A comprehensive study on the structural evolution of HfO2 thin films doped with various dopants

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.