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 degrees C, 500 degrees C, 600 degrees C, and 700 degrees 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 x 10(-4) Omega.cm, and a root mean square surface roughness of 2.49 nm, was obtained from the sample annealed at 500 degrees C. Then, CF(4) 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 x 10(-3) Omega.cm(2) from 9.07 x 10(-3) Omega.cm(2) via the CF(4) 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.