Semiconductor Light-emitting Diodes Research Articles

Influence of Y2O3 sintering aids on performance of c-BN-reinforced AlN/BN composite ceramics

AlN/h-BN composite ceramics exhibit considerable anisotropy, which significantly constrains their application in areas such as semiconductor packaging and light-emitting diodes. The introduction of c-BN can significantly enhance mechanical properties and effectively reduce the anisotropy. However, this results in lower thermal conductivity, which is important issue that needs to be addressed. In this study, hot-press sintering was employed to fabricate AlN-10 wt% h-BN-15 wt% c-BN-x wt.% Y2O3 (AB15CxY, with x = 0, 1, 2, 3, 4) composite ceramics. Effects of Y2O3 content on microstructure, anisotropy, thermal conductivity, and flexural strength of AlN/BN composite ceramics were systematically investigated. Results indicate that during high-temperature sintering, c-BN undergoes phase transformation into "onion-like'' h-BN, and Y2O3 reacts with Al2O3 film on the surface of AlN powder to form Y3Al5O12 (YAG) phase. YAG phase promotes sintering densification as well as directional arrangement of lamellar h-BN, endowing composite ceramics with relative density of 99.6 % (sample AB15C1Y) with increased degree of anisotropy (the index of orientation preference is |IOP| = 68–93). Thermal conductivity of AlN/BN composite ceramics is directly correlated with lattice oxygen content, and as Y2O3 content increases, lattice oxygen content decreases monotonically. Lattice oxygen content of sample AB15C3Y is as low as 0.35 wt%, and the maximum values of K∥ and K⊥ are 100 and 59 W‧m−1‧K−1, respectively, which represent increases of 64.8 % and 51.3 % compared with those of AB15C0Y ceramic, respectively. With the increase in Y2O3 content, AlN grains become gradually larger, and the proportion of YAG phase, which is characterized by brittle grain boundaries, increases, leading to gradual reduction in the flexural strength of AlN/BN composite ceramics.

Reduced Surface Recombination in Extended-Perimeter LEDs toward Electroluminescent Cooling.

III-V semiconductor light-emitting diodes (LEDs) are a promising candidate for demonstrating electroluminescent cooling. However, exceptionally high internal quantum efficiency designs are paramount to achieving this goal. A significant loss mechanism preventing unity internal quantum efficiency in GaAs-based devices is nonradiative surface recombination at the perimeter sidewall. To address this issue, an unconventional LED design is presented, in which the distance from the central current injection area to the device's perimeter is extended while maintaining a constant front contact grid size. This approach effectively moves the perimeter beyond the lateral spread of current at an operating current density of 101-102 A/cm2. In p-i-n GaAs/InGaP double heterojunction LEDs fabricated with varying sizes and perimeter extensions, a 19% relative increase in external quantum efficiency is achieved by extending the perimeter-to-contact distance from 25 to 250 μm for a front contact grid size of 450 × 450 μm2. Utilizing an in-house developed Photon Dynamics model, the corresponding relative increase in internal quantum efficiency is estimated to be 5%. These results are ascribed to a significant reduction in perimeter recombination due to a lower perimeter-to-surface area (P/A) ratio. However, in contrast to lowering the P/A ratio by increasing the front contact grid size of LEDs, the present method enables these improvements without affecting the required maximum current density in the microscopic active LED area under the front contact grid. These findings aid in the advancement of electroluminescent cooling in LEDs and could prove useful in other dedicated semiconductor devices where perimeter recombination is limiting.

