Utilizing Highly Crystalline Pyroelectric Material asFunctional Gate Dielectric in Organic Thin-Film Transistors By Nguyen Thanh Tien, Young Gug Seol, Le Huynh Anh Dao, Hwa Since the fir
Trang 1Utilizing Highly Crystalline Pyroelectric Material as
Functional Gate Dielectric in Organic Thin-Film
Transistors
By Nguyen Thanh Tien, Young Gug Seol, Le Huynh Anh Dao, Hwa
Since the first description of their use as potential elements for
electronic devices,[1] in 1987, organic thin-film transistors
(OTFTs) have been intensively studied, due to their potentially
lower cost, higher performance, and higher compatibility with
flexible electronic applications, as compared to conventional
silicon technology.[2–5] Recently, new functions of OTFTs and
their integrated circuits have been being considered, in an
attempt to take advantage of organic electronic devices in
different applications, such as memory,[6,7] radio-frequency
identification (RFID),[8]and sensors.[9–12]
For functional organic devices, organic smart materials with
ferroelectric, piezoelectric, and pyroelectric properties can be
directly integrated into the OTFT device structure Good
candidates are poly(vinylidene fluoride) (PVDF) and its
copoly-mer with trifluoroethylene, P(VDF-TrFE) The piezo- and
pyroelectricity of PVDF and P(VDF-TrFE) were studied in
depth,[13–22]and have been successfully applied in many research
fields,[23]but the applications of these properties in OTFTs are
limited to external sensing modules.[9,11]On the other hand, there
have been both theoretical and experimental reports of memory
applications based on the ferroelectricity of P(VDF-TrFE) in
OTFTs.[6,7,24–26] High current on-off ratio and fast switching
dipoles, which imply a small remnant polarization, are the key
aspects in this case In applications making use of the pyroelectric
and piezoelectric properties of P(VDF-TrFE), however, the
switching of small remnant polarization should be avoided,
and a stable large polarization is required instead Thus, physical
models based on the assumption of small and easy-to-switch
remnant polarizations[24–26] are not appropriate in interpreting
the experimental observation in this work, showing a very large
remnant polarization, and need to be modified in order to
accurately interpret the experimental information
In this report, we present for the first time the direct use of a
highly crystalline P(VDF-TrFE) material with a very large remnant
polarization as a pyroelectric gate-insulator layer in an OTFT
structure for temperature-sensing applications This has the
advantage of a simpler fabrication process compared to external
sensing modules A poling strategy based on step-wise poling
process[20]was required to enhance the effects of the pyroelec-tricity on the transistor performance (see Experimental) The output characteristics of the OTFTs were changed so as to exhibit
a linear current-voltage relationship, thus providing evidence of their large polarization We introduced a modified transistor equation to fully explain this phenomenon and related problems, such as the effect of the geometry on poling The thermal behavior of the functional OTFT was also investigated, and the results showed a linear response below the phase transition temperature of P(VDF-TrFE) The temperature response of the device was primarily attributed to the pyroelectric property of the highly crystalline P(VDF-TrFE) layer, rather than to temperature-dependent changes in the other parameters, such as field-effect mobility, gate capacitance, and contact resistance The positive pyroelectric coefficient extracted from the data is also the first such value reported for highly crystalline P(VDF-TrFE) material The poling operation mode was also introduced for the purpose of obtaining its stable and reliable performance in temperature-sensor applications
Figure 1a shows the output characteristics of the unpoled device, which are similar to those of conventional ferroelectric field-effect transistors (FeFETs).[25,26] After poling, the drain current-gate voltage (ID–VG) and gate current-gate voltage (IG–
VG) characteristics of the device were also measured at room temperature, by sweeping the gate voltage (VG) from 40 to 40 V
at various values of the drain voltage (VD), ranging from 0 to
40 V Repeated measurements of the ID–VGtransfer character-istics from the same device produced irreproducible curves due to
