GaN Light-emitting Diodes Research Articles

Properties of V-defect injectors in long wavelength GaN LEDs studied by near-field electro- and photoluminescence

The efficiency of multiple quantum well (QW) light emitting diodes (LEDs) to a large degree depends on uniformity of hole distribution between the QWs. Typically, transport between the QWs takes place via carrier capture into and thermionic emission out of the QWs. In InGaN/GaN QWs, the thermionic hole transport is hindered by the high quantum confinement and polarization barriers. To overcome this drawback, hole injection through semipolar QWs located at sidewalls of V-defects had been proposed. However, in the case of the V-defect injection, strong lateral emission variations take place. In this work, we explore the nature of these variations and the impact of the V-defects on the emission spectra and carrier dynamics. The study was performed by mapping electroluminescence (EL) and photoluminescence (PL) with a scanning near-field optical microscope in LEDs that contain a deeper well that can only be populated by holes through the V-defects. Applying different excitation schemes (electrical injection and optical excitation in the far- and near-field), we have shown that the EL intensity variations are caused by the lateral nonuniformity of the hole injection. We have also found that, in biased structures, the PL intensity and decay time in the V-defect regions are only moderately lower that in the V-defect-free regions thus showing no evidence of an efficient Shockley-–Read–Hall recombination. In the V-defect regions, the emission spectra experience a red shift and increased broadening, which suggests an increase of the In content and well width in the polar QWs close to the V-defects.

Dislocation half-loop control for optimal V-defect density in GaN-based light emitting diodes

V-defects are morphological defects that typically form on threading dislocations during epitaxial growth of (0001)-oriented GaN layers. A V-defect is a hexagonal pyramid-shaped depression with six {101¯1}-oriented sidewalls. These semipolar sidewalls have a lower polarization barrier than the polarization barriers present between the polar c-plane quantum wells and quantum barriers and can laterally inject carriers directly into quantum wells in GaN-based light emitting diodes (LEDs). This is especially important, as the high polarization field in c-plane GaN is a significant factor in the high forward voltage of GaN LEDs. The optimal V-defect density for efficient lateral carrier injection in a GaN LED (∼109 cm−2) is typically an order of magnitude higher than the threading dislocation density of GaN grown on patterned sapphire substrates (∼108 cm−2). Pure-edge dislocation loops have been known to exist in GaN, and their formation into large V-defects via low-temperature growth with high Si-doping has recently been studied. Here, we develop a method for pure-edge threading dislocation half-loop formation and density control via disilane flow, growth temperature, and thickness of the half-loop generation layer. We also develop a method of forming the threading dislocation half-loops into V-defects of comparable size to those originating from substrate threading dislocations.

Impact of Crystal Orientation on Modulation Bandwidth: Towards GaN LED-Based High-Speed Visible Light Communication

Light-emitting diodes (LEDs) with high modulation bandwidth are required for high-speed visible light communication applications. Crystal orientation in the GaN LED structure plays a key factor in its modulation bandwidth as the recombination lifetime is highly dependent on crystal orientation owing to the Quantum-Confined Stark Effect (QCSE). In this study, six different crystal orientation multi-quantum well (MQW) GaN LEDs are simulated to understand the impact of heterostructure orientation on modulation bandwidth, radiative recombination rates, and emission intensity. The results of this study demonstrate that semi-polar 101¯3¯ MQW LEDs provide the highest bandwidth in the current density range of 9–20 kA/cm2 compared to the other five orientations. For instance, the semi-polar 101¯3¯-based LED offers a modulation bandwidth of 912.7 MHz at 20 kA/cm2 current density. These results suggest that the semi-polar 101¯3¯ orientation-based LED has the potential to support a high-speed visible light communication system.

Dynamics of carrier injection through V-defects in long wavelength GaN LEDs

The efficiency of high-power operation of multiple quantum well (QW) light emitting diodes (LEDs) to a large degree depends on the realization of uniform hole distribution between the QWs. In long wavelength InGaN/GaN QW LEDs, the thermionic interwell hole transport is hindered by high GaN barriers. However, in polar LED structures, these barriers may be circumvented by the lateral hole injection via semipolar 101¯1 QWs that form on the facets of V-defects. The efficiency of such carrier transfer depends on the transport time since transport in the semipolar QWs is competed by recombination. In this work, we study the carrier transfer from the semipolar to polar QWs by time-resolved photoluminescence in long wavelength (green to red) LEDs. We find that the carrier transfer through the semipolar QWs is fast, a few tens of picoseconds with the estimated room temperature ambipolar diffusion coefficient of ∼5.5 cm2/s. With diffusion much faster than recombination, the hole transport from the p-side of the structure to the polar QWs should proceed without a substantial loss, contributing to the high efficiency of long wavelength GaN LEDs.

