Materials Science and Engineering B 136 (2007) 182–186 Investigation of InGaN/GaN light emitting diodes with nano-roughened surface by excimer laser etching method Hung-Wen Huang a,b , Chih-Chiang Kao a , Jung-Tang Chu a , Wei-Chih Wang a , Tien-Chang Lu a , Hao-Chung Kuo a , Shing-Chung Wang a,∗ , Chang-Chin Yu c , Shou-Yi Kuo d a Department of Photonics & Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, ROC b TrueLight Corporation, Hsinchu 300, Taiwan, ROC c Highlink Corporation, Hsinchu 300, Taiwan, ROC d Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu 300, Taiwan, ROC Received 17 September 2006; accepted 29 September 2006 Abstract The InGaN/GaN light emitting diodes (LEDs) with a nano-roughened top p-GaN surface fabricated by using Ni nano-masks and laser etching methods were demonstrated and analyzed. The experiment results observed the maximum light output power of the InGaN/GaN LED with a nano-roughened top p-GaN surface etched with the laser energy of 300 mJ/cm 2 was 1.55 times higher than that of a conventional LED, and the wall-plug efficiency was enhanced 68% at 20 mA. The series resistance of InGaN/GaN LED was reduced by 32% by the increase in the contact area of the nano-roughened surface. © 2006 Elsevier B.V. All rights reserved. Keywords: Gallium nitride (GaN); Light emitting diode (LED); Laser etching GaN-based materials have attracted considerable interest due to their potential use in optoelectronic devices, such as light emitting diodes (LEDs) and laser diodes (LDs) [1–4]. Recently, as the brightness of GaN-based LEDs has increased, applica- tions such as displays, traffic signals, backlights for cell phones, exterior automotive lighting and printers, have become possi- ble. However, the internal quantum efficiency of GaN-based LEDs is much less than 100% at room temperature because of the presence of non-radiative defects. Furthermore, because the refractive index of the nitride epitaxial layer differs greatly from that of the air, for which the refractive indexes of GaN and air are 2.5 and 1.0, respectively, the critical angle at which light generated in the InGaN–GaN active region can escape, is approximately [θ c = sin -1 (n air /n GaN )] ∼23 ◦ , which limits the external quantum efficiency of conventional GaN-based LEDs to only a few percent [5,6]. The light from LEDs can be enhanced either through the device surface or through the side walls of the device. Research on improving the light extraction efficiency (external quantum efficiency) and brightness in the LEDs [5–12] ∗ Corresponding author. E-mail address: scwang@cc.nctu.edu.tw (S.-C. Wang). has been intense. Recently, Chang et al. reported that cap layers grown at low temperature (800 ◦ C) increased the power output in the InGaN–GaN MQW LEDs by 10% [13]. Fujii reported an increased extraction efficiency in the GaN-based LEDs by surface roughening [12]. These processes all allow the photons generated within the LEDs to find the escape cone, by multiple scattering from a rough surface. We had reported that nano- roughened top surface of an InGaN/GaN LED using Ni clusters as a wet etching mask increased the wall-plug efficiency by 45% [5]. The large improvement in the light output power we have shown indicated that the use of metal clusters to fabricate a roughened p-GaN surface is an excellent means of making a high-power LED. In comparison to the wet etching, the excimer laser etching for p-GaN surfaces have advantages of the easy control and high etching rate. This investigation reported the fabrication of GaN LEDs with nano-roughened p-GaN surfaces using a KrF laser etching and self-assembled Ni metal clus- ters as the laser etching masks. The dimensions and density of the self-assembled Ni clusters can be controlled by the rapid thermal annealing (RTA) at temperatures from 750 to 900 ◦ C. Details of Ni cluster formation have been recently reported [14]. Fig. 1 depicts a schematic diagram of the LED with nano- roughened surface. The light output efficiency of LED with a 0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.09.030