Silicon carbide substrate, silicon carbide semiconductor device, and method for manufacturing silicon carbide substrate

A silicon carbide substrate, comprising:

• a silicon carbide single crystal substrate having a first conductivity type;
• a buffer layer provided over the silicon carbide single crystal substrate and having the first conductivity type;
• and
• a drift layer provided over the buffer layer and having the first conductivity type, wherein
• the buffer layer includes a fourth region and a fifth region,
• the fourth region is located over the fifth region,
• an impurity concentration of the fourth region is lower than an impurity concentration of the silicon carbide single crystal substrate, and
• an impurity concentration of the fifth region is higher than the impurity concentration of the silicon carbide single crystal substrate.

A method for manufacturing a silicon carbide substrate, the method comprising:

• forming, over a silicon carbide single crystal substrate having a first conductivity type, a silicon carbide epitaxial layer having the first conductivity type and having an impurity concentration higher than an impurity concentration of the silicon carbide single crystal substrate;
• forming a second region having the first conductivity type at a position apart from an upper surface of the silicon carbide epitaxial layer through ion implantation into the silicon carbide epitaxial layer;
• and
• after the formation of the second region, forming a drift layer having the first conductivity type over the silicon carbide epitaxial layer, wherein
• as the second region is formed, a first region is formed over the second region of the silicon carbide epitaxial layer,
• an impurity concentration of the first region is higher than an impurity concentration of the silicon carbide single crystal substrate, and
• an impurity concentration of the second region is higher than the impurity concentration of the first region.