An electrostatic force microscope was used to write and image localized dots of charge in a double barrier CeO2/Si/CeO2/Si(111) structure. By applying a relatively large tip voltage and reducing the tip to sample separation to 3–5 nm, charge dots 60–200 nm full width at half maximum of both positive and negative charge have been written. The total stored charge is found to be Q = ±(20–200)e per charge dot. These dots of charge are shown to be stable over periods of time greater than 24 h, with an initial charge decay time constant of tau ~ 9.5 h followed by a period of much slower decay with tau > 24 h. The dependence of dot size and total stored charge on various writing parameters such as tip writing bias, tip to sample separation, and write time is examined

Bridger, P. M.

Jones, J. T.

Marsh, O. J.

McGill, T. C.

Caltech Authors - Main

APPLIED PHYSICS LETTERS VOLUME 75, NUMBER 9 30 AUGUST 1999Charge storage in CeO2/Si/CeO2/Si111 structures by electrostatic
force microscopy
J. T. Jones, P. M. Bridger, O. J. Marsh, and T. C. McGilla)
Thomas J. Watson, Sr., Laboratory of Applied Physics, California Institute of Technology, Pasadena,
California 91125
~Received 26 March 1999; accepted for publication 8 July 1999!
An electrostatic force microscope was used to write and image localized dots of charge in a double
barrier CeO2 /Si/CeO2 /Si~111! structure. By applying a relatively large tip voltage and reducing the
tip to sample separation to 3–5 nm, charge dots 60–200 nm full width at half maximum of both
positive and negative charge have been written. The total stored charge is found to be Q
56(20– 200)e per charge dot. These dots of charge are shown to be stable over periods of time
greater than 24 h, with an initial charge decay time constant of t;9.5 h followed by a period of
much slower decay with t.24 h. The dependence of dot size and total stored charge on various
writing parameters such as tip writing bias, tip to sample separation, and write time is examined.
© 1999 American Institute of Physics. @S0003-6951~99!04035-8#Cerium oxide (CeO2) is an insulating material with a
lattice mismatch of only 0.35% to silicon ~Si! and an energy
band gap of ;5.5 eV. This attractive set of properties has the
potential to lead to a fully functional silicon heterojunction
technology. A significant amount of work has been done
examining the growth and characterization of CeO2 crystals
on Si,1–5 and the growth of single crystal Si on to CeO2 /Si
heterostructures6 has been recently reported. Based on these
promising results, a silicon resonant tunneling diode, an im-
proved silicon-on-insulator technology, and stacked silicon
electronics have all been proposed. A valuable and interest-
ing addition to this array of technologies would be the ca-
pacity for integrated electrostatic data storage.
In this letter, we describe the localized charging and sub-
sequent imaging of a double barrier CeO2 /Si/CeO2 /Si~111!
structure by electrostatic force microscopy ~EFM!.7–9 The
controllable writing of both positive and negative localized
dots of charge with long lifetimes is described and it is fur-
ther shown that these charge dots may be rewritten and re-
placed by charge of the opposite sign through the application
of an opposite charging bias. A simple analysis is presented
to quantify the total amount of charge stored in each charge
dot. The time evolution of these charge dots is studied, and
charge decay time constants are extracted. Finally, a study is
presented of various writing parameters such as tip bias, tip
to sample separation, and write time on the size and total
stored charge of the resultant charge dots.
Samples were produced from commercially available 3
in. Si~111! wafers, n-type with 3.0–4.3 V cm resistivity. Af-
ter being subjected to a standard acetone, isopropyl alcohol,
deionized water degrease in ultrasound, the wafer was etched
in 50:1 HF solution until hydrophobic, rinsed in deionized
