Wavelength Uniformity Research Articles

Ultra-high brightness Micro-LEDs with wafer-scale uniform GaN-on-silicon epilayers

Owing to high pixel density and brightness, gallium nitride (GaN) based micro-light-emitting diodes (Micro-LEDs) are considered revolutionary display technology and have important application prospects in the fields of micro-display and virtual display. However, Micro-LEDs with pixel sizes smaller than 10 μm still encounter technical challenges such as sidewall damage and limited light extraction efficiency, resulting in reduced luminous efficiency and severe brightness non-uniformity. Here, we reported high-brightness green Micro-displays with a 5 μm pixel utilizing high-quality GaN-on-Si epilayers. Four-inch wafer-scale uniform green GaN epilayer is first grown on silicon substrate, which possesses a low dislocation density of 5.25 × 108 cm−2, small wafer bowing of 16.7 μm, and high wavelength uniformity (standard deviation STDEV < 1 nm), scalable to 6-inch sizes. Based on the high-quality GaN epilayers, green Micro-LEDs with 5 μm pixel sizes are designed with vertical non-alignment bonding technology. An atomic sidewall passivation method combined with wet treatment successfully addressed the Micro-LED sidewall damages and steadily produced nano-scale surface textures on the pixel top, which unlocked the internal quantum efficiency of the high-quality green GaN-on-Si epi-wafer. Ultra-high brightness exceeding 107 cd/m2 (nits) is thus achieved in the green Micro-LEDs, marking the highest reported results. Furthermore, integration of Micro-LEDs with Si-based CMOS circuits enables the realization of green Micro-LED displays with resolution up to 1080 × 780, realizing high-definition playback of movies and images. This work lays the foundation for the mass production of high-brightness Micro-LED displays on large-size GaN-on-Si epi-wafers.

Uniform and large-size perovskite CsPb(BrxI1-x)3 quantum dot glass for laser display application

As a new display mode, laser display is becoming popular in the industry, and it has served our lives. In addition, a new material, perovskite quantum dots(QDs) is more capable of energizing laser displays with a narrow half-peak width, tunable emission and high color purity. Here, by incorporating perovskite CsPb(BrxI1-x)3 quantum dots into the glass, we explored the performance of large-size glass in laser display. It is demonstrated that the sheet of CsPbBr3 and CsPb(Br0.25I0.75)3 glasses has good wavelength uniformity, and the phosphor wheel with the sheets yields a wide color gamut (its color gamut reaches 124.3 % of NTSC and 91.3 % of Rec.2020), which will play a significant advantage in the field of laser projection.

The PAU survey: photometric calibration of narrow band images

ABSTRACT The physics of the accelerating Universe (PAU) camera is an optical narrow band and broad band imaging instrument mounted at the prime focus of the William Herschel Telescope. We describe the image calibration procedure of the PAU survey data. We rely on an external photometric catalogue to calibrate our narrow band data using stars that have been observed by both data sets. We fit stellar templates to the stellar broad-band photometry of the Sloan Digital Sky Survey and synthesize narrow band photometry that we compare to the PAUS narrow band data to determine their calibration. Consequently, the PAUS data are in the AB system as inherited from its reference calibrator. We do several tests to check the performance of the calibration. We find it self-consistent when comparing repeated observations of the same objects, with a good overall accuracy to the AB system which we estimate to be at the 2 per cent precision level and no significant trends as a function of narrow band filter or wavelength. Repeated observations allow us to build a spatial map of the illumination pattern of the system. We also check the wavelength dependence of the calibration comparing to stellar spectra. We find that using only blue stars reduces the effects of variations in the stellar template fitting to broad-band colours, improving the overall precision of the calibration to around 1 per cent and its wavelength uniformity. The photometric redshift performance obtained with the PAUS data attests to the validity of our calibration to reach the PAUS science goals.

Research Progress of Micro-LED Display Technology

Micro-LED display technology is considered to be the next generation of display technology, which has the characteristics of high miniaturization, thin film and integration, as well as the advantages of high brightness, high contrast, fast response speed and long service life. However, in the development of Micro-LED display technology, there are still some technical and cost problems to be solved. This paper focuses on the key technologies involved in Micro-LED display technology, such as chip technology, mass transfer, full-color display, bonding and driving technology, the research history and frontier progress of these technologies are reviewed in detail. For chip epitaxy technology, the wavelength uniformity, current density and defect control are emphasized; for chip process, the two main chip structures and the challenges brought by miniaturization are discussed emphatically; for chip integration, full-color display, backplane bonding and driving, several mainstream technical schemes are summarized in turn. Finally, the chip detection and repair technologies and commercial application are introduced.

