Enhanced ferroelectricity in epitaxial Hf 0.5 Zr 0.5 O 2 thin films integrated with Si(001) using SrTiO 3 templates
Résumé
SrTiO 3 templates have been used to integrate epitaxial bilayers of ferroelectric Hf 0.5 Zr 0.5 O 2 and La 2/3 Sr 1/3 MnO 3 bottom electrodes on Si(001). The Hf 0.5 Zr 0.5 O 2 films show enhanced properties in comparison to equivalent films on SrTiO 3 (001) single crystalline substrates. The films, thinner than 10 nm, have a very high remnant polarization of 34 lC/cm 2. Hf 0.5 Zr 0.5 O 2 capacitors at an operating voltage of 4 V present a long retention time well beyond 10 years and high endurance against fatigue up to 10 9 cycles. The robust ferroelectric properties displayed by the epitaxial Hf 0.5 Zr 0.5 O 2 films on Si(001) using SrTiO 3 templates pave the way for the monolithic integration on silicon of emerging memory devices based on epitaxial HfO 2. Published under license by AIP Publishing. https://doi.org/10.1063/1.5096002 Ferroelectricity in doped HfO 2 was recently demonstrated in polycrystalline films arising from a metastable orthorhombic phase (space group Pca2 1). 1,2 The phase, not formed in bulk ceramics, is generally stabilized by annealing capped amorphous films. The annealed films are polycrystalline and the orthorhombic (polar) phase coexists with the bulk stable monoclinic (nonpolar) phase. The relative amount of phases depends critically on capping, bottom electrode, and film thickness, and films thicker than around 20 nm usually present a high fraction of monoclinic phase. 2-5 Different factors, including surface energy and strain, have been proposed to be responsible for the stabilization of the orthorhombic phase. 5-9 In epitaxial films, where film-substrate interface energy plays an important role, the orthorhombic phase has also been stabilized. Remarkably, epitaxial orthorhombic Hf 1Àx A x O 2 (A ¼ Y or Zr) films have been grown on different oxide substrates: (001)-oriented yttria-stabilized zirconia (YSZ) [bare or indium tin oxide (ITO) coated], 10,12-14 ITO/YSZ(110), 15 ITO/ YSZ(111), 11,16 (001)-oriented La 2/3 Sr 1/3 MnO 3 (LSMO)/SrTiO 3 (STO), 17-19 LSMO/LaAlO 3 (001), 20 and insulating YSZ/Si(001). 21 Integration of epitaxial Hf 0.5 Zr 0.5 O 2 (HZO) on Si(001) in capacitor geometry has been reported recently using a complex multilayer buffer, LSMO/LaNiO 3 /CeO 2 /YSZ/Si(001). 22 A simpler epitaxial structure is desirable, and for this purpose, STO can be used as a template for the epitaxial integration of ferroelectric HfO 2. STO can be deposited by molecular beam epitaxy (MBE), permitting large area deposition, and it has excellent compatibility with functional perovskite oxides. Indeed, it is used as a buffer layer for deposition of ferroelectric perov-skites such as BaTiO 3 (Ref. 23) and Pb(Zr x Ti 1Àx)O 3 (Ref. 24) on Si(001). The structural compatibility permits epitaxial integration, but the thermal expansion mismatch between the oxides and silicon reduces strongly the out-of-plane polarization of these perovskites with respect to equivalent films on SrTiO 3 (001) single crystalline substrates. 23,24 The effectiveness of STO as a buffer layer for epitaxial growth of the ortho-rhombic phase of HfO 2 is unknown. To investigate it, epitaxial STO templates are used here as buffer layers to integrate ferroelectric capaci-tors with epitaxial HZO films and LSMO bottom electrodes on Si(001). We show that the orthorhombic phase is epitaxially stabilized in films of around 8 nm thickness and the ferroelectric properties are enhanced with respect to equivalent films on perovskite SrTiO 3 (001) substrates. The remnant polarization (P r) is around 34 lC/cm 2 , its retention is longer than 10 years, and its endurance against fatigue is up to around 10 9 cycles. Remarkably, both very long retention and high endurance are achieved using the same poling voltage. The epitaxial STO template of thickness t ¼ 26 nm was deposited by solid-source molecular beam epitaxy (MBE) on a Si(001) wafer.