Fabrication and Performance of Ti/Al/Ni/TiN Au-Free Ohmic Contacts for Undoped AlGaN/GaN HEMT

Abstract

We proposed a preparation method of a TiN capping layer compatible with the ohmic contacts process, and demonstrated the Au-free ohmic contacts of an undoped AlGaN/GaN high-electron-mobility transistor (HEMT) with a Ti/Al/Ni/TiN metal structure. TiN was prepared through depositing Ti thin film by a sputtering system and then annealing in N2 ambient by the rapid thermal annealing process. The thickness of Ti/Al, annealing temperature, and annealing time were investigated systematically. Using the Ti/Al/Ni/TiN structure, a low contact resistance ( $3.47 \times 10^{-5}\,\,\Omega $ cm2, $1.1~\Omega \cdot mm$ ) was obtained when annealed at 900 °C for 30 s in N2 ambient, which was comparable with conventional Au-based ohmic contacts ( $3.12 \times 10^{-5} \Omega \cdot cm^{2}$ , $1.05~\Omega \cdot mm$ ). In addition, the Ti/Al/Ni/TiN ohmic contacts showed smooth surface morphology with a surface roughness of 5.89 nm. AlGaN/GaN HEMT, based on Ti/Al/Ni/TiN Au-free ohmic contacts, was also fabricated and exhibited good dc characteristics. The reported Au-free AlGaN/GaN HEMT fabrication process can be used in standard Si fabs without the risk of contamination.