Synthesis and characterization of HfO2 thin films doped with Zr, Sr and Ca, based on low toxicity chemical solutions for piezoelectric applications

The recovery of residual vibrational energy (from cars, trains, bridges and even from the human body) to power microsensor and microdevices for everyday life and at large scale is still a futuristic concept that could become a reality with piezoelectric materials. Lead zirconate titatane PZT systems (Pb(ZrxTi1-x)O3) have produced large piezoelectric response and they are currently used in daily life technology. Unfortunately, lead is highly toxic. Interestingly, in 2011 it was found that piezoelectric response can be produced in hafnium dioxide (HfO2), an insulating material, with the introduction of dopants to stabilize an unexpected orthorhombic phase.

Physical techniques such as sputtering, atomic layer (ALD) and pulsed laser (PLD) deposition have produced highly piezoelectric doped-HfO2 thin films with Pr up to 45 µC/cm2. Therefore, doped-HfO2 has been regarded as a potential replacement of PZT. However, physical deposition techniques have some disadvantages such as high operating cost and toxic precursors. Chemical solution deposition methods have also produced piezoelectric doped-HfO2, although with lower Pr, but with promise to be applied on large surfaces at a much lower cost. Chemical processes still need to be optimized, reduced in toxicity, and scaled up.

Our current research focuses on the incorporation of Zr, Sr or Ca as dopants of HfO2. These elements are relatively abundant, and like Hf, they do not present risks for human health or for the environment. The synthesis is done by chemical routes and simple deposition methods, thus reducing the need for high-cost equipment. The purpose is to fabricate films with high piezoelectric response, of easy scalability, and with low-toxicity raw materials.