John David Baniecki

Chemisorption of water and carbon dioxide on nanostructured BaTiO3–SrTiO3(001) surfaces

The interaction of water and carbon dioxide with nanostructured epitaxial (Ba,Sr)TiO3(001) thin film and bulk single crystal SrTiO3(001) surfaces was studied using x-ray photoemission spectroscopy (XPS), thermal desorption spectroscopy (TDS), and density functional theory (DFT). On both surfaces, XPS and TDS indicate D2O and CO2 chemisorb at room temperature with broad thermal desorption peaks (423–723 K) and a peak desorption temperature near 573 K. A comparison of thermal desorption Redhead activation energies to adsorption energies calculated using DFT indicates that defect surface sites are important for the observed strong adsorbate-surface reactivity. Numerical calculations of the competetive adsorption/desorption equilibria for H2O and CO2 on SrTiO3(001) surfaces show that for typical atmospheric concentrations of 0.038% carbon dioxide and 0.247% water vapor the surfaces are covered to a large extent with both adsorbates. The high desorption temperature indicates that these adsorbates have the pote...

Investigation of the importance of interface and bulk limited transport mechanisms on the leakage current of high dielectric constant thin film capacitors

The importance of interface and bulk transport mechanisms on the leakage current of high dielectric constant thin film capacitors is examined by deriving an equation for the J–VA characteristic of a capacitor that includes the transport mechanisms of thermionic emission (TE), thermionic field emission (TFE), and carrier drift–diffusion (DD). The current is controlled by the slowest of three effective velocity parameters v1md, vD, and ṽ2dm characterizing electron injection into the dielectric at the cathode by TE and TFE, carrier DD in the film bulk, and electron ejection from the dielectric at the anode by TE and TFE, respectively. The effective velocity parameters are evaluated for a Pt/BST/Pt thin film capacitor that has been exposed to forming gas and it is shown that the dominant transport mechanism is interface limited TFE from the cathode with negligible influence of carrier transport by DD in the film bulk. Implications of these results on existing transport calculations for high dielectric constan...

H2O vapor-induced leakage degradation of Pb(Zr,Ti)O3 thin-film capacitors with Pt and IrO2 electrodes

We have investigated the influence of H2O vapor on the leakage characteristics of Pb(Zr,Ti)O3 (PZT) thin-film capacitors with Pt and IrO2 top electrodes by conducting electrical measurements ex situ in air and in situ in an 80% N2/20% O2 atmosphere with controlled humidity ranging from <0.1% to 95.5% relative humidity at 20 °C. The results show that the leakage characteristics of PZT thin-film capacitors are strongly sensitive to H2O vapor in the atmosphere. The onset to H2O-induced leakage degradation depended on the relative humidity and on the quantity of charge passed through the capacitor. A degradation mechanism based on the electrolysis of H2O that has adsorbed on the capacitor from the atmosphere and the role of the electrode material in the degradation process are discussed.

Surface core-level shifts of strontium observed in photoemission of barium strontium titanate thin films

Angle resolved x-ray photoelectron spectroscopy was used to investigate the surface electronic structure of barium strontium titanate (BST) films. In contrast to previous photoemission studies which identified two chemical states associated with only Ba in the near surface region, the authors have resolved core-level features from surface Sr atoms which provide new insight into the surface electronic structure of BST thin films. The surface Sr 3d features are found to lie approximately 1eV higher in binding energy than the bulk derived peaks. The effects of aqueous and annealing surface treatments and the origin of the Sr surface core-level shifts are discussed.

Effect of Y doping and composition-dependent elastic strain on the electrical properties of (Ba,Sr)TiO3 thin films deposited at 520 °C

We demonstrate that large and simultaneous improvements in permittivity, tunability, and leakage current density of (Ba,Sr)TiO3 (BST)-based thin-film capacitors can be achieved by yttrium doping. We have found that, for a low deposition temperature (520 °C) sputtering process, Y-doped BST capacitors exhibit tenfold lower leakage current density (<10−9A∕cm2 at 100KV∕cm) and 70% higher permittivity than nominally undoped BST-based capacitors. Furthermore, this work suggests an intriguing correlation between dopant concentration-dependent elastic strain in the films and their enhanced dielectric properties.

