Stabilization of epitaxial ZnMgN through oxygen incorporation: Properties of quaternary ZnMgNO

To maximize the efficiency of tandem solar cells and simultaneously decrease their environmental impact, absorbers with a band gap around 1.7 eV and synthesized with earth-abundant elements are needed. In this work single-crystalline ZnMgN thin films alloys with Mg concentrations up to 55% are epitaxially grown on MgO (100) substrates by plasma-assisted molecular beam epitaxy. While the band gap can be tuned from 1.1 eV for pure ${\mathrm{Zn}}_{3}{\mathrm{N}}_{2}$ to 2.1 eV, corresponding to (${\mathrm{Zn}}_{0.45}{\mathrm{Mg}}_{0.55}{)}_{3}{\mathrm{N}}_{2}$, across the optimum value of around 1.7 eV for silicon tandem solar cells, their long-term stability in air remains an open issue. Voluntary incorporation of oxygen during the molecular beam epitaxy growth and formation of ZnMgNO alloys is demonstrated to solve the stability issue while at the same time enabling control of charge carrier concentration without degrading the thin films' mobility ($50\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$). A thorough EDX-STEM-HAADF analysis, coupled with an x-ray photoelectron spectroscopy profiling study, reveals the homogeneous incorporation of oxygen throughout the whole layer thickness and the preservation of crystalline quality, opening the door for exploiting earth-abundant ZnMgNO alloys in tandem solar cells.