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 electrica l resistivity. For these films, the phase transitions were found to fo llow the sequence: alpha-phase (disordered face centred cubic, fcc, so lid solution); 5 at% Ge --> zeta-phase (disordered hexagonal close pac ked, hcp); 15 at% Ge --> zeta-phase + epsilon(1)-phase (ordered orthor hombic, Cu3Ge); 20 at% Ge --> epsilon(1)-phase; 25 at% Ge --> (epsilon (1)-phase + progressively increasing proportions of a disordered Ge-ri ch solid solution); 30-50 at% Ge. Germanium was found to have no marke d effect on grain size of all films studied excluding grain boundaries as electron scattering centres. Transition of the alpha-phase into th e zeta-phase was found to occur in a highly coherent manner, which cou ld be related to the reduced stacking fault energy of Cu by the additi on of Ge. Most evidence pointed out that the initial increase in resis tivity within the alpha-phase range was related to hcp scattering cent res, which could be associated with a localized high concentration of Ge. At 15 at% Ge, the resistivity reached a maximum value of about 50 mu Omega cm associated with the complete transformation of alpha-phase into the zeta-phase. With continued increase in Ge concentration, the resistivity was found to gradually decrease reaching a minimum value of about 10 mu Omega cm at 25 at % Ge, which was correlated with compl ete transition of the zeta-phase into the ordered epsilon(1)-phase (Cu 3Ge). It was shown that the superlattice of Cu3Ge could directly be de rived from the disordered zeta-phase by minor atom rearrangement on th e [0 0 0 1](hcp) plane. Even though, minor proportions of a Ge-rich so lid solution containing a small concentration of Cu were formed at Ge concentrations above 25 at %, the minimum resistivity of 10 Omega mu c m was maintained as the Ge concentration was increased to 35 at%. Subs equently, the resistivity was increased reaching about 46 mu Omega cm at 50 at% Ge.