Multifunctional Nanostructured Rare Earth-Doped HfO ₂ : Current Status and Future Perspectives

Hafnium oxide (HfO 2 ) is considered as the most promising candidate for replacement of conventional SiO 2 in complementary metal-oxide semiconductor (CMOS) technology. Owing to the appropriate high dielectric constant ( k ) of HfO 2 with excellent thermodynamic stability against silicon, it is capable of mitigating the major hurdle of high leakage current density encountered in the present CMOS industry. HfO 2 primarily exhibits three distinct phases such as monoclinic phase having k ∼ 15 stable at room temperature (RT) and tetragonal ( k ∼ 70) and cubic phases ( k ∼ 30) observed at ∼1700 and 2700 °C, respectively. The high-temperature tetragonal and cubic phases of HfO 2 are technologically more relevant due to their high k value and hence are sought after in electronic devices. For this reason, there have been systematic and extensive studies on the stabilization of cubic phase at RT in nanostructured rare earth (RE)-doped HfO 2 . Our comprehensive investigations reveal that after doping an optimum concentration of RE ions such as dysprosium (Dy) and/or samarium (Sm) into nanostructured HfO 2, it is possible to achieve the cubic phase at RT. Not only in RE-doped HfO 2 nanoparticles, the cubic phase even stabilizes at RT in thin films after doping with half of the dopant concentration. Apart from the evolution of structure, nanostructured RE ion-doped HfO 2 demonstrates captivating improvements in other physical properties like optical and magnetic properties, rendering a variety of potential applications. For example, while HfO 2 nanoparticles exhibit room-temperature ferromagnetism (RTFM), after doping even 1 atom % of Dy into HfO 2, RTFM is quenched. On the other hand, 1 atom % Dy-doped HfO 2 shows excellent luminescence in the visible region. Owing to the remarkable luminescence properties of RE-doped HfO 2, Dy and Sm codoped HfO 2 nanophosphors have been explored for latent fingerprint imaging in forensic sciences. These intriguing outcomes have motivated us to present a comprehensive review on nanostructured RE-doped HfO 2 dealing with robust multifunctional properties. Considering the widespread and consistent ongoing research work, this review provides a thorough assessment of fundamental understanding, current status, and perspectives for the near future work on nanostructured RE-doped HfO 2 that can be implemented in different promising industrial applications.