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Phys. Status Solidi B, 1–5 (2010) / DOI 10.1002/pssb.200983956
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basic solid state physics
Boron induced charge traps near the
interface of Si/SiO2 probed by second harmonic generation
,1
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1,6
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1,2
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Heungman Park* , Jingbo Qi , Ying Xu , Kalman Varga , Sharon M. Weiss , Bridget R. Rogers ,
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Gunter Lüpke , and Norman Tolk
1
Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA
Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, Tennessee 37235, USA
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Department of Chemical and Biomolecular Engineering, Nashville, Vanderbilt University, Tennessee 37235, USA
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Department of Applied Science, College of William and Mary, Williamsburg, Virginia 23187, USA
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National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA
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Zomega Terahertz Corporation, Troy, New York 12180, USA
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Received 15 December 2009, revised 27 February 2010, accepted 12 March 2010
Published online 15 June 2010
Keywords defects, frequency conversion, silicon, surfaces and interfaces
* Corresponding
author: e-mail [email protected], Phone: þ1 615 343 2219, Fax: þ1 615 343 7263
We review recent second harmonic generation (SHG) measurements for highly boron-doped Si/SiO2 systems. Using electric
field sensitive time-dependent SHG (TD-SHG), we determined
that the direction of the initial DC electric field at the interface
induced by boron induced charge traps is from oxide to silicon
thus demonstrating that the boron induced charge traps in the
oxide are positively charged. For a thin oxide ("2 nm) both
boron traps and O2 surface oxide traps contribute. However, for
a highly boron-doped Si/SiO2 sample with a thick thermally
grown oxide (thickness: 12 nm), the TD-SHG signal exhibits
a monotonic decrease arising from filling only the boron
charge traps. By fitting our data, we show that the interface
effective susceptibility jx(2)j is heavily dependent on doping
concentration.
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1 Introduction of electric field induced second
harmonic generation SHG has been established as an
effective method to study surface and interface electronic
properties of materials. Because SHG signals vanish for
inversion symmetric materials such as silicon and also
amorphous SiO2 under the dipole radiation approximation
[1–3], SHG signals arise primarily from surfaces and
interfaces. When there is an electric field present inside
these materials, the SHG signal contains an additional bulk
x(3) contribution [4–8]. Electric field induced SHG (EFISH)
can be described by the following equation [9, 10]
!
!2
I ð2vÞ ¼ !xð2Þ þ xð3Þ Edc ! ðI ðvÞ Þ2 :
(1)
x(2) and x(3) are non-linear second- and third-order electric
susceptibilities, respectively, and Edc is a quasi-static
electric field inside these materials. For Si/SiO2 systems,
if the oxide thickness is less than 10 nm, electrons from
the silicon valence band can be excited to the conduction
band of the oxide, usually by a multiphoton process. The
excited electrons can travel through the oxide and then
may be captured by oxygen molecules on the surface of the
oxide [10].
The remaining holes in the silicon and the electrons in the
oxygen molecules form a capacitor-like electric field. This
electric field is responsible for the EFISH signals in Si/SiO2
systems. If the fundamental laser beam wavelength is 800 nm
(1.55 eV), the typical EFISH measurement shows a slow
monotonic increase, which saturates with time as shown in
Fig. 1.
For 800 nm photons, a three-photon absorption process
is required to excite the electrons from the silicon valence
band to the oxide conduction band [10], a situation where the
electron excitation probability is very low and thus the
trapping rate is also low. Therefore Edc increases slowly and
the time-dependency can be easily detected by a timedependent SHG (TD-SHG) measurement. Edc in Eq. (1) is
expressed as a time-dependent electric field Edc(t) across the
interface of Si/SiO2. The EFISH technique has been
extensively used to investigate the interface properties of
semiconductor-oxide systems. In particular, it was used to
study electron and hole dynamics at the interface of Si/SiO2
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H. Park et al.: Boron induced charge traps near the interface of Si/SiO2 probed by SHG
SHG (a.u.)
