Research Projects

First Part – The project aims to develop a methodology to reproducibly stabilize the metastable ferroelectric phase in doped ZrO₂-based films using the facile, cheap and widely scalable chemical solution deposition (CSD) route on Si/SiO₂ substrate. Various growth parameters, post-growth treatments and Fe doping concentrations will be varied and their effect on the ferroelectric properties (like ferroelectric polarization and piezoelectric coefficient values) of the films on Si/SiO₂ substrate will be investigated. ZrO₂ and HfO₂ ultra-thin films have been widely used as high-k gate dielectric materials in Complementary-Metal-Oxide-Semiconductor (CMOS) device structures. The discovery of robust room-temperature ferroelectricity in ultra-thin ZrO₂/ HfO₂ films possessing a metastable orthorhombic phase with ferroelectric polarization values in the range 10-30 μC/cm² came as a big surprise to the community [T.S. Böscke et al. Appl. Phys. Lett. 99, 102903 (2011); J. Müller et al. Nano Lett. 12, 4318 (2012)]. The obtained ferroelectric polarization is already comparable or larger than that of BaTiO₃, yet it is on a binary CMOS compatible oxide. These results generated tremendous research interest in the ferroelectric materials community. However, it was subsequently realized that various aspects related to the robust ferroelectricity in these materials are unconventional and still remain poorly understood, leaving huge scope for research and development in this emergent field. Unlike traditional proper ferroelectrics, ZrO₂ and HfO₂ based ferroelectrics are improper ferroelectrics and the obtained ferroelectricity and piezoelectric responses are extremely susceptible to a number of process parameters, like extent of oxygen vacancies, doping and strain. For example, traditional ferroelectric materials possess positive piezoelectric coefficient and negative values of piezoelectric coefficients in ferroelectrics remain a rarity (only in some organic ferroelectrics) and are considered novel. Recently, it has been found that HfO₂ based ferroelectric films exhibit both signs of piezoelectric coefficients in different regions of the sample, which is extremely surprising and exotic. Further, it has been mostly considered that ferroelectricity in this fluorite structure films can only be stabilized in ultra-thin films, whereas some recent results on observation of ferroelectricity in thick films grown using CSD approaches suggest much broader phase space that still remains to be understood and explored. Among the two fluorite-based ferroelectrics, ZrO₂ is preferred over HfO₂ when considering its low thermal budget enabling easier integration into back-end-of-line processing and also since its greater natural abundance. CSD based film growth of doped ZrO2-based ferroelectric films have not been reported till date. Doping is expected to play a strong role on the phase stabilities as well as the obtained ferroelectric and sign of the piezoelectric properties, which will be investigated for the first time during this project. Second Part– We will also investigate the effect of strain on the ferroelectric and piezoelectric properties on free-stranding ZrO₂-based films placed on a flexible substrate. The later technique will involve developing CSD based approaches towards growth of ferroelectric ZrO₂-based film on Si/SiO₂ substrates using an intermediate sacrificial water-soluble layer, specifically water-soluble Ca-doped Sr₃Al2O₆ sacrificial layer. Subsequently, free-standing ZrO₂-based films will be obtained by dissolving the sacrificial intermediate film layer in water. This free-standing ZrO₂-based oxide film will then be transferred onto a flexible substrate. Ferroelectric and piezoelectric properties of ZrO₂-based particles and films in presence of strain from the flexible matrix/ substrate will be investigated for the first time and are expected to give many exciting results that can be corroborated with existing theoretical predictions.