Reference Light-emitting Diodes Research Articles

BAlGaN light‐emitting diode emitting at 350 nm

AbstractIn this work, the authors report the first demonstration of an ultraviolet light‐emitting diode (LED) with boron‐containing quantum wells (QWs). Secondary ion mass spectrometry measurements revealed a B concentration of 1% in the first QW with an increase in subsequent QWs and lingering decay into the p‐region, likely attributable to boron's surfactant nature. Electroluminescence emission from the BAlGaN LED was observed at 350 nm, with higher intensity compared to the AlGaN reference LED at low currents. A higher operating voltage compared to the reference LED was also observed which may be attributable to a nanomasking behaviour of boron in (Al)GaN alloys.

Performance Improvement of InGaN Red Light‐Emitting Diode by Using V‐Pits Layer and Step‐Graded GaN Barrier

The carrier‐transport properties of InGaN red light‐emitting diode (LED) with V‐pits layer and step‐graded GaN barrier are investigated. By using V‐pits layer technology, the inverted pyramid–shaped V‐pit layer forms an energy barrier around the dislocation, inhibits the lateral diffusion of electrons from active layer to the dislocation center, and reduces the probability of non‐radiative recombination rate of electrons and holes. With the increase of the horizontal opening width of the V‐pits layer, the uniformity of holes concentration distribution in InGaN/GaN multiple quantum well (MQWs)‐active region is improved significantly. Compared with the traditional InGaN/GaN MQWs‐active region with fixed well/barrier thickness, step gradient GaN barrier is adopted in the active layer of InGaN red LED, which can reduce the length of the transmission path and improve the injection efficiency of holes into deep quantum wells close to the n‐type region. At injection current density of 200 A cm−2, the internal quantum efficiency of LED D with V‐pits layer and step‐graded GaN barrier is improved by 56.4% as compared with that of reference LED A. In addition, the efficiency droop of LED D can also be effectively alleviated as compared with that of reference LED A.

Carrier Confinement in Ultraviolet-C AlGaN Light-Emitting Diodes

We have numerically analyzed three different light-emitting diodes (LEDs) namely LED S1, LED S2, and LED S3 in the ultraviolet (UV) region. The band diagram, radiative recombination, and carrier distribution have been studied for LED S2 and LED S3 in comparison to LED S1. Our simulation study has shown that both the proposed structures i.e., LED S2 and LED S3 prove to be better strategies for impeding electron overflow and improving hole inflow. LED S2 and LED S3 have been found to have a reduced efficiency droop at 200 A cm−2 than reference LED S1. Increase in carrier concentration and radiative recombination rate have directly affected the internal quantum efficiency (IQE) of LED S2 and LED S3. The emission spectra of proposed LEDs have increased markedly as compared to LED S1.

Arrays of TiO2 Nanosphere Monolayers on GaN-Based LEDs for the Improvement of Light Extraction

We report on the fabrication of TiO2 nanosphere (NS) monolayer arrays for the enhancement of light extraction quantum efficiency of GaN-based light-emitting diodes (LEDs). The fabricated TiO2 NSs monolayer arrays were composed of different phases of anatase (An-) and amorphous (Am-) TiO2. The arrays were transferred onto the topmost layer of LED chips via the facile icing transfer method. The LED chips covered with Am-TiO2 NS monolayer arrays showed 3.0- times enhanced light output power intensity compared with reference LED chips at a fixed injection current of 100 mA. The enhanced light extraction of LED chips by an Am-TiO2 NS monolayer can be attributed to a high transmittance (91.1%) in visible and increased light extraction probability of photons generated in LEDs, resulting from the enhanced light coupling efficiency by reduced total internal reflection (TIR). Finite-difference time-domain (FDTD) simulation results also agreed well with the experimentally observed results. Based on the experimental and theoretical results, our suggested Am- and An-TiO2 NS arrays can be considered a very facile and effective method to improve the device performance of various visible LED chips.

