AVS2010 Session NS-MoM: Oxide Based Nanoelectronics

Crystalline tungsten oxide nanoparticles were synthesized by hot-wire chemical vapor deposition (HWCVD). These materials are being examined for use in numerous energy related applications including electrochromic windows and fuel cells. It is possible to tune the particle morphology by changing key synthesis parameters including filament temperature, substrate temperature, and oxygen partial pressure. The resulting nanostructures are characterized by a number of techniques including transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. The dependence of nanoparticle size and morphology will be described both as a function of HWCVD synthesis conditions as well as post-deposition annealing treatments. The resulting nanoparticles are suspended in solution and used to form thin films on transparent conducting oxide coated glass substrates using an ultrasonic spray deposition process. Ultrasonic spray coating is a cost effective, scalable deposition process that offers an excellent route to achieve large-scale implementation of electrochromic films. Important ultrasonic spray variables include substrate temperature, precursor concentration, carrier solvent and other parameters related to solution atomization. The electrochromic properties of these films were characterized by performing cyclic voltammetry in registry with in situ measurements of optical transmission. Particular attention is paid to optimizing performance metrics such as coloration efficiency and cycling stability. Using the measurements described above, we will evaluate the important process-structure-performance relationships in these systems.

Advances in electrochemical energy storage science require the development of new or the refinement of existing in situ probes that can be used to establish structure – activity relationships for technologically relevant materials. The drive to develop reversible, high capacity electrodes from nanoscale building blocks creates an additional requirement for high spatial resolution probes to yield information of local structural, compositional, and electronic property changes as a function of the storage state of a material. In this paper, we describe a method for deconstructing a lithium ion battery positive electrode into its basic constituents of ion insertion host particles and a carbon current collector. This model system is then probed in an electrochemical environment using a combination of atomic force microscopy and tunneling spectroscopy to correlate local activity with morphological and electronic configurational changes. Cubic spinel Li_1+xMn_2-xO₄ nanoparticles are grown on graphite surfaces using vacuum deposition methods. The structure and composition of these particles are determined using transmission electron microscopy and Auger microprobe analysis. The response of these particles to initial de-lithiation, along with subsequent electrochemical cycling, is tracked using scanning probe microscopy techniques in polar aprotic electrolytes (lithium hexafluorophosphate in ethylene carbonate:diethylcarbonate). The relationship between nanoparticle size and reversible ion insertion activity will be a specific focus of this paper.

This work is funded within the Nanostructures for Electrical Energy Storage, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DESC0001160. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for US DOE’s NNSA under contract DE-AC04-94AL85000.

Electronic states with unusual properties can be promoted at interfaces between complex oxides [1]. A striking example is the interface between the band insulators LaAlO₃ and SrTiO₃, which displays conductivity with high mobility and 2D superconductivity [2,3]. We will discuss recent experiments that revealed the sensitivity of the normal and superconducting state to external electric fields. Using the electrostatic field effect, the phase diagram of the system has been mapped out, revealing a quantum phase transition between a superconducting state and an insulating state [4]. We will also lay out an example of an electronic property arising from the interfacial breaking of inversion symmetry, namely a large spin-orbit interaction, whose magnitude can be modulated by the application of an external electric field [5].

[1] E. Dagotto, Science 318, 1076 (2007)

[2] A. Ohtomo, H. Y. Hwang Nature 427, 423 (2004)

[3] N. Reyren, S. Thiel, A. D. Caviglia, L. F. Kourkoutis, G. Hammerl, C. Richter, C. W. Schneider, T. Kopp, A.-S. Ruetschi, D. Jaccard, M Gabay, D. A. Muller, J.-M Triscone, J Mannhart Science 317, 1196 (2007).

[4] A. D. Caviglia, S. Gariglio, N. Reyren, D. Jaccard, T. Schneider, M Gabay, S. Thiel, G. Hammerl, J. Mannhart, J.-M Triscone Nature 456, 624 (2008).

[5] A. D. Caviglia, M Gabay, S. Gariglio, N. Reyren, C. Cancellieri, J.-M Triscone Phys. Rev. Lett. 104, 126803 (2010).