a change in IDcaused by dipoles, and the measurements obtained using ‘‘short’’ (one measurement was made and taken as the measured value) and ‘‘long’’ sampling mode (128 measurements were made and the average value was taken as the measured value) also yielded different results (not shown) These variations reflect the hysteresis phenomenon in ferroelectric materials However, a consistent peak in the IG–VG characteristics was observed at a VG of around 30 V Figure 1b shows this peak at
VD¼ 40 V for the short and long sampling modes Notably, the applied electric field induced by a negative VGis parallel to the polarization of the poled P(VDF-TrFE) layer, while a positive VG produces an anti-parallel electric field Thus, this peak agreed well with previous results,[6] pointing to the existence of dipole switching when the applied electric field exceeds the coercive field
in the reverse direction This suggests that the value of VGshould not be allowed to exceed VD by more than 30 V during the measurement of our device, in order to avoid dipole switching (e.g., see the measurement conditions in Figure 1c) The use of
[*] N.-E Lee, N T Tien, Y G Seol, L H A Dao, H Y Noh
School of Advanced Materials Science and Engineering and Center
for Advanced Plasma Surface Technology, Sungkyunkwan University
Suwon, Kyunggi-do 440-746 (Korea)
E-mail: nelee@skku.edu
DOI: 10.1002/adma.200801831
Trang 2only a few measured points is also recommended, to minimize
hysteresis
Figure 1c shows the output characteristics of the poled device
measured at room temperature with data points at VG¼ 10,
20, 30, and 40 V and VDbetween 0 and 40V at 5V steps
These sampling conditions produced stable results during
repeated measurements, and were used for the measurement
of all other output characteristics of the poled devices in the remainder of this report The output characteristics of the poled device in Figure 1c are quite different from those reported in previous studies of the memory applications of ferroelectric FETs.[25,26] The ID–VD curves in Figure 1c show no current saturation in the region in which VDexceeds VG, where they were expected to be saturated The variation of IDwith VGis also small These phenomena imply that there is a high density of accumulated holes at the surface of the semiconductor layer that does not originate from the gate bias Such a large hole-density may come from the polarization of the P(VDF-TrFE) layer after it was poled, which is the main difference between our device and memory devices, showing no saturation and saturation in ID, respectively
Since the conventional transistor equation does not cover the assumption of such a large polarization in the gate dielectric, we introduce a modified equation, which is based on the following basic equation
where D is the dielectric displacement, e0and erare the vacuum and relative permittivities of P(VDF-TrFE), respectively, E is the electric field inside the gate dielectric layer, Pris the remnant polarization, and Ptotal is the total polarization For the sake of convenience, we define an equivalent voltage V0corresponding to
Pr, Pr¼ e0erV0/d, where d is the thickness of the gate dielectric layer
As derived in the Supporting Information S3, the modified ID–
VDcharacteristics equation is
L
1
D V0ð þ VGÞVD
(2)
where m is the field-effect mobility, C is the capacitance of the P(VDF-TrFE) layer, and W and L are the width and length of the channel, respectively
Table 1 shows the linear coefficient values in Equation 2 extracted by curve fitting at various VGvalues from the data in Figure 1c, and Figure 2 shows the plot of mCW(V0þ VG)/L versus
VG The linearity of mCW(V0þ VG)/L versus VG was clearly observed The V0value of 132 V was calculated for the poled device at room temperature, from the slope and the intercept of the curve in Figure 2
Noticeably, the measured capacitance of an as-deposited
500 nm P(VDF-TrFE) layer was about 16.23 nF cm2, of which the dielectric constant was 9.17 The measurement condition is
Figure 1 Effects of poling on device operation a) I D –V D output
charac-teristics before poling b) Dipole switching observed in I G –V G
character-istics c) I D –V D output characteristics after poling. Table 1.Coefficients of Equation 2, extracted from the ID –V D
character-istics in Figure 1c.
