La -Doped Hafnia Thickfims for Cryogenic Cooling

Cryogenic operation of devices is one of the essential requirement in the field of quantum technologies. Electrocaloric (EC) materials are promising as silent solid-state alternates to conventional refrigeration technologies. EC materials are generally Pb based materials, and work at their maximum efficiency only in a limited temperature range near the room temperature. The Pb-based ferroelectrics and polymers are reported to have large pyroelectric coefficients and output power while working in the vicinity of the phase transition temperature, however, struggle to generate voltage threshold (with thin films of nanometer scale) suitable for power electronics where thousands of volts are required. Thin film geometries offer a potential solution to this issue, as they can provide large, continuous power output and fast thermal cycling.<br/>Here we explore the efficacy of unconventionally ferroelectric Hafnia systems which are promising candidates for nanoelectronics due to their CMOS compatibility, robust ferroelectricity at nano scale thicknesses, low dielectric constant, large bandgap, simple chemical structure and less toxicity. The ferroelectricity in hafnia is originating from orthorhombic crystal structure which is stabilised by means of various parameters like dopants, temperature, electrodes, oxygen vacancies etc. Achieving ferroelectricity in Hafnia below 5nm and above 30 nm is quite challenging as the depolarization field effect becomes prominent while scaling down and the non -polar monoclinic phase accompanies the orthorhombic phase while scaling up.<br/>Our study presents the effect of dopant and electrode on achieving robust ferroelectricity in hafnia thick films prepared by solution deposition method. Defect based systems like hafnia generally requires thousands of wake-up cycles to produce the ferroelectricity which is a practical issue while integrating in to nanoelectronics. The solution processed- La doped hafnia thick films shows ferroelectricity with out the wake-up effect and the polarization values are stable till 10⁹ cycles. Surprisingly hafnia thick film shows a phase transition from ferroelectric to non- ferroelectric phase around 140K which is evident from the temperature dependent P-E loops and C-V loops. The pyroelectric properties and electrocaloric cooling effects in the system are well studied correlating with the electrode induced phase transition in hafnia. The electrocaloric cooling effect in hafnia is significant with ΔT=-2.2 (K) and offers new avenue to built cryogenic coolers at low cost.