Optical Properties and Two-Dimensional Electronic Structure in Wide-Gap Layered Oxychalcogenide:  La₂CdO₂Se₂

The optical and electronic properties of a wide-gap (∼3.3 eV) layered oxychalcogenide, La 2 CdO 2 Se 2, were examined using epitaxial thin films prepared by a reactive solid-phase epitaxy. Two optical absorption peaks due to exciton split by the spin−orbit interaction were observed at 3.43 and 3.61 eV at 10 K near the absorption edge. A sharp ultraviolet photoluminescence was observed even at room temperature, indicating that the free exciton had a large binding energy (estimated value = ∼40 meV) similar to Cu-based, layered oxychalcogenides LnCuOCh (Ln = lanthanide, Ch = chalcogen). The optical properties were compared to those of the Cu-based oxychalcogenides and low-dimensional CdSe-based materials. It is concluded that the two-dimensional crystal structure, which remarkably reduces the bandwidth of the conduction band, is a major origin for the wide band gap. Energy band calculations indicate that the holes are confined in the two-dimensional (CdSe 2 ) 2- layer, which is most likely responsible for the large binding energy of the exciton.