Mechanism of polarization switching in wurtzite-structured zinc oxide thin films

Abstract

The properties of a potentially new class of ferroelectric materials based on wurtzite-structured ZnO thin films are examined using the first-principles calculations. Theoretical P-E hysteresis loops were calculated using the fixed-D method for both unstrained and (biaxially) strained single crystals. Ferroelectric polarization switching in ZnO (S.G. P63mc) is shown to occur via an intermediate non-polar structure with centrosymmetric P63/mmc symmetry by displacement of cations relative to anions in the long-axis direction. The calculated coercive electric field (Ec) for polarization switching was estimated to be 7.2 MV/cm for defect-free monocrystalline ZnO. During switching, the short- and long-axis lattice parameters expand and contract, respectively. The large structural distortion required for switching may explain why ferroelectricity in this compound has not been reported experimentally for pure ZnO. Applying an epitaxial tensile strain parallel to the basal plane is shown to be effective in lowering Ec during polarization, with a 5% biaxial expansion resulting in a decrease of Ec to 3.5 MV/cm. Comparison with calculated values for conventional ferroelectric materials suggests that the ferroelectric polarization switching of wurtzite-structured ZnO may be achievable by preparing high-quality ZnO thin films with suitable strain levels and low defect concentrations.

Thermally Induced Wurtzite to h‐BN Structural Transition

“…If a system frequently switches between WZ and WZ' (WZ' is identical to WZ but with an opposite direction of the c-axis) through h-BN as shown in Figure 1, the averaged positions of the ions are the h-BN lattice site. This hypothetical transition is a new type of the ferroelectric-paraelectric phase transitions, and has been previously proposed by Moriwake et al [14,15] In this letter, we investigate the thermally induced WZ!h-BN transition in WZ crystals using MD simulations with a generic pair potential model. This model allows us to understand the behaviors and trends of the transition in different WZ crystals.…”

“…Although the number of good candidate materials showing the WZ! h-BN transition is very limited according to our DFT calculations, there is also a high possibility to observe this new transition in systems under an anisotropic pressure [7,14,15,[36][37][38] or in the doped systems. [39,40] Future experimental works are required to prove this new type of ferroelectric-paraelectric phase transition in important WZ crystals under extreme conditions (i.e., high temperatures).…”

“…We carried out DFT calculations for 14 WZ crystals to assess the possibilities of the occurrence of the WZ→h‐BN transition and found that the transition should take place in WZ ZnO crystals. Although the number of good candidate materials showing the WZ→h‐BN transition is very limited according to our DFT calculations, there is also a high possibility to observe this new transition in systems under an anisotropic pressure or in the doped systems . Future experimental works are required to prove this new type of ferroelectric–paraelectric phase transition in important WZ crystals under extreme conditions (i.e., high temperatures).…”

“…We chose this parameter set because it is coincidentally able to reproduce several physical properties of ZnO (see Table S2, Supporting Information). ZnO displays the h‐BN structure in 2D thin films, and it is one of the candidate materials that may undergo the WZ→h‐BN transition at high temperatures …”

“…ZnO displays the h-BN structure in 2D thin films, [29] and it is one of the candidate materials that may undergo the WZ!h-BN transition at high temperatures. [14,15] Three physical quantities, u, c/a and the enthalpy H, were used as order parameters to quantify the WZ$h-BN transitions. The coordinate u showing the degree of buckling of the hexagonal layers is the most direct order parameter that indicates the occurrence of the WZ!h-BN transition.…”