Chapter 2: Experiment Setup
2.3 Parameter Extraction
2.3.3 Subthreshold swing
Subthreshold swing represents how rapidly the device turn on from the off state. It is
related to the interface quality and the defect density of the device. We extract
subthreshold swing with
log
constant
VD
D G
I
S
V
Figure 2-1. Structure of OTFTs
Figure 2-2. Photo images of sensing system
Chapter 3
Result and Discussion
In this chapter, we first discussed the gas-sensing phenomenon of standard OTFTs.
Then, we introduced the UV-treated PMMA method to improve the sensitivity of
OTFTs. Besides, we also discussed the selectivity and the phenomenon of recovery.
Finally, we increased the relative humidity to find out the environment influence of
the water.
3.1 Ammonia-sensing Phenomenon of Standard OTFTs
3.1.1 Electrical Properties of Standard OTFTs
Before exposing the device on the analytic gas, we measured the electrical
characteristics of the device at constant drain bias, V 3 V, and sweeping gate
bias from 5 V to -40 V. With the Fig3-1, we can extract the threshold voltage as -16.4
V, the mobility as 0.3 cm2/Vs, and subthreshold swing as 0.66 V/decade. Those data
was taken as standard one to be compared with other data under gas-sensing.
3.1.2 Gas Diffusion Model
Fig3-2 presents the I-V curve under 1 ppm NH3 condition with different time. As
shown in Fig3-3, the variations of the turn-on current, field-effect mobility, and
threshold voltage shift were decreaed while the subthreshold swing was increased.
The turn-on current variation (Id/Id0), according to Eq(1), is affected by both threshold
voltage shift (Vth) and mobility variation (/0), where Id0 and 0 are the initial drain
current and the initial field-effect mobility.
The reason for the decreasing of mobility and threshold voltage is still not very
clear. A possible reason is that positive ammonia ions (NH4+) or polar ammonia
molecular (NH3) penetrated through the grain boundaries into the bulk of pentacene
layer and created scattering centers or traps [47], as Fig3-4. In addition, the polar
molecules may decrease the rate of charge transport in organic materials by increasing
the energetic disorder through charge–dipole interactions [48]. Because the
concentration of ammonia was fixed, the mobility decreased to saturate very soon.
On the other hand, the threshold voltage was decreased gradually. The reason
may be the hole-traps, which were attributed to the NH3 or NH4+ near dielectric
interface and caused lower concentration of gate-induced mobile carriers [49], as
Fig3-5. Therefore, it needs more negative gate voltage to induce holes in the
p-channel, as Fig3-6 Because it needed more time for the formation of the hole-traps
near the interface between pentacene and dielectric layer, the threshold voltage shift
could not as fast as the mobility variation. The increased subthreshold swing
conformed that the density of interface traps between pentacene and dielectric layer
were increased [46].
3.1.3 Ammonia Concentration Effect
To confirm the gas sensing model, we exposed the devices to different ammonia
concentration from 0.5ppm to 5ppm. Fig3-7 (a)(b) shows the threshold voltage shift
and mobility variation versus different ammonia concentration measured in different
time (1000 seconds and 2000 seconds).It clearly shows that the threshold voltage shift
and mobility variation were increased with the increasing of ammonia concentration,
which caused more NH4+ or NH3 penetrate through the grain boundaries of pentacene
film. We can also find that after waiting for 2000 seconds, the threshold voltage shift
more than waiting for 1000 seconds. However, the mobility variation did not change
much between 2000 seconds and 1000 seconds waiting time. The phenomenon
conformed to the gas sensing model we mentioned above.
For application to non-invasive diagnostic sensor for cirrhotic patients, it is
necessary to monitor ammonia concentration at 0.5 ppm or lower so that the breath
samples between healthy person (breath ammonia level: 0.278 ppm) and a patient
(breath ammonia level: 0.745 ppm) can be distinguished [50]. For the patients with
renal failure, we need to monitor ammonia concentration at 1 ppm (relieve) to 5 ppm
(dangerous) [51]. However, it was hard to distinguish the variation difference
between 0.5 ppm and 1 ppm ammonia with the standard OTFTs. Thus, we needed to
find another way to improve the sensing ability.
