Precise control of LaTiO3(110) film growth by molecular beam epitaxy and surface termination of the polar film

Abstract

Transition metal oxides exhibit abundant physical properties due to the electronic interactions between charge, orbit and spin degrees of freedom. Lanthanum titanate, LaTiO3, a typical strongly correlated electron material, shows Mott-type metal-insulator and antiferromagnetic transitions at low temperature. And these interesting behaviors can be tuned by adjusting the occupation of the t2g orbit of Ti3+, or introducing symmetry breaking or lattice strain into the heterointerfaces. Especially on LaTiO3(110) surface, the anisotropic structure as well as the surface polarity allows the flexible control of artificial low-dimensional structure. However, the instability induced by surface polarity hinders the growth of high-quality LaTiO3(110) film. Here we show that by keeping the growing surface reconstructed in the molecular beam epitaxy (MBE) process, the surface polarity can be effectively compensated for, allowing the high-quality layer-by-layer film growth. Moreover, the intensity of reflective high-energy electron diffraction (RHEED) pattern sensitively changes with the surface cation concentration. Therefore the relative deposition rates of La and Ti sources can be monitored and further be precisely calibrated in situ and in real-time. We first prepare the (2× 16) reconstruction on SrTiO3(110) surface by depositing La and Ti (2 ML for each) metals. Further increasing the Ti concentration on (2×16), i. e., the [Ti]/[La] ratio, results in the significant decrease of RHEED “1×” intensity and the increase of “2×” intensity. And the change of RHEED intensity is quantitatively reversible through reducing the [Ti]/[La] ratio by the same amount. We set the evaporation rate of Ti source to be slightly higher than that of La for the MBE film growth. And the shutter state of Ti source is controlled to be open or close, which is determined by the change of RHEED intensity. Precise cation stoichiometry is achieved in the LaTiO3(110) film. X-ray diffraction confirms the single crystallinity of the film while scanning tunneling microscope images indicate the atomically flat surface with (2×16) reconstruction that is responsible for the stabilization of the polar surface. The annealing of the sample in oxygen at 700 ℃ will oxidize the LaTiO3 film into the thermodynamically stable phase, i. e. , La2Ti2O7, although the as-grown LaTiO3 phase can be stable at room temperature. The high-resolution STM images reveal the detailed structural information of the (2×16) film surface–along the [001] direction, the tilt of TiO6 octahedron in LaTiO3 lattice results in the “2×” periodicity modulation on the (110) surface. The “×16” periodicity along [110] might be related to the rotation of TiO6 octahedron in (001) plane or to the strain relief on the surface. Both of the RHEED and STM observations indicate that the film surface is terminated by the TiO6 octahedron, i. e., the (O2) atom layer. Indeed the LaTiO3(110) polar surface can be stabilized by making two holes on the (O2) layer by oxidizing Ti3+ into Ti4+. On the contrary, due to the Coulomb repulsion between electrons on Ti3+ 3d orbit, the (110) surface is difficult to reduce (to introduce extra electrons). Therefore the (LaTiO) termination layer cannot be stable.