JP5626959B2 - Contact structure of organic semiconductor device, organic semiconductor device and manufacturing method thereof - Google Patents

Description

  The present invention relates to an organic device such as an organic semiconductor transistor, and particularly relates to reducing contact resistance with an organic semiconductor by a simple process.

  Organic field effect transistors (FETs) are known to have very large contact resistance compared to similar field effect transistors using inorganic semiconductor materials such as silicon, which hinders device operation. .

Several proposals and reports have been made on techniques for reducing this contact resistance. For example, Non-Patent Document 1 reports that by selectively inserting FeCl ₃ into the interface between the electrode and the organic semiconductor, the electrode interface that was a metal / insulator contact is changed to a metal / semiconductor contact. Non-Patent Document 2 reports that the contact resistance of an organic transistor was reduced by inserting a transition metal oxide layer (MoO ₃ , WO ₃ , V ₂ O ₅ ) at the electrode / organic semiconductor interface. Yes.

  Furthermore, the inventor of the present application has proposed a method of inserting a metal oxide film that is easily oxidized at the interface between an organic semiconductor and a metal electrode in Patent Document 1. A metal oxide film generally has a large band gap and a deep valence band level. Therefore, according to this method, the contact resistance of the organic FET is reduced by injecting charges through the valence band of the metal oxide film or by generating carriers due to charge transfer between the metal oxide film and the organic semiconductor. It was possible to do.

  An object of the present invention is to provide an element structure and a manufacturing method for reducing contact resistance in an organic semiconductor device with a simpler structure than the above method.

According to one aspect of the invention,
(A) electrodes,
(B) an organic semiconductor layer in contact with the electrode; and (c) a metal oxide layer in contact with the organic semiconductor layer provided adjacent to a contact portion between the edge of the electrode and the organic semiconductor layer. A contact structure for an organic semiconductor device is provided.

According to another aspect of the invention,
(A) electrodes,
(B) an organic semiconductor layer in contact with the electrode; and (c) an organic layer provided with a metal oxide layer in contact with the organic semiconductor layer provided adjacent to an edge of a region in which the electrode contacts the organic semiconductor layer. A contact structure for a semiconductor device is provided.

According to yet another aspect of the invention,
(A) a gate electrode;
(B) a gate insulating layer provided on the gate electrode;
(C) an organic semiconductor layer provided on the gate insulating layer;
(D) An electrode provided on the organic semiconductor layer and used as a source and a drain, respectively. (E) A metal oxide layer covering at least a contact portion between the surface of the organic semiconductor layer and the electrode is provided. Organic semiconductor devices are provided.

  The metal oxide layer may have a thickness of less than 1 nm.

  The metal oxide layer may have a thickness of 0.5 nm or less.

  The metal oxide layer may cover the surface of the organic semiconductor layer with a width of 1 nm from at least a boundary line between the surface of the organic semiconductor layer and an end of the electrode.

  The metal oxide layer may cover the surface of the organic semiconductor layer and the entire surface of the electrode.

  The metal oxide layer is made of nickel (Ni), aluminum (Al), beryllium (Be), chromium (Cr), cobalt (Co), zinc (Zn), copper (Cu), iron (Fe). It may consist of an oxide of a metal selected from

  The organic semiconductor layer may be composed of a compound selected from the group consisting of π-electron conjugated aromatic compounds, chain compounds, organic pigments, and organosilicon compounds.

  The organic semiconductor layer may be made of a compound selected from the group consisting of acene compounds such as pentacene, perylene compounds, oligothiophenes, polythiophenes, phthalocyanines such as copper phthalocyanine, and porphyrins.

According to yet another aspect of the invention,
(A) providing an organic semiconductor layer on the gate electrode covered with the gate insulating layer;
(B) providing a source electrode and a drain electrode on the organic semiconductor layer; and (c) providing a metal oxide layer covering at least a contact portion between the surface of the organic semiconductor layer and the surface of the source electrode and the drain electrode. An organic semiconductor device manufacturing method is provided.

Further, the step of providing the metal oxide layer may include a step of (c-1) providing a metal layer and (c-2) oxidizing the metal layer.

  The oxidizing step may include a natural oxidation step by exposing the metal layer to the atmosphere.

  The step of providing the metal oxide layer may be a step of directly forming the metal oxide.

  The metal oxide layer is made of nickel (Ni), aluminum (Al), beryllium (Be), chromium (Cr), cobalt (Co), zinc (Zn), copper (Cu), iron (Fe). It may consist of an oxide of a metal selected from

  The metal oxide layer may have a thickness of 1 nm or less.

  The metal oxide layer may have a thickness of 0.5 nm or less.

