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22 Nov 2004

Volume 85, Issue 21, pp. 4831-5106

Issue Cover Spotlight Figure

Appl. Phys. Lett. 85, 4845 (2004); http://dx.doi.org/10.1063/1.1823019 (3 pages)

Wounjhang Park and Jeong-Bong Lee
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Changes in the leakage currents in Ba0.8Sr0.2TiO3∕ZrO2 multilayers due to modulations in oxygen concentration

S. K. Sahoo, D. C. Agrawal, Y. N. Mohapatra, Subhasish B. Majumder, and Ram S. Katiyar

Appl. Phys. Lett. 85, 5001 (2004); http://dx.doi.org/10.1063/1.1825074 (3 pages) | Cited 17 times

Online Publication Date: 23 November 2004

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Use of multilayer dielectric thin films provides opportunities to optimize properties for different applications. In this work, the changes in the leakage current in Ba0.8Sr0.2TiO3 thin films upon introduction of ZrO2 layers of different thicknesses are studied. Unusual changes in the leakage current and the transition field with variation in the number and thickness of the ZrO2 layers are observed. Profile of the oxygen concentration across the sample, as determined by x-ray photoelectron spectroscopy, shows that the oxygen diffusion length controls the modulation in the concentration of the charged oxygen vacancies, and hence the depletion layer thicknesses, at the interfaces. A qualitative model for the observed behavior is provided.
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73.21.Ac Multilayers
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.55.-g Dielectric thin films
73.61.Ng Insulators
79.60.Bm Clean metal, semiconductor, and insulator surfaces
79.60.Dp Adsorbed layers and thin films

Fabrication of metal–oxide–semiconductor field-effect transistors using crystalline γ‐Al2O3 films as the gate dielectrics

Takayuki Okada, Kazuaki Sawada, Makoto Ishida, and Mohammad Shahjahan

Appl. Phys. Lett. 85, 5004 (2004); http://dx.doi.org/10.1063/1.1826228 (3 pages)

Online Publication Date: 23 November 2004

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Crystalline γ‐Al2O3 films were employed as high-κ gate dielectrics in metal–oxide–semiconductor field-effect transistors (MOSFETs) and characterization of these devices was performed. The crystalline dielectric was deposited with thicknesses of 4.0–4.5 nm by mixed source molecular beam epitaxy and the capacitance equivalent thicknesses obtained were 2.7–2.9 nm. The MOSFETs had exceptionally steep subthreshold slopes (63–67 mV∕decade), relatively low negative fixed charge densities (5–7×1012 cm−2) and interface state densities (2–3×1011 eV−1 cm−2). The maximum values of the effective carrier mobilities were 145 cm2∕V s for electrons and 85 cm2∕V s for holes.
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85.30.Tv Field effect devices
77.55.-g Dielectric thin films
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
72.20.Fr Low-field transport and mobility; piezoresistance
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology

Structural and dielectric properties of epitaxial Ba1−xSrxTiO3 films grown on LaAlO3 substrates by polymer-assisted deposition

Y. Lin, Jang-Sik Lee, H. Wang, Y. Li, S. R. Foltyn, Q. X. Jia, G. E. Collis, A. K. Burrell, and T. M. McCleskey

Appl. Phys. Lett. 85, 5007 (2004); http://dx.doi.org/10.1063/1.1827927 (3 pages) | Cited 22 times

Online Publication Date: 23 November 2004

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Epitaxial Ba1−xSrxTiO3 (BST) thin films with different Ba∕Sr ratio (x=0.1–0.9 with an interval of 0.1) have been grown on (001) LaAlO3 substrates using polymer-assisted deposition. Dielectric measurements show that the films have dielectric properties comparable to the BST films grown by the pulsed laser deposition. Systematic changes in the lattice parameters and dielectric behavior with x values have been measured. The highest dielectric constant (∼1010) and tunability (∼69%) at 1 MHz and room temperature have been obtained at x=0.3, which is at the phase boundary of tetragonal and cubic structures.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.55.-g Dielectric thin films
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
77.22.Ch Permittivity (dielectric function)
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)

