Cu₃Ge ohmic contacts to n-type GaAs

We show that Cu-Ge alloys prepared by depositing sequentially Cu and Ge layers onto GaAs substrates at room temperature followed by annealing at 400°C form a low-resistance ohmic contact to n-type GaAs over a wide range of Ge concentration that extends from 20 to 40 at.%. A contact resistivity of (4-6)×10^-7Ωcm² is obtained on n-type GaAs with doping concentrations of ∼1×10¹⁷ cm^-3. The contact resistivity is affected only slightly by varying the Ge concentration in the range studied and is not influenced by the deposition sequence of the Cu and Ge layers. In addition, the contacts are electrically stable during annealing at 450°C after contact formation. Structure and properties of Cu-Ge contact layers having lower and higher Ge concentrations from the stoichiometric Cu₃Ge composition are compared. High-resolution transmission electron microscopy and x-ray diffractometry have been used to study the ordering in the ε₁-Cu₃Ge (average lattice parameters: a_o = 5.30Å, b_o = 4.20Å, c_o = 4.56Å) which is responsible for orthorhombic distortion of the parent hexagonal ζ-phase. The results suggest that the formation of the ζ and ε₁-Cu₃Ge phases creates a highly doped n⁺-GaAs surface layer which leads to the low contact resistivity.