Sheng-Fu Yu

Inserting a p-InGaN layer before the p-AlGaN electron blocking layer suppresses efficiency droop in InGaN-based light-emitting diodes

In this study, we observed a dramatic decrease in the efficiency droop of InGaN/GaN light-emitting diodes after positioning a p-InGaN insertion layer before the p-AlGaN electron-blocking layer. The saturated external quantum efficiency of this device extended to 316 mA, with an efficiency droop of only 7% upon increasing the operating current to 1 A; in contrast, the corresponding conventional light-emitting diode suffered a severe efficiency droop of 42%. We suspect that the asymmetric carrier distribution was effectively mitigated as a result of an improvement in the hole injection rate and a suppression of electron overflow.

Enhanced Extraction and Efficiency of Blue Light-Emitting Diodes Prepared Using Two-Step-Etched Patterned Sapphire Substrates

Using a hot acid wet etching method, we have fabricated two types of patterned sapphire substrates: A pyramidal patterned sapphire substrate (PPSS) and a flat-top patterned sapphire substrate (FTPSS). After placing these samples into an atmospheric pressure metallorganic chemical vapor deposition system, we deposited standard InGaN light-emitting diode (LED) structures onto their surfaces. The crystal quality of these two surfaces was enhanced, as evidenced using X-ray diffraction (the full width at half-maximum decreased from 406.8 arcsec for the conventional sapphire to 356.4 and 349.2 arcsec for the PPSS and FTPSS samples, respectively). The output power of InGaN-based blue LEDs incorporating the PPSS and FTPSS improved to 17.9 and 18.7%, respectively, at 20 mA.

Ultralow threading dislocation density in GaN epilayer on near-strain-free GaN compliant buffer layer and its applications in hetero-epitaxial LEDs.

High threading dislocation (TD) density in GaN-based devices is a long unresolved problem because of the large lattice mismatch between GaN and the substrate, which causes a major obstacle for the further improvement of next-generation high-efficiency solid-state lighting and high-power electronics. Here, we report InGaN/GaN LEDs with ultralow TD density and improved efficiency on a sapphire substrate, on which a near strain-free GaN compliant buffer layer was grown by remote plasma atomic layer deposition. This “compliant” buffer layer is capable of relaxing strain due to the absorption of misfit dislocations in a region within ~10 nm from the interface, leading to a high-quality overlying GaN epilayer with an unusual TD density as low as 2.2 × 105 cm−2. In addition, this GaN compliant buffer layer exhibits excellent uniformity up to a 6” wafer, revealing a promising means to realize large-area GaN hetero-epitaxy for efficient LEDs and high-power transistors.

Characteristics of InGaN-based light-emitting diodes on patterned sapphire substrates with various pattern heights

The optical and electrical characteristics of InGaN-based blue light-emitting diodes (LEDs) grown on patterned sapphire substrates (PSSs) with different pattern heights and on planar sapphire by atmospheric-pressure metal-organic chemical vapor deposition were investigated. Compared with planar sapphire, it was found that the LED electroluminescence intensity is significantly enhanced on PSSs with pattern heights of 0.5 (21%), 1.1 (57%), 1.5 (81%), and 1.9 (91%) µm at an injected current of 20 mA. The increased light intensity exhibits the same trend in a TracePro simulation. In addition, it was also found that the level of leakage current depends on the density of V-shape defects, which were measured by scanning electron microscopy.

Efficiency Droop Characteristics in InGaN-Based Near Ultraviolet-to-Blue Light-Emitting Diodes

In this study, we prepared InGaN-based light-emitting diodes (LEDs) with peak emissions ranging from 400 to 445 nm and investigated their efficiency droop characteristics at injection currents of up to 1 A. We found that the external quantum efficiencies (EQEs) changed dramatically when the critical current increased from 0 to 350 mA, but exhibited a similar negative slope upon increasing the current from 350 mA to 1 A. The effects of piezoelectric polarization and different localized states in the active layer of the near UV-to-blue LEDs influenced the peak EQEs and the dramatic decays of the EQE droops at lower injection currents.

