Abstract
Pentacene organic thin-film transistors (OTFTs) were fabricated on a polyethersulfone (PES) substrate with an 
gate dielectric grown at 
by plasma enhanced atomic layer deposition. Due to the roughness of the 
surfaces, the OTFTs with 
of 
showed better performance compared to that with 
of 
. When OTFTs with a 
thick 
layer were fabricated on PES substrate, excellent electrical characteristics were obtained, including carrier mobility as large as 
, a subthreshold slope as low as 
/dec, and on-off current ratio as large as 
.
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Electronic devices based on organic materials have attracted much attention in recent years. Organic thin-film transistors (OTFTs) promise cheap, flexible, and disposable plastic electronics.1, 2 Low-temperature processability allows a variety of low-cost substrate materials to be used, including flexible polymeric materials.3 OTFTs have been studied for low-cost applications, such as liquid crystal display panels, active matrix organic light-emitting diodes, sensors, postage stamps, and radio-frequency identification.3 Most research related to the performance of OTFTs has focused on high mobility, low operating voltage, low subthreshold swing, and a threshold voltage close to 
.4–7 In particular, the high operating voltage in the range of 
is a serious obstacle for realizing practical devices. A high operating voltage not only results in high power consumption and a high cost of the driving circuit but also damages the organic materials.6 The key technique used to solve this problem concerns the gate insulator of OTFT. In order to induce larger number of carriers at a lower voltage, the gate insulator should be thinner and its dielectric constant should be larger.6 Therefore, high-κ materials such as 
and 
should be used in order to ensure a low operating voltage and a low subthreshold swing (SS), while low-κ materials such as 
and SiON should be used as gate dielectrics to obtain a low leakage current and high stability. Thus, in order to ensure a low operating voltage and high stability simultaneously, 
with a dielectric constant of 8 and a low leakage current may be a good candidate as an inorganic gate insulator for OTFT devices. Sputtered metal oxides such as 
, 
, 
, and 
have led to some improvement in carrier mobility, but the sputtering process is not compatible with OTFT manufacturing, and films have to be relatively thick to ensure that they are free of pinholes.7 Plasma enhanced atomic layer deposition (PEALD) is a suitable deposition method to grow high-quality, uniform, and thin layers of metal oxide even at low deposition temperatures.8, 9 Therefore, dense films grown by PEALD at temperatures lower than 
may be good candidates, particularly for the flexible device applications using plastic substances.
Although numerous studies concerning OTFT devices with inorganic gate insulators have been conducted, among them only a small number of studies have been conducted with plastic substrates. Given that the use of plastic substrates requires a low-temperature deposition process, it is difficult to obtain a high-density and high-quality film for a gate insulator. To deposit a gate insulator of high-κ materials on a flexible substrate, electrochemical anodization of metal thin films and electron beam evaporation have been used with low-temperature processes.6, 10, 11 A low operating voltage of 
was obtained using a 
gate insulator in the fabrication of OTFT devices on a plastic substrate. However, a drawback of this device was its low mobility.11
There are other issues preventing the achievement of a high-performance OTFT device when a plastic material is used as substrate. In general, when an inorganic film is deposited on a plastic substrate, stress is generated that originates from the mismatch in the thermal expansion coefficient between the two materials. Hence, a flexible substrate can be bent to relieve the stress generated by film deposition process.12 Another problem associated with the use of a plastic substrate is related to the surface roughness. The roughness of the surface of a plastic substrate has a strong effect on the performance of the OTFT device. Hence, it is essential to determine the gate insulator thickness to overcome this problem as well as to ensure excellent device characteristics. In this paper, a pentacene TFT was fabricated while varying the thickness of the 
gate insulator. This was accomplished using PEALD at 
on a polyethersulfone (PES) substrate. It was found that 
films grown by PEALD at 
exhibit a low leakage current.9 Hence, it is expected that OTFT with an 
gate insulator grown by PEALD will show good performance characteristics.
In this study, PES wafers were used as substrates followed by deposition of a 
Ti layer deposited by e-beam evaporation through a shadow mask. On top of the Ti gate metal, 
at thicknesses of 80, 120, and 
was deposited by PEALD at 
using trimethylaluminum (TMA) precursors and 
gas mixed with 
gas as a gate dielectric layer. In order to remove organic contaminants, a 
UV-ozone treatment was utilized; this procedure was determined through preliminary tests that involved changes of the exposure time. Following this, the 
surface was treated with hexamethyldisilazane (HMDS) by spin-coating at 
. Postbaking after spin-coating of the HMDS was conducted for 
using a hot plate at 
. The HMDS was used as a self-organizing material to improve the quality of the organic/dielectrics interface.13
The pentacene was purchased from Aldrich Chemical Co. and was purified using the train sublimation method at 
or lower. The pentacene thin film was deposited using a thermal evaporation method at a temperature of 
. The deposition rate was 
, and the total thickness was 
. On top of the pentacene layer, an 
thick Au film was deposited through a shadow mask to define source (
) and drain (
) electrodes. The channel length and width were 100 and 
, respectively. The structure of the OTFT device described above is shown in Fig. 1. The current-voltage 
measurements on the OTFT device were performed with a Keithley 4200 semiconductor parameter analyzer in darkness at room temperature. In order to observe the film roughness characteristics, 
was deposited at the thicknesses of 120 and 
on PES substrates with the aforementioned 
thick layer of Ti. The morphologies of the film surface were also observed using atomic force microscopy (AFM).
