Magnetron Sputter Epitaxy of CrB2/TiB2 Diboride Superlattice Thin Films

Abstract

Artificial superlattices with their exceptional properties have been popular in a broad range of applications such as electronic, magnetic, optical, and hard coating. Another potential application for single crystal artificial superlattices is highly efficient interference neutron optics, owing to an ultimate interface width of just ±½ atomic layer. Moreover, studies of superlattices have been instrumental in understanding the hardening mechanisms in transition metal nitrides and carbides while such studies on transition metal diborides is lacking, despite extensive studies on monolithic transition metal diboride thin films. This work is an initiative to grow CrB2/TiB2 (0001) diboride superlattices epitaxially onto Al2O3 (0001) substrates by direct current magnetron sputter epitaxy implementing two different approaches; compound diboride targets, and co-sputtering of a metal target with a compound target. Effects of substrate temperature, B stoichiometry (B/TM ratio), modulation period Λ = DCrB2 + DTiB2, layer thickness ratio Γ = D TiB2 D CrB2+ D TiB2, and relative applied power to magnetrons on the structural and interface quality of superlattices are studied and discussed.Using compound targets, superlattices with thickness ratio Γ = 0.3 and modulation periods Λ between 1 and 10 nm, and with Λ = 6 nm and thickness ratios between 0.2 to 0.8 were synthesized at the optimum sputter gas pressure of pAr = 4 mTorr and a substrate temperature of 600 °C. It is found that superlattices with Λ = 6 nm and Γ in the range of 0.2-0.4 exhibit the highest structural quality. However, B segregation in the over-stoichiometric TiBy layers (y > 2), grown from TiB2 compound target, results in narrow epitaxial superlattice columnar growth with structurally distorted B-rich boundaries. By co-sputtering from Ti and TiB2 targets, y could be reduced from 3.3 to 0.9 in TiBy layers through controlling the relative applied target power. Co-sputtered TiBy single layers and superlattices were grown at substrate temperatures between 600 and 900 °C. 300-nm-thick TiB2.3 single layers grown at 750 °C exhibited epitaxial domains about 10x larger than non-co-sputtered films. A significant enhancement for close-tostoichiometry CrB1.7/TiB2.3 superlattices with modulation periods Λ = 6 nm was achieved at 750 °C. X-ray diffraction, time of flight elastic recoil detection analysis, scanning transmission electron microscopy, electron energy loss spectroscopy, selected area electron diffraction, and nano-indentation are used for characterization. PrefaceThis licentiate thesis is a part of my PhD study under the supervision of Prof.