3.2 Ammonia-sensing Phenomenon of UV-treated PMMA OTFTs
To enhance the sensitivity, we used a UV-light irradiation on PMMA to modify
the dipole moment of the dielectric surface [52]. The PMMA functional end-groups
changed from −COOCH3 to −COOOH, which will result in the negative charge sites
near the PMMA surface. Fig3-8 is the simulation of PMMA and UV-treated PMMA
which were estimated by Gaussian 03 with ab initio calculation. The dipole moment
of standard PMMA and UV-treated PMMA were 1.81~1.91 Debye and 2.42~2.5
Debye, respectively.
3.2.1 Electrical Properties of UV-treated PMMA OTFTs
Fig3-9 is the I-V curve of UV-treated device which was measured at constant
drain bias, V 3 V, and sweeping gate bias from 5 V to -40 V. The threshold
voltage we extracted is -11.1 V, mobility is 0.31 cm2/Vs, and subthreshold swing is
0.68V/decade. Compared with standard device, UV-treated device was easily to be
turn on and its subthreshold swing was larger. It may due to the change of functional
end-groups (from –CH3 to –COOOH) produced by UV-treatment on PMMA surface,
which will result in the negative charged-states near PMMA surface. With the
comparing of the energy band of standard OTFTs Fig3-10(a) and UV-treated PMMA
OTFTs Fig3-10(b), the negative charge sites, which produced by UV treatment,
caused a surface potential change that induced a bending of the HOMO level at the
interface and increased the carrier density in the channel [53,54,55,56,57]. Thus, it did
not need so much negative gate voltage to induce holes for turn-on. However, the
mobility of both devices was the same. It indicated that the structure of the pentacene
film was not affected by UV treatment. Fig3-11 (a)(b) are the AFM images of
pentacene film deposited on PMMA and UV-treated PMMA, respectively. We can
find that the grains and roughness were almost the same.
To verify the hypothesis, we calculated the number of interface states Nss with
the equation [59],
where S.S. is the subthreshold swing, e is the Napierian logarithm, k is the
Boltzmann’s constant, C is the capacity of total device, q is the electric charge and T
is the absolute temperature.
For standard OTFTs (subthreshold swing = 0.64 V/decade), the number of interface
states were 1.431012 cm-2eV-1; for UV-treated OTFTs (subthreshold swing = 0.68
V/decade), the number of interface states were 1.531012 cm-2eV-1. It was reasonable
to observe that UV-treated device exhibited higher interface state density than that of
standard device.
3.2.2 Sensing Phenomenon of UV-treated PMMA OTFT
We measured the UV-treated PMMA devices at the same condition as standard
devices. After extracting the parameters from I-V curve, we found that the threshold
voltage shifted much, the mobility and drain current also decreased much compared
with standard OTFTs. Fig3-12(a)(b)(c) are the threshold voltage shift, mobility
variation and drain current versus ammonia concentration in different waiting time of
standard OTFTs and UV-treated PMMA OTFTs, respectively. We proposed that UV
radiation increased the dipole moment of dielectric surface and attracted ammonia gas
molecules to accumulate near the dielectric surface. Also, the negative charge sites
which caused by UV treatment enhanced the attraction of positive ammonia ions
(NH4+) gave stronger electric responses on UV-treated PMMA OTFTs than those of
standard OTFTs. The variation difference between 0.5 ppm and 1 ppm ammonia can
be large enough to be distinguished. Therefore, UV treatment can enhance the
ammonia sensitivity.
3.3 Selectivity of Gas Sensing
To research and design an ammonia gas sensor, it is very important to confirm
the sensing selectivity. Thus, we exposed the devices to five different kinds of gas that
may exist in human’s breath, including ammonia (1 ppm), methane (2 ppm), acetone
(more than 1 ppm), alcohol (more than 1 ppm) and carbon dioxide (1000 ppm). With
Fig3-13, we can find that at a fixed sensing time (2000 sec), the threshold voltage
shift was not distinct with increased time under the condition of methane (CH4),
acetone (CH3COCH3), alcohol (C2H5OH) and carbon dioxide (CO2). Although the
concentration of ammonia was the lowest, the threshold voltage shift was evident
after 1000 seconds. The variation of mobility under ammonia environment was also
the biggest. With the sensing index, threshold voltage shift and mobility variation, we
can confirm that both standard and UV-treated OTFTs exhibited good selectivity
between ammonia and other gases.
