Formation of Ta/Ti/Al/Mo/Au ohmic contacts to an AlGaN/AlN/GaN heterostructure grown on a silicon substrate

The features of the formation of Ta/Ti/Al/Mo/Au ohmic contacts to a Al$_{0.26}$Ga$_{0.74}$N/AlN/GaN heterostructure grown on semi-insulating Si(111) substrates are studied. The dependences of the contact resistance on the Al (90, 120, 150, 180 nm) and Ti (15, 30 nm) layer thickness and optimal temperature-time annealing conditions are determined for each studied metallization scheme. It is shown that the minimum achievable contact resistance monotonically increases from 0.43 to 0.58 $\Omega$ mm as the Al layer thickness increases from 90 to 180 nm at unchanged Ta, Ti, Mo, Au layer thicknesses. A change in the Ti layer thickness from 15 to 30 nm has no significant effect on the minimum contact resistance. The least contact resistance of 0.4 $\Omega$ mm is achieved for Ta/Ti/Al/Mo/Au layers with thicknesses of 10/15/90/40/25 nm, respectively. The optimal annealing temperature for this metallization variant is 825$^\circ$C at a process duration of 30 s. The grown ohmic contacts have smooth contact-area edges and flat morphology of their surface.