Semiconductor layer comprising cyclically stacked narrow, intermediate,

Abstract

A semiconductor layer for reflecting an optical beam, the semiconductor layer includes a plurality of film pairs of narrow and wide band gap semiconductor films (21, 22) having a refractive index difference and narrower and wider band gaps, an intermediate semiconductor film (31) is interposed between the narrow and the wide band gap semiconductor films of each film pair and is made to have an intermediate film band gap which is varied so as to be equal to the narrower and the wider band gaps at its interfaces contiguous to the narrow and the wide band gap semiconductor films. The narrow band gap semiconductor film and a portion adjacent thereto in the intermediate semiconductor film are called a first composite layer and given a first total thickness which is approximately equal to a quarter of a first wavelength had by the optical beam in the first composite layer. The wide band gap semiconductor film and a remaining portion adjacent thereto in the intermediate semiconductor film are called a second composite layer and given a second total thickness which is approximately equal to a quarter of a second wavelength had by the optical beam in the second composite layer. Preferably, the intermediate semiconductor film should have a film thickness which is substantially equal to a width which a potential barrier has perpendicularly of the film pairs if the intermediate semiconductor film were absent. More preferably, absorption of the optical beam in the narrow and the wide band gap semiconductor films is taken into account on determining their thicknesses.