Local Electrical Imaging of Tetragonal Domains and Field-Induced Ferroelectric Twin Walls in ConductingSrTiO3

Abstract

We demonstrate electrical mapping of tetragonal domains and electric field-induced twin walls in SrTiO 3 as a function of temperature and gate bias utilizing the conducting LaAlO 3 =SrTiO 3 interface and low-temperature scanning electron microscopy. Conducting twin walls appear below 105 K, and new twin patterns are observed after thermal cycling through the transition or on electric field gating. The nature of the twin walls is confirmed by calculating their intersection angles for different substrate orientations. Numerous walls formed when a large side-or back-gate voltage is applied are identified as field-induced ferroelectric twin walls in the paraelectric tetragonal matrix. The walls persist after switching off the electric field and on thermal cycling below 105 K. These observations point to a new type of ferroelectric functionality in SrTiO 3 , which could be exploited together with magnetism and superconductivity in a multifunctional context. DOI: 10.1103/PhysRevLett.116.257601 The emerging field of domain boundary engineering requires interfaces with unique functionalities [1] such as in the SrTiO 3 (STO)-based heterostructures [2][3][4][5]. STO is cubic at room temperature, but undergoes a ferroelastic transition to tetragonal structure at around 105 K. It does not become spontaneously ferroelectric at low temperatures despite its huge permittivity [6,7]. Nevertheless, electric order can be induced by stress, or by electric field (E) [8][9][10] at a threshold of 1.40 kV=cm at ∼5 K.When STO is used as a substrate or gate insulator for materials such as topological insulators and superconductors [11,12], films grown on STO are assumed to be biased uniformly. However, this is not really the case as nonuniformity can arise from the tetragonal domain structure. Twin boundaries between domains (twin walls) in STO are of particular interest, as they have been suggested to become conducting and ferroelectric at low temperatures [13][14][15][16][17][18][19]. Further, it has been demonstrated that ferroelectric "stripes" can be injected in ferroelectric thin films by increasing the applied voltage [20,21], which requires the material, or at least its domain walls, to be ferroelectric. Here, we image the ferroelastic twin walls in STO, and show that their response above the threshold field is strong evidence that it is the field-induced twin walls that become ferroelectric.STO can be made conducting by doping, by oxygen vacancies, or by electronic reconstruction induced by a polar oxide LaAlO 3 (LAO) that produces a few nanometers of two-dimensional electron gas (2DEG) below the interface [22][23][24], while the bulk of the STO remains insulating. Recently, an influence of the local tetragonal domain structure on the conductivity of the 2DEG at the LAO/ STO interface was demonstrated by probing the magnetic field [25] or electric potential [26] induced by the 2DEG, rather than mapping the electronic properties directly, which is difficult with a probe such as scanning tunneling microscopy because the 2DEG is...

Temperature dependent giant resistance anomaly in LaAlO3/SrTiO3 nanostructures

“…Even more recently Ma et al . 23 used low temperature scanning electron microscopy and electron beam induced current for the investigation of LAO/STO structures. They not only demonstrated that highly conducting filaments exist but they also showed that these filaments may be surrounded by insulating areas.…”

“…Since there is no restoring force this may require a long time and temperatures elevated well above the phase transition temperature as also observed by Ma et al . 23 . In our case an almost complete redistribution only seems to occur above 180 K which may indicate a thermal activation.…”

Low-Temperature Dielectric Anisotropy Driven by an Antiferroelectric Mode in SrTiO3

“…To explore this route, however, one first needs to understand whether (and how) the ferroelastic degrees of freedom can be harnessed via a suitable external field. As the ferroelastic domain walls have recently been shown to be polar [16][17][18][19][20][21][22], it is reasonable to expect that they will respond to an electrical bias. A detailed study on the evolution of the domain wall topology upon application of a voltage is currently missing, leaving the above question largely unanswered.…”

Infrared ellipsometry study of photogenerated charge carriers at the (001) and (110) surfaces of SrTiO3 crystals and at the interface of the corresponding LaAlO3Yazdi-Rizi1, Maršík2, Mallett3 et al. 2017Phys. Rev. B124140View full textAdd to dashboardCite

“…A very likely scenario is that they act as trapping centres for the photo-generated oxygen vacancies (or photo-mobilized oxygen vacancies which migrate from the bulk to the surface) which give rise to the itinerant electrons. The pinning of the oxygen vacancies may be caused by the local strain and, as was mentioned already in the introduction, by a polar charging of the domain boundaries due to a so-called flexoelectric effect [21][22][23]. The formation of oxygen vacancy clusters may also play an important role since it can give rise to a further reduction of the Coulomb-interaction with the itinerant electrons and thus enhance the lifetime of the photogenerated charge carriers.…”

“…For example, room temperature ferroelectricity has been observed in STO thin films that are grown under tensile strain on top of a DyScO 3 substrate [20]. Moreover, a polar order that is likely induced by a flexoelectric effect has been observed in the vicinity of the domain boundaries of the structural poly-domain state that appears below T* [21][22][23][24][25] and can even modify the …”