ABSTRACT: The alkaline earth stannates are touted for their wide band gaps and the highest room-temperature electron mobilities among all the perovskite oxides. CaSnO₃ has the highest measured band gap in this family and is thus a particularly promising ultra-wide band gap semiconductor. However, discouraging results from previous theoretical studies and failed doping attempts had written off this material as “undopable”. Here we redeem CaSnO₃ using hybrid molecular beam epitaxy (hMBE), which provides an adsorption-controlled growth for the phase-pure, epitaxial and stoichiometric CaSnO₃ films. By introducing lanthanum (La) as an n-type dopant, we demonstrate the robust and predictable doping of CaSnO₃ with free electron concentrations, n, from 3.3 × 10¹⁹ cm^-3 to 1.6 × 10²⁰ cm^-3. The films exhibit a maximum room-temperature mobility of 42 cm² V^-1s^-1 at n = 3.3 × 10¹⁹ cm^-3. Despite having a smaller radius than the host ion, La expands the lattice parameter. Using density functional calculations, this effect is attributed to the energy gain by lowering the conduction band upon volume expansion. Finally, we exploit the robust doping by fabricating the CaSnO₃ -based field-effect transistors. The transistors show promise for CaSnO₃’s high-voltage capabilities by exhibiting low off-state leakage below 20 pA/μm at a drain-source voltage of 100 V and on-off ratios exceeding 10⁶. This work opens the door to future studies on the semiconducting properties of CaSnO₃ and the many devices that could benefit from CaSnO₃’s exceptionally wide band gap.

This work was supported by the Air Force Office of Scientific Research FA9550-21-0127

[1] D. Du, et. al. Nano Lett.2022, 22, 21, 8647–8653

The ability to synthesize freestanding membranes of crystalline materials is critical for advancing the understanding of strain and strain gradient effects on materials properties. A common approach for the synthesis of freestanding membranes is epitaxial growth on graphene terminated substrates, where direct bonding between the film and substrate is prevented, allowing for exfoliation of the film from the substrate. However, challenges with wetting on the low surface energy graphene frequently prevents the growth of smooth epitaxial films. Here we demonstrate high quality growth of the flexomagnetic Heusler compound GdAuGe on graphene terminated germanium substrates using a cold-seeded growth approach. Scanning electron microscopy, atomic force microscopy, and x-ray diffraction experiments illustrate the tradeoffs resulting from growth at high and low temperatures and confirm that multistep growth approach comprised of a few-nanometer, amorphous deposition at room temperature followed by an anneal and subsequent growth at elevated temperature result in smooth, highly ordered films. This improved morphology and crystallinity enhances the ability to control the strain state in rippled GdAuGe membranes.

Rare earth-based Heuslers are prospective materials platforms for magnonics, topological spin texture, superconductivity, THz spintronics, etc. [1, 2]. The magneto-mechanical coupling in these materials allows for better control and manipulation of the primary order parameter and magnetic flexibility [3]. Here, we demonstrate novel flexomagnetic responses i.e., the coupling between strain gradient and magnetism, and strain-induced superconductivity, in GdAuGe Heusler membranes. The thin films of GdAuGe Heusler composition have been grown on monolayer Graphene/ Ge (111) by molecular beam epitaxy (MBE). GdAuGe films are then mechanically exfoliated to form free-standing rippled membranes.

GdAuGe shows an antiferromagnetic ordering below ~17 K, which is sustained when a homogeneous strain is applied. However, the application of strain gradient dramatically alters the magnetic ground state of GdAuGe in the rippled membranes. A phase diagram of the rippled GdAuGe membranes is shown in Fig. 1(a). Notably, a moderate strain gradient of a few tenths of a percentage transforms the ground state from antiferromagnetic to unconventional ferrimagnetic phases. These ferrimagnetic ground states in the rippled membranes offer the possibility of discovering spin reorientation and other unique magnetic phenomena; the most exciting observation is the emergence of superconductivity in GdAuGe membranes when a very large strain gradient is applied, with superconducting transitions occurring at low temperatures below ~3.5 K. Figure 1(b) shows the magnetic characterization of a superconducting GdAuGe rippled membrane.

At present, the microscopic origin of flexomagnetism and its effects on the thermodynamics of spin reorientation and phase transitions in these membranes remain unclear. Advanced spectroscopic measurements and magneto-transport experiments, combined with theoretical modeling, are planned to further investigate the phenomena in these rippled membranes.

References

1. Graf, Tanja, et al. "Simple rules for the understanding of Heusler compounds." Progress in solid state chemistry 39.1 (2011): 1-50.

2. Kawasaki, Jason K. "Heusler interfaces—Opportunities beyond spintronics?." APL Materials 7.8 (2019): 080907

3. Du, Dongxue, et al. "Epitaxy, exfoliation, and strain-induced magnetism in rippled Heusler membranes." Nature Communications 12.1 (2021): 1-7

Free-standing membranes have broad applications in the creation of symmetry-mismatched, non-equilibrium, and artificial heterostructures. We use sacrificial layer method to synthesize phase-pure epitaxial SrTiO₃ membranes. In this study, we will discuss the growth of strain-engineered SrTiO₃ films using different sacrificial layer(s) grown by hybrid MBE. We characterize the as-grown films using x-ray diffraction (XRD) and atomic force microscopy (AFM). We show exfoliation and transfer of films onto dissimilar substrates, followed by their structural characterization. Finally, we use impedance spectroscopy to characterize the dielectric properties and show a bulk-like dielectric constant of ≈300 for SrTiO₃ membranes transferred on Au coated Si substrate.