We report the results of studies on MAX phases [1,2] in the context of possible applications as thermally stable contact metallizations to GaN. The outstanding properties of these phases, such as good thermal and electrical conductivity, oxidation resistance, damage tolerance and thermal stability even at temperatures as high as 1000^oC [2,3] as well as their hexagonal structure, make these materials promising for such applications.

The properties mentioned above are a result of the nanolaminate hexagonal structure of these compounds [1,2], which consist of: a transition metal (M), an element from group IIIA or IVA (A) and carbon or nitrogen (X). Monocrystalline MAX phase is made of MX monolayers intertwined with monoatomic A layers.

Taking into account the fact, that Ti/Al bilayer yields low resistivity ohmic contacts to n‑GaN [4], our approach was to make contacts out of a Ti-Al-N MAX phase, which would improve the thermal stability of the contact system.

The conducted experiments involved: (1) depositing Ti/Al/TiN multilayers on n‑GaN substrate with subsequent heat treatment in an N₂ or Ar flow in 600^oC; (2) depositing Ti/Al multilayers on n‑GaN substrate followed by heat treatment in a N₂ flow in 600^oC. The resistivity of the metallization films was measured with a four-point probe and the ohmic contact characteristics were inferred from circular transmission line method. The samples’ microstructure was characterised with High Resolution X-Ray Diffractometry and Secondary Ion Mass Spectrometry depth profiling.

This study was partially supported by the EC under the project “Materials for Robust Gallium Nitride” CP-IP 214610-2 MORGaN.

References:

1.    H. Nowotny, Prog. Solid State Chem. 2, 27 (1970).

2.    M.W. Barsoum, Prog. Solid St. Chem. 28, 201 (2000).

3.    J. Emmerlich et al., Acta Materialia, 55, 1479 (2007).

4.    M.E. Lin et al., Appl. Phys. Lett. 64, 1003 (1994).