Which 3-layer is best? A study of ohmic contacts

Researchers from RMIT, La Trobe and Griffith Universities have worked together to create improved electrical contacts for high power devices using silicon carbide (SiC). The results of this close collaborative project have been published in Microelectronic Engineering.

Electrode pattern based on the two-contact circular test structure used in the measurement of contact resistance. The spacing in the gap between the inner circular and outer ring electrodes was (a) 7.5 μm, (b) 4.5 μm and (c) 1.5 μm.

The wide band-gap semiconductor SiC has shown attractive properties in devices operating at high temperature and high power. In particular, the 3C polytype of SiC has shown the unique ability for growth of epitaxial layers on Si substrates, which is of particular interest in making commercially viable devices that integrate with mature silicon wafer technology for applications in MEMS, photonics and thin membranes.

In the fabrication of devices using 3C-SiC, a critical requirement has been the formation of ohmic contacts with low diffusion at the metal/SiC interface. This project systematically examined—for the first time—the role of Cr in both the Au/Ni/Cr and Au/Cr/Ni 3-layer ohmic contact systems to n-type 3C-SiC as a function of annealing temperature. The team identified the effect of the 3-layered metal contact structure in controlling contact resistance of the metal to SiC interface. In n-SiC/Ni/Cr/Au contacts, the properties improved with higher anneal temperatures. The result show that using Cr as a barrier prevents the undesirable excessive diffusion of metal into the semiconductor.

Specific contact resistance, ρc, versus anneal temperature for Cr/Ni/Au and Ni/Cr/Au contacts to n-type 3C-SiC

“For this project, the film of SiC needed to be very highly doped to increase the conductivity while remaining a single crystal layer. The supply of the high quality films of SiC by Philip Tanner was essential for the project. The ANFF-Q epitaxial SiC production reactor was used for growth of the high quality wafers of SiC film. This reactor for the epitaxial growth of SiC on wafers has been developed at ANFF-Q’s facility by Leonie Hold and Alan Iacopi. This facility provides world-leading SiC on Si materials,” said Patrick Leech, lead author of this paper.

“The project is part of an ongoing interaction with ANFF-Q to develop metal/semiconductor contacts with lower contact resistance that withstand higher power levels and use less energy. We plan to develop devices with longer lifetimes, by taking advantage of the many superior properties of SiC.”