Gou-Chung Chi

Improvement of emission uniformity by using micro-cone patterned PDMS film

Micro-patterned PDMS film was fabricated and combined with LED chip on board (COB) package to improve the emission uniformity of LED chip. The micro scale patterned sapphire substrate (PSS) was used as a mold to fabricate micro-cone patterned PDMS (MC-PDMS) film. A strong scattering effect from this MC-PDMS film can be verified by the high haze ratio and the Bi-directional Transmission effect. The angle dependent color temperature measurement system was used to measure the ΔCCT of COB with and without MC-PDMS. The measurement results indicate that the ΔCCT was reduced from 1025K to 428K. This improvement can effectively eliminate the yellow ring effect of LED chip. This technology can be thus considered as a cost-effective way for the next generation of light source packages.

Efficiency and droop improvement in green InGaN/GaN light-emitting diodes on GaN nanorods template with SiO2 nanomasks

This study presents the green InGaN/GaN multiple quantum wells light-emitting diodes (LEDs) grown on a GaN nanorods template with SiO2 nanomasks by metal–organic chemical vapor deposition. By nanoscale epitaxial lateral overgrowth, microscale air voids were formed between nanorods and the threading dislocations were efficiently suppressed. The electroluminescence measurement reveals that the LEDs on nanorods template with SiO2 nanomasks suffer less quantum-confined Stark effect and exhibit higher light output power and lower efficiency droop at a high injection current as compared with conventional LEDs.

Efficiency Improvement of GaN-Based Light-Emitting Diode Prepared on GaN Nanorod Template

We demonstrated the formation of GaN-based nanorod (NR) structure by using self-assemble Ni nanoislands as the etching mask. It was found that crystal quality of the GaN epilayer prepared on an NR GaN template was significantly better than that prepared with a conventional low-temperature GaN nucleation layer. With the NR GaN template, it was found that 20-mA light-emitting diode (LED) output power can be enhanced by 39.8%, as compared to the conventional LED.

Efficiency improvement of GaN-based ultraviolet light-emitting diodes with reactive plasma deposited AlN nucleation layer on patterned sapphire substrate

The flip chip ultraviolet light-emitting diodes (FC UV-LEDs) with a wavelength of 365xa0nm are developed with the ex situ reactive plasma deposited (RPD) AlN nucleation layer on patterned sapphire substrate (PSS) by an atmospheric pressure metal-organic chemical vapor deposition (AP MOCVD). The ex situ RPD AlN nucleation layer can significantly reduce dislocation density and thus improve the crystal quality of the GaN epitaxial layers. Utilizing high-resolution X-ray diffraction, the full width at half maximum of the rocking curve shows that the crystalline quality of the epitaxial layer with the (RPD) AlN nucleation layer is better than that with the low-temperature GaN (LT-GaN) nucleation layer. The threading dislocation density (TDD) is estimated by transmission electron microscopy (TEM), which shows the reduction from 6.8u2009×u2009107xa0cm−2 to 2.6u2009×u2009107xa0cm−2. Furthermore, the light output power (LOP) of the LEDs with the RPD AlN nucleation layer has been improved up to 30xa0% at a forward current of 350xa0mA compared to that of the LEDs grown on PSS with conventional LT-GaN nucleation layer.

Enhanced Light Output Power and Growth Mechanism of GaN-Based Light-Emitting Diodes Grown on Cone-Shaped

In this study, we successfully transferred the patterns of a cone-shaped patterned sapphire substrate (CPSS) into SiO2 layer to fabricate a cone-shaped SiO2 patterned template by using nanoimprint lithography (NIL). The GaN-based light-emitting diodes (LEDs) were grown on this template by metal-organic chemical vapor deposition (MOCVD). The transmission electron microscopy (TEM) images suggest that the stacking faults formed near the cone-shaped SiO2 patterns during the epitaxial lateral overgrowth (ELOG) can effectively suppress the threading dislocations, which results in an enhancement of internal quantum efficiency. The Monte Carlo ray-tracing simulation reveals that the light extraction efficiency of the LED grown on cone-shaped SiO2 patterned template can be enhanced as compared with the LED grown on CPSS. As a result, the light output power of the LED grown on cone-shaped SiO2 patterned template outperformed the LED grown on CPSS.