Fabrication and Characterization of Nano-Structured ZnS:Cu LED

A wide range of Light Emitting Diodes (LEDs) applications, from general lighting to transmission sources of the Visual Light Communication (VLC) system, makes the LEDs very important to be developed. This research focuses on comparing LED performance due to the variation in surface size and shape of the LED. The research method is carried out with a simulation and an experimental approach. Before the experiment, the LED was simulated with nanopattern variations to determine the best fabrication parameter. The simulation method is carried out using Ansys Lumerical FDTD 2021. The experiment method used to fabricate nanopatterns on the surface of a semiconductor LED layer uses the nanoimprint lithography method. Stamps for nanoimprint lithography are made using Polydimethylsiloxane (PDMS), and the nanopattern sources are obtained from DVD and Blu-ray grating patterns. The characterization of nanoscale patterns was carried out using a Scanning Electron Microscope (SEM). The light emission intensity is measured using a lux meter at a series of emission angles. The results obtained from this research are that the smaller the width and the periodicity of the grating nanopattern, the light produced will be distributed at a wider angle, but the light intensity will decrease; conversely, for a planar surface without a grating nanopattern, level of focus and intensity of light will be higher. In addition, the thicker the ZnS:Cu layer, the better the intensity of the light produced.

Insight on the structural, electronic and optical properties of Zn, Ga-doped/dual-doped graphitic carbon nitride for visible-light applications

The density functional theory (DFT) was applied for the first time to study the doping and co-doping of Ga and Zn metals on graphitic carbon nitride (g-C3N4). The doping of these metal impurities into g-C3N4 leads to a significant decrease in the bandgap energy. Moreover, the co-doping leads to even lower bandgap energy than either individual Zn or Ga-doped g-C3N4. The theoretical electronic and optical properties including the density of state (DOS), energy levels of the frontier orbital, excited state lifetime, and molecular electrostatic potential of the doped and co-doped g-C3N4 support their application in UV–visible light-based technologies. The quantum mechanical parameters (energy band gap, binding energy, exciton energy, softness, hardness) and dipole moment exhibit higher values (ranging from 1.36 to 4.94 D) compared to the bare g-C3N4 (0.29 D), indicating better solubility in the water solvent. The time-dependent DFT (TD-DFT) calculations showed absorption maxima in between the UV–Vis region (309–878 nm). Additionally, charge transfer characteristics, transition density matrix (TDM), excited state lifetime and light harvesting efficiency (LHE) were investigated. Overall, these theoretical studies suggest that doped and co-doped g-C3N4 are excellent candidates for electronic semiconductor devices, light-emitting diodes (LEDs), solar cells, and photodetectors.

Flexible topographical design of light-emitting diodes realizing electrically controllable multi-wavelength spectra

Multi-wavelength visible light emitters play a crucial role in current solid-state lighting. Although they can be realized by combining semiconductor light-emitting diodes (LEDs) and phosphors or by assembling multiple LED chips with different wavelengths, these design approaches suffer from phosphor-related issues or complex assembly processes. These challenges are significant drawbacks for emerging applications such as visible light communication and micro-LED displays. Herein we present a platform for tailored emission wavelength integration on a single chip utilizing epitaxial growth on flexibly-designed three-dimensional topographies. This approach spontaneously arranges the local emission wavelengths of InGaN-based LED structures through the local In composition variations. As a result, we demonstrate monolithic integration of three different emission colors (violet, blue, and green) on a single chip. Furthermore, we achieve flexible spectral control via independent electrical control of each component. Our integration scheme opens the possibility for tailored spectral control in an arbitrary spectral range through monolithic multi-wavelength LEDs.

Effects of different artificial photosynthetically active radiation (PAR) sources and intensity on the growth and nutrient uptake in Ulva prolifera and Neopyropia yezoensis

Fluorescent lamps and semiconductor light-emitting diodes (LEDs) lamps are commonly used as artificial light sources in the indoor algae cultivation system. In this study, we tested the effect of two types of fluorescent lamps (T8 and T5) and white LED lamp on the growth, photosynthesis, and nutrient uptake rates of Neopyropia yezoensis and Ulva prolifera under three photosynthetically active radiation (PAR) conditions (80, 150 and 300 μmol photon m2 s−1). White LED and T8 fluorescent light induced the highest and the lowest growth rates, respectively, in both species under each PAR. The effect of T5 fluorescent light on growth rate depends on the species and PAR. Interestingly, the T8 fluorescent light significantly increased the uptake rate of phosphorus under all PARs and nitrogen uptake rate was enhanced under 150 μmol photon m2 s−1 in U. prolifera, which was not observed in N. yezoensis. Our results suggest that white LED lamp is a more economical and efficient artificial light source than fluorescent lamps because of their highest enhancement of growth. The optimal PAR of T5 fluorescent light for growth is lower than T8 fluorescent light in the cultivation of U. prolifera. T8 fluorescent increases phosphorous uptake rate in U. prolifera.

