Abstract
Contactless electroreflectance (CER) spectroscopy was applied to study optical transitions in ZnO/ZnMgO quantum wells (QWs) of various widths and barrier contents (Mg = 10, 20, and 30%) grown on sapphire substrates with a Zn0.9Mg0.1O or ZnO buffer layer. Spectral features related to absorption in ZnO QW region and ZnMgO barrier were clearly observed in CER spectra. The spectral position of these features correlates very well with photoluminescence peaks related to emission from ZnO QW and ZnMgO barrier. Besides CER resonances, which are associated with optical transitions, some spectral features related to the Fabry–Perot (F–P) interference effect were clearly observed for these samples. They appear for these structures due to a strong contrast of refractive index between the sapphire and Zn(Mg)O layers and they are observed in the region of transparency for Zn(Mg)O epilayers. For some samples the F–P related signal is quite strong and interferes with CER resonances related to QW transitions. However the two contributions (i.e., CER signal related to optical transitions and the Fabry–Perot effect) can be recognized when the proper analysis of CER spectra is performed. This analysis is presented and discussed in this paper.
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