A semiconductor structure having compound semiconductor (CS) device formed in a compound semiconductor of the structure and an elemental semiconductor device formed in an elemental semiconductor layer of the structure. The structure includes a layer having an elemental semiconductor device is disposed over a buried oxide (BOX) layer. A selective etch layer is disposed between the BOX layer and a layer for a compound semiconductor device. The selective etch layer enables selective etching of the BOX layer to thereby maximize vertical and lateral window etch process control for the compound semiconductor device grown in etched window. The selective etch layer has a lower etch rate than the etch rate of the BOX layer.