Growth Window of Ferroelectric Epitaxial Hf_0.5Zr_0.5O₂ Thin Films

Abstract

The metastable orthorhombic phase of hafnia is generally obtained in polycrystalline films, whereas in epitaxial films its formation has been much less investigated. We have grown Hf0.5Zr0.5O2 films by pulsed laser deposition and the growth window (temperature and oxygen pressure during deposition, and film thickness) for epitaxial stabilization of the ferroelectric phase is mapped. The remnant ferroelectric polarization, up to 24 C/cm 2 , depends on the amount of orthorhombic phase and interplanar spacing and increases with temperature and pressure for a fixed film thickness. The leakage current decreases with an increase in thickness or temperature, or when decreasing oxygen pressure. The coercive electric field (EC) depends on thickness (t) according the EC -t -2/3 scaling, which is observed by the first time in ferroelectric hafnia, and the scaling extends to thickness down to around 5 nm. The proven ability to tailor functional properties of high quality epitaxial ferroelectric Hf0.5Zr0.5O2 films paves the way toward understanding their ferroelectric properties and prototyping devices.

Engineering Ferroelectric Hf0.5Zr0.5O2 Thin Films by Epitaxial Stress

“…Additional information on growth conditions of HZO and LSMO is reported elsewhere. [18][19] Structural characterization: The crystal structure (crystal phases of HZO and lattice parameters of LSMO and HZO) was characterized by X-ray diffraction using Cu Kα radiation.…”

“…10 The resulting films are polycrystalline and contain paraelectric tetragonal and monoclinic phases besides the ferroelectric orthorhombic phase. 1,2,[11][12] The ferroelectric phase has been also grown epitaxially on a few substrates, including yttria-stabilized zirconia, [13][14][15][16] LaAlO3, 17 SrTiO3, [18][19][20][21] and buffered Si. 22 The research on epitaxial stabilization is just emerging in comparison with that on polycrystalline doped HfO2 films.…”

Direct Epitaxial Growth of Polar (1 – x)HfO2–(x)ZrO2 Ultrathin Films on Silicon

“…A prominent feature of hafnia-based materials is polymorphism 35 . While the ground state in the bulk HfO2 is a non-polar monoclinic (m-, P21/c) phase, a plethora of low-volume both polar and non-polar metastable states can be stabilized at ambient conditions via a combination of strategies such as cationic and anionic doping 1,[25][26][27]29,32 , thermal and inhomogeneous stresses 36,37 , nanostructuring 38 , epitaxial strain 16,20,22,23,26,29,[39][40][41][42] , and oxygen vacancy engineering 43,44 , all of which can be suitably engineered into thin-film geometries.…”

“…Such distinguishing characteristics 2 lead to an upsurge in application-oriented research as well as in curiosity-driven fundamental research to solve questions such as why these materials are capable of sustaining the unconventional ferroelectricity 13-32 , how these materials negate the effects of depolarization fields 33,34 , and whether such a new type of ferroelectricity can be replicated in other simple oxide systems.A prominent feature of hafnia-based materials is polymorphism 35 . While the ground state in the bulk HfO2 is a non-polar monoclinic (m-, P21/c) phase, a plethora of low-volume both polar and non-polar metastable states can be stabilized at ambient conditions via a combination of strategies such as cationic and anionic doping 1,[25][26][27]29,32 , thermal and inhomogeneous stresses 36,37 , nanostructuring 38 , epitaxial strain 16,20,22,23,26,29,[39][40][41][42] , and oxygen vacancy engineering 43,44 , all of which can be suitably engineered into thin-film geometries.Based on first-principles calculations 15,39,[45][46][47] at least five polar polymorphs (with space groups Pca21, Cc, Pmn21, R3 and R3m) can be identified as those that can be experimentally obtained.Owing to its relatively low energy, the orthorhombic (o-) Pca21 phase is widely observed in hafnia-based films grown via atomic layer deposition (ALD) 1,17,24,25 , chemical solution deposition (CSD) 28 , RF sputtering on Si 18,21 and pulsed-laser deposition (PLD) on selected substrates 19,23,26,31,[40][41][42] . A slightly higher energy rhombohedral (r-) phase (R3m or R3) has been recently observed on epitaxial Hf1/2Zr1/2O2 films grown on SrTiO3 (STO) 39 .…”

Magneto-ionic control of spin polarization in multiferroic tunnel junctions

“…Epitaxial growth of crystalline HZO can also be achieved [23] and has been recently also demonstrated on perovskite substrates with La 0.7 Sr 0.3 MnO 3 (LSMO) as bottom elecrode [24][25][26]. Four resistance states have been obtained in this type of junctions by both magnetic and electric field switching, but no ME coupling was reported.…”