J. H. Haeni

Dielectric functions and optical bandgaps of high-K dielectrics for metal-oxide-semiconductor field-effect transistors by far ultraviolet spectroscopic ellipsometry

A far ultraviolet (UV) spectroscopic ellipsometer system working up to 9 eV has been developed, and applied to characterize high-K-dielectric materials. These materials have been gaining greater attention as possible substitutes for SiO2 as gate dielectrics in aggressively scaled silicon devices. The optical properties of four representative high-K bulk crystalline dielectrics, LaAlO3, Y2O3-stabilized HfO2 (Y2O3)0.15–(HfO2)0.85, GdScO3, and SmScO3, were investigated with far UV spectroscopic ellipsometry and visible-near UV optical transmission measurements. Optical dielectric functions and optical band gap energies for these materials are obtained from these studies. The spectroscopic data have been interpreted in terms of a universal electronic structure energy scheme developed form ab initio quantum chemical calculations. The spectroscopic data and results provide information that is needed to select viable alternative dielectric candidate materials with adequate band gaps, and conduction and valence b...

Probing Nanoscale Ferroelectricity by Ultraviolet Raman Spectroscopy

We demonstrated that ultraviolet Raman spectroscopy is an effective technique to measure the transition temperature (Tc) in ferroelectric ultrathin films and superlattices. We showed that one-unit-cell-thick BaTiO3 layers in BaTiO3/SrTiO3 superlattices are not only ferroelectric (with Tc as high as 250 kelvin) but also polarize the quantum paraelectric SrTiO3 layers adjacent to them. Tc was tuned by ∼500 kelvin by varying the thicknesses of the BaTiO3 and SrTiO3 layers, revealing the essential roles of electrical and mechanical boundary conditions for nanoscale ferroelectricity.

Oxide nano-engineering using MBE

Abstract Molecular beam epitaxy (MBE) has achieved unparalleled control in the integration of semiconductors at the nanometer level; its use for the integration of oxides with similar nanoscale customization appears promising. This paper describes the use of reactive MBE to synthesize layered oxide heterostructures, including new compounds and metastable superlattices, involving monolayer-level integration of the dielectric and ferroelectric oxides SrO, SrTiO 3 , BaTiO 3 , PbTiO 3 , and Bi 4 Ti 3 O 12 . The controlled synthesis of such layered oxide heterostructures offers great potential for tailoring the dielectric and ferroelectric properties of materials. Oxide nano-engineering is accomplished by supplying the incident species in the desired layering sequence with submonolayer composition control. Comparisons between the growth of compound semiconductors and oxides by MBE are made.

Critical issues in the heteroepitaxial growth of alkaline-earth oxides on silicon

The critical aspects of the epitaxial growth of alkaline-earth oxides on silicon are described in detail. The step by step transition from the silicon to the alkaline-earth oxide as shown through reflection high energy electron diffraction is presented, with emphasis placed on the favorable interface stability, oxidation, structural, and strain considerations for each stage of the growth via molecular beam epitaxy.

Epitaxial growth of the first five members of the Srn+1TinO3n+1 Ruddlesden–Popper homologous series

The first five members of the Srn+1TinO3n+1 Ruddlesden–Popper homologous series, i.e., Sr2TiO4, Sr3Ti2O7, Sr4Ti3O10, Sr5Ti4O13, and Sr6Ti5O16, have been grown by reactive molecular beam epitaxy. A combination of atomic absorption spectroscopy and reflection high-energy electron diffraction intensity oscillations were used for the strict composition control necessary for the synthesis of these phases. X-ray diffraction and high-resolution transmission electron microscope images confirm that these films are epitaxially oriented and nearly free of intergrowths. Dielectric measurements indicate that the dielectric constant tensor coefficient e33 increases from a minimum of 44±4 in the n=1(Sr2TiO4) film to a maximum of 263±2 in the n=∞(SrTiO3) film.

RHEED Intensity Oscillations for the Stoichiometric Growth of SrTiO3 Thin Films by Reactive Molecular Beam Epitaxy

The growth of high quality multicomponent oxide thin films by reactive molecular beam epitaxy (MBE) requires precise composition control. We report the use of in situ reflection high-energy electron diffraction (RHEED) for the stoichiometric deposition of SrTiO3 (1 0 0) from independent strontium and titanium sources. By monitoring changes in the RHEED intensity oscillations as monolayer doses of strontium and titanium are sequentially deposited, the Sr:Ti ratio can be adjusted to within 1% of stoichiometry. Furthermore, the presence of a beat frequency in the intensity oscillation envelope allows the adjustment of the strontium and titanium fluxes so that a full monolayer of coverage is obtained with each shuttered dose of strontium or titanium. RHEED oscillations have also been employed to determine the doping concentration in barium- and lanthanum-doped SrTiO3 films.

Evolution of dislocation arrays in epitaxial BaTiO3 thin films grown on (100) SrTiO3

Dislocation arrays and dislocation half-loops in BaTiO3 thin films were characterized using transmission electron microscopy (TEM). BaTiO3 films with thicknesses ranging from 2 to 20 nm were grown on (100) SrTiO3 by reactive molecular beam epitaxy (MBE). The critical thickness for dislocations to occur in this system was found to lie between 2 and 4 nm. The misfit dislocations are mainly 〈100〉 type. The average spacing between the dislocations in the array becomes smaller when the film is thicker, which indicates gradual relaxation of mismatch strain with increasing film thickness.

Structural evidence for enhanced polarization in a commensurate short-period BaTiO3/SrTiO3 superlattice

A short-period (BaTiO3)6∕(SrTiO3)5 superlattice was characterized by x-ray diffraction and transmission electron microscopy. The superlattice is epitaxially oriented with the c axes of BaTiO3 and SrTiO3 normal to the (001) surface of the SrTiO3 substrate. Despite the large in-plane lattice mismatch between BaTiO3 and SrTiO3 (∼2.2%), the superlattice interfaces were found to be nearly commensurate. The crystallographic c∕a ratio of the superlattice was measured and the results agree quantitatively with first-principles calculations and phase-field modeling. The agreement supports the validity of the enhanced spontaneous polarization predicted for short-period BaTiO3∕SrTiO3 superlattices.

Multiferroic domain dynamics in strained strontium titanate

Multiferroicity can be induced in strontium titanate by applying biaxial strain. Using optical second harmonic generation, we report a transition from 4/mmm to the ferroelectric mm2 phase, followed by a transition to a ferroelastic-ferroelectric mm2 phase in a strontium titanate thin film. Piezoelectric force microscopy is used to study ferroelectric domain switching. Second harmonic generation, combined with phase-field modeling, is used to reveal the mechanism of coupled ferroelectric-ferroelastic domain wall motion. These studies have relevance to multiferroics with coupled polar and axial phenomena.

Thermal expansion of the new perovskite substrates DyScO 3 and GdScO 3

The thermal expansion coefficients of DyScO 3 and GdScO 3 were determined from298 to 1273 K using x-ray diffraction. The average thermal expansion coefficients of DyScO 3 and GdScO 3 were 8.4 and 10.9 ppm/K, respectively. No phase transitions were detected over this range, though the orthorhombicity decreased with increasing temperature. These thermal expansion coefficients are similar to other oxide perovskites (e.g., BaTiO 3 or SrTiO 3 ), making these rare-earth scandates promising substrates for the growth of epitaxial thin films of many oxide perovskites that have similar lattice spacing and thermal expansion coefficients.