3.4 Phenomenon of Recovery
In order to verify the sensing mechanism, we discuss the phenomenon of
recovery. At 0 second, both the standard and UV-treated devices were exposed to
nitrogen. Then, during 0 to 3000 seconds, the devices were exposed to 0 ppm
(nitrogen), 1 ppm, 3 ppm, 5ppm ammonia gas individually. After that, during 3000 to
4500 seconds, the ambience was purged with nitrogen.
With Fig3-14 (a)(b), we can find that the mobility of both the standard device
and UV-treated PMMA device had recovery. Although the value could not recover to
the beginning, the phenomenon were immediately when the ammonia was purged out.
The recovery of mobility may due to the decreasing of ammonia which may create
scattering centers or traps [47].
However, with Fig3-15 (a)(b), the threshold voltage shift of both devices did not
have evident recovery. Although the slope of threshold voltage shift was gradual when
the ammonia was purged out, the value could not be increased. The reason may be
that the ammonia molecular NH3 or ammonia ions NH4+ had been trapped on the
interface of the dielectric and pentacene layer. Thus, the reaction could not be
reversed.
3.5 Influences of Environment Humidity
In order to know the influence of humidity to the sensing model, we changed the
measure environment by increasing the relative humidity (RH) in the chamber. In the
wet nitrogen ambient (RH=50%), comparing with dry nitrogen ambient (RH=0%), we
can find that the threshold voltage of standard device had a negative shift, as
Fig3-16(a). However, in the same condition, the UV-treated PMMA device had a
positive threshold voltage shift, Fig3-16(b). It may due to different functional-end
group of dielectric. The polar -COOOH functional-end group of UV-treated PMMA
can interact with water molecules and create acceptor-like traps [60]. Because extra
holes were induced by trapped electrons, the threshold voltage turned positive [58].
Nevertheless, it is hard for less-polar -COCH3 functional-end group of standard
devices to interact with water molecules. When the polar water molecules diffused
into the interface between dielectric and pentacene layer, it may attribute hole-traps
near dielectric interface and caused lower concentration of gate-induced mobile
carriers. Therefore, it needs more negative gate voltage to induce holes in the
p-channel.
With the increasing of ammonia concentration, both the standard device and
UV-treated PMMA device had negative threshold voltage shift, especially in the wet
ambient (RH=50%). Because the threshold voltage shift of water and ammonia was
opposite for the UV-treated PMMA device, we can confirm that something must
happen in the wet ammonia ambient. The chemical reaction between water and
ammonia,
H O NH NH OH
, may be enhanced due to the increased H2O, which resulted in more NH4+ and less
H2O. Thus, the threshold voltage shifted much to the negative side in wet ammonia
ambient.
From Fig3-17(a), we can find that the mobility variations of both standard
devices and UV-treated PMMA device were decreased more in wet ambient than in
dry ambient. The reduction of carrier mobility was because polar water molecules
residing at grain boundaries interact with carriers [61]. Scattering effect or the field
screening effect may be the mechanism to describe interactions between polar water
molecules and carriers [62]. Because UV-treated PMMA devices have polar surface
which can attract more dipole water, the decreased difference between wet and dry
ambient of UV-treated PMMA devices were more than standard devices.
For the UV-treated PMMA devices, since threshold voltage moved to positive
value in wet nitrogen ambient, there should be an increase of the drain current
variation. However, from Fig3-17(b), the drain current variation of UV- treated device
was decreased in wet nitrogen ambient. The reduction of drain current may be mainly
attributed to a reduction of mobility [58]. But, we can see the influence of threshold
voltage in the difference of decreased current between wet and dry ambient, the
variation of UV-treated PMMA devices were smaller than standard devices.
