GaN LED Research Articles

(Invited) Heterogeneous Integration of IR Detectors with CMOS and Thin Film Transistors

Colloidal quantum dots (CQDs) show promising performance in IR detection because of their many unique advantages, including high material quality, flexibility in operation wavelength, easy fabrication process and suitability for heterogeneous integration with CMOS. Our presentation will cover three areas about colloidal quantum dot detectors: (a) the physical model of CQDs, (b) integration of CQDs with electronics, and (c) strategies of overcoming the limit of dark current for uncooled IR detection. For (a), we will present a physical model considering the unique transport properties of ligand connected quantum dot detectors. The model shows how the CQD detectors perform differently than conventional p-i-n detectors and why those popular parameters derived from Shockley’s diode model such as reverse saturation current and ideality factor cannot model the CQD detectors. For (b), we will show how CQD detectors can be integrated with IGZO thin film transistors on glass substrate, GaN LEDs for wavelength converters, and Si CMOS in a back-end-of-line process. For (c), we will discuss designs of colloidal quantum dot detectors for longer IR wavelengths with minimal effects of dark current to support uncooled operation.

Enhancing GaN LED Efficiency with Advanced Quantum Well Structure

Enhancing GaN LED Efficiency with Advanced Quantum Well Structure

Review Of the Light Extraction Efficiency of Gan Leds

Review Of the Light Extraction Efficiency of Gan Leds

Comparison of GaN LED Optical Simulation in 2D and 3D Space Based on k-Domain Analysis Method

Comparison of GaN LED Optical Simulation in 2D and 3D Space Based on k-Domain Analysis Method

Efficiently green-emitting and ultra-stable CsPbBr3@SiO2 nanocomposites for white light-emitting diodes

Inorganic oxide especially SiO2 encapsulation strategy is an effective way to improve the stability of all-inorganic cesium lead halide perovskite (CsPbX3, X = Cl, Br, I) nanocrystals (NCs). However, when CsPbX3 NCs are encapsulated in SiO2 by in situ tetraethyl orthosilicate (TEOS) hydrolysis, the unavoidable by-products (water and ethanol) of TEOS hydrolysis commonly damage the structural and optical properties of the CsPbX3 NCs. Herein, TDPA-CsPbBr3 NCs with the dense tetradecylphosphonic acid (TDPA) passivation were pre-prepared and subsequently encapsulated into SiO2 via in situ TEOS hydrolysis. Benefitting from the dense phosphonic passivation layer capable of resisting the erosion of polar by-products generated by TEOS hydrolysis, the obtained TDPA-CsPbBr3@SiO2 nanocomposites maintain the excellent optical properties of the colloidal CsPbBr3 NCs, showing near-unity photoluminescence quantum yield (PLQY) and ultra-narrow full width at half maximum (FWHM). Furthermore, the TDPA-CsPbBr3@SiO2 nanocomposites exhibit outstanding stability against ethanol, UV light, and heating treatment due to the dual protection of dense phosphonate ligand layers and inorganic SiO2 matrix. The white light-emitting diodes (WLED) device was fabricated by sequentially depositing green-emitting TDPA-CsPbBr3@SiO2 nanocomposites and red-emitting K2SiF6:Mn on blue-emitting GaN LED chip. The obtained WLED exhibits excellent luminous performances with standard white emission of CIE Coordinates (0.33, 0.33), a color gamut 138 % of the NTSC and 103 % of Rec. 2020, respectively. This study demonstrates that TDPA-CsPbBr3@SiO2 nanocomposites have a potential application in solid-state lighting and backlight displays.

