Thin-film transistor, method for fabricating thin-film transistor, and display device

Abstract

A thin-film transistor according to the present disclosure is capable of balancing excellent on-characteristics and excellent off-characteristics, and in which the electrical characteristics are symmetric even when the source electrode and the drain electrode are switched. The thin-film transistor includes: a substrate; a gate electrode; a gate insulating layer; a crystalline silicon layer above the gate insulating layer above the gate electrode; a non-crystalline silicon layer above the gate insulating layer and on both sides of the crystalline silicon layer, having a thickness smaller than a thickness of the crystalline silicon layer; a channel protective layer above the crystalline silicon layer; and a source electrode and a drain electrode.

1 . A thin-film transistor comprising:
 a substrate;   a gate electrode above the substrate;   a gate insulating layer above the gate electrode;   a crystalline silicon layer above the gate insulating layer;   a non-crystalline silicon layer above the gate insulating layer and on both sides of the crystalline silicon layer, having a thickness smaller than a thickness of the crystalline silicon layer;   a channel protective layer above the crystalline silicon layer;   a source electrode and a drain electrode; and   a contact layer between (i) at least a side surface of the crystalline silicon layer and an upper surface of the non-crystalline silicon layer and (ii) at least one of the source electrode and the drain electrode, the contact layer comprising non-crystalline silicon doped with impurity or polysilicon doped with impurity,   wherein the source electrode and the drain electrode are formed along at least the side surface of the crystalline silicon layer and the upper surface of the non-crystalline silicon layer and with the contact layer in between, the source electrode being above one portion of the non-crystalline silicon layer, and the drain electrode being above the other portion of the non-crystalline silicon layer.   
     
     
         2 . The thin-film transistor according to  claim 1 ,
 wherein an average grain size of crystals in the crystalline silicon layer is in a range from 10 nm to 1 μm.   
     
     
         3 . The thin-film transistor according to  claim 1 ,
 wherein the side surface of the crystalline silicon layer and a side surface of the channel protective layer are coplanar with each other.   
     
     
         4 . A thin-film transistor comprising:
 a substrate;   a gate electrode above the substrate;   a gate insulating layer above the gate electrode;   a first crystalline silicon layer above the gate insulating layer above the gate electrode;   a second crystalline silicon layer above the gate insulating layer and on both sides of the first crystalline silicon layer, having a thickness smaller than a thickness of the first crystalline silicon layer;   a channel protective layer above the first crystalline silicon layer;   a source electrode and a drain electrode; and   a contact layer between (i) at least a side surface of the first crystalline silicon layer and an upper surface of the second crystalline silicon layer and (ii) at least one of the source electrode and the drain electrode, the contact layer comprising non-crystalline silicon doped with impurity or polysilicon doped with impurity,   wherein the source electrode and the drain electrode are formed along at least the side surface of the first crystalline silicon layer and the upper surface of the second crystalline silicon layer and with the contact layer in between, the source electrode being above one portion of the second crystalline silicon layer, and the drain electrode being above the other portion of the second crystalline silicon layer,   wherein an average grain size of crystals in the first crystalline silicon layer is larger than an average grain size of crystals in the second crystalline silicon layer.   
     
     
         5 . The thin-film transistor according to  claim 4 ,
 wherein the average grain size of the crystals in the first crystalline silicon layer is in a range from 40 nm to 1 μm, and the average grain size of the crystals in the second crystalline silicon layer is at least 10 nm and smaller than 40 nm.   
     
     
         6 . The thin-film transistor according to  claim 4 ,
 wherein the side surface of the first crystalline silicon layer and a side surface of the channel protective layer are coplanar with each other.   
     
     
         7 . A method for fabricating a thin-film transistor, comprising:
 preparing a substrate;   forming a gate electrode above the substrate;   forming a gate insulating layer above the gate electrode;   forming a non-crystalline silicon layer above the gate insulating layer;   forming a channel protective layer above the non-crystalline silicon layer;   forming a projecting part by processing the non-crystalline silicon layer and the channel protective layer, the projecting part having an upper layer composed of the channel protective layer and the lower layer composed of the non-crystalline silicon layer;   irradiating, with a laser beam, the projecting part, a portion under the projecting part, and portions on both sides of the projecting part which are the non-crystalline silicon layer, the non-crystalline silicon layer in the projecting part and the portion under the projecting part being crystallized into a crystalline silicon layer, and the portions on both sides of the projecting part remaining as the non-crystalline silicon layer;   forming a contact layer between (i) at least a side surface of the crystalline silicon layer and an upper surface of the non-crystalline silicon layer and (ii) at least one of a source electrode and a drain electrode, the contact layer comprising non-crystalline silicon doped with impurity or polysilicon doped with impurity; and   forming the source electrode and the drain electrode along at least the side surface of the crystalline silicon layer and the upper surface of the non-crystalline silicon layer and with the contact layer in between, the source electrode being formed above one portion of the non-crystalline silicon layer, and the drain electrode being formed above the other portion of the non-crystalline silicon layer,   wherein when irradiating the non-crystalline silicon layer with the laser beam, absorptance of the non-crystalline silicon layer for the laser beam is greater in the projecting part and the portion under the projecting part of the non-crystalline silicon layer than in the portions on both sides of the projecting part of the non-crystalline silicon layer.   
     
