Tuning Properties of II‐IV‐Nitrides for Optoelectronic Applications

Abstract The earth‐abundant II‐IV‐nitrides ZnSnN ₂ and ZnGeN ₂ are direct bandgap semiconductors with a wurtzite‐derived crystal structure. Their alloys, ZnSn _x Ge _{1 − x} N ₂ , have bandgaps tunable across the full visible spectrum, making them interesting for many optoelectronic applications. Here, electrical, structural, and optical properties of near‐stoichiometric ZnSn _x Ge _{1 − x} N ₂ alloys, i.e., where [Zn]/([Zn]+[Ge]+[Sn]) ≈ 0.5, are reported, for samples synthesized by reactive magnetron sputtering. These results reveal unprecedentedly high electrical mobilities in Ge‐rich alloys, with values of 136 and 400 cm ² /Vs at room‐temperature and ≈100 K, respectively. The bandgaps are determined from optical absorption measurements combined with hybrid density functional calculations and reveal a significant Burstein–Moss shift in the Sn rich alloys. Finally, band alignments are determined in the sputter‐grown thin films by combining optical transmission measurements, hybrid density functional calculations, and UV photoelectron spectroscopy measurements, where the bandgap variation is predominantly caused by a shift of the conduction band edge. This work elucidates in unprecedented detail the tuning of optical and electrical properties in ZnSn _x Ge _{1 − x} N ₂ by variation of the chemical composition, where bandgap values of alloys with x ∈ [0.5−0.7] are suitable for top cell absorbers in two‐terminal tandem solar cells assuming a Si bottom cell.