Structural analysis of interfacial layers in Ti/Ta/Al ohmic contacts to n-AlGaN

Detailed structure of the interfacial layers of Ti/Ta/Al ohmic contacts to n-type AlGaN/GaN/sapphire are investigated by means of transmission electron microscopy. High-resolution electron microscopy (HREM), optical diffractograms, and computer simulations confirmed that TiN (∼10.0 nm) and ${\mathrm{Ti}}_{\mathrm{3}}\mathrm{AlN}$ (∼1.4 nm) interfacial layers form at the interface between the Ti layer and the ${\mathrm{Al}}_{\mathrm{0.35}}{\mathrm{Ga}}_{\mathrm{0.65}}\mathrm{N}$ substrate by a solid state reaction during annealing for 3 min in ${\mathrm{N}}_{\mathrm{2}}$ at 950 °C. The orientation relationship between ${\mathrm{Ti}}_{\mathrm{3}}\mathrm{AlN}$ and ${\mathrm{Al}}_{\mathrm{0.35}}{\mathrm{Ga}}_{\mathrm{0.65}}\mathrm{N}$ was found to be: ${\text{[011]}}_{{\mathrm{Ti}}_{\mathrm{3}}\mathrm{AlN}}{\text{∥[21̄1̄0]}}_{{\mathrm{Al}}_{\mathrm{0.35}}{\mathrm{Ga}}_{\mathrm{0.65}}\mathrm{N}}$ and ${\text{(11̄1)}}_{{\mathrm{Ti}}_{\mathrm{3}}\mathrm{AlN}}{\text{∥(0001)}}_{{\mathrm{Al}}_{\mathrm{0.35}}{\mathrm{Ga}}_{\mathrm{0.65}}\mathrm{N}}.$ The cubic ${\mathrm{Ti}}_{\mathrm{3}}\mathrm{AlN}$ interfacial layer has a lattice parameter of $\text{0.411±0.003\hspace{0.05em}}\mathrm{nm}$ with the space group $\text{Pm3m}$ matching that of ${\mathrm{Al}}_{\mathrm{0.35}}{\mathrm{Ga}}_{\mathrm{0.65}}\mathrm{N}.$ A model of the atomic configurations of the ${\mathrm{Ti}}_{\mathrm{3}}{\mathrm{AlN/Al}}_{\mathrm{0.35}}{\mathrm{Ga}}_{\mathrm{0.65}}\mathrm{N}$ interface is proposed. This model is supported by a good match between the simulated and the experimental HREM image of the ${\mathrm{Ti}}_{\mathrm{3}}{\mathrm{AlN/Al}}_{\mathrm{0.35}}{\mathrm{Ga}}_{\mathrm{0.65}}\mathrm{N}$ interface. The formation of TiN and ${\mathrm{Ti}}_{\mathrm{3}}\mathrm{AlN}$ interfacial layers appears to be responsible for the onset of the ohmic contact behavior in Ti/Ta/Al contacts.