Cambridge MIT Institute Grant CMI001
Optical Properties of Nanoscale Arrays
Professor James F. Scott, Department of Earth Sciences, University of Cambridge
Professor Keith A. Nelson, Department of Chemistry, MIT
Progress Report
October 22, 2003
Research conducted in the Cambridge and MIT groups since the project starting date of
2/1/01 are summarized. Plans for synthesizing the results from both groups are outlined
at the end.
Cambridge
The Cambridge efforts have been directed toward fabrication of novel ferroelectric
structures and two-dimensional, ordered arrays of such structures which are expected to
have important applications in photonics and other areas.
Initial efforts were directed toward exploration of sample preparation with Liquid
Source Misted Chemical Deposition (LSMCD) equipment. Flat Sr0.8Ta2.0Bi2.2O9 (SBT)
films were deposited successfully on Si-wafer fragments using Sr-, Bi-, Taethylhexanoate precursors. In a separate early effort, a homemade Rapid Thermal
Annealer (RTA) was built by conversion of a commercial tube furnace. This allowed the
thermal processing of films to be monitored to allow optimisation of film crystallisation.
Next, new method for introducing the liquid precursors into the mist generator was
designed and fitted. This allowed atmosphere sensitive precursor solutions such as
alkoxides to be deposited. (This was previously impossible due to the original design of
the LSMCD system). The new system allowed investigation of a novel alkoxide Biprecursor developed by Inorgtech (now Epichem): Bi (mmp)3, where mmp = 1-methoxy2-methyl-2-propoxide (OCMe2CH2OMe). We also investigated Hf and Zr tertiary
butoxide precursors to deposit HfO2 and ZrO2 films for gate oxide applications. It was
shown that Bi(mmp)3 was not as successful in producing crystalline SBT using this
method. Crystalline HfO2 films were successfully grown; the ZrO2 films were not as
crystalline.
An MIT summer student (Ryan Klinczak) visited for a 2-month summer project
investigating the use of LSMCD to deposit SBT films on Si/SiO2 and Si/SiO2/Ti/Pt wafers
using ethylhexanoate precursors.
First attempts at filling porous photonic silicon substrates with SBT using
ethylhexanoates were also carried out. The filling mechanism was shown to progress
by precursor coating the inside walls of pores rather than a “bottom-up” filling
mechanism. Crystalline SBT was successfully made inside the pores of the substrates.
The following patent application was made:
Filled Porous Substrates, Their Production and Use, Finlay D. Morrison and James
F. Scott, UK Patent Application GB0213235.5, filed 10 June 2002.
A summary of results for this period was presented as an invited talk at the 8 th
International Conference on Electronic Materials and appears as the publication:
Use of the "Mist" (Liquid-Source) Deposition System to Produce New HighDielectric Devices: Ferroelectric-filled Photonic Crystals and Hf-oxide and Related
Buffer Layers for Ferroelectric-Gate FETs, F.D. Morrison, J.F. Scott, M. Alexe, T.J.
Leedham, T. Tatsuta and O. Tsuji, Microelectronics Engineering, 66 (1-4), 591-599,
2003.
Given the pore wetting mechanism of SBT in porous photonic Si discovered
previously, attempts were made to use the Si substrate as a sacrificial template to
produce SBT tubes with nm thick walls and very high aspect ratio. After finding a
suitable etchant and etching conditions, perfectly registered arrays of uniform SBT tubes
were fabricated. The tubes were ca. 100 m long and 2-3 m in diameter with wall
thickness of ca. 200 nm. By careful control of the etching conditions the bottom of the
tubes were left embedded in the host Si matrix producing an array rather than freestanding tubes.
A final year Physics undergraduate from Cambridge (Miss Laura Ramsay)
completed a 2 month research project helping with attempts to fabricate tubes with
smaller dimensions.
A new sample stage was fitted to the LSMCD apparatus to allow application of a
dc voltage bias to the substrate in order to investigate electrostatic effects. The new
stage also allowed the substrate to be heated to ca. 100 °C.
Two more patent applications were filed during this period:
Deposition of Layers on Substrates, Finlay D. Morrison and James F. Scott, UK
Patent Application GB0302655.6, 5 February 2003.
Processes of Forming Small Diameter Rods and Tubes, Finlay D. Morrison and
James F. Scott, UK Patent Application GB0302654.9, 5 February 2003.
