Vertical Light-emitting Diodes Research Articles

Improved Performance of High-Voltage Vertical GaN LEDs via Modification of Micro-Cell Geometry

Vertical-type high-voltage light-emitting diodes (HV-LEDs) with 2 × 2 micro-cells were fabricated on Cu substrates, and the micro-cell geometry was modified to enhance the optoelectronic performance. The current spreading in micro-cell is most dominantly affected by the distance between electrode and edge of chip. When square cells were combined in a HV-LED, the device performance was poor due to an obvious current-crowding phenomenon that occurred near the electrodes. This was attributed that the electrodes in these four square micro-cells were all far away from the edges, resulting in the severe current-crowding phenomenon. On the contrary, as the HV-LED was prepared with four rectangle, triangle, or L-shaped micro-cells, the electrodes were close to the edges of micro-cells and the current spreading effect can be easily improved. Although a HV-LED connected with L-shaped cells possessed a better current spreading effect and a lower surface temperature, the light extraction was relatively low because of an electrode-shading loss effect. When triangular cells were used to prepare the HV-LED, the device achieved a superior optoelectronic performance compared with that of other cells because of a lower current-crowding effect and a more uniform light emission. After an epoxy package process, a lower forward voltage of 14.9 V and a higher output power of 353.2 mW were obtained using this HV-LED at an injection current of 80 mA. Additionally, the wall-plug efficiencies of this device at 20 and 80 mA were 41.1% and 29.7%, respectively. The results confirm that the design of triangular cell is beneficial for enhancing the optoelectronic performance of HV-LEDs. Furthermore, the fabrication processes of vertical LEDs have high potential for HV-LED applications.

Performances of p-side down vertical InGaN/GaN blue light-emitting diodes with chip size

In this paper, the p-side down vertical InGaN/GaN blue light-emitting diodes with different geometric patterns of n-electrodes of chip sizes of 300×300 µm2, 500×500µm2 and 1000×1000µm2 have been optimized for achieving a uniform distribution of current density and subsequently improved light extraction efficiency. An LED with a well-designed n-electrode pattern shows a uniform distribution of light emission with a relatively high output power due to uniform current spreading. It is observed that the n-electrode pattern has a marked influence on the current distribution and the light output power for the large area p-side down vertical LEDs.

Fabrication and application of indium-tin-oxide nanowire networks by polystyrene-assisted growth

The fabrication and application of polystyrene (PS)-assisted ITO nanowire (NW) networks are reported. The ITO-NW networks are fabricated by means of electron-beam deposition via PS. This method has the advantages of low-temperature (~300 °C), low-cost, facile and efficient operation. The growth mechanism of PS-assisted ITO NWs was analyzed in detail, and the morphology of which could be regulated by the size of PS. X-ray diffraction and high-resolution transmission electron microscope show that the ITO NWs are close to an integral cubic lattice. The transmittance of ITO-NW networks layer is above 90% after 400 nm and the sheet resistance is ~200 Ω/□. When they applied on vertical blue and green LEDs, the light output power all has been improved ~30%. And, the resistive switching behaviors of ITO-NWs were measured and analyzed in Ag/ITO-NW networks/Al capacitor. The application of ITO-NW networks on special morphological devices was discussed. The PS-assisted ITO-NW networks show a strong researching and application value.

Enhanced Performance of Large-Area Vertical Light-Emitting Diodes Treated by Laser Irradiation

Enhanced Performance of Large-Area Vertical Light-Emitting Diodes Treated by Laser Irradiation

Electrolyte-Gated Red, Green, and Blue Organic Light-Emitting Diodes.

