Gallium Nitride Light-emitting Diodes Research Articles

Non-destructive electroluminescence inspection for LED epitaxial wafers based on soft single-contact operation

Non-destructive and accurate inspection of gallium nitride light-emitting diode (GaN-LED) epitaxial wafers is important to GaN-LED technology. However, the conventional electroluminescence inspection, the photoluminescence inspection, and the automated optical inspection cannot fulfill the complex technical requirements. In this work, an inspection method and an operation system based on soft single-contact operation, namely, single-contact electroluminescence (SC-EL) inspection, are proposed. The key component of the SC-EL inspection system is a soft conductive probe with an optical fiber inside, and an AC voltage (70V pp , 100 kHz) is applied between the probe and the ITO electrode under the LED epitaxial wafer. The proposed SC-EL inspection can measure both the electrical and optical parameters of the LED epitaxial wafer at the same time, while not causing mechanical damage to the LED epitaxial wafer. Moreover, it is demonstrated that the SC-EL inspection has a higher electroluminescence wavelength accuracy than photoluminescence inspection. The results show that the non-uniformity of SC-EL inspection is 444.64%, which is much lower than that of photoluminescence inspection. In addition, the obtained electrical parameters from SC-EL can reflect the reverse leakage current (I s ) level of the LED epitaxial wafer. The proposed SC-EL inspection can ensure high inspection accuracy without causing damage to the LED epitaxial wafer, which holds promising application in LED technology.

Electric-driven flexible-roller nanoimprint lithography on the stress-sensitive warped wafer

Surface nanopatterning of semiconductor optoelectronic devices is a powerful way to improve their quality and performance. However, photoelectric devices’ inherent stress sensitivity and inevitable warpage pose a huge challenge on fabricating nanostructures large-scale. Electric-driven flexible-roller nanoimprint lithography for nanopatterning the optoelectronic wafer is proposed in this study. The flexible nanoimprint template twining around a roller is continuously released and recovered, controlled by the roller’s simple motion. The electric field applied to the template and substrate provides the driving force. The contact line of the template and the substrate gradually moves with the roller to enable scanning and adapting to the entire warped substrate, under the electric field. In addition, the driving force generated from electric field is applied to the surface of substrate, so that the substrate is free from external pressure. Furthermore, liquid resist completely fills in microcavities on the template by powerful electric field force, to ensure the fidelity of the nanostructures. The proposed nanoimprint technology is validated on the prototype. Finally, nano-grating structures are fabricated on a gallium nitride light-emitting diode chip adopting the solution, achieving polarization of the light source.

Neutron irradiation effects on gallium nitride-based blue LEDs

Gallium nitride (GaN) is a promising material for radiation-hardened electronics in outer space environment. This work focuses on radiation hardness evaluation of GaN device using commercial GaN quantum-well light-emitting diodes (LEDs). Both the electrical and optical properties of GaN LEDs were characterized before and after neutron irradiation. The light emission intensity was found to increase significantly after 1014n/cm2 of neutron irradiation in the Ohio State University Research Reactor (OSURR) at room temperature. The extracted values from the I-V curve after irradiation also demonstrate such optical improvements of the GaN LED after 1014n/cm2 of neutron irradiation.

Superior Optoelectronic Performance of N-Polar GaN LED to Ga-Polar Counterpart in the “Green Gap” Range

The low luminous efficiency of indium gallium nitride (InGaN) light-emitting diodes (LED) in the “green gap” range has been a long unsettled issue confounding the researchers. One of the main obstacles comes from the intrinsic polarization field in the incumbent Ga-polar LEDs (Ga-LEDs), where the polarization field will bend the energy band thus reducing the radiative recombination efficiency. The scenario will become different when we reverse the polarization field with the adoption of N-polar GaN, which should be a promising candidate to obtain LEDs with high luminous efficiency in the “green gap” range. In this study, the optical and electronic performances of InGaN LEDs in the “green gap” range with N- and Ga-polar have been numerically investigated. The results demonstrate that the light-output power of N-polar LED (N-LED) is ~1.69-fold higher than that of Ga-LED at a current density of 1250 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , thus leading to a significantly improved internal quantum efficiency. Meanwhile, the turn-on voltage of N-LED is lowered by ~17.3% compared to that of Ga-LED. As revealed by the energy band diagram, the superior optoelectronic performance of N-LED is mainly attributed to the stronger carrier confinement in the active region and the lower carrier injection barriers. This study suggests the prospective realization of high luminous efficiency InGaN LEDs in the “green gap” range by the implementation of N-LEDs.

