Study of contact formation in AlGaN/GaN heterostructures

The contact formation of Ti/Al and Ti metallization on AlGaN/GaN heterojunction field effect transistors (HFET) was investigated. It was found that ohmic contact formation is related to the low work function of the Ti contacting layer and the formation of a TiN phase at the Ti/nitride interface. Contact resistance as low as 1 Ω mm or less can be obtained on HFET samples with a $n_s\mu$ product of ${\text{∼0.8×10}}^{16}/\mathrm{V\hspace{0.17em}s}$ and on $n$ -GaN with a carrier concentration of ${\text{1.5×10}}^{18}/{\mathrm{cm}}^{\mathrm{3}}.$ Ti/Al bilayer contact scheme is superior to Ti-only contact due to a surface ${\mathrm{Al}}_{\mathrm{3}}\mathrm{Ti}$ layer in the bilayer contact, which may reduce the oxidation problem when annealed in ${\mathrm{N}}_{\mathrm{2}}$ at high temperatures. Preannealing the HFET samples at 850 °C for 1 h in ${\mathrm{N}}_{\mathrm{2}}$ appears to improve the ohmic contact in general, but not always observed. Our results indicate that Ti/Al contact scheme yields sufficiently low contact resistance on HFET structures for microwave applications.