Content uploaded by Chia-Ming Yang
Author content
All content in this area was uploaded by Chia-Ming Yang on Dec 08, 2015
Content may be subject to copyright.
Effects of CF
4
Plasma Treatment on pH and pNa Sensing Properties
of Light-Addressable Potentiometric Sensor with a 2-nm-Thick
Sensitive HfO
2
Layer Grown by Atomic Layer Deposition
Chi-Hang Chin
1;2
, Tseng-Fu Lu
2
, Jer-Chyi Wang
2
, Jung-Hsiang Yang
2
, Cheng-En Lue
2
,
Chia-Ming Yang
4
, Sheng-Shian Li
1
, and Chao-Sung Lai
2;3
1
Institute of NanoEngineering and MicroSystems, National Tsing Hua University, Hsinchu, Taiwan 300, R.O.C.
2
Department of Electronic Engineering, Chang Gung University, Taoyuan, Taiwan 333, R.O.C.
3
Biosensor Group, Biomedical Engineering Center, Chang Gung University, Taoyuan, Taiwan 333, R.O.C.
4
Inotera Memories, Inc., Taoyuan, Taiwan 333, R.O.C.
Received September 21, 2010; revised October 31, 2010; accepted November 15, 2010; published online April 20, 2011
We investigated the effect of the carbon tetrafluoride (CF
4
) plasma treatment on pH and pNa sensing characteristics of a light-addressable
potentiometric sensor (LAPS) with a 2-nm-thick HfO
2
film grown by atomic layer deposition (ALD). An inorg anic CF
4
plasma treatment with
different times was performed using plasma enhance chemical vapor deposition (PECVD). For pH detection, the pH sensitivity slightly decreased
with increasing CF
4
plasma time. For pNa detection, the proposed fluorinated HfO
2
film on a LAPS device is sensitive to Na
þ
ions. The linear
relationship between pNa sensitivity and plasma treatment time was observed and the highest pNa sensitiv ity of 33.9 mV/pNa measured from
pNa 1 to pNa 3 was achieved. Compared with that of the same structure without plasma treatment, the sensitivity was improved by twofold. The
response mechanism of the fluorinated HfO
2
LAPS is discussed according to the chemical states determined by X-ray photoelectron
spectroscopy (XPS) analysis. The analysis of F 1s, Hf 4f, and O 1s spectra gives evidence that the enhancement of pNa sensitivity is due to the
high concentration of incorporated fluorine in HfO
2
films by CF
4
plasma surface treatment. # 2011 The Japan Society of Applied Physics
1. Introduction
The measurement of hydrogen (H
þ
) and sodium (Na
þ
) ions
is becoming increasingly important for bio medical,
1)
industrial, food, and environmental applications. To monitor
the changes in concentration in real time, many types of
sensors have been proposed. Among the proposed sensors,
one of the best approaches to obtaining the target data is
by the use of silicon-based sensors with suitable sensing
membranes, such as ion-selective electrodes (ISEs),
2)
ion
sensitive field-effect transistors (ISFETs),
3,4)
and electro-
lyte–insulator–semiconductors (EISs).
5,6)
Recently, a new
type of silicon-based sensor, namely, the light-addressable
potentiometric sensor (LAPS), which was first introduced by
Hafeman in 1988, has been receiving much attention owing
to its addressability.
7,8)
The construction of the LAPS chip, which is similar to the
EIS device, is shown in Fig. 1. A dc bias was applied to form
a depletion region in the semiconductor. When the chip was
immersed in solutions of diff erent pHs or ion concentrations,
the width of the depletion region changed depending on the
changes in the surface potential of a sensitive layer. By
illuminating the semiconductor substrate with a modulated
light, the variation in the electrical field within the depletion
layer can be detected and measured in the form of an ac
photocurrent produced by illumination. Therefore, a change
in the analysis concentration can be directly determined by
measuring the amplitude of the ac photocurrent.
9)
Compared
with other silicon-based sensors mentioned above, LAPS
exhibits a number of advantages including the ability for
multi-ions sensing in one chip, simplicity structural,
simplicity of the fabrication process, and light addressa-
bility.
10,11)
Owing to its large and flat sensing area, LAPS is
also suitable for cell detection.
In our previous works, a single HfO
2
layer with high pH
sensitivity, low dri ft, and small-body effect was proposed
as a promising sensing membrane for pH detection.
12–14)
However, the unmodified HfO
2
sensing membrane cannot be
used for Na
þ
ion detection owing to its low sensitivity.
