JP2013175571A - Organic field effect transistor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce organic contact resistance of a source electrode and a drain electrode of an organic FET.SOLUTION: A contact part of a drain electrode or a source electrode adjoining a channel region is doped with acceptor substance. Thus, a depletion layer of an interface between an electrode and an organic semiconductor becomes thinner to reduce charge injection barrier wall resistance R, and further, an effect of trap on charge transfer near a contact is eliminated, to reduce access resistance R, and as a result of it, a contact resistance which is a total of both resistances significantly decreases.

Description

  The present invention relates to a field effect transistor (organic FET) using an organic semiconductor, and more particularly to an element structure for reducing contact resistance of an organic FET.

An organic field effect transistor (FET) having features such as lightness and flexibility is expected to be applied to various applications such as an RF-ID tag and an electronic paper backplane. On the other hand, the organic FET has an extremely high contact resistance _RC as compared with Si-MOSFET or the like at the electrode interface, which hinders element miniaturization and high-speed operation. The generally reported contact resistance value of the organic FET is about 1 to 100 kΩcm, which is 4 or 5 orders of magnitude higher than the value of the Si-MOSFET. For practical use of organic FETs, it is indispensable to greatly reduce the contact resistance. When an organic semiconductor with a large band gap is used, the contact resistance becomes higher due to a larger charge injection barrier. However, even when an organic semiconductor with a relatively small band gap is used, the contact resistance is still a big problem. .

Several proposals have conventionally been made on the structure of an organic FET for reducing the contact structure. Non-Patent Document 1 describes that the contact resistance was reduced and the device characteristics were successfully improved by inserting a molybdenum oxide layer at the electrode interface of the organic transistor. Non-Patent Document 2 describes that F ₄ TCNQ, which is an acceptor molecule, is inserted into the electrode interface of an organic transistor to reduce contact resistance (in the case of a p-type semiconductor). Non-Patent Document 2 proposed the concept that the contact resistance is composed of a resistance due to a charge injection barrier generated at the electrode / organic semiconductor interface and an access resistance existing between the electrode and the conduction channel. Furthermore, Non-Patent Document 3 describes that a self-assembled monolayer (SAM) is formed on the electrode surface of a bottom contact (BC) type organic FET to improve electrical characteristics. Yes. Although these improvements can reduce the contact resistance to some extent, they have not been sufficient for further application of organic FETs.

  An object of the present invention is to significantly reduce the contact resistance of an organic FET beyond the level that can be achieved with the prior art.

According to one aspect of the present invention, a source electrode, a drain electrode, a gate electrode, and an organic semiconductor having a channel region in contact with the source electrode and the drain electrode and controlled by an electric field by the gate electrode are provided. An organic FET is provided that performs doping of an acceptor material on an interface between the organic semiconductor and a portion adjacent to the channel region of at least one of the source electrode and the drain electrode.
Here, the organic FET may have a bottom contact / top gate structure, or may have a top contact / top gate structure.
The acceptor material may be selected from the group consisting of ferric chloride, TCNQ, F4TCNQ, fullerene and derivatives thereof, and a perylene n-type organic semiconductor.

  According to the present invention, by greatly reducing the contact resistance of the organic FET, the mobility and operational stability of the element can be improved, the threshold voltage can be controlled, the element size can be further reduced, and high-speed operation can be achieved.

The conceptual diagram which shows the component of contact structure _RC in organic FET of the top contact / bottom gate (TC / BG) structure which is a comparative example of this invention. FIG. 3 is a conceptual cross-sectional view in the vicinity of a contact for explaining the principle of the present invention. The conceptual sectional view showing the component of contact resistance _RC in the organic FET of the bottom contact / top gate (BC / TG) structure of one example of the present invention. 1 is a conceptual cross-sectional view showing components of contact resistance _RC in an organic FET having a top contact / top gate (TC / TG) structure according to an embodiment of the present invention. The figure which shows the channel length dependence of the ON resistance of the organic FET of the Example of this invention and comparative example in gate voltage -40V. The figure which shows the transfer characteristic of organic FET of the Example of this invention. The figure which shows the output characteristic of organic FET of the BC / TG structure of one Example of this invention. The figure which shows the output characteristic of organic FET of TC / TG structure of one Example of this invention.

In the present invention, based on the concept that the contact resistance _RC is composed of a resistance R _int due to a charge injection barrier generated at the electrode / organic semiconductor interface and an access resistance R _bulk existing between the electrode and the conduction channel, A new organic FET device structure capable of directly reducing the resistance factor is provided. Specifically, the charge injection barrier due to the resistance R _int is mainly reduced by performing the doping in the contact interface, reducing the access resistance R _bulk from the electrode to the conduction channel, R _int by performing the doping This is accomplished by placing the reduced interface adjacent to the channel region of the FET.

