The control of material interfaces at the atomic level has led to novel interfacial properties and functionalities. In particular, the study of polar discontinuities at interfaces between complex oxides lies at the frontier of modern condensed matter research. Here we employ a combination of experimental measurements and theoretical calculations to demonstrate the control of a bulk property, namely ferroelectric polarization, of a heteroepitaxial bilayer by precise atomic-scale interface engineering. More specifically, the control is achieved by exploiting the interfacial valence mismatch to influence the electrostatic potential step across the interface, which manifests itself as the biased-voltage in ferroelectric hysteresis loops and determines the ferroelectric state. A broad study of diverse systems comprising different ferroelectrics and conducting perovskite underlayers extends the generality of this phenomenon.complex oxide | heterostructure | interface physics | electronic reconstruction | polar discontinuity O ver the past few years, precisely constructed, atomically sharp perovskite oxide heterointerfaces have become focal points for condensed-matter-physics and materials science research (1-5). The incorporation and reconstruction of spin (6, 7), charge (8-10), and orbital (11) degrees of freedom across the heterointerfaces have led to novel electronic properties that are different from those inherent to the individual components. For example, pioneering work on the LaAlO 3 and SrTiO 3 (STO) heterostructures has revealed a nontrivial two-dimensional electron gas (2DEG) (10,12,13) at the interface, which also exhibits magnetic (14) and even superconductivity properties (15) that are induced by the polar discontinuity (16) (valence mismatch) across the interface.Motivated by this, research nowadays is primarily focused on probing and understanding the novel interfacial phenomena observed in complex-oxide heterostructures. However, the focus on interfacial properties sidesteps possible macroscopic implications of the interfacial atomic-scale control on the broad range of properties that are present in bulk complex oxides. On the other hand, in the semiconductor industry, atomic-scale interface engineering has long been used to improve the performance of devices through control of the threshold voltage (17), channel carrier mobility (18), Schottky barrier height (19), and so on. This forms the fundamental premise for this work: Can we control the bulk properties of a heterostructured system through the emergent state of matter at the interface? Such an approach could be particularly intriguing if one of the layers is highly polar and electrically switchable, i.e., ferroelectric in nature. Because functional ferroelectric systems, such as the nonvolatile memory (20), ferroelectric field effect transistor (21, 22), ferroelectric tunnel junction (23-27), and switching photo-diode (28), are strongly correlated with the interface electronic structures, it is of great importance to study how the interface atom...
Ionic crystals terminated at oppositely charged polar surfaces are inherently unstable and expected to undergo surface reconstructions to maintain electrostatic stability. Essentially, an electric field that arises between oppositely charged atomic planes gives rise to a built-in potential that diverges with thickness. Here we present evidence of such a built-in potential across polar LaAlO3 thin films grown on SrTiO3 substrates, a system well known for the electron gas that forms at the interface. By performing tunneling measurements between the electron gas and metallic electrodes on LaAlO3 we measure a built-in electric field across LaAlO3 of 80.1 meV/Å. Additionally, capacitance measurements reveal the presence of an induced dipole moment across the heterostructure. We forsee use of the ionic built-in potential as an additional tuning parameter in both existing and novel device architectures, especially as atomic control of oxide interfaces gains widespread momentum.As dictated by Maxwell's equations, the accumulation of screening charges at the boundary between dissimilar materials is one means 1 of ensuring a continuous electric displacement at the interface. 2 For instance, a layer of trapped screening charge forms the two dimensional electron gas that compensates a polarization mismatch at gallium nitride 3 and zinc oxide 4 based heterostructure interfaces. In insulating oxides, charge accumulation was observed at the interface between SrTiO 3 substrates with atomically precise surfaces and polar LaAlO 3 films. 5 LaAlO 3 thin films grown on singly terminated SrTiO 3 surfaces 6 comprise negatively charged AlO 2 and positively charged LaO end planes and are polar in the ionic limit. 1,7,8 When at least four unit cells (u.c.) of LaAlO 3 are deposited on TiO 2 terminated SrTiO 3 , an electron gas forms near the interface in SrTiO 3 . 5,9,10 It is often hypothesized that at a thickness of four u.c. the potential across LaAlO 3 exceeds the band gap of SrTiO 3 and electrons tunnel from the valence band of LaAlO 3 to the SrTiO 3 potential well, completely diminishing the potential across LaAlO 3 . 11 Thus within this picture, in the presence of an electron gas no field would be expected across the LaAlO 3 . However, if all the charge carriers do not lie precisely at the interface or have an extrinsic (oxygen vacancies 12 or cation doping 13 ) origin, the LaAlO 3 potential will not be fully screened and can thus be probed. 14 Alternatively, the precise band alignment between the LaAlO 3 and SrTiO 3 will also determine the strength of the residual fields in the LaAlO 3 . 15 Addressing this issue by determining the existence of an uncompensated built-in potential in LaAlO 3 is central to understanding the true nature of the polar LaAlO 3 /SrTiO 3 interface.We probe the potential landscape across the LaAlO 3 and the interface region in SrTiO 3 by employing a typical metal-insulator-metal capacitor geometry such that the LaAlO 3 thin films form the dielectric layer sandwiched between evaporated metallic electrodes an...
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