Low Au content thermally stable NiGe(Au)W ohmic contacts to n-type GaAs

Thermally stable low-resistance ohmic contacts to n-type GaAs incorporating a very thin layer of Au in conjunction with a layered Ni/Ge/W structure are reported. A minimum contact resistance of 0.16 Ω mm was obtained for contacts annealed at ∼650°C. The contact resistance was ∼0.3 Ω mn after thermal stressing at 400°C for 20 h. Cross-sectional transmission electron microscopy reveals a uniformly reacted layer only ∼34 nm deep, making these contacts significantly shallower and more homogeneous than eutectic-based AuGeNi contacts. X-ray diffraction shows the presence of NiGe, β-AuGa, and W phases in the reacted contacts. The volume fraction of the low melting point β-AuGa phase is considerably reduced from that reported for eutectic-based AuGeNi contacts. This, along with the presence of the high melting point NiGe compound, explains in part the improved thermal stability and morphology of the low Au content ohmic contacts.