  According to the present invention, contact resistance can be greatly reduced by using inexpensive and various materials without excessively complicating the manufacturing process of the organic semiconductor device. As a result, the contact resistance of the organic FET can be reduced by a very simple method and structure, and an organic device having excellent electrical characteristics can be provided by a low-cost process.

FIG. 3 is a schematic diagram showing current injection in the vicinity of a contact of an organic field effect transistor for explaining the principle of the present invention. Schematic which shows the device structure of the produced organic field effect transistor. The graph which shows the drain-source current-gate voltage characteristic of the organic transistor which used aluminum oxide and nickel oxide as a metal oxide layer. The graph (TLM) which plotted the on-resistance of the organic transistor which has a nickel oxide layer as a metal oxide layer, and the organic transistor which does not have a metal oxide layer with respect to channel length. The graph which shows the relationship between the contact resistance and gate voltage of the organic transistor which used the aluminum oxide and nickel oxide as a metal oxide layer calculated | required from the Y intercept of FIG. The graph which shows the drain-source current-gate voltage characteristic of the organic field effect transistor which has a metal oxide layer which carried out the natural oxidation of 0.5 nm of thickness and 1 nm of Ni.

  In an embodiment of the present invention described below, a conventional top contact organic field effect is formed by sequentially forming an organic semiconductor layer 3 and a source / drain electrode 4 on a gate insulating layer 2 formed on the surface of a gate electrode 1. After the transistor was fabricated, a metal oxide layer 5 was formed on the top surfaces of the organic semiconductor layer 3 and the source / drain electrodes 4. Thereby, compared with the case where a metal oxide layer is not provided, contact resistance can be reduced significantly.

  The mechanism by which the metal oxide layer formed on the upper surface of the device reduces the contact resistance of the device is considered as follows. As shown in FIG. 1, it is considered that the charge injection into the organic semiconductor layer 3 in the organic FET is mainly performed from the top contact electrode, that is, the end of the electrode 4, to bring about the effect of the present invention. That is, in a medium with non-uniform electrical conductivity, current flows through a path where the resistance from the start point to the end point is minimized, which is the cause of current concentration at the electrode end. On the other hand, due to charge transfer between an organic semiconductor such as pentacene and the metal oxide layer, a region rich in carriers and having few traps is formed up to the vicinity of the electrode end. Usually, the electrode 4 made of gold or the like has a much higher conductivity than that of pentacene or the like, so that current is conducted through the electrode 4 up to the end of the electrode and injected into the organic semiconductor layer 3 from the end of the electrode. It can be interpreted that the contact resistance is lowered by lowering the trap density in the vicinity thereof by the metal oxide layer 5.

  That is, the surface area of the organic semiconductor that is in contact with the electrode end (when considering the case where the resistance of this part is increased due to the presence of some other element between the vicinity of the electrode end and the organic semiconductor, It is sufficient that the metal oxide is in contact with the organic semiconductor in a very narrow range of about 1 nm (at the end of the contact region between the organic semiconductor and the organic semiconductor). Note that even if the metal oxide layer covers a wider area, there is usually no adverse effect. For example, the device manufacturing process is simplified, or the metal oxide layer has another role such as a protective layer. For the purpose, the metal oxide layer 5 can be provided in a wider range such as the entire device surface.

  In order to simplify the explanation, the principle of the present invention has been described above by taking an organic semiconductor device having a top contact structure as an example. However, after the source / drain electrodes are installed on the gate insulating film, Even in the case of a bottom contact structure in which an organic semiconductor layer is provided, the present invention can be similarly applied. In other words, as described above, the organic semiconductor layer is located at a position adjacent to the contact portion between the edge of the electrode (more generally, a portion of the electrode where current concentrates and flows out / inflow) and the organic semiconductor layer. It is important that the metal oxide is present in contact with the metal. Therefore, even in the bottom contact structure, for example, a metal oxide layer is provided between the gate insulating film and the electrode layer, or a structure in which the metal oxide layer is covered on the electrode is adjacent to the end of the electrode. The present invention can be applied by bringing a metal oxide into contact with an organic semiconductor at the position.

  It should be noted here that the terms “electrode end”, “electrode edge”, and “contact region end” are used. Even if it says in this way, it does not indicate the whole peripheral part, but indicates a specific part of the electrode or the like where current is concentrated. For example, in the organic semiconductor FET exemplified above, the portion closest to the drain electrode / source electrode in the sense that the resistance of the current path is the smallest among the source electrode / drain electrode (usually, the drain electrode among the source electrodes) It should be understood that it points to the end that faces the electrode side and has the smallest distance to it, and the end of the drain electrode that faces the source electrode side and has the smallest distance to it.