Equivalent analysis model of a GaN LED used as a receiver

In general, visible light communication (VLC) uses LEDs as transmitters. However, LEDs can serve as receivers to construct a simple duplex VLC system that uses only two LEDs instead of one LED and one photo-diode (PD). There is a lack of effective equivalent analysis models for characterizing and evaluating the inherent behavioral characteristics of LEDs used as receivers. This paper presents an equivalent analysis model for GaN LEDs as receivers. First, based on the proposed receiving equivalent circuit model, a third-order signal transmission mathematical analysis model is established, revealing the transmission relationship between the photocurrent and output voltage. Further research is conducted on the impact of parameter changes on the bandwidth, and the model can be simplified into a first-order low-pass mathematical analysis model under specific conditions, providing theoretical support for improving the bandwidth of LED receiving applications. The experimental results also confirm the theoretical predictions. This research result holds significant importance for revealing the intrinsic mechanisms and the improved optical communication performance of LEDs for effective reception.

Size-Dependent Characteristics of High-Bandwidth Photodetector Based on GaN Micro-LEDs and LEDs for High-Speed Visible Light Communication

Size-Dependent Characteristics of High-Bandwidth Photodetector Based on GaN Micro-LEDs and LEDs for High-Speed Visible Light Communication

Highly stable CsPbBr3/ PMA perovskite nanocrystals for improved optical performance

In this study, to address the stability issues, we synthesized a CsPbBr3-coated poly (maleic anhydride-alt-1-octadecene) (CsPbBr3/PMA) using a modified hot-injection method. The CsPbBr3/PMA perovskite nanocrystals (PNCs) exhibited effective green emission at 522 nm with an improved photoluminescence quantum yield (86.8 %) compared to traditional CsPbBr3 PNCs (54.2 %). The ligands in the polymer coating can bond with the uncoordinated Pb and Br ions on the surface of PNCs to minimize surface defects and avoid exposure to the external environment, enhancing the stability of the perovskites. Time-resolved photoluminescence spectra showed longer lifetimes for CsPbBr3/PMA PNCs, while transient absorption measurements provided valuable insights into the intraband hot-exciton relaxation and recombination. We demonstrate the potential application of CSPbBr3/PMA in a down-conversion white-light-emitting diode (LED) by coupling green CsPbBr3/PMA and red K2SiF6:Mn4+ phosphor-coated glass slides onto a 455-nm blue GaN LED. The white LED produced a white light with the International Commission on Illumination color coordinates of (0.323, 0.345), luminous efficiency of 58.4 lm/W, and color rendering index of 83.2. The fabricated, white-LED system obtained a wide color gamut of 125.3 % of the National Television Standards Committee and 98.9 % of Rec. 2020. The findings demonstrate that CsPbBr3/PMA can be an efficient down-conversion material for white LEDs and backlighting.

Micro-Raman for Local Strain Evaluation of GaN LEDs and Si Chips Assembled on Cu Substrates.

Integrated circuits are created by interfacing different materials, semiconductors, and metals, which are appropriately deposited or grown on substrates and layers soldered together. Therefore, the characteristics of starting materials and process temperatures are of great importance, as they can induce residual strains in the final assembly. Identifying and quantifying strain becomes strategically important in optimizing processes to enhance the performance, duration, and reliability of final devices. This work analyzes the thermomechanical local strain of semiconductor materials used to realize LED modules for lighting applications. Gallium Nitride active layers grown on sapphire substrates and Si chips are assembled by soldering with eutectic AuSn on copper substrates and investigated by Raman spectroscopy in a temperature range of -50 to 180 °C. From the Raman mapping of many different samples, it is concluded that one of the leading causes of strain in the GaN layer can be attributed to the differences in the thermal expansion coefficient among the various materials and, above all, among the chip, interconnection material, and substrate. These differences are responsible for forces that slightly bend the chip, causing strain in the GaN layer, which is most compressed in the central region of the chip and slightly stretched in the outer areas.