water, and immediately introduced into vacuum. Electron
beam evaporation was used to deposit material from an un-
doped Si charge and a 99.99% CeO2 charge to grow the
structures. Initially, a 200 Å Si buffer layer was grown and
examined by ~RHEED! reflection high-energy electron dif-
a!Electronic mail: tcm@ssdp.caltech.edu1320003-6951/99/75(9)/1326/3/$15.00
Downloaded 03 Apr 2006 to 131.215.225.171. Redistribution subject tfraction to assure the characteristic (737) reconstruction
was apparent, indicative of a clean Si surface ready for fur-
ther growth. Cerium oxide thin films were grown at a wafer
temperature of 550 °C, with chamber pressures ranging from
131027 – 231026 Torr due primarily to outgassing from
the CeO2 charge. Silicon thin films were also grown at a
wafer temperature of 550 °C, with chamber pressures of 5
31028 – 231027 Torr. A double barrier structure,
CeO2 /Si/CeO2 /Si~111!, was produced with symmetric 35 Å
CeO2 barriers and an intermediate 25 Å Si film. In situ
RHEED was again used to monitor film growth and showed
the CeO2 barriers and intermediate Si film to be polycrystal-
line.
The EFM data were collected using a Digital Instru-
ments Nanoscope IIIa controller and a Bioscope scanning
probe microscope operating in tapping mode. To detect the
electrostatic forces, a voltage is applied to commercially
available cobalt coated tapping mode atomic force micros-
copy ~AFM! tips which are scanned across the surface at a
constant tip to sample separation. Phase differences induced
by electrostatic forces on the oscillating tip during scanning
are detected and provide a measurement of the local charge
density. In all cases, topographical AFM images and electro-
static EFM images were recorded simultaneously.
By reducing the tip to sample separation and applying a
large charging bias to the EFM tip, dots of both positive and
negative charge can be written as shown in Fig. 1. Writing
was performed by restricting the EFM tip to a 1 nm2 area,
bringing it to within 5 nm of the sample, and applying a
charging bias of 610 V for 45 s. Subsequent imaging of the
resultant charge dots was performed over a 1 mm31 mm area
by increasing the tip to sample distance to 30 nm and imag-
ing at a tip bias of 1 V. No change in the topographical AFM
image was detected, but positive and negative dots of charge
115 nm full width at half maximum ~FWHM! are clearly
visible in the EFM image. Application of a positive charging
bias results in positive features in the subsequent EFM im-
aging at VEFM51 V. It was further shown that by repeating
the write sequence at the same location with a charging volt-6 © 1999 American Institute of Physics
o AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
1327Appl. Phys. Lett., Vol. 75, No. 9, 30 August 1999 Jones et al.age of opposite polarity, a charge dot could be effectively
rewritten with one of an opposite charge.
To compute the total stored charge, Q, we first use an
electrostatic analysis in similar fashion to Ref. 10 to compute
the localized stored charge, q, from a frequency shift, D f .
When the detected phase difference, Df, is very small, the
relationship between Df and D f is nearly linear and is found
to be D f ;3.5Df Hz/deg for our tip. The frequency shift,
D f , is related to the gradient of the force by the expression
D f 52 f 0 f 8(z0)/(2k), where z0530 nm is the tip to sample
separation during imaging, f 0559.8 kHz is the resonant fre-
quency of the tip, and k53 N/m is the estimated spring con-
stant of the tip. The force the tip feels under an applied direct
current ~dc! bias will be due to charge-charge interactions
and is given by
F~z !5
1
@z1~2dCeO2 /eCeO2!1~dSi /eSi!#
2
3S 2 dCeO22 q2eCeO22 e0a 1 2dCeO2qVEFMeCeO2 1 e0aVEFM
2
2 D ,
~1!
where z is again the tip to sample separation, d and e are the
thickness and dielectric constant of the CeO2 and Si films, as
indexed, VEFM is the bias applied to the tip, and a is the area
