Ferroelectricity in Rare-Earth Modified Hafnia Thin Films Deposited by Sequential Pulsed Laser Deposition

Abstract

Room temperature ferroelectricity in pulsed laser deposited rare-earth doped hafnium oxide (HfO 2 ) thin films is discussed. Maximum values of remnant polarizations (P r ) ∼13.5 and 12 μC/cm 2 along with coercive fields (E C ) ∼334 and 384 kV/cm are observed in 6 mol. % of rare-earth (Sm or Gd) doped-HfO 2 thin films (Sm:HfO 2 and Gd:HfO 2 ), respectively. Piezoresponse force microscopy measurements confirmed ferroelectric nature of films by showing phase hysteresis and butterfly amplitude loops. It is noticed that wake-up cycles improved the remnant polarization and found to be necessary for the forming of well saturated hysteresis loops. Our results showed potential toward realization of future highly scaled non-volatile ferroelectric memories. Ferroelectric materials are engendering considerable interest because of a variety of emerging applications including ferroelectric memories which have promising potentials for next generation high integration density and low power nonvolatile memory technology. 1,2The most studied material for ferroelectric memories, Pb[Zr x Ti 1-x ]O 3 (PZT), 1 is facing severe scaling limitations due to its complicated crystal structure which in turn hinders its applicability toward dense integration in the complementary metal oxide semiconductor (CMOS) technology. The lead (Pb) toxicity in PZT is also a snag due to ecological concerns.Recently, experimental as well as theoretical studies showed ferroelectricity in doped hafnia (HfO 2 ) thin films which are compatible with existing CMOS technology.3-10 Ferroelectric phenomenon in these doped-HfO 2 films deposited by various deposition techniques such as atomic layer deposition, 5-10 chemical solution deposition, 11 and sputtering 12 was mainly explained by the existence of noncentrosymmetric orthorhombic phase (space group of Pca2 1 ). 5,6,12 The dopant incorporation into monoclinic HfO 2 matrix results in a transformation to tetragonal/cubic phase with the appearance of an intermediate metastable orthorhombic (Pca2 1 ) phase. On the other hand, in some of the reports, ferroelectric nature in doped-HfO 2 films was attributed to the influence of "wake-up" effect. [11][12][13] In this paper, we report on the ferroelectric properties of rare earth doped Sm:HfO 2 (SHO) and Gd:HfO 2 (GHO) thin films fabricated by the sequential pulsed laser deposition (SPLD) technique. [14][15][16] The deposition was performed by periodic depositions from HfO 2 and Sm 2 O 3 /Gd 2 O 3 ceramic targets. The dopant concentration was obtained by controlling number of Sm 2 O 3 /Gd 2 O 3 ablation pulses relative to that of HfO 2 . To study the ferroelectric properties of the deposited samples, polarization-voltage (P-V), capacitance-voltage (C-V), and piezoresponse force microscopy (PFM) measurements were performed. ExperimentalPolycrystalline thin films of SHO and GHO of ∼60 nm thickness were fabricated on Pt/TiO 2 /SiO 2 /Si substrates by SPLD technique at a fixed temperature of 650• C and in an oxygen ambient partial pressure of ∼0.5 Pa. The KrF excimer laser...

Local structural investigation of hafnia-zirconia polymorphs in powders and thin films by X-ray absorption spectroscopy

“…The understandable doubts steadily reduced in the following years as reports of similar properties induced by a wealth of dopants and studies by different groups using unique fabrication methods and various electrode materials suggested some universalityat least for thin film samples. 11,12,13,14,15,16,17,18,19,20 Further "must-haves" for a ferroelectric, beyond a hysteretic curve of polarization P vs. electric field E, such as counter-clockwise switching in a ferroelectric field-effect transistor (FE-FET) 21,22 , piezo- 10,23 and pyroelectricity 24,25 , as well as the experimental proof of the presence of a polar phase 26,27 , solidified the original claim of Böscke et al 10 . The implementation of these new class of fluorite-type ferroelectrics in first devices for memory (transistor 21 and capacitor 28 based), energy harvesting and electrocaloric cooling 29 or supercapacitors 30,31 applications was even faster than the actual proof of the claimed phase behind.…”

Guidelines for the stabilization of a polar rhombohedral phase in epitaxial Hf0.5Zr0.5O2 thin films

“…While the monoclinic (P2 1 /c, m-) phase is the bulk ground state, other low-volume metastable polymorphs such as the tetragonal (t-), cubic (c-) or orthorhombic (o-) phases are responsible for the various functionalities in these materials [19,20]. These are high temperature, high pressure phases in the bulk, which can be stabilized at ambient conditions via nanostructuring [21], doping [1,10,[22][23][24][25], oxygen-vacancy engineering [26,27], thermal stresses [28,29] and epitaxial strain [22,25,[30][31][32][33][34][35][36][37], all of which can be suitably factored into thin film geometries. In particular, ferroelectric behavior results from the metastable polar phases.…”

Ferroelectricity in Lu doped HfO2 layers

“…Further "wake-up" cycles are needed to reach the maximum polarization of up to 11 lC/cm 2 and À10 lC/cm 2 , which is comparable to other PLD grown ferroelectric HfO 2 . 14,15 After 10 4 cycles, the maximum polarization is reached and does not increase with further cycling. From the measured CV curve, the relative permittivity is calculated [ Fig.…”

“…The determined P r is lower compared to other PLD grown ferroelectric HfO 2 like HfYO 2 (13 lC/cm 2 ) 14 and HfGdO 2 (12 lC/cm 2 ). 15 In the IV curve [s. Figs. 1(a)-1(c) red line], the so called "wake-up" of the ferroelectric domains is visible by the increasing current density.…”

“…(40) and PLD deposited HfGdO 2 15 (50). The reason for the lower permittivity could be an interfacial layer or a mixture of the ferroelectric phase and a low j phase.…”