Domain Pinning: Comparison of Hafnia and PZT Based Ferroelectrics

Abstract

Even though many studies on the field cycling behavior of ferroelectric hafnium oxide have recently been published, the issue is still not fully understood. The initial increase of polarization during first cycles is explained by different theoretical and empirical approaches. Field-induced phase changes as well as oxygen vacancy diffusion from interfacial layers toward the bulk are discussed. Trapped charges as well as the mentioned oxygen vacancy diffusion might cause a shift of the hysteresis along the voltage axis called imprint. Even though various studies connect this effect to charge diffusion with progression of cycling, a final experimental proof for the origin of wakeup and imprint is still missing. Based on the comprehensive comparative study of hafnia-zirconia and iron-doped lead zirconate titanate ferroelectrics, it is verified that the diffusion of oxygen vacancies is the main cause for both imprint and wake-up. Moreover, it is shown that a local seed inhibition of ferroelectric domains is most likely responsible for the reduced ferroelectric response in pristine state.

Analysis of Performance Instabilities of Hafnia‐Based Ferroelectrics Using Modulus Spectroscopy and Thermally Stimulated Depolarization Currents

“…[20] The value close to 1 for the constant phase parameter n (see Table 1) indicates quite homogeneous properties of this layer. [27] However, our very thin hafnia thin films consist of mainly columnar grains [22] with a high fraction of oriented grain boundaries [17] strongly decreasing the grain boundary contribution. In literature, where other material systems and thicker layers are investigated, this is often interpreted as the difference between grains and grain boundaries.…”

“…The presence of coherent monoclinic/ orthorhombic interphase boundaries and 90° orthorhombic domains in Hf 0.5 Zr 0.5 O 2 is consistent with findings of microstructure in Gd-doped hafnia thin films, [22] which suggests they possess similar microstructure features. [17] Annealing at higher temperatures initiates the generation of oxygen vacancies due to diffusion of oxygen into the TiN electrode. [20] During the ALD deposition process, oxygen from the hafnia film likely reacts with the TiN electrode to form an interfacial TiO x N region.…”

“…[17] suggested that these charges are most likely pinning domain walls at their seeds (seed inhibition), and that the interfacial layer between ferroelectric and electrode plays a crucial role for this effect. The loss of switchable polarization during cycling is strongly reduced with the increasing electric field.…”

“…[15] These charges can lead to multiple peaks in the current-voltage (I-V) characteristics, which merge during cycling. [17] This so-called imprint effect might cause a readout failure in memory devices due to a drastically reduced ferroelectric response. [16] In many cases, dielectric breakdown is then the final limiting factor in the device lifetime.…”

“…With further cycling, a reduction of ferroelectric response (fatigue) is observed. All three effects have been thought to relate to oxygen vacancy diffusion, [17] but also electronic contributions [18] and effects of phase change have been discussed. An additional detrimental effect is the shift of the polarization-voltage (P-V) hysteresis along the voltage axis after a capacitor is exposed to elevated temperatures.…”