MOVPE and its future production challenges

III-V Compound Semiconductors (CS) such as Gallium Arsenide (GaAs), Indium Phosphide (InP) and more latterly, Gallium Nitride (GaN) are critical Key Enabling Technologies, fundamentally underpinning several recent major technology revolutions. Two major current and likely-future markets within the burgeoning photonics industry ($7 billion) are 3D imaging and sensing and LIDAR applications, with high predicted market growth. Datacom and industrial markets add considerable further momentum to this market growth. Evidence of such expansion is provided in the manufacturing of VCSEL-based devices, which have in recent years, entered a rapid growth phase. For the mass-manufacturing of for example, GaAs/AlGaAs-based VCSEL MOVPE epitaxy, many areas require addressing in key underlying R&D, such as cost-reduction and in thickness and wavelength uniformity. In addition, scalability, (e.g. to 150 mm wafers), automation and in-situ process control will impact yield and throughput improvements.We will demonstrate the preparations for mass-manufacturing, through improvements implemented across the evolution of the VCSEL epiwafers from smaller diameters, through 100 mm and on to 150 mm wafers. More recently however, further developments have led to even larger diameter VCSEL wafer epitaxy, on both GaAs and Ge 200 mm substrates. Challenges in growing on such large diameter wafers will be presented from the epitaxy (layer uniformity and wafer bow) and device fabrication perspective. We will demonstrate how these improvements impact on the manufacturing readiness of production VCSEL wafers.

11.2: Invited Paper: Study of Green LED Growth for Advanced Display Applications

Advanced display technology has various potential applications with a huge potential market. Recently, mini‐LEDs and micro‐LEDs are attracting great attentions for advanced display applications because of the high dynamic range, high ambient contrast ratio, thin profile, and low power consumption [1]. Due to higher indium composition in the active layers, green LEDs' wavelength is more sensitive to temperature variation. Higher indium content InGaN layers also make green LEDs seriously suffer from quantum confined stark effect (QCSE). So it is much more difficult to get good wavelength uniformity and high efficiency for green LEDs. In this work, several factors including local temperature tuning, stress control during epitaxy growth, substrate pattern size and substrate orientation have been studied to improve green mini‐LED's wavelength uniformity and optical intensity.

A review of key technologies for epitaxy and chip process of micro light-emitting diodes in display application

The continuous miniaturization and integration of pixelated devices have become a main trend in the field of display. Micro light-emitting diode (micro-LED) display is composed of an array of LEDs that are sub-50-micrometers in length. It has huge advantages in brightness, resolution, contrast, power consumption, lifetime, response speed and reliability compared with liquid crystal display (LCD) and organic LED (OLED) display. Consequently, micro-LED display is regarded as the next-generation display technology with high potential applications, such as virtual reality (VR), augmented reality (AR), mobile phones, tablet computers, high-definition TVs and wearable devices. Currently, the combination of commercial 5G communication technology with VR/AR display, ultra high definition video technologies will further prompt the development of micro-LED display industry. However, some basic scientific and technological problems in micro-LED display remain to be resolved. As the chip size shrinks to below 50 μm, some problems that are not serious for large-sized LEDs appear for micro-LEDs. These problems include crystalline defects, wavelength uniformity, full-color emmision, massively tranferring and testing, etc. In the past two decades, various solutions to those problems have been proposed, which have greatly promoted the progress of micro-LED display. In this paper, an overview of micro-LED display since 2000 is given firstly, which includes the main research results and application achievements. Secondly the issues involved in the wafer epitaxy and chip process of micro-LEDs and possible solutions are discussed based on the display application in detail. The surface state induced by the dangling bonds and dry etching damages are concerned for the nonradiative recombination at a low injection level. The remedies are provided for those surface states, such as atomic-layer deposition and neutral beam etching. Some methods to reduce the threading dislocation and suppress the polarization field are summarized for micro-LED epitaxial growth. Moreover, the GaN-based LEDs on Si (100) substrate are also introduced for the future integration of micro-LEDs into the Si-based integrated circuits. As to the wavelength uniformity, the MOCVD equipment and growth technology including the laser treatment are discussed. In the chip processing part, the full-color display, mass transfer and effective inspection technology are discussed. Assembling RGB individual LEDs, quantum dot phosphor material and nanocoloumn LEDs are different routes for full-color display. Their trends in the future are provided. The pick and place, laser lift-off technologies, are strengthened in the massively transferring for micro-LEDs. In the massively and rapidly inspection technologies, the photoluminscence combined with Raman scattering, the electroluminescence combined with digital camera are discussed. Finally, the summary and outlook in these issues are also provided.

