Electrical and structural investigations of Ag-based Ohmic contacts for InAlAs∕InGaAs∕InP high electron mobility transistors

$\mathrm{Ge}∕\mathrm{Ag}∕\mathrm{Ni}$ and $\mathrm{Au}\mathrm{Ge}∕\mathrm{Ni}∕\mathrm{Au}$ Ohmic contacts on $\mathrm{In}\mathrm{Al}\mathrm{As}∕\mathrm{In}\mathrm{Ga}\mathrm{As}∕\mathrm{In}\mathrm{P}$ high electron mobility transistors with excellent contact resistance of $0.07\Omega \mathrm{mm}$ were obtained after annealing at 425 and $265°\mathrm{C}$ , respectively. The Ag-based contacts have a large processing window of $>130°\mathrm{C}$ . Structural analyses confirm that Ag and Au protrusions created during annealing effectively linked the two-dimensional electron gas layer with the metal contacts to produce excellent Ohmic characteristics. The formation of liquid AuGe eutectic phase in $\mathrm{Au}\mathrm{Ge}∕\mathrm{Ni}∕\mathrm{Au}$ at $300°\mathrm{C}$ is believed to cause overannealing. The eutectic temperature of Ag–Ge is $\sim 300°\mathrm{C}$ higher leading to a higher optimum annealing temperature and a wider processing window for the $\mathrm{Ge}∕\mathrm{Ag}∕\mathrm{Ni}$ contacts.