We report vertical electrolyte-gated red, green, and blue phosphorescent small-molecule organic light-emitting diodes (OLED), in which light emission was modified by tuning the electron injection via electrochemical doping of the electron injection layer 4,4-bis(N-carbazolyl)-1,1-biphenyl (CBP) under the assistance of a polymer electrolyte. These devices comprise an electrolyte capacitor on the top of a conventional OLED, with the interfacial contact between the electrolyte and electron injection layer CBP of OLEDs achieved through a porous cathode. These phosphorescent OLEDs exhibit the tunable luminance between 0.1 and 10 000 cd m-2, controlled by an applied bias at the gate electrode. This simple device architecture with gate-modulated luminance provides an innovative way for full-color OLED displays.

On the metallic bonding of GaN-based vertical light-emitting diode

In this work, metallic bonding in GaN-based vertical light-emitting diode (VLED) is systematically characterized by using combined methodology of transmission Kikuchi diffraction (TKD) and energy dispersive X-ray spectroscopy (EDS) in a scanning electron microscope (SEM). SEM-based TKD with EDS identifies chemical composition, grain morphology, orientation, and phases at metallic bonding, while transmission electron microscopy (TEM) provides nanoscale characteristics of metallic diffusion bonding, and its interface-related defects and nano-twinned boundaries. Our results from SEM-TKD and TEM techniques provide unparalleled insight into the metallic bonding, and its future optimization.

Effect of plasma surface treatment on embedded n-contact for GaN-based blue light-emitting diodes grown on Si substrate

Unlike the finger-like n-contact that is prepared after the wafer bonding and the N-polar GaN surface roughening for GaN-based vertical structure light-emitting diodes (LEDs) grown on Si substrates, the embedded via-like n-contact is formed prior to the wafer bonding. The high temperature process of the wafer bonding often causes the electrical characteristics of the via-like embedded n-contact to degrade. In this paper, we study in detail the effect of plasma treatment of the n-GaN surface on the forward voltage of GaN-based LED grown on Si substrate. It is shown that with no plasma treatment on the n-GaN surface, the forward voltage (at 350 mA) of the 1.1 mm1.1 mm chip with a highly reflective electrode of Cr (1.1 nm)/Al is 3.43 V, which is 0.28 V higher than that of the chip with a pure Cr-based electrode. The LED forward voltages for both kinds of n-contacts can be reduced by an O2 plasma treatment on the n-GaN surface. But the LED forward voltage with a Cr/Al-based electrode is still 0.14 V higher than that of the chips with a pure Cr-based electrode. However, after an Ar plasma treatment on the n-GaN surface, the LED forward voltage with a Cr/Al-based electrode is reduced to 2.92 V, which is equal to that of the chip with a pure Cr-based electrode. The process window of the n-GaN surface after the Ar plasma treatment is broader. X-ray photoelectron spectroscopy is used to help elucidate the mechanism. It is found that Ar plasma treatment can increase the concentration of N-vacancies (VN) at the n-GaN surface. VN acts as donors, and higher VN helps improve the thermal stability of n-contact because it alleviates the degradation of the n-contact characteristics caused by the high temperature wafer bonding process. It is also found that the O content increases slightly after the Ar plasma treatment and HCl cleaning. The O atoms are mainly present in the dielectric GaOx film before the Ar plasma treatment and the HCl cleaning, and they exist almost equivalently in the conductive GaOxN1-x film and the dielectric GaOx film after Ar treatment and HCl cleaning. The conductive GaOxN1-x film and the VN donors formed during the plasma treatment can reduce the contact resistance and the LED forward voltage.

6H-SiC衬底上AlGaN基垂直结构紫外LED的制备

6H-SiC衬底上AlGaN基垂直结构紫外LED的制备

Stress Engineering During the Fabrication of InGaN/GaN Vertical Light Emitting Diodes for Reducing the Quantum Confined Stark Effect