of the charged region. Simple modeling predicts the EFM
image of a localized charge to be 56 nm FWHM, with an
area a52450 nm2. The first term in the bracket in Eq. ~1! is
FIG. 1. A square array of 115 nm FWHM charge dots, 3 positive and 1
negative, written with an EFM to a CeO2 /Si/CeO2 /Si heterostructure. Ap-
plication of a positive tip bias during writing results in positive features in
the EFM image at VEFM51 V. Total charge on the dots is computed to be
Q5642e . No topographical changes are detected by AFM after writing,
but the charge dots are clearly visible in the EFM image. Charge dots of
both positive and negative charge have also been rewritten over top one
another by application of an opposite writing voltage.
TABLE I. A summary of the structures studied to examine the effect of the
lower CeO2 barrier on charge retention time t.
Sample
CeO2
~Å!
Si
~Å!
CeO2
~Å!
Si buffer
~Å! t ~h!
CE-41 35 25 35 200 9.8
CE-44 35 25 15 200 0.3
CE-43 35 fl fl 200 0.2Downloaded 03 Apr 2006 to 131.215.225.171. Redistribution subject tfound to be negligibly small,7 and the last term provides a
constant background independent of the charge. Using only
the middle term and the values given above, an approxima-
tion to the localized stored charge is then given by q
543Df e/deg. Computing the total stored charge in our
FIG. 2. Before ~a! and after ~b! images of a charge dot over time illustrating
the spreading and net decrease in total charge Q. A plot of the time evolu-
tion of Q in a separate dot ~c! exhibits an initial decay with t;9.5 h ~fit
line!, followed by a period of much slower decay with t.24 h.
FIG. 3. An EFM image of an array of dots written at different writing
voltages, tip to sample separations, and writing times as labeled. Local
charge density, q, and dot size are strongly dependent on writing voltage and
time, and more weakly dependent on tip to sample separation.o AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
1328 Appl. Phys. Lett., Vol. 75, No. 9, 30 August 1999 Jones et al.EFM images of area A, we find Q5q(A/a)e . For the dots in
Fig. 1, Df560.23° over an area of A513105 nm2, we
compute Q5642e .
To examine the time evolution of stored charge in our
system, single charge dots were written and monitored over
time. In all cases, the general trend was for a slow leakage of
charge accompanied by an increase in FWHM of the charge
dot, as shown in Figs. 2~a! and 2~b!. To estimate the lifetime
associated with the stored charge, a single charge dot was
written and continuously monitored over several days. After
writing at VEFM510 V, z53 nm, and t560 s, the resultant
charge dot was continuously imaged at VEFM51 V and z
530 nm at a read speed of 21 reads/h for the first 4 h, after
which imaging was performed intermittently for the remain-
der of the experiment. A plot of the total stored charge, Q, is
shown in Fig. 2~c!. There is an initial period of charge set-
tling and reorganization during which it is difficult to reliably
extract charge dot profiles. After this settling period and over
the first 10 h, an exponential fit to the charge decay exhibits
a time constant t;9.5 h, after which the rate of decay slows
down considerably to t.24 h. After a period of t.40 h; the
stored charge was no longer detectable by our instrument. As
the charge lifetime is found to be invariant to whether the tip
was continuously or intermittently engaged, as exhibited by
the uniform decay up until t510 h in Fig. 2~b!, charge leak-
age to the EFM tip is ruled out and the decay mechanism is
determined to be tunneling into the Si substrate.
To determine the effect of the lower CeO2 barrier on
charge retention time, two additional samples were studied: a
single barrier control sample consisting of 35 Å CeO2 atop
the crystalline silicon buffer layer, and a double barrier struc-
ture with a 35 Å CeO2 upper barrier, a 25 Å Si well, and a