Finally, a smaller array of uniform SBT tubes of diameter 800 nm was
successfully prepared. The tubes had a uniform wall thickness of ca. 40 nm and were
ca. 80 m long. Again, these were attached to the Si matrix at the tube base resulting
in a perfectly registered array. Different methods of coating the pore walls of the Si
substrate were investigated including dipping and blotting. To date neither have been
particularly successful. The application of a dc bias voltage to substrates was also
investigated. It appears that, under the fileds applied to date, there has been little or no
effect on the filling of the porous substrates.
Current efforts are directed at fabricating electrical contacts on the SBT tubes to
demonstrate ferroelectricity and to characterise them electrically. This involves
deposition of successive coatings of metal electrode and SBT on the pores to provide a
concentric electrode/SBT/electrode structure in each pore. A formulation of a
biodegradable polymeric host for the metal salt is required and also subsequent
integration with the SBT coating and crystallisation process.
Results to date have been presented at 2 large international conferences: 15th
International Symposium on Integrated Ferroelectrics, Colorado March 9-12 2003 and
10th European Meeting on Ferroelectricty, Cambridge August 4-8 2003. There have
also been 2 publications describing the work form this period: High-Aspect-Ratio
Piezoelectric Strontium-Bismuth-Tantalate Nanotubes, Finlay D. Morrison, Laura
Ramsay and James F. Scott, J. Phys. Cond. Matt., 15, L527-L532, 2003 and
Ferroelectric Nanotubes, F.D. Morrison, Y. Luo, I. Szafraniak, V. Nagarajan, R.B.
Wehrspohn, M. Steinhart, J.H. Wendorff, N.D. Zakharov, E.D. Mishina, K.A. Vorotilov,
A.S. Sigov, S. Nakabayashi, M. Alexe, R. Ramesh, and J.F. Scott, Rev. Adv. Mater. Sci.,
in press.
The figure below shows our most recent photonic arrays of porous nanotubes of
ferroelectric strontium bismuth tantalate. (a) is top view (plan) before etching away the
sacrificial Si substrate. (b) an SEM cross section showing photonic array embedded in
hexagonal Si pore array at bottom. (c) and (d) final photonic arrays, perfectly registered,
of order 1000x1000 (40 nm wall thicknesses).
MIT
The MIT efforts have been aimed at development of novel time-resolved optical
methods for characterization of the dynamical polarization responses of the ferroelectric
structures fabricated in Cambridge. The methods are based on the use of ultrashort light
pulses for generation of waves of terahertz (THz) frequency, called polariton waves, in
ferroelectric crystals. The techniques promise wide-ranging applications in optical
characterization manipulation of ferroelectric and other advanced materials. The MIT
CMI effort has included both computer simulations and experimental measurements of
polariton wave propagation in ferroectric structures. A central objective is the design and
fabrication of ferroelectric tips through which polariton waves can be focused for nearfield scanning microscopy at THz frequencies. This is particularly well suited for study of
the small structures fabricated by the Cambridge group.
In an early effort, experimental demonstration of THz polariton signal imaging with
second harmonic generation was achieved. This is likely to be the most promising
method since it can be applied to very small structures and very small total amounts of
material, appropriate for the samples fabricated at Cambridge.
Experimental observations were made of THz signal propagation through a twodimensional tip-like ferroelectric structure that was fabricated through femtosecond laser
machining. Although the size scale of this tip was too large for the applications of
primary interest, the observations confirmed the prospects for continued success at
much smaller scales. This requires far more care, however, as subwavelength scales
are approached and tip throughput becomes a major issue. Optimisation of the tip
material, including high dielectric constant for reduction of the THz wavelength, and
optimisation of the tip structure for waveguide mode selectivity and enhanced throughput
are major objectives.
Experimental control over THz polariton responses, including THz focusing,
waveform shaping, and propagation and guidance through patterned structures, was
demonstrated. These developments allow specified THz signals to be directed to
specified locations on a sample, with applications in control and characterization of the
structures fabricated by the Cambridge group.
A new femtosecond laser system optimised for improved generation, control, and
imaging of THz signals in complex ferroelectric structures was designed, ordered, and
constructed. The use of the system includes femtosecond pulse shaping through which
the optical outputs are optimised for generation of specified THz responses, and
polariton imaging through which the THz waves are monitored. The following paper that
describes important pulse shaping refinements has been accepted for publication.
Automated spatiotemporal diffraction of ultrashort pulses, J.C. Vaughan, T. Feurer,
and K.A. Nelson, Optics Letters, in press.