InGaN-Based Light-Emitting Diodes With an AlGaN Staircase Electron Blocking Layer

In this letter, we investigate the external quantum efficiency (EQE) and efficiency droop characteristics of InGaN-based light-emitting diodes (LEDs) incorporating AlxGa1-xN staircase electron blocking layers (EBLs). The LED featuring a composition-stepped EBL (x: 0.21, 0.14, and 0.07) exhibited the highest EQE under low current levels, but a severe efficiency droop at high current levels. In contrast, the LED with a composition-stepped EBL (x: 0.07, 0.14, and 0.21) exhibited a significant improvement in its onset of droop and a mitigated efficiency droop; we ascribe these features to the increased hole injection rate and the decreased built-in electric field.

Ultraviolet Electroluminescence from Nitrogen-Doped ZnO-Based Heterojuntion Light-Emitting Diodes Prepared by Remote Plasma in situ Atomic Layer-Doping Technique

Remote plasma in situ atomic layer doping technique was applied to prepare an n-type nitrogen-doped ZnO (n-ZnO:N) layer upon p-type magnesium-doped GaN (p-GaN:Mg) to fabricate the n-ZnO:N/p-GaN:Mg heterojuntion light-emitting diodes. The room-temperature electroluminescence exhibits a dominant ultraviolet peak at λ ≈ 370 nm from ZnO band-edge emission and suppressed luminescence from GaN, as a result of the decrease in electron concentration in ZnO and reduced electron injection from n-ZnO:N to p-GaN:Mg because of the nitrogen incorporation. The result indicates that the in situ atomic layer doping technique is an effective approach to tailoring the electrical properties of materials in device applications.

Using Pre-TMIn Treatment to Improve the Optical Properties of Green Light Emitting Diodes

We investigated the effects of pre-TMIn treatment on the optical properties of green light emitting diodes (LEDs). Although pre-TMIn treatment did not affect the epitaxial structure of quantum wells, it significantly improved the quality of the surface morphology relative to that of the untreated sample. Indium cluster can be seen by high-resolution transmission electron microscopy (HR-TEM), which is the explanation for the red-shift of photoluminescence (PL). Time-resolved photoluminescence measurements indicated that the sample prepared with pre-TMIn treatment had a shorter radiative decay time. As a result, the light output power of the treated green LED was higher than that of the conventional untreated one. Thus, pre-TMIn treatment appears to be a simple and efficient means of improving the performance of green LEDs.

Optimizing the multiple quantum well thickness of an InGaN blue light emitting diode

InGaN/GaN blue light emitting diodes with varied quantum well thickness from 2.4 nm to 3.6 nm are fabricated and characterized by atmosphere pressure metalorganic chemical vapor deposition (AP-MOCVD). Experimental results show that the exciton localization effect is enhanced from 21.76 to 23.48 by increasing the quantum well thickness from 2.4 nm to 2.7 nm. However, with the further increase of quantum well thickness, the exciton localization effect becomes weaker. Meanwhile, the peak wavelength of electroluminescence redshift with the increase of well thickness due to the larger quantum confined Stark effect (QCSE). In addition, the efficiency droop can be improved by increasing the well thickness.

The comprehensive characteristics of quaternary AlInGaN with various TMI molar rate

We demonstrated very thick (~400 nm) AlInGaN quaternary alloy grown on GaN epilayer by MOCVD. The Optical, electronic, crystalline quality and surface morphology of AlInGaN/GaN hetero structures with various TMI molar flow rate were be extensively discussed. With Al0.89In0.02GaN/GaN fully lattice matched structure, less and small V-defect pits and good crystal quality comparing with the other lattice mismatched AlInGaN/GaN hetero-structures were discovered. Finally, the difference AlInGaN quaternary epilayers could be directly applied to high power LED structure in the future by the same barrier growth conditions.