25‐2: Enabling MOCVD Technology for Micro LED High Volume Manufacturing

Epitaxy wafers are key to the technology development of Micro LED displays. As Micro LED chips are extremely small, the quality of epitaxy wafers including wavelength uniformity and low defect becomes crucial for further production process. We present progress on improvements to MOCVD technology of blue and red Micro LEDs based on Planetary Reactor® technology to meet the new challenging wavelength and defect requirements.

High-Power Single-Longitudinal-Mode DFB Semiconductor Laser Based on Sampled Moiré Grating

In this mymargin letter, we experimentally demonstrated a 1550-nm high-power single-longitudinal-mode (SLM) distributed feedback (DFB) semiconductor laser based on sampled moire grating (SMG). By designing the sampling structure with micrometer scale, moire grating (MG) can be equivalently realized along the laser cavity. Then, we can reduce the coupling coefficient near laser facet so as to increase the output power. The cavity length and ridge width of the fabricated laser are 1.0 mm and $3.0~\mu \text{m}$ , respectively. The measured threshold current and the slope efficiency are 30.0 mA and 0.36 mW/mA at the heat-sink temperature of 25 °C, respectively. When the injection current is 800.0 mA, the maximum output power is about 183.0 mW. The saturation power is significantly improved compared with conventional DFB laser with uniform sampled grating (USG), which was fabricated on the same wafer. In addition, a four-channel DFB laser array based on SMG was also fabricated on the same wafer, showing good wavelength uniformity.

Quantum Efficiency of Gallium Nitride–Based Heterostructures with GaInN Quantum Wells

An important parameter of light-emitting heterostructures is their external quantum efficiency. However, another strict requirement for the structures used in fabricating blue and white light-emitting diodes is that the wavelength in the emission spectrum peak and its spread over the entire structure must be 460 ± 5 nm. This is explained primarily by the most frequently used white light-emitting diode design being based on crystals coated with a luminescent layer of a certain composition excited by blue emissions. Deviating from the specified spectral parameters of heterostructures strongly degrades the light and color characteristics of light-emitting diodes. In this work, we solve the problem of optimizing the design and way of growing the active region of an emitting structure consisting of a set of Ga1 – xInxN quantum wells and wider-gap GaN barriers with a certain wavelength in the emission spectrum peak. The variation in the critical thickness of the pseudomorphic layer in the Poisson ratio ranges of 0 to 0.2 for GaAlN and 0 to 0.4 for GaInN is calculated. The emission wavelength is determined by both the bulk Ga1 – xInxN band gap, which depends on the molar fraction of In in quantum wells, and the quantum well thickness in quantum-sized layers. It is found from the obtained dependences that to obtain the required wavelength of 460 nm in the emission spectrum peak, the Ga1 – xInxN layers must be around 10.3% indium and have a quantum well thickness of about 2.5 nm. The effect the In distribution profile in quantum wells has on the external quantum efficiency, the uniformity of the emission wavelength distribution in the spectral peak, and the emission power distribution over the structure is considered. The best results are obtained for a trapezoidal In distribution, since it ensures the narrowest emission wavelength spread in the spectral peak and the most uniform emission power distribution over the structure. Studying the effect the number of quantum wells has on the properties of a heterostructure shows that the maximum external quantum efficiency corresponds to 4 to 5 quantum wells. The highest emission wavelength uniformity in the spectral peak over the structure is obtained at 5 to 7 quantum wells. The optimum number of quantum wells in the active region of the heterostructure is found to be 5.

Achromatic linear retarder with tunable retardance.

We present a universal design and proof-of-concept of a tunable linear retarder of uniform wavelength response in a broad spectral range. It consists of two half-wave retarders (HWR) between two quarter-wave retarders (QWRs), where the uniform retardance can be tuned continuously by simply rotating one of the HWRs. A proof-of-concept of this design is built by using commercially available Fresnel rhomb retarders that provide retardation with almost wavelength uniformity in the visible and near infrared from 450 to 1550nm. The design is universal, since other achromatic QWRs and HWRs could also be employed. The system is experimentally demonstrated to control the state of polarization of a supercontinuum laser.

A novel light source with tuneable uniformity of light distribution for artificial daylight photodynamic therapy.