Figure 1. Schematic diagram of a cross-sectional OTFT structure.
When an inorganic film is deposited onto a plastic substrate, the stress should be relieved. A previous work by the authors found that the stress level is likely insignificant in the deposition of 
films when using PEALD on a PES substrate due to the relatively low plasma power density and the low temperature.12 Another consideration is related to water penetration through the plastic substrate. A blocking layer such as 
may be required to prevent this.6 A previous work by the authors found that an 
film grown by PEALD prevented water penetration extremely well.14 Hence, the 
grown by PEALD is expected to serve as a good passivation layer and gate insulator.
On PES substrates, OTFT devices with 
gate dielectrics of 80, 120, and 
were fabricated and the device characteristics were compared. Figure 2 shows the electrical transfer characteristics of pentacene OTFTs with different film thicknesses of the 
gate dielectric material. The devices in Fig. 2 have a gate length of 
and a gate width of 
. In the case of the device with 
thick 
, turn-on characteristics were not observed. This result may be due to the existence of leaky and weak points in the 
gate dielectric resulting from its low thickness. However, the OTFT device with 
thick 
showed turn-on characteristics. The device with 
thick 
has a mobility rating of 
, a threshold voltage of 
, and a subthreshold slope of 
/decade, all of which are not satisfactory for high-performance devices. As the thickness of the gate oxide becomes higher, the device performance also increases; thus, the device with a 
thick 
layer exhibits a mobility of 
, a threshold voltage of 
, an on-off current ratio of 
, and a subthreshold slope of 
/decade. The data in this device are much better compared to the earlier results obtained using the plastic substrate that used 
films for the gate dielectrics.6, 10, 11 The main parameters of the mobility, the on-off ratio, the threshold voltage, and the subthreshold slope show great improvements over those using alumina gate dielectrics grown on an inorganic substrate.15, 16
Figure 2. (Color online) Electrical transfer characteristics of pentacene TFTs with a gate length of 
, a gate width 
, and gate dielectric thicknesses of 80, 120, and 
for 
.
The improvement of device performance in OTFT with 
thick 
can be explained by the roughness of the 
surface. The PES substrate used in these experiments is fairly rough compared to the Si substrate. The gate dielectric layer may also be rough as it is deposited onto a rough plastic substrate. In order to compare the roughness of 
surfaces, samples were fabricated with structures of 
/PES and of 
/PES with 120 and 
thick 
films, respectively. The morphologies of the 
surfaces on the Ti/PES and PES substrates were observed using AFM, as shown in Fig. 3. Figures 3a and 3b show images of the very rough surface of the 
thick 
layer on the Ti/PES and PES substrates. The root-mean-square roughness 
values for this sample were 
for the Ti/PES substrate and 
for the PES substrate, while those of the sample with the 
thick 
were 
for the Ti/PES substrate and 
for the PES substrates. Thus, it was found that the addition of the Ti electrode enhances the roughness of the gate dielectric layer. Accordingly, device performance is likely related to the surface morphology.6
Figure 3. (Color online) AFM images of 
films with thicknesses of 
on (a) Ti/PES and (b) PES substrates and of 
(c) on Ti/PES and (d) PES substrates. The 
values were (a) 
, (b) 
, (c) 
, and (d) 
.
Figures 4a and 4b show the electrical transfer and output characteristics of the TFT with a 
thick 
layer, respectively. For low power consumption with organic TFT, devices with near-zero threshold voltage ratings and low subthreshold slopes are desirable.2 The device has excellent turn-on characteristics and fulfills the conditions for low power consumption. From the results shown in Fig. 4b, drain current saturation is obtained at a relatively low drain voltage, and the device produces a drain current of 
at 
of 
and 
of 
(
, 
), which is much larger than what is required to drive an organic light-emitting diode.2
Figure 4. (Color online) Electrical (a) transfer and (b) output characteristics of the pentacene TFT with a gate dielectric thickness of 
. The data for 
are the same data as those shown in Fig. 2.
Device performance using a plastic substrate strongly depends on the thickness of the gate insulator. It is necessary for the gate insulator to be sufficiently thick to obtain satisfactory device performance in pentacene TFTs on PES substrate.
It was demonstrated that the performance of pentacene TFTs fabricated on the PES substrates strongly depends on the thickness of the gate insulator. A pentacene TFT with the 
thick 
gate insulator showed no turn-on characteristics, whereas the pentacene TFT fabricated with 150 and 
thick 
gate insulating layers showed a mobility rating of 0.62 and 
, a threshold voltage of 
and 
, an on-off current ratio of 
and 
, and a subthreshold slope of 0.4 and 
/decade, respectively. The difference in the performances of these devices with different 
thicknesses may be due to the different roughness levels of the 
surfaces. The high-performance results for TFT with a 
thick 
layer suggest that sufficiently thick insulating films with a high density can overcome the difficulties in using a plastic substrate.
Electronics and Telecommunications Research Institute assisted in meeting the publication costs of this article.