Jian-Jhong Chen

The aspect ratio effects on the performances of GaN-based light-emitting diodes with nanopatterned sapphire substrates

The nanopatterned sapphire substrates (NPSSs) with aspect ratio that varied from 2.00 to 2.50 were fabricated by nanoimprint lithography. We could improve the epitaxial film quality and enhance the light extraction efficiency by NPSS technique. In this work, the aspect ratio effects on the performances of GaN-based light-emitting diodes (LEDs) with NPSS were investigated. The light output enhancement of GaN-based LEDs with NPSS was increased from 11% to 27% as the aspect ratio of the NPSS increases from 2.00 to 2.50. Owing to the same improvement of crystalline quality by using various aspect ratios of NPSS, these results indicated that the aspect ratio of the NPSS is strongly related to the light extraction efficiency.

GaN-Based Light-Emitting Diodes Grown on Photonic Crystal-Patterned Sapphire Substrates by Nanosphere Lithography

GaN-based light-emitting diodes (LEDs) grown on photonic crystal-patterned sapphire substrates (PCPSS) have been demonstrated. PCPSS was fabricated by nanosphere lithography, and the photonic crystal structure was the hexagonal-lattice pattern. The forward voltages of PCPSS and patterned sapphire substrates (PSS) LEDs were smaller than that of conventional sapphire substrates (CSS) LED, and it infers the epitaxial film quality of PCPSS and PSS LEDs has been slightly improved. The luminance intensity of PCPSS LED was 1.63 and 1.51 times higher than those of CSS and PSS LED at 20 mA injection current. The enhancement in the luminance intensity of PCPSS LED is attributed to the photonic crystal structure.

Localized Surface Plasmon-Enhanced Nitride-Based Light-Emitting Diode With Ag Nanotriangle Array by Nanosphere Lithography

We describe a method to enhance the light output power of nitride-based light-emitting diodes (LEDs) through the coupling of multiple quantum wells (MQWs) with localized surface plasmon (LSP). The LSP was generated on an Ag nanotriangle array (NTA) on a 40-nm-thick p-type GaN layer beneath the p-pad of the LED, which was partially etched by inductively coupled plasma system. The Ag NTA was fabricated by nanosphere lithography. The resonant frequency of a generated LSP can be precisely controlled by changing the size of the polystyrene nanosphere and the Ag deposition thickness. Under the optimum conditions, the light output power of LED with an Ag NTA was 15.4% higher than LED without an Ag NTA at an inject current of 20 mA. The improvement in light output power can be attributed to the coupling effect between MQW and LSP.

Light Output Improvement of AlGaInP-Based LEDs With Nano-Mesh ZnO Layers by Nanosphere Lithography

AlGaInP-based light-emitting diodes (LEDs) with nano-mesh ZnO layers were fabricated by using nanosphere lithography. With 20-mA injection current, the output powers of the nano-mesh ZnO, planar ZnO, and conventional LEDs (LED-I, LED-II, and LED-III, respectively) were approximately 1.66, 1.58, and 1.44 mW, respectively. The improvement of output power in LED-I could be attributed to the nano-mesh ZnO layer that acts as light scattering centers at the surface. In addition, the intermediate refractive index (about n=2) of nano-mesh ZnO layer between those of the p-GaP window layer and air results in the broader critical angle and the reduction of the total internal reflection.

Efficiency Improvement of GaN-Based LEDs With

High-performance nitride-based light-emitting diodes (LEDs) grown with SiO2 microrod array have been demonstrated. The light output power of LEDs with SiO2 microrod array was 9.03% higher than conventional LEDs at the injection current of 20 mA. The improvement contributed to the enhancement of the light extraction efficiency, and epitaxial GaN film quality improved by direct heteroepitaxial lateral overgrowth with SiO2 microrod array. The light output power could be further enhanced by about 18.36% as compared with the conventional LEDs when adopting the textured sidewall surface which use buffered oxide etch to remove SiO2 microrod arrays and use NaOH to etch the sidewall again into an inverted pyramid shape. After the texturing process, the LEDs show higher electroluminescence intensity and broader far-field pattern. Furthermore, the LEDs with SiO2 microrod array and additional wet-etching process will not affect the electrical property.

\hbox{SiO}_{2}

We introduce a method to enhance the luminescence of GaN-based LEDs by combining the direct heteroepitaxy laterally overgrowth (DHELO) technique with selective wet etching process. The epitaxial overgrowth of GaN layers on sapphire substrate with SiO2 micro-rods array exhibited a reduced dislocation density and improved the crystal quality. The EL intensity of LEDs with SiO2 micro-rods array was 6.5% higher than conventional LEDs at 20 mA. The selective wet etching process was then used to texture the LED sidewalls into inverted pyramid shape. Finally, the EL intensity could be further enhanced about 12.5% as compared with LEDs with SiO2 micro-rods array when adopting the textured sidewalls. 2011 Published by Elsevier B.V.

Microrod Array and Textured Sidewalls

The wafer bonding process and chemical mechanical polishing (CMP) technique are used to remove the sapphire substrate for fabricating the thin-GaN light emitting diodes (LEDs). The stress effects in GaN epilayers on very thin sapphire substrates are analyzed by Raman spectroscopy and photoluminescence (PL). By reducing the thickness of the sapphire substrate from 450 µm to the range between 1.1 and 20.3 µm, the compressive stress in GaN epilayers will be almost released. By analyzing the main scattering signal of E2 (high) mode, the sample with thinnest sapphire of 1.1 µm exhibits 248.3 MPa stress relaxation which is obtained from the Raman shift Δω of 1.54 cm-1. By analyzing the variation of the biaxial stress with sapphire thickness, the relationship between the Raman wavenumbers ω (in cm-1) and residual sapphire thicknesses d (in µm) can be expressed by ω=569.51-1.92×exp (-d/5.81) (cm-1). Finally, an almost linear relationship between residual sapphire thickness and energy bandgap of GaN films was also observed during the CMP process.