A study of bottom-contact organic thin-film transistors based on source/drain tunneling structure

During recent years, OLED display has become one of the important products for consuming electronics and industrial domains. For AMOLED display, thin film transistor (TFT) is necessary for driving OLED devices. As the polysilicon and amorphous silicon TFTs have become mainstreams, organic thin film transistors (OTFTs) have also been proposed and shown their advantages in low-temperature process compatible with flexible substrates. However, OTFTs applicable to drive OLED generally adopt bottom-contact configuration. The performance of bottom-contact OTFTs are often inferior relative to top-contact OTFTs, due primarily to the high contact resistance of bottom-contact OTFTs. To overcome this disadvantage, the source/drain tunneling structure is proposed in this article. This tunneling structure can not only decorate the electrode/organic semiconductor interface to obtain a good metal-semiconductor contact, but also can form a MOS configuration at source/drain contact, so as to increase the carrier concentration at the contact to further reduce contact resistance. In this paper, a typical P-type high-mobility small molecule material of Pentacene is employed as the active layer of the OTFT, while CuO film, Polystyrene (PS) film is separately selected as the tunneling layer at source/drain contact. The diverse parameters, especially field effect mobility, were analyzed by analyzing electrical characteristics of the top-contact and bottom-contact OTFTs. In addition, the contact resistances were confirmed by line transfer method. The bottom-contact OTFTs incorporating tunneling layer own a maximum hole mobility of 0.143cm2/V·s. This suggested that the source/drain tunneling structure can indeed improve the electrical characteristics of bottom-contact OTFT, and further improvement is necessitated to achieve the high performance comparable to top-contact device.


Introduction
Being potential devices of display driving switches, thin film transistors has attracted great attention.Polysilicon [1,2] and amorphous silicon [3,4] thin film transistors have become mainstream in driving AMLCD and AMOLED due to their mature processing technology.In addition, organic thin film transistors (OTFT) [5][6][7] has also been proposed and developed during the last 30 years, due to their low-temperature process that is compatible with flexible substrates.Being the important evaluation parameter of OTFT, the carrier mobility has been significantly improved, for the conventional small molecule and polymer thin film transistor, the carrier mobility has reached to the level of a:Si thin film transistor, while for organic single crystal thin film transistor, the carrier mobility even reached the level of 10  2  •  ⁄ [8,9].However, these high-performance thin film transistors always adopted topcontact configuration, in order to obtain better organic semiconductor/electrode contact interface.In contrast, for OTFTs that drive AMOLED displays, bottom-contact configuration is usually employed, in order to avoid the destruction of organic semiconductor active layer during the following source/drain electrode definition step.Much effort has been delicated to realize high-performance bottom-contact transistors, notably with high carrier mobility, low threshold voltage, and high on/off ratio.One aspect inhibiting the realization of high-performance bottom-contact transistors lie in the electrode/organic contact [10,11].However, the electrical performance of bottom-contact OTFTs is inferior than topcontact OTFTs.The main factor that prohibits the high-performance bottom-contact OTFT is the inferior organic semiconductor/metal contact interface, especially the poor morphology at organic semiconductor/contact interface, which may lead to abundant interface states that limit the improvement of the transistor performance.In order to improve the electric parameters, such as carrier mobility, the most common approach is modifying electrode, for instance, by inserting buffer layer at the electrode, or by using other conducting materials as the graphene, polypyrrole, and so forth [12][13][14][15][16][17][18][19][20].These methods can not only improve the energy level, but also can reduce the interface defects, so as to reduce source/drain contact resistance.It is noteworthy of mentioning that, the total OTFT resistance of OTFT is made up of the sum of the channel resistance and the contact resistance, the latter being determined by the carrier injection at organic semiconductor/metal electrode interface [21].In this article, an electrode decorating method by electrode treatment and insulating tunneling layer introduction at source/drain contact is proposed to suppress large contact resistance due to low crystalline quality at organic semiconductor/metal contact interface of bottom-contact OTFT.Initially, the source/drain electrode is exposed to ultraviolet light, which introduces a thin metal-oxide layer, thereafter, another thin layer of organic insulator is deposited on the oxidized electrode, and the organic semiconductor active layer is formed on the organic insulator layer.By introducing metal oxide and thin organic insulating layer, the source/drain tunneling structure is obtained, which forms better contact between electrode and active layer, thus the contact resistance can be magnificently reduced.In this article, the representative P-type high-mobility small molecule material of Pentacene is adopted to build up OTFTs, and the effect of contact resistance on the characteristics of OTFTs is improved by introducing tunneling layer.The high-performance bottom-contact OTFTs is achieved, and the maximum hole mobility of Pentacene bottom-contact OTFTs achieved 0.143  2  •  ⁄ , which shows compariable performance with the top-contact OTFTs.