Impact of doped barriers on the recombination coefficients of c-plane InGaN/GaN single quantum well light-emitting diodes

Differential carrier lifetime measurements were performed on c-plane InGaN/GaN single quantum well (QW) light-emitting diodes (LEDs) of different QW indium compositions as well as with and without doped barriers. Mg-doped p-type and Si-doped n-type barriers close to the QW were used to reduce the net internal electric field in the QW, thereby improving the electron–hole wavefunction overlap on the LEDs. LEDs with doped barriers show short lifetimes and low carrier densities in the active region compared to the reference LEDs. The recombination coefficients in the ABC model were estimated based on the carrier lifetime and quantum efficiency measurements. The improvement in the radiative coefficients in the LEDs with doped barriers coupled with the blueshift of the emission wavelengths indeed indicates an enhancement in wavefunction overlap and a reduction of quantum confined Stark effect as a result of the reduced internal electric field. However, doped barriers also introduce non-radiative recombination centers and thereby increase the Shockley–Read–Hall (SRH) coefficient, although the increment is less for LEDs with high indium composition QWs. As a result, at high indium composition (22%), LEDs with doped barriers outperform the reference LEDs even though the trend is reversed for LEDs with lower indium composition (13.5%). Despite the trade-off of higher SRH coefficients, doped barriers are shown to be effective in reducing the internal electric field and increasing the recombination coefficients.

Nanoporous AlGaN-based distributed Bragg reflectors with enhanced luminescence for wafer-scale ultraviolet light-emitting devices

The large-area (20.3 cm2) NP-AlGaN DBRs, which are prepared via electrochemical (EC) etching in an acid solution, present high reflectivity (>90%) and wide spectral stopband. Then the nanoporous AlGaN Distributed Bragg reflectors (NP-AlGaN DBRs) are used as substrates for regrowth of ultraviolet light-emitting diodes (LEDs). Compared to those of the reference LEDs, the ultraviolet LEDs with the DBRs show sixfold enhancement and red-shift of the luminescence peak, as well as a rising PL lifetime. Luminescence enhancement is owing to the low defect density of the AlGaN/GaN MQWs and high light reflection effect of the NP-AlGaN DBRs. The red-shift is due to the stress relaxation in the AlGaN/GaN MQWs layer related to the wet etching and regrowth process.

Demonstration of Efficient Ultrathin Side-Emitting InGaN/GaN Flip-Chip Light-Emitting Diodes by Double Side Reflectors.

This work proposes an InGaN/GaN multiple-quantum-well flip-chip blue ultrathin side-emitting (USE) light-emitting diode (LED) and describes the sidewall light emission characteristics for the application of backlight units in display technology. The USE-LEDs are fabricated with top (ITO/distributed Bragg reflector) and bottom (Ag) mirrors that cause light emission from the four sidewalls in a lateral direction. The effect of light output power (LOP) on lateral direction is consistently investigated for improving the optoelectronic performances of USE-LEDs. Initially, the reference USE-LED suffers from very low LOP because of poor light extraction efficiency (LEE). Therefore, the LEE is improved by fabricating ZnO nanorods at each sidewall through hydrothermal method. The effects of ZnO nanorod lengths and diameters on LOP are systematically investigated for optimizing the dimensions of ZnO nanorods. The optimized ZnO nanorods improve the LEE of USE-LED, which thus results in increasing the LOP > 80% compared to the reference LED. In addition, the light-tools simulator is also used for elucidating the increase in LEE of ZnO nanorods USE-LED.

Performance improvement of InGaN LEDs by using strain compensated last quantum barrier and electron blocking layer

The influence of u-AlGaN/InGaN superlattices (SLs) last quantum barrier (LQB) and p-AlGaN/InGaN SLs electron blocking layer (EBL) on the performance of InGaN light-emitting diodes (LEDs) are analyzed experimentally and theoretically. Research results indicated that LED C with u-AlGaN/InGaN SLs LQB and p-AlGaN/InGaN SLs EBL exhibit the highest output power among the three groups of LED samples. At 200 mA, the output power of LED C is enhanced by 27.5% as compared with that of reference LED A with u-GaN LQB and p-AlGaN/GaN SLs EBL. The efficiency droop of LED C has also been suppressed effectively. At 200 mA, the efficiency droop of LED C was 28.5%, which was smaller than that of LED A 34%. By analyzing the distribution of electron/hole concentration in multiple quantum wells, it is indicated that the leakage of electrons from the active layer can be effectively reduced and the injection efficiency of holes into the active layer can be dramatically enhanced by using strain compensated AlGaN/InGaN SLs LQB and EBL. The reason is that the potential energy barriers for electrons and holes could be effectively regulated by using the strain compensated LQB and EBL, which is consistent with simulation results of energy band diagram.

