A method of forming a semiconductor device includes providing a substrate in a vacuum processing tool, the substrate having a strained Ge-containing layer on the substrate and a Si layer on the strained Ge-containing layer, maintaining the substrate at a temperature less than 700° C., and generating a soft plasma in the vacuum processing tool. The Si layer is exposed to the soft plasma to form a Si-containing dielectric layer while minimizing oxidation and strain relaxation in the underlying strained Ge-containing layer. A semiconductor device containing a substrate, a strained Ge-containing layer on the substrate, and an Si-containing dielectric layer formed on the strained Ge-containing layer is provided. The semiconductor device can further contain a gate electrode layer on the Si-containing dielectric layer or a high-k layer on the Si-containing dielectric layer and a gate electrode layer on the high-k layer.