Interfacial reactions of titanium/gold ohmic contacts with Sn-doped β-Ga₂O₃

Here we investigated interfacial reactions and interdiffusion of titanium/gold ohmic contacts with a tin-doped single-crystal β-Ga2O3 (010) substrate. After annealing at 470 °C for 1 min in N2 to form an ohmic contact, we studied the interface via scanning transmission electron microscopy and transmission electron microscopy with energy dispersive X-ray spectroscopy as well as electron energy loss spectroscopy. At the interface, annealing causes Ti to diffuse and oxidize, reducing Ga2O3 at the interface. This forms a defective β-Ga2O3 layer of 3-5 nm that has a relatively high Ti concentration. Above this is a 3-5 nm layer of Ti-TiOx that is partially lattice matched to the β-Ga2O3 substrate. The thermodynamic favorability of these redox reactions was explained by calculating Gibbs free energies of the reactions. In addition, the anneal causes interdiffusion of Ti and Au, until Au is in contact with the thin Ti-TiOx layer. A layer of Ti-rich nanocrystals, around 5 nm in diameter, is formed within the Au-Ti intermixed matrix, about 3 nm above the Ti-TiOx layer. Based on these observations, the ohmic properties are tentatively attributed to the interdiffusion of Ti and Au and the resulting thin Ti-TiOx layer, which helps band alignment. In addition, lattice matching of the defective Ga2O3 and Ti-TiOx layers to β-Ga2O3 facilitates the transport of carriers. A physical understanding of Ti/Au metallization can provide insights into future materials selection for thermally stable contacts in β-Ga2O3 power devices.