Paper EM-ThP10
Ohmic Contacts to n- and p-type GaN Based on TaN, TiN, and ZrN

Thursday, October 18, 2007, 5:30 pm, Room 4C

We examined the potential of transition metal nitrides as a diffusion barrier for Ti/Al-based Ohmic contacts to n-GaN and Ni/Au-based Ohmic contacts to p-GaN. The annealing temperature (600-1000°C) dependence of Ohmic contact characteristics using either Ti/Al/X/Ti/Au (n-Ohmics) or Ni/Au/X/Ti/Au (p-Ohmics) metallization schemes, where X is TaN, TiN, or ZrN, deposited by plasma-assisted sputtering were investigated by contact resistance and Auger Electron Spectroscopy (AES) measurements. For n-GaN (n~3x10¹⁷ cm^-3), the as-deposited contacts were rectifying and transitioned to Ohmic behavior for annealing at ≥600°C. A minimum specific contact resistivity of ~6x10^-5Ω.cm^-2 was obtained after annealing over a broad range of temperatures (600-900°C for 60 secs), comparable to that achieved using a conventional Ti/Al/Pt/Au scheme on the same wafer. The contact morphology became considerably rougher at the high end of the annealing range. The long-term reliability of the contacts was also examined-each contact structure showed an increase in contact resistance by a factor of 3-4 over 24 days at 350°C in air. AES profiling showed that long-term aging had little effect on the nitride-based contact structure. For annealing temperatures greater than 500°C, the contacts to p-GaN (p~10¹⁷ cm^-3) displayed Ohmic characteristics and reached a minimum specific contacts resistance of about 2x10^-4Ω.cm² after annealing at 700°C for 60 s. The specific contact resistance was stable on annealing up to at least 1000°C. However, at high temperatures the morphology of the contacts became very rough and a large degree of intermixing between the metallic layers was observed. The thermal stability of these contacts were found to be superior as compared to conventional Ni/Au, which displayed poor characteristics at anneal temperatures greater than 500°C.