Optical characterization of GaN/AlGaN bilayer by transmission and reflection spectra

A method has been proposed to study optical characteristics of GaN/Al0.45Ga0.55N bilayer on sapphire substrate at room temperature. Utilizing the incomplete absorption between 270 and 365 nm, absorption spectrum and extinction coefficient of wurtzite GaN both below and above energy gap were determined. Energy gaps of GaN and Al0.45Ga0.55N were located at 3.4 and 4.62 eV, respectively, in the same spectrum. Right above absorption edge (354 nm), the extinction coefficient was 0.269. Average total thickness calculated from transmission and reflection spectra was 1413 and 1407 nm, respectively, and thickness of GaN was calculated as 268.5 nm from the envelope of maximum reflection resulted from GaN/AlGaN interface. By comparison of two spectra, it was deduced that reflection curve had much larger oscillation amplitude than transmission spectrum, thanks to the larger contrast of interference fringes V. Additional half-wave loss was deduced at air/GaN interface, which resulted in the reverse of interference extrema.