Je Won Kim

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.

Enhanced optical power and low forward voltage of GaN-based light- emitting diodes with Ga-doped ZnO transparent conducting layer

Ga-doped ZnO (ZnO:Ga) films were grown by metalorganic chemical vapor deposition as transparent conducting layers for GaN light-emitting diodes (LEDs). The forward voltage of LEDs with ZnO:Ga was 3.3 V at 20 mA. The low forward voltage was attributed to the removal of a resistive ZnGa2O4 phase, decreased resistivity of ZnO:Ga films, and increased hole concentration in p-GaN by thermal annealing process. The light output power of LEDs with ZnO:Ga was increased by 25% at 20 mA compared to that of LEDs with Sn-doped indium oxide due to the enhanced transmittance and the increased hole concentration 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.

Improvement of light output power of InGaN/GaN light-emitting diode by lateral epitaxial overgrowth using pyramidal-shaped SiO 2

We report on the improvement of light output power of InGaN/GaN blue light-emitting diodes (LEDs) by lateral epitaxial overgrowth (LEO) of GaN using a pyramidal-shaped SiO(2) mask. The light output power was increased by 80% at 20 mA of injection current compared with that of conventional LEDs without LEO structures. This improvement is attributed to an increased internal quantum efficiency by a significant reduction in threading dislocation and by an enhancement of light extraction efficiency by pyramidal-shaped SiO(2) LEO mask.

Improvement of Light Extraction Efficiency in GaN-Based Light Emitting Diodes by Random Pattern of the p-GaN Surface Using a Silica Colloidal Mask

A two-dimensional silica colloidal particle was used to etch a p-GaN surface. By treating the p-GaN surface with polyelectrolyte (PE), mono-dispersed silica colloidal particles, 500 nm in diameter, could be uniformly distributed on a 2-in. p-GaN surface. The patterns on the p-GaN surface were produced by a plasma etching process using these colloidal particles as a mask. Etching depths of 150 and 200 nm were produced on the p-GaN surface of LED samples and an increase in the optical output power of 46.7% was observed compared to a reference sample without patterns on the p-GaN surface.

Enhanced light extraction efficiency in flip-chip GaN light-emitting diodes with diffuse Ag reflector on nanotextured indium-tin oxide

We investigated a flip-chip light emitting diode (FCLED) with a diffuse reflector fabricated by depositing a Ag film on a nanotextured indium-tin oxide (ITO) layer. The FCLED with a diffuse Ag reflector showed remarkably good adhesion and high reflectance than that with a specular Ag reflector deposited on the planar ITO layer. The optical output power of FCLED with the diffuse Ag reflector was enhanced by 161.3% at 300mA compared to that with the specular Ag reflector.

Confocal scanning electroluminescence spectro-microscope for multidimensional light-emitting property analysis

We report new type of micro-EL instrument and its applications for light emitting devices. Our new micro-EL, so-called confocal scanning electroluminescence sprctro-microscope (CSESM) has not only fast image acquisition time but also high image resolution. The newly developed CSESM is combined with confocal laser scanning photoluminescence micsoscope, i.e. micro-PL. Therefore, micro-EL distribution can be directly matched with micro-PL and mechanical chip structure of LED. It is fruitful for providing a fast and non-destructive method to analyze the homogeneity of LEDs in its completely proceeded form. Using this apparatus, we study local intensity and wavelength distribution of electroluminescence for InGaN/GaN blue LED chip. Our results represent that local fluctuations of electroluminescence intensity and wavelength position are closely connected with the fluctuation of local current density, i.e. current spreading features on LED chips.

Improving the Performance of Green LEDs by Low-Temperature Annealing of p-GaN with PdZn

This article reports the electrical properties of p-GaN annealed at low activation temperature by using a PdZn film in green InGaN/GaN multiquantum well (MQW) light-emitting diodes (LEDs). Electroluminescence (EL) intensity of green MQW LED annealed at 600°C using PdZn was improved by 33% at 20 mA compared to that annealed at 800°C without PdZn. These results are attributed to an increase of the hole concentration of p-GaN due to removal of hydrogen in p-GaN by PdZn and a decrease in thermal damage of MQW at low activation temperature.

Effect of PdZn film on the performance of green light-emitting diodes

PdZn was used to improve the electrical properties of p-GaN annealed at low activation temperature for high efficiency green light-emitting diodes (LEDs). A hole concentration of p-GaN annealed at 600 °C with PdZn was almost 28 times higher than that of p-GaN annealed at 800 °C without PdZn. SIMS analysis showed that hydrogen concentration in p-GaN annealed with PdZn is decreased compared to that without using PdZn because the PdZn enhances hydrogen desorption from the Mg-doped p-GaN film at low temperature. The green MQW LED annealed at 600 °C using PdZn showed improved electrical characteristic and optical output power compared to that annealed at 800 °C without using PdZn. These results are attributed to the increase of hole concentration of p-GaN due to removal of hydrogen in p-GaN by PdZn and the decrease in thermal damage of MQW at low activation temperature.

Convergence of optical spectroscopic system for characterization of InGaN/GaN multi-quantum well light-emitting diodes

We present a converged spectroscopic system design for performing photoreflectance (PR), electroreflectance (ER), electroluminescence (EL), photoluminescence (PL) and photovoltage (PV) measurements of semiconductors. The design of the experimental setup is described in detail. To test the performance of the system, measurements of a series of InxGa1-xN/GaN light emitting semiconductor with different indium composition of InGaN layer are carried out by use of this system. The experimental reflection and luminescence spectra are analyzed and discussed. The experimental results demonstrate the performance of this system. Optical and electrical properties of In0.15Ga0.85N/GaN multi-quantum well (MQW) light-emitting diodes (LEDs) with different quantum well (QW) thicknesses were investigated by electric-field dependent ER spectroscopy. From the ER measurements, we have observed the well-resolved transition peaks related to InGaN QW. Furthermore, the transitions related to yellow luminescence (YL) from Si-doped GaN and blue luminescence (BL) from Mg-doped GaN were observed in the ER spectra of In0.15Ga0.85N/GaN MQW LEDs. With increasing QW thickness, the additional transitions related to InGaN QW can be attributed to the recombination of excitons localized at the shallow potential states in InGaN QW, originating from the In-poor InGaN regions caused by indium phase separation in InGaN QW. By applying a reverse bias voltage, the ER features related to InGaN QW were shifted to higher energy, resulting from the reduction of quantum confined Stark effect in InGaN QW with increasing reverse bias voltage. On the other hand, the ER features from YL and BL band related to the deep and the shallow impurity state exhibit redshift and broaden with reverse bias voltage. These results can be attributed to the reduction of Coulomb interaction between donor and acceptor caused by the increase of depletion regions with increasing reverse bias voltage.