Achieving stable single-digit nanometer inverted domains in ferroelectric thin films is a fundamental issue that has remained a bottleneck for the development of ultrahigh density (>1  Tbit/in.^2) probe-based memory devices using ferroelectric media. Here, we demonstrate that such domains remain stable only if they are fully inverted through the entire ferroelectric film thickness, which is dependent on a critical ratio of electrode size to the film thickness. This understanding enables the formation of stable domains as small as 4 nm in diameter, corresponding to 10 unit cells in size. Such domain size corresponds to 40  Tbit/in.^2 data storage densitie

Collier, C. Patrick

Franklin, Nathan

Giapis, Konstantinos P.

Narui, Yoshie

Nishi, Yoshio

Tayebi, Noureddine

Zhang, Yuegang

English

Caltech Authors - Main

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Supplementary information for: 
 
Fully-inverted Single-Digit Nanometer Domains in Ferroelectric Films 
 
 
Noureddine Tayebi,1,2 Yoshie Narui,3 Nathan Franklin,1 C.Patrick Collier,3, 4 Konstantinos P. 
Giapis,3 Yoshio Nishi,2 and Yuegang Zhang1, 5, * 
 
1 Intel Corporation, 2200 Mission College Boulevard, Santa Clara, California 95054, USA. 
 
2 Department of Electrical Engineering, Stanford University, 420 Via Palou Mall, Stanford, 
California 94305, USA. 
 
3 Division of Chemistry and Chemical Engineering, California Institute of Technology, 
Pasadena, California 91125, USA. 
 
4 Oak Ridge National Laboratory, Center for Nanophase Materials Sciences, Oak Ridge, TN 
37831, USA. 
 
5 The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, 
USA. 
 
 
* Corresponding author, e-mail: yzhang5@lbl.gov 
 
 
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Effect of electrode size increase by gold nanoparticle reduction 
Figure S1 shows other nanopencils with individual SWNT electrodes (2–3 nm in diameter), 
which were exposed using a nanosecond pulse etching technique (see subsequent sections for 
procedure).  Using these nanopencils, no inverted domains were observed on the 50 nm thick 
PZT film even when 10 V pulses were applied for 100 µs.  Instead of decreasing the film 
thickness, an experiment was tried where the electrode size was increased by electrochemically 
forming a gold nanoparticle at the exposed tip (see subsequent sections for procedure).  Figures 
S1b,c show TEM images of the same probe with a 3nm SWNT electrode before and after gold 
nanoparticle formation, respectively.  The size of the electrode was thus increased from 3 nm to 
about 250 nm.  Note that gold reduction occurred only at the exposed end of the SWNT 
electrode, which confirms that the SWNT electrodes were electrically insulated by the SiOx 
sheath except at the exposed end.  This excludes current leakage through the sheath during the 
writing process using individual SWNT electrodes that might have been at the origin of the non-
inverted domains on this film.  The electrode with the gold nanoparticle was used to apply 10 V 
pulses on the 50 nm PZT film.  This time inverted domains of 82.5 nm were observed [Fig. S1d].  
The noisy nature of the inverted domains is due to the rough surface of the reduced gold 
nanoparticles, which affects the contact between the probe and PZT film.  The observation of 
domain inversion with the larger electrode reaffirms the dependence of domain stability on the 
ratio of electrode size to the ferroelectric film thickness. 
 
 
 
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Figure S1.  Effect of electrode size increase on domain inversion of 50 nm thick PZT film.  (a) 
TEM image of a nanopencil probe with a 2 nm SWNT electrode exposed using the pulse etching 
process (see Methods).  No inverted domains were observed on the 50 nm PZT film using this 
probe at a 10 V pulse.  (b) TEM image of a nanopencil probe with 3 nm SWNT electrode.  (c) 
TEM image of the nanopencil probe shown in b after electrochemical reduction of gold 
nanoparticles onto the exposed SWNT (see Methods).  The electrode size increased from 3 nm to 
250 nm.  (d) PFM height (top), amplitude (middle) and phase (bottom) images of a 50nm-thick 
PZT-film surface with 82.5 nm ferroelectric inverted domains formed by applying 10 V pulses to 
the film through the nanopencil shown in (c). 
 
(a) (b)
250 nm
82.5 nm
250 nm
250 nm
(c) (d)
250 nm
20 nm20 nm
2 nm 3 nm
50 nm
10 V 
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SWNT pick up and dielectric coating 
As grown SWNTs are attached to commercially available metal-coated conductive atomic force 
microscope (AFM) probe-tips (Nanosensors Inc.) using the pick-up technique developed by 
Lieber and co-workers (ref. S1).  Briefly, isolated and vertically aligned SWNTs are grown on an 
oxidized silicon substrate by chemical vapor deposition.  Silicon substrates are spin coated with 
several drops of iron catalyst solution at 3000 rpm until dry.  The substrates are then placed into 
a tube furnace at 900º C purged with 500 standard cubic centimeters (sccm) of argon and 
annealed for 10 min under a flow of 500 sccm argon and 100 sccm hydrogen.  Methane is added 
at 1000 sccm for 7 min followed by a return to annealing conditions for 10 min.  Then, the 
metal-coated tips are used to image the vertically grown SWNTs using a Digital Instruments 
multimode AFM with a Nanoscope IV controller.  During scanning, the probe-tips pick up 
individual SWNTs (or a bundle of SWNTs), which attach to the probe-tips by van der Waals 
forces and thus create SWNT-based probes. 
Subsequently, the SWNT probe-tips are placed on a grounded electrode in a low-pressure 
glass reactor.  An argon gas flow at 7.5 sccm is introduced to the reactor through a bubbler 
containing tetraethyl orthosilicate (TEOS) liquid kept at 20oC.  The reactor pressure is controlled 
at 95 mTorr.  An inductively coupled plasma is then struck upstream of the electrode at a power 
of 100 Watts and a frequency of 13.56 MHz.  The SiOx deposition continues for two minutes.  
Successful nanopencil fabrication is verified using a Phillips EM420 transmission electron 
microscope (TEM) operating at 120 kV. 
 