Effects of poling, and implications for metastable phase behavior in barium strontium titanate thin film capacitors

A. Lookman, J. McAneney, R. M. Bowman, J. M. Gregg, J. Kut, S. Rios, A. Ruediger, M. Dawber, and J. F. Scott

Appl. Phys. Lett. 85, 5010 (2004); http://dx.doi.org/10.1063/1.1827934 (3 pages) | Cited 11 times

Online Publication Date: 23 November 2004

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Barium strontium titanate (Ba0.5Sr0.5TiO3–BST) thin film capacitor structures were made using pulsed laser deposition, and their functional properties were monitored with varying temperature. It was found that poling at low temperature could induce distinct differences in the behavior of the dielectric constant and loss tangent, on heating. In relatively thick BST films (>∼650 nm), poling the sample at 80 K produced a change from a single broad anomaly to one in which three distinct anomalies could be observed. The temperatures of these anomalies (∼140, ∼200, and ∼260 K) were close to those known to be associated with phase transitions in bulk. Monitoring changes in polarization loops with temperature confirmed the likelihood that the dielectric anomalies observed were indeed the result of phase transitions in the films. Unusually, though, when the films were poled at 150 K, and then cooled to 80 K prior to collection of dielectric data on heating, the dielectric anomaly around 140 K was completely suppressed. The lack of a phase transition was confirmed by monitoring depolarization currents in the sample. It is suggested that poling has therefore allowed the phase state that existed at 150 K to persist metastably down to 80 K. For relatively thin BST films (<∼400 nm), poling at 80 K only induced two distinct anomalies in the dielectric response (at ∼200 and ∼290 K). Nevertheless, poling-related metastability could again be observed: when the samples were poled at 250 K and then cooled to 80 K prior to data collection on heating, the anomaly at ∼200 K was completely suppressed. These experiments suggest that metastable phase behavior could be commonplace in thin film ferroelectrics.
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84.32.Tt Capacitors
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
77.55.-g Dielectric thin films
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.22.Ej Polarization and depolarization
77.22.Ch Permittivity (dielectric function)
77.22.Gm Dielectric loss and relaxation
81.15.Fg Pulsed laser ablation deposition
77.80.B- Phase transitions and Curie point

Interface-oxygen-loss-controlled voltage offsets in epitaxial Pb(Zr0.52Ti0.48)O3 thin-film capacitors with La0.7Sr0.3MnO3 electrodes

Wenbin Wu, K. H. Wong, and C. L. Choy

Appl. Phys. Lett. 85, 5013 (2004); http://dx.doi.org/10.1063/1.1827929 (3 pages) | Cited 12 times

Online Publication Date: 23 November 2004

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Epitaxial Pb(Zr0.52Ti0.48)O3 (PZT) thin-film capacitors with La0.7Sr0.3MnO3 (LSMO) electrodes have been grown on (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (001) substrates by pulsed-laser deposition. The process-induced imprint behavior in the ferroelectric capacitors was examined by in situ and ex situ annealing at various conditions. It was found that for the capacitors in situ annealed at reduced oxygen pressures, where the LSMO electrodes are stable, voltage offsets in the polarization-electric field hysteresis loops were observed only for those treated at temperatures higher than the Curie temperature. At lower temperatures, the oxygen loss may be suppressed by stresses arising primarily from the paraelectric-to-ferroelectric transformation. However, for the capacitors ex situ annealed at the same low temperature, large voltage offsets were induced due to the oxygen instability of the LSMO electrodes. We show evidence that the imprint is caused by oxygen loss at the PZT∕LSMO interface, and closely related to the variation of the PZT structure.
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84.32.Tt Capacitors
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
77.22.Ej Polarization and depolarization
77.80.Dj Domain structure; hysteresis
77.80.B- Phase transitions and Curie point
68.55.-a Thin film structure and morphology
81.40.Gh Other heat and thermomechanical treatments
68.60.Bs Mechanical and acoustical properties

