Comparison of Ti/Au, Ni/Au, and Sc/Au ohmic contact metal stacks on (Al0.18Ga0.82)2O3

Abstract

Three different metal stacks, namely Ti/Au, Ni/Au and Sc/Au, were examined as Ohmic metal contacts to Si-doped, n-type (4.1 × 10¹⁹ cm⁻³), 300-nm thick (Al_0.18Ga_0.82)₂O₃ layers grown by metal-organic chemical vapor deposition. This is a typical composition used for (Al_xGa_1-x)₂O₃ /Ga₂O₃ heterostructure field effect transistors. The effects of postdepositional annealing (300–475 °C) were examined through circular transfer length method (CTLM) measurements to determine both the transfer resistance and specific contact resistivity. The lowest resistances were achieved with Ti/Au, with specific contact resistivity 1.2 × 10^–4 Ω·cm² and transfer resistance 3.82 Ω·mm for as-deposited contacts. Annealing was found to degrade both of these resistances in all cases from the as-deposited values, even though the AGO sheet resistance decreased slightly, from 1191 Ω/□ to 905 Ω/□ after annealing at 475 °C. The temperature dependence of specific contact resistivity is also investigated.