reduced 15 Å CeO2 lower barrier, as shown in Table I. In
both CE-43 and CE-44 it was found that significantly less
charge was stored under standard write conditions of VEFM
510 V, z53 nm, and t560 s, and charge decay time con-
stants were much shorter. The control sample, CE-43, was
found to have a charge decay time constant of t;0.2 h, and
the reduced barrier sample, CE-44, was found to have a
charge decay time constant of t;0.3 h. These results clearly
show that the lower CeO2 barrier is serving as an inhibitor to
the primary decay mechanism which is concluded to be tun-
neling into the Si substrate. Furthermore, the ability to store
charge in the control sample indicates that charge is being
stored at crystalline defects within the upper CeO2 barrier or
at the upper CeO2 /Si interface, ruling out the polycrystalline
Si well as a potential storage site.
To examine the effects of writing parameters on charge
dot size and total stored charge, an array of positive chargeDownloaded 03 Apr 2006 to 131.215.225.171. Redistribution subject tdots was written at various EFM tip biases, tip to sample
distances, and write times as shown in Fig. 3. Line scans
were taken across the low pass filtered image to extract
charge dot sizes and local charge densities, q. It is found in
general that both the size and q of the charge dots scale
linearly with the same slope in relation to the various writing
parameters. From a nominal writing parameter start point of
VEFM510 V, t560 s, and z55 nm, it is found that the size
and q of a charge dot may be expected to decrease 10% for
each 0.5 V decrease in VEFM , decrease 10% for each 10 s
decrease in t, and increase 10% for a 2 nm decrease in z.
In conclusion, an EFM was used to write and image
localized dots of charge in a double barrier
CeO2 /Si/CeO2 /Si~111! structure. By applying relatively
large tip voltages of VEFM56(6 – 10) V and reducing the tip
to sample separation to z53 – 5 nm, charge dots 60–200 nm
FWHM of both positive and negative charge have been writ-
ten. The total stored charge is found to be Q
56(20– 200)e per charge dot. These charge dots are shown
to be stable over periods of time greater than a day, with an
initial charge decay time constant of t;9.5 h followed by a
period of much slower decay with t.24 h. The dependence
of charge dot size and total stored charge on various writing
parameters such as tip bias, tip to sample separation, and
write time has been examined and linear dependencies ex-
tracted.
This work was supported by the Defense Advanced Re-
search Project Agency, and monitored by the Air Force Of-
fice of Scientific Research under Grant No. F49620-96-1-
0021.
1 T. Inoue, Y. Yamamoto, S. Koyama, S. Suzuki, and Y. Ueda, Appl. Phys.
Lett. 56, 1332 ~1990!.
2 L. Luo, X. Wu, R. Dye, R. Muenchansen, S. Foltyn, Y. Coulter, C. Mag-
giore, and T. Inoue, Appl. Phys. Lett. 59, 2043 ~1991!.
3 T. Inoue, Y. Yamamoto, M. Satoh, A. Ide, and S. Katsumata, Thin Solid
Films 281-282, 24 ~1996!.
4 T. Chikyow, S. Bedair, L. Tye, and N. El-Masry, Appl. Phys. Lett. 65,
1030 ~1994!.
5 S. Yaegashi, T. Kurihara, H. Hoshi, and H. Segawa, Jpn. J. Appl. Phys.,
Part 1 33, 270 ~1994!.
6 J. Jones, E. Croke, C. Garland, O. Marsh, and T. McGill, J. Vac. Sci.
Technol. B 16, 2686 ~1998!.
7 D. Sarid, Scanning Force Microscopy ~Oxford University Press, New
York, 1991!.
8 W. Nabhan, B. Equer, A. Broniatowski, and G. DeRosny, Rev. Sci. In-
strum. 68, 3108 ~1997!.
9 M. Nonnenmacher, M. P. O’Boyle, and H. K. Wickramasinghe, Appl.
Phys. Lett. 58, 2921 ~1991!.
10 D. Schaadt, E. Yu, S. Sankar, and A. Berkowitz, Appl. Phys. Lett. 74, 472
~1999!.o AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp


Charge storage in CeO2/Si/CeO2/Si(111) structures by electrostatic force microscopy

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Charge storage in CeO2/Si/CeO2/Si(111) structures by electrostatic force microscopy

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