An electroencephalogram microdisplay to visualize neuronal activity on the brain surface.

Functional mapping during brain surgery is applied to define brain areas that control critical functions and cannot be removed. Currently, these procedures rely on verbal interactions between the neurosurgeon and electrophysiologist, which can be time-consuming. In addition, the electrode grids that are used to measure brain activity and to identify the boundaries of pathological versus functional brain regions have low resolution and limited conformity to the brain surface. Here, we present the development of an intracranial electroencephalogram (iEEG)-microdisplay that consists of freestanding arrays of 2048 GaN light-emitting diodes laminated on the back of micro-electrocorticography electrode grids. With a series of proof-of-concept experiments in rats and pigs, we demonstrate that these iEEG-microdisplays allowed us to perform real-time iEEG recordings and display cortical activities by spatially corresponding light patterns on the surface of the brain in the surgical field. Furthermore, iEEG-microdisplays allowed us to identify and display cortical landmarks and pathological activities from rat and pig models. Using a dual-color iEEG-microdisplay, we demonstrated coregistration of the functional cortical boundaries with one color and displayed the evolution of electrical potentials associated with epileptiform activity with another color. The iEEG-microdisplay holds promise to facilitate monitoring of pathological brain activity in clinical settings.

Si-Based GaN Light-Emitting Diodes with MoS2 Nanosheet-Decorated Fabry–Perot Waveguide Structure

Si-Based GaN Light-Emitting Diodes with MoS₂ Nanosheet-Decorated Fabry–Perot Waveguide Structure

Optimizing performance and energy consumption in GaN(n)/InxGa1-xN/GaN/AlGaN/GaN(p) light-emitting diodes by quantum-well number and mole fraction

High performance and safe light-emitting devices (LEDs) are needed. Highly efficient IIIV nitride semiconductors are known for short-wavelength LEDs. Multiple-quantum well (MQW) are considered in LEDs. Influence of MQW and indium concentration on LED performance are studied here in GaN(n)/InxGa1-xN(i)/GaN(i)/AlGaN(p)/GaN(p) LEDs, where GaN(n) and GaN(p) have different dopants to formulate junctions, InxGa1-xN(i) is a 3 nm-thick intrinsic QW, GaN(i) is barrier intrinsic layer and AlGaN(p) is a 15 nm-thick electron blocking layer (EBL). Simulation is performed by Tcad-Silvaco. Current versus voltage (I-V) plots, luminosity power, band diagram, spectrum response, radiative recombination rate and electric field effect, are investigated to rationalize effects of InxGa1- xN(i) QW number and x. Increasing (x) improves radiative recombination rate, spectral power and band gap at less current. Devices with 6 quantum wells and x= 0.16 or 0.18 exhibit best performance. Minimizing x at 0.16, at high performance, is described.

Pulsed-Mode Metalorganic Vapor-Phase Epitaxy of GaN on Graphene-Coated c-Sapphire for Freestanding GaN Thin Films.

We report the growth of high-quality GaN epitaxial thin films on graphene-coated c-sapphire substrates using pulsed-mode metalorganic vapor-phase epitaxy, together with the fabrication of freestanding GaN films by simple mechanical exfoliation for transferable light-emitting diodes (LEDs). High-quality GaN films grown on the graphene-coated sapphire substrates were easily lifted off by using thermal release tape and transferred onto foreign substrates. Furthermore, we revealed that the pulsed operation of ammonia flow during GaN growth was a critical factor for the fabrication of high-quality freestanding GaN films. These films, exhibiting excellent single crystallinity, were utilized to fabricate transferable GaN LEDs by heteroepitaxially growing InxGa1-xN/GaN multiple quantum wells and a p-GaN layer on the GaN films, showing their potential application in advanced optoelectronic devices.

Wafer-Scale Transferrable GaN Enabled by Hexagonal Boron Nitride for Flexible Light-Emitting Diode.