15,16)
The innovation presented in this work is the development
and application of carbon tetrafluoride (CF
4
) plasma on a
thin hafnium oxide film grown by ALD based on LAPS for
Na
þ
ion detection. The pH and pNa sensing characteristics
were both examined using a LAPS measurement system
with a lock-in amplifier. In our investigation, the proposed
low-power CF
4
plasma surface modification by plasma-
enhanced chemical vapor deposition (PECVD) can enhance
the pNa sensitivity of ALD-HfO
2
thin films. The changes in
the surface chemical states of ALD-HfO
2
thin films with
or without plasma treatment were studied by the X-ray
photoelectron spectroscopy (XPS) technique. On the basis of
the analysis of F 1s, O 1s, and Hf 4f spectra, the possible
sensing behavior due to fluorine incorporation by plasma
treatment was proposed.
2. Experimental Procedure
2.1 Devices fabrication
To investigate the sensing characteristics of Na
þ
ions, a
Fig. 1. (Color online) Schematic cross-sectional view of the LAPS chip.
E-mail address: cslai@mail.cgu.edu.tw
Japanese Journal of Applied Physics 50 (20 11) 04DL06
04DL06-1
# 2011 The Japan Society of Applied Physics
REGULAR PAPER
DOI: 10.1143/JJAP.50.04DL06
LAPS device with single HfO
2
thin film as pH and pNa
sensing membrane was fabricated using an ALD system.
The process flow and schematic cross-sectional view of the
LAPS structure are shown in Fig. 2. A 4-in. (100) p-type
silicon wafer with resistivities of 4–6 cm was used as the
substrate. Then, a 2-nm-thick HfO
2
layer was directly grown
on the Si substrate with standard RCA clean processing at
200
C using the ALD system. For the ALD deposition,
high-purity tetrakis(ethylmethylamino)hafnium (TEMAH) is
used as the precursor. H
2
O vapor served as the oxidant
and Ar gas was supplied as the purge and carrier gases,
respectively. The thickness was estimated according to the
deposition rate (1
A/cycle) and checked using an ellips-
ometer. Next, the 2-nm-thick HfO
2
layer grown by ALD was
fluorinated using CF
4
plasma surface treatment for 1, 3, and
5 min at 300
C using a plasma enhance chemical vapor
deposition (PECVD) system. The rf power was set at 30 W
and the processing pressure was controlled at 500 mtorr.
Finally, a 300-nm-thick Al layer as the contact electrode was
evaporated on the back side of the wafer. To define the
illumination area of LAPS, the wet etching process was used
to open the back side Al electrode after photolithography
processing.
2.2 Test solutions
To extract the pH sensitivity, standard pH buffer solutions
of pHs 4, 7, and 10 were purchased from Merck Inc. To
activate the surface sites, all LAPS samples were immersed
in RO water for 12 h before measurement. To investigate
the sensing properties of Na
þ
ions, 5 mM tris(hydroxy-
methyl)aminomethane (Tris)/HCl solution was prepared as
the buffered electrolyte. The concentrations of Na
þ
in the
range between 10
3
and 10
1
M were controlled by injecting
0.1 M NaCl/ Tris–HCl and 1 M NaCl/Tris–HCl into the
buffered electrolyte using a micropipette. In order to obtain
stable pNa responses, all the LAPS sample s were immerse d
in 5 mM Tris/HCl for 12 h before the measurement.
2.3 LAPS measurement system
Figure 3 shows the LAPS measurement system with the
reference electrode, measurement cell, lock-in amplifier
(Stanford Research Systems SR510), and computer with the
DAQ card. For signal analysis, the lock-in amplifier as the
filter was used to increase the amplitude of the received
signal. Figure 4(a) shows the photovoltage –bias voltage
characteristics of the unfluorinated-HfO
2
LAPS which were
measured in pH buffer solutions from pHs 2 to 10. The
photocurrent, which is the output signal, was determined
according to the sine wave root-mean-square (RMS) values
using LabVIEW software. The photovoltage was obtained
by translat ing photocurrent through a resistance. To
determine the inversion point, diff erentiation was performed
2 times on the curves, as shown in Fig. 4(b). From the
obtained results shown in Fig. 4(b), the shift of the inverting
points in different buffer solutions could be considered as the
pH sensing responses, and the pH sensitivity was calculated
by the linear fitting of the responsive voltages, as shown in
Fig. 4(c). The definition of linearity is the relationship
Fig. 2. (Color online) Process flow and schematic of LAPS device with
ALD-HfO
2
thin film.
