Methods of making thin film transistors comprising zinc-oxide-based semiconductor materials

Abstract

A thin film transistor comprises a zinc-oxide-containing semiconductor material. Such transistors can further comprise spaced apart first and second contact means or electrodes in contact with said material. Further disclosed is a process for fabricating a thin film transistor device, wherein the substrate temperature is no more than 300° C. during fabrication.

1 . A method of making a zinc-oxide-based thin film semiconductor, for use in a transistor, comprising:
 (a) applying, to a substrate, a seed coating comprising a colloidal solution of zinc-oxide-based nanoparticles having an average primary particle size of 5 to 200 nm;   (b) drying the seed coating to form a porous layer of zinc-oxide-based nanoparticles;   (c) optionally annealing the porous layer of zinc-oxide-based nanoparticles at a temperature higher than the temperature of step (a) or (b);   (d) applying, over the porous layer of nanoparticles, an overcoat solution comprising a soluble zinc-oxide-precursor compound that converts to zinc oxide upon annealing, to form an intermediate composite film;   (e) drying the intermediate composite film; and   (f) annealing the dried intermediate composite film at a temperature of at least 50° C. to produce a semiconductor film comprising zinc-oxide-based nanoparticles supplemented by additional zinc oxide material formed by the conversion of the zinc-oxide-precursor compound during the annealing of the composite film.   
     
     
         2 . The method according to  claim 1 , wherein the temperature of annealing in step (f) is about 130 to 300° C. 
     
     
         3 . The method according to  claim 1 , wherein the semiconductor film is annealed n step (f) for about 10 seconds to about 10 minutes. 
     
     
         4 . The method of  claim 1  wherein the porous layer of nanoparticles is annealed in step (c) before applying the overcoat solution. 
     
     
         5 . The method according to  claim 1 , wherein the semiconductor film is optionally annealed in step (c) and/or annealed in step (f) by a laser annealing. 
     
     
         6 . The method of  claim 1  wherein the colloidal solution of zinc-oxide-based nanoparticles is applied to the substrate at a first temperature and then annealed in step (c) at a higher temperature of 50 to 500° C., prior to applying the overcoat solution. 
     
     
         7 . The method of  claim 1  wherein the temperatures of the substrate during step (a) is 300° C. or less. 
     
     
         8 . The method of  claim 1  wherein the optional annealing in step (c) and the annealing in step (f) is in the presence of air under ambient conditions. 
     
     
         9 . The method of  claim 1  wherein the semiconductor film has a mobility that is substantially greater than that of the seed coating alone, if annealed without the overcoat solution. 
     
     
         10 . The method of  claim 1  wherein the semiconductor film is capable of exhibiting electron mobility greater than 0.01 cm 2 /Vs. 
     
     
         11 . The method of  claim 1  wherein the semiconductor film exhibits a band-gap of less than about 5 eV. 
     
     
         12 . The method of  claim 1  wherein the zinc-oxide-based nanoparticles, upon heating, form a layer comprising a substantially transparent layer. 
     
     
         13 . The method of  claim 1  wherein the nanoparticles are comprised of undoped zinc oxide. 
     
     
         14 . The method of  claim 1  wherein the nanoparticles or the precursor solution comprises acceptor dopant. 
     
     
         15 . The method of  claim 1  wherein the seed coating is applied to the substrate at a level of 0.02 to 1 g/m 2  of nanoparticles, by dry-weight, or the overcoat solution is applied at a level of 2×10 4  to 0.01 moles/m 2  of precursor compound. 
     
     
         16 . The method of  claim 1  wherein the seed coating or the overcoat solution is applied by an inkjet printer. 
     
     
         17 . The method of  claim 1  wherein the seed coating or the overcoat solution is applied by spin coating, extrusion coating, hopper coating, dip coating, or spray coating. 
     
     
         18 . The method of  claim 1  wherein the substrate comprises a dielectric layer made from an inorganic or organic electrically insulating material. 
     
     
         19 . The method of  claim 1  comprising depositing the seed coating onto at least a portion of a surface of the dielectric layer. 
     
     
         20 . The method according to  claim 18  wherein the dielectric layer comprises a substantially transparent material. 
     
     
         21 . The method according to  claim 18  further comprising depositing on the dielectric layer at least one material for forming a source and a drain prior to depositing the seed coating. 
     
     
         22 . The method of  claim 1  for fabricating a thin film semiconductor device, comprising subsequent to step (f):
 (g) forming a spaced apart source electrode and drain electrode, wherein the source electrode and the drain electrode are separated by, and electrically connected with, the semiconductor film; and.   (h) forming a gate electrode spaced apart from the semiconductor film.   
     
     
         23 . The method of  claim 1  comprising, not necessarily in order, the following steps:
 (a) providing the substrate;   (b) providing a gate electrode material over the substrate   (c) providing a dielectric layer over the gate electrode material;   (d) forming the semiconductor film over the gate dielectric; and   (e) providing a source electrode and a drain electrode contiguous to the semiconductor film.   
     
     
         24 . The method of  claim 23  wherein the method further comprises step (f) wherein metal contacts are applied to the upper surface of the semiconductor film. 
     
     
         25 . The method of  claim 1  wherein the substrate is flexible. 
     
     
         26 . The method of  claim 1  wherein steps (a) through (f) are carried out in its entirety below a peak temperature of 150° C. 
     
     
         27 . A method of inkjet printing a semiconductor film on a substrate element comprising:
 a) providing an inkjet printer that is responsive to digital data signals;   b) loading a first printhead with the seed solution of  claim 1 ;   c) printing on the substrate using the seed solution in response to the digital data signals;   d) loading a second printhead with the overcoat solution of  claim 1 ;   e) printing over the first coating using the overcoat solution in response to the digital data signals; and   f) annealing the printed substrate.