Patent Publication Number: US-2007102697-A1

Title: Junction structure of organic semiconductor device, organic thin film transistor and fabricating method thereof

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention generally relates to a semiconductor device and a fabricating method thereof. More particularly, the present invention relates to an organic thin film transistor and a fabricating method thereof.  
      2. Description of Related Art  
      Because the organic semiconductor device can be formed on a flexible plastic substrate or a metal substrate, it has advantages of light weight, low cost and flexibility. Thus, organic thin film transistors have been in valued in recent years.  
      In fabricating methods of the organic thin film transistor, the material for the source/drain is usually metal, such as gold, palladium or platinum. The junction between an electrode made of these metals and an organic semiconductor is Schottky contact, and thus a higher junction resistance is existed. Several methods have been disclosed to resolve the problem of high contact resistance in the organic thin film transistor. For example, a method of doping impurities in the junction between the metal electrode and the organic semiconductor is disclosed in the prior art. However, the organic semiconductor doped with impurities has poor stability. Another conventional method is forming a buffer layer having high carrier density between the organic semiconductor and the metal electrode. Fermi level of the buffer layer is between the levels of the organic semiconductor layer and the metal electrode so as to reduce the energy barrier when carriers inject into the organic semiconductor from the metal electrode. However, this method causes the reduction of the on-off ratio of the organic thin film transistor.  
      In the references of U.S. Pat. No. 6,335,539 B1 and U.S. Pat. No. 6,569,707 B2, a thiol compound is formed on the metal electrode to form a self-assembled monolayer so as to reduce the junction resistance in the organic thin film transistor. However, only a few of metals (metal electrode) can be used in this method. In addition, the self-assembled monolayer formed on the metal electrode is formed by immersion method. Hence, this method can only be applied to particular type organic thin film transistors.  
     SUMMARY OF THE INVENTION  
      Accordingly, the present invention is directed to a junction structure of an organic semiconductor device capable of reducing the junction resistance in the organic semiconductor device.  
      The present invention is directed to an organic thin film transistor and a fabricating method thereof capable of reducing the junction resistance in the organic thin film transistor.  
      A junction structure of an organic semiconductor device is provided. The junction structure includes an organic semiconductor layer, a conductive layer and a modifying layer. The modifying layer is formed between the organic semiconductor layer and the conductive layer, wherein the material for the modifying layer comprises an inorganic compound or an organic complex compound.  
      According to an embodiment of the present invention, said material for the organic semiconductor layer comprises a small molecule organic semiconductor, a polymer semiconductor or an oligomer semiconductor.  
      According to an embodiment of the present invention, said material for the conductive layer comprises a metal, a metal oxide or a conductive polymer.  
      According to an embodiment of the present invention, said inorganic compound comprises LiF, CsF, LiO 2 , LiBO 2 , K 2 SiO 3 , Cs 2 CO 3  or Al 2 O 3 .  
      According to an embodiment of the present invention, said organic complex compound comprises CH 3 COOLi , CH 3 COONa, CH 3 COOK, CH 3 COORb or CH 3 COOCs.  
      An organic thin film transistor including a gate, a source/drain, a dielectric layer, an organic semiconductor layer and at least a modifying layer is also provided. The gate is electrically isolated from the source/drain. The dielectric layer is disposed between the gate and the source/drain. The organic semiconductor layer is disposed between the source and the drain. The modifying layer is disposed between the organic semiconductor layer and the source/drain, wherein the material for the modifying layer comprises an inorganic compound or an organic complex compound.  
      According to an embodiment of the present invention, said material for the gate comprises a metal, a metal oxide, a conductive polymer or a doped silicon material.  
      According to an embodiment of the present invention, said material for the source/drain comprises a metal, a metal oxide, a conductive polymer or a doped silicon material.  
      According to an embodiment of the present invention, said the dielectric layer comprises SiO 2 , Si 3 N 4 , TiO 2 , LaO 2 , Al 2 O 3 , polyimide, polymethylmethacrylate, polyamide or parylene.  
      According to an embodiment of the present invention, said the material for the organic semiconductor layer comprises a small molecule organic semiconductor, a polymer semiconductor or an oligomer semiconductor.  