Hexagonal Boron Nitride Passivation Layer for Improving the Performance and Reliability of InGaN/GaN Light-Emitting Diodes

We introduce a low temperature process for coating InGaN/GaN light-emitting diodes (LEDs) with h-BN as a passivation layer. The effect of h-BN on device performance and reliability is investigated. At −5 V, the leakage current of the h-BN passivated LED was −1.15 × 10−9 A, which was one order lower than the reference LED’s leakage current of −1.09 × 10−8 A. The h-BN layer minimizes the leakage current characteristics and operating temperature by acting as a passivation and heat dispersion layer. With a reduced working temperature of 33 from 45 °C, the LED lifetime was extended 2.5 times following h-BN passivation. According to our findings, h-BN passivation significantly improves LED reliability.

Deep Ultraviolet AlGaN-Based Light-Emitting Diodes with p-AlGaN/AlGaN Superlattice Hole Injection Structures

The p-AlGaN/AlGaN superlattice (SL) hole injection structure was introduced into deep ultraviolet (DUV) light-emitting diodes (LEDs) to enhance their performances. The period thicknesses of the p-Al0.8Ga0.2N/Al0.48Ga0.52N SLs affected the performances of the DUV LEDs. The appropriate period thickness of the p-Al0.8Ga0.2N/Al0.48Ga0.52N SL may enhance the hole injection of DUV LEDs. Therefore, compared with the reference LEDs, the DUV LEDs with the 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) SL presented forward voltage reduction of 0.23 V and light output power improvement of 15% at a current of 350 mA. Furthermore, the 10-pair Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) SL could slightly suppress the Auger recombination and current overflow of the DUV LEDs in a high-current operation region. In addition to improved carrier injection, the DUV LEDs with the p-Al0.8Ga0.2N/Al0.48Ga0.52N SL hole injection structure showed reduced light absorption at their emission wavelength compared with the reference LEDs. Therefore, the DUV LEDs with p-Al0.8Ga0.2N/Al0.48Ga0.52N SL may exhibit better light extraction efficiency than the reference LEDs. The enhancement of p-Al0.8Ga0.2N (1 nm)/Al0.48Ga0.52N (1 nm) SL may contribute to improvements in light extraction and hole injection.

Improvement of Radiative Recombination Rate and Efficiency Droop of InGaN Light Emitting Diodes with In‐Component‐Graded InGaN Barrier

The influences of In‐component‐graded InGaN barrier on the performances of InGaN light emitting diodes (LEDs) are studied. The results show that LEDs with In‐component linearly graded InGaN barrier exhibit better performances of radiative recombination rate and efficiency droop than reference LED with constant‐In‐component InGaN barrier. At 150 mA, the external quantum efficiencies for LED A with constant‐In‐component InGaN barrier, LED B with the In‐component linearly increasing in the InGaN barrier, and LED C with the In‐component linearly decreasing in the InGaN barrier are 30.30%, 36.66%, and 34.03%, respectively. Among the three sets of LED samples, LED B, with the In‐component linearly increasing in the InGaN barrier, has the highest radiative recombination rate due to suppression of quantum confinement stark effect and enhancement of carrier confinement effect in the active layer. The efficiency droop for LED A, LED B, and LED C is 34.83%, 25.67%, and 16.67%, respectively. Analysis indicates that the efficiency droop is mainly determined by the hole injection efficiency and electron leakage from the active layer. LED C with the In‐component linearly decreasing in the InGaN barrier has the lowest efficiency droop due to improved hole injection efficiency and reduced electron leakage.

The Effect of Nanometer‐Scale V‐Pit Layer on the Carrier Recombination Mechanisms and Efficiency Droop of GaN‐Based Green Light‐Emitting Diodes

The effects of nanometer‐scale V‐pits layer with different parameters on the carrier recombination mechanisms and efficiency droop of GaN‐based green light‐emitting diodes (LEDs) are investigated. Under optimized growth conditions, the threading dislocations (TDs) are surrounded by V‐pits layer with six (10–11) side wall facets. The V‐pits layer contains sidewall multiple quantum wells (MQWs) with bandgap several hundred millielectronvolts higher than that of c‐plane MQWs. This higher bandgap acts as a barrier for carriers and prevents them from reaching the dislocations, which can effectively suppress the nonradiative recombination. Analysis suggests that the V‐pits enhance hole injection into deeper MQWs, which is confirmed by the secondary ion mass spectroscopy profiles of Mg and Al. With the aid of the V‐pits layer, holes can be injected into deeper MQWs, which reduce the carrier concentration within individual QWs. Therefore, the Auger‐related efficiency droop can be suppressed. The results show that LED with 200 nm LT‐GaN + 40 nm n‐GaN V‐pits layer is able to deliver an improvement in the light output power by 189% as compared with reference LED without V‐pits layer. At 200 mA, the efficiency droop for LED with200 nm LT‐GaN + 40 nm n‐GaN V‐pits is 25.9%, which is much smaller than 34.7% for reference LED without V‐pits.

