Origin of anomalous polarization offsets in compositionally graded Pb(Zr,Ti)O-3 thin films

Ferroelectric films with composition gradients normal to the substrate have recently been reported to exhibit anomalously large polarization offsets w hen hysteresis loops are driven with an ac voltage and measured using a Saw yer-Tower (ST) circuit. These offsets have been reported in graded Pb(Zr, T i)O-3 (over 400 mu C/cm(2)) and graded (Ba, Sr)TiO3 (30 mu C/cm(2)) films. In this work, it was found that the offset observed in graded Pb(Zr, Ti)O-3 films can be attributed to development of a dc voltage across the ac-volta ge driven film, rather than a polarization offset. Recognition of this resu lt reduces these previously reported offset values by a factor of C-s/C-ref , where C-ref and C-s are the reference and sample capacitances, respective ly, used in the ST circuit. These dc-voltage offsets still represent a phen omenon which may lead to novel device applications. (C) 1999 American Insti tute of Physics. [S0003-6951(99)03002-8].