      According to an embodiment of the present invention, said inorganic compound comprises LiF, CsF, LiO 2 , LiBO 2 , K 2 SiO 3 , Cs 2 CO 3  or Al 2 O 3 .  
      According to an embodiment of the present invention, said organic complex compound has a formula of RX, wherein R is an organic function group and X is a metal element. For example, the organic complex compound comprises CH 3 COOLi , CH 3 COONa, CH 3 COOK, CH 3 COORb or CH 3 COOCs.  
      According to an embodiment of the present invention, said gate is formed under the source/drain.  
      According to an embodiment of the present invention, said the gate is formed above the source/drain. In another embodiment, the organic thin film transistor further comprises another gate formed under the source/drain.  
      In addition, a method for forming an organic thin film transistor is also provided. The method includes forming a gate, a dielectric, an organic semiconductor layer and a source/drain, characterized in that forming a modifying layer between the organic semiconductor layer and the source/drain, wherein the material for the modifying layer comprises an inorganic compound or an organic complex compound.  
      According to an embodiment of the present invention, the method for forming the modifying layer comprises performing a depositing process with a shadow mask.  
      According to an embodiment of the present invention, the method for forming the modifying layer comprises performing a depositing process collocated with a photolithography and etching process.  
      Since the modifying layer made of an inorganic compound or an organic complex compound is formed between the organic semiconductor layer and the conductive layer, the junction resistance between the organic semiconductor layer and the conductive layer can be reduced.  
      The modifying layer formed between the organic semiconductor layer and the conductive layer is made of inorganic compound or an organic complex compound so that it can be applied to various organic thin film transistors.  
      The modifying layer formed between the organic semiconductor layer and the conductive layer is formed through a shadow mask or photolithography and etching process so that it can be applied to various organic thin film transistors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
       FIG. 1  is a cross-section view showing a junction structure of an organic semiconductor device according to an embodiment of the present invention.  
       FIG. 2  is a cross-section view showing an organic thin film transistor according to an embodiment of the present invention.  
       FIGS. 3-7  are cross-section views showing other organic thin film transistors according to embodiments of the present invention. 
    
    
     DESCRIPTION OF THE EMBODIMENTS  
      Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.  
      Junction Structure of an Organic Semiconductor Device  
       FIG. 1  is a cross-section view showing a junction structure of an organic semiconductor device according to an embodiment of the present invention. As shown in  FIG. 1 , the junction structure of an organic semiconductor device includes an organic semiconductor layer  40 , a conductive layer  55  and a modifying layer  60 . The organic semiconductor device is, for example, a metal-oxide-semiconductor (MOS) device, a metal-insulator-semiconductor (MIS) device, a thin film transistor (TFT) or an organic thin film transistor (OTFT). In an embodiment, the material for the organic semiconductor layer  40  comprises a small molecule organic semiconductor, a polymer semiconductor or an oligomer semiconductor. The small molecule organic semiconductor is, for example, tetracene, pentacene or phthalocyanine. The polymer semiconductor is, for example, polythiophene, polyfluorene, polyphenylenevinylene or a derivative thereof, such as poly(3-octyl)thiophene, poly(dioctylfluroene) or poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene]. The oligomer semiconductor is, for example, α-sexithiophene.  
      As shown in  FIG. 1 , the material for the conductive layer  55  is, for example, a metal, a metal oxide or a conductive polymer. The metal can be aluminium, titanium, nickel, copper, gold or chromium, for example. The metal oxide can be, for example, indium stannum oxide or indium zinc oxide. The conductive polymer can be, for example, a mixture of 3,4-polyethylenedioxythiophene and polystyrenesulfonate (PEDOT:PSS) or polyaniline.  
      In order to reduce the junction resistance between the conductive layer  55  and the organic semiconductor layer  40 , a modifying layer  60  is formed therebetween. The material for the modifying layer  60  comprises an inorganic compound or an organic complex compound. The inorganic compound can be, for example, LiF, CsF, LiO 2 , LiBO 2 , K 2 SiO 3 , Cs 2 CO 3 , Al 2 O 3  and the like. The organic complex compound has a formula of RX, wherein R is an organic function group, such as CH 3 COO—, and X is a metal element. For example, the organic complex compound comprises CH 3 COOLi , CH 3 COONa, CH 3 COOK, CH 3 COORb or CH 3 COOCs.  