Uniting GaN Electronics and Photonics on A Single Chip

The bandgap energies of the group III-V semiconductor gallium nitride (GaN) and its alloys cover emission wavelengths ranging from the ultraviolet to the visible. Concurrently, GaN has enabled high performance transistors to provide attractive solutions in the high voltage and high frequency regimes. Both GaN optoelectronics and electronics have been successfully developed, and the two technologies are often dependent on each other in many real-life applications. In the simplest case, both the GaN light-emitting diode (LED) and photodiode have to be driven or amplified by transistor-based circuits. However, these circuits are separately made on the Si platform. What if GaN optoelectronics and electronics can be integrated onto the same platform? That would result in significant reduction in material costs, processing costs and packaging costs. At the same time, the performance of the monolithically integrated system will be significantly improved, due to reduced resistances and parasitic capacitances. In view of such prospects, we propose and characterize monolithically integrated GaN metal-oxide-semiconductor field effect transistors (MOSFETs), transmitter, waveguide, and receiver, which are fabricated onto a conventional InGaN/GaN LED wafer without involving re-growth or post-growth doping (diffusion or ion implantation). The capability of integrating optoelectronics (transmitter, waveguide, receiver) with MOSFETs would inevitably open up new horizons for the GaN platform.

Polarization-insensitive GaN metalenses at visible wavelengths

The growth of wide-bandgap materials on patterned substrates has revolutionized the means with which we can improve the light output power of gallium nitride (GaN) light-emitting diodes (LEDs). Conventional patterned structure inspection usually relies on an expensive vacuum-system-required scanning electron microscope (SEM) or optical microscope (OM) with bulky objectives. On the other hand, ultra-thin metasurfaces have been widely used in widespread applications, especially for converging lenses. In this study, we propose newly developed, highly efficient hexagon-resonated elements (HREs) combined with gingerly selected subwavelength periods of the elements for the construction of polarization-insensitive metalenses of high performance. Also, the well-developed fabrication techniques have been employed to realize the high-aspect-ratio metalenses working at three distinct wavelengths of 405, 532, and 633 nm with respective diffraction-limited focusing efficiencies of 93%, 86%, and 92%. The 1951 United States Air Force (USAF) test chart has been chosen to characterize the imaging capability. All of the images formed by the 405-nm-designed metalens show exceptional clear line features, and the smallest resolvable features are lines with widths of 870 nm. To perform the inspection capacity for patterned substrates, for the proof of concept, a commercially available patterned sapphire substrate (PSS) for the growth of the GaN LEDs has been opted and carefully examined by the high-resolution SEM system. With the appropriately chosen metalenses at the desired wavelength, the summits of structures in the PSS can be clearly observed in the images. The PSS imaging qualities taken by the ultra-thin and light-weight metalenses with a numerical aperture (NA) of 0.3 are comparable to those seen by an objective with the NA of 0.4. This work can pioneer semiconductor manufacturing to choose the polarization-insensitive GaN metalenses to inspect the patterned structures instead of using the SEM or the bulky and heavy conventional objectives.

High quality GaN tetrapodal structures hetero-integrated on 3D Si surfaces

Gallium nitride (GaN) light-emitting diodes (LEDs) on silicon (Si) substrates have been highlighted due to the potentials to achieve significant cost reduction, effective thermal management, and highly directional light emission, by replacing the conventional sapphire substrates with the opportunities to adopt the well-established Si-based semiconducting technologies. However, large mismatch of physical properties between Si and GaN considerably lowers the material quality of GaN and the corresponding device performances. Herein, we employ three-dimensional faceted Si surfaces (3D faceted Si) to effectively release the generated strain energy during the hetero-epitaxial growth of GaN on Si for fabricating high performance GaN-on-Si LEDs. The resulting outcome not only includes the unique morphology of 3D tetrapodal GaN layers on 3D faceted Si to form space-filling arrays for efficient device utilization, but also contains the excellent material quality of the hetero-integrated GaN layers, comparable to that grown on planar sapphire substrates. Consequently, those 3D tetrapodal GaN-on-Si LEDs exhibit the excellent light extraction characteristics due to their shapes to effectively suppress total internal reflection of generated light. In addition, the anisotropic formation of active layers in the 3D tetrapodal GaN-on-Si LEDs performs multi-color emission characteristics with tunable color capabilities.

