Optical constants of Al_0.85Ga_0.15As_0.56Sb_0.44 and Al_0.79In_0.21As_0.74Sb_0.26

Digital alloy Al_0.85Ga_0.15As_0.56Sb_0.44 and random alloy Al_0.79In_0.21As_0.74Sb_0.26 avalanche photodiodes with a 1-μm multiplication layer exhibit low excess noise under 543-nm laser illumination comparable to Si avalanche photodiodes. This has motivated a study of the optical characteristics of these materials. The absorption coefficients and complex refractive indices were extracted via variable-angle spectroscopic ellipsometry. Features of three semiconductor layer fitting approaches are compared, and a Kramers–Kronig-consistent basis spline function was chosen due to its flexibility and accuracy of approximating optical constants of quaternary materials. The external quantum efficiency has been calculated based on the extracted absorption coefficients and is shown to agree well with the measured external quantum efficiency.