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

摘要

PROBLEM TO BE SOLVED: To provide an epitaxial substrate in which an Si substrate is used as a base substrate, and with which a HEMT device with a high breakdown voltage can be achieved.SOLUTION: An epitaxial substrate, in which a group of group-III nitride layers is 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 which is made of AlN and a layer with many crystal defects configured of at least one kind from a columnar or granular crystal or domain and having an average film thickness of 40-200 nm, and has an interface between a second group-III nitride layer and the first group-III nitride layer of a three-dimensional uneven surface, while a projecting part whose side wall is formed by a (10-11) plane or a (10-12) plane of AlN is present at a density of 5×10-5×10/cmand at an average interval of 45-140 nm or less; and a third group-III nitride layer which is epitaxially formed on the second group-III nitride layer and is a graded composition layer where the proportion of existence of Al is smaller in a portion closer to a fourth group-III nitride.