Effect of phase transitions in copper-germanium thin film alloys on their electrical resistivity

An investigation was carried out to study the phase transitions in Cu-Ge thin films (80-200 nm in thickness) containing 0, 5, 15, 20, 25, 30, 35, 40, 45 and 50 at% Ge, and the corresponding effects on electrical resistivity. For these films, the phase transitions were found to follow the sequence: α-phase (disordered face centred cubic, fcc, solid solution); 5 at% Ge → ζ-phase (disordered hexagonal close packed, hcp); 15 at% Ge → ζ-phase + e{open}₁-phase (ordered orthorhombic, Cu₃Ge); 20 at% Ge → e{open}₁-phase; 25 at% Ge → (e{open}₁-phase + progressively increasing proportions of a disordered Ge-rich solid solution); 30-50 at% Ge. Germanium was found to have no marked effect on grain size of all films studied excluding grain boundaries as electron scattering centres. Transition of the α-phase into the ζ-phase was found to occur in a highly coherent manner, which could be related to the reduced stacking fault energy of Cu by the addition of Ge. Most evidence pointed out that the initial increase in resistivity within the α-phase range was related to hcp scattering centres, which could be associated with a localized high concentration of Ge. At 15 at% Ge, the resistivity reached a maximum value of about 50 μΩ cm associated with the complete transformation of α-phase into the ζ-phase. With continued increase in Ge concentration, the resistivity was found to gradually decrease reaching a minimum value of about 10 μΩ at 25 at% Ge, which was correlated with complete transition of the ζ-phase into the ordered e{open}₁-phase (Cu₃Ge). It was shown that the superlattice of Cu₃Ge could directly be derived from the disordered ζ-phase by minor atom rearrangement on the [0 0 0 1]_hcp plane. Even though, minor proportions of a Ge-rich solid solution containing a small concentration of Cu were formed at Ge concentrations above 25 at%, the minimum resistivity of 10 Ωμ cm was maintained as the Ge concentration was increased to 35 at%. Subsequently, the resistivity was increased reaching about 46 μΩ cm at 50 at% Ge.