A semiconductor layer with a low density of trap states is provided. A transistor with stable electrical characteristics is provided. A transistor having high field-effect mobility is provided. A semiconductor device including the transistor is provided. A method for evaluating a semiconductor layer is provided. A method for evaluating a transistor is provided. A method for evaluating a semiconductor device is provided. Provided is, for example, a semiconductor layer with a low defect density which can be used for a channel formation region of a transistor, a transistor including a semiconductor layer with a low defect density in a channel formation region, or a semiconductor device including the transistor.