Fabrication process
Source/drain electrode insulating layers are created by dissolving polystyrene (PS) in toluene solvent.The doped silicon wafers 50 nm  2 coverage are used as the gate electrode and gate dielectric layer, respectively.They were cleaned in acetone, dehydrated alcohol, and deionized water, respectively, for 15 minutes, with the aid of ultrasonic oscillating.To remove any remaining solvent, the samples were heated at 110°C for 30 minutes after being spin-coated with a 40 mg/mL PS solution for 1 minute at 3000 rpm at room temperature.After that, copper source and drain were deposited at a moderate deposition rate with the vacuum level of 1.5× 10 −4 Pa and patterned by shadow masks, the forming film thickness is approximately 100 nm.Then the samples were exposed to ultraviolet light for 30 minutes, which forms a thin  layer, and thereafter a PS layer with thin thickness is deposited.The combination of  layer and PS layer work as the source/drain tunnelling layer.Finally, organic semiconductor Pentacene layer is deposited with a moderate rate of 0.15 nm/s at the vacuum level of 1×10 -4 Pa, and the grown thickness is approximately 30 nm.All the electrical characterization is carried out by dualchannel Keithley 2450 source-meter at room temperature in air.

Results and Discussion
Pentacene is a representative P-type organic semiconductor material (Fig. 1a).Fig. 1(b) shows the typical configuration of Pentacene bottom-contact OTFTs, which includes bottom-contact OTFTs with and PS layer as tunneling layer.By observing, the semiconductor layer is gray and the electrode is brownish yellow (Fig. 1c).1(d) demonstrates that the surface of the PS film is relatively uniform and smooth, and the grains are continuous.This also allows the pentacene film to be directly deposited on the surface of the PS film to grow a high-performance layer.These are of great importance, as the PS layer helps to improve the charge transport property, that is, the tunnelling effect of source/drain and interface quality of active layer, which is important for improving the electrical characteristics of the bottom contact OTFT.2(c)-(d) is the images of crystal morphologies on the surface of the electrode.It can be seen that the overall crystal size is slightly larger than the one in the channel.And it also becomes larger when the source/drain is decorated with tunneling layers.This shows that the carrier transport capability can increase with the tunneling structure.This verifies the improvement of tunneling layer in improving OTFT performance with bottom contact configuration.To verify the improved property of bottom-contact OTFT by introducing electrode treatment and inserting organic dielectric layer at the electrode, the electrical characterizations were carried out, and the top-contact OTFT is also employed as the reference device.Fig. 3 shows the transfer property of different devices at a drain-source voltage Vds = 1 V at room temperature.Fig. 3(a)-(c) shows the transfer characteristics of top-contact OTFT, bottom-contact OTFT, oxidization treated copper electrode and PS tunneling layer bottom-contact OTFT, respectively.For top-contact OTFT, the on-state current is in the order of 10 -6 A at Vds = -1 V, with a current ratio of approximately 10 5 , and the on-state current increases to the order of 10 -5 A at Vds = -15 V.For different Vds, the flat-band voltage is fixed at approximately 5 V.For bottom-contact OTFT without tunneling layer, the on-state current reduced to the order of 10 -8 A at Vds =-1 V, and this value increases to the order of 10 -7 A at Vds = -15 V.The inferior property of bottom-contact OTFTs can be ascribed to the large contact resistance with poor morphology at electrode/semiconductor active layer interface.From fig. 3(c), when introducing copper oxidization and PS tunneling layer, the on-state current increases to the order of 10 -6 A at Vds = -1V, and this value increases to approximately 10 -5 A at Vds =-15V, which demonstrated the effect of the source/drain tunneling layer.Note that, with the introduction of oxidized copper layer and PS tunneling layer, transfer The comparison of transfer property for various devices at Vds = -1V.is shown in Fig. 3(d).Table 1 provided a list of the electrical parameters.The threshold voltage, nominal mobility for top-contact OTFTs are 1.3 V, and 0.135 cm 2 /V.s and the transconductance is 2.68nS, respectively.For the conventional bottom-contact OTFT, the nominal mobility is 0.0026 cm 2 /V.s, the threshold voltage is -1.6 V, and the transconductance is 2.68 nS .In contrast, for the ameliorated bottom-contact OTFT with source/drain tunneling structure, the electrical parameters have been improved.For the bottom-contact OTFT with CuO and PS tunneling layers, the nominal mobility is 0.143 cm 2 /V.s, threshold voltage is -0.6 V, transconductance is 148 nS.It is concluded that with the improvement of structure and fabrication process, the performance distinction between bottom-contact OTFT and top-contact OTFT has been narrowed, and for certain bottom-contact devices, the performance can even compare with top-contact device.This indicates that the excellent organic semiconductor/metal electrode contact has been obtained, which efficiently reduces the influence of contact resistance.