2
600
400
200
0
0
200
400
600
Time (s)
Figure 1 TD-SHG measurement from a Sb doped Si wafer with a
native oxide (resistivity: 0.005–0.02 V cm) [11].
systems [9, 12], and also for Si/Al2O3 [13, 14], Si/HfO2 [13,
15], Si/MgO [16], Si/ZrO2 [13, 17] and SOI [18] systems. In
addition to these material systems, Scheidt et al. investigated
highly boron-doped Si/SiO2 systems [19]. We also reported
independently similar results with a somewhat different
interpretation [20]. In particular, Scheidt et al. used
rotational anisotropic SHG (RA-SHG) to determine the
interface electric field direction. The RA-SHG results
showed a phase difference between initial and saturated
SHG signals, which confirmed that the built-in boron
induced charge traps and the oxygen charge traps create
DC electric fields in opposite directions. In our previous
paper, we focussed on the formation mechanism and stability
of boron induced charge traps, and filling process by internal
photo-emission. In this paper we discuss our experimental
results and interpretations.
2 Experimental setup Our experimental setup is
shown in Fig. 2. A Mira Ti:Sapphire laser is used to generate
a P-polarized 800 nm beam. The beam is focused onto the
samples at 458. The laser repetition rate is 75 MHz with a
pulse width of about 150 fs. A prism is used to separate the
resulting 400 nm beam. The 400 nm SHG beam is directed to
a photo-multiplier-tube. The SHG intensity is measured by a
photon counter. The plane-of-incidence is parallel to the
h100i sample crystal plane. When the sample was rotated to
λ =800 nm
(1.55 eV)
λ =400nm
the h110i crystal orientation, the same results were observed.
All measurements were performed in air at room temperature
and in dark conditions. Table 1 is a description of the silicon
(100) wafers used in this study, which outlines the dopant
species, resistivity, and dopant concentrations, respectively.
Ellipsometry was used to measure the thicknesses of the
native oxide layers. All wafers were found to have a 2 nm
thick oxide within experimental error. A laser power of
400 mW corresponds to an approximately 8.8 GW/cm2 peak
intensity on the sample surface.
3 Boron induced charge traps An initial sharp
decrease as a function of time was observed in our TD-SHG
measurement for highly boron-doped silicon wafers as
shown in Fig. 3. However, measurements on highly
antimony-doped wafers did not show a similar decrease
(Fig. 1). Only wafers B and C showed an initial decrease in
these TD-SHG measurements. It was clear that wafer A
would also show the initial SHG decrease if SHG data
acquisition rate were fast enough. The time-dependency of
the SHG measurements arises directly from the timedependent electric field across the interface of Si/SiO2.
Thus we suggest that the initial decrease in the SHG
measurement comes from the presence of a substantial preexisting electric field across the Si/SiO2 interface, which was
created during the growth of the oxide. We attribute this
electric field to the presence of B- ions in the silicon and the
presence of Bþ ions in the oxide, forming a capacitor-like
field across the interface.
The initial SHG signal at t ¼ 0 comes from
x(2) þ x(3)E(t ¼ 0) as shown in Eq. (1). The E(t ¼ 0) is the
initial electric field which is present at the interface. This
initial electric field comes from boron charge traps as shown
by in Fig. 3. The initial electric field direction is from the
oxide to the silicon substrate. During intense laser illumination, photo-excited electrons in the silicon substrate fill the
boron Bþ charge traps and also fill the oxygen traps on the
oxide surface. The electrons captured by the Bþ charge traps
decrease the magnitude of the initial electric field. The
electrons captured by the oxygen traps increase an additional
electric field, which is opposite to the direction of the initial
boron-induced electric field. Therefore the magnitude of the
PMT
Figure 2 (online color at: www.pss-b.com) Experimental setup for
SHG measurements.
SHG (a.u.)
400
Sample
Ԙ
ԙ
300
E=0
Ԝ
ԛ
200
Table 1 Silicon sample description.
Ԛ
wafer label
dopant
resistivity
(V cm)
concentration (cm&3)
wafer
wafer
wafer
wafer
boron
boron
boron
antimony
5–15
0.01–0.02
0.001–0.009
0.005–0.020
2.7 ' 1014–9 ' 1015
3.1 ' 1018–8.4 ' 1018
9.7 ' 1018–1.3 ' 1020
1.2 ' 1018–1.1 ' 1019
A
B
C
D
E≠ 0
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0
200
400
Time (s)
600
800
E≠ 0
Si
Ԙ
ԙ
Ԛ
ԛ
Ԝ
SiO2
Figure 3 Left: TD-SHG measurement from highly boron doped
silicon (resistivity: 0.01–0.02 V cm), right: interpretation of the SHG
measurement regarding interface electric fields (white arrow: preexisting built-in electric filed due to boron charge traps, dark arrow:
induced electric field due to surface oxygen charge traps) [11].