Fig. 3. (Color online) LAPS measurement system with the reference
electrode, measurement cell, lock-in amplifier, and computer with DAQ
card.
246810
0.3
0.4
0.5
0.6
0.7
-60
-40
-20
0
20
40
(b)
Second Differential
Photovoltage
0.0 0.4 0.8 1.2
-1
0
1
2
3
4
(a)
pH2
pH4
pH6
pH8
pH10
Photovoltage (V)
Voltage Bias (V)
(C)
2 nm-HfO
2
LAPS
Output Voltage (V)
pH
Sensitivity=42.6 mV/pH
Linearity=99.2%
Fig. 4. (Color online) (a, b) I–V curves and (c) pH-sensing response.
C.-H. Chin et al.
Jpn. J. Appl. Phys. 50 (2011) 04DL06
04DL06-2
# 2011 The Japan Society of Applied Physics
between the X-axis and the Y -axis. The high linearity in
Fig. 4(c) indicates that the relationship between pH (X-axis)
and resp onsive voltage (Y-axis) is high.
2.4 Physical characterization
To study the composition and chemical state of the HfO
2
films with or without CF
4
plasma post-treatment, XPS
measurements were carried out (V. G. Scientific Microlab-
350) using a standard Mg K (1253.6 eV) X-ray source.
XPS profiles were collected with a pass energy of 40 eV and
step of 0.05 eV.
3. Results and Discussion
3.1 XPS surface analysis of fluorinated HfO
2
sensing
membrane
To analyze the composition and chemical states of the ALD-
deposited HfO
2
film with or without CF
4
plasma treatment,
XPS analysis was performed, as shown in Figs. 5 (F 1s
profiles), 6 (Hf 4f profiles), and 7 (O 1s profiles). All F 1s,
O 1s, and Hf 4f XPS profiles were calibrated by setting the
C 1s peak at 284.5 eV to eliminate the charge-up effect. The
background line was defined using a tougaard-type shape
and the peaks were fitted with a Lorentzian–Gaussian
function.
Figure 5 shows the F 1s profiles of (a) the ALD-HfO
2
thin
film with CF
4
plasma treatment for 5 min and (b) the ALD-
HfO
2
thin film without plasma treatment. For the samp le
with plasma treatment, it can be easily observed that a peak
with a strong intensity corresponding to fluorine incorpora-
tion by CF
4
plasma was detected.
The Hf 4f profiles of the ALD-HfO
2
thin film with plasma
treatment for 5 min and the ALD-HfO
2
thin film without
plasma treatment are both shown in Fig. 6. In the case of
Hf 4f profiles, the profile shape of the sample with plasma
treatment is different from that of the control sample. For the
control sample shown in Fig. 6(b), two major peaks at the
binding posi tions of 16.1 and 17.8 eV, corresponding to the
oxidized Hf 4f
7=2
and Hf 4f
5=2
bindings, respectively, were
observed. However, changes in the binding ener gy of the
Hf 4f spectrum are obser ved after plasma treatment. For the
sample with plasma treatment shown in Fig. 6(a), higher
binding energies of 17.7 and 19.8 eV of the oxidized
Hf 4f
7=2
and Hf 4f
5=2
, respectively, were obtained. The
peaks shifted positively by about 2 eV, compared with the
control sample, which should be due to the incorporation of
fluorine atoms into the HfO
2
film. To fit the profile well, two
additional peaks at binding energies of 18.95 and 21.3 eV
corresponding to the fluorinated HfO
2
bindings were added.
From the results of the XPS analysis of Hf 4f profiles, the
fluorine incorporation into the ALD-HfO
2
film induced by
CF
4
plasma surface treatment was confirmed.
Figure 7 shows the O 1s profiles of (a) the ALD-HfO
2
thin film with CF
4
plasma treatment for 5 min and (b) the
ALD-HfO
2
thin film without plasma treatment. In Fig. 7(b),
two major peaks can be found for the untreated HfO
2
film,
one at a binding energy of 530 eV, corresponding to the
chemical bond of HfO
2
, and the other at a larger binding
682 684 686 688 690
(b)
F 1s - W/O
Binding Energy (eV)
Intensity (arb. unit)
F 1s - plasma 5min
(a)
Fig. 5. (Color online) F 1s XPS profiles of (a) the ALD-HfO
2
thin film
with CF
4
plasma for 5 min and (b) the ALD-HfO
2
thin film without plasma
treatment.