Trang 30.1 V AC voltage at 1 kHz, while the DC bias ranged from 20 to
20 V This value was slightly smaller than previously reported
dielectric-constant values, ranging from 10–12.[6,11] The result
could be subjected to the high crystallinity of the material It is
necessary to emphasize that the capacitance was determined by
the field-induced polarization of nonpolar material, which
vanishes when the applied electric field is removed In the case
of the P(VDF-TrFE) material, this field-induced polarization
originates from polar molecules in the amorphous phase, and
should be distinguished from the remnant polarization, Pr, which
comes from the dipole moments between the polymer chains in
the crystalline b phase.[13–15] In our case, recrystallizing the
P(VDF-TrFE) layer from the melt insured a large fraction of highly
crystalline b phase, or a small fraction of amorphous phase This
explained why we obtained such a small capacitance value
More interestingly, the capacitance value of a poled
P(VDF-TrFE) layer in the same thickness was reduced by a factor of 4,
3.98 nF cm2, compared to that of the as deposited thin-film The
reduction may be due to the high internal electric field caused by
the remnant polarization As this internal electric field was
rather high, which was equivalent to 132 V bias, the field-induced
polarization was prevented from adapting with small applied
voltages (20 þ 20 V) This prevention resulted in the
reduced capacitance The calculated surface-charge density was
5.25 mC m2, and was in the range similar to that reported by a
previous work.[27]It is noticeable that, under a poling electric field
of 60–80 MV m1, a completely poled P(VDF-TrFE) layer would
result in a remanent polarization value of 50–80 mC m2.[6,27]
Although our sample was poled under the electric field of
80 MV m1, such a smaller value of semiconductor
surface-charge density than expected is presumably ascribed to the
limited hole density in the pentacene layer in the device structure
The ID–VD curves of the current device in Figure 1c also
showed a small tendency to bend upwards, whereas they should
bend downwards according to the physical meaning Actually,
both the bending upwards and downwards of the curves (not
shown) were observed, and this variation came from the
transistor geometry features in the poling process Since the
channel length is much larger than the P(VDF-TrFE) layer
thickness, the poling electric field was not uniformly distributed
over the entire channel Thus, there are higher electric fields near
the source/drain electrodes, and these result in the larger
polarization of the P(VDF-TrFE) layer in these regions Since the source/drain electrodes were deposited by a shadow mask, misalignments can occur, causing overlapped areas with the gate electrode Depending on how much the difference of the overlap
is, the ID–VDcurves can bend upwards or downwards However, the fact that the extracted quadratic coefficients in the Equation 2 are a hundred times smaller than the linear coefficients (see Supporting Information S4) indicates the minority of geometry effect This may be due to the conducting property of the pentacene layer in the poling condition, which greatly reduced the nonuniformity in the poling electric field
According to previous works, it was argued that the pyroelectricity of P(VDF-TrFE) with a low crystallinity originates from the thermal vibration of the dipoles in the amorphous phase at high temperature, which causes a reduction of the average dipole moment in the poling direction.[13–16]However, those works did not investigate highly crystalline materials, whose dipole moments are drastically affected by the thermal expansionof the b phase crystals, since the remnant polarization
of highly crystalline P(VDF-TrFE) comes from dipole moments between the polymer chains in the crystalline b phase.[13–16]In this report, we distinguish between the contributions of the two phenomena, thermal vibration and thermal expansion, to the pyroelectricity of P(VDF-TrFE) Time-dependent ID measure-ments were performed at fixed VGand VDvalues right after the poling process, while the temperature was varied The tempera-ture was kept at 25 8C for 5 min, and then gradually raised up to
80 8C The sample was maintained at this high temperature for
5 min, and then cooled down to room temperature
Figure 3a shows the result obtained at VG¼ 40 V and
VD¼ 20 V In Figure 3a, there was a large decrease of ID in region III, in which the temperature was maintained at 80 8C Actually, the same phenomenon took place at room temperature