      The efficiency of that carriers inject into the organic semiconductor layer  40  form the conductive layer  55  can be improved because of the formation of the modifying layer  60 , and the junction resistance between the organic semiconductor layer  40  and the conductive layer  55  can be reduced.  
      Organic Thin Film Transistor  
       FIG. 2  is a cross-section view showing an organic thin film transistor according to an embodiment of the present invention. As shown in  FIG. 2 , the organic thin film transistor  10   a  comprises a gate  20 , a source  50  and a drain  51 , a dielectric layer  30 , an organic semiconductor layer  40  and at least a modifying layer  60 . The organic thin film transistor  10   a  is a bottom gate thin film transistor, and thus the gate  20  is formed under the source  50  and the drain  51 . The dielectric layer  30  is formed between the gate  20  and the source  50 /drain  51  so as to isolate the gate  20  and the source  50 /drain  51 . The organic semiconductor layer  40  is formed between the source  50  and the drain  51  so as to form a semiconductor channel. In addition, the modifying layer  60  is formed between the organic semiconductor layer  40  and the source  50 /drain  51  to reduce the junction resistance between the organic semiconductor layer  40  and the source  50 /drain  51 . In particular, the material for the modifying layer  60  comprises an inorganic compound or an organic complex compound.  
      The fabricating method for the thin film transistor  10   a  is first providing a substrate  12 , such as a glass substrate, a sapphire substrate, a semiconductor substrate or a polymer plastic substrate. The semiconductor substrate, for example, is germanium silicide (SiGe) or a semiconductor doped with P or N type impurities. The polymer plastic substrate is, for example, polyethylene teraphthalate (PET) or polycarbonate (PC).  
      Next, as shown in  FIG. 2 , a gate  20  is formed on the substrate  12 . The material for the gate  20  includes, for example, a metal, a metal oxide, a conductive polymer or a doped silicon material. The metal can be aluminium, titanium, nickel, copper, gold or chromium, for example. The metal oxide can be, for example, indium stannum oxide or indium zinc oxide, for example. The conductive polymer can be, for example, a mixture of 3,4-polyethylenedioxythiophene and polystyrenesulfonate (PEDOT:PSS) or polyaniline. Thereafter, a dielectric layer  30  is formed over the substrate  12  and the gate  20 . The material for the dielectric layer  30  comprises silicon oxide (SiO 2 ), silicon nitride (Si 3 N 4 ), titanium oxide (TiO 2 ), lanthanum oxide (LaO 2 ), aluminum oxide (Al 2 O 3 ), polyimide, polymethylmethacrylate, polyamide or parylene. Then, an organic semiconductor layer  40  is formed on the dielectric layer  30 . The material for the organic semiconductor layer  40  comprises a small molecule organic semiconductor, a polymer semiconductor or an oligomer semiconductor. The small molecule organic semiconductor is, for example, tetracene, pentacene or phthalocyanine. The polymer semiconductor is, for example, polythiophene, polyfluorene, polyphenylenevinylene or a derivative thereof, such as poly(3-octyl)thiophene, poly(dioctylfluroene) or poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene]. The oligomer semiconductor is, for example, α-sexithiophene.  
      Thereafter, a modifying layer  60  is formed on the organic semiconductor layer  40 . The modifying layer  60  can be formed by performing a depositing process with a shadow mask so as to form the modifying layer  60  covering the first region  42  and the second region  44 . Then, a source  50  and a drain  51  are formed in the first region  42  and the second region  44  by using the same shadow mask. The modifying layer  60  formed between the organic semiconductor layer  40  and the source  50 /drain  51  can improve the efficiency of that carriers (electrons or holes) inject into the organic semiconductor layer  40  from the source  50  and the drain  51 , and the junction resistance between the organic semiconductor layer  40  and the source  50 /drain  51  can be reduced. In addition, the modifying layer  60  formed in the first and second regions  42 ,  44  can also be formed by a depositing process collocated with a photolithography and etching process, and then the source  50  and the drain  51  are formed on the modifying layer  60  using a photo-mask that is the same to that used in the photolithography process for the modifying layer  60 . The material for the modifying layer  60  comprises an inorganic compound or an organic complex compound. The inorganic compound can be, for example, lithium fluoride (LiF), cesium fluoride (CsF), lithium oxide (LiO 2 ), lithium boron oxide (LiBO 2 ), potassium sulfite (K 2 SiO 3 ), cesium carbonate (Cs 2 CO 3 ), aluminum oxide (Al 2 O 3 ) and the like. The organic complex compound has a formula of RX, wherein R is an organic function group, such as acetate (CH 3 COO—), and X is a metal element. For example, the organic complex compound comprises CH 3 COOLi , CH 3 COONa, CH 3 COOK, CH 3 COORb or CH 3 COOCs. The material for the source  50  and the drain  51  includes, for example, a metal, a metal oxide, a conductive polymer or a doped silicon material.  
