Engineering Ferroelectricity in HfxZr1-XO2
Authors: Weeks Stephen , Pal Ashish , Narasimhan Vijay , Littau Karl , Pramanik Dipankar , Chiang Tony 
Published: February 2020

Recent work has shown that ferroelectric (FE) behavior in doped HfO2 thin films can be achieved by stabilizing the non-centrosymmetric orthorhombic phase (Pbc21) in films for thicknesses under 10 nm,1 which has led to renewed interest in these films for non-volatile memory (NVM) applications. Following the initial work on doped HfO2, researchers were able to demonstrate that the polarization response in these films could be changed from paraelectric to ferroelectric (FE) and ultimately antiferroelectric (AFE) by increased blending of ZrO2 in a solid solution.2,3 Beyond composition, the FE response of the HfxZr1-xO2 solid solution is a complex function of a number of parameters including thickness,4 the starting substrate,5 annealing conditions (both temperature and ambient gas),6,7,8 as well as the presence of a capping material during annealing.3

While there are a number of reports on mapping the FE response of HfxZr1-xO2 with respect to these parameters individually, there is still no individual report detailing optimization of the FE response with respect to all of these variables across the HfxZr1-xO2 composition space. Here we report on the optimization of the FE response in HfxZr1-xO­2 solid solutions relative to composition, physical thickness, top electrode processing and post metallization annealing conditions using TiN and Ir bottom electrodes (BEL). We utilize a combinatorial approach to rapidly identify an optimized workflow with respect to these variables and investigate the relationship between film microstructure and FE properties. Additionally, we demonstrate the compositionally driven transition from FE to AFE in HfxZr1-xO2 is sensitive to the choice of bottom electrode.
Figure 1 shows the polarization response of 8.5 and 8 nm Hf.25Zr.75O2 thin films grown on Ir and TiN BEL respectively. These films were subjected to identical annealing treatments (10 minutes at 500 °C in N2), demonstrating the influence of the Ir and TiN BEL in determining the polarization characteristics. A triangular voltage waveform was utilized and both samples were subjected to a stress of ±3 MV/cm pulses at 1 KHz for 1 second before measurement. Devices using a TiN BEL exhibited AFE-like behavior similar to previous reports for this composition,2 while devices using an Ir BEL showed strong FE characteristics and a remnant polarization (Pr) of 19 µC/cm2. Previously, such high remnant polarization (Pr>16 μC/cm2
) has only been reported for more Hf rich Hf.5Zr.5O2 films.9 This suggests that the bottom electrode influences the composition at which the HfxZr1-xO2 transforms from a FE to an AFE film.
These results show that the starting surface has a significant impact in determining the composition at which a strong FE response can be obtained in the HfxZr1-xO2 films, and that proper choice of workflow process conditions can lead to improvement in the exhibited FE properties. Further optimization of the FE response and detailed structural characterization of this material system will be discussed as a function of composition, thickness and annealing conditions. These findings are particularly important for NVM technology, where the remnant polarization and memory window play major roles in determining the usefulness of the film.

References

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[2] J. Müller et al, Nano Letters, vol. 12, p. 4318, 2012.
[3] M. H. Park et al, Adv. Energy Mat., vol 4, p.1811, 2014.
[4] M.H. Park et al, Appl. Phys. Lett., vol. 104, p. 072901, 2014.
[5] M. H. Park et al, Appl. Phys. Lett., vol. 105, p. 072902, 2014.
[6] M. H. Park et al, Appl. Phys. Lett., vol. 102, p. 242905, 2013.
[7] T. Shimizu et al, Appl. Phys. Lett., vol. 106, p. 112904, 2015.
[8] M.H. Park et al, Appl. Phys. Lett., vol. 102, p. 112914, 2013.
[9] H.J. Kim et al, Nanoscale, vol. 8, p. 1383, 2016.

Figure 1