Accurate interband-energy measurements from Ellipsometric spectra

The optical constants of AlGaInP, AlGaInP (Si-doping) and AlGaInP (Mg-doping) grown by MOCVD were measured in the region of visible light at room temperature using null spectroscopic ellipsometry, then the dielectric function spectra were obtained. By numerical differentiation, the third-derivative spectra of the dielectric function for three samples were evaluated. Based on the third-derivative theory, it was proved that the third-derivative spectra were proportional to the electroreflectance (ER) spectra under the low-field modulation condition. Thus, the third-derivative spectra given by the ellipsometric spectra were directly related to the band structure of material. Analyzing the third-derivative spectra of the imaginary part of the dielectric function with the three-point method used in ER spectra, the E_g and E_g+Δ₀ values of three samples were gained accurately. The experimental results were in good agreement with those published.