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 ${\mathrm{(E}}_{\mathrm{FX}})$ of AlGaN alloys in ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N/GaN/sapphire}$ 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 ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N/GaN}$ heterostructures. On the basis of the above analysis, the feasibility of the reflectance measurement for such heterostructures is confirmed. At room temperature, the $E_{\mathrm{FX}}\mathrm{s}$ 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 $E_{\mathrm{FX}},$ the spectral feature of exciton-LO phonon interaction was observed in the reflectance spectrum record for low Al composition $\text{(x⩽0.16).}$ 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 $\text{0⩽x<0.3,}$ the resulting bowing parameter shows a downward value of 0.53 eV.