Ultrashort Pulse Laser Lift-Off Processing of InGaN/GaN Light-Emitting Diode Chips

Gallium nitride (GaN) film delamination is an important process during the fabrication of GaN light-emitting diodes (LEDs) and laser diodes. Here, we utilize 520 nm femtosecond laser pulses, exploiting nonlinear absorption rather than single-photon absorption such as in conventional laser lift-off (LLO) employing excimer or Q-switched laser sources. The focus of this study is to investigate the influence of laser scanning speed and integrated fluence corresponding to laser energy per area during the LLO processing of GaN LED chips and their resulting structural properties. Because both the sapphire substrate and InGaN/GaN heterostructures are fully transparent to the emission of the laser system, a key question is related to the impact of laser pulses on the quality of a thin film structure. Therefore, several characterization methods (i.e., scanning electron microscopy, atomic force microscopy, X-ray diffraction, Raman spectroscopy, and electroluminescence spectroscopy) were employed to understand the material modifications made by femtosecond LLO (fs-LLO). We demonstrated that by adjusting the laser scanning speed, smooth GaN surfaces and good crystal quality could be obtained regardless of the existing delamination of metal contact, which then slightly downgraded the LED performance. Here, the integrated fluence level was set in the range of 2.6–4.4 J/cm2 to enable the fs-LLO process. Moreover, two mitigation strategies were developed and proven to improve the optoelectrical characteristics of the lifted-off LEDs (i.e., modification of the processing step related to the metal creation and reduction of laser energy).

Numerical investigation of traps and optical response in III-V nitride quantum LED

The impact of trapping on the optical properties of Gallium Nitride (GaN) Light Emitting Diodes (LED) has been analyzed in this work. The analysis is carried out by using Technology Computer Aided Design (TCAD) physical simulator. GaN LED with four quantum Wells have been considered with donor trap density ranging from 5 × 1015 cm−3 to 1 × 1019 cm−3. Polarization, radiative recombination and Auger recombination models were used in the simulation. The TCAD simulation results indicate that luminous power and quantum efficiency of GaN LEDs decreases with increase of trap density.

Characterization of the Interdependence Between the Light Output and Self-Heating of Gallium Nitride Light-Emitting Diodes

Abstract With the advent of gallium nitride (GaN) as an enabling material system for the solid-state lighting industry, high-power and high-brightness light-emitting diodes (LEDs) with wavelengths ranging from near ultraviolet to blue are being manufactured as part of a tremendously large and ever-increasing market. However, device self-heating and the environment temperature significantly deteriorate the LED's optical performance. Hence, it is important to accurately quantify the LED's temperature and correlate its impact on optical performance. In this work, three different characterization methods and thermal simulation were used to measure and calculate the temperature rise of an InGaN/GaN LED, as a result of self-heating. Nanoparticle-assisted Raman thermometry was used to measure the LED mesa surface temperature. A transient Raman thermometry technique was utilized to investigate the transient thermal response of the LED. It was found that under a 300 mW input power condition, self-heating is negligible for an input current pulse width of 1 ms or less. The temperature measured using nanoparticle-assisted Raman thermometry was compared with data obtained by using the forward voltage method (FVM) and infrared (IR) thermal microscopy. The IR and Raman measurement results were in close agreement whereas the data obtained from the widely accepted FVM underestimated the LED temperature by 5–10%. It was also observed that an increase in environment temperature from 25 °C to 100 °C would degrade the LED optical power output by 12%.

Room-temperature one-pot synthesis of highly stable SiO2-coated Mn-doped all-inorganic perovskite CsPb0.7Mn0.3Br0.75Cl2.25 quantum dots for bright white light-emitting diodes

In this paper, we report a simple one-pot strategy to synthesize highly stable SiO2-coated orange-yellow CsPb0.7Mn0.3Br0.75Cl2.25 quantum dots with strong orange-yellow photoluminescence. The microstructure characterization indicates that the CsPb0.7Mn0.3Br0.75Cl2.25 quantum dots were effectively encapsulated by the SiO2. The SiO2-coated CsPb0.7Mn0.3Br0.75Cl2.25 quantum dots show higher humidity stability and thermal stability than the CsPb0.7Mn0.3Br0.75Cl2.25 quantum dots without SiO2 coating. The white light-emitting diodes (WLEDs) were obtained by coating orange-yellow CsPb0.7Mn0.3Br0.75Cl2.25@SiO2 and blue CsPbBrCl2@SiO2 quantum dots on the ultraviolet gallium nitride light-emitting diode chip. The as-prepared white light-emitting diodes show bright white light emission with a color coordinate of (0.3347, 0.2825), color rendering index of 82 and correlated color temperature of 5329 K. The results provide a simple method to fabricate stable orange-yellow luminescent material for white light-emitting diodes.