Transfer-free rapid growth of 2-inch wafer-scale patterned graphene as transparent conductive electrodes and heat spreaders for GaN LEDs

A technique for the transfer-free growth of 2-inch wafer-scale patterned graphene directly on GaN LED epilayers is introduced. High-quality graphene as transparent electrodes and heat spreaders is synthesized directly on GaN by PECVD at only 600 °C deposition temperature and within 3 min growth time. Co acts as both the catalyst for graphene growth and the dry etching mask for GaN mesas, which greatly improves the efficiency of the semiconductor device process. Elegantly, the graphene growth is in accordance with the shape of Co, which offers a lithography-free patterning technique of the graphene. Afterward, using our penetration etching method through the PMMA and graphene layers, the Co is peacefully removed, and in-situ Ohmic contact is achieved between the graphene and p-GaN where the contact resistivity is only 0.421 Ω cm2. The graphene sheet resistance is as low as 631.2 Ω sq−1. The device is also superior to the counterpart graphene-free LED in terms of heat spreading behavior, as evidenced by the lower junction temperature and thermal resistance. Most importantly, the developed technique produces graphene with excellent performance and is intrinsically more scalable, controllable, and semiconductor industry compatible than traditionally transferred graphene.

Facile Microwave Synthesis of Efficient Green Emissive Carbon Dots Powder and Their Application in Visible Light Communication and White Light Emitting Devices

AbstractHighly green emissive solid‐state carbon dots (CDs) with photoluminescence quantum efficiency of 58% are prepared through a rapid microwave assisted heating method. Due to the spatial confinement from the biuret crystal matrix, aggregation among CDs is effectively suppressed, thus allowing the CDs to give efficient emission in the solid‐state. The CDs show excitation independent emission and mono‐exponential decay characteristics with a nearly constant lifetime of ≈13 ns upon varying the detected emissions, indicating the presence of a single type of emissive state in the CDs. Due to their high quantum efficiency and short lifetime, the obtained CDs are applied as the color conversion layer of a near‐ultraviolet micro light‐emitting diode (µLED) chip (405 nm) for visible light communication, achieving a modulation bandwidth of 165 MHz, which is much higher than the bandwidth of the conventional combination of Ce3+‐doped yttrium aluminum garnet phosphor with GaN LED. Moreover, the green emitting solid‐state CDs are applied to fabricate a prototype white LED device, which exhibits good lighting suitability with a color rendering index of 90.6 and a Commission Internationale de L'Eclairage chromaticity coordinates of (0.327, 0.332).

Computational Investigation of c-GaN/GaAs1–x N x /GaAs Heterojunction Solar Cell

GaN-based electronics have witnessed an increase in both research and industrial activities, first spurred by the successful demonstration of GaN LEDs, and are now expanding into transistors and photovoltaic cells. In addition, GaN/GaAs heterojunction devices are currently receiving much interest. In this study, we conduct rigorous optoelectronic computational analysis of cubic phase GaN (c-GaN)/GaAs heterojunction solar cells for a comprehensive understanding of the cell. We utilize a compositionally graded GaAs1–xNx buffer layer to reduce defect states at the heterojunction interface caused by a significant lattice mismatch between c-GaN and GaAs. Furthermore, we enhance the performance of the cell by optimizing GaAs absorber layer thickness and c-GaN buffer layer doping concentration. Moreover, we examine the effects of GaAs1–xNx/GaAs interface recombination velocity (IRV) on the cell. Overall, we achieve ~23% power conversion efficiency within 1.25- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> thin-film GaAs at low GaAs1–xNx/GaAs IRV. The analyses and results presented in this study demonstrate the vast application potential of c-GaN/GaAs heterojunction in high-efficiency solar cells.