The scope for reengineering the stress in vertical light-emitting diodes (VLEDs) grown Si(111) substrates during the substrate removal and wafer bonding steps in fabrication processing to improve internal quantum efficiency (IQE) is demonstrated. The tensile stress in the GaN layers adjacent to an InGaN/GaN multiple quantum well (MQW) emissive region can be controllably varied in such a way to compensate the pseudomorphic compressive stress in the InGaN layers, and thereby counteract the reduction in the IQE due to the quantum confined Stark effect. A quadratic dependence on the MQW photoluminescence emission wavelength and wafer bow is obtained to enable straightforward strain reengineering through monitoring the change in the wafer bow of the LED epitaxy. The external quantum efficiency of stress-engineered VLEDs bonded onto a 475- $\mu \text{m}$ -thick Si carrier wafer was 25% higher at 300-mA drive current than that of their counterparts bonded onto 675- $\mu \text{m}$ -thick Si substrates using otherwise identical processing.

Investigation of n-ohmic contact of vertical GaN-based light-emitting diodes on graphite substrate with Ag-In bonding

Vertical light-emitting diodes (VLEDs) were successfully transferred from a GaN-based sapphire substrate to a graphite substrate by using low-temperature and cost-effective Ag-In bonding, followed by the removal of the sapphire substrate using a laser lift-off (LLO) technique. One reason for the high thermal stability of the AgIn bonding compounds is that both the bonding metals and Cr/Au n-ohmic contact metal are capable of surviving annealing temperatures in excess of 600°C. Therefore, the annealing of n-ohmic contact was performed at temperatures of 400°C and 500°C for 1min in ambient air by using the rapid thermal annealing (RTA) process. The performance of the n-ohmic contact metal in VLEDs on a graphite substrate was investigated in this study. As a result, the final fabricated VLEDs (chip size: 1000µm×1000µm) demonstrated excellent performance with an average output power of 538.64mW and a low operating voltage of 3.21V at 350mA, which corresponds to an enhancement of 9.3% in the light output power and a reduction of 1.8% in the forward voltage compared to that without any n-ohmic contact treatment. This points to a high level of thermal stability and cost-effective Ag-In bonding, which is promising for application to VLED fabrication.

Device characteristics and thermal analysis of GaN-based vertical light-emitting diodes with different types of packages

We investigated the device characteristics of GaN-based blue vertical light-emitting diodes (VLEDs) with two different package structures (i.e., lead frame with metal/plastic body (MPLF package) and lead frame with metal body (MLF package)) under various measurement conditions. In comparison with the MPLF packaged VLEDs, the MLF packaged VLEDs exhibited relatively lower junction temperature and thermal resistance values due to the better heat dissipation capability, leading to further improved optical, spectral, and thermal device characteristics. Thermal simulations of the VLEDs with two different packages were performed using three-dimensional steady-state device models to theoretically calculate their thermal and mechanical behaviours. The maximum temperatures, internal temperature distributions, and thermomechanical stresses were analysed by a finite element method.

Enhanced light extraction of GaN-based vertical LEDs with patterned trenches and nanostructures

An efficient surface texturing technique that uses patterned trench etching and the selective formation of GaN nanostructures on the trench bottoms to improve the light extraction of vertical GaN-based light-emitting diodes (VLEDs) is proposed and demonstrated. Theoretical and experimental results that show the effectiveness of the proposed surface roughening scheme in improving light output power (LOP) and wall-plug efficiency (WPE) are presented and discussed. Compared with conventional VLEDs, significant improvements in LOP and WPE at 350mA of about 37.6% and 5.1%, respectively, are obtained for the proposed VLEDs. The effective lateral light emission harvested by patterned trenches and the strongly enhanced angular randomization of photons that minimizes the total internal reflection at the GaN/air interface are responsible for the LOP and WPE improvements.

