Epitaxial Integration on Si(001) of Ferroelectric Hf_0.5Zr_0.5O₂ Capacitors with High Retention and Endurance

Abstract

Epitaxial ferroelectric Hf0.5Zr0.5O2 films have been successfully integrated in a capacitor heterostructure on Si(001). The orthorhombic Hf0.5Zr0.5O2 phase, [111] out-of-plane oriented, is stabilized in the films. The films present high remnant polarization Pr close to 20 μC/cm 2 , rivaling with equivalent epitaxial films on single crystalline oxide substrates. Retention time is longer than 10 years for writing field of around 5 MV/cm, and the capacitors show endurance up to 10 9 cycles for writing voltage of around 4 MV/cm. It is found that the formation of the orthorhombic ferroelectric phase depends critically on the bottom electrode, being achieved on La0.67Sr0.33MnO3 but not on LaNiO3. The demonstration of excellent ferroelectric properties in epitaxial films of Hf0.5Zr0.5O2 on Si(001) is relevant towards fabrication of devices that require homogeneity in the nanometer scale, as well as for better understanding of the intrinsic properties of this promising ferroelectric oxide.

Growth Window of Ferroelectric Epitaxial Hf0.5Zr0.5O2 Thin Films

“…3,[6][7][8][9][10][11][12][13] The ferroelectric orthorhombic phase can be also stabilized in epitaxial films. [14][15][16][17][18][19][20][21] In epitaxial films, the orthorhombic phase is generally formed during deposition at high temperature, without need of annealing. [14][15][16][17][18][19][20] Epitaxial films are of high interest for better understanding of the properties of ferroelectric hafnia, as well as for prototyping devices with ultrathin films or having small lateral size, for which the higher homogeneity of epitaxial films respect to polycrystalline films is an advantage.…”

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

“…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. 2,5,8,[10][11][12]23 However, epitaxial HfO2 films are of huge interest as their properties can be better controlled than those of polycrystalline samples.…”

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.…”

“…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 .…”

“…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 .…”