Control of ternary alloy composition during remote epitaxy on graphene

Abstract

Understanding the sticking coefficient $σ$, i.e., the probability of an adatom sticking to a surface, is essential for controlling the stoichiometry during epitaxial film growth. However, $σ$ on monolayer graphene-covered surfaces and its impact on remote epitaxy are not understood. Here, using molecular-beam epitaxial (MBE) growth of the magnetic shape memory alloy Ni$_2$MnGa, we show that the sticking coefficients for metals on graphene-covered MgO (001) are less than one and are temperature and element dependent, as revealed by ion backscattering spectrometry (IBS) and energy dispersive x-ray spectroscopy (EDS). This lies in stark contrast with most transition metals sticking on semiconductor and oxide substrates, for which $σ$ is near unity at typical growth temperatures ($T<800\degree$C). By initiating growth below $400 \degree$ C, where the sticking coefficients are closer to unity and wetting on the graphene surface is improved, we demonstrate epitaxy of Ni$_2$MnGa films with controlled stoichiometry that can be exfoliated to produce freestanding membranes. Straining these membranes tunes the magnetic coercive field. Our results provide a route to synthesize membranes with complex stoichiometries whose properties can be manipulated via strain.

Figures (4)

• Figure 1: (a) Ion Beam Scattering (IBS) for nominally 20 nm thick Ni$_2$MnGa films grown on graphene/MgO and MgO at $600\degree$ C, showing reduced sticking for Ni and Mn on graphene. (b) Energy dispersive X-ray spectroscopy (EDS) measurements for Ni$_2$MnGa films with nominal thickness 80 nm on gr/MgO and on MgO. Both IBS and EDS sample was were capped with a protective layer of Au. (c) EDS intensity ratios $I_{graphene}/I_{MgO}$, tracking temperature and element dependent changes in the cumulative sticking coefficient for Ni$_2$MnGa on graphene-covered MgO. Error bars are standard deviations on multiple regions of a given sample. (d,e) SEM images of the nominally 80 nm thick films grown at room remperature on graphene/MgO and MgO and capped with Au. (f,g) SEM images of the nominally 80 nm thick films grown at $625\degree$C on graphene/MgO and MgO and capped with Au.
• Figure 2: Out of plane X-ray diffraction scans (Cu $K\alpha$) of Ni$_2$MnGa films grown on MgO and on graphene/MgO at $600\degree$C, $400\degree$C, and $370\degree$C, compared to a film grown directly on MgO. Asterisks * denote MgO substrate reflections and "x" denotes secondary phase reflections.
• Figure 3: (a) Azimuthal $\phi$ scans for a Ni$_2$MnGa film grown on graphene/MgO (001). The off axis $010$ reflections track the in-plane orientation of Ni$_2$MnGa domains with (110) out of plane orientation. The $101$ reflections track the in-plane orientation of the (001) domain. (b) Domain orientations of Ni$_2$MnGa (blue) with respect to MgO (001) (black) determined from (a).
• Figure 4: (a) X-ray diffraction before and after membrane exfoliation. (b) SQUID magnetometry of a relaxed Ni$_2$MnGa film on graphene/MgO (dark blue), and on the same sample after exfoliation and rippling to create a strained Ni$_2$MnGa membrane (light blue). The measurement was performed at 100 K with field oriented within the film plane.