A method of fabricating a semiconductor device includes etching a semiconductor substrate having a top surface to form a trench having sidewalls and a bottom surface that extends from the top surface into the semiconductor substrate. A dielectric liner of a first dielectric material is formed on the bottom surface and sidewalls of the trench to line the trench. A second dielectric layer of a second dielectric material is deposited to at least partially fill the trench. The second dielectric layer is partially etched to selectively remove the second dielectric layer from an upper portion of the trench while preserving the second dielectric layer on a lower portion of the trench. The trench is filled with a fill material which provides an electrical conductivity that is at least that of a semiconductor.