Enhancement of photo- and electro-luminescence of GaN-based LED structure grown on a nanometer-scaled patterned sapphire substrate Hyoungwon Park a , Kyeong-Jae Byeon a , Jong-Jin Jang b , Okhyun Nam b , Heon Lee a,⇑ a Department of Materials Science and Engineering, Korea University, 5-1 Anam-Dong, Seongbuk-Ku, Seoul 136-701, South Korea b LED Technology Center, Department of Nano-Optical Engineering, Korea Polytechnic University, Shiheung 429-839, South Korea article info Article history: Received 30 August 2010 Received in revised form 16 June 2011 Accepted 29 July 2011 Available online 22 August 2011 Keywords: Nanoimprint lithography (NIL) GaN-based light-emitting diodes (LEDs) Nanometer-scaled patterned sapphire substrate (NPSS) Photoluminescence (PL) Electroluminescence (EL) abstract In this study, a 2 in. sized a highly periodic nanometer-scaled patterned sapphire substrate (NPSS) was fabricated using nanoimprint lithography (NIL) and inductively coupled plasma etching to improve the light-extraction efficiency of GaN-based light-emitting diodes (LEDs). A blue LED structure was grown on the nanometer-scale patterned sapphire substrates, and the photoluminescence (PL) and electrolumi- nescence (EL) were measured to confirm the effectiveness of the nanometer-scaled patterns on sapphire. An improvement in luminescence efficiency was observed when NPSS was applied; 2 times stronger PL intensity and 2.8 times stronger EL intensity than the LED structure grown on the unpatterned sapphire wafers were measured. These results show highly periodic nanometer-scaled patterns create multi- photon scattering and effectively enhance the light-extraction efficiency of LEDs. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Single crystalline sapphire wafers are used widely as substrates for group III nitride based light emitting diodes (LEDs) [1–3]. The surface of the sapphire wafer is often textured or patterned on the micrometer scale to improve the crystalline quality of the GaN epi-layers and enhance the photon extraction efficiency of the group III nitride based LEDs [4–6]. By forming micrometer- scaled cone-shaped patterns on sapphire wafers, the so-called pat- terned sapphire substrate (PSS) photons can be scattered at the patterned surface, and the escape probability and extraction effi- ciency of the LED increases [7,8]. Moreover, the GaN epitaxial layer that is grown on the PSS exhibits better crystalline quality, which enhances the internal quantum efficiency of the LED [9,10]. The solid-state lighting industry is growing rapidly, and larger wafer size processes will be needed to increase the production of GaN-based LEDs. On the other hand, some barriers must be over- come in order to achieve GaN epitaxial growth on larger wafers with diameters of up to 6 in. A thick un-doped GaN epitaxial layer should be formed to flatten the GaN surface prior to the growth of the full LED structures, including n-GaN, multi-quantum-well, and p-GaN layers. Therefore, the micrometer-scaled PSS can be applied to the GaN-based LED fabrication. As the thickness and area of GaN films increase, residual stresses, which are caused by the lattice and thermal mismatch between the sapphire substrate and GaN film, also increase causing severe wafer bowing. Consequently, for the GaN epitaxial growth on a larger wafer, the film thickness of the GaN epitaxial layer should be minimized while maintaining the PSS for device performance. When the sapphire substrate is patterned on the nano-scale, the required thickness of the un-doped GaN layer can be reduced, while maintaining its effectiveness for enhancing the light extrac- tion of the device. Recently, several studies have examined the application of nanometer-scaled PSS (NPSS) [10–12], and while the device performance was enhanced significantly, the pattern shapes were still random. In this study, a uniform and periodic nanometer-scaled sapphire pattern was fabricated using the nanoimprint lithography (NIL) technique and the inductively-coupled plasma (ICP) etching pro- cess. A blue LED structure was grown on a NPSS wafer and the photo- and electro-luminescence and current–voltage (I–V) char- acteristics were measured to assess its impact on the GaN-based LED performance. 2. Experiments A 6 in. large Si master template was fabricated by conventional photolithography and reactive ion etching (RIE). The pattern of the Si master template was psudo-hexagonal array of 340 nm diameter holes with pitches of 800 nm. Approximately 270 nm of the hole depths were confirmed by cross-sectional SEM. The Si master tem- plate was then coated with a hydrophobic anti-stiction layer to 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.07.014 ⇑ Corresponding author. Tel.: +82 2 3290 3284; fax: +82 2 928 3584. E-mail address: heonlee@korea.ac.kr (H. Lee). Microelectronic Engineering 88 (2011) 3207–3213 Contents lists available at SciVerse ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee