Si:SrTiO3-Al2O3-Si:SrTiO3 multi-dielectric architecture for metal-insulator-metal capacitor applications

Metal-insulator-metal (MIM) capacitors comprised of amorphous Si:SrTiO3-Al2O3-Si:SrTiO3 multi-dielectric architecture have been fabricated employing a combination of pulsed laser and atomic layer deposition techniques. The voltage linearity, temperature coefficients of capacitance, dielectric and electrical properties upon thickness were studied under a wide range of temperature (200 – 400 K) and electric field stress (± 1.5 MV/cm). A high capacitance density of 31 fF/µm2, a low voltage coefficient of capacitance of 363 ppm/V2, a low temperature coefficient of capacitance of < 644 ppm/K and an effective dielectric constant of ~133 are demonstrated in a MIM capacitor with ~1.4 nm capacitance equivalent thickness in a ~40 nm thick ultra high-k multi-dielectric stack. All of these properties make this dielectric architecture of interest for next generation highly scaled MIM capacitor applications.PostprintPeer reviewe

Disorder driven structural and dielectric properties of silicon substituted strontium titanate

Financial support from NSF Grant No. NSF-RII-0701525 was acknowledged. S.D. is thankful to DOD for doctoral fellowship under Grant No. W911NF-11-1-0204. S.P.P. is grateful to NSF for financial assistance under Grant No: NSF-EFRI RESTOR # 1038272.A systematic study on structural, microstructural, optical, dielectric, and electrical properties of phase-pure silicon-modified SrTiO3 polycrystalline electroceramics synthesized using high energy solid state reaction techniques is presented. The asymmetry and splitting in the x-ray diffractometer spectra and the observation of first order transverse optical TO2 and longitudinal optical LO4 modes in Raman spectra (nominally forbidden) revealed the distortion in the cubic lattice as a result of breaking of inversion symmetry due to doping. A bandgap Eg of 3.27 eV was determined for the sample by diffuse reflectance spectroscopy. A high dielectric constant of -400 and very low dielectric loss of -0.03 were obtained at 100 kHz near ambient conditions. The temperature dependence of the dielectric data displayed features of high temperature relaxor ferroelectric behavior as evidence of existence of polar nano-regions. The ac conductivity as a function of frequency showed features typical of universal dynamic response and obeyed a power law σac = σdc+Aωn . The temperature dependent dc conductivity followed an Arrhenius relation with activation energy of 123 meV in the 200–500 K temperature range. The linear dielectric response of Pt/SrSi0.03Ti0.97O3/Pt dielectric capacitors was well characterized. The measured leakage current was exceptionally low, 13 nA/cm2 at 8.7 kV/cm, revealing an interface blocked bulk conduction mechanism.Publisher PDFPeer reviewe

Effect of off-center ion substitution in morphotropic lead zirconate titanate

This research was supported by the DOD Grant No. FA9550–16-1–0295. S.P.P. gratefully acknowledges the financial assistance from IFN-NSF under the Grant No. EPS-1002410.A detailed study of the effect of off-center donor ion (Sc3+) substitution on structural, microstructural, optical, dielectric, electrical and ferroelectric properties of morphotropic lead zirconate titanate electroceramics with the stoichiometric formula Pb0.85Sc0.10Zr0.53Ti0.47O3 (PSZT), synthesized using a high-energy solid-state reaction technique, was carried out. Powder x-ray diffractometry was used to identify the stabilized tetragonal phase (space group P4mm) with considerably reduced tetragonal strain, c/a = 1.005. An analysis of the thermal dependence of the Raman results indicated a smooth order-disorder displacive (ferroelectric-paraelectric) phase transition as revealed by the observed disappearance of the soft modes A1 (1TO) and A1 (2TO) above 460 K. The dielectric response of Pt/PSZT/Pt metal-ferroelectric-metal (MFM) capacitors was probed over a wide range of thermal excursions (85-600 K) and ac signal frequencies (102-106 Hz). Thermally activated dynamic and static conduction processes indicate hopping conduction mechanism (Eact ≤ 0.015 eV) and the formation of small polarons caused by the electron and/or hole-lattice (phonon) interaction (Eact ≥ 0.1 eV) at low (100−300 K) and high temperatures (300−600 K), respectively. The reduction in remnant polarization obtained is in good agreement with the largely reduced tetragonal strain observed in this sample, ( Pr ∝√(c/a-1) ). DC current conduction is dominated by Poole-Frenkel mechanism that assumes a Coulombic attraction between de-trapped electrons and positively charged stationary defects in the polycrystalline matrix.PostprintPeer reviewe

Ferroelectric photovoltaic properties in doubly substituted (Bi0.9La0.1)(Fe0.97Ta0.03)O3 thin films

This work was supported by the DOE-EPSCoR Grant No. DE-FG02-08ER46526. Acknowledgment is also due to NSF Grant No. #1002410 for providing fellowships to R.K.K., D.B., and J.S.Y.Doubly substituted [Bi0.9La0.1][Fe0.97Ta0.03]O3 (BLFTO) films were fabricated on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition. The ferroelectric photovoltaic properties of ZnO:Al/BLFTO/Pt thin film capacitor structures were evaluated under white light illumination. The open circuit voltage and short circuit current density were observed to be ∼0.20 V and ∼1.35 mA/cm2, respectively. The band gap of the films was determined to be ∼2.66 eV, slightly less than that of pure BiFeO3 (2.67 eV). The PVproperties of BLFTO thin films were also studied for various pairs of planar electrodes in different directions in polycrystalline thin films.Publisher PDFPeer reviewe

Unipolar resistive switching in planar Pt/BiFeO3/Pt structure

This work was supported by the NASA EPSCoR Grant # NNX13AB22A. Financial support to various researchers from IFN-NSF grant # EPS 1002410 (RSK, DB, YS and BRW) is gratefully acknowledged. S. K. is thankful to UGC, India for a Raman fellowship under Indo-US 21st century knowledge initiatives (No:5-53/2013(I.C)).We report unipolar resistive switching suitable for nonvolatile memory applications in polycrystalline BiFeO3 thin films in planar electrode configuration with non-overlapping Set and Reset voltages, On/Off resistance ratio of similar to 10(4) and good data retention (verified for up to 3,000 s). We have also observed photovoltaic response in both high and low resistance states, where the photocurrent density was about three orders of magnitude higher in the low resistance state as compared to the high resistance state at an illumination power density of similar to 100 mW/cm(2). Resistive switching mechanisms in both resistance states of the planar device can be explained by using the conduction filament (thermo-chemical) model. (C) 2015 Author(s).Publisher PDFPeer reviewe

Freestanding n-Doped Graphene via Intercalation of Calcium and Magnesium into the Buffer Layer - SiC(0001) Interface

The intercalation of epitaxial graphene on SiC(0001) with Ca has been studied extensively, yet precisely where the Ca resides remains elusive. Furthermore, the intercalation of Mg underneath epitaxial graphene on SiC(0001) has not been reported. Here, we use low energy electron diffraction, x-ray photoelectron spectroscopy, secondary electron cut-off photoemission and scanning tunneling microscopy to elucidate the physical and electronic structure of both Ca- and Mg-intercalated epitaxial graphene on 6H-SiC(0001). We find that Ca intercalates underneath the buffer layer and bonds to the Si-terminated SiC surface, breaking the C-Si bonds of the buffer layer i.e. 'freestanding' the buffer layer to form Ca-intercalated quasi-freestanding bilayer graphene (Ca-QFSBLG). The situation is similar for the Mg-intercalation of epitaxial graphene on SiC(0001), where an ordered Mg-terminated reconstruction at the SiC surface and Mg bonds to the Si-terminated SiC surface are formed, resulting in Mg-intercalated quasi-freestanding bilayer graphene (Mg-QFSBLG). Ca-intercalation underneath the buffer layer has not been considered in previous studies of Ca-intercalated epitaxial graphene. Furthermore, we find no evidence that either Ca or Mg intercalates between graphene layers. However, we do find that both Ca-QFSBLG and Mg-QFSBLG exhibit very low workfunctions of 3.68 and 3.78 eV, respectively, indicating high n-type doping. Upon exposure to ambient conditions, we find Ca-QFSBLG degrades rapidly, whereas Mg-QFSBLG remains remarkably stable.Comment: 58 pages, 10 figures, 4 tables. Revised text and figure