In order to systematically study the electrical characteristics, a list of bottom-contact Pentacene OTFTs with channel lengths varying from 0.2 mm to 1.8 mm has been fabricated and their electrical performances have been analyzed.Fig. 4(a) described the variation of maximum mobilities with different channel lengths.Compared with the conventional bottom-contact OTFT, the maximum hole mobility of the bottom OTFT with CuO and PS tunneling layers increases approximately 4 times.The increase of the mobility is due to the reduction and suppression of contact resistances.For the bottomcontact device without tunneling layer, the maximum hole mobility is 0.193 cm 2 /V.s.For the bottomcontact device with CuO and PS tunneling layers, the maximum hole mobility is 0.741 cm 2 /V.s.In comparison to the top-contact Pentacene OTFT, the maximum hole mobility is still relatively small for the short channel bottom-contact transistor, however, with the improvement of fabrication process and device structure, the mobility of bottom-contact device approached or even surpassed the mobility of the top-contact OTFT.The relationship between the mobility and channel length can be explained by the interaction between channel resistance and contact resistance, which is caused by a variance of the morphological change.The improved transistor property can be illustrated by the relationship between contact resistance and the channel length.The contact resistance linearly reduced with the decreased channel length.In order to clearly demonstrate the variation of resistance, the line transfer method is employed to extract contact resistance.Fig. 4(b)-(c) show the relationship of resistances with different channel lengths for topcontact Pentacene OTFT and bottom-contact Pentacene OTFT without decoration of electrodes.Figure 4(d) shows the bottom-contact Pentacene OTFTs with electrode modifications.It is concluded that the total resistance shows linear growth relationship with channel length.The channel resistance should rise with the length of the channel for longer channels, and resistance of contact may be uniform.The total resistance has a fairly linear relationship associating with the channel length, with the possible exception of shorter channels [23].Note that, comparing with the bottom-contact OTFT, the bottom-contact OTFT with electrode modification shows much lower total resistance, for large channel length, the total resistance of conventional bottom-contact OTFT is about 10 6 Ω•cm, while this value reduced to approximately 10 4 Ω•cm for the electrode-decorated bottom-contact OTFT, which can compare with the top-contact OTFT.These results clearly indicate that the introduction of electrode treatment and dielectric tunneling layer can efficiently reduce contact resistance due to the morphological improvement of contact interface.

Conclusion
In summary, we have proposed a tunnelling structure of decorated electrode, which improves contact interface morphology and achieves high-performance bottom-contact OTFT with pentacene active layer.The improvement of bottom contact performance is attributed to UV exposure treatment of electrode and introducing organic dielectric tunnelling layer, which improved film quality at electrode/active layer interface, wand provides excellent drain/source contact for OTFT.Therefore, the maximum efficient mobility of bottom-contact Pentacene OTFT with electrode modification layer and with channel length of 100  is about 0.143 cm 2 /V.s.Electrode decoration greatly lowers the bottom contact OTFT's total resistance.The morphology improvement at contact interface is crucial for reducing contact resistance, and this simple improvement provides practical approach for obtaining high-performance bottomcontact OTFT.

Figure 1 .
Figure 1.(a) Molecular structures of the organic semiconductors OTFT, (b) Structures of decorated bottom-contact OTFT, (c) Photos of OTFT under light, (d)Cross-sectional SEM images of the bottom OTFT with CuO and PS tunneling layers.Fig.1(d) demonstrates that the surface of the PS film is relatively uniform and smooth, and the grains are continuous.This also allows the pentacene film to be directly deposited on the surface of the PS film to grow a high-performance layer.These are of great importance, as the PS layer helps to improve the charge transport property, that is, the tunnelling effect of source/drain and interface quality of active layer, which is important for improving the electrical characteristics of the bottom contact OTFT.

