Integration and Reliability Aspects of Low-Temperature and Au-free Ta/Al-based Ohmic Contacts for AlGaN/GaN MIS-HEMTs

In this paper, low temperature and gold (Au)-free Ta/Al-based ohmic contacts fabricated by sputtering on AlGaN/GaN heterostructures are demonstrated on 150 mm GaN-on-Si substrates. The Au-free manufacturing process of Ta/Al-based ohmic contacts implemented by sputtering on large area substrates is shown to give comparable results to conventional Ti/AI-based ohmic contacts and therefore to be suitable for the integration of GaN-based devices in the Si technology production lines. Homogeneous contact resistance (R
c
) values with a low mean value of
$1.2\ \Omega$
mm are obtained over the entire substrate using Ta/Al/TaN metal stacks annealed at 550 °C. Low temperature and Au-free Ta/Al/TaN ohmic contacts are also compared to conventional high temperature annealed and Au-containing Ti/AI-based ohmic contacts in terms of sheet resistance (R
sh
) stability before and after passivation. The temperature dependence of R
c
in both metal schemes is then investigated supporting different transport mechanisms and posing novel reliability challenges related to their integration in metal-insulator-semiconductor (MIS)-high electron mobility transistors (HEMTs). For this reason, the two ohmic contact schemes are integrated into MIS-HEMTs which are compared in terms of on-resistance (R
DS, on
) at high-temperature operation.