Implementation of daylight photodynamic therapy (dPDT) is somewhat limited by variable weather conditions. Light sources have been employed to provide artificial dPDT indoors, with low irradiances and comparable treatment times to dPDT. Uniform light distribution across the target area is desirable in effective treatment planning, particularly for large areas. A novel light source is developed with tuneable direction of light emission in order to meet this challenge. Wavelength composition of the novel light source is controlled such that the protoporphyrin-IX (PpIX) weighted spectra of both the light source and daylight match. The uniformity of the light distribution is characterised on a flat surface, a model head and a model leg. For context, a typical conventional PDT light source is also characterised. Additionally, the wavelength uniformity across the treatment site is characterised. The PpIX-weighted spectrum of the novel light source matches the PpIX-weighted daylight spectrum, with irradiance values within the bounds for effective dPDT. By tuning the direction of light emission, improvements are seen in the uniformity across large anatomical surfaces. Wavelength uniformity is discussed. We have developed a light source that addresses the challenges in uniform, multiwavelength light distribution for large area artificial dPDT across curved anatomical surfaces.

InGaN/GaN Blue Light Emitting Diodes Using Freestanding GaN Extracted from a Si Substrate

We demonstrate the first InGaN/GaN blue light emitting diodes (LEDs) on freestanding GaN grown using a Si substrate. Transmission electron microscopy and X-ray diffraction analysis revealed that the InGaN/GaN multi quantum wells (MQWs) on freestanding GaN grown using Si substrates have excellent structural properties suitable for high-performance optical devices. Photoluminescence measurements confirm the high crystal quality of the InGaN/GaN MQWs and remarkable emission wavelength uniformity with a standard deviation of 0.68%. Light–current–voltage characteristics indicate that the InGaN/GaN LEDs on freestanding GaN grown using a Si substrate exhibit a forward voltage of 3.75 V at a current of 20 mA and rectifying characteristics with very low leakage current and high breakdown voltage. Furthermore, they provide stable blue electroluminescence (λ = 460 nm) with a small variation in the emission wavelength of 0.2% over a 2 in. area. The internal quantum efficiency of InGaN/GaN LEDs on freestanding GaN gro...

Effect of back-surface roughness of sapphire substrate on growth of GaN thin films

The change in the full width at half maximum (FWHM) of the X-ray diffraction rocking curve of sapphire (0006) reflections as a function of the slit-width of the incident X-rays was measured for GaN-on-sapphire substrates at elevated temperatures from 25 to 800°C to study the effect of back-surface roughness of sapphire substrate on the bowing behavior of GaN-on-sapphire substrates at elevated temperature. The extent of bowing of the GaN-on-sapphire substrate was estimated by determining the rocking curve broadening effect from the measured FWHMs, because the rocking curve will be broadened if the crystal has curvature in its physical shape. It was found that the GaN-on-sapphire substrate was typically bowed in the convex direction at room temperature, and this convex bowing was reduced and became flat and then concave with increasing temperature. The clear difference in the temperature at which the GaN-on-sapphire substrate became flat was shown as a function of the back-surface roughness of the sapphire substrates. The back-surface roughness of the sapphire substrate has been suggested to have a direct impact on the uniformity of the LED wavelength across the substrate diameter.

非故意掺杂GaN层厚度对蓝光LED波长均匀性的影响

非故意掺杂GaN层厚度对蓝光LED波长均匀性的影响

Control of lateral thickness gradients of Mo-Si multilayer on curved substrates using genetic algorithm.

An inversion method based on a genetic algorithm has been developed to control the lateral thickness gradients of a Mo-Si multilayer deposited on curved substrates by planar magnetron sputtering. At first, the sputtering distribution of the target is inversed from coating thickness profiles of flat substrates at different heights. Then, the speed profiles of substrates sweeping across the target are optimized according to the desired coating thickness profiles of the primary and secondary mirrors in a two-bounce projection system. The measured coating thickness profiles show that the non-compensable added figure error is below 0.1 nm rms, and the wavelength uniformity across each mirror surface is within ±0.2% P-V. The inversion method introduced here exhibits its convenience in obtaining the sputtering distribution of the target and efficiency in coating iterations during process development.