  As the metal oxide layer 5, it is possible to directly form a metal oxide by a method such as sputtering or vacuum vapor deposition, or by exposing to the atmosphere after forming a metal thin film that is easily oxidized, by surrounding oxygen. It can also be used as a natural oxide film. The ambient oxygen can be supplied by exposure to air when the device is removed from the fabrication vessel after completion of the above steps.

In the present invention, any type of metal oxide can be used as the metal oxide layer, and any type can be used as long as it is a stable metal oxide. In addition, even when the metal oxide layer is formed by a natural oxidation phenomenon of metal in the atmosphere, any material can be used as long as it is a material that easily undergoes oxidation and generates a stable metal oxide. That is, the metal oxide layer is not particularly limited in addition to nickel (Ni) and aluminum (Al) specifically shown below. For example, beryllium (Be), chromium (Cr), cobalt ( Co), zinc (Zn), copper (Cu), iron (Fe) and other oxide layers can be used.
The thickness of the metal oxide layer is preferably less than 1 nm when a conductive metal oxide such as Ni oxide is used. This is because when the metal oxide layer is conductive, if the film thickness is too thick, the source and drain electrodes are short-circuited, and the off-current of the organic FET increases (details will be described later). If a large leakage current does not flow between the source and drain through this layer, such as by providing a metal oxide layer only around the necessary electrodes instead of over the entire device surface, conductive metal There is no limit to the layer thickness due to the use of oxides. However, normally, if there is a film thickness of 0.5 nm or less, an effect sufficient to reduce the contact resistance can be obtained.

  When an insulating material such as an Al oxide is used as the metal oxide layer, the off-current is not deteriorated even if the film thickness is 1 nm or more, and a thicker film can be used. Even in this case, if there is a film thickness of 0.5 nm or less, an effect sufficient to reduce the contact resistance can be obtained.

  In the case of a structure in which the metal oxide layer is localized at the edge of each of the source electrode and the drain electrode instead of covering the entire device surface, the conductivity of each of the localized oxide films is between As long as a high substance is not present, the above-described countermeasure against the leakage current through the oxide film is not particularly necessary.

  Although pentacene is used as a specific example of a typical organic semiconductor, the effect of the present invention is effective for all p-type organic semiconductors and is not limited to pentacene. Examples of currently known organic semiconductors include π-electron conjugated aromatic compounds, chain compounds, organic pigments, and organosilicon compounds. More specifically, acene compounds such as pentacene, pentacene derivatives, anthracene and anthracene derivatives, perylene compounds, oligothiophenes, polythiophenes and derivatives thereof, phthalocyanines and derivatives thereof such as copper phthalocyanine and zinc phthalocyanine, porphyrins and derivatives thereof There are derivatives and the like. Of course, the organic semiconductor usable in the present invention is not limited to these.

  In addition, since the effect of the present invention is at the interface between the electrode and the organic semiconductor, a current injected from the electrode to the organic semiconductor such as having a source and a drain electrode in contact with the upper portion of the organic semiconductor layer as shown in FIG. Any other device structure may be used as long as the device structure concentrates on the electrode end. Therefore, for example, the present invention can be applied to general organic semiconductor devices having the above-described bottom contact structure.

  Further, although gold is used as the electrode material, the effect of the present invention is also effective for other electrode materials, regardless of the type.

  An organic FET having a metal oxide layer shown in FIG. 2 was produced as follows. A substrate obtained by forming a 200 nm silicon oxide film serving as the gate insulating film 2 on the surface of a highly doped silicon wafer serving as the gate electrode 1 was used. After modifying the surface of the silicon oxide film with phenylenetrichlorosilane, pentacene was vacuum-deposited as the organic semiconductor layer 3 under a condition of a film thickness of 40 nm, and gold was patterned as a source / drain electrode 4 with a film thickness of 40 nm. Furthermore, Al or Ni was vacuum-deposited with a film thickness of 0.5 nm on the upper surface of the device, and then exposed to the atmosphere, whereby the metal oxide layer 5 was formed by natural oxidation of the metal under the atmosphere.

  As a result of conducting electrical measurement of the manufactured organic FET in a nitrogen atmosphere, as shown in the graph of FIG. 3, the element having an Al oxide layer or a Ni oxide layer as a metal oxide layer does not have a metal oxide layer. Compared with the device, the drain current increased.

