Single-atomic-layered quantum wells built in wide-gap semiconductors $\mathrm{Ln}\mathrm{CuO}\mathrm{Ch}$ $(Ln=\mathrm{lanthanide},$ $Ch=\mathrm{chalcogen})$

$\mathrm{Ln}\mathrm{CuO}\mathrm{Ch}$ $(Ln=\mathrm{lanthanide},Ch=\mathrm{chalcogen})$ layered oxychalcogenides are wide-gap $p$-type semiconductors composed of alternately stacked ${(Ln}_2{\mathrm{O}}_2{)}^{2+}$ oxide layers and $({\mathrm{Cu}}_2{\mathrm{Ch}}_2{)}^{2-}$ chalcogenide layers. Energy band calculations revealed that $\mathrm{Cu}-Ch$ hybridized bands only spread in the $({\mathrm{Cu}}_2{\mathrm{Ch}}_2{)}^{2-}$ layers, which suggests that hole carriers in these bands are confined by the potential barriers formed by the ${(Ln}_2{\mathrm{O}}_2{)}^{2+}$ layers. Stepwise absorption spectra of a series of $\mathrm{Ln}\mathrm{CuO}\mathrm{Ch}$ experimentally verified that an exciton in the $({\mathrm{Cu}}_2{\mathrm{Ch}}_2{)}^{2-}$ layers shows a two-dimensional behavior. These theoretical and experimental results indicate that $\mathrm{Ln}\mathrm{CuO}\mathrm{Ch}$ has “natural multiple quantum wells” built into its layered structure.