**3. Ferroelastic materials**

Stress-induced changes in the local asymmetric switching behavior and piezoelectric proper‐ ties of the sol-gel Pb(Zr0.30Ti0.70)O3 and Mn-doped Pb(Zr0.30Ti0.70)O3 ferroelectric films have been investigated by Koval *et al.*[62] A modified nanoindentation system with a conductive spherical indenter tip was used for the simultaneous application of driving voltage and mechanical loading. It was shown that the switching charge versus applied voltage (Q-V) hysteresis loops shift gradually along the voltage axis with increasing indentation force (100 – 500 mN). The effect of spherical nanoindentation on the asymmetric switching behaviour is shown in Fig. 10a, which compares the Q-V loops obtained at different indentation loads for the Mn-doped PZT (PMZT) film. In addition, a progressive hysteresis gap of the charge – voltage loops is displayed in the figure inset. The parameter of horizontal loop asymmetry *δ* (Fig. 10b) was found to increase almost linearly with the force by an increment of about 0.4-0.5 x10-3 per 100 mN during the application of a sinusoidal signal of 25 V-, 37 V- and 50 V-peak

l asymmetric s Mn-doped Pb(Z ed nanoindentati lication of drivi pplied voltage ( ation force (100 ehaviour is show s for the Mn-do ge loops is displ found to increa uring the applica quency.

switching behav Zr0.30Ti0.70)O3 fe ion system with ng voltage and (Q-V) hysteresis 0 – 500 mN). T wn in Fig. 10a, w ped PZT (PMZ ayed in the figur se almost linearl ation of a sinusoi

vior and piezoe erroelectric film a conductive sp mechanical load s loops shift gra The effect of sp which compares T) film. In addi re inset. The par rly with the force idal signal of 25

electric ms have pherical ding. It adually pherical the Qition, a rameter e by an V-, 37

teresis loop of th rom Koval *et al*

he Mn*l*. [62].

bute to ent and omains hanced e space Koval *et*  nt upon versible al and hat the librium

nce and an *et al.* T-based ation of via the

rallel and contrib dipole realignme ng effect on do results in the enh may lead to more domain walls. K ectric coefficien -enhanced irrev h the theoretic ] have shown th d from its equil

ility, performan force. Gruverma 1)-oriented PZT that the applica nent of the film

of the Q-V hyst h permission fr

at may act in par ops: i) defect-d of the clampin ter mechanism r nd potentially m fect-stabilized d ffective piezoele it to the stressconsistent with Emelyanov [66] n wall is shifted

cate that reliabi by mechanical f rinting of (111 they observed t arization compon

*l*.

3] and He *et a* ishing LLC.

ges in the local 0.30Ti0.70)O3 and *l.*[62] A modifie multaneous app charge versus ap creasing indenta etric switching b ndentation loads e charge – voltag (Fig. 10b) was 3 per 100 mN du s and 50 Hz freq

drive voltages and 50 Hz frequency.

out 0.4-0.5 x10-3 ak drive voltages

voltage shift an n film under na , AIP Publishing

nd (b) the asymm anoindentation. R

metry parameter Reproduced with

**Figure 10.** (a) The voltage shift and (b) the asymmetry parameter of the Q-V hysteresis loop of the Mn-doped PZT thin film under nanoindentation. Reproduced with permission from Koval *et al*. [62]. Copyright 2005, AIP Publishing LLC.

These results are interpreted in Ref. 62 by two mechanisms that may act in parallel and contribute to the indentation-driven voltage shift of the Q-V hysteresis loops: i) defect-dipole realignment and concurrent space charge rearrangement, and ii) reduction of the clamping effect on domains controlled by a variation of the internal residual stress. The later mechanism results in the enhanced polarization state of the film, the so-called interfacial poling, and potentially may lead to more space charge trapping at the bottom electrode due to liberation of defect-stabilized domain walls. Koval *et al.* in their early work [63] reported on an increase of the effective piezoelectric coefficient upon nanoindentation in the PZT film capacitors and attributed it to the stress-enhanced irreversible movement of ferroelastic domain walls. This scenario is consistent with the theoretical and experimental works of other researchers.[64, 65] Pertsev and Emelyanov [66] have shown that the residual stresses can change significantly

mechanisms tha V hysteresis loo ii) reduction al stress. The lat rfacial poling, an liberation of de crease of the ef and attributed i his scenario is 5] Pertsev and en a 900

domain

thin films indic ficantly affected oling and impr roscopy (PFM) the in-plane pola

Ref. 62 by two shift of the Q-V rangement, and internal residua he so-called inter electrode due to ported on an inc ilm capacitors main walls. Th esearchers.[64,65 gnificantly whe

in ferroelectric ors can be signifi tress-induced p onse force micr ess can change t

15

 domain wall is shifted from its equilibrium position by an external field. Stress-driven effects in ferroelectric thin films indicate that reliability, performance and lifetime of ferroelectric capacitors can be significantly affected by mechanical force. Gruverman *et al.*

g LLC.

e interpreted in -driven voltage ce charge rearr variation of the te of the film, th at the bottom e y work [63] rep n in the PZT fi ferroelastic dom orks of other re s can change si xternal field. -driven effects electric capacito orted on the st using piezorespo ive or tensile str

p

permission from [50]. Copyright

properties of th been investigate indenter tip was was shown that along the voltag nanoindentation V loops obtaine progressive hyst of horizontal loo increment of abo V- and 50 V-pea

Stress-

m D. Hesse [53 2005, AIP Publi

222 Ferroelectric Materials – Synthesis and Characterization


p b i w a n V p o i V

F d C

Fig. 10. (a) The doped PZT thin Copyright 2005,

These results ar the indentationconcurrent spac controlled by a polarization stat charge trapping *al.* in their early nanoindentation movement of f experimental wo residual stresses position by an ex

Stress

lifetime of ferro [67] have repo capacitors. By u either compressi

when a 900

T t c c p c *a* n m e r p

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