A method for forming a semiconductor structure comprising a silicon-on-insulator layer structure with crystalline silicon oxide SiOx as the insulator material comprises: providing a crystalline silicon substrate having a substantially clean deposition surface in a vacuum chamber; heating the silicon substrate to an oxidation temperature To in the range of 550 to 1200 ° C.; supplying, while keeping the silicon substrate in the oxidation temperature, with an oxidation pressure Po in the range of 1·10−8 to 1·10−4 mbar in the vacuum chamber, molecular oxygen O2 into the vacuum chamber with an oxygen dose Do in the range of 0.1 to 1000 Langmuir; whereby a crystalline silicon oxide layer with a thickness of at least two molecular layers is formed within the silicon substrate, between a crystalline silicon base layer and a crystalline silicon top layer. Related semiconductor structures are described.