Epitaxy growth and mechanical transfer of high-quality III-nitrides using 2D materials, weakly bonded by van der Waals force, becomes an important technology for semiconductor industry. In this work, wafer-scale transferrable GaN epilayer with low dislocation density is successfully achieved through AlN/h-BN composite buffer layer and its application in flexible InGaN-based light-emitting diodes (LEDs) is demonstrated. Guided by first-principles calculations, the nucleation and bonding mechanism of GaN and AlN on h-BN is presented, and it is confirmed that the adsorption energy of Al atoms on O2 -plasma-treated h-BN is over 1eV larger than that of Ga atoms. It is found that the introduced high-temperature AlN buffer layer induces sufficient tensile strain during rapid coalescence to compensate the compressive strain generated by the heteromismatch, and a strain-relaxation model for III-nitrides on h-BN is proposed. Eventually, the mechanical exfoliation of single-crystalline GaN film and LED through weak interaction between multilayer h-BN is realized. The flexible free-standing thin-film LED exhibits ≈66% luminescence enhancement with good reliability compared to that before transfer. This work proposes a new approach for the development of flexible semiconductor devices.

Electrically pumped optomechanical beam GaN-LED accelerometer based on the quantum-confined Stark effect

Micro-nano optomechanical accelerometers are widely used in automobile, aerospace, and other industrial applications. Here, we fabricate mechanical sensing components based on an electrically pumped GaN light-emitting diode (LED) with a beam structure. The relationship between the blueshift of the electroluminescence (EL) spectra and the deformation of the GaN beam structure based on the quantum-confined Stark effect (QCSE) of the InGaN quantum well (QW) structure is studied by introducing an extra mass block. Under the equivalent acceleration condition, in addition to the elastic deformation of GaN-LED, a direct relationship exists between the LED’s spectral shift and the acceleration’s magnitude. The extra mass block (gravitational force: 7.55×10−11 N) induced blueshift of the EL spectra is obtained and shows driven current dependency. A polymer sphere (PS; gravitational force: 3.427×10−12 N) is placed at the center of the beam GaN-LED, and a blueshift of 0.061 nm is observed in the EL spectrum under the injection current of 0.5 mA. The maximum sensitivity of the acceleration is measured to be 0.02 m/s2, and the maximum measurable acceleration is calculated to be 1.8×106 m/s2. It indicates the simultaneous realization of high sensitivity and a broad acceleration measurement range. This work is significant for several applications, including light force measurement and inertial navigation systems with high integration ability.

Highly Stable CsPbI3 Perovskite Quantum Dots Enabled by Single SiO2 Coating toward Down-Conversion Light-Emitting Diodes

All-inorganic CsPbI3 perovskite quantum dots (PeQDs) have sparked widespread research due to their excellent optoelectronic properties and facile synthesis. However, attaining highly stable CsPbI3 perovskite quantum dots (PeQDs) against heat and polar solvents still remains a challenge and hinders any further practical application. Here, by exploiting (3-aminopropyl) triethoxysilane (APTES) as the sole silica (SiO2) precursor, we report a one-step in situ synthesis of single SiO2-coated CsPbI3 (SiO2-CsPbI3) PeQDs, namely that one SiO2 particle only contains one CsPbI3 PeQD particle. The obtained SiO2-CsPbI3 PeQDs are cubic in shape, have a more uniform size distribution, and possess narrow emission, with near unit photoluminescence quantum yields of up to 97.5%. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy confirm the formation of SiO2 through the hydrolysis of APTES on the CsPbI3 PeQDs surface. Furthermore, they have a significantly improved stability against storage, heat, and ethanol. By combining purple-emission GaN light-emitting diodes, the SiO2-CsPbI3 PeQDs were successfully employed as down-conversion emitters and exhibited considerable enhanced luminous performance and excellent stability, demonstrating their promising future in the practical application of solid-state lighting fields.

