Performance of InGaN Light-Emitting Diodes Fabricated on Patterned Sapphire Substrates with Modified Top-Tip Cone Shapes

Hindawi Publishing Corporation International Journal of Photoenergy Volume 2014, Article ID 796253, 7 pages http://dx.doi.org/10.1155/2014/796253 Research Article Performance of InGaN Light-Emitting Diodes Fabricated on Patterned Sapphire Substrates with Modified Top-Tip Cone Shapes Hsu-Hung Hsueh,1 Sin-Liang Ou,2 Chiao-Yang Cheng,3 Dong-Sing Wuu,2 and Ray-Hua Horng1,4 1 Graduate Institute of Precision Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung 40227, Taiwan Department of Materials Science and Engineering, National Chung Hsing University, Taichung 40227, Taiwan 3 Wafer Works Optronics Corporation, Taoyuan 32542, Taiwan 4 Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 70101, Taiwan 2 Correspondence should be addressed to Ray-Hua Horng; Received 14 February 2014; Revised 5 June 2014; Accepted 6 June 2014; Published 19 June 2014 Academic Editor: Hao-Chung Kuo Copyright © 2014 Hsu-Hung Hsueh et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. InGaN light-emitting diodes (LEDs) were fabricated on cone-shaped patterned sapphire substrates (PSSs) by using low-pressure metalorganic chemical vapor deposition. To enhance the crystal quality of the GaN epilayer and the optoelectronic performance of the LED device, the top-tip cone shapes of the PSSs were further modified using wet etching. Through the wet etching treatment, some dry-etched induced damage on the substrate surface formed in the PSS fabrication process can be removed to achieve a high epilayer quality. In comparison to the LEDs prepared on the conventional sapphire substrate (CSS) and cone-shaped PSS without wet etching, the LED grown on the cone-shaped PSS by performing wet etching for 3 min exhibited 55% and 10% improvements in the light output power (at 350 mA), respectively. This implies that the modification of cone-shaped PSSs possesses high potential for LED applications. 1. Introduction In recent years, InGaN light-emitting diodes (LEDs) have been used in a wide range of optoelectronic applications such as traffic signals, automobiles, full-color displays, solidstate lighting, and backlights of liquid-crystal displays [1– 3]. To apply for these applications, an LED device with high luminescence efficiency is required. Nevertheless, depositing a GaN epilayer on lattice-mismatched substrates consisting of sapphire and silicon carbide has resulted in a high threading dislocation (TD) density with a range from 109 to 1011 cm−2 [4, 5]. The high TD density created in the GaN epilayer causes a considerable deterioration of LED performance, including electron mobility, device lifetime, and the quantum efficiency of radiative recombination. Consequently, decreasing the TD density may be a key process in achieving high-efficiency LEDs. To reduce the TD density of the GaN epilayer to a range of 106 -107 cm−2 , several methods including epitaxial lateral overgrowth (ELOG), pendeoepitaxy, and facet-controlled ELOG have been developed. Moreover, because of its singlegrowth process with no interruption, the patterned sapphire substrate (PSS) technique is another promising method for achieving a GaN epilayer with high crystal quality. However, as the GaN epilayer is grown on PSS, a long period is required for merging the GaN epilayers grown on etched and nonetched sapphire and subsequently obtaining a smooth film surface. Based on previous reports [6–8], InGaN/GaN epilayers with high crystal quality can be achieved by using metalorganic chemical vapor deposition (MOCVD) on a 2 cone-shaped PSS. At the first step of GaN growth on a coneshaped PSS, the epitaxial film is merely deposited on the flat basal of the sapphire substrate. Furthermore, the GaN growth on the cone regions has no preferential orientation. This indicates that the growth time of a GaN epilayer with a smooth surface on a cone-shaped PSS is less than that required for using the conventional PSS. In this study, cone-shaped PSSs were fabricated using dry and wet etching processes and then employed for growing InGaN LED epitaxial structures. In addition, to improve the epilayer quality and LED performance, the wet etching process was applied to modify the top-tip shape of the PSSs by changing the etching time. The epilayer quality, light extraction characteristic, and optoelectronic performance were investigated in detail for these InGaN LEDs fabricated on PSSs with modified top-tip cone shapes. 2. Experimental Procedure For the fabrication of PSSs with modified top-tip cone shapes, the dry and wet etching processes were employed in sequence, which are described as follows. First, a thick photoresist was deposited on the (001) sapphire through spincoating, and then a thermal photoresist reflow process was used to create the cone-shaped pattern array. The photoresist array was used as the mask layer to transfer the pattern on the sapphire by applying an inductively coupled plasma reactive ion etching (ICP-RIE) system using reactive Cl2 gas. The diameter, interval, and height of each cone-shaped pattern were set to 2.4, 0.5, and 1.5 𝜇m, respectively. After performing ICP-RIE, the cone-shaped PSSs were further chemically etched using a mixture of H2 SO4 : H3 PO4 (3 : 1) solution at 250∘ C for 3, 5, 7, and 10 min to form the various top-tip shapes. To clean the substrate surface, these PSSs were soaked in the H2 SO4 : H2 O2 (3 : 1) solution at 120∘ C for 10 min and in deionized water at room temperature for 10 min in sequence. For growing the LED epitaxial structure, the epilayers were on these PSSs and on a conventional sapphire substrate (CSS) employed as the contrast sample by using low-pressure MOCVD. The LED epitaxial structure included a 3 𝜇m thick layer of undoped GaN (u-GaN), a 2 𝜇m thick layer of n-type GaN:Si, 6 periods of InGaN/GaN multiple quantum wells, a 100 nm thick p-type AlGaN layer, and a 0.2 𝜇m thick p-type GaN:Mg layer. To fabricate the LED device, a 24 × 45 mil.2 mesa pattern was defined and prepared using standard photolithography and dry etching processes. Subsequently, the ITO film used as the transparent conducting layer was deposited on the p-type GaN layer. Finally, the Cr/Au metal was prepared as both the n- and p-pad electrodes. The epilayer quality of the flat u-GaN was measured using X-ray diffraction (XRD) combined with the analyses of the (002) and (102) planes. The surface morphology and pattern feature of these PSSs were observed using scanning electron microscopy (SEM). The microstructure and electron diffraction pattern of the GaN epilayers grown on PSSs were investigated using transmission electron microscopy (TEM). The typical current-voltage (𝐼-𝑉) characteristic of the International Journal of Photoenergy fabricated InGaN LED device was analyzed using a semiconductor parameter analyz (...truncated)