Initial attempts at simulation of polariton signal propagation in ferroelectric
structures were carried out based on single PC capabilities and low dimensional models
for reduced computational demands. Simulation of polaritons in two-dimensional models
for bulk samples, thin films, rectangular waveguides, resonators, and other functional
elements was carried out. These simulations were shown to reveal some but not all of
the important properties of THz signals in complex three-dimensional structures. A Ph.D.
thesis on THz propagation through complex ferroelectric structures, including
experiments and two-dimensional simulations, was completed. A detailed paper based
on this work is currently in preparation.
A multi-node Beowulf computer cluster for simulation of THz polariton signal
propagation in three-dimensional ferroelectric structures was designed, purchased, and
assembled. Code for simulation of three-dimensional structures, involving parallel
computation with all 24 nodes, was written. Three-dimensional simulations have been
optimized for THz polariton propagation in ferroelectric materials. The results have been
checked successfully against experiment and against analytical theory in cases where
the latter can be applied. These simulations are now fully operational for complex
structures including waveguides, patterned devices, and hybrid material assemblies.
Three-dimensional simulations revealed the THz polariton wavevector component
in the third dimension (along the direction of the optical excitation pulse used to produce
the THz wave) and its consequences for our optical control and imaging experiments.
Experimental results from THz polaritons in resonator structures, with associated
three-dimensional simulations, have been completed. The experiments include different
optical excitation fields for narrowband or broadband THz responses in the resonators,
and they include multiple excitation pulses with scanning repetition rates to find
resonance frequencies. These results are being written up currently.
Discussions were held with a patent attorney, Dr. Marc Wefers of Fish &
Richardson, concerning patenting of THz near-field imaging based on ferroelectric tips,
and other IP issues. Three-dimensional simulations of tip structures are called for to
optimise the design of tips for this application. These simulations have been coded and
initiated. Investigation of prior art for THz near-field microscopy was carried out and a
draft disclosure for the MIT Technology Licensing Office (TLO) is nearly complete.
Plans for continued work
Both the Cambridge and MIT groups are producing scientific results and IP that
will continue to generate interest. At the same time, synthesis of the efforts is
accelerating. Several of the novel samples fabricated at Cambridge have been sent to
the MIT group for study. The properties of these samples were not optimal for the MIT
measurements, and both the samples and the measurements are being modified to
permit coupling to each other. The objectives are optical characterization of the fast
(GHz-THz) polarization responses of the Cambridge materials, to assess their suitability
for high-bandwidth applications, and, most ambitiously, optical control over the
ferroelectric states of the materials with potentially novel applications in memory,
switching, and signal processing.
Publications:
Use of the "Mist" (Liquid-Source) Deposition System to Produce New High-Dielectric Devices:
Ferroelectric-filled Photonic Crystals and Hf-oxide and Related Buffer Layers for Ferroelectric-Gate
FETs, F.D. Morrison, J. F. Scott, M. Alexe, T.J. Leedham, T. Tatsuta and O. Tsuji, “Proceedings of IUMRS
8th International Conference on Electronic Materials, 2002, Xi’an, China”, Microelectronics Engineering,
66 (1-4), 591-599, 2003.
High-Aspect-Ratio Piezoelectric Strontium-Bismuth-Tantalate Nanotubes, Finlay D. Morrison, Laura
Ramsay and James F. Scott, J. Phys.: Condens. Matt., 15, L527-L532, 2003.
Ferroelectric Nanotubes, F. D. Morrison, Y. Luo, I. Szafraniak, V. Nagarajan, R. B. Wehrspohn, M.
Steinhart, J. H. Wendroff, N. D. Zakharov, E. D. Mishina, K. A. Vorotilov, A.S. Sigov, S. Nakabayashi, M.
Alexe, R. Ramesh, and J. F. Scott, Rev. Adv. Mat. Sci., 4 (2), 114-122, 2003.
Automated spatiotemporal diffraction of ultrashort pulses, J.C. Vaughan, T. Feurer, and K.A. Nelson,
Opt. Lett., in press.
Patents:
Filled Porous Substrates, Their Production and Use, Finlay D. Morrison and James F. Scott, UK
Patent Application GB0213235.5, filed 10 June 2002.
Deposition of Layers on Substrates, Finlay D. Morrison and James F. Scott, UK Patent Application
GB0302655.6, 5 February 2003.
Processes of Forming Small Diameter Rods and Tubes, Finlay D. Morrison and James F. Scott, UK
Patent Application GB0302654.9, 5 February 2003.