On intrinsic Stokes shift in wide GaN/AlGaN polar quantum wells
The interpretation of electromodulated reflectance (ER) spectra of polar quantum wells (QWs) is difficult even for homogeneous structures because of the built-in electric field. In this work we compare the room temperature contactless ER and photoluminescence (PL) spectra of polar GaN/AlGaN QWs with the effective-mass band structure calculations. We show that the emission from the ground state transition is observed in PL but the ER is dominated by transitions between excited states. This effect results from the polarization-induced built-in electric field in QW that breaks the selection rules that apply to square-like QWs, allowing many optical transitions which cannot be separately distinguished in the ER spectrum. We develop the guidelines for the identification of optical transitions observed in PL and ER spectra. We conclude that an intrinsic Stokes shift, i.e., a shift between emission and absorption, is present even for homogeneous GaN/AlGaN QWs with large width, where the electron–hole wavefunction overlap for the fundamental transition is weak.