However, in order to keep the OFF current of the organic transistor at a low value, the doping is performed only on the contact portion and not on the channel portion. Therefore, in a conventionally used top contact / bottom gate (TC / BG) structure, an element structure (FIG. 1) in which a doped portion and a channel region are not adjacent to each other as in the case where doping is performed on the contact interface is performed in the bulk region. The resistance component of the portion where R _bulk reduction due to doping does not occur becomes dominant with R _bulk . The present invention allows the doping to the contact to greatly reduce both R _int and R _bulk by making the doped interface adjacent to the channel region, resulting in a significant reduction in contact resistance _RC. It is something to be made.

Factors for reducing contact resistance _RC due to contact doping (which is not intended to be limited in the following, but will be described with ferric chloride (FeCl ₃ ) as an example) as a dopant include: electrode / organic For example, the charge injection barrier at the semiconductor interface is reduced and R _int is reduced, and the charge density in the access region of the device is increased due to the generation of charges, and the trap level is occupied and R _bulk is reduced. The TC / BG structure, which is a conventionally used structure, will be described as an example. The contact interface resistance R _int and the contact to channel access resistance R _bulk are connected in series as shown in an equivalent circuit in FIG. (Non-Patent Documents 1 and 2). FIG. 2 schematically shows the energy state of the contact interface. The charge injection barrier existing at the contact interface is explained as a Schottky barrier mainly caused by the energy difference between the Fermi level of the electrode and the HOMO level of the p-type organic semiconductor. When the metal and the p-type organic semiconductor are in contact with each other, a depletion layer is formed in the vicinity of the interface in the organic semiconductor due to charge transfer at the contact interface (FIG. 2A). This depletion layer serves as a charge injection barrier to the organic semiconductor. By doping the contact interface, the generated carriers reduce the width of the depletion layer. As a result, the carrier performs tunnel injection in the reduced depletion layer, so that the resistance R _int due to the charge injection barrier is greatly reduced (FIG. 2B).

In addition, the charge generated by the doping also affects the access region. By occupying the trap level existing in this region, there is an effect of reducing R _bulk in the vicinity of the contact (see FIG. 2B). . Effect of the _{R bulk} of the contact doping can be seen in dependence on the gate voltage _{V G} of the contact resistance _{R C.} In TC / BG organic FET, dependency on _{V G} of _{R C} is often described as a result of the charge transport through the region rich in traps (non-patent documents 2 and 4). According to the findings of the present inventors, the contact resistance R _C of the undoped device has been found to exhibit a strong dependence on the gate voltage V _G, which corresponds to the trend that has been previously reported. On the other hand, the contact resistance R _C in the case of the device of FeCl ₃ doped There was also found that hardly depends on the gate voltage V _G. The contact resistance _RC is constant with respect to the gate voltage V _G because charge carriers are generated in the access region by FeCl ₃ doping. That is, by doping FeCl ₃ , an acceptor level is formed during the forbidden band, and holes are generated at the HOMO level end. This induced hole greatly increases the charge density in the access region and occupies its active trap, so that charge transport through this region is not affected by the charge trap (FIG. 1 (b)). . Thus, greatly improved bulk conductivity in the access area, dependent on the gate voltage V _G of the contact resistance R _C is lowered. Therefore, R _bulk is also greatly reduced by FeCl ₃ doping. Eventually, the contact resistance _RC due to doping of the contact with FeCl ₃ is reduced by R _int due to tunnel injection through the thinned depletion region in the contact interface, and R due to reduction of active traps in the access region. It is concluded that this is due to a decrease in _bulk .

In the organic FET having a conventional TC / BG structure, the R _bulk near the electrodes by doping the contact surface can be reduced, because had still higher bulk resistance in the region away from the contact, reduction of R _C by doping The effect was limited. In the present invention, an extremely low _RC can be realized by further reducing the R _bulk by making the region where the effect of doping is adjacent to the channel region. Examples of specific cross-sectional structures of such organic FETs are shown in FIGS.

FIGS. 3A and 3B are cross-sectional views showing the conceptual structures of organic FETs having a BC / TG structure and a TC / TG structure, respectively. In these cross-sectional views, the contact resistance _RC is composed of a resistance R _int due to a charge injection barrier generated at the interface between the electrode and the organic semiconductor, and an access resistance R _bulk existing between the electrode and the conduction channel. Also shown is an equivalent circuit of source-drain resistance based on the above model. That is, the resistance between the source and the drain is ideally the resistance _Rch of the channel region, but actually the contact resistance _RC is present at the interface between the source electrode and the organic semiconductor and at the interface between the drain electrode and the organic semiconductor. appear. Here, the contact resistance _RC appears as a series connection of the charge injection barrier resistance R _int and the access resistance R _bulk, and the resistance R _{ch of the} channel region enters in series. It is expressed as follows.
R _int + R _bulk + R _ch + R _bulk + R _int

