Effect of SrTiO₃ oxygen vacancies on the conductivity of LaTiO₃/SrTiO₃ heterostructures

In this work, we report the formation of a conductive layer through oxygen vacancies in an underlying SrTiO₃ (STO) layer due to the growth of LaTiO₃ (LTO) and the resulting LTO thickness-dependent conductivity of the LTO/STO system. Crystalline LTO films were grown by molecular beam epitaxy on TiO₂-terminated STO(001) single-crystal substrates and 8-unit-cell (u.c.) STO template layers grown on Ge(001), under partial pressures of molecular oxygen ranging from 10^-10 to 10^-7 Torr. Film crystallinity was studied by in situ reflection high-energy electron diffraction, ex situ X-ray diffraction, and ex situ transmission electron microscopy. Film composition and the existence of oxygen vacancies were confirmed by in situ X-ray photoelectron spectroscopy. LTO films grown on STO substrates at oxygen partial pressures of 10^-10 Torr were optimally oxidized (1:1:3 La:Ti:O). However, LTO films grown on 8-u.c. templates of STO on Ge with oxygen partial pressures less than 10^-7 Torr showed extensive reduction of the Ti oxide and desorption of Sr/SrO in the STO layer. LTO films began to over-oxidize when grown on STO single-crystal substrates at oxygen partial pressures greater than 10^-10 Torr but were nearly optimally oxidized when grown on STO templates on Ge at oxygen partial pressures of 10^-7 Torr. Electrical characterization showed a dependence of conductivity on the thickness of the LTO films, with sheet carrier densities reaching ∼5 × 10¹⁶ cm^-2 for 20-u.c. (8-nm-thick) LTO/STO grown at 10^-10 Torr of oxygen, suggesting that significant conduction occurred throughout the STO substrate due to the formation of oxygen vacancies.