Epitaxial growth and resistivity of θ -CuAl2(001)/MgO(001) films

CuAl2 layers are deposited on MgO(001) substrates by ultrahigh vacuum dc magnetron cosputtering to explore the epitaxial growth of the tetragonal θ-CuAl2 intermetallic compound. Increasing the deposition temperature from Ts = 100 to 400 °C causes a transition from 110- to 001-oriented films, with a pure 110-oriented two-domain epitaxy for Ts = 100 °C, a polycrystalline microstructure for Ts = 200 °C, and epitaxial single crystals with CuAl2(001) || MgO(001) and CuAl2[110] || MgO[100] for Ts ≥ 300 °C. The measured resistivity ρ of d = 350 nm thick layers decreases with increasing Ts = 100–400 °C from 8.14 to 6.39 μΩ cm at 298 K and from 1.92 to 1.35 μΩ cm at 77 K. However, layers with a smaller d = 50 nm exhibit coalesced microstructures with rough surfaces for Ts ≤ 250 °C and discontinuous microstructures for Ts ≥ 300 °C, leading to high effective film resistances. The overall results provide insight into the epitaxial growth of an intermetallic compound, demonstrating epitaxy of the tetragonal θ-CuAl2 phase but also revealing challenges in achieving continuous θ-CuAl2 films which, in turn, prohibit direct measurement of the intrinsic resistivity size effect.