of the color rendering index of a phosphor-conversion white light-emitting diode

We experimentally investigated the temperature dependence of the color rendering index (CRI) of a phosphor-conversion (PC) white light-emitting diode (LED) composed of a GaN-based blue LED chip and yttrium aluminum garnet doped with Ce3+ (YAG) phosphor. For the 15 CRI values from R1 to R15, 12 CRI values were found to increase with temperature from 20 to 80 °C. The general CRI (Ra) was increased from 84.6 to 86.1 as the temperature increased from 20 to 80 °C. R9 which represents the color rendering property for the deep red was found to increase by more than 10% and some CRI values exhibited more than 20% increase in this temperature range. This increasing behavior of CRI with temperature is contrary to the case of a multi-chip-based white LED, where the Ra was reported to decrease with increasing temperature. The improvement in the CRI with temperature for the PC white LED is attributed to the relative increase of the spectral power distribution (SPD) in the wavelength range from 460 to 520 nm and the overall broadening of SPD with increasing temperature. As a result of the increase in the CRI with temperature, the PC white LEDs are expected to be advantageously used for stable high-temperature operation of LED lighting.

Low-frequency generation-recombination noise behaviors of blue/violet-light-emitting diode

During the past two decades, GaN-based light-emitting diode has been used as a high-quality light-source. Low-frequency noise as a diagnostic tool for quality control and reliability estimation has been widely accepted and used for semiconductor devices. Understanding the origin of efficiency-droop effect is key to developing the ultimate solid-state light source. Various mechanisms that may cause this effect have been suggested, including carriers’ escape, loses due to dislocations, and the Auger effect. In this study, we investigate the low-frequency noise behaviors of GaN-based blue light-emitting diode with InGaN/GaN multiple quantum wells. The measured currents range from 0.1 mA to 180 mA. According to the characteristics of power spectral density of current noise and the generation-combination mechanism between electrons and holes in the active region of light-emitting diode (LED), we adopt the well-known model of low-frequency noise to fit the relationship between power spectral density of current and frequency, and find that there exists a transition between generation-combination and 1/<i>f</i> noise when the light-emitting diode begins to work. In other words, it can be derived that the low-frequency noise behaviors are dominated by generation-combination noise when the currents are between 0.1 mA and 27 mA; with the current gradually increasing, the origin source of low-frequency noise in blue/violet-light LED will transit to the 1/<i>f</i> noise. Through the analysis of the transport and recombination mechanism of the carriers, and combination with the model of low-frequency noise, we analyze the corner frequency of the generation-recombination noise. The results of this paper provide an effective tool and method to study the conversion of light-emitting diodes.

Performance improvement of light-emitting diodes with double superlattices confinement layer * * Project supported by the National Key Research and Development Program of China (No. 2016YFB0400102), the Beijing Municipal Science and Technology Project (Nos. Z161100002116032, D12110300140000), the National Basic Research Program of China (No. 2011CB301902), the Guangzhou Science & Technology Planning Project of Guangdong Province, China (Nos. 201604016095, 201604030035), the

In this study, the effect of double superlattices on GaN-based blue light-emitting diodes (LEDs) is analyzed numerically. One of the superlattices is composed of InGaN/GaN, which is designed before the multiple quantum wells (MQWs). The other one is AlInGaN/AlGaN, which is inserted between the last QB (quantum barriers) and p-GaN. The crucial characteristics of double superlattices LEDs structure, including the energy band diagrams, carrier concentrations in the active region, light output power, internal quantum efficiency, respectively, were analyzed in detail. The simulation results suggest that compared with the conventional AlGaN electron-blocking layer (EBL) LED, the LED with double superlattices has better performance due to the enhancement of electron confinement and the increase of hole injection. The double superlattices can make it easier for the carriers tunneling to the MQWs, especially for the holes. Furthermore, the LED with the double superlattices can effectively suppress the electron overflow out of multiple quantum wells simultaneously. From the result, we argue that output power is enhanced dramatically, and the efficiency droop is substantially mitigated when the double superlattices are used.