Dynamic response-based LEDs health and temperature monitoring

This paper presents a number of novel methods to measure the junction temperature and to estimate the health of gallium nitride light-emitting diodes (LEDs). The methods are based on measurements of the dynamic impedance and optical output. Our experimental analysis reveals temperature sensitive parameters of the electrical and optical responses. Moreover, a correlation between the non-radiative current characterizing the active region defects and the small-signal impedance is shown. The demonstrated methods can be applied to enhance existing temperature-monitoring techniques. The derived dependencies also build a foundation for advanced in-field health monitoring of the LEDs using the infrastructure of visible light communication systems. Such methods and techniques are valuable for predictive maintenance of solid state lighting systems.

Ambient Temperature–Corrected Mechanical Stress Mapping of Gallium Nitride and Aluminum Indium Gallium Phosphide Films by Raman Scattering Spectroscopy for Characterization of Light‐Emitting Diodes

Raman spectroscopy can provide a detailed analysis of mechanical stress in crystalline materials through the measurement of shifts in vibrational frequency. However, the ambient temperature drift during measurements can lead to inaccurate mechanical stress values. Herein, the 2D Raman mapping analysis of gallium nitride (GaN) and aluminum indium gallium phosphide (AlInGaP) epitaxial films within packaged light‐emitting diode (LED) devices reveals micrometer‐scale localized stress distributions. A temperature‐corrected measurement method is developed to evaluate the mechanical stress of InGaN LED dies at various LED operating conditions. The impact of die design on mechanical stress is studied for both GaN on Al2O3 (chip scale package, CSP) and GaN thin‐film flip chip (TFFC) designs. Raman mapping of AlInGaP LEDs is also demonstrated and shows comparable resolution with the GaN LED results.

Damage-Free Transfer of GaN-Based Light-Emitting Devices and Reuse of Sapphire Substrate

Gallium nitride (GaN) based semiconductors have been gaining worldwide attention in optoelectronics application, notably information displays and solid-state lighting. GaN light-emitting diodes (LEDs) epilayers are typically grown on sapphire substrates with metal-organic chemical vapor deposition (MOCVD) technique due to the absence of native nitride substrates. In this paper, the mesh-patterned sapphire substrates are fabricated to speed up the laser lift-off efficiency for the separation of GaN films from sapphire substrates. Damage-free GaN thin films fabricated on the sapphire substrates were successfully released and transferred onto Si substrates with sufficient quality through a Ti/Pt/Au/In bonding and subsequent laser lift-off processes. The GaN film mounted on Si substrates exhibits almost no attenuation in PL intensity before and after its transfer. The released sapphire substrate can be reprocessed by annealing & chemical mechanical polishing (CMP) process for reuse as epitaxial substrate with considerable cost reduction, featuring a comparable photoluminescence performance with fresh sapphire substrates.

Beyond solid-state lighting: Miniaturization, hybrid integration, and applications of GaN nano- and micro-LEDs

Gallium nitride (GaN) light-emitting-diode (LED) technology has been the revolution in modern lighting. In the last decade, a huge global market of efficient, long-lasting, and ubiquitous white light sources has developed around the inception of the Nobel-prize-winning blue GaN LEDs. Today, GaN optoelectronics is developing beyond solid-state lighting, leading to new and innovative devices, e.g., for microdisplays, being the core technology for future augmented reality and visualization, as well as point light sources for optical excitation in communications, imaging, and sensing. This explosion of applications is driven by two main directions: the ability to produce very small GaN LEDs (micro-LEDs and nano-LEDs) with high efficiency and across large areas, in combination with the possibility to merge optoelectronic-grade GaN micro-LEDs with silicon microelectronics in a hybrid approach. GaN LED technology is now even spreading into the realm of display technology, which has been occupied by organic LEDs and liquid crystal displays for decades. In this review, the technological transition toward GaN micro- and nanodevices beyond lighting is discussed including an up-to-date overview on the state of the art.