Machine vision inspection of early failure and line-shaped defects of blue InGaN/GaN light emitting diodes soaked in liquid nitrogen for cryogenic tests

Cryogenic tests were conducted on blue InGaN/GaN light-emitting diodes (LEDs) by immersing the device in liquid nitrogen (LN2). To study the degradation of the LEDs, a combination of optical, electrical, and material characterizations were performed on the LN2-soaked device. The results indicate that noticeable emission spectral shifts were observed during the LED immersion due to lattice deformation. Moreover, clustering of cracks was generated in the cross-section, and line-shaped defects appeared on the surface of the GaN LED, which was different from the defects on AlInGaP LEDs after LN2 immersion. To track the trend of device damage, computer-assisted OpenCV-Python software was used to zoom in on nuanced variations in certain areas of the emission images. Early degradation of certain areas can be detected by computer-assisted machine vision, which has the potential to provide an early warning and protective circuits for the system consisting of the initial degraded LEDs. Reliability studies on LN2-soaked InGaN/GaN LEDs show promise in studying the reliability issues of GaN device applications in Arctic regions and space exploration.

Enhancing light extraction efficiency of GaN LED by combining complex‐period photonic crystals with doping

AbstractThe light extraction efficiency (LEE) of GaN‐based light‐emitting diodes (LEDs) was limited by intense total internal reflection and the photothermal effect. In order to solve this problem, a method to synergistically control the LEE of GaN‐based LEDs by ​​combining the complex‐period photonic crystals (PhCs) with M (M = Al, In) material doping is proposed. The forbidden band width of two‐dimensional (2D) PhC array, three‐dimensional (3D) LED model, M doping, and electromagnetic field distribution are investigated respectively. By doping the M, the LED emission wavelength range is regulated to achieve the dual‐band emission. Furthermore, a triangular complex‐period photonic structure is introduced to establish stacked or etched PhCs models. By combining the plane wave expansion and finite difference time domain algorithm, the structural parameters of PhCs and M concentration dependent LEE are investigated, and the electromagnetic field distribution is explored also. The results show that the optimal LEEs can be achieved are 19.08% and 13.96% for blue light and ultraviolet, respectively, which are larger than that of traditional flat‐panel LED (4%). This work provides theoretical results and technical support for the design of LEDs with high luminous efficiency.

Simulation of GaN-based light emitting diodes incorporating composition fluctuation effects

III-Nitride light emitting diodes (LEDs) are widely used in a range of high efficiency lighting and display applications, which has enabled significant energy savings in the last decade. Despite the wide application of GaN LEDs, transport mechanisms across InGaN/GaN heterostructures in these devices are not well explained. Fixed polarization sheet charges at InGaN/GaN interfaces lead to large interface dipole charges, which create large potential barriers to overcome. One-dimensional models for transport across such heterostructures predict turn-on voltages that are significantly higher than that found in real devices. As a result, conventional models for transport cannot predict the performance of new designs such as for longer wavelength LEDs or for multi-quantum well LEDs. In this work, we show that incorporating low and high indium compositions within quantum wells at the submicrometer scale can provide an accurate prediction of the characteristics of GaN/InGaN light emitting diodes.

Effective integration of a MOSFET phototransistor to a GaN LED for UV sensing.

In this Letter, we report an effective monolithic integration of a metal oxide semiconductor field effect (MOSFET) phototransistor (PT) and a light-emitting diode (LED) on a GaN-on-Si LED epitaxial (epi) wafer. Avoiding additional growth or Si diffusion, the PT was directly fabricated on the LED epi layer, providing a cost-effective and facile method. As a driver, the PT could modulate both peak value of the light intensity and output current of the integrated LED. As an ultraviolet (UV) detector, our PT showed sufficient responsivity. It was found that the gate-voltage-dependent photocurrent-response of the device had a shorter response time, and a higher responsivity was obtained at a higher gate-voltage bias. The device demonstrated a switching effect that the photoinduced current from the PT drove the LED when the UV lamp was turned on, whereas the photoinduced current stopped driving upon powering off the UV lamp. The experiment proved that the integrated device working as a UV detector exhibited a fast response time and a longstanding stability. We anticipate that such an approach could have potential applications for UV light detection and visible light communication (VLC).

