Reduced Surface Recombination in Extended-Perimeter LEDs toward Electroluminescent Cooling

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

Graphene as a Transparent Conductive Electrode in GaN-Based LEDs

Graphene combines high conductivity (sheet resistance down to a few hundred Ω/sq and even less) with high transparency (>90%) and thus exhibits a huge application potential as a transparent conductive electrode in gallium nitride (GaN)-based light-emitting diodes (LEDs), being an economical alternative to common indium-based solutions. Here, we present an overview of the state-of-the-art graphene-based transparent conductive electrodes in GaN-based LEDs. The focus is placed on the manufacturing progress and the resulting properties of the fabricated devices. Transferred as well as directly grown graphene layers are considered. We discuss the impact of graphene-based transparent conductive electrodes on current spreading and contact resistance, and reveal future challenges and perspectives on the use of graphene in GaN-based LEDs.

Promoted Hole Transport Capability by Improving Lateral Current Spreading for High-Efficiency Quantum Dot Light-Emitting Diodes

Carrier imbalance resulting from stronger electron injection from ZnO into quantum-dot (QD) emissive layer than hole injection is one critical issue that constrains the performance of QDs-based light-emitting diodes (QLEDs). This study reports highly efficient inverted QLEDs enabled by periodic insertion of MoO3 into (4,4'-bis(N-carbazolyl)-1,1'-biphenyl) (CBP) hole transport layer (HTL). The periodic ultrathin MoO3/CBP-stacked HTL results in improved lateral current spreading for the QLEDs, which significantly relieves the crowding of holes and thus enhances hole transport capability across the CBP in QLEDs. Comprehensive analysis on the photoelectric properties of devices shows that the optimal thickness for MoO3 interlayer inserted in CBP is only ≈1 nm. The resulting devices with periodic two insertion layers of MoO3 into CBP exhibit better performance compared with the CBP-only ones, such that the peak current efficiency is 88.7 cd A-1 corresponding to the external quantum efficiency of 20.6%. Furthermore, the resulting QLEDs show an operational lifetime almost 2.5 times longer compared to CBP-only devices.

High-Power GaN-Based Vertical Light-Emitting Diodes on 4-Inch Silicon Substrate

We demonstrate high-power GaN-based vertical light-emitting diodes (LEDs) (VLEDs) on a 4-inch silicon substrate and flip-chip LEDs on a sapphire substrate. The GaN-based VLEDs were transferred onto the silicon substrate by using the Au-In eutectic bonding technique in combination with the laser lift-off (LLO) process. The silicon substrate with high thermal conductivity can provide a satisfactory path for heat dissipation of VLEDs. The nitrogen polar n-GaN surface was textured by KOH solution, which not only improved light extract efficiency (LEE) but also broke down Fabry-Pérot interference in VLEDs. As a result, a near Lambertian emission pattern was obtained in a VLED. To improve current spreading, the ring-shaped n-electrode was uniformly distributed over the entire VLED. Our combined numerical and experimental results revealed that the VLED exhibited superior heat dissipation and current spreading performance over a flip-chip LED (FCLED). As a result, under 350 mA injection current, the forward voltage of the VLED was 0.36 V lower than that of the FCLED, while the light output power (LOP) of the VLED was 3.7% higher than that of the FCLED. The LOP of the FCLED saturated at 1280 mA, but the light output saturation did not appear in the VLED.

Improvement in Light Output of Ultraviolet Light-Emitting Diodes with Patterned Double-Layer ITO by Laser Direct Writing

A patterned double-layer indium-tin oxide (ITO), including the first unpatterned ITO layer serving as current spreading and the second patterned ITO layer serving as light extracting, was applied to obtain uniform current spreading and high light extraction efficiency (LEE) of GaN-based ultraviolet (UV) light-emitting diodes (LEDs). Periodic pinhole patterns were formed on the second ITO layer by laser direct writing to increase the LEE of UV LED. Effects of interval of pinhole patterns on optical and electrical properties of UV LED with patterned double-layer ITO were studied by numerical simulations and experimental investigations. Due to scattering out of waveguided light trapped inside the GaN film, LEE of UV LED with patterned double-layer ITO was improved as compared to UV LED with planar double-layer ITO. As interval of pinhole patterns decreased, the light output power (LOP) of UV LED with patterned double-layer ITO increased. In addition, UV LED with patterned double-layer ITO exhibited a slight degradation of current spreading as compared to the UV LED with a planar double-layer ITO. The forward voltage of UV LED with patterned double-layer ITO increased as the interval of pinhole patterns decreased.

Microtube Light-Emitting Diode Arrays with Metal Cores

We report the fabrication and characteristics of vertical microtube light-emitting diode (LED) arrays with a metal core inside the devices. To make the LEDs, gallium nitride (GaN)/indium gallium nitride (In(x)Ga(1-x)N)/zinc oxide (ZnO) coaxial microtube LED arrays were grown on an n-GaN/c-aluminum oxide (Al2O3) substrate. The microtube LED arrays were then lifted-off the substrate by wet chemical etching of the sacrificial ZnO microtubes and the silicon dioxide (SiO2) layer. The chemically lifted-off LED layer was then transferred upside-down on other supporting substrates. To create the metal cores, titanium/gold and indium tin oxide were deposited on the inner shells of the microtubes, forming n-type electrodes inside the metal-cored LEDs. The characteristics of the resulting devices were determined by measuring electroluminescence and current-voltage characteristic curves. To gain insights into the current-spreading characteristics of the devices and understand how to make them more efficient, we modeled them computationally.

Hybrid Tunnel Junction-Graphene Transparent Conductive Electrodes for Nitride Lateral Light Emitting Diodes

Graphene transparent conductive electrode (TCE) applications in nitride light emitting diodes (LEDs) are still limited by the large contact resistance and interface barrier between graphene and p-GaN. We propose a hybrid tunnel junction (TJ)-graphene TCE approach for nitride lateral LEDs theoretically and experimentally. Through simulation using commercial advanced physical models of semiconductor devices (APSYS), we found that low tunnel resistance can be achieved in the n(+)-GaN/u-InGaN/p(+)-GaN TJ, which has a lower tunneling barrier and an enhanced electric field due to the polarization effect. Graphene TCEs and hybrid graphene-TJ TCEs are then modeled. The designed hybrid TJ-graphene TCEs show sufficient current diffusion length (Ls), low introduced series resistance, and high transmittance. The assembled TJ LED with the triple-layer graphene (TLG) TCEs show comparable optoelectrical performance (3.99 V@20 mA, LOP = 10.8 mW) with the reference LED with ITO TCEs (3.36 V@20 mA, LOP = 12.6 mW). The experimental results further prove that the TJ-graphene structure can be successfully incorporated as TCEs for lateral nitride LEDs.