Enhanced Spontaneous Polarization in Ultrathin SnTe Films with Layered Antipolar Structure

Abstract

2D SnTe films with a thickness of as little as 2 atomic layers (ALs) have recently been shown to be ferroelectric with in‐plane polarization. Remarkably, they exhibit transition temperatures (Tc ) much higher than that of bulk SnTe. Here, combining molecular beam epitaxy, variable temperature scanning tunneling microscopy, and ab initio calculations, the underlying mechanism of the Tc enhancement is unveiled, which relies on the formation of γ‐SnTe, a van der Waals orthorhombic phase with antipolar inter‐layer coupling in few‐AL thick SnTe films. In this phase, 4n − 2 AL (n = 1, 2, 3…) thick films are found to possess finite in‐plane polarization (space group Pmn21), while 4n AL thick films have zero total polarization (space group Pnma). Above 8 AL, the γ‐SnTe phase becomes metastable, and can convert irreversibly to the bulk rock salt phase as the temperature is increased. This finding unambiguously bridges experiments on ultrathin SnTe films with predictions of robust ferroelectricity in GeS‐type monochalcogenide monolayers. The observed high transition temperature, together with the strong spin‐orbit coupling and van der Waals structure, underlines the potential of atomically thin γ‐SnTe films for the development of novel spontaneous polarization‐based devices.

From an atomic layer to the bulk: Low-temperature atomistic structure and ferroelectric and electronic properties of SnTe films

“…6 displays experimental (upper row) and computational (lower row) results supporting the antiferroelectric coupling of 4 AL SnTe. 71 There, the empirical relation dI/dV 0.5 × DOS (established in Fig. 2(a)) can be seen again.…”

“…Band bending is larger on a ferroelectrically-coupled (AA) 4 AL SnTe film when compared to an antiferroelectrically-coupled (AB ) 4 AL SnTe film, because the electric field lines cancel out at the exposed edge on the latter case. 70,71 Here, we use energetics and the experimental values of ∆α for 2 AL and 4 ALs, to demonstrate an antiferroelectric coupling on 4 AL SnTe that is at odds with previous claims of ferroelectric coupling 68,89 and consistent with experiment. 70,71 In the present calculations, the AA structure shown in Fig.…”

“…70,71 Here, we use energetics and the experimental values of ∆α for 2 AL and 4 ALs, to demonstrate an antiferroelectric coupling on 4 AL SnTe that is at odds with previous claims of ferroelectric coupling 68,89 and consistent with experiment. 70,71 In the present calculations, the AA structure shown in Fig. 4(a) has two 2 ALs relatively displaced along the z−direction.…”

“…And while common theoretical approaches assume a slab can be obtained by cutting two opposing surfaces of bulk rocksalt 73 or rhombic 59,68,89 bulk samples, the present work is aimed to explore the structural evolution of a freestanding SnTe slab containing 2n ALs by the successive addition of a 2 ALs in the overall lowest-energy conformation to the slab containing 2(n−1) ALs with n a positive integer, complementing the experimental results of Ref. 71.…”

“…At the same time, interest on ultrathin SnTe originated due to theoretical predictions of ferroelectricity on these films [63][64][65][66][67][68][69] and their experimental fabrication 70,71 . These SnTe slabs have not been created by capping a bulk sample, but grown from the bottom up [70][71][72] .…”