Van der Waals integration of GaN light-emitting diode arrays on foreign graphene films using semiconductor/graphene heterostructures

We report the van der Waals integration of micropatterned GaN light-emitting diodes (LEDs) onto foreign graphene films. GaN micro-LEDs were selectively grown on a graphene substrate using a patterned SiO2 mask, and then the whole device structure was laterally fixed by a polyimide insulator to form a united layer. After device fabrication, the LED/graphene heterostructure device was piled on the foreign graphene layers using a typical wet transfer technique of 2D crystals where the bottom graphene layer of the heterostructure was adhered to the foreign graphene only by van der Waals interactions. The transferred micro-LEDs showed well-aligned crystallographic orientations as well as reliable device performances, including strong light emissions, good rectifying behaviors of the current density–voltage curve, and good simultaneity between the electroluminescence intensity and the applied currents, ensuring reliable electrical connections and mechanical adhesions of the light-emitting layer to the foreign graphene films. Furthermore, the reliable adhesiveness allowed us to achieve device wearability, while the LEDs exhibited homogeneous light emissions under various bending conditions because of negligible external stress in the discrete micro-LEDs.

A Novel GaN LED Structure With Both P and N Discontinuous Ohmic Contacts and Discontinuous CBL

In this paper, a novel kind of GaN light-emitting diode (LED) with P and N discontinuous ohmic contact electrodes and discontinuous current blocking layer (CBL) is designed and prepared. Multiple contact windows under the N- and P-electrodes formed by etching help realize the discontinuous ohmic contacts with the N-GaN layer and the ITO layer, respectively. Similarly, by etching a plurality of discontinuous openings with the same pitch in the CBL, a discontinuous CBL structure is formed. As compared with traditional structures, the discontinuous ohmic contacts can maximize the number of current paths which allow the carriers to be collected from all four directions, leading to a more uniform current spreading and thermal management. Furthermore, as the contacts are not continuous any longer, some of the otherwise removed materials (such as the active region) lying between the patches are saved. The optoelectronic characteristics, junction temperature, thermal resistance, and infrared thermography measurements are conducted and the current distribution is simulated. The results show that the novel LED structure has better optoelectronic performances and current spreading ability compared to traditional devices. At 150 mA current, the luminous efficiency is 15.2% higher than the traditional LED, the junction temperature is reduced by 4.9%, and the thermal resistance is reduced by 12%.

Quantum dot-integrated GaN light-emitting diodes with resolution beyond the retinal limit

Near-eye display technology is a rapidly growing field owing to the recent emergence of augmented and mixed reality. Ultrafast response time, high resolution, high luminance, and a dynamic range for outdoor use are all important for non-pixelated, pupil-forming optics. The current mainstream technologies using liquid crystals and organic materials cannot satisfy all these conditions. Thus, finely patterned light-emissive solid-state devices with integrated circuits are often proposed to meet these requirements. In this study, we integrated several advanced technologies to design a prototype microscale light-emitting diode (LED) arrays using quantum dot (QD)-based color conversion. Wafer-scale epilayer transfer and the bond-before-pattern technique were used to directly integrate 5-µm-scale GaN LED arrays on a foreign silicon substrate. Notably, the lithography-level alignment with the bottom wafer opens up the possibility for ultrafast operation with circuit integration. Spectrally pure color conversion and solvent-free QD patterning were also achieved using an elastomeric topographical mask. Self-assembled monolayers were applied to selectively alter the surface wettability for a completely dry process. The final emissive-type LED array integrating QD, GaN, and silicon technology resulted in a 1270 PPI resolution that is far beyond the retinal limit.

Mechanism and quantum efficiency of yellow emission at the organic-inorganic F8T2/p-GaN interface

An organic/inorganic F8T2/gallium nitride (GaN) hybrid light-emitting device is fabricated and found to emit white light. The white light emission is determined to be the combination of the blue emission of the GaN light-emitting diode and the yellow emission of the F8T2 layer. Analysis of the results of this work reveals the following: (1) the yellow emission of the F8T2 layer in the F8T2/GaN light-emitting device is caused by both electroluminescence and photoluminescence and (2) the origin of the majority of the yellow emission is the electroluminescence mechanism.A potential barrier in the valence band at the F8T2/p-GaN interface has been observed and determined to be 1.05 eV by ultraviolet photoelectron spectroscopy analysis. The number of hole carriers flowing over the potential barrier (1.05 eV) into the valence band of GaN; multiple quantum wells (MQWs) governs the quantum efficiency of the yellow emission. Because the percentage of the overflowed hole carriers in the valence band increases with the current input, the quantum efficiency of the active region in the F8T2 layer increases with the current input; this observed behavior is opposite to that of existing solid-state light-emitting devices.

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.