Low resistance Ti/Al/Mo/Au Ohmic contacts for AlGaN/GaN heterostructure field effect transistors

A metallization scheme consisting of Ti/Al/Mo/Au was utilized to develop low-resistance ohmic contacts to AlGaN/GaN heterostructure field effect transistors (HFET). A contact resistance as low as 0.20 Ωmm and a specific contact resistivity as low as 4.5 × 10^-7 Ωcm² were obtained using a pre-metallization surface treatment with SiCl₄ plasma at a self-bias voltage of -300 V in a reactive ion etching (RIE) system. X-ray photoelectron spectroscopy (XPS) measurements of the SiCl₄ plasma-treated surface revealed an increase in the N vacancies thereby increasing the donor concentration at the surface. Also a blue shift of the peak energy of the Ga 3d photoelectrons was observed showing that the Fermi level moved closer to the conduction band at the surface of the AlGaN.