Interface electron transfer and thickness dependent transport characteristics of La_0.7Sr_0.3VO₃ thin films

La_0.7Sr_0.3VO₃ (LSVO) thin films, 5-30 unit cells (u.c.) in thickness, have been epitaxially deposited on (0 0 1) SrTiO₃ (STO) single crystal substrates. Although LSVO is metallic in bulk, insulating behavior is observed, from 2 to 390 K, in LSVO films less than 9 u.c. in thickness, while thicker films show a metal-insulator transition with the critical temperature increasing with the decrease of film thickness. X-ray absorption spectra reveal a charge transfer across the LSVO/STO interface for a continuous increase of V valence in LSVO, as well as a decrease of Ti valence in interfacial STO, with the LSVO film thickness increases. The transport characteristics are discussed in terms of enhanced electron localization due to the reduction of film thickness and V 3d band filling induced by the charge transfer.