Zinc oxide film and method for making
A method for depositing a solid film of ZnO onto a substrate from a reagent solution includes a reservoir of reagent solution maintained at a sufficiently low temperature to inhibit homogeneous reactions within the reagent solution. The reagent solution contains a source of Zn, a source of O, and multiple ligands to further control solution stability and shelf life. The chilled solution is dispensed through a showerhead onto a substrate. The substrate is positioned in a holder that has a raised structure peripheral to the substrate to retain or impound a controlled volume (or depth) of reagent solution over the exposed surface of the substrate. The reagent solution is periodically or continuously replenished from the showerhead so that only the part of the solution directly adjacent to the substrate is heated. A heater is disposed beneath the substrate and maintains the substrate at an elevated temperature at which the deposition of a desired solid phase from the reagent solution may be initiated. The showerhead may also dispense excess chilled reagent solution to cool various components within the apparatus and minimize nucleation of solids in areas other than on the substrate. The deposited film may be annealed after deposition and may be doped to enhance selected characteristics. The ZnO films made by the process have distinctive electrical and optical properties and are suitable for a variety of electronic and optical devices.
1. A method for depositing a solid ZnO film onto a substrate from a reagent solution comprising the steps of:
providing a supply of reagent solution containing a source of Zn, a source of O, and at least three ligands comprising sodium citrate, ammonium hydroxide, and potassium nitrilotriacetate, and maintained at a first temperature at which homogeneous reactions are substantially inhibited within said reagent solution;
dispensing a controlled flow of said reagent solution from a showerhead assembly;
positioning said substrate to receive at least a portion of said controlled flow of said reagent over a selected area of said substrate;
providing a raised structure peripheral to said selected area whereby a controlled volume of said reagent solution may be maintained upon said substrate and replenished at a selected rate; and,
heating said substrate and said controlled volume of said reagent solution upon said substrate to a second temperature, higher than said first temperature, whereby deposition of zinc oxide from said reagent solution may be initiated.
2. The method of claim 1 wherein said second temperature is at least 60° C. higher than said first temperature.
3. The method of claim 1 wherein said source of Zn comprises a soluble Zn salt.
4. The method of claim 3 wherein said soluble Zn salt is selected from the group consisting of: chlorides; nitrates; citrates; sulfates; and acetates.
5. The method of claim 1 further including an additional ligand selected from the group consisting of: acetic acid; triethanolamine; citric acid; ammonium chloride; and hydantoin.
6. The method of claim 1 further comprising the step of:
annealing said coated substrate at a selected temperature in a selected atmosphere.
7. The method of claim 6 wherein said selected temperature is in the range of 300 to 1000° C. and said selected atmosphere is selected from the group consisting of: Ar; N 2 ; H 2 ; O 2 ; air; and combinations thereof.
8. The method of claim 1 wherein said reagent solution further comprises a soluble salt of a dopant species having a valence different from that of Zn(II).
9. The method of claim 8 wherein said soluble salt is selected from the group consisting of: In(NO 3 ) 3 ; InCl 3 ; Ga(NO 3 ) 3 ; Al(NO 3 ) 3 ; Al 2 (SO 4 ) 3 ; salts of other group III elements; LiNO 3 ; AgNO 3 ; salts of other group I elements; and salts of other group IV elements.
10. The method of claim 1 further comprising the step of:
treating said ZnO film by ion implantation of a species selected from the group consisting of: In; Ga; Al; and N.