A phase change structure includes a first phase change material layer pattern and a second phase change material layer pattern. The first phase change material layer pattern may partially fill a high aspect ratio structure, and the second phase change material layer pattern may fully fill the high aspect ratio structure. The first phase change material layer pattern may include a first phase change material, and the second phase change material layer pattern may include a second phase change material having a composition substantially different from a composition of the first phase change material.