Lanthanum-Doped Hafnium Oxide: A Robust Ferroelectric Material

Abstract

Recently simulation groups have reported the lanthanide series elements as the dopants that have the strongest effect on the stabilization of the ferroelectric non-centrosymmetric orthorhombic phase in hafnium oxide. This finding confirms experimental results for lanthanum and gadolinium showing the highest remanent polarization values of all hafnia-based ferroelectric films until now. However, no comprehensive overview that links structural properties to the electrical performance of the films in detail is available for lanthanide-doped hafnia. La:HfO appears to be a material with a broad window of process parameters, and accordingly, by optimization of the La content in the layer, it is possible to improve the performance of the material significantly. Variations of the La concentration leads to changes in the crystallographic structure in the bulk of the films and at the interfaces to the electrode materials, which impacts the spontaneous polarization, internal bias fields, and with this the field cycling behavior of the capacitor structure. Characterization results are compared to other dopants like Si, Al, and Gd to validate the advantages of the material in applications such as semiconductor memory devices.

On the Origin of the Large Remanent Polarization in La:HfO2

“…[7] It should further be noted that La is one of the dopants that increase the crystallization temperature of HfO 2 the most. A P r of >20 µC cm -2 could be achieved with annealing temperature ranging from 600 and 1000 °C for samples with the composition and film thickness.…”

“…In Supporting Information S1, the computed lattice parameters of the doped Pca2 1 unit cells are compared to GIXRD data for pure zirconia, hafnia-zirconia, pure hafnia, and hafnia with different dopants. Although Figure 1 indicates that La doping of HfO 2 increases the polarization by about 10% in comparison to pure HfO 2 , this cannot explain the measured polarization improvement of around 50% [1,7] and leaves this question open for further discussion in the following. This is an important prerequisite for the reliability of the spontaneous polarization (P s ) derived in the next step.…”

“…A high-throughput study by Falkowski et al [9] confirmed the results with a much higher number of relaxed structures. [7] This result means that lanthanum's stabilizing qualities of the ferroelectric phase can explain the wider concentration range reported by Schroeder et al, but it does not explain the increased P r . [7] Ignoring undesired artifacts such as dead/passive layers [10][11][12] or internal bias fields, [13][14][15] the macroscopic P r of a sample may be imagined as governed by three main factors: 1) the fraction of the material exhibiting a polar symmetry, 2) the spontaneous polarization of the unit cell, and 3) the collective orientations of the unit cells with respect to the direction of the applied electric field.…”

“…[2][3][4][5][6][7] A physical explanation for these results has remained elusive. [2][3][4][5][6][7] A physical explanation for these results has remained elusive.…”

“…While a reproduction of the remarkable P r of 40 µC cm -2 has never been reported in subsequent publications, a recent work by Schroeder et al achieved P r between 25 and 30 µC cm -2 for a wide dopant concentration range. [7] In addition, it should be mentioned that only dipoles taking part in switching contribute to the measured remanent polarization.…”