Calculation of DC Circuits Containing the Child-Langmuir Diodes

The DC circuits containing the Child-Langmuir (CL) diodes as their components are proposed for the first time for studying and calculation. The CL-diodes have a volt-ampere characteristic in the form of the power law with an exponent of 3/2. The devices that should be related to the category of CL-diodes include vacuum thermionic diodes with direct currents limited by the electron beam own space charge, solar cells, semiconductor light-emitting diodes, high-power diodes of direct-acting electron accelerators, etc. Circuits containing several CL-diodes have not been considered before anywhere. Formulas for general perveance with parallel- and series-connected CL-diodes having different perveances are given. Two calculation examples of the general perveance of the circuit containing three and four CL-diodes are presented. The methods and results of this study can be used for calculating the volt-ampere characteristics of, e.g., complex multicomponent circuits of solar cells or light emitting diodes.

Towards intelligent illumination systems: from the basics of light science to its application

The development and design of lighting systems is closely linked to the physiology of the human visual system. Whereas with the first generation of light sources, the visual appearance of objects in an illuminated environment was only possible by adjusting the level of illuminance. In contrast, with modern semiconductor light-emitting diode (LED) systems, the emitted spectrum can be flexibly varied. This new degree of freedom has led to an interdisciplinary field of research, aiming to explore the effect of light on humans in terms of physiological, psychological and cognitive parameters and to model their mechanisms or make them quantifiable via mathematical metrics. Today’s quality assessment of light spectra is composed of metrics that combine colour perception, contrast sensitivity, visual sensation, non-visual responses and cognitive preference. A lighting system that takes these aspects into account is commonly referred to as an integrative lighting solution or Human Centric Lighting. This article describes the current knowledge about the human eye’s visual and non-visual processing system, the development of colour rendering metrics, and the light-induced effect on nocturnal melatonin suppression. Then, the basic concept of an intelligent and individually adaptable lighting system will be discussed.Practical Relevance: This article deals with the basics of light science and covers the fundamental aspects of intelligent lighting systems, which with the help of multi-channel LED luminaires, could address the visual properties of light and the human circadian system separately via metameric spectra.

ARM-Based Indoor RGB-LED Visible Light Communication System

As a new green solid-state light source, semiconductor light-emitting diodes (LEDs) have the advantages of low power consumption, small size, long life, short response time, and good modulation performance. At the same time, the frequency band to which LED light sources belong does not require regulatory registration, thus alleviating the current problem of spectrum scarcity for wireless communications. However, white LED-based visible light communication (VLC) systems suffer from limited bandwidth and low energy efficiency. Therefore, an ARM-based indoor RGB-LED VLC system is proposed. Firstly, the three RGB colours are mixed into white light, thus obtaining a larger modulation bandwidth than normal white LEDs while illuminating normally. Secondly, the S3C6410 processor is used to modulate and demodulate the RGB-LEDs with biased light OFDM, thus obtaining a high spectrum utilisation while ensuring system transmission stability. Then, according to the characteristics of the light source of the VLC system, the leading and window functions used in the optical network transceiver module are designed to improve the communication energy efficiency of the system. Finally, functional tests were carried out on an ARM development board. The experimental results show that with a single RGB-LED light source, the maximum transmission distance is 5 cm, the maximum average delay is 68 ms, the maximum throughput is 25 Mbps, and the BER is controlled below 3.2 × 10−3, which meets the basic communication requirements.