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Original
Paper
Phys. Status Solidi B (2010)
3
4000
b)
Wafer B
3000
396mW
2000
307mW
1000
SHG (a.u.)
SHG (a.u.)
a)
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500
SHG (a.u.)
total electric field decreases as measured by the decreasing
SHG signal, indicated by in Fig. 3. At the minimum of the
SHG signal, the net total electric field becomes zero, though
the two separate electric fields may not be zero, as shown
by in Fig. 3. After reaching a minimum, the SHG signal
increases as more electrons are transferred from silicon into
the oxygen charge traps on the oxide surface and into the Bþ
charge traps until both traps are saturated, as shown in and
in Fig. 3. Because the native oxide layer is relatively thin,
electrons can be easily detrapped from the oxygen charge
traps due to quantum mechanical tunneling. When the laser
beam is blocked, the electrons captured by the oxygen
molecules on the surface of the oxide can tunnel back to
empty states in the valence band of the silicon substrate,
which results in a decrease of the electric field at the interface
[9, 12]. We may expect that during the blocking the boron
charge traps also release electrons to silicon substrate.
However when the beam was unblocked after being blocked
for a short period, no initial SHG decrease was observed as
shown in Fig. 4. Even the beam was blocked for about 1 h, no
significant recovery of the initial SHG decrease was
observed. From the observation we can infer that the
detrapping rate for the boron charge traps is much less than
the detrapping rate for the surface oxygen traps or the filled
Bþ charge traps become stable after filled by electrons. This
interpretation is supported by a density functional calculation which shows that neutral boron (B) is more stable in
amorphous SiO2 than positively ionized boron (Bþ) [21].
One paper suggested that B atoms diffuse into SiO2 as Bþ
ions [22]. Even more recently, another group showed that
boron has various stable and meta-stable forms in crystalline
and defective SiO2 [23, 24]. This group also showed that Bþ
is one of these stable forms. They showed results in
400
300
200
initial laser blocking
100
0
0
600 800 1000 1200 1400
Time (s)
agreement with the previous research [21] in amorphous
SiO2.
4 Boron doping concentration dependent
second order electric susceptibility: x(2) The magnitude of the initial decrease in our SHG measurements
correlates with boron doping concentration, as shown in
Fig. 5. When laser power is sufficiently high, the initial SHG
decrease is not observed for wafer B, because the oxygen
charge traps are filled very quickly by the excited electrons
(Fig. 5(a): 396 mW). However, when the laser power is very
low, the oxygen charge traps are filled very slowly due to
the fact that a three-photon excitation process is required (the
three-photon excitation probability is proportional to the
cube of laser power). This is illustrated in Fig. 5(a): 154 mW
and Fig. 5(b): 155 mW. Thus if laser power is low enough,
4000
Wafer C
3000
397mW
2000
305mW
1000
155mW
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60
80
100
0
20
1400
40
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Time (s)
Time (s)
d)
Wafer B (307mW)
1200
SHG (a.u.)
SHG (a.u.)
400
0
0
c)
200
Figure 4 TD-SHG measurement from highly boron doped silicon
(resistivity: 0.01–0.02 V cm). During the measurement the laser
beam is blocked [11].
154mW
0
second laser blocking
1000
800
600
2400
Wafer C (305mW)
2000
1600
1200
400
0
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20
40
60
Time (s)
80
100
0
20
40
60
Time (s)
80
100
Figure 5 (online color at: www.pssb.com) (a) SHG results from boron doped
silicon (resistivity: 0.01–0.02 V cm) by
three different laser powers [11]. (b)
SHG results boron doped silicon (resistivity: 0.001–0.009 V cm) by three different
laser powers [11]. (c) Fitting result of
307 mW data of (a). (d) Fitting result of
305 mW data of (b). All data were taken
using identical experimental conditions.
