Depolarization-Field-Induced Instability in Thin Ferroelectric Films—Experiment and Theory

The existence of depolarization fields in thin ferroelectric films is experimentally demonstrated by investigating an unconventional electrode-ferroelectric configuration consisting of a triglycine sulphate (TGS) film sandwiched between a gold and a doped-silicon electrode. Due to nonidentical electrodes, the compensating-charge distribution for opposite polarization directions is asymmetric and consequently depolarization fields cannot be sufficiently reduced by domain formation. These depolarization fields manifest themselves by reducing the intrinsic polarization in thin ferroelectric films with respect to the bulk value and this aspect is experimentally confirmed. For a sufficiently thin film, depolarization fields become too strong and a polarization instability is observed. These effects are simulated by altering the compensation-charge extension in the silicon electrode by photoillumination. Other causes, like impurities, and structural defects, which can lead to reduction of polarization, are ruled out. Theoretical model calculations including the depolarization field are reported. The calculation predicts an intrinsic polarization reduction in thin ferroelectric films and is in good agreement with experiments reported here.