We have fabricated (La, Sr)MnO3 thin films on n-type SrTiO3 substrates doped with Nb by means of the pulsed laser deposition technique. It is shown that hole carriers photogenerated in the substrates under UV light irradiation are injected efficiently to the. lms. By measuring the out-of-plane voltage of the heterojunction the surface density of hole carriers injected to (La, Sr)MnO3 is estimated: It is at most 1.6 x 10(13) cm(-2), but is improved up to 3.1 x 10(13) cm(-2) by modifying the junction structure, i.e., inserting a pure SrTiO3 layer between the film and the substrate. (C) 2004 Elsevier B.V. All rights reserved.