Light-Emitting Diodes Based on InGaN/GaN Nanowires on Microsphere-Lithography-Patterned Si Substrates.

The direct integration of epitaxial III-V and III-N heterostructures on Si substrates is a promising platform for the development of optoelectronic devices. Nanowires, due to their unique geometry, allow for the direct synthesis of semiconductor light-emitting diodes (LED) on crystalline lattice-mismatched Si wafers. Here, we present molecular beam epitaxy of regular arrays n-GaN/i-InGaN/p-GaN heterostructured nanowires and tripods on Si/SiO2 substrates prepatterned with the use of cost-effective and rapid microsphere optical lithography. This approach provides the selective-area synthesis of the ordered nanowire arrays on large-area Si substrates. We experimentally show that the n-GaN NWs/n-Si interface demonstrates rectifying behavior and the fabricated n-GaN/i-InGaN/p-GaN NWs-based LEDs have electroluminescence in the broad spectral range, with a maximum near 500 nm, which can be employed for multicolor or white light screen development.

Application of Response Surface Methodology for Optimization of Nanosized Zinc Oxide Synthesis Conditions by Electrospinning Technique.

Zinc oxide (ZnO) is a well-known semiconductor material due to its excellent electrical, mechanical, and unique optical properties. ZnO nanoparticles are widely used for the industrial-scale manufacture of microelectronic and optoelectronic devices, including metal oxide semiconductor (MOS) gas sensors, light-emitting diodes, transistors, capacitors, and solar cells. This study proposes optimization of synthesis parameters of nanosized ZnO by the electrospinning technique. A Box–Behnken design (BB) has been applied using response surface methodology (RSM) to optimize the selected electrospinning and sintering conditions. The effects of the applied voltage, tip-to-collector distance, and annealing temperature on the size of ZnO particles were successfully investigated. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images confirm the formation of polyvinylpyrrolidone-zinc acetate (PVP-ZnAc) fibers and nanostructured ZnO after annealing. X-ray diffraction (XRD) patterns indicate a pure phase of the hexagonal structure of ZnO with high crystallinity. Minimal-sized ZnO nanoparticles were synthesized at a constant applied potential of 16 kV, with a distance between collector and nozzle of 12 cm, flow rate of 1 mL/h, and calcination temperature of 600 °C. The results suggest that nanosized ZnO with precise control of size and morphology can be fabricated by varying electrospinning conditions, precursor solution concentration, and sintering temperature.

AlGaN nanowire deep ultraviolet light emitting diodes with graphene electrode

Despite graphene being an attractive transparent conductive electrode for semiconductor deep ultraviolet (UV) light emitting diodes (LEDs), there have been no experimental demonstrations of any kind of semiconductor deep UV LEDs using a graphene electrode. Moreover, although aluminum gallium nitride (AlGaN) alloys in the format of nanowires are an appealing platform for surface-emitting vertical semiconductor deep UV LEDs, in particular, at short wavelengths, there are few demonstrations of AlGaN nanowire UV LEDs with a graphene electrode. In this work, we show that transferred graphene can serve as the top electrode for AlGaN nanowire deep UV LEDs, and devices emitting down to around 240 nm are demonstrated. Compared to using metal, graphene improves both the light output power and external quantum efficiency. Nonetheless, devices with a graphene electrode show a more severe efficiency droop compared to devices with metal. Here, we attribute the heating effect associated with the large contact resistance to be the major reason for the severe efficiency droop in the devices with a graphene electrode. Detailed scanning electron microscopy and Raman scattering experiments suggest that the nanowire height nonuniformity is the main cause for the large contact resistance; this issue could be potentially alleviated by using nanowires grown by selective area epitaxy that is able to produce nanowires with uniform height. This work, therefore, not only demonstrates the shortest wavelength LEDs using a graphene electrode but also provides a viable path for surface-emitting vertical semiconductor deep UV LEDs at short wavelengths.

A High-Efficiency Wavelength-Tunable Monolayer LED with Hybrid Continuous-Pulsed Injection.