14 16 18 20 22 24
(b)
Hf 4f - W/O
Binding Energy (eV)
F-Hf-O
HfO
2
Hf 4f - plasma 5min
HfO
2
F-Hf-O
Intensity (arb. unit)
(a)
Fig. 6. (Color online) Hf 4f XPS profiles of (a) the ALD-HfO
2
thin film
with CF
4
plasma for 5 min and (b) the ALD-HfO
2
thin film without plasma
treatment.
528 530 532 534 536 538
Binding Energy (eV)
Intensity (arb. unit)
O 1s - W/O
(b)
(a)
O 1s -
plasma 5min
HfO
2
HfO
x
F
y
contamination
Fig. 7. (Color online) O 1s XPS profiles of (a) the ALD-HfO
2
thin film
with CF
4
plasma for 5 min and (b) the ALD-HfO
2
thin film without plasma
treatment.
C.-H. Chin et al.
Jpn. J. Appl. Phys. 50 (2011) 04DL06
04DL06-3
# 2011 The Japan Society of Applied Physics
energy of 531.8 eV, corresponding to the contaminant. For
the sample with plasma treatme nt for 5 min shown in
Fig. 7(a), a third peak is applied to fit the O 1s profiles well
after plasma treatment. The corr esponding O 1s spectrum of
the sample with plasma treatment for 5 min showed three
peaks at binding energies of 531.2, 532.45, and 538.85 eV.
The peak with the lowest binding energy in Fig. 7(a), which
is positively shifted from the 530 eV of the untreated HfO
2
film, is attributed to the peak of HfO
2
. The second peak with
a binding energy of 532.45 eV in Fig. 7(a), which is near the
binding energy of 531.8 eV of the untreated HfO
2
film, is the
contaminant peak. The peak with highest binding energy of
538.85 eV in Fig. 7(a) is considered to be the peak of
HfO
x
F
y
.
17)
This is probably attributed to the bond formed
between O and F in the HfO
2
film after plasma treatment. A
similar formation of Al(OF)
x
and Si(OF)
x
layers after plasma
treatment was obtained in previous studies.
18,19)
Thus,
according to the report from Ding et al. and,
17)
a similar
HfO
x
F
y
film may be formed during CF
4
plasma treatment.
From the results of the XPS analysis, fluorine incorporation
in the HfO
2
film after CF
4
plasma treatment is confirmed.
3.2 pH sensitivity of fluorinated HfO
2
LAPS
To obtain pH sensitivity, the current–voltage (I–V) curves of
LAPS were measured in standard pH buffer solutions
(Merck) at pHs of 4, 7, and 10. The actual pHs of standard
buffer solutions were examined before and after measure-
ment using a commercial pH electrode for pH sensitivity
calculation. Figure 8 shows the photovoltage–bias voltage
characteristics of (a) HfO
2
-LAPS without plasma treatment
and (b) HfO
2
-LAPS with CF
4
plasma treatment for 5 min.
The pH sensitivity and linearity were obtained by the linear
fitting of the responsive voltages at pHs of 4, 7, and 10. The
pH sensitivity of HfO
2
LAPS without plasma treatment was
28.1 mV/pH and the linearity was 99.9%. This shows that
the thin ALD-HfO
2
layer is usable for pH detection. For
HfO
2
LAPS with CF
4
plasma treatment for 5 min, the
extracted pH sensitivity is 22.4 mV/pH with a linearity of
99.9%. The detailed distributions of pH sensitivity and
linearity as a function of CF
4
plasma time are shown in
Fig. 9. The pH sensitivity decreased with increasing plasma
treatment time and nearly saturated when the plasma
treatment time was more than 3 min. Both the HfO
2
-LAPS
samples with and without post-CF
4
plasma treatment
showed a stable pH sensitivity between 20–30 mV/pH,
which shows good linearity over 99%. It could be attributed
to the fluorinated bond (F–O bond) formation on a thin
ALD-HfO
2
surface after CF
4
plasma treatment, which was
confirmed by XPS analysis. According to the site binding
theory, fluorinated bond formation could reduce the number
of active sites (N
A
, N
A
¼ HfO
þ HfOH þ HfOH
2
þ
). With
a decrease in the number of these reactive groups, the pH
sensitivity decreased.
