Evidence for In-Plane Electrical Polarization in 3R-β'-InSe Thin Films Grown by Molecular Beam Epitaxy.

β-InSe has been identified as a potential ferroelectric. This work describes the growth of β-InSe via molecular beam epitaxy along with a description of its properties, including the search for switchable polarization in 3R-β'-InSe. The thin-film morphology, crystal structure, and phase maps of β-InSe on Si(111) and AlO(0001) were evaluated by atomic force microscopy, X-ray diffraction, and Raman spectroscopy as a function of the atomic Se/In flux ratio and growth temperature. Smooth β-InSe thin films were successfully realized on Si(111) at a substrate temperature of 150 °C using a Se/In flux ratio of 5.7, as well as on AlO(0001) at 450 °C using a Se/In flux ratio of 5.5. Scanning transmission electron microscopy (STEM) confirms the 3R polytype of β-InSe in films on both substrates, with a minor disorder associated with the 2H polytype at the interface. Indications of in-plane Se atom displacements characteristic of the 3R-β'-InSe polytype were found by STEM and second harmonic generation analysis in films on AlO(0001), but not in films on Si(111). However, attempts at electrical polarization switching did not produce compelling evidence for ferroelectricity. Instead, electrical transport measurements demonstrated locally varying anisotropic responses with the applied electric fields along different in-plane directions, with some hysteresis associated with the trapping of charges.