Thin-film heterostructure thermoelectrics in a group IIa and IV-VI materials system
Embodiments of a thin-film heterostructure thermoelectric material and methods of fabrication thereof are disclosed. In general, the thermoelectric material is formed in a Group IIa and IV-VI materials system. The thermoelectric material includes an epitaxial heterostructure and exhibits high heat pumping and figure-of-merit performance in terms of Seebeck coefficient, electrical conductivity, and thermal conductivity over broad temperature ranges through appropriate engineering and judicious optimization of the epitaxial heterostructure.
1. A thin-film thermoelectric material comprising:
a first well layer; and
a barrier layer on a surface of the first well layer, the barrier layer having a bandgap that is greater than a bandgap of the first well layer; and
a second well layer on a surface of the barrier layer opposite the first well layer, the second well layer having a bandgap that is less than the bandgap of the barrier layer;
wherein each of the first well layer, the barrier layer, and the second well layer comprises a Group IIa and IV-VI material.
2. The thin-film thermoelectric material of claim 1 comprising a heterostructure formed in a Group IIa and IV-VI materials system, the heterostructure comprising a plurality of barrier layers including the barrier layer separated by plurality of well layers including the first and second well layers such that the plurality of barrier layers and the plurality of well layers form an alternating series of barrier and well layers.
3. The thin-film thermoelectric material of claim 2 wherein each well layer of the plurality of well layers comprises one or more layers of a first material in the Group IIa and IV-VI materials system, and each barrier layer of the plurality of barrier layers comprises one or more layers of a second material in the Group IIa and IV-VI materials system having a bandgap that is greater than that of the first material in the Group IIa and IV-VI materials system of all adjacent well layers in the heterostructure.
4. The thin-film thermoelectric material of claim 2 wherein at least one barrier layer of the plurality of barrier layers comprises a short period superlattice formed in the Group IIa and IV-VI materials system, and at least one well layer of the plurality of well layers comprises a short period superlattice formed in the Group IIa and IV-VI materials system.
5. The thin-film thermoelectric material of claim 2 wherein at least one barrier layer of the plurality of barrier layers comprises a short period superlattice formed in the Group IIa and IV-VI materials system.
6. The thin-film thermoelectric material of claim 5 wherein the short period superlattice comprises a plurality of superlattice barrier layers separated by a plurality of superlattice well layers such that the plurality of superlattice barrier layers and the plurality of superlattice well layers form an alternating series of superlattice barrier and superlattice well layers.
7. The thin-film thermoelectric material of claim 6 wherein each superlattice well layer of the plurality of superlattice well layers comprises one or more layers of a first material in the Group IIa and IV-VI materials system, and each superlattice barrier layer of the plurality of superlattice barrier layers comprises one or more layers of a second material in the Group IIa and IV-VI materials system having a bandgap that is greater than a bandgap of the first material in the Group IIa and IV-VI materials system of all adjacent superlattice well layers in the short period superlattice.
8. The thin-film thermoelectric material of claim 7 wherein thicknesses of the plurality of superlattice barrier layers are substantially equal such that the at least one barrier layer of the plurality of barrier layers has a bandgap that is substantially constant over a thickness of the at least one barrier layer.
9. The thin-film thermoelectric material of claim 7 wherein thicknesses of the plurality of superlattice barrier layers are varied across the short period superlattice to create a bandgap energy gradient that increases cross-plane effective carrier density transport.
10. The thin-film thermoelectric material of claim 7 wherein thicknesses of the plurality of superlattice barrier layers increase across the short period superlattice to thereby increase a bandgap of the at least one barrier layer of the plurality of barrier layers over distance in a desired direction of carrier flow.
11. The thin-film thermoelectric material of claim 2 wherein at least one well layer of the plurality of well layers comprises a short period superlattice formed in the Group IIa and IV-VI materials system.
12. The thin-film thermoelectric material of claim 11 wherein the short period superlattice comprises a plurality of superlattice barrier layers separated by a plurality of superlattice well layers such that the plurality of superlattice barrier layers and the plurality of superlattice well layers form an alternating series of superlattice barrier and superlattice well layers.
13. The thin-film thermoelectric material of claim 12 wherein each superlattice well layer of the plurality of superlattice well layers comprises one or more layers of a first material in the Group IIa and IV-VI materials system, and each superlattice barrier layer of the plurality of superlattice barrier layers comprises one or more layers of a second material in the Group IIa and IV-VI materials system having a bandgap that is greater than a bandgap of the first material in the Group IIa and IV-VI materials system of all adjacent superlattice well layers in the short period superlattice.
14. The thin-film thermoelectric material of claim 13 wherein thicknesses of the plurality of superlattice barrier layers are substantially equal such that at least one barrier layer has a bandgap that is substantially constant over a thickness of the at least one barrier layer.
15. The thin-film thermoelectric material of claim 13 wherein thicknesses of the plurality of superlattice barrier layers are varied across the short period superlattice to create a bandgap energy gradient that increases cross-plane effective carrier density transport.
16. The thin-film thermoelectric material of claim 13 wherein thicknesses of the plurality of superlattice barrier layers increase across the short period superlattice to thereby increase a bandgap of the at least one barrier layer over distance in a desired direction of carrier flow.
17. A method of fabricating a thin-film thermoelectric material comprising:
forming a first well layer; and
forming a barrier layer on a surface of the first well layer, the barrier layer having a bandgap that is greater than a bandgap of the first well layer; and
forming a second well layer on a surface of the barrier layer opposite the first well layer, the second well layer having a bandgap that is less than the bandgap of the barrier layer;
wherein each of the first well layer, the barrier layer, and the second well layer comprises a Group IIa and IV-VI material.