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 degrees 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 X 10(-7) Omega cm(2) on n-type GaAs with doping concentrations of 1 x 10(17) 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 xi and epsilon(1)-Cu3Ge phases which correlate with the low contact resistivity. The xi and epsilon(1)-Cu3Ge phases have a planar and structurally abrupt interface with the GaAs substrate without any inter-facial 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 xi and epsilon(1)-Cu3Ge phases demonstrate a higher transconductance compared to devices with AuGeNi contacts.