LaTiO3(110)薄膜分子束外延生长的精确控制和表面截止层的研究

LaTiO₃ 是一种典型的强关联电子材料, 其(110) 薄膜为通过晶格对称性、应变等的设计调控外延结构的物理性质提供了新的机会. 本文研究了SrTiO₃(110) 衬底表面金属La 和Ti 沉积所引起的微观结构变化, 进而利用电子衍射信号对分子束外延薄膜生长表面阳离子浓度的灵敏响应, 发展了原位、实时、精确控制金属蒸发源沉积速率的方法, 实现了高质量LaTiO₃(110) 薄膜的生长和对阳离子化学配比的精确控制. 由于LaTiO₃中Ti³⁺ 3d 电子的库仑排斥作用, 氧原子层截止的(110) 表面更容易实现极性补偿, 因此生长得到的薄膜表面暴露出单一类型的氧截止面.

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

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 t_2g 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.