      It should be noted that the material for the modifying layer  60  formed in the first region  42  can also be different from that of the modifying layer  60  formed in the second region  44 . For example, the modifying layer  60  formed in the first region  42  is LiF while the modifying layer  60  formed in the second region  44  is CsF. In addition, the modifying layer  60  formed between the organic semiconductor layer  40  and the source  50 /drain  51  can also be a multi-layer structure.  
      The modifying layer can also be applied to other type organic thin film transistors.  FIGS. 3-7  are cross-section views showing other organic thin film transistors according to embodiments of the present invention. Because the modifying layer  60  can be formed through a depositing process with a shallow mask or a depositing process collocated with a photolithography and etching process, it is applied to various organic thin film transistors.  
      As shown in  FIG. 3 , the organic thin film transistor  10   b  formed on a substrate  12  comprises a gate  20 , a source  50  and a drain  51 , a dielectric layer  30 , an organic semiconductor layer  40  and at least a modifying layer  60 . The gate  20  is formed under the source  50  and the drain  51 , and thus the organic thin film transistor  10   b  is also a bottom gate thin film transistor. In particular, the organic semiconductor layer  40  and the modifying layer  60  are disposed over the source  50  and the drain  51 .  
      Please refer to  FIG. 4 , the gate  20  is formed under the source  50  and the drain  51 , and thus the organic thin film transistor  10   c  is also a bottom gate thin film transistor. In particular, the source  50  and the drain  51  are formed at two levels. Hence, a vertical channel is formed from the organic semiconductor layer  40 .  
      As shown in  FIG. 5 , the organic thin film transistor  10   d  formed on a substrate  12  comprises a gate  20 , a source  50  and a drain  51 , a dielectric layer  30 , an organic semiconductor layer  40  and at least a modifying layer  60 . The gate  20  is formed above the source  50  and the drain  51 , and thus the organic thin film transistor  10   d  is a top gate thin film transistor.  
      As shown in  FIG. 6 , the organic thin film transistor  10   e  formed on a substrate  12  comprises a gate  20 , a first source  50  and a first drain  51 , a first dielectric layer  30 , an organic semiconductor layer  40 , a second source  50   a  and a second drain  51   a , a second dielectric layer  30   a , at least a first modifying layer  60  and at least a second modifying layer  60   a . The gate  20  is disposed above the first source  50 /drain  51  and the second source  50   a /drain  51   a , and thus the organic thin film transistor  10   e  is a top gate thin film transistor. In particular, the first source  50 /drain  51  and the second source  50   a /drain  51   a  are not disposed at the same level, and thus a vertical channel is formed from the organic semiconductor layer  40 .  
      Please refer to  FIG. 7 , the organic thin film transistor  10   f  formed on a substrate  12  comprises a first gate  20 , a source  50  and a drain  51 , a first dielectric layer  30 , an organic semiconductor layer  40 , a second dielectric layer  30   a , at least a modifying layer  60  and a second gate  20   a . The organic thin film transistor  10   f  is a double gate thin film transistor. The first gate  20  is formed above the source  50  and the drain  51  while the second gate  20   a  is formed under the source  50  and the drain  51 .  
      The modifying layer made of the inorganic compound or organic complex compound used in the organic semiconductor device can reduce the junction resistance. In addition, the modifying layer can be applied to various organic thin film transistors having different organic semiconductor layer.  
      The modifying layer is formed by performing a depositing process with a shadow mask or a depositing process collocated with a photolithography and etching process. Hence, the modifying layer can be formed in various type organic thin film transistors.  
      It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.