Polarization Direction Dependence of Thermodynamic Stability of Ferroelectric BiAlO₃(0001) Polar Surfaces

Abstract

Polar surfaces of ferroelectric oxides are of great importance for both fundamental and practical interest. In this report, we present a theoretical study to determine the thermodynamic stability and electronic properties of ferroelectric BiAlO3(0001) surfaces. According to the calculated surface phase diagram, we predict that the equilibrium stoichiometries are distinct for the oppositely polarized BiAlO3 surfaces under the same chemical conditions. In addition to nonstoichiometry of the surface chemical compositions, we find that anomalous filling of the surface states can also result in surface metallization in order to compensate the remarkable surface polarity. Besides providing atomic-scale understanding of the BiAlO3(0001) surfaces, we also put forward the practical implications in novel magnetoelectric devices and advanced surface chemistry.

Electrostatic Modulation and Mechanism of the Electronic Properties of Monolayer MoS2 via Ferroelectric BiAlO3(0001) Polar Surfaces

“…This is due to the characteristics of metastability of the polarization-reversed BAO(0001) surface terminations relative to the thermodynamically preferred polarization-unreversed BAO Z+ and Z− surface terminations. 22 More interestingly, different from the so-called polarization compensation mechanism of ferroelectric BAO(0001) substrates, 4,13,43,48 an unusual charge transfer opposite the polarization compensation can be seen in the MoS 2 overlayer of the MoS 2 /BAO Z− heterostructure, as shown in Figure 5. In other words, the MoS 2 overlayer on the down-polarized (Z−) ferroelectric BAO(0001) surface is electrostatically n-doped.…”

“…It can be observed that after full structure relaxation, the BAO Z+ substrate is still terminated with the −Al−O 3 −Bi atomic layer (as shown in Figure 2a), which is the same as the initial thermodynamically preferred BAO Z+ surface termination without MoS 2 adsorption. 22,25 Such information indicates that the adsorption of MoS 2 has no significant influence on the atomic structures of the BAO Z+ surface. Similar conclusions can also be obtained for the relaxed MoS 2 /BAO Z+↓ heterostructure (as shown in Figure 2b).…”

“…In addition, a recent theoretical study indicated that due to the coexistence and coupling of bulk Rashba spin-splitting and ferroelectricity, bulk BAO can be an attractive ferroelectric Rashba semiconductor. Note that in our previous reports , regarding BAO(0001) polar surfaces, the thermodynamic surface phase diagram and the corresponding electronic properties and spin textures were determined. It was found that the thermodynamically preferred positive (Z+) and negative (Z−) polar surfaces are terminated by the −Al–O 3 –Bi atomic layer and the −O 3 –Bi 2 layer, respectively.…”

“…19−21 The theoretical spontaneous polarization of BAO is about 80 μC/cm 2 . 22 Epitaxial BAO thin films can be fabricated using the pulsed laser deposition (PLD) method. 23 In addition, a recent theoretical study 24 indicated that due to the coexistence and coupling of bulk Rashba spin-splitting and ferroelectricity, bulk BAO can be an attractive ferroelectric Rashba semiconductor.…”

“…Ferroelectric BiAlO 3 (BAO) with the R 3 c space group exhibits outstanding piezoelectric and ferroelectric properties along the [0001] direction and is a promising alternative material to the lead-based Pb­(Zr, Ti)­O 3 (PZT) piezoelectric material. − The theoretical spontaneous polarization of BAO is about 80 μC/cm 2 . Epitaxial BAO thin films can be fabricated using the pulsed laser deposition (PLD) method .…”