Epitaxial Growth of Single-Phase Magnesium Dihydride Thin Films

We describe the epitaxial growth process of single-phase magnesium dihydride (MgH₂) thin films on MgO(100) substrates, achieved by reactive magnetron sputtering. We find that direct growth at substrate temperatures higher than 100 °C leads to partial MgH₂ decomposition to Mg, hindering single-phase epitaxy of MgH₂. To improve the crystallinity and suppress the decomposition of Mg, we optimize MgH₂ growth using a two-step process, consisting of (1) precursor growth at room temperature and (2) postdeposition annealing at 380 °C, under a pressure of 1.0 × 10⁵ Pa with H₂ (4%)/Ar (96%) premixed gases. Using this two-step process, we obtain single-phase MgH₂ epitaxial films with high crystallinity, transparency, and resistivity. Further, the application of this method to grow MgH₂ thin films on different MgF₂ and Al₂O₃ substrates enables us to use the epitaxial effects to control the growth orientation of MgH₂ thin films; we show that MgH₂(100) and MgH₂(001) epitaxial thin films can be grown on Al₂O₃(001) and MgF₂(001) substrates, respectively.