Cho et al. Nanoscale Research Letters 2011, 6:578 http://www.nanoscalereslett.com/content/6/1/578 2.08 and 0.004, respectively, at 450 nm, the blue LED emission wavelength. The transmittance of the high refractive index is > 90% at the blue LED emission wave- length [20]. Analysis of the morphologies and the property of the LED The morphologies of micro- and nanoscale complex struc- tures of the high refractive index polymer were analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The effect of the micro- and nanoscale complex structure on the enhancement of the LED light extraction efficiency was confirmed by electrolumines- cence measurement. The electrical properties of the LED devices were measured using current-voltage (I-V) characteristics. Results and discussion Fabrication sub-micron polymer structure on the LEDs The AFM analysis was performed to determine the mor- phology and height of the micro- and nanoscale complex structure. The AFM images of the micro- and nanoscale complex structure that formed on the N-face n-GaN sur- face, replicated polymer mold, and LED device are shown is Figure 2a,c, respectively. Figure 2 clearly shows that a continuous array of micro- and nanoscale structures was formed on the LED devices with high fidelity. According to AFM analysis, the height of the micro- and nanoscale complex polymer pattern ranged from 0.18 to 1.2 μm. Since the actual LED device has a mesa structure, con- trolling the residual layer was extremely difficult. The resi- dual polymer layer can have a detrimental effect on the transparency of the ITO layer of the GaN LED, so the spin-coating speed of the high refractive index polymer resin was carefully adjusted. As shown in Figure 3a,c, in case of lower spin-coating speeds, high fidelity pattern transfer was achieved and the thickness of the residual layer was also quite high. In cases of higher spin-coating speed, the micro- and nanoscale complex structure was not completely transferred due to the lack of an imprint resin. However, the thickness of the residual layer was drastically decreased. In this study, residual layer thickness control was accomplished by spin-coating speed adjust- ment rather than by the RIE. With a spin-coating speed of 5,000 rpm, micro- and nanoscale pattern transfer was achieved with high fidelity and the residual layer thickness was minimized. In order to investigate the effect of the micro- and nanoscale polymer structures on the LED photon extrac- tion efficiency, we chose two identical GaN-based blue LED devices that were grown on PSS and formed the micro- and nanoscale complex polymer structure on one wafer using the nanoimprint lithography process. The cross-sectional SEM micrographs of the LED structure Page 3 of 6 Figure 2 Three-dimensional atomic force microscopy image (inset is a 2 dimensional image) of a micro- and nanoscale complex structure of (a) the N-faced GaN surface, (b) replicated polydimethylsiloxane stamp, and (c) surface of light-emitting diode device after the nanoimprint lithography process. with the micro- and nanoscale complex polymer structure on the PSS are shown in Figure 4a. Figure 4b, c show that the micro- and nanoscale complex polymer structures formed on the metal electrode were clearly removed via the RIE etching process to ensure stable current injection. Analysis of properties of the LEDs We measured the electroluminescence (EL) intensities at 20 mA of injection current LED devices with and without