  Further, in order to elucidate the cause of the increase in the drain current, an analysis was performed by a transmission line model (TLM) method. The on-resistance of the transistor was plotted in the graph of FIG. 4 with the Y axis and the channel length as the X axis, and the contact resistance value was calculated from the Y intercept of the fitting line as shown in FIG. As a result, at a gate voltage of −40 V, the contact resistance value of the element not having the metal oxide layer was 22300 Ωcm, whereas the element having the Al oxide layer was 15400 Ωcm, and the element having the Ni oxide layer was 8600 Ωcm. there were. It was found that the contact resistance of the element having the metal oxide layer was greatly reduced as compared with the element having no metal oxide layer. (Since the TLM method used here is a method well known to those skilled in the art, details of the method itself will not be described in this specification, but if necessary, refer to, for example, Non-Patent Document 3.)

  Furthermore, in order to investigate the influence of the thickness when a conductive metal oxide such as Ni oxide is used as the metal oxide layer on the characteristics of the organic FET, the thickness of the Ni oxide layer is 2 nm of 1 nm and 0.5 nm. In each case, a device having the structure shown in FIG. 2 was prepared, and its gate voltage-drain current characteristics were measured. The result is shown in FIG. As can be seen from this figure, the thickness of this layer is preferably less than 1 nm. This is because when the metal oxide layer is conductive, if the film thickness is too thick, the off-current of the organic FET increases due to the leakage current between the source and drain electrodes.

  According to the present invention, since the contact resistance can be greatly reduced by a simpler organic semiconductor device structure and manufacturing process than those proposed so far, the present invention is greatly improved in practical use of organic semiconductor devices. Useful.

DESCRIPTION OF SYMBOLS 1 Gate electrode 2 Gate insulating layer 3 Organic-semiconductor layer 4 Source / drain electrode 5 Metal oxide layer

Japanese Patent Application No. 2009-268309

Applied Physics Letters, 84, 1004 (2004) R.A. Schroeder, et al. , ‘Improving organic transistor performance with Schottky contacts’ Applied Physics Letters, 87, 193508 (2005) C.I. -W. Chu, et al. , ‘High-performance organic thin-film transducers with metal oxide / metal bilayer electrode’ Applied Physics Letters 91, 053508 (2007) T .; Minari, et al. , ‘Charge injection process in organic field-effect transistors’

Claims (11)

An organic semiconductor device provided with the following (a) to (e).
(A) A gate electrode.
(B) A gate insulating layer provided on the gate electrode.
(C) An organic semiconductor layer provided on the gate insulating layer.
(D) An electrode provided on the organic semiconductor layer and used as a source and a drain, respectively.
(E) A metal oxide layer covering the surface of the organic semiconductor layer with a width of 1 nm from at least a boundary line between the surface of the organic semiconductor layer and the end of the electrode. The organic semiconductor device according to claim 1, wherein the metal oxide layer has a thickness of less than 1 nm. The organic semiconductor device according to claim 2, wherein the metal oxide layer has a thickness of 0.5 nm or less. The metal oxide layer is selected from the group consisting of nickel (Ni), aluminum (Al), beryllium (Be), chromium (Cr), cobalt (Co), zinc (Zn), copper (Cu), and iron (Fe). The organic-semiconductor device in any one of Claims 1-3 consisting of the oxide of the obtained metal . The organic semiconductor layer according to any one of claims 1 to 4, wherein the organic semiconductor layer is made of a compound selected from the group consisting of a π-electron conjugated aromatic compound, a chain compound, an organic pigment, and an organosilicon compound. device. The organic semiconductor layer is made of a compound selected from the group consisting of acene compounds such as pentacene, perylene compounds, oligothiophenes, polythiophenes, phthalocyanines such as copper phthalocyanine, and porphyrins . An organic semiconductor device according to any one of the above. An organic semiconductor device manufacturing method provided with the following steps (a) to (c).
(A) An organic semiconductor layer is provided on a gate electrode covered with a gate insulating layer.
(B) A source electrode and a drain electrode are provided on the organic semiconductor layer.
(C) The following steps (c-1) and (c-2) are provided, and a metal oxide layer covering at least a contact portion between the surface of the organic semiconductor layer and the surfaces of the source electrode and the drain electrode is provided.
(C-1) A metal layer is provided.
(C-2) The metal layer is oxidized. The organic semiconductor device manufacturing method according to claim 7, wherein the oxidizing step includes a step of spontaneous oxidation by exposing the metal layer to the atmosphere. The metal oxide layer is selected from the group consisting of nickel (Ni), aluminum (Al), beryllium (Be), chromium (Cr), cobalt (Co), zinc (Zn), copper (Cu), and iron (Fe). The organic-semiconductor device manufacturing method of Claim 7 or Claim 8 which consists of an oxide of the obtained metal. The method for producing an organic semiconductor device according to claim 7, wherein the metal oxide layer has a thickness of 1 nm or less. The organic semiconductor device manufacturing method according to claim 7, wherein the metal oxide layer has a thickness of 0.5 nm or less.