Analysis of the light-extraction efficiency of SiC substrate-based flip-chip vertical light-emitting diodes with embedded photonic crystals

To improve the light-extraction efficiency (LEE) of flip-chip vertical light-emitting diodes (LEDs) grown on silicon carbide (SiC) substrate, embedded photonic crystals (PhCs) were alternatively introduced into the n-GaN layer of LEDs, since etching of the SiC substrate was very difficult. The finite-difference time-domain (FDTD) method was employed to investigate the combination effects of the micro-cavity and the embedded PhCs. The influences of the PhCs configurations on the LEE of LEDs were also examined to get an optimal structure. With the optimized parameters, about 20% enhancement of LEE was achieved comparing to planar SiC substrate-based flip-chip vertical LEDs. The LEE of conventional surface PhCs LEDs and double layer PhCs LEDs were also investigated for comparison. The results indicated that LEDs with carefully designed embedded PhCs could provide more LEE than surface PhCs LEDs. The structures proposed here offered scopes for the design of high-efficiency, high-power LEDs.

High Light Extraction Efficiency AlGaInP LEDs With Proton Implanted Current Blocking Layer

Improving light extraction efficiency is the key issue for light-emitting diodes (LEDs). Nowadays, a vertical structure design dominates LEDs. However, the light from the active region just below the p-electrode is severely blocked by the metal contact. In this letter, we use proton implantation with a depth all the way to the active region to turn the part beneath the p-pad insulating, which constitutes the most-effective-ever current blocking method. Earlier particle implantation studies never reached the device active region. Our experimental results show that the H+-implanted LEDs improve the light output power by 75% compared with non-implanted counterparts and the light intensity increases by 64.48%. By virtue of indium tin oxide current spreading film, the increase in working voltage is negligible. Analyzing the reverse leakage current, the side effect associated with the implantation is limited to an acceptable range. Numerical simulation is performed to support the experiment. Our results represent a new and simple method for solving the light blocking problem in vertical LEDs, without introducing the seemingly existing severe implantation damage to the device structure.

Controlling the defect density to improve the output power of InGaN/GaN-based vertical light-emitting diodes by using substrates patterned with SiO2 lenses

In this study, we investigated the effect of SiO2 lenses on the output power of InGaN/GaN-based vertical light-emitting diodes (VLEDs; wavelength = 445 nm) and compared the results to those of reference VLEDs without the SiO2 lenses (planar samples). Arrays of SiO2 lenses (pitch = 3 μm, width = 2.5 μm, height = 1.0 μm) were formed on c-plane sapphire substrates. The external quantum efficiency (EQE) of the packaged VLEDs with planar and patterned substrates was characterised. At 5 mA, the EQE of the patterned samples was 150% higher than that of the planar samples. A patterned, N-polar, n-GaN sample contained far fewer nanopipes (approximately 2.2 × 105 cm−2) than a planar n-GaN sample (approximately 2.4 × 106 cm−2). Furthermore, the patterned samples contained far fewer threading dislocations (approximately 1.0 × 108 cm−2) than the planar samples (approximately 5.0 × 108 cm−2). Scanning electron microscopy (SEM) images showed that the photoelectrochemical (PEC)-etched patterned samples contained cones that were 150% larger than that of the PEC-etched planar samples. In addition, SEM images, cathode luminescence measurements and finite-difference time-domain simulations were used to characterise the improved light output of the patterned samples.

Electroless Silver Plating Reflectors to Boost the Performance of Vertical Light-Emitting Diodes

In this paper, we experimentally demonstrated a novel approach known as electroless silver plating (ESP) for the fabrication of Ag-only reflectors on vertical light-emitting diodes (VLEDs). The Ag reflector that was obtained through the ESP approach shows a higher reflectance of 83% and a lower contact resistivity of $6\ast 10^{-3} \Omega\cdot\text{cm}^{2}$ with p-GaN, compared with that fabricated through the conventional electron beam (EB) approach, which shows a reflectance of 75% and a contact resistivity of $6.2\ast 10^{-2} \Omega\cdot\text{cm}^{2}$ , respectively. Improvements in the light output intensity-current (L-I) and current-voltage (I-V) results in VLEDs, which originated from the high reflectance and low-contact resistivity of the incorporated ESP–Ag reflector. Our result opens up an alternative and promising way to substitute the traditional EB approach for Ag-based reflector fabrication and, thus, is practically meaningful in real device fabrication, such as with VLEDs.

