Nano-imprinting of refractive-index-matched indium tin oxide sol-gel in light-emitting diodes for eliminating total internal reflection

Abstract

Refractive-index (RI)-matched nanostructures are implemented in GaN-based light-emitting diodes (LEDs) for enhancing light output efficiency. The RI-matched indium tin oxide (ITO) nanostructures are successfully implemented in GaN-based lateral LEDs by using ITO sol-gel and nanoimprint lithography. The ITO sol-gel nanostructures annealed at 300 °C have RI of 1.95, showing high transparency of 90% and high diffused transmittance of 34%. Consequently, the light output power in LEDs with the RI-matched nanostructures increases by 8% in comparison with that in LEDs containing flat ITO. Ray tracing and finite-difference time-domain (FDTD) simulations show that the RI-matched nanostructures on the transparent current spreading layer dramatically reduce Fresnel reflection loss at the interface of the current spreading layer with the nanostructure and extract confined waveguide lights in LEDs.

Fig. 1. (a) SEM images of photochemically etched GaN with 2, 4, 8, and 16 M-KOH for master mold. (b) Average value of bottom diagonal of hexagonal pyramids expressed as a function of KOH molar concentration.

Fig. 2. (a) Schematic illustration of the nano-imprinted LEDs with pyramid-shaped ITO nanostructures formed by RI-matched resin. (b) Calculated light extraction efficiency with refractive index; the inset shows schematic explanation of the mechanism for improving light extraction with different RI layers. (c) Calculated far-field intensity with nanostructures having different refractive indices by ray-tracing (ref, bare ITO). The wavelength was fixed at 450 nm.

Fig. 3. (a) Refractive index, (b) GIXRD and (c) sheet resistance results of ITO sol–gel films as functions of the annealing temperature. (d) SEM images of ITO sol–gel films. At 400 °C and 500 °C, ITO sol–gel films show porous morphology. The inset shows the cross-sectional images. (e) X-ray photoelectron spectra (O(1s) peak) of as-coated, 300 °C, and 500 °C annealed ITO sol–gel films. (∼530 eV: M–O–M lattice oxygen, ∼531 eV: M–OH metal hydroxide oxygen, and ∼532 eV: adsorbed oxygen species) The annealing condition was fixed in an N₂ ambient for 3 min.

Fig. 4. SEM images of 1^st replica mold and 2^nd replica mold as a function of remaining ethanol solvent after volatilization.

Fig. 5. Photographs and SEM images of nano-structures shaped as different hexagonal pyramid patterns. (a) Master mold of photochemically etched GaN with 2 M, 8 M, and 16 M-KOH. (b) Inverted-patterned 1^st replica polymer mold imprinted from the master mold. (c) 2^nd replica mold of ITO sol–gel imprinted from the 1^st replica mold. (d) Distribution of the length of diagonal in the 2^nd replica ITO sol–gel. (e) Average diagonal of the nanostructure on the 2^nd replica ITO sol–gel.

Fig. 6. (a) Total (line) and diffused (dot) transmittance spectra of nano-imprinted ITO replica with different sizes of hexagonal pyramids as a function of wavelength. (b) Average diffusive and specular transmittance (380–860 nm) of nano-imprinted ITO replica as a function of average bottom diagonal. (c) Photographs of scattered light by nano-imprinted ITO replica using monochromatic laser with wavelengths of 460 nm (blue), 530 nm (green) and 660 nm (red). Light intensity was set constant at 60 mW cm⁻².

Fig. 7. (a) Current–voltage (I–V) characteristics of LEDs. (b) Radiant flux of LEDs as a function of injection current (5–100 mA). (c) Electroluminescence (EL) spectra of LEDs with different surface structures at a 20 mA injection current: flat, pyramid-shaped nanostructures with different sizes formed using PCE. (d) Measured light output power of LEDs as a function of hexagonal pyramid size of a nanostructure.

Fig. 8. (Left) Calculated cross-sectional electric field distribution of the LEDs with (a) flat ITO structure, nanostructured ITO with (b) 2 M, (c) 8 M, and (d) 16 M KOH. (Right) Enlarged view of the electric field distribution between ITO and air.