Novel Pulsating-DC High-Voltage Linear Driving Scheme for GaN LED General Lighting

This work investigates a novel pulsating DC high-voltage linear driving scheme for GaN-based Light-emitting diode (GaN LED) general lighting to save costs and alleviate flicker. The superiority and practicality of this scheme in three-phase AC power grids were demonstrated for the first time. Compared to applications for single-phase AC grids, linear driving of GaN LEDs for three-phase AC grids can provide superior performance for general lighting. The DC component of the three-phase AC rectified voltage reaches 90.7%, which effectively alleviates the flicker problem. In this paper, we balanced GaN LED power and driving efficiency by optimizing the GaN LED distribution of the linear multi-string GaN LED driving scheme while taking the effects of grid voltage fluctuations into account. In addition, we constructed a double-string GaN LED lighting system as a modular prototype with scalability. The experimental results exhibit high driving efficiency (~94% @380 V line voltage), high power factor (~0.952), flicker-free, and high reliability at a very low cost (~$0.005/W).

Self-filtering illumination source and application in fluorescence imaging

To date, fluorescence imaging systems have all relied on at least one beam splitter (BS) to ensure the separation of excitation light and fluorescence. Here, we reported SiO2/TiO2 multi-layer long pass filter integrated GaN LED. It is considered as the potential source for imaging systems. Experimental results indicate that the GaN LED shows blue emission peaked at 470.3 nm and can be used to excite dye materials. Integrating with a long pass filter (550 nm), the light source can be used to establish a real-time fluorescence detection for dyes that emit light above 550 nm. More interestingly, with this source, a real-time imaging system with signature words written with the dyes, such as ‘N J U P T’, can be converted into CCD images. This work may lead to a new strategy for integrating light sources and BS mirrors to build mini and smart fluorescence imaging systems.

Ultra High Luminous Efficiency p-Type Surface Defect Structure GaN LED

In this paper, we propose a double-layer SiO2 photonic crystal LED with a linear defect structure in the [Formula: see text]-GaN layer, the purpose is to solve the problem of low light extraction efficiency caused by diffusion scattering effect. We used FDTD modeling to analyze the light-emitting characteristics of GaN LED, after optimizing the structural parameters of two-dimensional photonic crystals, the light extraction efficiency is improved from 19.4% to 35.2%, the light extraction efficiency is increased by 1.8 times, meanwhile the light output power is also increased by 1.86 times. The introduction of the defect structure does not affect the [Formula: see text]–[Formula: see text] characteristics of the LED, and the output power of the LED is increased by 11% under the current of 300[Formula: see text]mA.

Low Melting Point Phosphor-In-Glass (PiG) Realizing at a Normal Atmosphere and Packaged Performance with GaN LED

In this study, a low-temperature normal atmosphere phosphor composite manufacturing technique based on the Sn-P-F-O glass system is proposed to solve the phosphor silicon difficulties of easy aging, low color rendering, oxygen and heat stability. To prepare ((BaSr)2SiO4:Eu2+) Green and (CaAlSiN3:Eu2+) Red phosphor in glass (PiG), the Sn-P-F-O glass precursor was sintered for 1 hour at 800 °C, then mixed with G/R phosphors. X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence spectroscopy (PL) were used to investigate the crystal phase, microstructure, excitation and emission spectra of PiG. To investigate the mechanism of phosphor and matrix glass mixing ratio on optical performance of PiG-LED, the G/R PiG plate was utilized to encapsulate GaN LED chips and its optoelectronic performance was examined. Excited by current of 350 mA, G/R PiG-LED emits 6506 K correlated color temperature (CCT) white light when G/R ratio is 9:1, color rendering index as high of 90.4 when G/R ratio is 13:1.