     
         8 . The method for fabricating the thin-film transistor according to  claim 7 ,
 wherein when irradiating the non-crystalline silicon layer with the laser beam, the crystalline silicon layer including crystals having an average grain size in a range from 10 nm to 1 μm is formed by the laser irradiation.   
     
     
         9 . The method for fabricating the thin-film transistor according to  claim 7 ,
 wherein when irradiating the non-crystalline silicon layer with the laser beam, a wavelength of the laser beam is in a range from 473 nm to 561 nm.   
     
     
         10 . A method for fabricating a thin-film transistor, comprising:
 preparing a substrate;   forming a gate electrode above the substrate;   forming a gate insulating layer above the gate electrode;   forming a non-crystalline silicon layer above the gate insulating layer;   forming a channel protective layer above the non-crystalline silicon layer;   forming a projecting part by processing the non-crystalline silicon layer and the channel protective layer, the projecting part having an upper layer composed of the channel protective layer and the lower layer composed of the non-crystalline silicon layer;   irradiating, with a laser beam, the projecting part, a portion under the projecting part, and portions on both sides of the projecting part which are the non-crystalline silicon layer, the non-crystalline silicon layer in the projecting part and the portion under the projecting part being crystallized into a first crystalline silicon layer, and the portions on both sides of the projecting part being crystallized into a second crystalline silicon layer;   forming a contact layer between (i) at least a side surface of the first crystalline silicon layer and an upper surface of the second crystalline silicon layer and (ii) at least one of a source electrode and a drain electrode, the contact layer comprising crystalline silicon doped with impurity or polysilicon doped with impurity; and   forming the source electrode and the drain electrode along at least the side surface of the first crystalline silicon layer and the upper surface of the second crystalline silicon layer and with the contact layer in between, the source electrode being formed above one portion of the second crystalline silicon layer, and the drain electrode being formed above the other portion of the second crystalline silicon layer,   wherein when irradiating the non-crystalline silicon layer with the laser beam, absorptance of the non-crystalline silicon layer for the laser beam is greater in the projecting part and the portion under the projecting part of the non-crystalline silicon layer than in the portions on both sides of the projecting part of the non-crystalline silicon layer, and   the first crystalline silicon layer formed has crystals having an average grain size larger than an average size of crystals in the second crystalline silicon layer.   
     
     
         11 . The method for fabricating the thin-film transistor according to  claim 10 ,
 wherein the average grain size of the crystals in the first crystalline silicon layer is in a range from 40 nm to 1 μm, and the average grain size of the crystals in the second crystalline silicon layer is at least 10 nm and smaller than 40 nm.   
     
     
         12 . The method for fabricating the thin-film transistor according to  claim 11 ,
 wherein when irradiating the non-crystalline silicon layer with the laser beam, the wavelength of the laser beam is 473 nm, and   when forming the projecting part,   the projecting part is formed such that a thickness of the non-crystalline silicon layer from a bottom surface to an upper surface on a side of the projecting part of the non-crystalline silicon layer is smaller, by at least 13 nm, than a thickness of the non-crystalline silicon layer from the bottom surface to an upper surface in the projecting part.   
     
     
         13 . The method for fabricating the thin-film transistor according to  claim 11 ,
 wherein when irradiating the non-crystalline silicon layer with the laser beam, the wavelength of the laser beam is 532 nm, and   when forming the projecting part,   the projecting part is formed such that a thickness of the non-crystalline silicon layer from a bottom surface to an upper surface on a side of the projecting part of the non-crystalline silicon layer is smaller, by at least 15 nm, than a thickness of the non-crystalline silicon layer from the bottom surface to an upper surface in the projecting part.   
     
     
         14 . The method for fabricating the thin-film transistor according to  claim 10 ,
 wherein when irradiating the non-crystalline silicon layer with the laser beam, the wavelength of the laser beam is 561 nm, and   when forming the projecting part,   the projecting part is formed such that a thickness of the non-crystalline silicon layer from a bottom surface to an upper surface on a side of the projecting part of the non-crystalline silicon layer is smaller, by at least 16 nm, than a thickness of the non-crystalline silicon layer from the bottom surface to an upper surface of the projecting part.   
     
     
         15 . The method for fabricating the thin-film transistor according to  claim 10 ,
 wherein when irradiating the non-crystalline silicon layer with the laser beam,   a difference between absorptance of the non-crystalline silicon layer in the projecting part and a part under the projecting part for the laser beam and absorptance of the non-crystalline silicon layer in portions on both sides of the projecting part for the laser beam is at least 3%.   
     
