High brightness GaN-based light-emitting diodes (LEDs) have been widely used for solid-state lighting sources due to their low power consumption, long lifetime, compact form factor, and eco-friendly nature [1–3]. The internal quantum efficiency (η
int) of GaN-based LEDs has been drastically improved by the progress of GaN-based epitaxial growth and device fabrication technologies [4, 5]. Accordingly, many attempts have been made to maximize the external quantum efficiency (photon extraction efficiency) of LEDs. However, much room remains for improvement of the external quantum efficiency.
One of the biggest issues surrounding the current high brightness LEDs is their low light extraction efficiency (η
ext) due to the total internal reflection (TIR) of light at the interface of the LED structure with ambient . Various attempts, including surface roughening [7, 8], the formation of photonic crystals [9, 10], the use of patterned sapphire substrates (PSS) [11, 12], and the use of an air-gap structure inside the LED , were made to suppress the TIR.
The TIR can be minimized by the scattering of light at the interface, which was enhanced by forming the photonic crystal structure or other micro- and nanoscale complex structures. In order to form those structures, plasma processing, such as reactive ion etching (RIE) or inductive coupled plasma etching, is inevitably used along with the lithography process and this can deteriorate the LED's electrical performance [14–16]. Therefore, micro- or nanoscale complex structures need to be formed on the LED structure without plasma processing.
In this study, micro- and nanoscale complex structures made of high refractive index polymer were formed to enhance the LED light extraction efficiency. The micro- and nanoscale structures were obtained from the photo-electro chemical (PEC)-etched surface of the N-faced GaN. The GaN epilayer of a vertical LED was detached from the sapphire substrate and placed over metallic heat sink; thus, the N-faced GaN surface was exposed. In order to improve the photon extraction efficiency of the vertical LED, the N-faced GaN surface was etched using the PEC process to form micro- and nanoscale structure . Micro- and nanoscale patterns of N-faced GaN was replicated using a polydimethylsiloxane (PDMS) molding process and transferred to the ITO electrode surface of conventional edge emitting type GaN blue LED devices using nanoimprint lithography. Due to the micro- and nanoscale complex structures that had formed on the ITO layer, the TIR could be suppressed and more photons could be extracted by scattering with the structure.