20)
3.3 pNa sensitivity of fluorinated HfO
2
LAPS
To obtain the pNa sensitivity, the I–V curves of LAPS were
measured in the prepared pNa solutions from pNa 1 to
pNa 3. The photovoltage–bias voltage characteristics of
HfO
2
-LAPS without plasma treatment and HfO
2
-LAPS with
CF
4
plasma treatment for 5 min are shown in Figs. 10(a) and
10(b), respectively. As shown in Figs. 10(a) and 10(b), the
I–V curves shifted positively along the X-axis, correspond-
ing to the increase in pNa. This positive shift means that
the positive charge is the main species that determine the
potential between the electrolyte and the insulator. For pNa
sensitivity determination, in order to avoid the deviation
caused by the pH variation, the actual sensing voltages of
sodium ions were obtained with pH calibration using a
commercial pH electrode. The pH sensitivity of HfO
2
LAPS
without plasma treatment extracted from Fig. 10(a) was
17.8 mV/pNa with a linearity of 98.4%. Owing to the low
pNa sensitivity and low linearity, a thin ALD-HfO
2
layer
is not suitable for pNa detection. For HfO
2
LAPS with
CF
4
plasma treatment for 5 min shown in Fig. 10(b), the
determined pNa sensi tivity is 34.8 mV/pH and the linearity
0
2
4
6
0.0 0.2 0.4 0.6 0.8 1.0
0
2
4
6
pH 4
pH 7
pH 10
2 nm ALD-HfO
2
W/O
Sensitivity = 28.1 mV/pH
Linearity = 99.9%
(a)
Photovoltage (V)
Voltage Bias (V)
pH 4
pH 7
pH 10
2 nm ALD-HfO
2
CF
4
plasma 5 min.
Sensitivity = 22.4 mV/pH
Linearity = 99.9%
(b)
Fig. 8. (Color online) Photovoltage-bias voltage characteristics of (a) the
ALD-HfO
2
thin film with CF
4
plasma for 5 min and (b) the ALD-HfO
2
thin
film without plasma treatment for different pHs of 4, 7, and 10.
20
24
28
32
531
Different plasma times
Sensitivity
Linearity
Plasma Time (min)
Sensitivity (mV/pH)
Control
decreased 7 mV/pH
96
98
100
Linearity (%)
Fig. 9. (Color online) pH sensitivity and linearity distributions of the
fluorinated HfO
2
thin film on LAPS as a function of CF
4
plasma treatment
time.
C.-H. Chin et al.
Jpn. J. Appl. Phys. 50 (2011) 04DL06
04DL06-4
# 2011 The Japan Society of Applied Physics
is high at 99.9%. The proposed inorganic CF
4
plasma
treatment of ALD-HfO
2
thin films is highly feasible for pNa
detection. The relationship between pNa sensitivity and
plasma treatment time is shown in Fig. 11. The pNa
sensitivity increased with increasing CF
4
plasma time and
the highest pNa sensitivity of 33.9 mV/pNa was obtained
from the sample with plasma treatment for 5 min. The active
site of the Na
þ
ion based on the site binding theory is HfO
(HfO
þ Na
þ
¼ HfONa). However, more formation of
fluorinated bonds could reduce the number of active sites
(N
A
, N
A
¼ HfO
þ HfOH þ HfOH
2
þ
), including the HfO
site. Therefore, we think that the fluorinated bond (HfOF) is
possibily one that may be active toward the Na
þ
ion. The
enhancement of pNa sensitivity could be a result of the
attraction of Na
þ
ions induced by the fluorinated bonds. To
determine the exact reason for this, further investigation of
the Na
þ
sensing mechanism of the fluorinated HfO
2
layer
should be carried out.
4. Conclusions
In this work, 2-nm-thick ALD-HfO
2
thin films subjected to
fluorinated treatment using CF
4
plasma on LAPS for H
þ
and
Na
þ
ion detection were studied. For pH detection, the pH
sensitivity decreased with increasing plasma treatment time .
For pNa detection, the proposed LAPS with fluorinated
HfO
2
thin films was found sensitive to the concentration
change of Na
þ
ions with good linearity (>99%). The pNa
sensitivity enhancement of fluorinated HfO
2
LAPS reveals
a linear dependence on the plasma time and the highest
pNa sensitivity of 33.9 mV/pNa is achieved with a plasma
treatment time of 5 min in our study. The possible
mechanism of pNa sensing based on the changes of surface
sites due to the fluorine incorporation by plasma treatment is
proposed, as confirmed by XPS analysis. The measured
results offer a useful guideline for the sensing performance
optimization and development of multi-ion LAPS.