Uniformity investigation of MOCVD‐grown LED layers

Doped or undoped gallium nitride compounds (GaN/InGaN), usually grown by metal‐organic chemical vapor deposition (MOCVD) method, are at the heart of blue and green light emitting diodes (LEDs). Growth uniformities, such as the excited wavelength, luminous intensity and film thickness, critically influence their application in LED devices. In this paper, growth of GaN compounds in a MOCVD reactor, capable of a one‐time production of 36 × 2” wafers of nitrides, has been investigated. To examine growth uniformity across the wafer and from wafer to wafer, the reactor is divided into Zone A, Zone B and Zone C according to distance to the center of the graphite susceptor. Comparative analysis of each zone offers a straightforward view of the mean excitation wavelength, luminous intensity, film thickness and their standard deviations. Conformity of the growth uniformity in each zone is further checked comprehensively through averaging across‐wafer and wafer‐to‐wafer variables and their standard deviations. Zone B is found to retain excellent wavelength uniformity, since it is located at the middle of the susceptor with weaker effects of the susceptor edge and of the inlet gas flow. Zone A, at the center of the reactor, has the best mean intensity and thickness uniformities due to a well control of the infrared temperature measurement during the growth. And Zone C is worst in all uniformities and should be the main focus when optimizing the reactor. The above experimental analysis reveals the principles common to the MOCVD technique, and provides a basic for further optimization of the process window to improve the cycles with considerable reduction of the costs.

High Brightness, Large Scale GaN Based Light-Emitting Diode Grown on 8-Inch Si Substrate

A crack-free, uniform InGaN/GaN light-emitting diode (LED) structure with strain-engineered buffer layer was grown on an 8-inch diameter Si(111) substrate. The full width at half maximum (FWHM) of (002) and (102) ω-scan is 280 and 420 arcsec, respectively. For LED on 8-inch Si, multiple quantum well (MQW) photoluminescence (PL) wavelength uniformity of 0.55% (2.4 nm) has been achieved by using proper curvature engineered wafer carrier. We demonstrated high brightness 1×1 mm2 LED devices utilizing vertical chip process then evaluated their device properties. The electro-optical characteristics of the fabricated vertical LED (VLED) shows around 1 W light output power at 1 A injection current with operating voltage of 4.0 V.

(Invited) High Brightness, Large Scale GaN Based Light-Emitting Diode Grown on 8-Inch Si Substrate

GaN based light-emitting diodes(LEDs) grown on a Si substrate is one of promising future technologies in LED industry due to its scalability and costeffectiveness compare to the conventional sapphire based LEDs.One of major technical issues in GaN/Si LED is the severemismatch oflattice and thermal expansion coefficientsbetween GaNand Si, which induces significant tensile strain in the GaNepilayer that often leads to wafer cracking. Optical loss by the absorbing Si substrate is another barrier to overcome for realizing high-brightness LEDs competitive to those on sapphire or SiC substrates.In this study, we demonstrated blue LEDs grown on 8-inch diameter Si substrate and fabricated high brightness devicesutilizing vertical chip process Thecrack-free InGaN/GaN LED waferswith strain-engineered buffer structure were grown on 8-inch diameter Si(111) substrates by metalorganic chemical vapor deposition (MOCVD) system (Veeco TurboDisk k465i).The optimized growth conditions of AlN buffer layer improved the crystalline quality of GaNepilayers. The FWHM of (002) and (102) ω-scan is 280 and 420 arcsec, respectively. These values are comparable to the best reported so far for GaN on silicon. The surface morphology of GaN layer investigated using atomic force microscopy (AFM) exhibits very smooth surface and clear atomic step flow patterns. The root-mean-square (RMS) roughness is 0.15 nm in a 5x5 μm2 scan area.For LED on 8-inch Si, MQW PL wavelength uniformity of ~0.5% (2.4 nm) has been achieved using the proper strain/curvature engineering with optimized wafer carrier designs. These are among the lowest values reported to date.After the epitaxial growth with silicon substrate, a 1mm × 1mm sized vertical type LED(VLED) was fabricated using both wafer bonding process and Si removing process. While the GaN on sapphire employs a laser lift-off technique to remove the substrate, we developed the wet etching process in order to remove the silicon substrate without any damageon the surface of LED structure. The electro-optical characteristics of the fabricated VLED in this work shows good efficiency(almost 1W light power at 1A inject current) with operating voltage of 4.0V.

(Invited) High Brightness, Large Scale GaN Based Light-Emitting Diode Grown on 8-Inch Si Substrate

A crack-free, uniform InGaN/GaN light-emitting diode (LED) structure with strain-engineered buffer layer was grown on an 8-inch diameter Si(111) substrate. The full width at half maximum (FWHM) of (002) and (102) ω-scan is 280 and 420 arcsec, respectively. For LED on 8-inch Si, multiple quantum well (MQW) photoluminescence (PL) wavelength uniformity of 0.55% (2.4 nm) has been achieved by using proper curvature engineered wafer carrier. We demonstrated high brightness 1 x 1 mm2 LED devices utilizing vertical chip process then evaluated their device properties. The electro-optical characteristics of the fabricated vertical LED (VLED) shows around 1 W light output power at 1 A injection current with operating voltage of 4.0 V.