A dicing-free substrate technology was proposed and demonstrated to simplify the fabrication of vertical-structured metal substrate GaN-based light-emitting diodes (VM-LEDs) using a Sn-based solder screen printing technique with patterned laser lift-off technology. As compared with conventional sapphire substrate GaN-based LEDs, VM-LEDs with an effective emission area of 1000×1000 µm2 were found to have a 0.38 (0.87) V reduction in forward voltage at 350 (700) mA. In addition, their enhancement in light output power in the current range of 350–700 mA was found to successively increase from 55 to 76%. By considering these results, the power conversion efficiency of VM-LEDs was found to be 2.14 times that of regular LEDs at 700 mA.

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Introduction Continuous efforts have been made to promote GaN-based LEDs as a boost to white-light LEDs in the applications of flashlight, backlight source of liquid crystal display television, and even solid-state lighting [1], [2]. Among these, GaN-based flip-chip LED (FC-LEDs) is very attractive for high brightness (HB) applications because of better thermal management and the relatively large critical angle for light extraction up to 33.8 o as compared with those top-emitting structures on sapphire substrates (i.e., light emitted from the GaN/air interface) [3]. Recently, attempts to further improve the light extraction efficiency of FC-LEDs by means of chemical wet etching, focus ion beam (FIB), and inductively coupled plasma (ICP) dry etching techniques to roughen sapphire substrates and increase surface scattering have been reported and significant advancements have been demonstrated [4]-[5]. Nevertheless, these techniques were time-consuming (e.g. 20−30 min using ICP and ~10 hrs using chemical wet etching [4]-[5]) or not cost-effective [6]. In this work, we reported a fast surface roughening technology using CO 2 laser irradiation on sapphire substrate for light extraction enhancement of FC-LEDs. Through the control of the irradiation power and scanning speed, the surface of sapphire substrate could be uniformly roughened within 5 min for a 2" wafer. Advantages include simplicity and fast fabrication, and cost effectiveness were demonstrated. Superior electrical and optical characteristics of the fabricated laser-roughening LEDs (LR-LEDs) were reported and investigated as well. 2. Sample preparation The GaN wafer used in this work were epitaxially grown on 2 inch c-plane 400-μm-thick sapphire substrates by metal-organic chemical vapor deposition (MOCVD). For the detailed epitaxial structures, please refer to ref.7. A schematic drawing illustrating the fabrication processes of the proposed surface roughening technology using CO 2 laser is shown in Fig. 1. The p-GaN was partially etched until the n-type GaN layer as exposed using an inductively coupled plasma (ICP) etching system. Subsequently, the deposition/oxidization of Ni/Au layer structure and deposition of indium zinc oxide (IZO) film as the transparent conductive layer were carried out in

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