Copper indium gallium selenide (CIGS) thin films with composition controlled by co-sputtering
A method and apparatus for forming a thin film of a copper indium gallium selenide (CIGS)-type material are disclosed. The method includes providing first and second targets in a common sputtering chamber. The first target includes a source of CIGS material, such as an approximately stoichiometric polycrystalline CIGS material, and the second target includes a chalcogen, such as selenium, sulfur, tellurium, or a combination of these elements. The second target provides an excess of chalcogen in the chamber. This can compensate, at least in part, for the loss of chalcogen from the CIGS-source in the first target, resulting in a thin film with a controlled stoichiometry which provides effective light absorption when used in a solar cell.
1. A method for forming a film comprising:
providing first and second targets in a common sputtering chamber, the first target comprising a Cu—In—Ga—Se alloy and the second target comprising a chalcogen;
co-sputtering the first and second targets towards a substrate to form a film comprising polycrystalline CIGS material on the substrate which incorporates chalcogen from the second target.
2. The method of claim 1 , wherein the chalcogen is selected from selenium, sulfur, tellurium, and combinations thereof.
3. The method of claim 2 , wherein the chalcogen in the second target is predominantly selenium.
4. The method of claim 1 , wherein the second target has a higher concentration of chalcogen than the first target.
5. The method of claim 1 , wherein the second target comprises at least 60 atomic % chalcogen.
6. The method of claim 1 , wherein at least one of the first and second targets includes a dopant.
7. The method of claim 6 , wherein the dopant is selected from sodium, lithium, chromium, nickel, titanium and combinations thereof.
8. The method of claim 1 , wherein the polycrystalline CIGS material has the general formula: Cu a (Z)M y , where Z comprises a combination of In and Ga or a combination of In, Ga and Al; M comprises Se, a combination of Se and S, a combination of Se and Te, or a combination of Se, S, and Te; 0.8≦a≦1; and 1.6≦y≦2.4.
9. The method of claim 1 , wherein the first target has an oxygen concentration of less than 10 ppm.
10. The method of claim 1 , wherein the sputtering comprises supplying energy to the first and second targets at a ratio which provides an amount of the chalcogen in the chamber which is in excess of a stoichiometric amount of the chalcogen in the polycrystalline CIGS material.
11. The method of claim 1 , further comprising, during the sputtering, maintaining the substrate at a temperature in excess of a melting point of the chalcogen.
12. The method of claim 1 , further comprising providing a third target in the sputtering chamber, the third target comprising a CIGS material different from the Cu—In—Ga—Se alloy in the first target, the method including sputtering from the third target towards the substrate.
13. The method of claim 1 , wherein the sputtering from the first and second targets provides a film which has a higher chalcogen concentration than is achievable when only the first target is sputtered.
14. The method of claim 1 , wherein the method includes varying a ratio of energy supplied to the first and second targets during the sputtering resulting in a varying film stoichiometry with depth.
15. The method of claim 1 , further comprising:
forming a solar cell comprising the thin film.