X-ray photoemission for probing charging/discharging dynamics

Article abstract of DOI:10.1021/jp0644006

A novel technique is introduced for probing charging/discharging dynamics of dielectric materials in which X-ray photoemission data is recorded while the sample rod is subjected to ± 10.0 V square-wave pulses with varying frequencies in the range of 10^-3 to 10³ Hz. For a clean silicon sample, the Si2p(Si⁰) peak appears at correspondingly -10.0 eV and +10.0 eV binding energy positions (20.0 eV difference) with no frequency dependence. However, the corresponding peak of the oxide (Si⁴⁺) appears with less than 20.0 eV difference and exhibits a strong frequency dependence due to charging of the oxide layer, which is faithfully reproduced by a theoretical model. In the simplest application of this technique, we show that the two O1s components can be assigned to SiO_x and TiO _y moeties by correlating their dynamical shifts to those of the Si2p and Ti2p peaks in a composite sample. Our pulsing technique turns the powerful X-ray photoemission into an even more powerful impedance spectrometer with an added advantage of chemical resolution and specificity.

Full text of DOI:10.1021/jp0644006

19112
2006, 110, 19112-19115
Published on Web 09/06/2006
X-ray Photoemission for Probing Charging/Discharging Dynamics
,†
‡
Sefik Suzer* and Aykutlu D aˆ na
Chemistry and Physics Departments, Bilkent UniVersity, 06800 Ankara, Turkey
ReceiVed: July 12, 2006; In Final Form: August 21, 2006
A novel technique is introduced for probing charging/discharging dynamics of dielectric materials in which
X-ray photoemission data is recorded while the sample rod is subjected to (10.0 V square-wave pulses with
-
3
3
0
varying frequencies in the range of 10 to 10 Hz. For a clean silicon sample, the Si2p(Si ) peak appears at
correspondingly -10.0 eV and +10.0 eV binding energy positions (20.0 eV difference) with no frequency
4
+
dependence. However, the corresponding peak of the oxide (Si ) appears with less than 20.0 eV difference
and exhibits a strong frequency dependence due to charging of the oxide layer, which is faithfully reproduced
by a theoretical model. In the simplest application of this technique, we show that the two O1s components
x y
can be assigned to SiO and TiO moeties by correlating their dynamical shifts to those of the Si2p and Ti2p
peaks in a composite sample. Our pulsing technique turns the powerful X-ray photoemission into an even
more powerful impedance spectrometer with an added advantage of chemical resolution and specificity.
Charge accumulation and dissipation (charging/discharging)
in dielectric materials are vital processes for design and function
of various devices and sensors. This is especially important
dynamical information can also be extracted if the voltage
stimuli are applied in the form of pulses, as we have recently
1
30,33
reported.
Since an electron source cannot be turned on and
for SiO2, as the thickness of the dielectric layer is expected to
shrink down to few atomic layers for the next generation of
metal oxide semiconductor (MOS) devices. Charge accumula-
off rapidly, the application of the voltage stress to the sample
rod brings an important advantage for dynamical measurements.
2
In this contribution, we extend our investigation to analysis
of XPS data recorded by applying the voltage stress in the form
of bipolar square pulses for probing charging/discharging
dynamics of thin dielectric surface structures. We also supple-
ment our experimental data with a theoretical model. Previously
reported dynamical photoemission measurements have been
either in the ultrafast, subpicosecond regime using laser
tion in the oxide layer occurs via various trapping mechanisms
and is normally probed by electrical current-voltage and/or
current-capacitance measurements.3 Photoemission utilizing
UV, X-rays, and lasers has also been employed for probing very
-6
-
9
7-9
fast (<10 s) charging dynamics.
Core-level X-ray photo-
emission, XPS, is especially attractive, since additional chemical
information can also be derived from the line positions of the
corresponding peaks. However, the measured line positions are
severely altered by local potentials developed due to the
uncompensated charges resulting from photoelectron emission,
7-9
1
4
excitations
or in much longer time regimes (10 -10
14,21,33-36
minutes) with conventional XPS measurements.
This
is the first time where XPS measurements are reported using
voltage pulses for probing charging/discharging dynamics of
especially for poorly conducting samples or regions (layers or
-3
3
dielectric materials in the range of 10 to 10 s, matching those
domains) within such samples.1
0-16
Various methods of charge
3
7
of many chemical-biochemical processes.
compensation have been developed using low-energy electrons,
ions, and/or photons.17,18
The SiO2 layers were grown thermally on HF-cleaned Si-
(
100) substrates at 600 °C in air. Thickness of the overlayer
On the other hand, one can also utilize XPS for understanding
the mechanisms leading to and/or controlling of the charging/
discharging processes in materials which offer great possibilities
for researchers in all fields.19 Several applications have been
reported, which utilize the charging, called controlled surface
38
was estimated from the angular dependency of the XPS data.
A Kratos ES300 electron spectrometer with MgKR X-rays (non-
monochromatic) was used for XPS measurements. In the
standard geometry, the sample accepts X-rays at 45° and ejects
photoelectrons at 90° with respect to the surface plane. A
filament placed nearby the sample provides low-energy electrons
for neutralization. A typical sample is a ca. 2 mm silicon wafer
charging, for extracting chemical, physical, structural, and
electrical parameters of various surface species.2
0-26
Using a
slightly different strategy and by applying voltage stress to the
sample rod while recording XPS spectra, we have shown that
2
with dimensions of 4 × 8 mm , electrically connected from
both the top (oxide layer) and the bottom (silicon substrate) to
the sample holder, which is grounded or pulsed by a square
the extent of charging can be controlled and various analytical
and electrical information can be extracted.2
7-32
In addition to
-3
3
wave in the range of 10 to 10 Hz while recording the spectra.
For mimicking the charging behavior, a series resistor (1 Mohm)
and a parallel capacitor (0.1-1000 nF) are connected externally.
The composite SiOx + TiOy sample was obtained by coating
the silicon substrate with a very thin layer of Ti.
static information derived from application of dc voltage stress,
*
Corresponding author (suzer@fen.bilkent.edu.tr).
Chemistry Department.
Physics Department.
†
‡
1
0.1021/jp0644006 CCC: $33.50 © 2006 American Chemical Society

