Low Resistance and Highly Reflective Sb-Doped SnO 2 Õ Ag Ohmic Contacts to p-Type GaN for Flip-Chip LEDs Dong-Seok Leem, a June-O Song, a Hyun-Gi Hong, a J. S. Kwak, b Y. Park, b and Tae-Yeon Seong c,z a Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea b Photonics Lab., Samsung Advanced Institute of Technology, Suwon 440-600, Korea c Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea We have investigated high-quality Sb-doped SnO 2 /Ag ohmic contacts to p-GaN for use in flip-chip light emitting diodes LEDs. The Sb-doped SnO 2 /Ag contacts produce specific contact resistances of 10 -4  cm 2 upon annealing at 430 and 530°C for 1 min in air. It is shown that InGaN blue LEDs fabricated with the Sb-doped SnO 2 /Ag contacts give a forward-bias voltage of 3.18 V at 20 mA, while LEDs with Ag contacts show 3.36 V. It is further shown that the LEDs made with the Sb-doped SnO 2 /Ag contact layers show higher light output power compared with the LEDs with the Ag contacts. © 2004 The Electrochemical Society. DOI: 10.1149/1.1789853 All rights reserved. Manuscript submitted December 29, 2003; revised manuscript received March 11, 2004. Available electronically September 7, 2004. Group III-nitride semiconductors have been extensively investi- gated because of their technological importance for the fabrication of light emitting diodes LEDs, laser diodes, and photo-detectors. 1-3 In particular, GaN-based LEDs are important for practical applica- tions such as solid-state lighting and full-color displays. 4,5 Commer- cially available GaN-based LEDs, however, have some limits to the application for solid-state lighting. For commercial top-emitting GaN-based LEDs grown on sapphire substrates, light is extracted through a semi-transparent current spreading layer on the top of p-GaN, which limits the light extraction efficiency. 4 To overcome this problem, flip-chip LEDs have recently been introduced. 6,7 In this configuration, LEDs are fabricated with highly reflective metals and light is extracted through the transparent sapphire substrate rather than a partially absorbing current spreading layer. Ag and Al are the most commonly used reflectors for flip-chip LEDs, although Ag was shown to be better than Al due to its higher reflectivity and better ohmic behavior. 4,8 However, Ag suffers from poor adhesion to p-GaN and agglomeration upon annealing, which degrade its elec- trical and thermal behaviors. 4,8,9 Thus, recently, multilayer contacts consisting of a thin oxidized Ni/Au bilayer overcoated with a thick Al or Ag layer were investigated to minimize the drawbacks of single Ag contacts. 8 However, because Au itself absorbs and scatters the extracted light, and contributes to the lowering of light output, Au-free contact layers have been investigated. For example, Gess- mann et al., 10 investigating an ITO/Ag contact to p-GaN, showed that LEDs with the new scheme produces significantly improved output power compared to that with the Ni/Au contact. Therefore, in this work, we have also investigated a transparent conducting oxide Sb-doped SnO 2 ) -based contact to p-GaN. The Sb-doped SnO 2 layer was employed as a first layer because of its high transmittance of about 95% at 460 nm. The Sb-doped SnO 2 /Ag contacts produce a good ohmic contact with specific contact resistances of 10 -4  cm 2 when annealed at 430 and 530°C in air ambient. The electrical and optical performances of LEDs fabricated with the annealed Sb- doped SnO 2 / Ag contacts are characterized and compared to those of LEDs with the annealed Ag contact. Metallorganic chemical vapor deposition grown 1.0 m thick p-GaN layers (5  10 17 cm -3  were ultrasonically degreased using trichloroethylene, acetone, methanol, and DI water for 5 min in each step, followed by N 2 blowing. Prior to lithography, the samples were treated with a buffered oxide etch BOE solution for 20 min and rinsed in DI water. Circular transfer length method CTLM patterns were defined by the standard photolithographic technique for mea- suring specific contact resistance. 11 The outer dot radius was fixed to be 75 m and the spacing between the inner and outer radii varied from 4 to 24 m. The Sb-doped SnO 2 films referred to here as SnO 2 :Sb) were deposited by pulsed laser ablation of a 1 atom % Sb-doped SnO 2 target, using a KrF 248 nm Excimer laser. Ag films were then deposited on the SnO 2 :Sb layer by electron-beam evaporation. For comparison, a single Ag contact layer was also deposited on p -GaN. Some of the samples were then rapid-thermal- annealed at 430 and 530°C for 1 min in air ambient. Current-voltage I-V measurements were performed using a parameter analyzer HP 4155A. Auger electron spectroscopy AES was carried out using a PHI 670 Auger microscope with an electron beam of 10 keV and 0.0236 A. The film morphologies of the samples were character- ized by scanning electron microscopy SEM, Hitachi S-4700. Fur- thermore, to evaluate if the Sb-doped SnO 2 /Ag scheme is suitable for GaN-based LEDs, InGaN/GaN multiple-quantum-well MQW blue LEDs were fabricated and examined. Figure 1 shows the typical I-V characteristics of the SnO 2 :Sb 5 nm/Ag 175 nm and Ag 175 nm contacts as a function of the annealing temperature, measured between the metal pads with a spacing of 4 m. The as-deposited SnO 2 :Sb/Ag contact revealed rectifying behavior not shown due to the insulating nature of the as-deposited SnO 2 :Sb. However, annealing the samples at tempera- z E-mail: tyseong@gist.ac.kr Figure 1. Typical I-V characteristics of Sb-doped SnO 2 /Ag and Ag contacts as a function of the annealing temperature. Electrochemical and Solid-State Letters, 7 10 G219-G221 2004 1099-0062/2004/710/G219/3/$7.00 © The Electrochemical Society, Inc. G219