Metal oxide semiconductor field-effect transistors, formed using silicon dioxide and silicon, have undergone four decades of staggering technological advancement. With fundamental limits to this technology close at hand, alternatives to silicon dioxide are being pursued to enable new functionality and device architectures. We achieved ferroelectric functionality in intimate contact with silicon by growing coherently strained strontium titanate (SrTiO3) films via oxide molecular beam epitaxy in direct contact with silicon, with no interfacial silicon dioxide. We observed ferroelectricity in these ultrathin SrTiO3 layers by means of piezoresponse force microscopy. Stable ferroelectric nanodomains created in SrTiO3 were observed at temperatures as high as 400 kelvin.
Emergent phenomena, including superconductivity and magnetism, found in the two-dimensional electron liquid (2-DEL) at the interface between the insulators lanthanum aluminate (LaAlO 3 ) and strontium titanate (SrTiO 3 ) distinguish this rich system from conventional 2D electron gases at compound semiconductor interfaces. The origin of this 2-DEL, however, is highly debated, with focus on the role of defects in the SrTiO 3 , while the LaAlO 3 has been assumed perfect. Here we demonstrate, through experiments and firstprinciple calculations, that the cation stoichiometry of the nominal LaAlO 3 layer is key to 2-DEL formation: only Al-rich LaAlO 3 results in a 2-DEL. Although extrinsic defects, including oxygen deficiency, are known to render LaAlO 3 /SrTiO 3 samples conducting, our results show that in the absence of such extrinsic defects an interface 2-DEL can form. Its origin is consistent with an intrinsic electronic reconstruction occurring to counteract a polarization catastrophe. This work provides insight for identifying other interfaces where emergent behaviours await discovery.
Abstract:Superlattices of (LaMnO 3 ) 2n /(SrMnO 3 ) n (1≤ n ≤ 5), composed of the insulators LaMnO 3 and SrMnO 3 , undergo a metal-insulator transition as a function of n, being metallic for n≤2 and insulating for n≥3. Measurements of transport, magnetization and polarized neutron reflectivity reveal that the ferromagnetism is relatively uniform in the metallic state, and is strongly modulated in the insulating state, being high in LaMnO 3 and suppressed in SrMnO 3 . The modulation is consistent with a Mott transition driven by the proximity between the (LaMnO 3 )/(SrMnO 3 ) interfaces. Disorder localizes states at the Fermi level at the interfaces for n ≥ 3. We suggest that this disorder is due to magnetic frustration at the interfaces.
A variety of three-constituent superlattice patterns were made in atomic layer-by-layer films, with patterns breaking inversion symmetry giving effective permanent bias fields ranging up to about 200 kV/cm. Dielectric constants at room temperature were nearly 10(3), with loss tangents under 0.01. Most of the response came from discrete dipoles comprising multiple unit cells, but without any ferroelectric phase transition.
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