Effects of growth orientations and epitaxial strains on phase stability of HfO2 thin films

Abstract

The discovery of ferroelectricity in both pure and doped HfO 2 -based thin films have revitalized the interest in using ferroelectrics for nanoscale device applications. To take advantage of this silicon-compatible ferroelectric, fundamental questions such as the origin of ferroelectricity and better approach to controlled realization of ferroelectricity at the nanoscale need to be addressed. The emergence of robust polarization in HfO 2based thin films is considered as the cumulative effect of various extrinsic factors such as finite-size effects and surface/interface effects of small grains, compressive stress, dopants, oxygen vacancies, and electric fields. The kinetic effects of phase transitions P bca, become thermodynamically stable in (111)-oriented thin films over a wide range of epitaxial strain conditions. This work suggests a potential avenue to better stabilize the ferroelectric phase in HfO 2 thin films through substrate orientation engineering.

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

“…-1 with respect to the ground state) 39 39 and the films grown on Si(111) in the current work satisfy these conditions, and thus exhibit a polar r-phase (and not the low-energy o-phase). These observations, however, are quite contradictory to the theoretical predictions of Liu and Hanrahan 22 , possibly owing to the absence of the r-phases in their calculations. It must also be noted that films grown in (001) orientation in this work, show a preference for the o-phase, and not the r-phase.…”

“…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 .…”

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.…”

Interplay between oxygen defects and dopants: effect on structure and performance of HfO2-based ferroelectrics

“…In this picture of phase transitions without the creation of a phase boundary, the transition rate would depend on the barrier height from the initial to the final phase. The energy landscape for the transition between the relevant phases and the transition states characterizing the height of the barriers have been calculated with and without 38 the inclusion of oxygen defects. 16 The results indicate that oxygen vacancies do not affect the height of the barrier significantly, but oxygen interstitial lowers the barrier of the metastable tetragonal phase or even completely removes this barrier towards the m-phase.…”