Shih-Cheng Huang

Defect reduction and efficiency improvement of near-ultraviolet emitters via laterally overgrown GaN on a GaN/patterned sapphire template

An approach to improve the defect density and internal quantum efficiency of near-ultraviolet emitters was proposed using a combination of epitaxial lateral overgrowth (ELOG) and patterned sapphire substrate (PSS) techniques. Especially, a complementary dot array pattern corresponding to the underlying PSS was used for the ELOG-SiO2 mask design. Based on the transmission-electron-microscopy and etch-pit-density results, the ELOG∕SiO2∕GaN∕PSS structure can reduce the defect density to a level of 105cm−2. The internal quantum efficiency of the InGaN-based ELOG-PSS light-emitting diode (LED) sample showed three times in magnitude as compared with that of the conventional GaN/sapphire one. Under a 20mA injection current, the output powers of ELOG-PSS, PSS, and conventional LED samples were measured to be 3.3, 2.9, and 2.5mW, respectively. The enhanced output power could be due to a combination of the reduction in dislocation density (by ELOG) and improved light extraction efficiency (by PSS). Unlike the previ...

Fabrication of Pyramidal Patterned Sapphire Substrates for High-Efficiency InGaN-Based Light Emitting Diodes

In this study, a wet-etched pyramidal patterned sapphire substrate (PSS) was used to fabricate the near-ultraviolet InGaN-based light-emitting diodes (LEDs). The pyramidal PSS was etched using a 3H 2 SO 4 :1H 3 PO 4 mixture solution and the activation energy of this reaction is determined to be 28.2 kcal/mol. Three symmetric sidewall facets of the etched pyramidal hole were {112k} on the (0001) sapphire. It was found that the GaN epi layer grew laterally from the top of the pyramid pit and overhung the cavity. An evident reduction in dislocation density of the GaN-on-PSS sample can be confirmed by the etch-pit-density, double-crystal X-ray, and micro photoluminescence measurement results. Under a 20 mA forward injection current, the output power of the conventional and pyramidal PSS LEDs (in epoxy lamp form, λ D = 400 nm) were 7.45 and 9.35 mW, respectively. A 25% enhancement in output power was achieved in the pyramidal PSS LED as compared with that of the conventional LED sample. The enhanced output power is not only due to the improvement of the internal quantum efficiency upon decreasing the dislocation density, but also due to the enhancement of the extraction efficiency using a pyramidal PSS. From light-tracing calculation, the pyramidal reflector arrays can offer more probability of escaping photons from the GaN/sapphire interface, resulting in an increase in light extracting efficiency.

Investigation of efficiency droop for InGaN-based UV light-emitting diodes with InAlGaN barrier

The efficiency droop in InGaN-based UV light emitting device (LED) with AlGaN and InAlGaN barrier is investigated. Electroluminescence results indicate that the light performance of quaternary LEDs can be enhanced by 25% and 55% at 350 mA and 1000 mA, respectively. Furthermore, simulations show that quaternary LEDs exhibit 62% higher radiative recombination rate and low efficiency degradation of 13% at a high injection current. We attribute this improvement to increasing of carrier concentration and uniform redistribution of carriers.

High efficiency light emitting diode with anisotropically etched GaN-sapphire interface

We report the fabrication and study of high efficiency ultraviolet light emitting diodes with inverted micropyramid structures at GaN-sapphire interface. The micropyramid structures were created by anisotropic chemical wet etching. The pyramid structures have significantly enhanced the light output efficiency and at the same time also improved the crystal quality by partially relieving the strain and reducing the dislocation defects in GaN. The electroluminescent output power at normal direction was enhanced by 120% at 20 mA injection current and the output power integrated over all directions was enhanced by 85% compared to a reference sample.

Enhanced Output Power of Near-Ultraviolet InGaN/AlGaN LEDs With Patterned Distributed Bragg Reflectors

A 400-nm near-ultraviolet InGaN/AlGaN light-emitting diode (LED) with a patterned distributed Bragg reflector (PDBR) mask is the subject of this paper. The design of the PDBR mask on the GaN/sapphire substrate attempts to reduce the threading dislocation density in the epitaxial template and enhance light extraction efficiency via the reflective behavior of the DBR. Under an injection current of 20 mA, the forward voltages of the PDBR and conventional LEDs were 3.51 and 3.52 V, respectively. This result indicates that the operating voltage of the PDBR LED does not arise by this PDBR mask design. In addition, the leakage current of the PDBR LED sample (1.36 nA at -5 V) is found to be lower than that of the conventional LED (11 nA). We also discovered that the light output power for the PDBR LED was approximately 39% higher (at 20 mA) than the conventional LED, and this significant improvement in performance is attributed not only to the GaN template crystalline quality reform but also due to the light extraction enhancement via the PDBR mask.

