Effects of stoichiometry and individual layer thickness ratio on the quality of epitaxial CrBx/TiBy superlattice thin films

Studies of single crystal artificial superlattices (SLs) of transition-metal (TM) diborides, which is instrumental to understand hardening mechanisms at nanoscale, is lacking. Here, CrBx/TiBy (0001) diboride SLs [x,y ∈ 1.7–3.3] are grown epitaxially on Al2O3(0001) substrates by direct-current magnetron sputter epitaxy. Growth conditions for obtaining well-defined SLs with good interface quality are found at 4 mTorr Ar pressure and 600 °C. 1-µm-thick SL films deposited with modulation periods Λ between 1 and 10 nm, and Λ = 6 nm SLs with TiBy-to-Λ layer thickness ratios Γ ranging from 0.2 to 0.8 are studied. SLs with Λ = 6 nm and Γ in the range of 0.2–0.4, with a near stoichiometric B/TM ratio, exhibit the highest structural quality. The effects of Γ and stoichiometries (B/TM ratio) on the distribution of B in the SL structures are discussed. By increasing the relative thickness of TiBy, the crystalline quality of SLs starts to deteriorate due to B segregation in over-stoichiometric TiBy, resulting in narrow epitaxial SL columnar growth with structurally-distorted B-rich boundaries. Moreover, increasing the relative thickness of under-stoichiometric CrBx enhances the SL quality and hinders formation of B-rich boundaries. The SLs are found to exhibit hardness values in the range of 29–34 GPa.