A method for fabricating a semiconductor device includes performing thermal cleaning for a surface of a silicon substrate in an atmosphere including hydrogen under a condition that a thermal cleaning temperature is higher than or equal to 700° C. and is lower than or equal to 1060° C., and a thermal cleaning time is longer than or equal to 5 minutes and is shorter than or equal to 15 minutes; forming a first AlN layer on the substrate with a first V/III source ratio, the forming of the first AlN layer including supplying an Al source to the surface of the substrate without supplying a N source, and supplying both the Al source and the N source; forming a second AlN layer on the first AlN layer with a second V/III source ratio that is greater than the first ratio; and forming a GaN-based semiconductor layer on the second AlN layer.