Cho et al. Nanoscale Research Letters 2011, 6:578 http://www.nanoscalereslett.com/content/6/1/578 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. Experimental procedure Fabrication of micro and nano complex structures on the GaN-based LED The overall process flow used to form the polymeric micro and nano complex structure on LED device is described in Figure 1. Details of the fabrication of the LED devices have been shown elsewhere [18]. As shown in Figure 1, a micro and nano complex polymer structure was formed on the completed LED structure, which has n and p electrodes and a mesa structure using the nanoimprint lithography process. Since a flexible PDMS stamp was used as an imprint template, a micro- and nanoscale complex polymer structure was uniformly formed on the LED despite the step height between the n- and p-GaN regions. The process details of PDMS replication are available elsewhere [19]. The polymer structure on the electrodes of the p- and n-GaN layers was selectively removed by photolithography and RIE. GXR601, which purchased from AZ Electronic Materials Page 2 of 6 (Stockley Park, UK), was used as a positive photo-resist. Since only metal electrodes were exposed and the ITO surface was not exposed to the plasma, plasma-induced damage to the LED device was avoided. Fabrication of mico- and nanoscale complex structure using PEC etching process The complicated micro- and nanoscale structure origi- nated from the photochemical etched N-face GaN epitax- ial layer. The GaN layer was epitaxially grown on (0001) sapphire substrate with a thickness of a few micrometers and was lifted off using a laser. The N-faced GaN surface was then exposed and etched with 5 M KOH solution at 60°C. To enhance the etching, the solution was continu- ously stirred and ultraviolet light was illuminated simulta- neously onto the surface during the etching process [17]. Details of material used as micro- and nanoscaled complex structure A high refractive index resin containing TiO2 nanoparti- cles was purchased from Brewer Science Inc. (Rolla, MO, USA) and used as an imprint resin to form the micro- and nanoscale complex structures since its n and k values are Figure 1 Schematic diagram of fabrication of micro- and nanoscale complex polymer structure on the LED device.