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) have shown ferroelectricity (FE) attributed to the orthorhombic (o) phase of HfO with space group 2. 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 could be stabilized in Gd-doped bulk polycrystalline HfO. Rietveld analysis of XRD data shows that the relative population of o-phases in the presence of the monoclinic (m) phase of HfO 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. Stabilization of Gd in both and 2 phases of HfO was supported by defect formation energy calculations using density functional theory (DFT). The present study has important implications in future applications of HfO in ferroelectric devices and in understanding the role of dopants in stabilizing the o-phase of HfO in the bulk.