An atomic-force microscope (AFM) is used to locally inject, detect, and quantify the amount and location of charge in SiO2 films containing Si nanocrystals (size ~2–6 nm). By comparison with control samples, charge trapping is shown to be due to nanocrystals and not ion-implantation-induced defects in samples containing ion-beam-synthesized Si nanocrystals. Using an electrostatic model and AFM images of charge we have estimated the amount of charge injected in a typical experiment to be a few hundred electrons and the discharge rate to be ~35±15 e/min

Atwater, Harry A.

Bell, L. D.

Boer, Elizabeth A.

Brongersma, Mark L.

Flagan, Richard C.

English

Caltech Authors - Main

APPLIED PHYSICS LETTERS VOLUME 79, NUMBER 6 6 AUGUST 2001Localized charge injection in SiO2 films containing silicon nanocrystals
Elizabeth A. Boer,a) Mark L. Brongersma, and Harry A. Atwater
Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena,
California 91125
Richard C. Flagan
Department of Chemical Engineering, California Institute of Technology, Pasadena, California 91125
L. D. Bell
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91125
~Received 14 July 2000; accepted for publication 14 May 2001!
An atomic-force microscope ~AFM! is used to locally inject, detect, and quantify the amount and
location of charge in SiO2 films containing Si nanocrystals ~size ;2–6 nm!. By comparison
with control samples, charge trapping is shown to be due to nanocrystals and not
ion-implantation-induced defects in samples containing ion-beam-synthesized Si nanocrystals.
Using an electrostatic model and AFM images of charge we have estimated the amount of charge
injected in a typical experiment to be a few hundred electrons and the discharge rate to be ;35615
e/min. © 2001 American Institute of Physics. @DOI: 10.1063/1.1383574#Future developments in nanoscale silicon electronics re-
quire tools for local manipulation and probing of stored
charge. Charge storage and manipulation at the nanoscale is
particularly important to nonvolatile floating gate memory
devices such as nanocrystal-based memories1 in which the
floating gate consists of a dense array of Si nanocrystals
embedded in the gate oxide. Such devices potentially offer
both superior nonvolatile memory performance relative to
conventional flash memories, and a simple design that is
scalable to nanometer dimensions.2 Silicon, with its mature
technology, is the material of choice for these devices as
structures can be fabricated cheaply and reliably. In general,
however, nanoparticle synthesis methods introduce defects in
the oxide host which can affect charge injection and
retention.3
Traditional device analysis techniques such as
capacitance–voltage measurements3 give important macro-
scopic device information, but in order to probe the local
properties of a device, a technique capable of detecting the
presence and motion of a few localized charges is necessary.
Conducting-tip atomic-force microscopy ~AFM! is sensitive
to a variety of forces, including electrostatic, thus making it
a good tool for mapping weak electrostatic potentials4,5 and
capacitance6 on the nanometer scale. It can also be used to
inject electrons or holes into a localized region in materials
such as polymers,7 thin insulating films,8 double-barrier
CeO2 /Si/CeO2 /Si structures,9 and Co nanoclusters embed-
ded in SiO2.10 For these reasons, an AFM is a useful tool to
study the injection and dissipation of charge in SiO2 films
containing ion-beam-synthesized Si nanocrystals.11 In this
letter, we report on charge injection in silicon nanocrystal
floating gates and show that charge trapping is due to the
presence of nanocrystals and not defects from the implanta-
tion process. We develop and use a generalized force model
of the tip–sample interaction and combine electrostatic mod-
a!Electronic mail: Elizabeth.Boer@alcatel.fr7910003-6951/2001/79(6)/791/3/$18.00
Downloaded 03 Apr 2006 to 131.215.225.171. Redistribution subject teling and AFM imaging to investigate the discharging dy-
namics of injected charge.
The samples investigated consist of a 100 nm wet ther-
mally grown silicon-dioxide layer on light B-doped Si sub-
strates that was implanted at room temperature with 35 keV
Si1 ions to a fluence of 431016 Si/cm2. The samples were
annealed at 1100 °C for 10 min in vacuum ~base pressure
,831027 Torr! to allow the nucleation and growth of sili-
con nanocrystals ~size ;2–6 nm, as determined from trans-
mission electron microscopy!. Control samples consisted of
~a! unimplanted and ~b! Ar-implanted SiO2 films, implanted
at room temperature with 50 keV Ar1 ions to a fluence of
2.931016 Ar/cm2. The inert element argon and the particular
ion energy and fluence were chosen so as to produce samples
