Large photoconductivity and light-induced recovery of the insulator-metal transition in ultrathin La0.7Ce0.3MnO3-δ films

Tetravalent-ion-doped lanthanum manganite films typically suffer from overoxygenation in the as-prepared state, which in turn leads to an effective hole doping instead of the nominal electron doping. This problem can be overcome by post-deposition annealing in a reducing atmosphere, which, however, suppresses the phase transition from an insulating to a metallic phase at the magnetic ordering temperature so that the films are insulating in the whole temperature range. In the present work, reduced La0.7Ce0.3MnO3-delta thin films were investigated with respect to their transport characteristics under photoexcitation. While the films are insulating in the dark, even the exposure to diffuse daylight recovers the insulator-metal transition (IMT). Excitation with continuous visible laser light further decreases the resistance by up to seven orders of magnitude and shifts the IMT to higher temperatures. The spectral, temporal, intensity, and temperature dependences of the photoconductivity have been investigated. The results suggest that (i) the manganite film shows a light-induced IMT and large photoconductivity, (ii) the substrate has an influence on the photoconductivity (through carrier injection into the film and/or substrate photoconduction) that grows with decreasing wavelength of the light, and (iii) an electron-doped metal state might be present under photoexcitation.