Here, when the BC / TG structure of FIG. 3A is seen, since the organic semiconductor thin film is formed on the substrate provided with the electrodes, the organic semiconductor thin film has a stepped shape as shown in the figure. The substrate and the electrode surface are covered with the shape. Since the height of the electrode and the film thickness of the semiconductor layer are approximately the same, this structure allows the channel formed on the gate insulating layer side surface of the organic semiconductor thin film existing between the source electrode and the drain electrode to be doped. It will be very close to the area and can be seen as adjacent to the channel. Here, since the current flows through a path where the resistance is minimum, it should be noted that the doped regions that substantially contribute to the current are the upper right corner of the source electrode and the upper left corner of the drain electrode in the drawing. As a result, both the charge injection barrier resistance R _int and the access resistance R _{bulk in} the above equation are extremely small, and thus a very small contact resistance _RC is obtained. On the other hand, in the conventional structure shown in FIG. 1, the film thickness of the semiconductor layer acts as a resistance as it is. Accordingly, not only the charge injection barrier resistance R _int but also the access resistance R _bulk is increased.

In the TC / TG structure of FIG. 3B, the interface between the source and drain electrodes and the organic semiconductor and the channel region exist on the same side surface of the organic semiconductor film, so that these interfaces are almost directly channel regions. It can comprise so that it may touch. Therefore, the distance through which the current flowing through the organic FET passes through the region contributing to the access resistance R _bulk is extremely short, which makes the access resistance R _bulk very small or substantially zero. Therefore, in FIG. 3B, the entire resistance between the source and the drain is expressed as follows.
R _int + R _ch + R _int

  Note that even when the bottom contact / bottom gate (BC / BG) structure is adopted, the lower end of the electrode is considered to be in contact with the channel. However, a problem with this structure is that since an adhesive layer for improving adhesion is generally used in the lower layer of the electrode, it is impossible to inject charges directly from the electrode (Non-patent Document 5). Furthermore, in the BC / BG structure, it is actually difficult to dope the lower part of the electrode, and it is therefore difficult to create a structure in which the doped region and the channel are in contact with each other.

As described above, in the present invention, the charge injection barrier resistance R _int and the access resistance R _bulk , which are the resistance components constituting the contact resistance _RC , can be obtained in both the structure of FIG. 3A and the structure of FIG. Both are significantly reduced as described above, and thus the contact resistance _RC is greatly reduced. As for the access resistance R _bulk , simply reducing the distance from the interface to the channel region does not only contribute to it. Since the organic semiconductor region contributing to the access resistance is limited to the very vicinity of the doped interface due to the shortening of the distance, the resistance per unit length is greatly reduced, and the access resistance R _bulk can be further reduced by multiplying the two. Is done.

In the following examples, C8-BTBT is used as the organic semiconductor, but the effect of the present invention is effective for all organic semiconductors and is not limited to C8-BTBT. Further, although FeCl ₃ is used as a doping material, any material may be used as long as it is a material that generates carriers by charge transfer with an organic semiconductor. Examples of substances that can be used as the doping material include TCNQ, F4TCNQ, fullerene and derivatives thereof, and perylene-based n-type organic semiconductors. Similarly, the type of insulating layer material does not matter. Furthermore, although the vacuum evaporation method is used for forming the electrode and the organic semiconductor layer in the embodiment, the kind of the film forming method is not limited. As film forming methods other than the vacuum evaporation method, for example, screen printing, gravure printing, ink jet printing, and the like are conceivable.

  In addition, by performing doping at least in the vicinity of the channel region in the interface between the electrode and the organic semiconductor, the effects and effects of the present invention can be achieved. You may go.