without nanocrystals, but with the same ion-implantation-
damage profile as the Si-implanted samples @as calculated
using TRIM ~Ref. 12!#. The control samples were annealed as
above. All samples were etched with buffered hydrofluoric
acid13 approximately halfway through the film, as measured
by Rutherford backscattering spectroscopy, to eliminate con-
tamination or surface defects from the vacuum anneal.14
The atomic-force microscope and highly doped Si tips
were from commercial sources.15 All images were taken in
noncontact mode, with the sample stage grounded, in a box
purged with N2 gas ~resulting humidity ,10%!. Charge was
transferred from the AFM tip to the samples by applying a
potential to the tip ~typically, 210 V!, disengaging the feed-
back of the microscope, then lowering the tip toward the
sample surface. The distance between the tip and sample was
monitored by observing the damping of the tip oscillations
on an oscilloscope.16 After contact and charge transfer, the
feedback was reengaged, the AFM tip grounded, and images
made ~Fig. 1!. Conventional AFM imaging was used to de-
tect the injected charge.
Figure 1~a! and 1~c! show AFM images of etched Si
nanocrystal and control samples. The morphology of the
nanocrystal sample is different from that of the flat control
sample, confirming the presence of nanocrystals. The particle© 2001 American Institute of Physics
o AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
792 Appl. Phys. Lett., Vol. 79, No. 6, 6 August 2001 Boer et al.density is seen to be ;631010 cm22, similar to that mea-
sured from transmission electron micrographs. The features
are 1–4 nm high. Figure 1~b! shows localized charging in the
nanocrystal samples, while the Ar-implanted control sample
annealed at 1100 °C does not show this type of behavior
@Fig. 1~d!#. This result indicates that nanocrystals are neces-
sary for localized charging. The AFM image of charge ap-
pears as a protrusion on the surface due to the electrostatic
interaction between the image charge in the grounded tip and
the charge in the sample.
Figure 2~a! shows a discharging time series—vertical
cross sections of subsequent AFM images are plotted on the
same axes. In the inset to Fig. 2~a!, the ‘‘apparent height’’ of
the injected charge decreases from ;17 to 3 nm in 500 s.
Figure 2~b! consists of the corresponding fits to the data,
obtained by an iterative method explained below. In Fig. 3,
the amount and spatial extent of the charge as a function of
time is plotted, demonstrating that the spatial extent of the
injected charge decreases with time. These results were de-
termined using an electrostatic model as follows. The AFM
tip was approximated by a grounded metallic sphere, and the
electrostatic force interaction between the injected charge on
the sample and the image charge in the grounded tip was
calculated. The charge was assumed to be only on the sample
surface, to reduce the complexity of the computation, but in
principle the model could be easily expanded to include
three-dimensional charge distributions. The charge distribu-
tion consisted of a disk upon which was imposed a grid
~typical grid spacing, 5 nm!. In principle, any arbitrary two-
dimensional charge distribution may be assigned to this grid,
but for a first approximation, the total assigned charge was
uniformly distributed over the grid points. Polarization ef-
fects were ignored, as were effects on the tip motion due to
the surrounding medium ~such as hydrodynamic damping!,17
FIG. 1. Atomic-force microscope image of ~a! an etched silicon-dioxide
film containing silicon nanocrystals made by Si ion implantation and anneal-
ing before charging and ~b! the same sample after charge transfer. Note that
the injected charge is imaged as a protrusion on the surface. The lateral
dimension for ~a! and ~b! is 0.5 mm. The vertical scale ~black to white! is 15
nm for ~a! and 25 nm for ~b!. ~c! and ~d! represent AFM images before and
after charging of an etched silicon-dioxide film implanted with Ar1 ions and
annealed. Note that in this sample no localized charging is seen, indicating
that charge is not trapped in defects from the implantation process. The
lateral dimension for ~c! and ~d! is 1 mm. The vertical scale ~black to white!
for both is 1.5 nm.Downloaded 03 Apr 2006 to 131.215.225.171. Redistribution subject tand the nanocrystal topography was replaced with a flat
plane. The electrostatic interaction between the tip and
sample was then found by taking each charged grid point,
and finding the resultant image charge induced in the
grounded tip. The total Coulomb force was subsequently cal-
culated by summing up the interaction of each grid-point
charge with each image charge, i.e.,