High-efficiency and wavelength-tunable light-emitting diode (LED) devices will play an important role in future advanced optoelectronic systems. Traditional semiconductor LED devices typically have a fixed emission wavelength that is determined by the energy of the emission states. Here, a novel high-efficiency and wavelength-tunable monolayer WS2 LED device, which operates in the hybrid mode of continuous-pulsed injection, is developed. This hybrid injection enables highly enhanced emission efficiency (>20 times) and effective size of emission area (>5 times) at room temperature. The emission wavelength of the WS2 monolayer LED device can be tuned over more than 40nm by driving AC voltages, from exciton emission to trion emission, and further to defect emission. The quantum efficiency of defect electroluminescence (EL) emission is measured to be more than 24.5 times larger than that from free exciton and trion EL emission. The separate carrier injection in the LED also demonstrates advantages in allowing defect species to be visualized and distinguished in real space. Those defects are assigned to be negatively charged defects. The results open a new route to develop high-performance and wavelength-tunable LED devices for future advanced optoelectronic applications.

Crystal growth, structure and optical properties of Pr3+-doped yttria-stabilized zirconia single crystals* *Project supported by the National Natural Science Foundation of China (Grant No. 11975004) and the Key Research and Development Plan Project of Guangxi, China (Grant No. Guike AB18281007).

The development of blue semiconductor light-emitting diodes (LEDs) has produced potential applications for Pr-doped materials that can absorb blue light, especially crystals, and we now report structure and optical properties for high-quality Pr-doped single crystals of yttria-stabilized zirconia (YSZ) grown by the optical floating zone (FZ) method. X-ray diffraction (XRD) and Raman spectroscopy showed that all of the single crystal samples were in the cubic phase, whereas the corresponding ceramic samples contained a mixture of monoclinic and cubic phases. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) spectroscopy showed that Pr was present as the Pr3+ ion in ceramic rods and single crystals after heating to high temperatures. The absorption and photoluminescence excitation (PLE) spectra of the Pr-doped YSZ crystals measured at room temperature showed strong absorption of blue light, while their photoluminescence (PL) spectra showed five emission peaks at 565 nm, 588 nm, 614 nm, 638 nm, and 716 nm under 450 nm excitation. The optimum luminescence properties were obtained with the crystal prepared using 0.15 mol% Pr6O11, and those with higher concentrations showed evidence of quenching of the luminescence properties. In addition, the color purity of Pr-doped YSZ single crystal reached 98.9% in the orange–red region.

Task failure prediction for wafer-handling robotic arms by using various machine learning algorithms

Industries are increasingly adopting automatic and intelligent manufacturing in production lines, such as those of semiconductor wafers, optoelectronic devices, and light-emitting diodes. For example, automatic robot arms have been used for pick-and-place workpiece applications. However, repairing automatic robot arms is time-consuming and increases the downtime of equipment and the cycle time of manufacturing. In this study, various machine learning (ML) models, such as the general linear model (GLM), random forest, extreme gradient boosting, gradient boosting machine, and stacked ensemble, were used to predict the maximum Cartesian positioning shift (i.e. the maximum eccentric distance) in the next handling time period (e.g. 1 min). A charge-coupled-device-based fault diagnostic system was developed to measure the critical positions of the robotic arm when transferring wafers. A novel data augmentation method was used to determine the correlation parameters in the dataset for the ML models. The prediction error for each algorithm was determined using the root mean square error (RMSE). The results revealed that the GLM exhibited the lowest prediction errors. The RMSEs of the GLM were 0.024, 0.032, and 0.046 mm for 3421 pickups, the last 1000 pickups, and 100 pickups, respectively, for the prediction target. Thus, the GLM is a promising model for predicting the task failure of wafer-handling robotic arms.