Strain Relaxation Effect on the Peak Wavelength of Blue InGaN/GaN Multi-Quantum Well Micro-LEDs

In this paper, the edge strain relaxation of InGaN/GaN MQW micro-pillars is studied. Micro-pillar arrays with a diameter of 3–20 μm were prepared on a blue GaN LED wafer by inductively coupled plasma (ICP) etching. The peak wavelength shift caused by edge strain relaxation was tested using micro-LED pillar array room temperature photoluminescence (PL) spectrum measurements. The results show that there is a nearly 3 nm peak wavelength shift between the micro-pillar arrays, caused by a high range of the strain relaxation region in the small size LED pillar. Furthermore, a 19 μm micro-LED pillar’s Raman spectrum was employed to observe the pillar strain relaxation. It was found that the Raman E2H mode at the edge of the micro-LED pillar moved to high frequency, which verified an edge strain relaxation of = 0.1%. Then, the exact strain and peak wavelength distribution of the InGaN quantum wells were simulated by the finite element method, which provides effective verification of our PL and Raman strain relaxation analysis. The results and methods in this paper provide good references for the design and analysis of small-size micro-LED devices.

57‐4: Invited Paper: Progress on Key Innovations in Direct‐View μLED Display Manufacturing

Applied Materials reported earlier on an innovative approach to μLED‐based front plane technology using a single wavelength GaN LED emitter with Cd‐free quantum dots for color conversion. This proprietary μLED front plane technology incorporates several key innovations such as the use of high external quantum efficiency dice in ultraviolet wavelength spectrum (vs. red/green/blue), highest photon extraction efficiency at small die‐size, fewer number of mass transfer steps with high yield, and a four‐subpixel backplane architecture to allow for a simple, cost‐effective repair strategy. A picture of this display is shown below.

An Investigation on CCT and Ra Optimization for Trichromatic White LEDs Using a Dual-Weight-Coefficient-Based Algorithm.

Spectral optimization is applied as an effective tool in designing solid-state lighting devices. Optimization speed, however, has been seldomly discussed in previous reports as regards designing an algorithm for white light-emitting diodes (WLEDs). In this study, we propose a method for trichromatic WLEDs to obtain the optimal Ra under target correlated color temperatures (CCTs). Blue-, yellow-, and red-color monochromatic spectra, produced by the GaN LED chip, YAG:Ce3+ phosphors, and CdSe/ZnSe quantum dots, respectively, are adopted to synthesize white light. To improve the effectiveness of our method, the concept of dual weight coefficients is proposed, to maintain a numerical gap between the proposed floating CCT and the target CCT. This gap can effectively guarantee that Ra and CCT ultimately move toward the targeting value simultaneously. Mechanisms of interaction between CCT, Ra, and dual-weight coefficients are investigated and discussed in detail. Particularly, a fitting curve is drawn to reveal the linear relationship between weight coefficients and target CCTs. This finding effectively maintains the accuracy and accelerates the optimization process in comparison with other methods with global searching ability. As an example, we only use 29 iterations to achieve the highest Ra of 96.1 under the target CCT of 4000 K. It is hoped that this study facilitates technology development in illumination-related areas such as residential intelligent lighting and smart planting LED systems.

Hybrid Device of Blue GaN Light-Emitting Diodes and Organic Light-Emitting Diodes with Color Tunability for Smart Lighting Sources.

A lighting device with a wide color-tunable range is still a challenge for lighting based on either organic light-emitting diodes (OLEDs) or inorganic LEDs. In this work, we first proposed a novel hybrid device of organic LEDs and inorganic blue GaN LEDs to achieve full white and other colors. Organic LEDs were stacked with green and red emissive layers and connected with blue GaN LEDs in parallel but in opposite polarity voltage. Under the alternate-current (AC) driving, the hybrid structure can be controlled independently by applying timing variable opposite voltages to emit the light from either blue LEDs or the stacked OLEDs for forming mixed colors. The hybrid device can generate white light, varying in a wide range by changing the amplitude and duty ratio (DR) of AC-driving signals, from cold white to standard white and to warm white (3668–11 833 K). When an AC voltage of (4.80 V, −2.45 V) was applied, the device has a high color gamut of 95.24% National Television System Committee (NTSC) and a high color rendering index (Ra) of 92.4%. The novel hybrid device with the blue LED and OLED in opposite polarity exhibits potential applications in smart solid-state lighting, display, and light communication.