EPITAXIAL SUBSTRATE FOR SEMICONDUCTOR ELEMENT, SEMICONDUCTOR ELEMENT, AND METHOD FOR PRODUCING EPITAXIAL SUBSTRATE FOR SEMICONDUCTOR ELEMENT

摘要

Provided is an epitaxial substrate in which a silicon substrate is used as a base substrate, that allows a HEMT device with a high breakdown voltage to be achieved. An epitaxial substrate, in which a group of group-III nitride layers are formed on a (111) single crystal Si substrate such that a (0001) crystal plane of the group of group-III nitride layers is substantially in parallel with a surface of the substrate, includes: a first group-III nitride layer that is made of AlN and that is a layer with many defects configured of at least one kind from a columnar or granular crystal or domain; a second group-III nitride layer whose interface with the first group-III nitride layer is shaped into a three-dimensional concave-convex surface; a third group-III nitride layer that is epitaxially formed on the second group-III nitride layer; and the third group-III nitride layer that is a graded composition layer in which the proportion of existence of Al is smaller in a portion closer to a fourth group-III nitride.