Giant Electromechanical Response from Defective Non-Ferroelectric Epitaxial BaTiO₃ Integrated on Si (100)

Recently giant electromechanical (EM) responses have been observed in several defective systems due to the formation of site deficiency which is obtained either by creating geometrical or compositional modification. The idea of obtaining a giant EM response in the defective system gained tremendous attention owing to the observation of<i> d</i>₃₃ value of ~200,000 pm/V even in centrosymmetric gadolinium-doped CeO_2−<i>x</i> films at milli Hz frequency owing to the mass migration of point defects under application of electric field leading to a local structural change from a cubic to tetragonal. Since large range motion of ions results in large EM response in Gd:CeO₂, its EM response is confined to very low frequency (~milli Hz). This response degrades to 25000 pm/V at 100 Hz and beyond that the response is almost zero, with different defect-induced mechanisms at play in different frequencies. <u>Here we demonstrate silicon compatible Barium Titanate (BTO) system that shows an equivalent large electromechanical response at much higher frequencies (&gt;1 kHz)</u>. These films are Barium and oxygen-deficient nature as revealed from XPS and TEM studies. X-ray Diffraction shows that the thermal expansion coefficient of these defective films is 3×10^-5 (K^-1) one of the highest reported. We measure out-of-plane displacement while applying in-plane field using Laser Doppler Vibrometry (LDV) and <u>report highest second order effective M₃₁ (of the order 10^-14 (m/V)²)for any material at high frequencies (&gt;1000 Hz)</u>. We further performed small signal and large signal impedance measurements and correlate them with our structural data from TEM and XRD. Through simulations, we assess the effects of joule heating in mechanical displacements and attribute the rest to defect-based electro-strictive behavior.