Band offsets at the epitaxial SrTiO3/SrZrO3 (0 0 1) heterojunction

The interface formation between SrTiO3 and SrZrO3 has been studied using in situ photoelectron spectroscopy. Epitaxial SrZrO3 thin films were grown on (0?0?1)-oriented SrTiO3?:?Nb single crystals via pulsed-laser deposition. The epitaxial SrZrO3 growth was verified via x-ray diffraction and Cs-corrected high angle annular dark field scanning transmission electron microscopy. A type I straddling configuration has been found for the interface analysed with a valence band and conduction band offset of 0.5?eV and 1.9?eV, respectively.

A study of current transport in (BaxSr1−x)Ti1+yO3+z thin-film capacitors containing a voltage-dependent interface state charge distribution

J–V–T characteristics, measured over a very wide temperature (50–441 K) range, are analyzed simultaneously with C–V measurements to investigate conduction mechanisms and the state of charge of Pt/BaxSr1−xTi1+yO3+z(BST)/Pt thin-film capacitors during an applied voltage or current stress. A time- and voltage-dependent state of charge of the Pt/BST/Pt capacitors is inferred from stress-induced voltage shifts in the C–V curves. The voltage and temperature dependence of the C–V curve shifts is shown to be consistent with a voltage-dependent charge in interface states resulting from a change in potential across interfacial dipole layers. An intimate contact Schottky barrier model incorporating a voltage-dependent charge in interface states at both cathode and anode contacts is used to investigate conduction mechanisms in Pt/BST/Pt thin-film capacitors. The basic transport mechanisms of drift-diffusion, thermionic field emission, and Fowler–Nordeim tunneling are shown to dominate leakage in limiting voltage and ...

Density functional theory and experimental study of the electronic structure and transport properties of La, V, Nb, and Ta doped SrTiO3

The electronic structure and transport properties of donor doped SrTiO3 are studied using density functional theory with spin-orbit coupling and conductivity, Hall, and Seebeck effect measurements over a wide temperature range (100 K to 600 K). Split-off energies ΔSO are tunable through the dopant SO interaction strength and concentration varying from 28.1 meV for pure STO to 70.93 meV for SrTi0.5Nb0.5O3. At lower carrier concentrations and temperatures, SO coupling has a marked effect on both the filling dependence of the density-of-states mass as well as the temperature dependence of the Seebeck coefficient, with quantitative theoretical predictions based on DFT calculations that include the SO interaction in closer agreement to the experimental data. Moreover, the results suggest that the predictive power of the current theory is not unlimited, with less accuracy for the calculated S predicting the magnitude of the experimental S data at lower dopant concentrations than for degenerately doped systems. ...

Microstructural and Electrical Properties of (Ba x Sr 1 − x )Ti 1 + y O 3 + z Thin Films Prepared by RF Magnetron Sputtering

The electrical and microstructural properties of (Ba x Sr 1 m x )Ti 1+y O 3+z (BST) thin films prepared by RF magnetron sputtering were investigated as a function of deposition temperature over the range of 100 C to 650 C. Films deposited above approximately 350 C on Pt/TiO 2 /SiO 2 /Si substrates were polycrystalline with relative permittivites of 100 nm thick BST thin films varying from 100 at 350 C to 600 at 650 C. For deposition temperatures below approximately 350 C, the electrical properties were strongly influenced by the presence of a less crystalline BST layer. Films deposited at 250 C were comprised of a multilayer polycrystalline/less crystalline BST structure. The less crystalline BST layer strongly affected both measured dielectric permittivity and leakage properties. Leakage characteristics of Pt/BST(250C)/Pt capacitors exhibited a power law dependence on voltage or an exponential dependence on square root of the applied voltage depending on whether the top Pt electrode adjacent ot the polycrystalline BST layer was biased at a high or low electric potential, respectively. Mechanisms for the observed leakage behavior are discussed.

Electronic transport behavior of off-stoichiometric La and Nb doped SrxTiyO3−δ epitaxial thin films and donor doped single-crystalline SrTiO3

Above room temperature electronic transport properties of SrxTiyO3−δ films with cation A/B = (La + Sr/Nb + Ti) ratios of 0.9 to 1.2 are compared to STO single crystals with combined Hall carrier densities of 3 × 1016 cm−3 ≤ nH ≤ 1022 cm−3. In contrast to Hall mobility which is single crystal-like (μH ≈ 6 cm2/Vs) only near A/B = 1, the Seebeck coefficient (S) is single crystal-like over a range of nonstoichiometry. For nH   1021 cm−3, S is metallic-like with m∗/mo ∼ 8. No marked increase in m∗ with decreasing nH owing to a carrier filling dependence is observed.