An unexplored antipolar phase in HfO2 from first principles and implication for wake-up mechanism

Abstract

Perturbing tetragonal HfO2 supercells in simulation with dopants or interstitial oxygen in the context of the ferroelectric Pca21 No. 29 (oIII-phase) formation frequently leads to an unexplored, lower energetic orthorhombic crystal phase with space group Pnma No. 62 (oV-phase). The crystal structure is inequivalent to cottunite oII-phase with the same space group but is closely related to the second ferroelectric Pmn21 No. 31 (oIV-phase) space group, as antipolar alignment. To explore the possibility of oIV- or oV-phase formation, we calculate the free energy in harmonic approximation and the energy landscape. Starting from the tetragonal P42/nmc No. 137 phase (t-phase), according to energy and activation barrier, the formation of the oV-phase is more favorable than that of the oIV-phase. Furthermore, exploring the energy landscape of the oIV–oV system, we find possible antiferroelectric-like behavior in HfO2 and Hf0.5Zr0.5O2, but not in ZrO2. We propose the formation of an oV-phase fraction as a possible structural explanation for a contribution to the wake-up phenomenon in HfO2. X-ray diffraction results on 10 nm Hf1−xZrxO2 thin films with varying Zr content are consistent with the claim.

Characteristics of Low‐Energy Phases of Hafnia and Zirconia from Density Functional Theory Calculations

“…[ 8 ] The dipole interaction energy lowers the energy relative to the polar o IV. [ 13,15 ] The structure is inequivalent to the high‐pressure phase o II, which is energetically too unfavorable to be listed in Table 1, as discussed in detail in the study by Azevedo Antunes et al [ 8 ] . Next in line for HfO 2 is the t ‐phase (, no.…”

“…The energy landscape of the higher‐energetic polymorphs o IV and o V has recently been discussed by Azevedo Antunes et al [ 8 ] in the context of a possible contribution to the wake‐up effect. Again, the transition involves shear motion of the structures.…”

“…Here, the free energy differences between the higher‐symmetric structures and the m ‐phase show a significant temperature dependence with the consequence of temperature‐driven phase transitions, although the total energy differences are large at T = 0 K. An exception is probably the o IV‐phase and o V‐phase. In contrast to ZrO 2 , these structures in HfO 2 are farther away from symmetry because the pseudocubic cell shows much stronger distortion, [ 8 ] indicating less symmetry. The t ‐phase for ZrO 2 and HfO 2 shows a strong free energy difference and gives the explanation for the m ‐phase‐to‐ t ‐phase transition at elevated temperatures.…”

“…HfO 2 and ZrO 2 with their metastable but polar and ferroelectric phase are materials that possess excellent properties for microelectronic and technical applications, [ 1 ] which range from dense nonvolatile memories [ 2,3 ] over thin‐film piezoelectric transducers [ 4 ] to thin‐film pyroelectric devices. [ 5 ] The polymorphism of the phases in HfO 2 and ZrO 2 is still surprising as new relevant phases have been found only recently by simulations, [ 6–8 ] and some experimental findings are still not consistent with the conventionally considered phases, [ 9 ] which are the monoclinic ground‐state m ‐phase ( P2 1 /c , no. 14), the polar‐orthorhombic o III‐phase ( Pca2 1 , no.…”

Novel Functionality in Switchable Polar Materials

“…This suppression in the thin-film geometry is analogous to the stabilization of the tetragonal phase by substitution in ZrO 2 (Y-substituted ZrO 2 , YSZ). While the primary polar phase has been identified as Pca2 orthorhombic (also referred to as oIII), there is experimental and theoretical evidence for competing polar phases [38,39,43] with polarization in other high symmetry directions, as well as antipolar phases [44,45] (Figure 3).…”