     
         16 . The method for fabricating the thin-film transistor according to  claim 9 ,
 wherein when irradiating the non-crystalline silicon layer with the laser beam, the wavelength of the laser beam is 473 nm, and   when forming the projecting part,   the projecting part is formed such that a thickness of the non-crystalline silicon layer from a bottom surface to an upper surface on a side of the projecting part of the non-crystalline silicon layer is smaller, by at least 4 nm, than a thickness of the non-crystalline silicon layer from the bottom surface to an upper surface in the projecting part.   
     
     
         17 . The method for fabricating the thin-film transistor according to  claim 9 ,
 wherein when irradiating the non-crystalline silicon layer with the laser beam, the wavelength of the laser beam is 532 nm, and   when forming the projecting part,   the projecting part is formed such that a thickness of the non-crystalline silicon layer from a bottom surface to an upper surface on a side of the projecting part of the non-crystalline silicon layer is smaller, by at least 5 nm, than a thickness of the non-crystalline silicon layer from the bottom surface to an upper surface in the projecting part.   
     
     
         18 . The method for fabricating the thin-film transistor according to  claim 9 ,
 wherein when irradiating the non-crystalline silicon layer with the laser beam, the wavelength of the laser beam is 561 nm, and   when forming the projecting part,   the projecting part is formed such that a thickness of the non-crystalline silicon layer from a bottom surface to an upper surface on a side of the projecting part of the non-crystalline silicon layer is smaller, by at least 5 nm, than a thickness of the non-crystalline silicon layer from the bottom surface to an upper surface in the projecting part.   
     
     
         19 . The method for fabricating the thin-film transistor according to  claim 7 ,
 wherein when irradiating the non-crystalline silicon layer with the laser beam,   an absorptance of the non-crystalline silicon layer in the projecting part and the part under the projecting part for the laser beam is at least 30%, and   an absorptance of the portions of the non-crystalline silicon layer on both sides of the projecting part for the laser beam is at most 20%.   
     
     
         20 . The method for fabricating the thin-film transistor according to  claim 7 ,
 wherein when irradiating the non-crystalline silicon layer with the laser beam,   X and Y satisfy the following (Expression 1) and (Expression 2),
   0.50 m≦Y≦ 0.40+0.50 m   (Expression 1)
 
   −4.00( X− 0.50 l )+1.92+0.50 m≦Y≦− 4.00( X− 0.50 l )+2.68+0.50 m,   (Expression 2)
 
   where l and m denote integers starting from 0,   X denotes a value obtained by dividing, by a wavelength of the laser beam, an optical thickness of the non-crystalline silicon layer calculated by multiplying a thickness of the non-crystalline silicon layer from a bottom surface to an upper surface in the projecting part and a refractive index of the non-crystalline silicon layer, and   Y denotes a value obtained by dividing, by the wavelength of the laser beam, an optical thickness of the gate insulating layer obtained by multiplying a thickness of the gate insulating layer and a refractive index of the gate insulating layer.   
     
     
         21 . The method for fabricating the thin-film transistor according to  claim 7 ,
 wherein when forming the gate insulating layer,   the gate insulating layer including a silicon nitride layer and a silicon oxide layer on the silicon nitride layer is formed,   when irradiating the non-crystalline silicon layer with the laser beam,   X and Y satisfy the following (Expression 3) and (Expression 4) or (Expression 5) and (Expression 6),
   0.226 ≦Y≦ 0.26  (Expression 3)
 
   −2.90( X− 0.5 n )+1.39 ≦Y≦− 2.90( X− 0.5 n )+1.97  (Expression 4)
 
   0.340 ≦Y≦ 0.543  (Expression 5)
 
   −2.90( X− 0.5 n )+1.70 ≦Y≦− 2.90( X− 0.5 n )+2.28,  (Expression 6)
 
   where n denotes integers starting from 0,   X denotes a value obtained by dividing, by the wavelength of the laser beam, an optical thickness of the non-crystalline silicon layer calculated by multiplying a thickness of the non-crystalline silicon layer from a bottom surface to an upper surface in the projecting part and a refractive index of the non-crystalline silicon layer, and   Y denotes a value obtained by dividing, by a value obtained by multiplying a refractive index of the silicon oxide layer and the wavelength of the laser beam, an optical thickness of the gate insulating layer including the silicon nitride layer and the silicon oxide layer obtained by a conversion using a refractive index of the silicon oxide layer.   
     
     
         22 . The method for fabricating the thin-film transistor according to  claim 7 ,
 wherein when forming the projecting part,   Z satisfies the following (Expression 7),
   0.5×( k+ 0.3)≦ Z≦ 0.5×( k+ 0.7),
 
   where Z denotes a value obtained by dividing, by the wavelength of the laser beam, an optical thickness of the channel protective layer obtained by multiplying a thickness of the channel protective layer and a refractive index of the channel protective layer, and k denotes an integer starting from 0.