Fabrication of extremely thermal-stable GaN template on Mo substrate using double bonding and step annealing process**Project supported by the Guangdong Innovative Research Team Program (No. 2009010044), the China Postdoctoral Science Foundation (No. 2014M562233), the National Natural Science Foundation of Guangdong, China (No. 2015A030312011), and the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (No. IOSKL2014KF17).

A new layer transfer technique which comprised double bonding and a step annealing process was utilized to transfer the GaN epilayer from a sapphire substrate to a Mo substrate. Combined with the application of the thermal-stable bonding medium, the resulting two-inch-diameter GaN template showed extremely good stability under high temperature and low stress state. Moreover, no cracks and winkles were observed. The transferred GaN template was suitable for homogeneous epitaxial, thus could be used for the direct fabrication of vertical LED chips as well as power electron devices. It has been confirmed that the double bonding and step annealing technique together with the thermal-stable bonding layer could significantly improve the bonding strength and stress relief, finally enhancing the thermal stability of the transferred GaN template.

Performance of GaN-on-Si-based vertical light-emitting diodes using silicon nitride electrodes with conducting filaments: correlation between filament density and device reliability.

Transparent conductive electrodes with good conductivity and optical transmittance are an essential element for highly efficient light-emitting diodes. However, conventional indium tin oxide and its alternative transparent conductive electrodes have some trouble with a trade-off between electrical conductivity and optical transmittance, thus limiting their practical applications. Here, we present silicon nitride transparent conductive electrodes with conducting filaments embedded using the electrical breakdown process and investigate the dependence of the conducting filament density formed in the transparent conductive electrode on the device performance of gallium nitride-based vertical light-emitting diodes. Three gallium nitride-on-silicon-based vertical light-emitting diodes using silicon nitride transparent conductive electrodes with high, medium, and low conducting filament densities were prepared with a reference vertical light-emitting diode using metal electrodes. This was carried to determine the optimal density of the conducting filaments in the proposed silicon nitride transparent conductive electrodes. In comparison, the vertical light-emitting diodes with a medium conducting filament density exhibited the lowest optical loss, direct ohmic behavior, and the best current injection and distribution over the entire n-type gallium nitride surface, leading to highly reliable light-emitting diode performance.

Polystyrene-catalytic indium–tin–oxide nanorods grown on green light-emitting diodes for enhancing light extraction

A novel technique for fabricating indium–tin–oxide (ITO) nanorods (NRs) on hexagonal-pyramid-surface green vertical light-emitting diodes (VLEDs) is demonstrated using electron-beam deposition with polystyrene spheres catalysis. The ITO NRs have high optical transmittance (>90%) in green wavelength and good crystal quality with a cubic structure. The VLED with ITO NRs has a 31% enhancement of light output power at 200 mA, compared with those without ITO NRs. Finite-difference time-domain simulations suggest that the power enhancement is attributed to the gradient refractive indices of the ITO NRs, and that the light extraction enhancement is caused by changes in ITO NR heights.

Reduced efficiency droop of nonpolar a-plane (11-20) GaN-based light-emitting diodes by vertical injection geometry

Vertical nonpolar a-plane (11-20) InGaN/GaN light-emitting diodes (LEDs) have been demonstrated by using laser lift-off technique. The forward voltage of the a-plane vertical LEDs was 4.3 V at 350 mA, which was reduced by 0.8 V compared to that of the a-plane lateral LEDs. The vertical geometry of the a-plane LEDs produced the higher quantum efficiency with a low efficiency droop and also enhanced the output power by more than 40%, when compared to those of a-plane lateral LEDs. These results can be attributed to the high thermal dissipation as well as uniform current spreading of the vertical geometry of the a-plane LEDs. Furthermore, elimination of the highly defected GaN nucleation layer after removing the sapphire substrates during the fabrication process can also enhance current injection efficiency, followed by the increase in the output power.