Electrical and microstructural characteristics of Ge/Cu ohmic contacts to n-type GaAs

It is shown 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 15 to 40 at. %. The contacts exhibit a specific contact resistivity of 7 × 10⁻⁷ Ω cm² on n-type GaAs with doping concentrations of 1 × 10¹⁷ cm⁻³. The contact resistivity is unaffected by varying the Ge concentration in the range studied and is not influenced by the deposition sequence of the Cu and Ge layers. Cross-sectional high-resolution transmission electron microscopy results show that the addition of Ge to Cu in this concentration range causes Cu to react only with Ge forming the ξ and ε₁–Cu₃Ge phases which correlate with the low contact resistivity. The ξ and ε₁–Cu₃Ge phases have a planar and structurally abrupt interface with the GaAs substrate without any interfacial transition layer. It is suggested that Ge is incorporated into the GaAs as an n-type impurity creating a highly doped n⁺-GaAs surface layer which is responsible for the ohmic behavior. n-channel GaAs metal-semiconductor field-effect transistors using ohmic contacts formed with the ξ and ε₁–Cu₃Ge phases demonstrate a higher transconductance compared to devices with AuGeNi contacts.