Artificial ferroelectricity in perovskite superlattices

Takaaki Tsurumi, Takakiyo Harigai, Daisuke Tanaka, Song-Min Nam, Hirofumi Kakemoto, Satoshi Wada, and Keisuke Saito

Appl. Phys. Lett. 85, 5016 (2004); http://dx.doi.org/10.1063/1.1825057 (3 pages) | Cited 36 times

Online Publication Date: 23 November 2004

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Artificial superlattices of SrZrO3(SZO)∕SrTiO3(STO) were fabricated by molecular-beam epitaxy. Lattice parameters showed that the lattice distortion (c∕a ratio) attained a maximum value in the [(SZO)10∕(STO)10]4 superlattice. Dielectric relaxation was observed in the [(SZO)1∕(STO)1]40 and [(SZO)10∕(STO)10]4 superlattices in the low frequency domain. Dielectric permittivity of the SZO∕STO superlattices was over 10 000 at 110 MHz. The SZO∕STO superlattices showed clear Q–V hysteresis curves, which indicated that ferroelectricity was induced artificially in the superlattices in spite of the paraelectric nature of SZO and STO. The origin of the ferroelectricity was related to the anisotropic lattice distortion in the superlattice structure.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
68.65.Cd Superlattices
77.22.Gm Dielectric loss and relaxation
77.80.Dj Domain structure; hysteresis
77.22.Ch Permittivity (dielectric function)
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Positive temperature coefficient of resistivity in Pt∕(Ba0.7Sr0.3)TiO3∕YBa2Cu3O7-x capacitors

Hao Yang, Bin Chen, Jun Miao, Li Zhao, Bo Xu, Xiaoli Dong, Lixin Cao, Xianggang Qiu, and Bairu Zhao

Appl. Phys. Lett. 85, 5019 (2004); http://dx.doi.org/10.1063/1.1827928 (3 pages) | Cited 4 times

Online Publication Date: 23 November 2004

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Pt∕(Ba0.7Sr0.3)TiO3(BST)∕YBa2Cu3O7-x(YBCO) capacitors were fabricated on (001) LaAlO3 substrates. The leakage current density-electric field characteristics were measured in the temperature range from 20 to 300 K. In the electric field region of >0.7 MV∕cm for positively biased Pt electrode and >1.4 MV∕cm for negatively biased Pt electrode, the leakage current shows space-charge-limited-current behavior from 20 to 250 K. Meanwhile a positive temperature coefficient of resistivity (PTCR) in the temperature range from 20 to 150 K was observed and attributed to the strong negative temperature dependence of the dielectric constant of BST thin films.
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84.32.Tt Capacitors
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
77.55.-g Dielectric thin films
77.22.Jp Dielectric breakdown and space-charge effects
77.22.Ch Permittivity (dielectric function)

Effective masses and complex dielectric function of cubic HfO2

J. C. Garcia, L. M. R. Scolfaro, J. R. Leite, A. T. Lino, V. N. Freire, G. A. Farias, and E. F. da Silva

Appl. Phys. Lett. 85, 5022 (2004); http://dx.doi.org/10.1063/1.1823584 (3 pages) | Cited 11 times

Online Publication Date: 23 November 2004

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The electronic band structure of cubic HfO2 is calculated using an ab initio all-electron self-consistent linear augmented plane-wave method, within the framework of the local-density approximation and taking into account full-relativistic contributions. From the band structure, the carrier effective masses and the complex dielectric function are obtained. The Γ-isotropic heavy and light electron effective masses are shown to be several times heavier than the electron tunneling effective mass measured recently. The imaginary part of the complex dielectric function ϵ2(ω) is in good agreement with experimental data from ultraviolet spectroscopic ellipsometry measurements in bulk yttria-stabilized HfO2 as well as with those performed in films deposited with the tetrakis diethylamido hafnium precursor for energies smaller than 9.5 eV.
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77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
71.20.Ps Other inorganic compounds
77.22.Ch Permittivity (dielectric function)
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
78.40.Ha Other nonmetallic inorganics
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