{\hbox{SiO}}_{2}

In this paper, we demonstrated the high performance GaN-based LEDs by using a high aspect ratio cone-shape nano-patterned sapphire substrate (HAR-NPSS). We utilized nano-imprint lithography (NIL) and dry-etching system to fabricate a high depth HAR-NPSS. The micro-scale patterned sapphire substrate (PSS) was also used for comparison. A great enhancement of light output was observed when GaN-based LEDs were grown on a HAR-NPSS or a PSS. The light output power of LEDs with a HAR-NPSS and LEDs with a PSS were enhanced of 49 and 38% compared to LEDs with a unpatterned sapphire substrate. The high output power of the LED with a HAR-NPSS indicated that the technology of NAR-NPSS not only can improve the crystalline quality of GaN-based LEDs but also a promising development to a NPSS.

Patterned Template

X-ray absorption near-edge structure (XANES), extended x-ray absorption fine structures (EXAFS), and photoluminescence measurements were used to elucidate the microstructural and photoluminescence properties of ZnO nanowires (ZnO-NWs) that had been treated with Eu by thermal diffusion. The Ou2009K- and Euu2009L3-edge XANES and EXAFS spectra at the Znu2009K- and Euu2009L3-edge verified the formation of Eu2O3-like layer on the surface of ZnO-NWs. X-ray diffraction, XANES and EXAFS measurements consistently suggest the lack of substitutional doping of Eu ions at the Zn ion sites in the interior of ZnO-NWs. The clear sharp and intense emission bands in the range 610–630 nm of Eu-treated ZnO-NWs originated from the intra-4f transition of Eu ions in the Eu2O3-like surface layer.

Investigation and Comparison of the GaN-Based Light-Emitting Diodes Grown on High Aspect Ratio Nano-Cone and General Micro-Cone Patterned Sapphire Substrate

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.

Determination of the microstructure of Eu-treated ZnO nanowires by x-ray absorption

Large-area graphene of high quality and uniformity was successfully grown by chemical vapor deposition (CVD) using surface oxidation treatment of copper foil prior to the graphene growth step. The graphene was transferred to the polyethylene terephthalate (PET) substrate (G/PET) to act as a transparent front electrode of hybrid heterojunction photovoltaic (PV) cells; these cells were based on a structural motif of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) as the p-type semiconductor, GaAs (1 0 0) as the inorganic n-type semiconductor, and a silver comb electrode atop of PEDOT:PSS. By using G/PET as a transparent front electrode, the power conversion efficiency (PCE), under simulated AM1.5G illumination conditions, was greatly enhanced by up to 26% (from 6.85% to 8.60%). All PV characteristics, including open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF), contributed to this PCE enhancement. The reflectance, external quantum efficiency, and dark current were investigated to explain this observed PCE enhancement. Although two layers of graphene can efficiently reduce the sheet resistance, the reduction of transmittance in multilayer cells resulted in lower short-circuit current density, leading to lower PCE, in comparison with those with only one layer of graphene.

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 work, we investigate blade-coated organic interlayers at the rear surface of hybrid organic-silicon photovoltaics based on two small molecules: Tris(8-hydroxyquinolinato) aluminium (Alq(3)) and 1,3-bis(2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl) benzene (OXD-7). In particular, soluble Alq(3) resulting in a uniform thin film with a root-mean-square roughness < 0.2nm is demonstrated for the first time. Both devices with the Alq(3) and OXD-7 interlayers show notable enhancement in the open-circuit voltage and fill-factor, leading to a net efficiency increase by over 2% from the reference, up to 11.8% and 12.5% respectively. The capacitance-voltage characteristics confirm the role of the small-molecule interlayers resembling a thin interfacial oxide layer for the Al-Si Schottky barrier to enhance the built-in potential and facilitate charge transport. Moreover, the Alq(3) interlayer in optimized devices exhibits isolated phases with a large surface roughness, in contrast to the OXD-7 which forms a continuous uniform thin film. The distinct morphological differences between the two interlayers further suggest different enhancement mechanisms and hence offer versatile functionalities to the advent of hybrid organic-silicon photovoltaics.