Ja-Yeon Kim

Fabrication of photonic crystal structures on light emitting diodes by nanoimprint lithography

Photonic crystal (PC) structures on green light emitting diodes (LED) were successfully fabricated using nanoimprint lithography. The stamp, with a two-dimensional pillar structure, was fabricated using laser interference lithography through a double exposure technique. To achieve PC structures with a precise dimension, thermal treatment of the photoresist was employed during stamp fabrication; this proved to be effective for the control of the diameter and shape of the hole. The two-dimensional PC structures, with a period of 295 nm, diameter of 180 nm and depth of 100 nm, on the green LEDs resulted in nine-fold enhancement of the photoluminescence intensity compared to the as-deposited samples without PC structures.

Improvement of light extraction efficiency of flip-chip light-emitting diode by texturing the bottom side surface of sapphire substrate

A high light-extraction efficiency was demonstrated in the flip-chip light-emitting diode (FCLED) with a textured sapphire substrate. The bottom side of a sapphire substrate was patterned using a dry etching process to increase the light-extraction efficiency. Light output power measurements indicated that the scattering of photons emitted in the active layer was considerably enhanced at the textured sapphire substrate resulting in an increase in the probability of escaping from the FCLED. The light-output power of the FCLED was increased by 40.2% for a 0.4-mum deep FCLED with a periodic distance of 13-mum mesh-type texture on the bottom side of the sapphire substrate

Selective wet etching of p-GaN for efficient GaN-based light-emitting diodes

The selective wet etching of a p-GaN layer by using a solution of KOH in ethylene glycol (KE) was studied to enhance the optical and electrical performance of the GaN-based light-emitting diodes (LEDs). The surface of the p-GaN, which was selectively etched in the KE solution, showed hexagonal-shaped etch pits. The light-output power of etched LEDs was improved by 29.4% compared to that of the nonetched LED. This improvement was attributed to the increase in the probability of photons to escape due to the increased surface area of textured surface and the reduction in contact resistance of the ohmic layer resulting from the increased contact area and hole concentration on the textured p-GaN. The reverse leakage current of the LED was also greatly decreased due to the surface passivation and the removal of defective regions from the p-GaN.

Thermally stable and highly reflective AgAl alloy for enhancing light extraction efficiency in GaN light-emitting diodes

The properties of a AgAl alloy reflector layer deposited on a p-GaN layer for use in high-efficiency GaN flip-chip light-emitting diodes (FCLEDs) were investigated. The AgAl layer showed good adhesion properties compared to a layer of Ag on p-GaN. In addition, no agglomeration was found, indicating that the AgAl layer is thermally stable due to the formation of oxidized Al on the surface and at the interface of the AgAl layer. The InGaN∕GaN multiquantum well light-emitting diode with the annealed AgAl layer showed good I-V characteristic and an enhanced optical output power compared to that with an annealed Ag layer due to the high reflectivity (86.7% at 465 nm), smooth surface after annealing, and good Ohmic property of AgAl. These results clearly indicate that a AgAl layer on p-GaN constitutes a promising reflector and Ohmic scheme for achieving high-brightness FCLEDs.

Enhanced emission efficiency of GaN∕InGaN multiple quantum well light-emitting diode with an embedded photonic crystal

A photonic crystal (PC) structure of periodic SiO2 pillar cubic array is embedded in n-GaN layer of InGaN∕GaN multiple quantum well (MQW) blue (480nm) light-emitting diode (LED). The diameter, period, and depth of SiO2 pillar are 124±6, 230±10, and 130±10nm, respectively. The increments of 70% for external quantum efficiency, 17% for internal quantum efficiency, and 45% for light extraction efficiency from photoluminescence measurement, and 33% for optical output power at 20mA are observed for LEDs with an embedded PC layer. This improvement can be attributed to the increased extraction efficiency by PC effect as well as increased internal quantum efficiency due to the decrease of dislocation density in n-GaN layer because of an epitaxial lateral over-growth process.

Green light-emitting diodes with self-assembled In-rich InGaN quantum dots

A green light-emitting diode (LED) was fabricated using self-assembled In-rich InGaN quantum dots (QDs). The photoluminescence studies showed that the QDs provide thermally stable deeply localized recombination sites for carriers with negligibly small piezoelectric field. The electroluminescence spectra of the LED showed a peak in the green spectral range and the dominant peak was blueshifted with increasing injection current due to the distribution of depth of the potential wells of QDs. The output power of the LED increased with increasing injection current, indicating that the potential wells are thermally stable and deeply localized in the QDs.

Enhanced light extraction from GaN-based green light-emitting diode with photonic crystal

This letter reports the properties of GaN-based green light-emitting diodes (LEDs) having a p-GaN photonic crystal layer with a photonic bandgap (PCWG) and without a photonic bandgap (PCOG). With decreasing the photoluminescence (PL) detection angle from 140° to 60°, the enhancement of PL intensity of LED with PCWG was largely increased from 9 to 25 times, compared to that of LEDs without a patterned structure, while the PL intensity of LED with PCOG was increased from 4.6 to 5.6 times. The electroluminescence output power of green LEDs with a PCWG was enhanced about two times compared to LEDs with a PCOG. These results suggest that the light extraction of green LEDs can be greatly increased by using PCWG instead of PCOG.

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.