Epitaxial growth and electrical transport properties of Cr2GeC thin films

Abstract

Cr${}_{2}$GeC thin films were grown by magnetron sputtering from elemental targets. Phase-pure Cr${}_{2}$GeC was grown directly onto Al${}_{2}$O${}_{3}$(0001) at temperatures of 700--800 \ifmmode^\circ\else\textdegree\fi{}C. These films have an epitaxial component with the well-known epitaxial relationship Cr${}_{2}$GeC(0001)//Al${}_{2}$O${}_{3}$(0001) and ${\mathrm{Cr}}_{2}\mathrm{GeC}(11\overline{2}0)//{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(1\overline{1}00)$ or ${\mathrm{Cr}}_{2}\mathrm{GeC}(11\overline{2}0)//{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(\overline{1}2\overline{1}0).$ There is also a large secondary grain population with $(10\overline{1}3)$ orientation. Deposition onto Al${}_{2}$O${}_{3}$(0001) with a TiN(111) seed layer and onto MgO(111) yielded growth of globally epitaxial Cr${}_{2}$GeC(0001) with a virtually negligible $(10\overline{1}3)$ contribution. In contrast to the films deposited at 700--800 \ifmmode^\circ\else\textdegree\fi{}C, the ones grown at 500--600 \ifmmode^\circ\else\textdegree\fi{}C are polycrystalline Cr${}_{2}$GeC with $(10\overline{1}0)$-dominated orientation; they also exhibit surface segregations of Ge as a consequence of fast Ge diffusion rates along the basal planes. The room-temperature resistivity of our samples is 53--66 \ensuremath{\mu}\ensuremath{\Omega}cm. Temperature-dependent resistivity measurements from 15--295 K show that electron-phonon coupling is important and likely anisotropic, which emphasizes that the electrical transport properties cannot be understood in terms of ground state electronic structure calculations only.