Nanoscale Laterally Modulated Properties of Oxide Ultrathin Films by Substrate Termination Replica through Layer-by-Layer Growth

Abstract

Modulation of oxide properties in a direction perpendicular to surfaces can be easily achieved by advanced thin film deposition tools. In contrast, simultaneous nanoscale modulation of properties in directions perpendicular and parallel to surfaces, as required for building three-dimensional (3D) nanometric devices, has remained elusive. Though bottom-up approaches allow obtaining controlled lateral growth, these techniques fail to achieve long-range order or do not allow simultaneous modulation of properties in the perpendicular direction. Here, we show that, by exploiting a dual strategy based on the spontaneous surface restructuration of some perovskite substrates and the layer-by-layer growth of nanometric heteroepitaxial perovskite layers, surfaces with laterally modulated surface relief, chemical termination, electrostatic potential, and electrical conductance can be obtained. As illustrative examples, conducting ferromagnetic manganite La2/3Sr1/3MnO3 and ferroelectric BaTiO3 ultrathin films have been grown on (001)SrTiO3 and (001)(La0.18Sr0.82)(Al0.59Ta0.41)O3 substrates with self-ordered chemical terminations.

Instability and Surface Potential Modulation of Self-Patterned (001)SrTiO3 Surfaces

“…These results match exceptionally well with those obtained in N 2 atmosphere at low humidity conditions in as-prepared samples (see Supporting Information). It is worth stressing that data collected either in as-prepared samples as sample 1a, and samples prepared by a mild low temperature annealing (200 °C) under UHV conditions (sample 1b), consistently indicate that the work function of A is smaller than that of B regions, in agreement with the theoretical trend for A corresponding to SrO and B to TiO 2 [7]. However, high resolution nc-AFM images (Figure 2b) show some degree of heterogeneity with fairly periodic stripes of aligned protrusions ≈0.1−0.2 nm high and 10 nm wide (Figure 2c) which should not exist on ideally TiO 2 surfaces.…”

Oxide Single-Crystal Surfaces: A Playground for Self-assembled Oxide Nanostructures

“…This has been observed on SrTiO 3 [56], on LaAlO 3 [57], on DyScO 3 [52], and on LSAT [11]. Surface potential differences between both terminations in SrTiO 3 have been predicted to be around 2.3 eV but measured with lower value [32]. These chemically-patterned surfaces have then been used to elaborate different nanostructures by selective growth (see Figure 2).…”

“…In the case of low energy interface, 2D epitaxial growth can occur without preferential nucleation with the chemical terminations replicating themselves at the film surface (see Figure 2A). In addition to the possible modulation of the properties in the out-of-plane direction with layered heterostructures elaborated by advanced deposition techniques, self-assembled chemical terminations have allowed to laterally modulate the interface and surface properties of heterostrucutres [32], generating for instance a 2DEG only at the specific (LaO/TiO 2 ) laterally-confined regions on the singleterminated terrace width of the LaAlO 3 /SrTiO 3 interface [62], as predicted [63]. More generally, that has allowed to study both chemical terminations of various functional perovskite oxides, such as ferromagnetic (La,Sr)MnO 3 and ferroelectric BaTiO 3 , which are difficulty accessible even with advanced and widely used oxide deposition techniques such as pulsed laser deposition (PLD) [32].…”

“…In addition to the possible modulation of the properties in the out-of-plane direction with layered heterostructures elaborated by advanced deposition techniques, self-assembled chemical terminations have allowed to laterally modulate the interface and surface properties of heterostrucutres [32], generating for instance a 2DEG only at the specific (LaO/TiO 2 ) laterally-confined regions on the singleterminated terrace width of the LaAlO 3 /SrTiO 3 interface [62], as predicted [63]. More generally, that has allowed to study both chemical terminations of various functional perovskite oxides, such as ferromagnetic (La,Sr)MnO 3 and ferroelectric BaTiO 3 , which are difficulty accessible even with advanced and widely used oxide deposition techniques such as pulsed laser deposition (PLD) [32]. The chemical terminations can also have a dramatic effect on the crystallographic orientation and the morphology of thin films, as YSZ on SrTiO 3 (001) surface where it has been shown that 2D growth of YSZ film only occurs on the SrO termination with (001)-orientation whereas dome-shaped tilted (111)-oriented YSZ islands of a few tens of nanometers in diameter and a few nanometers in height epitaxially grow on the TiO 2 termination [64].…”

“…In addition to these mechanisms, the chemical termination of single-crystalline perovskite oxide substrates has played a major role in the conducted epitaxial growth of high quality oxide films and nanostructures. The progress made to control the chemical termination by chemical and thermal treatments of the most used oxide single-crystal substrates, SrTiO 3 (001) in particular [28,29], has opened doors toward the control, enhancement and emergence of original physical properties [30][31][32], with the most famous example of the formation of a high-mobility two-dimension electron gas (2DEG) at the LaAlO 3 /SrTiO 3 interface appearing only on the TiO 2 termination of the SrTiO 3 (001) surface [30]. In addition, we will see that self-assembly of the chemical terminations of (001)-oriented perovskite singlecrystals can also be used to generate different nanostructures.…”