Electronic Band Structure of In-Plane Ferroelectric van der Waals β′-In₂Se₃

Abstract

Layered indium selenides (In2Se3) have recently been discovered to host robust out-of-plane and inplane ferroelectricity in the α and β´ phases, respectively. In this work, we utilise angle-resolved photoelectron spectroscopy to directly measure the electronic bandstructure of β´-In2Se3, and compare to hybrid density functional theory (DFT) calculations. In agreement with DFT, we find the band structure is highly two-dimensional, with negligible dispersion along the c-axis. Due to n-type doping we are able to observe the conduction band minima, and directly measure the minimum indirect (0.99 eV) and direct (1.53 eV) bandgaps. We find the Fermi surface in the conduction band is characterized by anisotropic electron pockets with sharp in-plane dispersion about the M ̅ points, yielding effective masses of 0.21 m0 along KM ̅̅̅̅̅ and 0.33 m0 along ΓM ̅̅̅̅ . The measured band structure is well supported by hybrid density functional theory calculations. The highly two-dimensional (2D) bandstructure with moderate bandgap and small effective mass suggest that β´-In2Se3 is a potentially useful new van der Waals semiconductor. This together with its ferroelectricity makes it a viable material for highmobility ferroelectric-photovoltaic devices, with applications in non-volatile memory switching and renewable energy technologies.

Wafer‐Scale Fabrication of 2D β‐In2Se3 Photodetectors

“…[41] Since this material has a nearly 2D band structure, a modest increase in the bandgap is expected due to the thickness reduction. [11] Thus, the observed value for five QLs closely corresponds to the one expected for bulk β-In 2 Se 3 [32] and a notable difference must occur only in thinner films (as monolayer or bilayer). From the responsivity, we determine the external quantum efficiency (EQE) to 0.67%.…”

“…More recently, 2D indium selenides (In x Se y ) have gained attention, [ 9 ] due to their bandgap in the visible spectral region, comparable to TMDCs, but also due to several novel properties: InSe exhibits one of the largest mobilities of 2D semiconductor materials, [ 10 ] and β‐In 2 Se 3 shows good mobility, [ 11 ] excellent photoresponsivity, [ 12 ] and exotic ferroelectricity [ 13 ] (the ordered ferroelectric phase is also called β′‐In 2 Se 3 by some authors). [ 11,13 ] Transistors with high mobility and on/off ratio have already been realized.…”

“…More recently, 2D indium selenides (In x Se y ) have gained attention, [ 9 ] due to their bandgap in the visible spectral region, comparable to TMDCs, but also due to several novel properties: InSe exhibits one of the largest mobilities of 2D semiconductor materials, [ 10 ] and β‐In 2 Se 3 shows good mobility, [ 11 ] excellent photoresponsivity, [ 12 ] and exotic ferroelectricity [ 13 ] (the ordered ferroelectric phase is also called β′‐In 2 Se 3 by some authors). [ 11,13 ] Transistors with high mobility and on/off ratio have already been realized. [ 14 ] Therefore, 2D In 2 Se 3 materials have the potential to address several limitations of the current silicon (Si) and III–V technologies, such as improved mobility and overall performance of transistors for electronics, as well as integrated photodetectors and light emitters in the same material system (same die), all possible on virtually any substrate, including transparent and flexible substrates.…”

Review on Recent Developments in 2D Ferroelectrics: Theories and Applications

“…Based on the thorough first‐principles calculations and ab initio molecular dynamic simulations of In 2 Se 3 monolayer, two degenerate ground states (α‐In 2 Se 3 ) and a metastable state (β‐In 2 Se 3 ), with Se‐In‐Se‐In‐Se quintuple layer structures, are identified to be ferroelectric. [ 18b ] The two degenerate ground states have both in‐plane and out‐of‐plane ferroelectric polarization, which are locked with each other, that is, when either polarization is switched, the other one will also be switched simultaneously. An experimentally verified new mechanism of the switching process proposed that the Se atoms in the middle layer move laterally about 100 pm, which is one order larger than that of the traditional ferroelectrics, while breaking and forming of the SeIn bonds ( Figure ).…”

“…The 2D vdW materials exhibit various properties such as durability against huge strain, stable surface with less dangling bonds, and easy functionalization of surface atoms, which may be favorable for sustaining the ferroelectric polarization. [ 13 ] So far, various 2D vdW materials with the thickness of several layers or even monolayer have been reported to be intrinsically ferroelectric, such as MX 2 (M = W Mo, X = S, Se, Te) transition metal dichalcogenides (TMDs), [ 14 ] group‐IV monochalcogenides, [ 15 ] metal triphosphates, [ 16 ] layer perovskites, [ 17 ] indium selenide (In 2 Se 3 ), [ 18 ] and others, however, only their small parts are experimentally verified. Meanwhile, various origins of ferroelectric polarization in 2D vdW ferroelectrics have been discovered.…”