Low density (≤ 4 Mb) non-volatile ferroelectric random access memories (FeRAMs) are now in the market in “smart cards” competing successfully against established non-volatile memories such as FLASH and EEPROM. The next frontier in fundamental and applied science relevant to FeRAMs is to develop film synthesis processes, materials integration strategies, device architecture, and fabrication processes needed to produce ferroelectric nanocapacitors and study ferroelectric performance at the nanoscale in real device conditions. Critical parameters that need to be measured are polarization switching, fatigue, retention, and imprint and leakage currents, since all these parameters play a critical role in the performance of a reliable FeRAM. Until now, most studies of polarization domain configuration and dynamics in ferroelectric nanocapacitors have been done using the Atomic Force Microscope piezoresponse force microscopy (PFM) technique that images the polarization via the piezoelectric deformation of the polarized region. However, nanocapacitors performance relevant to high density (≥ 1 Gb) FeRAMS need to be evaluated via measurements of all the parameters mentioned above. This paper describes our effort to fabricate BiFeO₃ nanocapacitors (≤ 200 nm in diameter) in a matrix address configuration to measure the ferroelectric properties described above at the nanoscale. BiFeO₃ is explored in this work as the ferroelectric capacitor layer, because it exhibits high polarization (90-120 µC/cm²), it does not contain Pb as PbZr_xTi_1-xO₃ (PZT), and has much higher polarization than SrBi₂Ta₂O₉ (SBT) (~ 30 µC/cm²), which is a leading ferroelectric material in “smart cards” based on FeRAMS. The BFO nanocapacitors involve a SrRuO₃ (SRO)/BFO/SRO layered film structure grown on Strontium Titanate (STO) single crystal substrate. All films are grown by off-axis magnetron sputter-deposition. Nanocapacitors are fabricated with top and bottom electrodes in a matrix line configuration, using electron beam lithography and reactive ion etch processes developed by our group. The BFO nanostructure is at the crossing of the top and bottom electrode lines. The characterization of the nanocapacitor’s performance involves measurement of polarization using electrical excitation via voltage applied between the bottom and top electrodes. Correlations between the film thickness and diameter with the nanocapacitor’s polarization characteristics will be discussed.

Use of the Center for Nanoscale Materials is supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

The intrinsic coupling of soft-phonon order parameters and electron transport in ferroic materials can usher a wide range of novel physical phenomena with potential for new applications in information technology, energy harvesting and quantum computing. In this talk we will present local conductivity and piezoresposne measurements on the surfaces of uniaxial (Pb(Zr_0.2Ti_0.8)O₃) and multiaxial (BiFeO₃) perovskite ferroelectrics, with film thicknesses ranging from 30 nm to 100 nm. Conductive atomic force microscopy revealed that most of these films possesses highly non-linear, and often hysteretic current-voltage characteristics, and in many cases the hystereses could be correlated to local polarization switching induced by the electric field of the AFM tip. In lead zirconate titanate, the large spontaneous polarization produced up to 500-fold enhancement of local conductivity, and the film remained sufficiently conducting in the bias-region significantly smaller than the switching voltage. As a result, this effect can be used for a non-destructive and resistive read-out of the polarization state on length-scales down to 10 nm, implementing a prototypical memory function. Extending the I-V measurements to low-temperatures revealed a strong exponential dependence of the conductivity. We developed a novel analysis scheme, which enabled identifying trap-assisted Fowler-Nordheim tunneling and Poole-Frenkel hopping as two dominant mechanisms behind non-linear I-V curves. Curiously, we have been able to separate the contributions due to interface- and bulk-limited conduction, as well as to visualize spatially-resolved variations due to each transport regime.

We will further discuss the peculiarities of local electron transport through BiFeO₃, and in particular the mechanism behind local conductivity of 109^o ferroelastic domain walls. Based on a statistical analysis of I-V curves and simultaneous measurements of local transport and piezoresponse, we suggest that the domain wall is not a static conducting object under a biased tip, but instead that a transient, local and microscopically reversible topological distortion of polarization structure at the wall contributes to enhanced electron transport. In particular, it produces a seminal example of ferroic memristive functionality.

The measurements were conducted at the Center for Nanophase Materials Sciences sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. DOE.

[1] P. Maksymovych, S. Jesse, P. Yu, R. Ramesh, A. P. Baddorf, S. V. Kalinin,

Science 324 (2009) 1421.

[2] P. Maksymovych, J. Seidel et. al, submitted (2010)