in regions I and V, but they are too small to be observable on a large scale This phenomenon is attributed to the decrease in the remnant polarization of the P(VDF-TrFE) layer, and agreed well with the conclusion reached in previous reports[13–16] that the reduction in the average remnant polarization resulting from the thermal vibration is the origin of the pyroelectricity of P(VDF-TrFE) This thermal vibration phenomenon resulted in the usually negative pyroelectric coefficients of P(VDF-TrFE), which imply a smaller remnant polarization at higher temperatures Moreover, we also observed in Figure 3a that the value of ID increased fairly rapidly as the temperature increased in region
II, and similar behavior also occurred in the backward direction
in region IV The data of regions II and IV in Figure 3a were replotted in Figure 3b, in terms of IDversus temperature By a rough estimation, the ID behavior in Figure 3b can be distinguished into a linear component, which is proportional
to temperature, and a nonlinear one Since the nonlinear signal was inversely proportional to temperature and decays by time, it
is reasonable to attribute this component to a negative pyroelectric phenomenon It is also important to point out that the different behaviors of ID in the forward and backward directions in Figure 3b are mainly due to the decay of ID in region III indicated in Figure 3a So, if the negative pyroelectric phenomenon can be minimized, a low thermal hysteresis and linear performance can be achieved for OTFT-based tempera-ture-sensor applications
Figure 2 Linear contribution of V 0 to I D corresponding to V G
Trang 4We introduced a poling operation mode in order to eliminate
negative pyroelectric phenomenon for sensing purposes, in
which the device was continually poled except when ID was
measured The measurements were taken at VG¼ 30 V and
VD¼ 10, 20, 30, and 40 V The temperature was in the
range from 25 to 60 8C, in 5 8C intervals The experimental data
were shown in Figure 4a and, as expected, the responses of IDto
temperature were linear V0 values obtained at different
temperatures indicate a linear temperature-sensitive response,
as shown in Figure 4b
According to Equation 2, three parameters that could be
attributed to the temperature-dependent behavior of ID in
Figure 4a are C, m, and V0 Associated with the measured
capacitance of the P(VDF-TrFE) layer at different temperatures,
dependence of m on temperature could be calculated from the
slope of mCW(V0þ VG)/L versus VG, in Figure 2 The measured
capacitance of the P(VDF-TrFE) layer, indeed, showed an increase
by a factor of 1.2 from 25 to 60 8C (see Supporting Information
S5) However, the calculated m values did decrease by a similar
factor, leading to an invariance of the mC values over temperature
The observed opposite temperature-dependent behaviors of
capacitance and mobility from 25 to 60 8C in our P(VDF-TrFE)
OTFT structure implied their minor total effect in the
temperature response of the device Therefore, a linear response
of ID to temperature variation in P(VDF-TrFE) device can be primarily attributed to the temperature sensitive change of V0
We would like to emphasize that the obtained m values, in this situation, are an effective mobility due to probable temperature dependence of other parameters, such as, for example, contact resistance between source/drain electrode and pentacene layer and carrier transport This temperature dependence, however, depends strongly on device geometry and measurement conditions According to previous works,[28,29] contribution of Au/pentacene contact resistance to the total resistance becomes negligible compared to that of the channel resistance at a high gate bias Hence, contribution of contact resistance to the temperature-dependent behavior of device at high gate bias turned into a less important component, like our case The thermally activated hopping transport was known to occur in pentacene below room temperature, in which hole mobility increases with temperature following an Arrhenius beha-vior.[30,31]However, as the temperature increased above a certain elevated value, mobility in pentacene eventually started to decrease.[32–35]This phenomenon is attributed to the increased carrier scattering occurring at elevated temperature.[33] Our comparative experiment re-performed with nonpyroelectric gate
Figure 3 Temperature response of the poled device in continuous
measurement: a) proportional I D with temperature and its decay were
both observed and b) data of proportional I D with temperature were
re-plotted.
Figure 4 Temperature response of the poled device in poling operation mode: a) temperature response of I D and b) temperature sensitivity of the poled device.
