Growth and properties of thin films of the doped infinite-layer compounds Ca_1-xLn_xCuO₂ (Ln=Sm, Nd, Y)

The pulsed laser deposition (PLD) technique has been utilized for thin film synthesis of lanthanide-doped infinite-layer compounds Ca1-xLnxCuO2 (Ln=Sm, Nd, and Y). The films are grown epitaxially on (100)-oriented SrTiO3 substrates in a low pressure oxygen ambient with in situ monitoring using reflection high-energy electron diffraction (RHEED). For Sm3+ and Nd3+ substitutions, the infinite-layer structure of CaCuO2 is retained for x≤0.15, with a small but systematic decrease in the c-axis parameter of the films being observed with increasing dopant concentration. For higher substitution levels, a defect structure analogous to Sr14Cu24O41 is stabilized. Doping of the infinite-layer phase has also been achieved with Y3+ substitution, however the phase transformation to the defect structure occurs at lower concentrations (x<0.10). A significant decrease in the room-temperature resistivity, with metallic temperature dependence, is observed for optimally doped CaCuO2 films after they are post-annealed in vacuum. Nevertheless, no evidence of superconductivity has been detected in any of the films down to 5 K. Possible reasons for the absence of superconductivity in the Ca1-xLnxCuO2 films are discussed. © 1994.