F ~electrostatic!5(i , j

Fi j5(i , j
qsiqt j
4p«0Di j
2 dˆ i j , ~1!
with qsi the ith grid-point charge on the sample, qt j the jth
image charge in the tip, and Di j the distance between them
~and dˆ i j a unit vector!. Thus, the total force was made up of
two terms: one term due to the van der Waal interaction
~between a sphere and plane!,18 and the second from the
electrostatic contribution @Eq. ~1!#. Only the z component of
FIG. 2. ~a! Cross-section time series of AFM images of trapped charge in a
SiO2 film containing silicon nanocrystals and ~b! the corresponding fits to
the data. The first trace is recorded 4063 after charge injection, and each
subsequent trace is recorded at 57 s intervals. The inset shows how the
‘‘apparent height’’ ~or AH!, from the maximum of the traces in ~a!, de-
creases with time. From the fits in ~b!, the amount and location of the
trapped charge seen in each trace is determined and plotted in Fig. 3.
FIG. 3. Quantity and location of the injected charge as a function of time, as
determined from the fits @in Fig. 2~b!# to the AFM discharging time series
data @Fig. 2~a!#. Note the result that the area over which the charge is located
decreases as the charge dissipates.o AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
793Appl. Phys. Lett., Vol. 79, No. 6, 6 August 2001 Boer et al.the total force was included since only the vertical forces on
the tip were measured. It was assumed that the microscope
feedback loop maintained the tip at a constant force gradient,
set by and estimated from the imaging parameters, an as-
sumption valid for small tip oscillation amplitudes ~as com-
pared to the tip–sample separation distance!. This assump-
tion holds when FCoulomb@Fvan der Waals , i.e., in the charged
region. The equation
S ]Fz]z D
tot
5S ]Fz]z D
vdW
1(
i , j
S ]Fz]z D i j5const, ~2!
was solved ~using Newton’s method! for the height of the tip
above the flat plane as it scanned over the charged region.
This calculated profile was then compared to the data. By
adjusting the initially assumed charge and charged area itera-
tively, a ‘‘best fit’’ was found.
From this analysis, it was determined that in a typical
experiment ;350690 electrons ~of ;4 electrons per par-
ticle! are injected and leak away at a rate of ;35
615e/min. The decrease of the spatial extent of the charge
with time suggests the main dissipation path is to the sub-
strate ~at a rate of ;0.2 nA/cm2!. Vertical transport may be
via nanoparticles or via defects ~from ion implantation or
charge injection!. Clear lateral dissipation was only seen in
experiments done in air ~i.e., humid environment!.
In summary, we have shown that AFM is a useful tech-
nique to inject and detect charge on the nanoscale in techno-
logically relevant materials such as Si nanoparticle-
containing SiO2 films. Using an atomic-force microscope, we
have shown that the charge trap centers in annealed, Si-
implanted silicon-dioxide films are due to the presence of
nanocrystals and not simply due to process-induced defects
in SiO2. We have done calculations and have determined the
magnitude of the injected charge and the rate of its dissipa-
tion.Downloaded 03 Apr 2006 to 131.215.225.171. Redistribution subject tThe research described in this letter was jointly spon-
sored by the National Aeronautics and Space Administration
~NASA! and the Jet Propulsion Laboratory Director’s Re-
search and Development Fund, and by the National Science
Foundation under Grant No. DMR 98-71850.
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Localized charge injection in SiO_2 films containing silicon nanocrystals

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Localized charge injection in SiO_2 films containing silicon nanocrystals

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