Forming a plurality of initial trenches that extend through a layer of silicon-germanium and into a substrate to define an initial fin structure comprised of a portion of the layer of germanium-containing material and a first portion of the substrate, forming sidewall spacers adjacent the initial fin structure, performing an etching process to extend the initial depth of the initial trenches, thereby forming a plurality of final trenches having a final depth that is greater than the initial depth and defining a second portion of the substrate positioned under the first portion of the substrate, forming a layer of insulating material over-filling the final trenches and performing a thermal anneal process to convert at least a portion of the first or second portions of the substrate into a silicon dioxide isolation material that extends laterally under an entire width of the portion of the germanium-containing material.