Ultrafast intraband Auger process in self-doped colloidal quantum dots

Summary Investigating the separate dynamics of electrons and holes has been challenging, although it is critical for the fundamental understanding of semiconducting nanomaterials. n-Type self-doped colloidal quantum dots (CQDs) with excess electrons occupying the low-lying state in the conduction band (CB) have attracted a great deal of attention because of not only their potential applications to infrared optoelectronics but also their intrinsic system that offers a platform for investigating electron dynamics without elusive contributions from holes in the valence band. Here, we show an unprecedented ultrafast intraband Auger process, electron relaxation between spin-orbit coupling states, and exciton-to-ligand vibrational energy transfer process that all occur exclusively in the CB of the self-doped β-HgS CQDs. The electron dynamics obtained by femtosecond mid-infrared spectroscopy will pave the way for further understanding of the blinking phenomenon, disproportionate charging in light-emitting diodes, and hot electron dynamics in higher quantum states coupled to surface states of CQDs.

Van der Waals Integrated Silicon/Graphene/AlGaN Based Vertical Heterostructured Hot Electron Light Emitting Diodes.

Silicon-based light emitting diodes (LED) are indispensable elements for the rapidly growing field of silicon compatible photonic integration platforms. In the present study, graphene has been utilized as an interfacial layer to realize a unique illumination mechanism for the silicon-based LEDs. We designed a Si/thick dielectric layer/graphene/AlGaN heterostructured LED via the van der Waals integration method. In forward bias, the Si/thick dielectric (HfO2-50 nm or SiO2-90 nm) heterostructure accumulates numerous hot electrons at the interface. At sufficient operational voltages, the hot electrons from the interface of the Si/dielectric can cross the thick dielectric barrier via the electron-impact ionization mechanism, which results in the emission of more electrons that can be injected into graphene. The injected hot electrons in graphene can ignite the multiplication exciton effect, and the created electrons can transfer into p-type AlGaN and recombine with holes resulting a broadband yellow-color electroluminescence (EL) with a center peak at 580 nm. In comparison, the n-Si/thick dielectric/p-AlGaN LED without graphene result in a negligible blue color EL at 430 nm in forward bias. This work demonstrates the key role of graphene as a hot electron active layer that enables the intense EL from silicon-based compound semiconductor LEDs. Such a simple LED structure may find applications in silicon compatible electronics and optoelectronics.

Ag Localized Surface Plasma-Modulated NBE Emissions from ZnCdO Thin Films

Recently, metal surface plasma technology has been widely used to improve the quantum efficiency of optoelectronic devices, such as ZnO light-emitting diodes (LEDs). In this work, Ag films with the appropriate thickness were prepared by evaporation and annealed, and a layer of ZnCdO films with different Cd concentrations was grown on the Ag films by the pulsed laser deposition method. During this process, the changes in the Cd concentrations caused the near-band edge (NBE) emission of the ZnCdO films to be continuously adjusted in the ultraviolet to green wavelength range by controlling the oxygen pressure. The prepared Ag nanoparticles and ZnCdO composite films were used to study the metal localized surface plasmon (LSP) effect. The results show that the enhanced or reduced photoluminescence intensity of ZnCdO NBE depends on the probability of electrons transport to either the LSP level or the Fermi level of the metal. This discovery provides an important theoretical basis for the preparation of ZnCdO semiconductor LEDs in the future.

Understanding the p-Type GaN Nanocrystals on InGaN Nanowire Heterostructures

The efficiency of semiconductor light-emitting diodes (LEDs) has been largely limited by the extremely inefficient p-type conduction on InGaN heterostructures. Here we report highly efficient p-typ...

Visualizing the Intensity Distribution of a Microwave Field Using a Microstrip Rectenna with a Schottky Diode

A microwave radiation intensity indicator based on an antenna-coupled detector (rectenna), an amplifier, and a semiconductor light-emitting diode was constructed and tested. This device is a hybrid integrated circuit, where a microstrip log-periodic antenna on a dielectric substrate also serves as a mechanical carrier. The possibility of visual determination of the shape of the antenna radiation pattern and the shape of an object based on the reflected radiation intensity was demonstrated experimentally at 10 and 30 GHz.