The GaN-based wide band-gap semiconductor have attracted considerable interest, in terms of application for high brightness optical device, such as light emitting diodes (LEDs), solid-state lighting, high performance back light units in liquid crystal displays and traffic lights. In general, the lig...
The GaN-based wide band-gap semiconductor have attracted considerable interest, in terms of application for high brightness optical device, such as light emitting diodes (LEDs), solid-state lighting, high performance back light units in liquid crystal displays and traffic lights. In general, the light output power of a multiple quantum well (MQW) light emitting diodes (LEDs) depend on the internal quantum efficiency (IQE) and the light extraction efficiency (LEE). Recent crystal growth technology has improved the internal efficiency, while the extraction efficiency still remains low because of the total reflection at the surfaces of LEDs. The high refractive index of GaN (n~2.5) prohibits light beyond a critical angle from being extracted according to the conditions of Snell’'s law. To improve the LEE, attempts have been made to overcome the significant photon loss resulting from total internal reflection inside the LEDs. To enhance the escape probability of photons generated in the active layer of the LED, a large critical angle or rough surface is required. Although the refractive index of GaN cannot be changed, light output can be enhanced by roughening the top surface of a GaN sample to increase the effective critical angle and hence the escape cone. Angular randomization of photons can be achieved by surface scattering from the roughened top surface. To increase the light extraction efficiency of LEDs, several approaches have been attempted. Recently, ZnO nanotips/nanorods grown on GaN substrate by metal orgarnic chemical vapor deposition (MOCVD) have been used to enhance the LEE. In addition, improvement in light extraction by ZnO nanorod arrays grown on ITO films using an aqueous solution method at a temperature of 90℃ have been reported. Although these methods improve light output power without using dry etching or lithography, the forward voltage of GaN LEDs with ZnO nanotips/nanorods increases due to thermal damage of the transparent p-electrode during ZnO nanotip/nanorod growth at high temperature. In this study, improvement of light output of GaN-based LEDs using Zinc Oxide (ZnO) nanoparitlces on and under Induim tin oxide (ITO) films, respectively. The first, ZnO nanoparticles were deposited on ITO film using spin-coating process. ITO films were composed of flat and hemispherical shaped ITO, hemispherical shaped ITO on flat ITO film was made wet and dry etching. It has been found that the light output power of ZnO textured LED was 21%, 34% greater than that of a conventional LED with a flat ITO, at 20mA of current injection, respectively. The light output power of ZnO textured LEDs is significantly improved owing to a reduction of the total internal reflection effect and a possibility of a high level of light scattering at the textured surface. The second, different density of ZnO nanoparitcles were deposited on p-GaN surface using spin-coating process. upon deposition of ITO film over the dispersed ZnO nanoparticles, the ITO surface tends to attain nano-rough morphology due to the presence of ZnO nanoparticles. The light output power of the ITO/ZnO textured LEDs was 39% higher than that of a conventional LED with a planar ITO, at 20mA of current injection. This is attributed to the improved light output power favored by the light scattering tendency of ZnO nanoparticles and the nano-roughed ITO film.
The GaN-based wide band-gap semiconductor have attracted considerable interest, in terms of application for high brightness optical device, such as light emitting diodes (LEDs), solid-state lighting, high performance back light units in liquid crystal displays and traffic lights. In general, the light output power of a multiple quantum well (MQW) light emitting diodes (LEDs) depend on the internal quantum efficiency (IQE) and the light extraction efficiency (LEE). Recent crystal growth technology has improved the internal efficiency, while the extraction efficiency still remains low because of the total reflection at the surfaces of LEDs. The high refractive index of GaN (n~2.5) prohibits light beyond a critical angle from being extracted according to the conditions of Snell’'s law. To improve the LEE, attempts have been made to overcome the significant photon loss resulting from total internal reflection inside the LEDs. To enhance the escape probability of photons generated in the active layer of the LED, a large critical angle or rough surface is required. Although the refractive index of GaN cannot be changed, light output can be enhanced by roughening the top surface of a GaN sample to increase the effective critical angle and hence the escape cone. Angular randomization of photons can be achieved by surface scattering from the roughened top surface. To increase the light extraction efficiency of LEDs, several approaches have been attempted. Recently, ZnO nanotips/nanorods grown on GaN substrate by metal orgarnic chemical vapor deposition (MOCVD) have been used to enhance the LEE. In addition, improvement in light extraction by ZnO nanorod arrays grown on ITO films using an aqueous solution method at a temperature of 90℃ have been reported. Although these methods improve light output power without using dry etching or lithography, the forward voltage of GaN LEDs with ZnO nanotips/nanorods increases due to thermal damage of the transparent p-electrode during ZnO nanotip/nanorod growth at high temperature. In this study, improvement of light output of GaN-based LEDs using Zinc Oxide (ZnO) nanoparitlces on and under Induim tin oxide (ITO) films, respectively. The first, ZnO nanoparticles were deposited on ITO film using spin-coating process. ITO films were composed of flat and hemispherical shaped ITO, hemispherical shaped ITO on flat ITO film was made wet and dry etching. It has been found that the light output power of ZnO textured LED was 21%, 34% greater than that of a conventional LED with a flat ITO, at 20mA of current injection, respectively. The light output power of ZnO textured LEDs is significantly improved owing to a reduction of the total internal reflection effect and a possibility of a high level of light scattering at the textured surface. The second, different density of ZnO nanoparitcles were deposited on p-GaN surface using spin-coating process. upon deposition of ITO film over the dispersed ZnO nanoparticles, the ITO surface tends to attain nano-rough morphology due to the presence of ZnO nanoparticles. The light output power of the ITO/ZnO textured LEDs was 39% higher than that of a conventional LED with a planar ITO, at 20mA of current injection. This is attributed to the improved light output power favored by the light scattering tendency of ZnO nanoparticles and the nano-roughed ITO film.
주제어
#light emitting diodes(LEDs)+ZnO nanoparticles
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