Effect of Annealing Temperature on the Sheet Resistance of AlN/GaN HEMTs

AlN/ GaN high electron mobility transistors (HEMTs) intrinsically have high two-dimensional electron gas (2DEG) densities with good mobility and high breakdown fields, due to both, higher band gap and charge polarization at AlN/ GaN interface. Enabling the use of ultrathin barrier structures, which offers extreme gate scalability for high frequency applications, figure1 shows a schematic layer structure of AlN/GaN wafer involved in this work. As high Al content in the barrier allows such many advantages, relatively high ohmic contact resistance (Rc) and very sensitive barrier are main problems for such devices. Conventional Ti/Al/Ni/Au ohmic contacts are known to exhibit very low Rc, however, it requires annealing at high temperatures (>800 oC) beyond the melting point of Aluminum. This causes the sheet resistance (Rsh) to largely deteriorate especially in GaN HEMTs with AlN barriers, since they have 100% Aluminum content. In this paper, a different metallization stack based on Molybdenum for the ohmic contact will be reported. The two stacks of Mo/Al/Mo/Al/Ti/Al and Mo/Al/Mo/Au, require a much lower temperature down to 550 oC. The conventional Ti/Al/Ni/Au based stack which is annealed at 800 oC results in an increase of Rsh to ~1.1 KΩ/□, while the Mo based stack which is annealed at 550 oC results in a sheet resistance of 377 Ω/□ as shown in table I below. Rc and Rsh values were obtained via circular transmission line measurement (CTLM), while Annealing conditions were optimized to give a good compromise between Rc and Rsh