Structures including electrical isolation and methods of forming a structure including electrical isolation. A first polycrystalline layer is located in a substrate, and a second polycrystalline layer is positioned between the first polycrystalline layer and a top surface of the substrate. The substrate includes a first portion of the single-crystal semiconductor material that is positioned between the second polycrystalline layer and the top surface of the substrate. The substrate includes a second portion of the single-crystal semiconductor material that is positioned between the first polycrystalline layer and the second polycrystalline layer. The first polycrystalline layer has a thickness. The second polycrystalline layer has a portion with a thickness that is greater than the thickness of the first polycrystalline layer.