Ni/Ag as low resistive ohmic contact to p-type AlGaN for UV LEDs

Reflective (thick) and semi-transparent (thin) Ni/Ag/Ni contacts were prepared on GaInN-based light-emitting diodes (LEDs) and p-GaN/p-Al0.15Ga0.85N layer sequences. A light output power enhancement of 41% and forward voltage reduction of 0.59 V were obtained compared to a Ni/Au contact for LEDs emitting at 400 nm with thick p-GaN contact layers. The specific contact resistance of the Ni/Ag/Ni contacts on p-GaN/p-Al0.15Ga0.85N with varying p-GaN thickness (5-20 nm) were determined by transmission line method and compared to Ni/Au contacts. Low resistive ohmic contacts were obtained for a p-GaN thickness of less than 10 nm. The p-GaN layer can be completely omitted for the reflective Ni/Ag/Ni contact. In addition, reflection and transmission of the Ni/Ag/Ni metallization schemes were investigated in the ultra-violet spectral range. Thick Ni/Ag/Ni and thin Ni/Ag/Ni covered by Al are promising to serve as reflective contacts for ultra-violet LEDs. The former for wavelength around 350 nm and the latter for wavelengths below 350 nm.