Interfacial AlN formation of Si/Ti/Al/Cu Ohmic contact for AlGaN/GaN high-electron-mobility transistors

We report on the electrical and microstructural analysis on Si/Ti/Al/Cu Ohmic contact for AlGaN/GaN high-electron-mobility transistors grown on Si (111) substrate. With optimized Si and Ti thickness in Si/Ti/Al/Cu Ohmic metallization, a minimum Ohmic contact resistance of 0.44 Ω mm and specific contact resistance of 3 × 10^{- 6} -cm² with smooth surface morphology were achieved. Significant change in electrical performance and morphology showed high dependence of Ti and Si thickness on the multilayer metal scheme. Importantly, refractory behavior was shown in low annealing temperature, though uniform and continuous TiSi_x with low work function was formed on AlGaN. However, Ohmic behavior was shown in high annealing temperature, because thin AIN surrounding TiN promotes further N vacancies in GaN than the conventional Ti/Al-based Ohmic contact does. An outer Cu layer has low resistivity and the interfacial Si layer forms TiSi_x, which works as only a barrier to prevent Cu in-diffusion, not to transport current. As a result, we revealed that Ohmic contact mechanism in Si/Ti/Al/Cu is governed mainly by field emission near the Fermi level or themionic-field emission. Microstructural study on metal/semiconductor interface region was conducted by using transmission electron microscopy (TEM).