Study of 375 nm ultraviolet InGaN/AlGaN light-emitting diodes with heavily Si-doped GaN transition layer in growth mode, internal quantum efficiency, and device performance

High performance 375 nm ultraviolet (UV) InGaN/AlGaN light-emitting diodes (LEDs) were demonstrated with inserting a heavy Si-doped GaN transition layer by metal-organic chemical vapor deposition. From transmission electron microcopy (TEM) image, the dislocation densities were significantly reduced due to the existence of the heavily Si-doping growth mode transition layer (GMTL), which results in residual stress relaxation and 3D growth. The internal quantum efficiency (IQE) of the LEDs with GMTL was measured by power-dependent photoluminescence (PL) to be 40.6% higher than ones without GMTL. The GMTL leads to the superior IQE performance of LEDs not only in decreasing carrier consumption at nonradiative recombination centers but also in partially mitigating the efficiency droop tendency. When the vertical-type LED chips (size: 1 mm × 1 mm) was driven with a 350 mA injection current, the output powers of the LEDs with and without GMTL were measured to be 286.7 and 204.2 mW, respectively. A 40.4% enhanceme...

Power Enhancement of 410-nm InGaN-Based Light-Emitting Diodes on Selectively Etched GaN/Sapphire Templates

A selectively etched (SE) GaN template for high-power 410-nm InGaN-based LEDs was fabricated, where 2- μm-thick undoped GaN was grown on recess patterned sapphire substrates (PSSs). This was followed by H3PO4 selective etching, 0.5-μm-thick SiO2 film deposition, and a final chemical-mechanical polishing process to remove the excess SiO2. Three kinds of substrates, i.e., conventional sapphire substrates (CSSs), recess PSSs, and SE GaN templates, were used to grow the near-UV LEDs for comparison. The etched pit density of n-type GaN could then be reduced to 105 cm-2, whereas the number of screw-type and edge-type threading dislocations decreased by 16.5% and 49.9%, respectively, since SiO2 fillings on the SE GaN template hindered their propagation. The output power of a near-UV LED fabricated on the SE GaN template was 13% and 46% higher than that of a near-UV LED fabricated on recess PSSs and CSSs, respectively. A substantial improvement in performance can be attributed to the improved epilayer crystallinity and, also, the increased light scattering within the LED induced by SiO2 fillings.

Improved Output Power of InGaN-Based Ultraviolet LEDs Using a Heavily Si-Doped GaN Insertion Layer Technique

In this paper, a high quality ultraviolet light-emitting diodes (UV-LEDs) at 375 nm was developed using a heavy Si-doping technique with metal organic chemical vapor deposition. By using high-resolution X-ray diffraction, the full width at half-maximum of the rocking curve shows that the GaN film inserting a heavily Si-doped GaN layer (Si-HDL) had high crystalline quality. From the transmission electron microscopy image, the threading dislocation density was decreased after inserting a Si-HDL between undoped and n-doped GaN layers by nanoscale epitaxial lateral overgrowth. As a result, a much smaller reverse current and a higher light output were achieved. The improvement of light output at an injection current of 20 mA was enhanced by 40%. Therefore, we can use an in-situ nano pattern without complex photolithography and etching process and improve the internal quantum efficiency of UV-LEDs.

Demonstration of InGaN Light-Emitting Diodes by Incorporating a Self-Textured Oxide Mask Structure

A 460-nm InGaN light-emitting diode (LED) with a self-textured oxide mask (STOM) array structure upon an additional low-temperature GaN interlayer is demonstrated. As compared with a conventional LED, the electroluminescence (EL) spectrum of an STOM-LED displays a manifest peak profile without Fabry-Pérot interference fringes, one-fold increment in EL intensity, and 43% enhancement in total output power at an injection current of 20 mA. High-density light-emitting dots across the STOM-LED surface are discovered at a small injection current of 20 μA because the STOM array can act as scattering centers to enhance the light extraction.

Improved Output Power of 380 nm InGaN-Based LEDs Using a Heavily Mg-Doped GaN Insertion Layer Technique

High-performance InGaN-based 380 nm UV LEDs are fabricated by using a heavily Mg-doped GaN insertion layer (HD-IL) technique. Based on the transmission electron microscopy, etch pit density, and cathodoluminescence results, the HD-IL technique can substantially reduce the defect density of GaN layer. The double-crystal X-ray diffraction results are in good agreement with those observations. The internal quantum efficiency of LED sample with an HD-IL shows around 40% improvement compared with the LED sample without the use of HD-IL. When the vertical-type LED chips (size: 1 mm times 1 mm) are driven by a 350 mA current, the output powers of the LEDs with and without an HD-IL are measured to be 203.4 and 158.9 mW, respectively. As much as 28% increased light output power is achieved.