GaAs-based optoelectronics grown on GaAs/Si virtual substrates with multiple spaced thermal-cycle annealing

This paper describes the growth of a GaAs-based light-emitting diode (LED) on a GaAs/Si virtual substrate by metalorganic chemical vapor deposition. A GaAs/Si virtual substrate (or template) was formed by depositing a 3.5-μm-thick GaAs epilayer on a (100) Si substrate with the employment of multiple spaced thermal-cycle annealing (TCA). The number of annealing cycle was zero to five in this study to examine crystallinity variation of a GaAs/Si template. High-resolution X-ray rocking curve analysis showed that (400) GaAs crystallinity in terms of full width at half maximum (FWHM) improved significantly with the number of the spaced TCA and then reached a minimum at four annealing cycles. There seems to be no significant difference in crystallinity between the GaAs/Si templates with four and five spaced annealing cycles. The GaAs/Si template with four spaced annealing cycles demonstrated not only an excellent (400) FWHM value of 94.6 arcsec but also a reduced dislocation density of 2.4 × 107 cm−2 evaluated from plan-view transmission electron microscopic observation. Electroluminescence spectra of GaAs-based LEDs grown on GaAs/Si templates showed that emission peak intensity increased monotonically with the number of the spaced annealing cycles. Furthermore, the emission peak intensity of a LED grown on the GaAs/Si template with five spaced annealing cycles reached 80% of a reference LED grown on a GaAs substrate, indicating that the presented multiple spaced TCA method may be useful in obtaining superior crystallinity of a GaAs/Si template for the potential application of monolithic integration of optoelectronic devices with Si.

Improved Performance of GaN-Based Light-Emitting Diodes Grown on Si (111) Substrates with NH3 Growth Interruption.

We demonstrate the highly efficient, GaN-based, multiple-quantum-well light-emitting diodes (LEDs) grown on Si (111) substrates embedded with the AlN buffer layer using NH3 growth interruption. Analysis of the materials by the X-ray diffraction omega scan and transmission electron microscopy revealed a remarkable improvement in the crystalline quality of the GaN layer with the AlN buffer layer using NH3 growth interruption. This improvement originated from the decreased dislocation densities and coalescence-related defects of the GaN layer that arose from the increased Al migration time. The photoluminescence peak positions and Raman spectra indicate that the internal tensile strain of the GaN layer is effectively relaxed without generating cracks. The LEDs embedded with an AlN buffer layer using NH3 growth interruption at 300 mA exhibited 40.9% higher light output power than that of the reference LED embedded with the AlN buffer layer without NH3 growth interruption. These high performances are attributed to an increased radiative recombination rate owing to the low defect density and strain relaxation in the GaN epilayer.

III-nitride blue light-emitting diodes utilizing hybrid tunnel junction with low excess voltage

Tunnel junctions (TJs) offer alternative designs and promise in some cases improved performances for nitride-based light-emitting diode (LEDs) and laser diodes (LDs) and are widely used in academic studies. However, the voltage penalty of the LEDs and LDs, in comparison with standard contact technologies, has been a major concern especially for commercial applications. In this study, we investigated methods to achieve low excess voltage. Using ammonia molecular beam epitaxy (NH3 MBE), GaN TJs were grown on commercial metalorganic chemical vapor deposition (MOCVD) grown blue LED wafers. Atom probe tomography (APT) and secondary ion mass spectrometry (SIMS) indicate 1 min buffered HF (BHF) clean of the regrowth interface reduced Mg and impurity incorporation into the n++ regrown TJ layers. The wafers were processed and measured in parallel to reference wafers using both university processes and industry processes. At 20 A cm−2, TJ LEDs grown with Si δ-doping at the junction interface processed in the university cleanroom had a forward voltage of 3.17 V in comparison to 2.86 V for LEDs processed with a standard indium tin oxide (ITO) contact. Unencapsulated TJ LEDs processed by industrial process without ITO or current blocking layer had about 0.3 V excess voltage compared to reference LEDs. The TJ LEDs also had more uniform light emission profile. The low excess voltage and consistent results acquired in both settings suggest that TJ can be scaled for industrial processes.

AlGaN Deep-Ultraviolet Light-Emitting Diodes with Localized Surface Plasmon Resonance by a High-Density Array of 40 nm Al Nanoparticles.

