Stacked transistors having isolation layers between epitaxial structures

A structure, comprising:

• a first epitaxial structure on a substrate;
• a first stack structure adjacent to the first epitaxial structure;
• an isolation structure on the first epitaxial structure, wherein the isolation structure comprises a first dielectric layer and a second dielectric layer on the first dielectric layer, wherein the first dielectric layer is oxygen free, and wherein a thickness of the second dielectric layer is greater than a thickness of the first dielectric layer;
• an other isolation layer on the first stack structure, wherein a bottom surface of the other isolation layer is coplanar with a bottom surface of the first dielectric layer;
• and
• a second epitaxial structure on the isolation structure, wherein an interface between the second epitaxial structure and the isolation structure is curved.

A structure, comprising:

• a first epitaxial structure on a substrate;
• a dielectric layer, comprising:
• a first layer on the first epitaxial structure and comprising an oxygen-free layer and a planar top surface;
• and
• a second layer on the planar top surface of the first layer and comprising a curved top surface, wherein the second layer has a lower dielectric constant than a dielectric constant of the first layer;
• and
• a second epitaxial structure on the dielectric layer.