Improved Electrical and Optical Properties of Vertical GaN LEDs Using Fluorine-Doped ITO/Al Ohmic Reflectors

In this paper, we report on the effects of indium-doped tin oxide (ITO) film surface conditions on the electrical and optical properties of ITO/Aluminium(Al) ohmic reflectors used in vertical GaN light-emitting diodes (LEDs). First, four ITO films annealed at 400°C, 500°C, 600°C, and 700°C for 5 min by a rapid thermal process were characterized; the best performance, with a transmittance of 91% at 460 nm, a resistivity of 8.2 × 10^-4 Ω·cm, and a root mean square surface roughness of 2.49 nm, was obtained from the sample annealed at 500°C. Then, CF₄ plasma was applied to the surface of the ITO before the Al deposition, not only to clean the ITO surface, and thereby increase the reflectance of the ITO/Al, but also to reduce the contact resistivity by preventing the Al-O bonding and inducing an Al-F bonding. The ITO work function is effectively increased by fluorine doping, which eventually reduces the Schottky barrier height between the ITO and the p-GaN. As a result, the reflectance of the ITO/Al increased up to 85% from 71% at 460 nm; its contact resistivity was down to 2.03 × 10^-3 Ω·cm² from 9.07 × 10^-3 Ω·cm² via the CF₄ plasma treatment. Finally, we applied the fluorinated ITO/Al metal to vertical GaN LEDs, which enabled the light output power to be enhanced by 72% at 60 mA compared with those with nontreated ITO/Al reflectors.