Chu-Young Cho

Large-scale patterned multi-layer graphene films as transparent conducting electrodes for GaN light-emitting diodes

This work demonstrates a large-scale batch fabrication of GaN light-emitting diodes (LEDs) with patterned multi-layer graphene (MLG) as transparent conducting electrodes. MLG films were synthesized using a chemical vapor deposition (CVD) technique on nickel films and showed typical CVD-synthesized MLG film properties, possessing a sheet resistance of [Formula: see text] with a transparency of more than 85% in the 400-800 nm wavelength range. The MLG was applied as the transparent conducting electrodes of GaN-based blue LEDs, and the light output performance was compared to that of conventional GaN LEDs with indium tin oxide electrodes. Our results present a potential development toward future practical application of graphene electrodes in optoelectronic devices.

Effect of electron blocking layer on efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes

The effect of an electron blocking layer (EBL) on the efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes (LEDs) is investigated. At low current density, the LEDs with a p-AlGaN EBL show a higher external quantum efficiency (EQE) than LEDs without an EBL. However, the EQE of LEDs without an EBL is higher than LEDs with an EBL as injection current density is increased. The improved EQE of LEDs without an EBL at high current density is attributed to the increased hole injection efficiency.

Enhanced optical output power of green light-emitting diodes by surface plasmon of gold nanoparticles

We demonstrate the surface plasmon (SP) enhanced green light-emitting diodes (LEDs). The Au nanoparticles were embedded in the p-GaN of LEDs. The photoluminescence and electroluminescence measurements showed improved optical properties of LEDs with Au nanoparticles due to an increase in the spontaneous emission rate by resonance coupling between the excitons in multiple quantum wells and localized surface plasmons in Au nanoparticles. The optical output power of SP-enhanced green LEDs with Au nanoparticles was increased by 86% without showing degradation of the electrical characteristics of LEDs compared to LEDs without Au nanoparticles.

Surface plasmon-enhanced light-emitting diodes using silver nanoparticles embedded in p-GaN

We demonstrate the surface plasmon-enhanced blue light-emitting diodes (LEDs) using Ag nanoparticles embedded in p-GaN. A large increase in optical output power of 38% is achieved at an injection current of 20 mA due to an improved internal quantum efficiency of the LEDs. The enhancement of optical output power is dependent on the density of the Ag nanoparticles. This improvement can be attributed to an increase in the spontaneous emission rate through resonance coupling between the excitons in multiple quantum wells and localized surface plasmons in Ag nanoparticles embedded in p-GaN.

Effect of Mg doping in the barrier of InGaN/GaN multiple quantum well on optical power of light-emitting diodes

We report on Mg doping in the barrier layers of InGaN/GaN multiple quantum wells (MQWs) and its effect on the properties of light-emitting diodes (LEDs). Mg doping in the barriers of MQWs enhances photoluminescence intensity, thermal stability, and internal quantum efficiency of LEDs. The light output power of LEDs with Mg-doped MQW barriers is higher by 19% and 27% at 20 and 200 mA, respectively, than that of LEDs with undoped MQW barriers. The improvement in output power is attributed to the enhanced hole injection to well layers in MQWs with Mg-doped barriers.

Surface plasmon-enhanced light-emitting diodes with silver nanoparticles and SiO2 nano-disks embedded in p-GaN

We report the fabrication and characterization of surface plasmon (SP)-enhanced blue light-emitting diodes (LEDs) with Ag nanoparticles and SiO2 nano-disks embedded in the p-GaN layer. Compared with LEDs without Ag nanoparticles, the optical output power increases for the SP-enhanced LEDs with and without SiO2 nano-disks are 72% and 49%, respectively. The greater increase in optical output power for the SP-enhanced LEDs with SiO2 nano-disks compared with SP-enhanced LEDs without SiO2 nano-disks can be attributed to the SiO2 nano-disks in the p-GaN layer, which reduce the Ag nanoparticle-induced defects and enhance the light extraction efficiency of the LEDs.

Phosphor-free white light-emitting diode with laterally distributed multiple quantum wells

A phosphor-free white light-emitting diode (LED) was fabricated with laterally distributed blue and green InGaN∕GaN multiple quantum wells (MQWs) grown by a selective area growth method. Photoluminescence and electroluminescence (EL) spectra of the LED showed emission peaks corresponding to the individual blue and green MQWs. The integrated EL intensity ratio of green to blue emission varied from 2.5 to 6.5 with the injection current below 300mA, but remained constant at high injection currents above 300mA. The stability of the emission color at high currents is attributed to parallel carrier injection into both MQWs.

InGaN∕GaN multiple quantum wells grown on microfacets for white-light generation

We report the white color electroluminescence (EL) emission from InGaN∕GaN multiple quantum wells (MQWs) grown on GaN microfacets. The white color was realized by combining EL emission from InGaN∕GaN MQWs on c-plane (0001), semipolar {11−22}, and {1−101} microfacets of trapezoidal n-GaN arrays. The color of EL emission was changed from reddish to bluish color with injection current and showed a white color in the current range of 180–230mA. The variation in the color of EL emission was attributed to differences in current injection and quantum efficiency of MQWs grown on c-plane (0001) and semipolar GaN microfacets.

Effect of InGaN quantum dot size on the recombination process in light-emitting diodes

The effect of InGaN quantum dot (QD) size on the performance of light-emitting diodes (LEDs) was investigated by varying the QD size from 1.32to2.81nm. The electroluminescence peak of the LEDs containing small QDs (1.32nm) was redshifted with increasing input current while that of large QDs (2.81nm) was blueshifted up to 40mA due to the screening effect of the piezoelectric field. The optical output power of LEDs fabricated with small QDs was much higher compared to those with large QDs. These results were attributed to a weaker piezoelectric field and enhanced quantum confinement in small QDs.

Improvement of efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes with trapezoidal wells

We investigated InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) with trapezoidal wells to improve the efficiency droop. MQW LEDs with trapezoidal wells showed a lower operating voltage and an improved efficiency droop with a low crossover current density of 5 A cm−2, which was a significant improvement over conventional LEDs that use rectangular wells. The external quantum efficiency was increased by 20% at a current density of 70 A cm−2. The improvement in efficiency droop of the MQWs with trapezoidal wells can be attributed to an increased internal quantum efficiency due to the enhanced overlap of the electron and hole wave functions at high current densities.