We present a remarkable improvement in the efficiency of AlGaN deep-ultraviolet light-emitting diodes (LEDs) enabled by the coupling of localized surface plasmon resonance (LSPR) mediated by a high-density array of Al nanoparticles (NPs). The Al NPs with an average diameter of ∼40 nm were uniformly distributed near the Al0.43Ga0.57N/Al0.50Ga0.50N multiple quantum well active region for coupling 285 nm emission by block copolymer lithography. The internal quantum efficiency is enhanced by 57.7% because of the decreased radiative recombination lifetime by the LSPR. As a consequence, the AlGaN LEDs with an array of Al NPs show 33.3% enhanced electroluminescence with comparable electrical properties to those of reference LEDs without Al NPs.

Effect of a thin Au and ZnO layer on optical properties of 1D PhC structures patterned in LED surface

1D patterns with different periods were fabricated by Electron Beam Direct Write (EBDW) lithography with the aim to improve the Light Emitting Diode (LED) efficiency, i.e. the light out coupling. Three sets of 1D PhC LED samples with bare PhC, thin Au, and ZnO layers were prepared. The highest Light Extraction Enhancement (LEE) was achieved for 600 nm Photonic Crystals (PhC) period in the case of bare and Au deposited PhC, and for 500 nm PhC period in the case of ZnO deposited PhC, respectively. LEE was studied by measuring L-I characteristics of untreated reference LEDs compared to patterned LEDs.

Error Analysis on Indoor Localization with Visible Light Communication

Affected by the complexity of the indoor environment, accurate indoor positioning is challenging in many localization based services (LBS). Recently, it has been recognized that, visible light communication (VLC) is promising for indoor navigation and positioning, due to the low implementation cost with marginal modification to the existing infrastructure and the possibility to achieve high accurate positioning results. Provided that the positions of the light emitting diodes (LEDs) are known to the receiver, the angle of arrival (AOA) of the light signal is able to be estimated by a camera embedded in a smart phone, and thus the position of the smart phone can be derived based on the triangulation. In this paper, the performance of the positioning accuracy is analyzed based on indoor positioning with VLC, and the analytical upper bound of location error is derived. Extensive simulation results have verified the theoretical analysis on the VLC-based localization approach in different indoor scenarios. In order to obtain better location performance, the principles of choosing reference LED and localization LED are also given.

Investigation on Modulation Speed of Photon-Recycling White Light-Emitting Diodes With Vertical-Conduction Structure

The speed performance of a high lumen and high color rendering index (CRI) gallium nitride-based white-light light-emitting diode (LED) has been investigated in detail by measuring its electrical-to-optical frequency responses with different color filters at the receiver end. The ultra-violet/blue/green multiple quantum wells (MQWs) are stacked in the studied LED chips, which have a vertical conducting structure and red phosphor coating for generating white light with high CRI. The typical studied LED exhibits a smaller 3 dB bandwidth (0.6 MHz versus 2 MHz) compared with a reference LED with ultra-violet/blue MQWs and a yellow phosphor coating. This finding suggests that the fluorescent decay time of red phosphor is much longer than that of yellow ones (approximately 270 ns versus approximately 80 ns). In addition, the extracted carrier lifetime inside blue/green InxGa1-xN/GaN MQWs under UV light pumping/modulation is as long as approximately 50 ns, which is much slower than that of forward-biased current modulation (approximately 50 ns versus on–off keying modulation scheme are demonstrated under a moderate operation bias current (approximately 0.75 A) of studied LEDs.

Fabrication and properties of high quality InGaN-based LEDs with highly reflective nanoporous GaN mirrors

Distributed Bragg reflectors (DBRs) are essential components for the development of optoelectronic devices. In this paper, we first report the use of the nanoporous GaN (NP-GaN) DBR as a template for regrowth of InGaN-based light-emitting diodes (LEDs). The wafer-scale NP-GaN DBR, which is fabricated by electrochemical etching in a neutral solution, has a smooth surface, high reflectivity (>99.5%), and wide spectral stop band width (>70 nm). The chemical composition of the regrown LED thin film is similar to that of the reference LED, but the photoluminescence (PL) lifetime, PL intensity, and electroluminescence intensity of the LED with the DBR are enhanced several times compared to those of the reference LED. The intensity enhancement is attributed to the light reflection effect of the NP-GaN DBR and improved crystalline quality as a result of the etching scheme, whereas the enhancement of PL lifetime is attributable to the latter.