Systems and methods for integrating a-axis oriented barium titanate thin films on silicon (001) via strain control
Various embodiments of the present technology enable growth of a-axis oriented barium titanate (BTO) films by inserting a relaxed strain control layer having a larger lattice constant than the c-axis of BTO and a similar thermal expansion mismatch. As a result, in-plane tensile stress causes BTO to grow with its ferroelectric polarization in-plane. Some embodiments allow for BTO films to immediately be grown on silicon with a-axis orientation, and without the need to create thick layers for relaxation. Using various embodiments of the present technology, the BTO can be grown in-plane with minimal dislocation density that is confined to the interface region.
1. A wafer comprising:
a silicon substrate;
a strain control layer formed on the silicon substrate; and
a strained a-axis oriented barium titanate (BTO) film formed on the strain control layer,
wherein a short, a-axis for BTO crystals in the film are normal to a plane defined by a surface of the silicon substrate in contact with the strain control layer, and
wherein a long, c-axis for BTO crystals in the film are:
parallel to the plane, and
in a direction defined by a substrate flat of the silicon substrate.
2. The wafer of claim 1 , further comprising a buffer layer of strontium titanate formed between the silicon substrate and the strain control layer.
3. The wafer of claim 1 , wherein the BTO film is epitaxially grown on the strain control layer with a ferroelectric polarization in-plane with respect to the BTO film.
4. The wafer of claim 1 , wherein residual strain of the strained a-axis oriented BTO film varies in proportion with a thickness of the strain control layer.
5. The wafer of claim 1 , further comprising one or more integrated optoelectronic devices formed in or on the strained a-axis oriented BTO film.
6. The wafer of claim 1 , wherein the strain control layer includes a relaxed layer formed on the silicon substrate, the relaxed layer formed of one of: a perovskite, a spinel, a fluorite, a rocksalt, and a bixbyite oxide.
7. The wafer of claim 6 , wherein a lattice constant of the relaxed layer is larger than a lattice constant of the c-axis of BTO.
8. The wafer of claim 1 , wherein the strain control layer includes a layer of barium stannate.
9. A wafer comprising:
a substrate;
a relaxed layer of barium stannate formed on the substrate; and
a strained a-axis oriented barium titanate (BTO) film formed on the layer of barium stannate,
wherein a short, a-axis for BTO crystals in the film are normal to a plane defined by a surface of the substrate in contact with the relaxed layer, and
wherein a long, c-axis for BTO crystals in the film are:
parallel to the plane, and
in a direction defined by a substrate flat of the substrate.
10. A method comprising:
forming a Zintl template on a silicon substrate;
growing a seed layer of strontium titanate (STO) on the silicon substrate;
growing a strain control layer on the seed layer; and
growing a layer of a-axis oriented barium titanate (BTO) on the strain control layer.
11. The method of claim 10 , wherein growing a seed layer of STO comprises: growing the seed layer of STO to a thickness of 2 nm at a temperature below 300° C., and then increasing the temperature to greater than or equal to 300° C. to facilitate crystallization of the seed layer of STO.
12. The method of claim 10 , wherein growing a layer of a-axis oriented BTO comprises depositing the a-axis oriented BTO on the strain control layer with a long crystal axis of the a-axis oriented BTO in-plane with a substrate flat of the silicon substrate.
13. The method of claim 10 , wherein growing a layer of a-axis oriented BTO comprises forming the layer of a-axis oriented BTO on the strain control layer in the absence of strain of the BTO to the STO.
14. The method of claim 10 , wherein growing a layer of a-axis oriented BTO comprises depositing the a-axis oriented BTO on the strain control layer using one of: sputtering, pulsed laser deposition, chemical vapor deposition, and atomic layer deposition.
15. The method of claim 10 further comprising forming one or more integrated optoelectronic devices formed in or on the layer of a-axis oriented BTO.
16. The method of claim 10 , wherein growing a layer of a-axis oriented BTO comprises epitaxially growing the layer of a-axis oriented BTO having a ferroelectric polarization in-plane with respect to the strain control layer.
17. The method of claim 10 , wherein growing a strain control layer comprises growing the strain control layer with a layer of barium stannate.
18. The method of claim 10 , wherein growing a strain control layer comprises growing the strain control layer with a relaxed layer formed on the silicon substrate.
19. The method of claim 18 , wherein the relaxed layer is formed of one of: a perovskite, a spinel, a fluorite, a rocksalt, and a bixbyite oxide.
20. The method of claim 18 , wherein a lattice constant of the relaxed layer is larger than a lattice constant of a c-axis of BTO.