Letters
J. Phys. Chem. B, Vol. 110, No. 39, 2006 19113
Figure 2. (a) XPS spectrum of the Si2p region of the clean silicon
sample recorded at 0.1 Hz together with the same sample connected to
the pulse generator through a RC (1 Mohm + 56 nF) circuit recorded
at 0.1 and 50 Hz. (b) The frequency dependence of the separation
between the split peaks plotted in a semilogarithm fashion for the clean
Figure 1. XPS spectrum of the Si2p region recorded when grounded
(olive) and when subjected to 10.0 V square pulses at 0.02 (red) and
2
Si, 10 nm SiO , and clean Si tied through the RC circuit.
4
0 (blue) Hz frequencies: (a) a clean silicon sample and (b) a sample
containing ca. 10 nm thermal silicon oxide layer. The square-wave
modulation splits the Si2p peak into two components, and the separation
between them displays a frequency dependence only for the 10 nm
SiO sample. The peaks labeled as pl. and sat. correspond to the plasmon
2
and KR3,4 satellite peaks of the Si2p, respectively. The pulsing scheme
is shown in the inset.
frequency and containing an external RC (R ) 1 Mohm, C )
56 nF), the separation becomes as low as 17.5 eV, which
recovers at higher frequencies. The frequency dependence of
the separation between the split peaks is plotted in Figure 2b
for the SiO2, the clean Si, and the clean Si tied through an
external RC in a semilog fashion to emphasize the remarkable
resemblance between the frequency dependence of the Si +
2
In Figure 1a, we display the Si2p region of the XPS spectra
of a clean Si wafer together with another one containing ca. 10
nm thermal oxide layer, both recorded under voltage stress
in the form of bipolar square pulses with two different
RC to that of the SiO . As far as the frequency dependence is
concerned, the SiO layer acts like a simple RC circuit. One
2
can further the experimental simulation by combining the two
0
frequencies. For the clean sample, the Si peak is split into two
and measuring the frequency dependence of the very same 10
nm oxide sample connected this time to an external RC circuit.
The experimental results are shown in Figure 3, together with
our theoretical simulation results to be described below, again
yielding a perfect agreement. Doubling of the S-type curve is
the signature of two different charging RC elements [(i) SiO2
layer, (ii) Si + RC], which is exactly reproduced by our
theoretical simulations.
Our XPS measurements can be modeled with a lumped circuit
as shown in Figure 4a, where the electron gun and the
photoelectron currents are modeled by a voltage-controlled
current source (VCCS). The nonlinear voltage-current curve
for the VCCS was extracted through analysis of a series of XPS
measurements by applying dc voltages to the clean silicon
sample in series with a known external resistor. The VCCS
current-voltage dependence is assumed to be the same through-
out the entire dynamic measurements. The excitation voltage
source VEX(t) can be a time-varying voltage with any arbitrary
shape. To calculate the XPS spectra, we need to solve the
components appearing at correspondingly -10.0 and +10.0 eV
with respect to its original position, since the sample spends
5
0% of its time at 10.0 V higher and lower potentials in the
negative and positive cycles, respectively. For the oxide, the
4
+
Si peak is similarly split (as shown in Figure 1b), but the
shifts are not only smaller than 10.0 eV but also exhibit strong
frequency dependence, due to dynamical charging/discharging
of the oxide layer under the influence of the voltage stimuli, as
was also pointed out in our previous work.3 Furthermore, the
O1s peak exhibits exactly the same frequency dependence. The
technique enables us to determine the impedance of different
surface species.
3
This dynamical behavior can be experimentally simulated by
introducing a simple RC element tied externally to the clean Si
sample which displays a frequency dependence as depicted in
Figure 2a. In the spectrum of Si recorded at 0.1 Hz and without
any external element, the separation between the split peaks is
exactly 20.0 eV, whereas in the spectrum recorded at the same

