Abstract
The origin of the unexpected ferroelectricity in doped HfO2 thin films is now considered to be the formation of a non-centrosymmetric Pca21 orthorhombic phase. Due to the polycrystalline nature of the films as well as their extremely small thickness (∼10 nm) and mixed orientation and phase composition, structural analysis of doped HfO2 thin films remains a challenging task. As a further complication, the structural similarities of the orthorhombic and tetragonal phase are difficult to distinguish by typical structural analysis techniques such as X-ray diffraction. To resolve this issue, the changes in the grazing incidence X-ray diffraction (GIXRD) patterns of HfO2 films doped with Si, Al, and Gd are systematically examined. For all dopants, the shift of o111/t101 diffraction peak is observed with increasing atomic layer deposition (ALD) cycle ratio, and this shift is thought to originate from the orthorhombic to P42/nmc tetragonal phase transition with decreasing aspect ratio (2a/(b + c) for orthorhombic and c/a for the tetragonal phase). For quantitative phase analysis, Rietveld refinement is applied to the GIXRD patterns. A progressive phase transition from P21/c monoclinic to orthorhombic to tetragonal is confirmed for all dopants, and a strong relationship between orthorhombic phase fraction and remanent polarization value is uniquely demonstrated. The concentration range for the ferroelectric properties was the narrowest for the Si-doped HfO2 films. The dopant size is believed to strongly affect the concentration range for the ferroelectric phase stabilization, since small dopants can strongly decrease the free energy of the tetragonal phase due to their shorter metal-oxygen bonds.
| Original language | English |
|---|---|
| Pages (from-to) | 4677-4690 |
| Number of pages | 14 |
| Journal | Journal of Materials Chemistry C |
| Volume | 5 |
| Issue number | 19 |
| DOIs | |
| State | Published - 2017 |
Funding
Authors gratefully acknowledge funding from the Army Research Office through contract number W911NF-15-1-0593. M. H. P. is supported by DRESDEN Junior Fellowship. The DRESDEN Fellowship Program of the TU Dresden is part of its Institutional Strategy, funded by the Excellence Initiative of the German Federal and State Governments. T. S., U. S. and T. M. gratefully acknowledge the German Research Foundation (Deutsche Forschungsgemeinschaft) for funding part of this research in the frame of the "Inferox" project (MI 1247/11-2). E. D. G. and J. M. L. gratefully acknowledge funding from the National Science Foundation (Award No. DMR-1350273). E. D. G. acknowledges support for this work through a National Science Foundation Graduate Research Fellowship (Grant DGE-1252376). This work was performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (award number ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI).