An Overview of Ferroelectric Hafnia and Epitaxial Growth

Abstract

Hafnia thin films have been under intensive research during the past few years due to its robust ferroelectricity under very thin limit and good compatibility with silicon. The polar crystal structure critical to ferroelectricity in hafnia thin films is metastable, and is generally obtained in polycrystalline thin films, coexisting with other nonpolar phases. Recently, much attention has been focused on epitaxial ferroelectric hafnia thin films to get rid of the nonpolar phases, to investigate the more intrinsic factors to ferroelectricity, and its potential applications. Herein, recent progress on the growth of epitaxial hafnia thin films is reviewed. The epitaxial growth mechanism is explored, in particular, the interface matching, phase stability under temperature and oxygen pressure, followed by discussions on thickness dependency of ferroelectricity, and wake-up effect in hafnia. Finally, an outlook on ferroelectric hafnia both on fundamental studies and future applications is discussed.

Improved polarization and endurance in ferroelectric Hf0.5Zr0.5O2 films on SrTiO3(110)

“…The remanent polarization, P r = 33 μC cm −2 , is much higher than the usual values reported for HZO. 2,3 Remarkably, P r is 50% higher than the equivalent epitaxial film grown simultaneously on STO(001). The larger polarization is likely the consequence of the higher amount of the orthorhombic phase in the film on LSMO/STO(110).…”

“…Epitaxial films are more convenient than polycrystalline samples to advance towards this knowledge. [2][3][4][5][6][7] Significant progress has recently been made by studying epitaxial films of doped HfO2 on La0.67Sr0.33MnO3 (LSMO) buffered (001)-oriented SrTiO3 (STO) substrates. Relevant findings reported with epitaxial films include: demonstration of high polarization, endurance and retention in films less than 5 nm; 8 proof of ferroelastic domain switching; 9 demonstration of thickness (t) dependence on the coercive electric field (EC) according to EC -t -2/3 scaling; 8,[10][11][12] estimation of the Curie temperature by X-ray diffraction measurements; 13 control of the relative amount of ferroelectric and paraelectric phases by substrate selection; 14 and improvement of the endurance in presence of parasitic monoclinic phase.…”

Epitaxial ferroelectric interfacial devices

“…However, a growing number of studies have reported the growth of epitaxial ferroelectric HfO 2 films. [387][388][389][390][391][392][393][394][395][396][397][398] One example is the epitaxial growth of 15-20 nm YO 1.5 -substituted HfO 2 films by pulsed laser deposition on [100]-oriented yttrium oxide-stabilized zirconium oxide (YSZ) substrates, 387 and on YSZ(110) single crystals using Sn-doped In 2 O 3 (ITO) as a bottom electrode, 389 where large ferroelectric polarizations and an estimated critical temperature of 723 K were observed. A cross-sectional annular bright field STEM image of a ca.…”

Intrinsic ferroelectricity in Y-doped HfO2 thin films

“…The calculated ferroelectric polarization of the orthorhombic Pca21 phase of HfO2 is about 50.2 µC/cm 2 , which is consistent with the previous theoretical studies. [10][11][12][13][14] The polarization is directed along the c-axis, as enforced by the symmetry of the crystal, so that the a-and bcomponents of polarization are zero. The 5% Y doping slightly reduces the polarization down to about 49.9 µC/cm 2 , which indicates that Y does not play a decisive intrinsic role in high polarization values observed in our experiments, but rather helps to stabilize the orthorhombic Pca21 phase of hafnia.…”

“…The small crystal grain sizes lead to proliferation of structural defects (low crystallinity) in the form of grain boundaries, which are expected to undermine and obscure ferroelectric properties. 9 Indeed, it has been challenging to elucidate the crystal structure of the ferroelectric o-phase; to date, most of the experimentally observed spontaneous polarization values are significantly lower than the theoretically predicted ones (40-60 µC/cm 2 ) [10][11][12][13][14] .…”