Methods of manufacturing a semiconductor device including a semiconductor substrate and a hetero semiconductor region including a semiconductor material having a band gap different from that of the semiconductor substrate and contacting a portion of a first surface of the semiconductor substrate are taught herein, as are the resulting devices. The method comprises depositing a first insulating film on exposed portions of the first surface of the semiconductor substrate and on exposed surfaces of the hetero semiconductor material and forming a second insulating film between the first insulating film and facing surfaces of the semiconductor substrate and the hetero semiconductor region by performing a thermal treatment in an oxidizing atmosphere.