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SWNT electrode exposure using the sharpening process 
The SWNT electrode exposure, or nanopencil “sharpening”, was performed using an Asylum 
Research MFP-3D scanning probe microscope.  This is achieved by repetitively scanning a 
20×20 µm area of a conductive diamond film with a root mean squared roughness of 9 nm (a 
part of the base of a conductive diamond AFM tip from Nanosensors inc.).  The scanning was 
performed under contact mode at a speed of 100 µm/s while applying a 0.5 V bias between the 
nanopencil and the diamond surface.  The height and current signals were simultaneously 
monitored and recorded [Fig. 2 in manuscript] using the conductive AFM module.  A force of 5 
nN was used for all the SWNT electrode exposures.  Scanning is stopped once the current starts 
to flow indicating that the SWNT electrode is exposed.  A smaller (1×1 µm area) scan was then 
performed at a 2.5 µm/s speed and I–V curves were taken at highly conductive spots identified in 
the current distribution scan to confirm the nanopencil conductivity.  Successful electrode 
exposures were further verified using a Phillips EM420 TEM operating at 120 kV. 
 
PZT ferroelectric film growth 
Atomically smooth (RMS = 0.17˗0.2 nm), single crystal PZT films are deposited on a 50 nm 
single crystal SrRuO3 (SRO) film deposited on SrTiO3(100) using metal-organic chemical vapor 
deposition (MOCVD) at a 605 oC temperature under a 5 torr pressure with Ti, Zr, Pt precursor 
fluxes of 0.07 ml/min, 0.025 ml/min and 0.165 ml/min, respectively (ref. 14 in manuscript). 
 
Data write and read 
The write/read experiments were performed using an Asylum Research MFP-3D scanning probe 
microscope.  The nanopencils are used as conducting probes to write inverted domain dots on the 
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PZT films following the same method described in ref. 7 in manuscript.  The ferroelectric PZT 
films are initially polarized in an upward direction and placed on a grounded electrode.  A 
nanopencil probe is first brought into contact with the film surface.  Positive electrical pulses 
(varying from 3 to 10 V) with a pulse width of at most 100 µs are then applied to the PZT 
surface through the conductive nanopencil core.  The ferroelectric domains are imaged using the 
same nanopencil probes under the piezoresponse-force microscopy (PFM) mode where domain 
walls show dark contrast in amplitude images and the inverted domains show 180° phase shift to 
the background film. 
 
Electrical characterization of PZT films 
25 nm thick platinum pads (20×20 μm2 or 25×25 μm2 in size) are deposited on the PZT films to 
serve as a top electrode.  The SRO substrate is used as a bottom electrode.  The capacitance vs 
bias measurements of the metal-PZT-metal capacitor are carried out using an Agilent 
Technology LCR-meter at a 1 MHz frequency with a 50 mV ac bias.  The polarization vs bias 
measurements are carried out using a Radiant Technology system at a 10 KHz frequency. 
 
Modeling of electric field mapping within the PZT film 
The modeling of the electric field component along the polarization axis was modeled using the 
COMSOL-MULTTIPHYSICS package.  The SWNT electrode of various sizes is modeled as a 
metallic wire sheathed with a 65 nm thick SiOx film in contact with the PZT film on a grounded 
metallic electrode.  The properties of the PZT films determined experimentally are used in the 
simulations.  The bias is applied to the electrode and fixed charge and fixed field assumptions are 
adopted. Axial symmetry is assumed in the model. 
 7
 
SWNT electrode exposure using the high voltage (HV) pulse etching process 
A nanosecond HV-pulse etching method was also used to remove the SiOx located at the apex of 
a fully insulated SWNT probe [Figs. 3a,b in manuscript].  The probe is brought within a few 
nanometers of a grounded platinum surface and a pulse of at least 20 V is then applied.  The 
probes are mounted into an electrostatic force microscopy (EFM) tip holder, which allows for the 
direct probe biasing.  The separation distance between the tip and the surface (z-position) is 
controlled by a feedback loop in force calibration mode.  Using a signal access module (break-
out box), the low voltage z-position signal is monitored with an SR 830 lock-in amplifier, which 
triggers an Agilent waveform generator.  When the nanotube is within a few nanometers of the 
platinum surface, the waveform generator triggers an Agilent 8114A pulse generator to apply a 
100 ns pulse to the tip.  The applied potential is varied until a significant change in the z-position 
is observed. 
 
Gold nanoparticle reduction at the exposed end of the SWNT electrode 
Gold nanoparticles are electrochemically reduced at the exposed end of the SWNT electrode 
using a similar technique to the one described in ref. S1.  The exposed nanopencil probes are 
immersed in aqueous solution of AuCl4– held at – 200 mV.  Gold nanoparticles are 
electrodeposited potentiostatically by applying a ~ 40 V pulse.  No particle formation was 
observed on the SiOx-coated nanotube sidewalls or AFM tip support. 
 
References 
 
S1. J. H. Hafner, C. L. Cheung, T. H. Oosterkamp, and C. M. Lieber, J. Phys. Chem. B 105, 743 
(2001). 
S2. B. M. Quinn, C. Dekker, S. G. Lemay, J. Am. Chem. Soc. 127, 6146 (2005) 


Fully inverted single-digit nanometer domains in ferroelectric films

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Fully inverted single-digit nanometer domains in ferroelectric films

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