Kun-Ching Shen

High performance of Ga-doped ZnO transparent conductive layers using MOCVD for GaN LED applications

High performance of Ga-doped ZnO (GZO) prepared using metalorganic chemical vapor deposition (MOCVD) was employed in GaN blue light-emitting diodes (LEDs) as transparent conductive layers (TCL). By the post-annealing process, the annealed 800°C GZO films exhibited a high transparency above 97% at wavelength of 450 nm. The contact resistance of GZO decreased with the annealing temperature increasing. It was attributed to the improvement of the GZO crystal quality, leading to an increase in electron concentration. It was also found that some Zn atom caused from the decomposition process diffused into the p-GaN surface of LED, which generated a stronger tunneling effect at the GZO/p-GaN interface and promoted the formation of ohmic contact. Moreover, contrast to the ITO-LED, a high light extraction efficiency of 77% was achieved in the GZO-LED at injection current of 20 mA. At 350 mA injection current, the output power of 256.51 mW of GZO-LEDs, corresponding to a 21.5% enhancement as compared to ITO-LEDs was obtained; results are promising for the development of GZO using the MOCVD technique for GaN LED applications.

An 83% enhancement in the external quantum efficiency of ultraviolet flip-chip light-emitting diodes with the incorporation of a self-textured oxide mask

We demonstrate here a 380-nm ultraviolet InGaN flip-chip (FC) light-emitting diode (LED) with self-textured oxide mask (STOM-FCLED) structures fabricated in a large-area (1125 × 1125 μm2) FC configuration. An 83% enhancement in the external quantum efficiency was achieved for the STOMFCLEDs when compared with FCLEDs without the STOM structure operating at an injection current of 350 mA. For STOM-FCLEDs operating at an injection current of 1000 mA, a light output of approximately 400 mW was obtained. These results could be attributed to the introduction of the STOM structure, which not only reduces the density of threading dislocation but also intensifies the LED light extraction.

Pulsed laser deposition of hexagonal GaN-on-Si(100) template for MOCVD applications

Growth of hexagonal GaN on Si(100) templates via pulsed laser deposition (PLD) was investigated for the further development of GaN-on-Si technology. The evolution of the GaN growth mechanism at various growth times was monitored by SEM and TEM, which indicated that the GaN growth mode changes gradually from island growth to layer growth as the growth time increases up to 2 hours. Moreover, the high-temperature operation (1000 °C) of the PLD meant no significant GaN meltback occurred on the GaN template surface. The completed GaN templates were subjected to MOCVD treatment to regrow a GaN layer. The results of X-ray diffraction analysis and photoluminescence measurements show not only the reliability of the GaN template, but also the promise of the PLD technique for the development of GaN-on-Si technology.

GaN light emitting diodes with wing-type imbedded contacts

A wing-type imbedded electrodes was introduced into the lateral light emitting diode configuration (WTIE-LEDs) to reduce the effect of light shading of electrode in conventional sapphire-based LEDs (CSB-LEDs). The WTIE-LEDs with double-side roughened surface structures not only can eliminate the light shading of electrode and bonding wire, but also increase the light extraction and light output power. Contrast to CSB-LEDs, a 79% enhancement of output intensity in the WTIE-LED was obtained at 100 mA injection current. Similarly, the output power of packaged WTIE-LEDs was enhanced 59% higher compared with the packaged CSB-LEDs at the same injection condition. Therefore, using the imbedded contact to reduce light shading would be a promising prospective for LEDs to achieve high output power.

Thin Film GaN LEDs Using a Patterned Oxide Sacrificial Layer by Chemical Lift-Off Process

A high-quality GaN-based vertical light-emitting diode (LED) was successfully fabricated and transferred to an electroplated Cu substrate using strip-patterned silicon dioxide (SiO2) as a sacrificial layer in a chemical lift-off (CLO) process. The SiO2 strip patterns not only provide the sacrificial structure during the detachment process, but also improve the quality of GaN epilayers through epitaxial lateral overgrowth. Compared with conventional LEDs, the CLO-LEDs have a higher output power and a lower forward voltage. The CLO-LED has a decrease in forward voltage of 0.42 V (at 20 mA) as compared with the conventional LED. In addition, at a drive current of 350 mA, the output power of CLO-LEDs is enhanced ~ 2.2 fold, compared with that of conventional LEDs.

