Ferroelectricity in Simple Binary ZrO₂ and HfO₂

Abstract

The transition metal oxides ZrO(2) and HfO(2) as well as their solid solution are widely researched and, like most binary oxides, are expected to exhibit centrosymmetric crystal structure and therewith linear dielectric characteristics. For this reason, those oxides, even though successfully introduced into microelectronics, were never considered to be more than simple dielectrics possessing limited functionality. Here we report the discovery of a field-driven ferroelectric phase transition in pure, sub 10 nm ZrO(2) thin films and a composition- and temperature-dependent transition to a stable ferroelectric phase in the HfO(2)-ZrO(2) mixed oxide. These unusual findings are attributed to a size-driven tetragonal to orthorhombic phase transition that in thin films, similar to the anticipated tetragonal to monoclinic transition, is lowered to room temperature. A structural investigation revealed the orthorhombic phase to be of space group Pbc2(1), whose noncentrosymmetric nature is deemed responsible for the spontaneous polarization in this novel, nanoscale ferroelectrics.

The origin of ferroelectricity in Hf1−xZrxO2: A computational investigation and a surface energy model

“…According to the data of Yashima 62 a diameter of 24 nm is sufficient whereas in the model a size of 36 nm is necessary to yield the m-phase to have the lowest energy. The results of the free energy model for the 9 nm film data series of Müller 11 phase is dominant such that no f-phase is stable, although it is energetically close. The energy disadvantage to the t-phase is small so that it can be overcome with an electric field (see Sec.…”

“…Furthermore, the calculated results depend on the chosen basis set and the number of kpoints which were not always well documented in previous studies. In 11 . In all cases, except for the c-phases, our structural values have proved to be better than 1 % compared to experimental data without taking expansion effects from zero point motions into account.…”

“…TABLE VI contains the calculated results for a 9 nm thin film at 80 K, 300 K and a 900 K anneal temperature modeling the experimental results of Müller 11 . For all temperatures the 9 nm thick film is calculated to be monoclinic even at anneal temperatures.…”

“…ZrO2 was also studied extensively in the context of functional thin films since its tetragonally stabilized form has a high dielectric constant. Müller 11 showed, that ZrO2 behaves antiferroelectric: above a critical field in the order of 1 MV/cm, a temperature dependent field induced phase transformation to the f-phase occurs. A recent review of the current status of HfO2, ZrO2 and HZO based ferroelectric films can be found in 10 .…”

Polarization switching behavior of Hf–Zr–O ferroelectric ultrathin films studied through coercive field characteristics

“…These values are consistent with those of other reports. [6][7][8][9][10] In the currentvoltage (I-V ) measurement, however, a large leakage current is notable and a hard breakdown occurs at around ±4 V, that is, 4 MV=cm. In the operation of Hf-Zr-O capacitors, attention must be paid to the maximum electric field in order to avoid breakdown.…”