Pyroelectric response of ferroelectric thin films

Abstract

A thermodynamic formalism is developed to calculate the pyroelectric coefficients of epitaxial (001) Ba0.6Sr0.4TiO3 (BST 60/40) and Pb0.5Zr0.5O3 (PZT 50/50) thin films on (001) LaAlO3, 0.29 LaAlO3:0.35(Sr2TaAlO6) (LSAT), MgO, Si, and SrTiO3 substrates as a function of film thickness by taking into account the formation of misfit dislocations at the growth temperature. The role of internal stress is discussed in detail with respect to epitaxy-induced misfit and thermal stresses arising from the difference between the thermal expansion coefficients of the film and the substrates. It is shown that the pyroelectric coefficients steadily increase with increasing film thickness for BST 60/40 and PZT 50/50 on LSAT and SrTiO3 substrates due to stress relaxation by misfit dislocations. Large pyroelectric responses (∼1.1 μC/cm2 K for BST 60/40 and ∼0.3 μC/cm2 K for PZT 50/50) are theoretically predicted for films on MgO substrates at critical film thicknesses (∼52 nm for BST 60/40 and ∼36 nm for PZT 50/50) corresponding to the ferroelectric to paraelectric phase transformation. Analysis shows that the pyroelectric coefficients of both BST 60/60 and PZT 50/50 epitaxial films on Si substrates are an order of magnitude smaller than corresponding films on LaAlO3, LSAT, MgO, and SrTiO3 substrates.