University of Illinois Non-linear Electrodynamic Response of Dielectric Materials microwave applications (radar, etc) phase shifters tuned filters voltage controlled oscillators optical applications (wdm, etc) amplitude modulators phase modulators frequency shifters these things are cm long better to be microns long! PowerPoint Presentation by: Professor Jim Eckstein Department of Electrical and Computer Engineering

University of Illinois What gives rise to non-linear response? Saturation of ionic and electronic polarization In materials with inversion symmetry  (2) is zero amorphous materials vapors simple cubic crystals In materials lacking inversion symmetry  (2) is nonzero ferroelectrics poled organic films (with non-centrosymmetric molecules)

P E Dielectric polarization of a ferroelectric, which has broken inversion symmetry. Easy to see that this is not zero in materials with broken symmetric response. But, two solutions or branches.  (2) diverges as T  T c University of Illinois P0P0

P E For non-linear optical and other field tuning applications would prefer a characteristic like this. Stable, single solution can’t de-pole Permanently polarized dielectric with big  (2) Use molecular nanostructuring to make such a material (MBE) (once you figure out what matters!) University of Illinois What you would like for non-linear modulators, etc…

Artificial structures using ALL-MBE to synthesize materials and heterostructures not found in nature broken symmetry throughout film favors one polarization permits stable operation nearer to Curie temperature obtain larger response at zero bias Grow crystal using ferroelectric and related phases: stack with structurally broken c-axis inversion symmetry lattice property, e.g. strain, T c favored polarization unfavored polarization supercell Controlling material properties via epitaxial strain Producing new “materials” by modulated heterostructure growth Tensile strain-induced magnetic anisotropy in magnetic oxide anisotropy energy surface (La 0.7 Ca 0.3 MnO 3 on SrTiO 3 substrate) University of Illinois

Introducing the actors, perovskite titanate phases BaTiO 3 ferroelectric (order-disorder, weakly 1 st order) SrTiO 3 non ferroelectric, but would like to be well known substrate CaTiO 3 ferroelectricity, what’s that?? 4.0 A 3.9 A 3.8 A Combine these in single crystal heterostructures to investigate the effects of compositional (strain) symmetry breaking 

Inversion symmetry? CTO STO BTO 222+NO STO BTO CTO 222- NO CTO STO BTO 1212YES CTO STO BTO 422+NO CTO STO BTO 622+NO supercell nanostructure (each rectangle is one monolayer)

Atomic Layer-by-Layer Molecular Beam Epitaxy Ozone generator Oxygen Pump Ca Sr Ba Al La Y Ti Mn Cu Bi rotating substrate positioner quartz crystal monitor RHEED electron gun hollow cathode lamp photomultiplier tube quadrupole mass spectrometer turbo pump load lock shutters hollow cathode lamp photomultiplier tube substrate holder atomic absorption spectroscopy for feedback control ozone oxidation in-situ RHEED with digital video We have control over the source fluxes to better than 1% accuracy (AA, RHEED) ozone still RHEED reveals surface crystal structure

a a a aaa RHEED images at different points of the super cell growth Start of Super Cell CTO Surface End of Super Cell CTO Surface After 1 ML BTOAfter 1 ML STO After 0.5 ML STO Specular Spot Oscillation from 1 Super Cell 1 ML BTO 2 ML BTO 1 ML STO 2 ML STO 1 ML CTO 2 ML CTO

Growth and Processing of Capacitor Devices Substrate STO or NGO Dielectric superlattice LSMO base electrode LSMO top electrode In-situ Au Top LSMO Ex-situ Au Dielectric superlattice Base LSMO SiO 2 Few 100  m

University of Illinois  (E;T) for 222+  (E;T) for 1212  (E;T) for 222-

University of Illinois Now, add more BaTiO 3 this has large  at higher T in bulk 422+ shows large range of linear response at zero bias from peak in  estimate P no hysteresis, single solution 622+ shows larger linear response at zero bias from peak in  estimate P no hysteresis, single solution Both show temperature indepen- dent effect from 180 to 360 K

P E P0P0 Measure Obtain P 0 by integrating  dE P sat should be the same size at + and – infinite bias field University of Illinois Obtain permanently polarized material with tunable dielectric constant