Tae Ki Kim

GaN-Based Light-Emitting Diodes on Micro-Lens Patterned Sapphire Substrate

GaN-based light-emitting diodes (LEDs) were fabricated on a micro-lens patterned sapphire substrate (ML-PSS). ML patterning on the sapphire substrate was carried out by using photolithography with photo-resist reflow technique and dry etching process using chlorine based inductively coupled plasma. The ML-PSS was prepared using a periodic ML pattern with diameters of 3 µm and spacing of 2, 4, and 5 µm, respectively, on the c-plane sapphire substrate. The leakage current of the LEDs fabricated on the ML-PSS greatly decreased compared to that of a conventional LED and it decreases with increasing ML-pattern spacing; it decreases from 1.8 to 0.2 µA at reverse voltage of 15 V as the ML-pattern spacing is increased from 2 to 5 µm. The output power of the LED with 5 µm spacing was about 155% higher than that of a conventional LED and about 10% higher than that of the LED on the PSS with spacing of 2 µm. This improvement of the output power is contributed not only by reduction of dislocation density depending on spacing of patterning but also by the enhancement of light extraction efficiency with outcoupling via the ML patterned facets on sapphire substrate.

GaN microcavity structure with dielectric distributed Bragg reflectors fabricated by using a wet-chemical etching of a (111) Si substrate

GaN microcavity structure with SiO2∕ZrO2 dielectric distributed Bragg reflectors was fabricated by means of transferring an InGaN∕GaN multiple quantum well (QW) structure from the (111) Si substrate onto a sapphire carrier and wet-chemical etching of the substrate. A dip in the reflectivity spectrum of the microcavity structure is observed at a wavelength of 411nm indicating the cavity resonance mode. Also, the strong influence of the cavity on the QW photoluminescence has been observed. A sharp emission spectrum, with a linewidth of 3.5nm, occurs at a wavelength of 411nm coincided with the position of the cavity resonance mode.

Enhancement of Light Extraction from GaN-Based Light-Emitting Diodes by Coating Surface with Al2O3 Powder

GaN-based light-emitting diodes (LEDs) were fabricated by coating Al2O3 powder on a p-GaN and a semitransparent p-contact metal surface to increase the extraction probability of the internal reflected photons through the Al2O3 powder. The density of the ~300-nm-diameter Al2O3 powder was about 1.9 ×1013 cm-2 on both surfaces. The forward voltages of the LEDs coated with Al2O3 powder on the p-GaN surface and the semitransparent p-contact metal surface were 4.15 and 3.42 V at 20 mA, respectively. The light output powers of both LED structures with Al2O3 powder coated on the p-GaN and semitransparent p-contact metal surfaces were increased by almost 30% compared with the conventional LED structure without Al2O3 powder coating.

Al2O3 Powder Coating and Surface Texturing for High Efficiency GaN-Based Light Emitting Diodes

For high efficiency GaN-based light-emitting diodes (LEDs), Al2O3 powder coating and surface texturing were investigated using natural lithography and dry-etching methods to improve the light-output power of the GaN-based LED. The 300-nm-size Al2O3 powder is coated on the indium tin oxide (ITO) surface at various conditions using a spin-coating method. The morphologies of the ITO surface were observed using a scanning electron microscope. The Al2O3 powder is left on the ITO surface after surface-texturing to increase extraction efficiency. The light output powers of the surface-textured GaN-based LEDs coated with the Al2O3 powders are enhanced by ~66% compared with the conventional LED at 20 mA.