Comprehensive study on the origin of orthorhombic phase stabilization in Gd-doped HfO and DFT calculations

In recent times, ultra-thin films of hafnium oxide (HfO 2 ) have shown ferroelectricity (FE) attributed to the orthorhombic (o) phase of HfO 2 with space group Pca 2 1 . This polar o-phase could be stabilized in the doped thin film of the oxide. In the present work, both polar and non-polar o-phases of HfO 2 could be stabilized in Gd-doped bulk polycrystalline HfO 2 . Rietveld analysis of XRD data shows that the relative population of o-phases in the presence of the monoclinic (m) phase of HfO 2 increases with increasing Gd-content. The local environment around the host atom has been investigated by time differential perturbed angular correlation (TDPAC) spectroscopy, synchrotron based X-ray near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) measurements showed a reduction in grain size with increasing Gd-dopant indicating a solute drag effect. It could be established that the segregation of the Gd-dopant in the grain boundary is a thermodynamically favorable process and the solute drag effect plays an important role in nucleation of the o-phase in bulk HfO 2 . Stabilization of Gd in both Pbca and Pca 2 1 phases of HfO 2 was supported by defect formation energy calculations using density functional theory (DFT). The present study has important implications in future applications of HfO 2 in ferroelectric devices and in understanding the role of dopants in stabilizing the o-phase of HfO 2 in the bulk. The grain size of HfO 2 is reduced from ∼50 nm to ∼25 nm by a Gd-dopant (1-5 at%) with concomitant stabilization of the orthorhombic phase over the monoclinic phase.