19114 J. Phys. Chem. B, Vol. 110, No. 39, 2006
Letters
in Figure 4a, and IS(VS) is the surface-potential-dependent
current of the electron flood gun. Solutions can be obtained by
numerical integration for arbitrary excitations VEX(t). For a
periodic excitation of period T, one can calculate a line-shape
function that gives the intensity of the XPS signal at a voltage
shift of V as
1
T
T
g(V) )
∫
δ[V (t) - V] dt
(2)
S
0
∞
-
Note that ∫ g(V) dV ) 1, and hence, the line-shape function
∞
is properly normalized. The XPS spectrum recorded by the
spectrometer SXPS(V) can now be calculated by convolution of
the line-shape function g(V) with the original spectrum S0(V)
obtained by grounding the sample.
∞
SXPS(V) )
∫
g(V - λ)S (λ) dλ
(3)
0
-
∞
Figure 3. The frequency dependence of the separation between the
split peaks plotted in a semilogarithm fashion for the 10 nm SiO only
black), tied through 1 Mohm resistance (red), tied through 1 Mohm
resistance and 10 nF (blue), and 56 nF (green) capacitance. The dashed
curves are results of the theoretical simulation (see text).
2
By numerically solving eq 1 for the values of R ) 1 MΩ and
C ) 56 nF for square-wave excitation, we calculate the line-
shape functions through eq 2 as shown in Figure 4b. It can be
seen that, depending only on the period of the excitation and
charge-discharge time constants of the sample system, the left
and right peaks of the line-shape function shift. If the excitation
signal period is much higher than the sample time constants,
we are working in a regime similar to dc. In this case, the peaks
are shifted almost to the positions that would be obtained by
applying -10 or +10 V (dc) to the sample, respectively. Also,
the broadening is almost zero. If the excitation frequency is
high, the average surface potential is shifted due to the nonlinear
nature of the VCCS, but the separation between the left and
right peaks is closer to 20 V. Also, there is a greater broadening
of the peaks. It is seen through calculations that, for the square-
wave case, the peaks have maximum broadening, as half of the
period of excitation coincides with the charging/discharging time
constant of the sample. The broadening for the square-wave
case goes to zero as excitation frequency goes to zero or infinity.
Resulting spectra for two square-wave excitations are also shown
in Figure 4c together with the experimental data. Agreement
between calculated and measured data is again remarkable. The
theory works equally well for square-wave and sinusoidal
excitation voltages as tested on the same 10 nm SiO2 sample
(
(not reproduced here), showing that it can be used to calculate
the XPS spectra under any arbitrary waveform excitation.
This method measures an effective time constant comprising
an effective resistance and capacitance of the films under X-rays
and low-energy electron exposure. The charging and discharging
of the films are nonlinearly dependent on the surface electrical
field created, since they are described by tunneling or hopping
processes. The simple RC model can be replaced with a more
accurate model that takes into account the distribution of charge
traps using a distributed network. However, the ultimate model
has to include E-field and position-dependent charge-discharge
rates as well as local density of trap states.
Figure 4. (a) The circuit used for simulation of the frequency
dependence measured and given in Figures 2 and 3. (b) Calculated
line shapes. (c). Experimental and simulated spectra of the clean Si
tied through the RC (1 Mohm + 56 nF) circuit at two different
frequencies.
The charging and discharging processes we probe are related
to trapping and detrapping of the holes created in the valence
-
12
band of the oxide following the very fast (<10
s) photo-
2
1
differential equation describing the change of the surface
potential VS(t) given by
emission process. Therefore, the time constants we measure
are comparable to the time constants derived by time-dependent
leakage currents determined for MOS systems under X-ray
3
-5
and/or using scanning capacitance microscopy.6
d(V - V ) VS - VEX
exposure
S
EX
C
+
+ I (V ) ) 0
(1)
S
S
The dynamical measurement paves the way to a number of
analytical tools for extracting information about charging/
discharging of surface structures. As the simplest application
of this technique, we investigated a composite surface containing
dt
R
where, R and C are, respectively, the effective resistance and
the capacitance between the surface and the source as shown

Letters
J. Phys. Chem. B, Vol. 110, No. 39, 2006 19115
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