Critical thickness in LaNiO3 ultrathin films: the influence of underlayer

LaNiO3 ultrathin films were deposited on (001) SrTiO3 substrates by pulsed laser deposition. Transport properties of these LaNiO3 films were studied as a function of film thickness. A metal-insulator transition is observed as the film thickness is less than a critical value, 5 unit cells (u.c.). This transition is due to a thickness-driven carrier localization. The critical thickness can be influenced by inserting a 2 u.c.-thick SrTiO3 underlayer between the LaNiO3 film and the substrate. SrTiO3 underlayers, deposited under a low O-2 pressure, may keep the 5 u.c. LaNiO3 film metallic. However, the critical thickness of LaNiO3 with a SrTiO3 underlayer post-annealed in O-2 at 950 A degrees C is still 5 unit cells. It is proposed that electrons transferred to the LaNiO3 film, induced by oxygen vacancy doping in the underlayer, reduce the critical thickness for carrier localization.