Determination of exciton transition energy and bowing parameter of AlGaN alloys in AlGaN/GaN heterostructure by means of reflectance measurement

The normal-incidence reflectance measurement was employed to obtain the free exciton transition energy (E_FX) of AlGaN alloys in Al_xGa_1-xN/GaN/sapphire heterostructure grown by metalorganic chemical vapor deposition. It was found that the thickness variation of the AlGaN layer may cause a noticeable change in the line shape of reflectance spectrum and impede the identification of the desired excitonic position. By using a reflection model of two absorbing layers with a transparent substrate, the experimental reflectance spectra were theoretically simulated and utilized to explain the reflection mechanism in AlxGa_1-xN/GaN heterostructures. On the basis of the above analysis, the feasibility of the reflectance measurement for such heterostructures is confirmed. At room temperature, the E_FXs obtained from the fitting showed an excellent agreement with the corresponding peak energies in the photoluminescence spectra. Furthermore, at the optical energy position about 100 meV above the E_FX, the spectral feature of exciton-LO phonon interaction was observed in the reflectance spectrum record for low Al composition (x≤0.16). Using the Al mole fraction derived from x-ray diffraction measurement, the bowing parameter of the epitaxial AlGaN layer was determined. In the range of 0≤x<0.3, the resulting bowing parameter shows a downward value of 0.53 eV.