Memory and method for preparing memory

A method for preparing a memory, comprising:

• providing a substrate, wherein the substrate comprises a first N-type active region and a first P-type active region;
• forming an epitaxial layer covering the first P-type active region, wherein the epitaxial layer exposes the first N-type active region;
• simultaneously forming a first gate dielectric layer covering the first N-type active region and a second gate dielectric layer covering the epitaxial layer, wherein a thickness of the first gate dielectric layer is same as a thickness of the second gate dielectric layer;
• and forming a first gate covering the first gate dielectric layer to form a first N-channel Metal Oxide Semiconductor (NMOS) device, and forming a second gate covering the second gate dielectric layer to form a first P-channel Metal Oxide Semiconductor (PMOS) device;
• wherein the epitaxial layer comprises at least a first sub-layer and a second sub-layer, and a constituent material of the epitaxial layer comprises silicon and germanium and wherein forming the epitaxial layer covering the first P-type active region comprises:
• forming the first sub-layer covering the first P-type active region;
• and forming the second sub-layer covering the first sub-layer, wherein a mass fraction of silicon in the second sub-layer is greater than or equal to a mass fraction of silicon in the first sub-layer;
• and wherein a mass fraction of germanium in the first sub-layer gradiently changes in a direction perpendicular to the substrate.