Bulk Depolarization Fields as a Major Contributor to the Ferroelectric Reliability Performance in Lanthanum Doped Hf_0.5Zr_0.5O₂ Capacitors

Abstract

After the discovery of ferroelectricity in HfO2, many dopants have been incorporated into the material to improve the ferroelectric properties. The binary mixture of HfO2 and ZrO2, HfxZrx−1O2, showed the widest process window in terms of polarization, but other memory related aspects still need improvement. Recently, the co‐doping of La into a mixed Hf0.5Zr0.5O2, La:HZO, was reported to improve the endurance properties further but the explanation spanning both structural and electrical characteristics of La:HZO and their interaction is still missing. In this work, an extensive study of La:HZO with La content ranging from 0 to 4.3 mol% is conducted and resultant stabilization of nonpolar tetragonal phase, coercive field reduction, endurance improvement, stronger retention loss, and less imprinted hysteresis loop is reported with increasing La concentration. The model simultaneously explaining the electrical and structural properties is presented. In ferroelectric capacitor structures, the depolarization fields originating from nonferroelectric layers at the metal/ferroelectric interface are discussed extensively in previous studies but, here, for the first time, the impact of depolarization fields from nonferroelectric regions in the bulk of the ferroelectric material is reported, which is an important element to explain all the observed trends.

Epitaxial Ferroelectric La-Doped Hf0.5Zr0.5O2 Thin Films

“…15,17,21 Similar reduction of monoclinic phase was observed in polycrystalline La-doped HZO films. 5,7 In summary, XRD confirms that the orthorhombic phase present in the La-doped HZO films has grown epitaxially, but exhibits crystal variants and coexists with a minority monoclinic phase. Therefore, the films are not monocrystalline: the majority orthorhombic phase presents crystal variants and coexists with the minority monoclinic phase, existing grain boundaries between crystal variants and between the two phases.…”

“…Imprint field, critical in the asymmetry, may depend on the presence of monoclinic phase, more important in thicker films, and also on the characteristics of the voltage pulses applied (amplitude, time and polarity). 7 Current leakage decreases monotonically with thickness, which favors a better retention in thicker films. However, the effect of leakage depends on spatial inhomogeneities, mainly due to grain boundaries, and on the possible coexistence of electronic and ionic contributions with different time scales.…”

“…The higher d o-HZO(111) in La doped films differs from the reported slight shrinkage for polycrystalline HZO films doped with similar La content. 5,7 The surface of the films is very flat. The atomic force microscopy (AFM) topographic images of La:HZO films on STO(001) ( The effect of leakage is twofold: it can add to switching current and polarization can be overestimated, but on the contrary, it does not allow full saturation of the loop and therefore the polarization is underestimated.…”

Wake‐Up Mechanisms in Ferroelectric Lanthanum‐Doped Hf0.5Zr0.5O2 Thin Films

“…[7] Lanthanum (La) doping of Hf 0.5 Zr 0.5 O 2 improves the cycling behavior for high cycle numbers, but a very pronounced increase in the wake-up effect is reported. [6,8,9] The wake-up effect is not only limited to La:Hf 0.5 Zr 0.5 O 2 but is also reported for ferroelectric HfO 2 stabilized using other dopants such as La, Si, and Al-doped HfO 2 . [10,11] Therefore, understanding the cause of these degradation behaviors is an important task for incorporating these materials into reliable memory technologies.…”

“…Several issues in complementary metal–oxide–semiconductor (CMOS) process integration of perovskite‐based ferroelectrics such as contamination caused by lead‐based oxides, a high thermal budget required to achieve the ferroelectric phase, [ 1 ] and hydrogen sensitivity [ 2 ] lead to limited scalability, [ 3 ] and therefore hinder the progress of ferroelectric integration into state‐of‐the‐art CMOS technologies. In contrast, HfO 2 ‐based ferroelectrics promise to solve the previously mentioned obstacles, [ 4 ] but do suffer from defects, [ 5 ] a high coercive field leading to high switching voltages and low endurance, [ 6 ] and charge trapping. [ 7 ]…”