Determining the Dielectric and Piezoelectric Response of SnSe

Tin selenide (SnSe) is a layered chalcogenide which is predicted to exhibit a piezoelectric coefficient greater than conventional lead-free piezoelectrics such as MoS₂and AlN. However, bulk SnSe forms in the centrosymmetric <i>Pnma</i> structure while piezoelectricity can only occur in non-centrosymmetric crystal structures. Thus, SnSe can only become piezoelectric by breaking the center of symmetry by either scaling the material down to the monolayer limit or inducing stacking faults. In this work, we demonstrate layer control in SnSe down to the monolayer limit with micron scale lateral growth for the determination of the intrinsic piezoelectric response of this material.<br/> <br/>SnSe films were grown over a temperature range of 250 – 300 °C on (100) MgO by molecular beam epitaxy. X-ray photoelectron spectroscopy shows that films grown using a Se:Sn flux ratio of 1.17:1 or greater exhibit 1:1 Se:Sn stoichiometry. Raman spectroscopy fits to the <i>Pnma</i> phase of SnSe for thick films, with a clear shift in the A¹_g and B¹_3g upon scaling down to the monolayer. The monolayer thickness is further verified by atomic force microscopy showing a discrete step height of approximately 0.5 nm per layer. Transmission electron microscopy shows stacking faults in films thicker than 10 monolayers that break the center of symmetry along the basal plane direction of the 2D material, creating a net symmetry breaking that enables piezoelectricity beyond a monolayer or through surface effects. Perpendicular sets of planar top contacts were then deposited onto both monolayer and thick SnSe films for a comparison of the dielectric and piezoelectric measurements. The dielectric response is highly anisotropic and thickness dependent with a minimum tanδ of 0.06 when measured in the dark. The impact of layer scaling on the piezoelectric response was measured through wafer flexure. Overall, this work provides insight into the layer control synthesis and electrical anisotropy of a unique class of 2D materials.