P. R. Wang

A Screen-Printed Sn-Based Substrate Technology for the Fabrication of Vertical-Structured GaN-Based Light-Emitting Diodes

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.

The Preparation of SiO 2 Nanotubes with Controllable inner/outer Diameter and Length using Hydrothermally Grown ZnO Nanowires

Diameter and Length using Hydrothermally Grown ZnO Nanowires Der-Ming Kuo, Shui-Jinn Wang, Kai-Ming Uang, Wei-Chih Tsai, Wen-I Hsu, Wei-Chi Lee, Pei-Ren Wang, Chih-Ren Tseng Institute of Microelectronics, Dept. of Electrical Eng., National Cheng Kung Univ., Tainan, Taiwan Department of Electrical Engineering, WuFeng Institute of Technology, Chia-yi, Taiwan *Phone: +886-6-2757575-62351, Fax: +886-6-2763882, E-mail: sjwang@mail.ncku.edu.tw

A Screen Printed Sn-based Dicing-Free Metal Substrate Technology for the Fabrication of Vertical-Structured GaN-based Light-Emitting Diodes

A Screen Printed Sn-Based Dicing-Free Metal Substrate Technology for the Fabrication of Vertical-Structured GaN-Based Light-Emitting Diodes Pei-Ren Wang , Po-Hung Wang, Hon-Yi Kuo, Kai-Ming Uang, Tron-Min Chen, Der-Ming Kuo, and Shui-Jinn Wang Institute of Microelectronics, Dept. of Electrical Eng., National Cheng Kung Univ., Tainan, Taiwan Dept. of Electrical Eng., Wu Feng Institute of Technology, Chia-yi, Taiwan *Phone: +886-6-2757575-62351, Fax: +886-6-2763882, E-mail: sjwang@mail.ncku.edu.tw

A Fast Sapphire Substrate Surface Roughening Technology using CO 2 Laser for Enhancing Light Extraction of GaN-Based Flip-Chip Light-Emitting Diodes

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 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 CO2 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 CO2 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 sequence onto the p-GaN layer, and bonding pads were formed on the TCL and n-type GaN. In this stage, 300 ×300 μm regular top-emitting GaN-LEDs were fabricated (Fig. 1(a)). Afterwards, CO2 laser (Universal Laser system Inc./M300) with wavelength of 10.5 μm was used to roughen the surface of sapphire substrate [Fig. 1(b)]. The laser was focused on the surface of sapphire, and the irradiation power and scanning speed used in this work were 30% and 50% respectively [8]. During the process, most CO2 laser would induce thermal decomposition of Al2O3 on sapphire surface, and this would give rise to surface roughening [9]. After a surface clean in ultrasonic acetone bath, the fabrication for the LR-LEDs was completed (Fig. 1(c)). It’s worth mentioning here, in present work, roughening a 2” wafer was done within 5 min. Note that, top-emitting GaN-LEDs (namely regular LEDs) were also fabricated on the same wafer simultaneously for comparisons. 3. Results and discussion The SEM images of the surface morphology of the samples at some specific processing stages were shown in Fig. 2. Figure 2(a) shows the surface of the sapphire substrate before roughening with CO2 laser irradiation. A flat and even surface was obtained. Figure 2(b) shows the surface of the sapphire substrate after roughened and Fig. 2(c) shows the magnification of Fig. 2(b). As is evident from the figure, popcorn-like nano-posts formed on the sapphire surface. Figure 2(d) shows the cross-sectional view of sapphire substrate. It is noted that a few micro-cracks appeared on the surface of the sapphire substrate. The average penetration depth of these micro-cracks from the surface of sapphire substrate is about 10 μm. The comparisons of the typical current-voltage (I-V) characteristics of LR-LEDs and regular LEDs were shown in Fig. 3. At 20 mA, the forward voltages (VF) of LR-LEDs and regular LEDs have the same value of 3.2 V. The inset shows the reverse I-V characteristics. No noticeable difference in VF and comparably good reverse characteristics reveal that the proposed surface roughening technology using CO2 laser irradiation did not affect electrical performance of devices. Figure 4 shows the comparison of typical light output power-current (Lop-I) characteristics of LR-LEDs and regular LEDs. At 20 mA, it is seen that a considerable improvement in Lop by 40.2% (i.e., Extended Abstracts of the 2008 International Conference on Solid State Devices and Materials, Tsukuba, 2008,

Use of Current-Blocking Layer to Enhance Performance of Vertical GaN-Based Light-Emitting Diodes with a Ni-Plating Substrate

A great progress in display technology and solid-state lighting sources has been made by the use of GaN-based light-emitting diodes (LEDs) [1-2]. Above all, the improvement in efficiency and light power is important features for LEDs to promote the new era applications in photonics. Nevertheless, current spreading is still an imperative challenge for GaN-based LEDs due to p-GaN layer of relatively lower doping concentration and lateral-conducting structure using insulated sapphire. To alleviate the current crowding effect (CCE) and/or enhance the external quantum efficiency of conventionally lateral GaN-LEDs, efforts have been made by means of flip-chip technique [1], vertical conducting structure [3], or surface texturing [4], etc. Recently, the authors’ group has developed Vertical-structure Metallic-substrate GaN-based Light-Emitting Diodes named as VM-LEDs [5-6], as shown in Fig. 1(a). Such an n-side-up vertical structure shows a significant alleviation for thermal and CCE of lateral LEDs. However, a substantial increase in efficiency must be achieved for competing with conventional lighting sources. Furthermore, a portion of injected carriers will be confined under the contact pad owing to its shortest current path. Accordingly, a considerable amount of generated light will be absorbed or reflected by the opaque metal pad. To solve this problem, a selective activation of p-GaN layer [7] or an insulating SiO2 current blocking layer (CBL) [8] under the p-pad electrode was adopted for the lateral-conducting GaN-based LEDs. To further enhance the light output of regular VM-LEDs, we developed a CBL scheme for vertical GaN-based LEDs. In this work, the VM-LEDs with SiO2 film as a CBL were fabricated by patterned LLO and Ni-plating technologies. The effectiveness of the CBL for vertical-structured GaN LEDs was also simulated and examined.