1. Fabrication of device A device having the structure shown in FIGS. 3A and 3B was fabricated as follows. A glass substrate was washed with acetone and isopropanol, and a substrate having a parylene thin film of about 300 nm was used as a substrate in order to make the surface hydrophobic. In the case of the BC / TG structure (FIG. 3A), a Ti (3 nm) / Au (37 nm) electrode is vacuum-deposited on a substrate through a metal mask, and ferric chloride (FeCl _{3) is} formed on the electrode surface without removing the mask. ) As a dopant. Further, C8-BTBT (Nippon Kayaku Co., Ltd.) was vacuum deposited as an organic semiconductor with a film thickness of 40 nm. In the case of the TC / TG (FIG. 3B) structure, C8-BTBT is first vacuum-deposited with a film thickness of 40 nm as an organic semiconductor, and then FeCl ₃ as a dopant is vapor-deposited with a thickness of 0.3 nm through a metal mask. Au was deposited to 40 nm to form an electrode. In both cases, Cytop (Asahi Glass Co., Ltd.) was spin-coated with a film thickness of 500 nm as a gate insulating layer and naturally dried in the atmosphere. Finally, a Ti (3 nm) / Au (37 nm) electrode was vacuum deposited as a top gate electrode to form an organic FET. As a reference element, a TC / BG structure with and without a doping layer at the contact interface (FIG. 1) was prepared as follows. The glass substrate was washed with acetone and isopropanol, and Ti (3 nm) / Au (37 nm) was vacuum deposited as a bottom gate electrode. A parylene thin film having a thickness of about 300 nm was formed as a gate insulating layer by chemical vapor deposition. Further, C8-BTBT (Nippon Kayaku Co., Ltd.) was vacuum deposited as an organic semiconductor with a film thickness of 40 nm. For a device having a dopant layer, a source / drain electrode was formed by depositing FeCl ₃ as a dopant by 0.3 nm through a metal mask and further depositing Au by 40 nm. For an element having no dopant layer, Au was evaporated to 40 nm through a metal mask to form an electrode.

2. Evaluation of contact resistance The contact resistance _RC was evaluated using a TLM method (Transfer Line Method). FIG. 4 shows the channel length dependence of the on-resistance of each element at a gate voltage of −40V. The contact resistance _RC can be calculated from the y-intercept of this plot. Here, the on resistance of the TC / BG element doped with the reference element and the on resistance of the BC / TG and TC / TG elements are shown. It can be seen that the contact resistance _RC of the BC / TG and TC / TG elements is significantly reduced with respect to the reference element. The contact resistance at the gate voltage of −40V of the undoped TC / BG device manufactured as the reference device, the TC / BG device with electrode interface doping, the BC / TG with the electrode interface doping, and the TC / TG device is 200 respectively. , 8,000, 8,800, 220, and 100 Ωcm. A summary of these results is shown in Table 1.

3. Evaluation of electrical properties The fabricated devices were evaluated for electrical properties under light shielding and vacuum. The transfer characteristics of the manufactured element are shown in FIG. The drain current values of the BC / TG and TC / TG elements doped at the electrode interface increased sharply in the subthreshold region, and the current hysteresis could be almost eliminated. When the average value of the mobility of each element was calculated from this characteristic, it was 5.5 and 5.7 cm ² / Vs for the BC / TG and TC / TG elements subjected to electrode interface doping, respectively. 6A and 6B show a comparison of the output characteristics of each element. Also here, the drain current value of the doped BC / TG and TC / TG elements increases completely linearly even in the low drain voltage region, and an ideal current-voltage characteristic without the influence of contact resistance is obtained. .

  According to the present invention, a remarkable reduction in the contact resistance of an electrode in an organic FET can be achieved. Therefore, the present invention is expected to greatly contribute to the practical application of organic semiconductor devices that have been described in many fields. Is done.

Improvement of subthreshold current transport by contact interface modification in p-type organic field-effect transistors, M. Kano, T. Minari, and K. Tsukagoshi, Applied Physics Letters, 94 143304 (2009). Charge injection process in organic field-effect transistors, T. Minari, T. Miyadera, K. Tsukagoshi, Y. Aoyagi, and H. Ito, Applied Physics Letters, 91, 053508 (2007). Pentacene TFT with improved linear region characteristics using chemically modified source and drain electrodes, D. J. Gundlach, L. L. Jia, and T. N. Jackson, IEEE Electron Device Lett. 22, 571 (2001). Gated four-probe measurements on pentacene thin-film transistors: Contact resistance as a function of gate voltage and temperature, P. V. Pesavento, R. J. Chesterfield, C. R. Newman, and C. D. Frisbie, Journal of Applied Physics, 96, 7312 (2004). Reduction of contact resistance in pentacene thin-film transistors by direct carrier injection into a-few-molecular-layer channel, N. Yoneya, M. Noda, N. Hirai, K. Nomoto, M. Wada, and J. Kasahara, Applied Physics Letters, 85, 4663 (2004).

Claims (4)

A source electrode;
A drain electrode;
A gate electrode;
In an organic field effect transistor provided with an organic semiconductor in contact with the source electrode and the drain electrode and having a channel region controlled by an electric field by the gate electrode,
An organic field effect transistor in which an acceptor material is doped at an interface between the organic semiconductor and a portion of at least one of the source electrode and the drain electrode adjacent to the channel region.   The organic field effect transistor according to claim 1, having a bottom contact / top gate structure.   The organic field effect transistor according to claim 1, having a top contact / top gate structure.   4. The organic field effect transistor according to claim 1, wherein the acceptor material is selected from the group consisting of ferric chloride, TCNQ, F 4 TCNQ, fullerene and derivatives thereof, and a perylene-based n-type organic semiconductor. 5.