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H. Park et al.: Boron induced charge traps near the interface of Si/SiO2 probed by SHG
the three-photon excitation probability can be considered
negligible. In this case, only a monotonic decrease in our TDSHG measurement is observed which indicates that the
boron-induced electric field decreases monotonically. This
observation implies that it may require two-photon or onephoton excitation to fill the boron charge traps.
If the oxide thickness is greater than 10 nm, excited
electrons from the silicon valence band cannot reach the
surface of the oxide. In this case oxygen charge traps do not
play a role in the measured TD-SHG signals [10]. In
particular, the excited electrons fill only the boron charge
traps, which results in a decreasing built-in DC electric field
as the traps are filled. Therefore for highly boron-doped Si/
SiO2 with a thick oxide layer, it is expected that only the
boron charge traps are involved in a TD-SHG measurement
and a monotonic decrease should be shown in the
measurement. An experimental result is shown in Fig. 6
using wafer C with a thermally grown oxide layer (thickness:
12 nm). It showed only a monotonic decrease even with high
laser beam intensity (526 mW) as expected.
Relative information on the susceptibilities is obtained
by fitting Eq. (1) to our data. In general, x(2) and x(3) are
complex numbers therefore Eq. (1) can be modified as
!
!2
(2)
I ð2vÞ ¼ !jxð2Þ j þ jxð3Þ jeiu EðtÞ! ðI ðvÞ Þ2 ;
where u is the relative phase between the x(2) and x(3). The
time-dependent electric field E(t) can be obtained by a rate
equation which can be expressed by
EðtÞ ¼ E0 e&t=t1 & E1 ð1 & e&t=t2 Þ;
SHG (a.u.)
1500
1250
750
500
250
526 mW
Wafer C
Thermally grown oxide
Thickness = 12 nm
0
100
200
Time (s)
300
Figure 6 SHG results measured in highly boron-doped Si/SiO2
with a thermally grown oxide layer (oxide thickness: 12 nm, resistivity: 0.001–0.009 V cm).
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5 Summary We observed an initial SHG amplitude
decrease in TD-SHG measurements on highly boron-doped
silicon wafers indicating a time-dependent decrease in an
electric field at the Si/SiO2 interface. This effect is attributed
to a pre-existing static electric field at the interface of Si/
SiO2, which is postulated to be induced by boron charge traps
created during the growth of the oxide. Using a tunable ultrafast laser system, the boron charge traps were filled by an
internal photoelectron emission process. We determined the
direction of the electric field to be from the oxide to the
silicon substrate. No significant recovery was observed on
the boron charge traps after blocking the laser beam, which
indicates that the boron charge traps are meta-stable or
require a significant recovery time, longer than several hours.
From a fitting of our SHG data, we find that the effective
second order susceptibility increases in a non-linear manner
as the boron concentration increases.
(3)
where E0 is the magnitude of the initial electric field arising
from boron charge traps, E1 the magnitude of the electric
field from oxygen charge traps, t1 the detrapping time
constant of the boron induced charge trap, and t2 is the
trapping time constant of the oxygen charge traps. One
should also take into account an electron detrapping
constant for the oxygen traps for accurate fitting.
However, the effective value of jx(2)j is not affected by
the detrapping constant for the oxygen traps and the value of
jx(3)j cannot be separated from the fitting because it is
combined with the electric fields, E0 and E1. In Fig. 5(c, d),
1000
Eqs. (2) and (3) are fit to the two data sets (307 mW and
305 mW).
The fitting result shows that jx(2)j increases as the doping
concentration increases. Van der Pauw four-point probe
measurements were used to determine the resistivities of
wafers B and C, which gave 0.0165 V cm and 0.00519 V cm,
respectively. We compared the relative ratios of jx(2)j and
doping concentration which result in jx(2)jwafer C / jx(2)jwafer B
¼ 1.55, nwafer C / nwafer B ¼ 4.72. The results indicate that the
effective jx(2)j increased non-linearly with respect to boron
concentration.
Acknowledgements This work was supported by the
Department of Energy (DOE), Basic Energy Sciences, Grant No.
DE-FGO2-99ER45781.
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Phys. Status Solidi B (2010)
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