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 sputtering apparatus comprising:
a sputtering chamber;
a first target positioned in the sputtering chamber, which is energizable to sputter a Cu—In—Ga—Se alloy towards a substrate, the first target comprising a Cu—In—Ga—Se alloy of formula
Cu a (In,Ga)Se y
in which
0.8≦a≦1.0; and
1.6≦y≦2.4, wherein y varies from an exact stoichiometric amount, and
wherein Se is provided in an amount ranging from 40 atomic % to 60 atomic %; and
a second target positioned in the sputtering chamber, which is energizable to sputter chalcogen towards the substrate, the second target comprising at least 80 atomic % chalcogen, wherein the chalcogen is selected from the group consisting of S, Se, Te, and combinations thereof,
whereby by energizing the first and second targets, a layer of CIGS material is provided on the substrate, the layer incorporating, within the CIGS material polycrystalline structure, chalcogen from the second target.
2. The sputtering apparatus of claim 1 , further comprising a source of a sputtering gas connected with the chamber.
3. The sputtering apparatus of claim 1 , wherein the first and second targets are separately powered by a respective power source.
4. The sputtering apparatus of claim 1 , wherein the first target comprises a bulk polycrystalline CIGS material.
5. The sputtering apparatus of claim 1 , wherein the first target and second target are angled to each other to sputter a common area of the substrate.
6. The sputtering apparatus of claim 1 , wherein the chalcogen in the second target predominantly comprises selenium.
7. The sputtering apparatus of claim 1 , wherein at least one of the first and second targets includes a dopant.
8. The sputtering apparatus of claim 7 , wherein the dopant is selected from the group consisting of sodium, lithium, chromium, nickel, titanium, and combinations thereof.
9. The sputtering apparatus of claim 1 , wherein the layer of CIGS material provided on the substrate has a 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.
10. The sputtering apparatus of claim 1 , wherein the first target has an oxygen concentration of less than 10 ppm.
11. The sputtering apparatus of claim 1 , wherein the second target has an oxygen concentration of less than 10 ppm.
12. The sputtering apparatus of claim 1 , wherein the first and second targets are supplied with energy at a ratio which provides an amount of the chalcogen in the chamber that is in excess of a stoichiometric amount of the chalcogen in the polycrystalline CIGS material.
13. The sputtering apparatus of claim 1 , further comprising one or more controllers for variably adjusting a ratio of energy supplied by first and second power sources powering the first and second targets during sputtering.
14. The sputtering apparatus of claim 1 , further comprising a heater for maintaining a substrate at a temperature in excess of the melting point of the chalcogen during the sputtering.
15. The sputtering apparatus of claim 1 , further comprising a third target comprising a CIGS material different from the CIGS material in the first target, wherein the third target is energized to sputter towards the substrate.
16. The sputtering apparatus of claim 1 , wherein the first target comprises at least 20 atomic % Cu.
17. The sputtering apparatus of claim 1 , wherein the first target comprises at least 20 atomic % Ga/In/Al.
18. The sputtering apparatus of claim 1 , wherein the second target comprises at least 90 atomic % chalcogen.
19. The sputtering apparatus of claim 1 , wherein the second target comprises at least 99 atomic % chalcogen.
20. The sputtering apparatus of claim 1 , wherein the first target comprises an atomic ratio Se:(Se+Cu+Ga/In/Al) is from 1:1.8 to 1:2.2.
21. The sputtering apparatus of claim 1 , wherein the first target comprises an atomic ratio Se:(Se+Cu+Ga/In/Al) is from 1:1.9 to 1:2.1.