Hee Kwan Lee

Effect of AZO seed layer on electrochemical growth and optical properties of ZnO nanorod arrays on ITO glass

We report the structural and optical properties of ZnO nanorod arrays (NRAs) grown by an electrochemical deposition process. The ZnO NRAs were grown on indium tin oxide (ITO) coated glass substrates with a thin sputtered Al-doped ZnO (AZO) seed layer and compared with ones directly grown without the seed layer. The growth condition dependence of ZnO NRAs was investigated for various synthetic parameters. The morphology and density of the ZnO NRAs were accordingly controlled by means of zinc nitrate concentration and growth time. From photoluminescence results, the ultraviolet emission was significantly enhanced after thermal treatment. For ZnO NRAs grown on ITO glass without the seed layer, the diffuse transmittance was enhanced despite the reduction in the total transmittance, indicating a high haze value. By using a thin AZO seed layer, the well-aligned ZnO NRAs on AZO/ITO glass are controllably and reproducibly synthesized by varying the growth parameters, exhibiting a total transmittance higher than 91% in the visible wavelength range as well as good optical and crystal quality.

Light-Extraction Enhancement of Large-Area GaN-Based LEDs With Electrochemically Grown ZnO Nanorod Arrays

We reported the improvement in light-extraction efficiency of large-area InGaN/GaN blue light-emitting diodes (LEDs) with zinc oxide (ZnO) nanorod arrays (NRAs), together with theoretical analysis. The ZnO NRAs with high transmittance were grown on the indium-tin-oxide surface with a thin Al-doped ZnO seed layer by electrochemical deposition. The vertically well-aligned ZnO NRAs exhibited an excellent suppression of internal reflections as well as a good transmittance. For LEDs with optimized ZnO NRAs, the light output power was improved by ~20% at 350 mA compared to that of the conventional LED.

Light-extraction enhancement and directional emission control of GaN-based LEDs by self-assembled monolayer of silica spheres

The light extraction of 1 × 1 mm(2) GaN-based blue light-emitting diodes (LEDs) was enhanced by a self-assembled monolayer (SAM) of silica submicron spheres. The silica spheres were synthesized with various spherical sizes via the ammonia-catalyzed hydrolysis and condensation of tetraethyl orthosilicate in water/ethanol solutions. Hexagonal closely-packed (HCP) silica sphere monolayer was formed onto the indium tin oxide layer of the LED by a spin coating process. The size effect of silica spheres on the light-extraction efficiency (LEE) of GaN-based LEDs was theoretically studied and their optimum size was determined. The simulation results showed that the use of silica spheres can improve the LEE by 1.1-1.32 times compared to the conventional LEDs. The light output power of the LED with 650-nm-thick SAM of HCP silica spheres was experimentally enhanced by 1.28 and 1.23 times under the injection currents of 100 and 350 mA, respectively. By employing the SAM of HCP silica spheres, the directional emission pattern was relatively converged, indicating a reasonable consistency with the simulation result.

Improved light extraction of InGaN/GaN blue LEDs by GaOOH NRAs using a thin ATO seed layer.

We investigated the effect of gallium oxide hydroxide (GaOOH) nanorod arrays (NRAs) on the light extraction of InGaN/GaN multiple quantum well blue light-emitting diodes (LEDs). GaOOH NRAs were prepared on an indium tin oxide electrode (ITO) layer of LEDs by electrochemical deposition method. The GaOOH NRAs with preferred orientations were grown on the ITO surface by sputtering a thin antimony-doped tin oxide seed layer, which enhances heterogeneous reactions. Surface density and coverage were also efficiently controlled by the different growth voltages. For LEDs with GaOOH NRAs grown at −2 V, the light output power was increased by 22% without suffering from any serious electrical degradation and wavelength shift as compared with conventional LEDs.

Light Output Extraction Enhancement in GaN-Based Green LEDs With Periodic AZO Subwavelength Nanostructure Arrays

We experimentally and theoretically demonstrate the enhancement in the light extraction efficiency of InGaN/GaN multiple quantum well green light-emitting diodes (LEDs) through periodic aluminum-doped zinc oxide (AZO) subwavelength nanostructure arrays (SNAs). The AZO SNAs are formed on the surface of indium tin oxide electrodes of LEDs by laser interference lithography and a subsequent dry etching after AZO film deposition. For LEDs with AZO SNAs, the light output power is increased by 19% at 100 mA compared to the conventional LED on patterned sapphire substrate. Also, there is no distinct degradation in the electrical characteristics of LEDs.

Enhanced Light Extraction of GaN-Based Green Light-Emitting Diodes With GaOOH Rods

We reported the enhanced light extraction efficiency in InGaN/GaN multiple quantum well green light-emitting diodes (LEDs) with gallium oxide hydroxide (GaOOH) rods. The GaOOH rods were prepared by an aqueous gallium nitrate solution at 80°C and then coated on the surface of indium tin oxide electrodes of LEDs by a simple drop coating process. The synthesized GaOOH rods indicated a rhombus-shaped rod structure with average lengths of 2 μm and lateral dimensions of 50-500 nm. For LEDs with GaOOH rods, the light output powers were increased by 24.3% and 26.4% compared to the conventional LED on patterned sapphire substrate at 20 mA and 100 mA, respectively. Also, there was no distinct degradation in electrical characteristics of LEDs with GaOOH rods.

Improved Light Extraction of GaN-Based Blue Light-Emitting Diodes with ZnO Nanorods on Transparent Ni/Al-Doped ZnO Current Spreading Layer

We reported the light-extraction properties of InGaN/GaN multiple quantum wells blue light-emitting diodes (LEDs) with ZnO nanorod arrays (NRAs) on Ni/Al-doped ZnO (AZO) films as a current spreading layer (CSL). The Ni/AZO bilayer exhibited a high optical transmittance of ~80% at λ~460 nm. The electrical property of AZO films was improved by inserting a thin Ni layer, which leads to the better current–voltage characteristics of LEDs. The ZnO nanorods can be easily grown on the AZO surface of Ni/AZO CBL as the same materials by a simple wet chemical growth. For 450 ×450 µm2 LED with Ni/AZO CSL, the incorporation of ZnO NRAs into the AZO surface improved the light output power by ~14% at 100 mA without causing any electrical degradation compared to the conventional LED without ZnO NRAs.

Optoelectronic and thermal characteristics of GaN-based monolithic light emitting diode arrays

We investigated the optoelectronic and thermal characteristics of InGaN/GaN monolithic light emitting diode (LED) arrays operating at λ~ 470 nm. The optical output power (Pout) and forward voltage (VF) were almost linearly increased with the number of LEDs for series arrays. In the case of parallel LED arrays, the maximum operating current was increased by increasing the number of devices, but the VF was kept almost constant. Around 235 mA, the maximum Pout of 55.6 mW was obtained for the 1 × 3 series LED array fabricated with a separation distance of 1000 µm, while the Pout was 20.4 mW for the 1 × 3 parallel LED array. From the measured light–current–voltage data, the maximum internal temperature (Tmax), i.e. a maximum value of internal temperature rise within the devices under operation, was theoretically determined using a three-dimensional steady-state thermal heat dissipation model based on the finite element method. Also, the temperature profiles were obtained for various separation distances and array sizes. At injection current of 220 mA, the Tmax was theoretically calculated as 325.6 K and 306.6 K for 1 × 3 series and parallel LED arrays with a 1000 µm separation distance, respectively. The Tmax was increased and decreased on decreasing the separation distance and substrate thickness, respectively, and its dependence on Tmax became more significant at higher injection current.