Transfer-Free Graphene-Like Thin Films on GaN LED Epiwafers Grown by PECVD Using an Ultrathin Pt Catalyst for Transparent Electrode Applications.

In this work, we grew transfer-free graphene-like thin films (GLTFs) directly on gallium nitride (GaN)/sapphire light-emitting diode (LED) substrates. Their electrical, optical and thermal properties were studied for transparent electrode applications. Ultrathin platinum (2 nm) was used as the catalyst in the plasma-enhanced chemical vapor deposition (PECVD). The growth parameters were adjusted such that the high temperature exposure of GaN wafers was reduced to its minimum (deposition temperature as low as 600 °C) to ensure the intactness of GaN epilayers. In a comparison study of the Pt-GLTF GaN LED devices and Pt-only LED devices, the former was found to be superior in most aspects, including surface sheet resistance, power consumption, and temperature distribution, but not in optical transmission. This confirmed that the as-developed GLTF-based transparent electrodes had good current spreading, current injection and thermal spreading functionalities. Most importantly, the technique presented herein does not involve any material transfer, rendering a scalable, controllable, reproducible and semiconductor industry-compatible solution for transparent electrodes in GaN-based optoelectronic devices.

Heterogeneous Integration of GaN and BCD Technologies

Light-emitting diodes (LEDs) are solid-state devices that are highly energy efficient, fast switching, have a small form factor, and can emit a specific wavelength of light. The ability to precisely control the wavelength of light emitted with the fabrication process enables LEDs to not only provide illumination, but also find applications in biology and life science research. To enable the new generation of LED devices, methods to improve the energy efficiency for possible battery operation and integration level for miniaturized lighting devices should be explored. This paper presents the first case of the heterogeneous integration of gallium nitride (GaN) power devices, both GaN LED and GaN transistor, with bipolar CMOS DMOS (BCD) circuits that can achieve this. To validate this concept, an LED driver was designed, implemented and verified experimentally. It features an output electrical power of 1.36 W and compact size of 2.4 × 4.4 mm2. The designed fully integrated LED lighting device emits visible light at a wavelength of approximately 454 nm and can therefore be adopted for biology research and life science applications.

Enhancing GaN LED Efficiency through Nano-Gratings and Standing Wave Analysis.

Based on our recent work, this paper reviews our theoretical study on gallium nitride (GaN) light-emitting-diode (LED). The focus of the paper is to improve LED light extraction efficiency through various nano-grating designs. The gratings can be designed at different locations, such as at the top, the middle, and the bottom, on the LED. They also can be made of different materials. In this study, we first present a GaN LED error-grating simulation model. Second, nano Indium Tin Oxide (ITO) top gratings are studied and compared with conventional LED (CLED) using standing wave analysis. Third, we present results related to a patterned sapphire substrate (PSS), SiO2 Nanorod array (NR), and Ag bottom reflection layer. Finally, we investigate the nano-top ITO grating performance over different wavelengths to validate our design simulation, which focusing on a single wavelength of 460 nm.

Fabrication of GaOx Confinement Structure for InGaN Light Emitter Applications

An indium gallium nitride (InGaN) light-emitting diode (LED) with an embedded porous GaN reflector and a current confined aperture is presented in this study. Eight pairs of n+-GaN:Si/GaN in stacked structure are transformed into a conductive, porous GaN/GaN reflector through an electrochemical wet-etching process. Porous GaN layers surrounding the mesa region were transformed into insulating GaOx layers in a reflector structure through a lateral photoelectrochemical (PEC) oxidation process. The electroluminescence emission intensity was localized at the central mesa region by forming the insulating GaOx layers in a reflector structure as a current confinement aperture structure. The PEC-LED structure with a porous GaN reflector and a current-confined aperture surrounded by insulating GaOx layers has the potential for nitride-based resonance cavity light source applications.

Investigation of Nanoscale Photonic Crystal Arrays on Gallium Nitride Blue Light-emitting Diodes

Nanoscale photonic crystal arrays have been investigated by simulation to enhance the light extraction efficiency of blue gallium nitride light-emitting diodes. The plane wave expansion method and finite difference time domain method were used to reveal the photonic energy band gaps with the relevant structure parameters. The results showed suitable band gaps at the wavelength range of 454~606 nm for triangular lattice array with lattice constant of 200 nm. It would facilitate the design process for high-efficient diodes.