Composite substrate and method for producing a composite substrate

Abstract

A composite substrate is provided that exhibits excellent reflection properties at the interface of a wavelength conversion layer.The composite substrate comprises: a wavelength conversion layer configured to convert incident light into light of a different wavelength; and a multilayer film arranged adjacent to the wavelength conversion layer, the multilayer film comprising a plurality of refractive index layers, the wavelength conversion layer having a thickness-directed end section on one side on which the multilayer film is arranged, and in the thickness-directed end section of the wavelength conversion layer forming a region in which the abundance of an inert gas atom is 0.5 at% or more, and in the multilayer film a first refractive index layer among the plurality of refractive index layers being positioned closest to the wavelength conversion layer, and the abundance of the inert gas atom in the first refractive index layer being less than 0.5 at%.