High thermal stability of high indium content InGaN films grown by pulsed laser deposition

Thermal stability on the structural and optical properties of high indium content InGaN films grown using pulsed laser deposition (PLD) was investigated through long-duration and high-temperature annealing. X-ray diffraction and cathode- luminescence measurements of the 33% indium InGaN revealed no differences in the line-shape and peak position even after annealing at 800°C for 95 min; similar structural stability was found for the 60% samples after annealing for 75 min. The higher thermal stability is attributed to nanoscale InN domains with different orientations create mixed-polarity InGaN/InN interfaces, resulting in higher activation energies at interfaces and increasing the thermal stability of the material. Furthermore, the InGaN films were subjected to metalorganic chemical vapor deposition treatment to regrow a GaN layer; results are promising for the development of high thermal stability InGaN films using the PLD technique.

High-Efficiency and Crack-Free InGaN-Based LEDs on a 6-inch Si (111) Substrate With a Composite Buffer Layer Structure and Quaternary Superlattices Electron-Blocking Layers

In this paper, a composite buffer layer structure (CBLS) with multiple AlGaN layers and grading of Al composition/u-GaN1/(AlN/GaN) superlattices/u-GaN2 and InAlGaN/AlGaN quaternary superlattices electron-blocking layers (QSLs-EBLs) are introduced into the epitaxial growth of InGaN-based light-emitting diodes (LEDs) on 6-inch Si (111) substrates to suppress cracking and improve the crystalline quality and emission efficiency. The effect of CBLS and QSLs-EBL on the crystalline quality and emission efficiency of InGaN-based LEDs on Si substrates was studied in detail. Optical microscopic images revealed the absence of cracks and Ga melt-back etching. The atomic force microscopy images exhibited that the root-mean-square value of the surface morphology was only 0.82 nm. The full widths at half maxima of the (0002) and (101̅2) reflections in the double crystal X-ray rocking curve were ~330 and 450 respectively. The total threading dislocation density, revealed by transmission electron microscopy, was <; 6× 108 cm-2. From the material characterizations, described above, blue and white LEDs emitters were fabricated using the epiwafers of InGaN-based LEDs on 6-inch Si substrates. The blue LEDs emitter that comprised blue LEDs chip and clear lenses had an emission power of 490 mW at 350 mA, a wall-plug efficiency of 45% at 350 mA, and an efficiency droop of 80%. The white LEDs emitter that comprised blue LEDs chip and yellow phosphor had an emission efficiency of ~110 lm/W at 350 mA and an efficiency droop of 78%. These results imply that the use of a CBLS and QSLs-EBL was found to be very simple and effective in fabricating high-efficiency InGaN-based LEDs on Si for solid-state lighting applications.

High indium content InGaN films grown by pulsed laser deposition using a dual-compositing target

High indium compositions InGaN films were grown on sapphires using low temperature pulse laser deposition (PLD) with a dual-compositing target. This target was used to overcome the obstacle in the InGaN growth by PLD due to the difficulty of target preparation, and provided a co-deposition reaction, where InGaN grains generated from the indium and GaN vapors deposit on sapphire surface and then act as nucleation seeds to promote further InGaN growth. The effects of co-deposition on growth mechanisms, surface morphology, and electrical properties of films were thoroughly investigated and the results clearly show promise for the development of high indium InGaN films using PLD technique with dual-compositing targets.

ZnO Nanowires Embedded in Epoxy Resin Separating from the Substrate for Wearable Electronics Applications

A novel lift-off method to obtain ZnO nanowires embedded in flexible films for wearable electronics application has been fabricated in this study. Traditional peel-off method using polydimethylsiloxane (PDMS) polymer to transfer ZnO nanowires to flexible films, however, it remains some ZnO nanowires on the original substrate. The novel lift-off strategy will overcome the previous situation and is a candidate for wearable electronics application. For finger bending application, the maximum open-circuit voltage and closed-circuit current density can be obtained about 115 mV and 59 nA/cm2, respectively. Moreover, the lift-off fabrication is a promising method for Si substrate recycling usage and for large area fabrication.

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...