Trang 5dielectric material, poly(4-vinyl phenol) (PVP), in the same OTFT
structure and poling operation mode, also showed a similar
behavior to previous works[32–35] (see Supporting Information
S6) In the case of our P(VDF-TrFE) device, high V0values may
increase hole concentration in the pentacene layer, producing
high scattering even at room temperature Thus, increasing
temperature is expected to cause higher scattering, and as a result
reduce the effective mobility
As mentioned, the highly crystalline b-phase P(VDF-TrFE)
material also showed a positive pyroelectricity, which means
higher Prat elevated temperature, due to the thermal expansion of
crystals Linear temperature dependence of extracted V0, shown
in Figure 4b, confirmed this assumption Thus, it is reasonable to
attribute the temperature behavior of ID in Figure 4a to this
positive pyroelectricity of the highly crystalline b-phase
P(VDF-TrFE) materials
Since the variation of ID with temperature depends on the
measurement conditions, as shown in Figure 4a, it is better to
define the device sensitivity in terms of dV0/dT, which is
4.38 V 8C1 according to Figure 4b The deviation of the
calculated V0increased by time during repeated measurements,
and is about 29.81 V, or 6.8 8C after 5 h This may be due to the
quality degradation of the pentacene layer when being exposed to
air for a long time
This is the first report on the direct use of a pyroelectric
material as a gate dielectric layer in OTFTs and their
temperature-sensitive behavior A decrease in the polarization due to thermal
vibration was observed at high temperatures Poling operation
mode was employed to avoid this problem, and allow the use of
the large positive pyroelectric contributions of highly crystalline
b-phases, by thermal expansion mechanisms The linear
response of the crystalline phase polarization was extracted from
a modified model The linear response of the device in a certain
temperature range suggests its potential application in
pyro-electric thermal sensors Further works should be done on
obtaining higher stability and a larger dynamic range The use of a
built-in pyroelectric gate dielectric would reduce the complexity of
the fabrication process Integrating various smart organic
dielectric materials directly into the OTFT device structure would
enable the production of a vast range of functional or smart
organic electronic devices
Experimental
Fabrication: Figure 5 shows the structure of the OTFTs used in this
report An Ni gate electrode was deposited on a clean polyimide (PI) film by
the electroplating method [36] The material of the gate dielectric is a
P(VDF-TrFE) (65 mol% VDF) purchased from Piezotech S.A A solution
(20 wt%) of P(VDF-TrFE) dissolved in dimethylformamide (DMF) solvent was spun on the Ni gate electrode to a layer thickness of 500 nm, followed
by drying at 60 8C to remove the DMF solvent Next, the gate dielectric layer was annealed on a hot plate at temperatures of up to 200 8C, in order to melt it completely This layer was maintained at 140 8C for 2 h, and then naturally cooled down to room temperature in nitrogen ambient, to enhance the crystallinity of the b phase A pentacene layer followed by Au source/drain electrodes were deposited by a thermal evaporator [36] Characterizations: In order to investigate the pyroelectric property of P(VDF-TrFE) in the OTFT device, a poling strategy based on step-wise poling process was employed A basic step-wise poling process was carried out on the device at room temperature with grounded source/drain electrodes, by biasing the gate electrode up to 40 V The sample was then poled continuously at 40 V, while heating it from room temperature to
80 8C, maintaining it at this temperature for 2 h, and finally cooling it down
to room temperature to maximize the poling effect.
The output characteristics of the device placed on a hot chuck at different temperatures ranging from 25 to 80 8C were measured using an
HP 1415B semiconductor parameter analyzer, to explore the pyroelectric behavior of the highly crystalline b phase P(VDF-TrFE) material in the OTFT device The channel dimensions chosen for the device characterization were 35 mm in length and 800 mm in width The measurements were taken with only a few measuring points, in order to prevent large changes in the polarization of the P(VDF-TrFE) layer The capacitance of the P(VDF-TrFE) material was measured using an Agilent E4980A precision LCR meter in a MIM structure.
Acknowledgements
This work was supported by a Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2006-311-D00574) This research was also financially supported by the Ministry of Knowledge Economy (MKE) and Korea Industrial Technology Foundation (KOTEF), through the Human Resource Training Project for Strategic Technology N.T.Tien also wishes to thank N T Xuyen for her valuable discussions Supporting Information is available online from Wiley InterScience or from the author.
Received: July 1, 2008 Revised: September 5, 2008 Published online: December 18, 2008
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