Figure 3-1. Transfer characteristics of standard OTFTs
Figure 3-2. I-V curve of standard OTFTs under 1ppm NH3 with different time
Figure 3-3. The parameters variation of OTFTs under 1 ppm NH3 with different time
0 -10 -20 -30 -40
0 500 1000 1500 2000
0.900.95
Figure 3-4. Illustration of scattering effect and traps for gas sensing
Figure 3-5. Illustration of screen effect for gas sensing
(a) (b)
Figure 3-6. Energy band of (a) standard OTFTs and (b) Ammonia sensing PMMA OTFTs
(a) (b)
Figure 3-7. (a) Threshold voltage shift and (b) mobility variation versus different ammonia concentration measured in 1000 seconds and 2000 seconds
NH3 concentration (ppm) 0 second
NH3 concentration (ppm) 0 second
1000 seconds 2000 seconds
Figure 3-8. Simulation of PMMA and UV-treated PMMA estimated by Gaussian 03 with ab initio calculation.
Figure 3-9. Transfer characteristics of UV-treated PMMA OTFTs
(a) (b)
Figure 3-10. Energy band of (a) standard OTFTs and (b) UV-treated PMMA OTFTs
(a)
(b)
Figure 3-11. AFM images of pentacene film deposited on (a) PMMA and (b) UV-treated PMMA
(a)
NH3 concentration (ppm) 0 (second)
NH3 concentration (ppm) 0 (second)
NH3 concentration (ppm) 0 (second)
NH3 concentration (ppm) 0 (second)
1000 (second) 2000 (second)
(c)
Figure 3-12. (a) Threshold voltage shift (Vth) (b) mobility variation (/0) and (c) drain current variation (I/I0) versus ammonia concentration in different waiting
time of standard OTFTs and UV-treated PMMA OTFTs
0 1 2 3 4 5
Figure 3-13. Threshold voltage shift (Vth) and mobility variation ((0-)/0) percentage for standard and UV-treated OTFTs when devices were exposed to
different kinds of gas molecules.
(a) (b)
Figure 3-14. Mobility variation versus different time of (a) standard OTFTs and (b) UV-treated PMMA OTFTs under different ammonia concentration from 0
second to 3000 seconds
(a) (b)
Figure 3-15. Threshold voltage shift versus different time of (a) standard OTFTs and (b) UV-treated PMMA OTFTs under different ammonia concentration
0 1500 3000 4500
(a) (b)
Figure 3-16. Threshold voltage shift versus different NH3 concentrations in the dry ambient (RH=0%) and in the wet ambient (RH=50%) of (a) standard OTFTs and (b)
UV-treated PMMA OTFTs
NH3 concentration (ppm) RH = 0%
NH3 concentration (ppm) RH = 0%
RH = 50%
UV-treated
(a)
(b)
Figure 3-17. (a) Mobility variation and (b) drain current variation versus different NH3 concentrations in the dry ambient (RH=0%) and in the wet ambient
(RH=50%) of standard OTFTs or UV-treated PMMA OTFTs
0 1 2 3 4 5
NH3 concentration (ppm) RH = 0%
NH3 concentration (ppm) RH = 0%
NH3 concentration (ppm) RH = 0%
NH3 concentration (ppm) RH = 0%
RH = 50%
Chapter 4 Conclusion
A pentacene-based OTFT was shown to be highly sensitive for ammonia sensing
from 0.5 to 5 ppm, a critical range for the diagnosis of patients with chronic liver
diseases and renal failure. This demonstrated that OTFT devices, which can be
fabricated by simple and cheap process and exhibited channel length and width as
large as several hundreds of microns, are useful as non-invasive biomedical sensors.
This is on the contrary to inorganic MOSFET devices that require high fabrication
cost and complicated fabrication process to scale down its dimension to the range of
nanometers to increase the gas sensing sensitivity. The sensitivity and selectivity of
OTFTs as gas sensor can be further improved by the modification of the PMMA
dielectric layer, selecting suitable measuring parameters and providing additional
local electric field. Due to the simple fabrication processes of the devices, OTFTs are
promising to be developed to a portable and disposable gas sensor.
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