A composite substrate includes a substrate, a high-resistance layer located on the substrate, the high-resistance layer comprising a first low-temperature aluminum nitride (AlN) layer, a high-temperature AlN layer and a second low-temperature AlN layer which are stacked in sequence, and a growth substrate located on a side, away from the substrate, of the high-resistance layer. Under the action of the first low-temperature AlN layer, a tensile stress on the high-temperature AlN layer may be reduced, to reduce a dislocation, and further improve a crystal quality of the high-temperature AlN layer and ensure resistivity of the high-temperature AlN layer; and an element of Al in the high-temperature AlN layer is prevented from diffusing into the growth substrate, to protect the crystal quality of the high-temperature AlN layer and improve a bonding effect between the high-resistance layer and the growth substrate. Thus, stability and reliability of the composite substrate are greatly improved.

A method includes forming a semiconductor layer over a substrate; etching a portion of the semiconductor layer to form a first recess and a second recess; forming a first masking layer over the semiconductor layer; performing a first thermal treatment on the first masking layer, the first thermal treatment densifying the first masking layer; etching the first masking layer to expose the first recess; forming a first semiconductor material in the first recess; and removing the first masking layer.