Figure 2 .
Figure 2. (a) the morphology of the channel surface of bottom-contact OTFT, (b) the morphology of the channel surface of bottom-contact OTFT with CuO and PS tunneling layer, (c) the morphology of the electrode surface of bottom-contact OTFT, (d) the morphology of the electrode surface of bottom-contact OTFT with CuO and PS tunneling layer.Fig. 2(a)-(b) demonstrates the morphology for the surface of conventional bottom-contact OTFT, bottom-contact OTFT with oxidization treated copper electrode and PS tunneling layer.The crystals of the decorated active layer are obviously larger than that of the unmodified active layer, and the PS modification results in a large area of island-like crystals.Fig.2(c)-(d) is the images of crystal morphologies on the surface of the electrode.It can be seen that the overall crystal size is slightly larger than the one in the channel.And it also becomes larger when the source/drain is decorated with tunneling layers.This shows that the carrier transport capability can increase with the tunneling structure.This verifies the improvement of tunneling layer in improving OTFT performance with bottom contact configuration.

Figure 3 .
Figure 3. (a) transfer characteristics of top-contact OTFT, (b) transfer characteristics of undecorated bottom-contact OTFT, (c) transfer property of bottom-contact OTFT with CuO and Ps tunneling layers, (d) comparison of transfer property of different OTFT structures.To verify the improved property of bottom-contact OTFT by introducing electrode treatment and inserting organic dielectric layer at the electrode, the electrical characterizations were carried out, and the top-contact OTFT is also employed as the reference device.Fig.3shows the transfer property of different devices at a drain-source voltage Vds = 1 V at room temperature.Fig.3(a)-(c) shows the transfer characteristics of top-contact OTFT, bottom-contact OTFT, oxidization treated copper electrode and PS tunneling layer bottom-contact OTFT, respectively.For top-contact OTFT, the on-state current is in the order of 10 -6 A at Vds = -1 V, with a current ratio of approximately 10 5 , and the on-state current increases to the order of 10 -5 A at Vds = -15 V.For different Vds, the flat-band voltage is fixed at approximately 5 V.For bottom-contact OTFT without tunneling layer, the on-state current reduced to the order of 10 -8 A at Vds =-1 V, and this value increases to the order of 10 -7 A at Vds = -15 V.The inferior property of bottom-contact OTFTs can be ascribed to the large contact resistance with poor morphology at electrode/semiconductor active layer interface.From fig.3(c), when introducing copper oxidization and PS tunneling layer, the on-state current increases to the order of 10 -6 A at Vds = -1V, and this value increases to approximately 10 -5 A at Vds =-15V, which demonstrated the effect of the source/drain tunneling layer.Note that, with the introduction of oxidized copper layer and PS tunneling layer, transfer

Figure 4 .
Figure 4. (a) variation of mobilities with channel lengths for different OTFT structures, (b) line transfer method of resistance deduction for top-contact device, (c) line transfer method of resistance deduction for bottom-contact device without electrode decoration, (d) line transfer method of resistance deduction for bottom-contact device with CuO and PS tunneling layers.The relationship between the mobility and channel length can be explained by the interaction between channel resistance and contact resistance, which is caused by a variance of the morphological change.The improved transistor property can be illustrated by the relationship between contact resistance and the channel length.The contact resistance linearly reduced with the decreased channel length.In order to clearly demonstrate the variation of resistance, the line transfer method is employed to extract contact resistance.Fig.4(b)-(c) show the relationship of resistances with different channel lengths for topcontact Pentacene OTFT and bottom-contact Pentacene OTFT without decoration of electrodes.Figure4(d)shows the bottom-contact Pentacene OTFTs with electrode modifications.It is concluded that the total resistance shows linear growth relationship with channel length.The channel resistance should rise with the length of the channel for longer channels, and resistance of contact may be uniform.The total resistance has a fairly linear relationship associating with the channel length, with the possible exception of shorter channels[23].Note that, comparing with the bottom-contact OTFT, the bottom-contact OTFT with electrode modification shows much lower total resistance, for large channel length, the total resistance of conventional bottom-contact OTFT is about 10 6 Ω•cm, while this value reduced to

Table 1 .
[22]acteristics shift positively with increased Vds, with the variations of threshold voltage and efficient mobility.The positively shift of transfer characteristics with Vds is maybe due to that source/drain tunneling structure forms metal-oxide-semiconductor (MOS) configuration that works as the doublegate OTFT[22]Electrical parameters for different configurations 7