Polarization properties at rocksalt/wurtzite oxide interfaces

The binary oxides MgO, CaO, ZnO, and CdO have a variety of band gaps which enables band-gap engineering over a wide range. ZnO takes the wurtzite structure, while MgO, CaO, and CdO take the rocksalt structure. We examine the formal polarization in these materials and in particular the formal polarization difference at the interface between ZnO and each of the rocksalt oxides. We find that any rocksalt oxide (including alloys) strained to ZnO will exhibit a very large polarization charge discontinuity of $\ensuremath{-}0.907\phantom{\rule{0.16em}{0ex}}{\mathrm{Cm}}^{\ensuremath{-}2}$. Heterostructures between ZnO and the rocksalt oxides can therefore host a free-carrier gas with sheet densities up to $5.7\ifmmode\times\else\texttimes\fi{}{10}¹⁴\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. Such large polarization fields and carrier densities may allow applications in tunnel junctions or as contact layers for UV devices.