Acknowledgment
This work was supported by the National Science Council of
the Republic of China under contract no. NSC 98-2221-E-
182-057-MY3.
1) A. Errachid, J. Bausells, N. Zine, H. Jaffrezic, C. Martelet, N. Jaffrezic-
Renault, and M. Charbonnier: Mater. Sci. Eng. C 21 (2002) 9.
2) R. P. Buck and E. Lindner: Anal. Chem. 73 (2001) 88A.
3) P. Bergveld: IEEE Trans. Biomed. Eng. 17 (1970) 70.
4) C. D. Fung, P. W. Chenug, and W. H. Ko: IEDM Tech. Dig., 1980, p. 689.
5) M. Turek, L. Ketterer, M. Cla en, H. K. Berudt, G. Elbers, P. Kru
¨
ger, M.
Keusgen, and M. J. Scho
¨
ning: Sensors 7 (2007) 1415.
6) M. J. Scho
¨
ning, D. Brinkmann, D. Rolka, C. Demuth, and A. Poghoss ian:
Sens. Actuators B 111–112 (2005) 423.
7) T. Yoshinobu, M. J. Scho
¨
ning, R. Otto, K. Furuichi, Yu. Mourzina, Yu.
Ermolenko, and H. Iwasaki: Sens. Actuators B 95 (2003) 352.
8) T. Wagner, T. Yoshinobu, C. Rao, R. Otto, and M. J. Scho
¨
ning: Sens.
Actuators B 117 (2006) 472.
9) M. Nakao, T. Yoshinobu, and H. Iwasaki: Sens. Actuators B 20 (1994)
119.
10) S. Inoue, T. Yoshinobu, and H. Iwasaki: Sens. Actuators B 32 (1996) 23.
11) B.-K. Sohn, B.-W. Cho, C.-S. Kim, and D.-H. Kwon: Sens. Actuators B 41
(1997) 7.
12) M.-N. Niu, X.-F. Ding, and Q.-Y. Tong.: Sens. Actuators B 37 (1996) 13.
13) C.-S. Lai, C.-E. Lue, C.-M. Yang, J.-H. Jao, and C.-C. Tai: Sens. Actuators
B 130 (2008) 77.
14) C.-S. Lai, C.-M. Yang, and T.-F. Lu: Jpn. J. Appl. Phys. 45 (2006) 3807.
15) C.-S. Lai, C.-M. Yang, and T.-F. Lu: Electroch em. Solid-State Lett. 9
(2006) G90.
16) S. J. Ding, Y. J. Huang, Q. Q. Sun, and W. Zhang: ECS Trans. 25 [6]
(2009) 209.
17) A. C. Miller, F. P. McCluskey, and J. A. Taylor: J. Vac. Sci. Technol. A 9
(1991) 1461.
18) J. H. Thomas III: J. Vac. Sci. Technol. B 7 (1989) 1236.
19) K.-I. Ho, T.-F. Lu, C.-P. Chang, C.-S. Lai, and C.-M. Yang: Proc. IEEE
Sensors, 2009, 5398575.
15
20
25
30
35
40
531
Different plasma times
Sensitivity
Linearity
Plasma Time (min)
Sensitivity (mV/pNa)
Control
increased 15.7 mV/pH
96
98
100
Linearity (%)
Fig. 11. (Color online) pNa sensitivity and linearity distributions of the
fluorinated HfO
2
thin film on LAPS as a function of CF
4
plasma treatment
time.
0
1
2
3
4
0.2 0.4 0.6 0.8
0
1
2
3
4
pNa 1
pNa 2
pNa 3
2nm ALD-HfO
2
W/O
Sensitivity = 17.8 mV/pNa
Linearity = 98.4%
(a)
Photovoltage (V)
Voltage Bias (V)
pNa 1
pNa 2
pNa 3
2 nm ALD-HfO
2
CF
4
plasma 5 min.
Sensitivity = 34.8 mV/pNa
Linearity = 99.9%
(b)
Fig. 10. (Color online) Photovoltage-bias voltage characteristics of
(a) the ALD-HfO
2
thin film with CF
4
plasma for 5 min and (b) the ALD-
HfO
2
thin film without plasma treatment for different pNa’s from 1 to 3.
C.-H. Chin et al.
Jpn. J. Appl. Phys. 50 (2011) 04DL06
04DL06-5
# 2011 The Japan Society of Applied Physics
