There is provided a technique to form a single crystal semiconductor thin
film or a substantially single crystal semiconductor thin film. A heat
treatment is carried out for an amorphous semiconductor thin film, to
thereby obtain a crystalline semiconductor thin film. After the
crystalline semiconductor thin film is irradiated with ultraviolet light
or infrared light, a heat treatment at a temperature range of 900 to
1200.degree. C. is carried out in a reducing atmosphere. The surface of
the crystalline semiconductor thin film is extremely flattened through
this step, defects in crystal grain boundaries and crystal grains
disappear, and the single crystal semiconductor thin film or
substantially single crystal semiconductor thin film is obtained.