from corrosion by fluorine gas fluorine thus introduced is attached on the surface and enters into gaps of the semiconductor layer where fluorine atoms substitute for hydrogen atoms binding with silicon to create si f bindings in places of si h bindings also the fluorine atoms on the layer expel hydrogen atoms away from the semiconductor layer and bind with silicon atoms instead because the electric negativity of fluorine is 4 0 larger than 3 5 of oxygen in this process radiation from a window 16 is effective so that fluorine gas is sufficiently excited furthermore thermal annealing may be carried out on the fluorine gas introduced so that fluorine atoms can permeate into the semiconductor layer and substitute a large number of fluorine atoms binding with silicon atoms after evacuating the reaction chamber 11 by the turbo molecular pump 9 the substrate 10 on the heater 12 is transported from the reaction chamber 11 to the first pre stage chamber 1 through the second pre stage chamber 2 with the gate valves 3 and 4 opened then with the gate valve 4 closed and the gate valve 5 opened the pressure in the first pre stage chamber 1 is kept at a prescribed negative pressure by the cryosorption pump 6 the negative pressure may be less than 10 3 torr preferably 10 6 to 10 9 torr for the semiconductor layer 10 light irradiation annealing and thermal annealing are repeated for examining the change of the conductivity of the semiconductor layer 10 the thermal annealing is carried out with halogen light 100 mw cm 2 and the photo annealing is carried out supplying power from the heater the measuring of the conductivity is accomplished.