Piezoelectric thin films investigated by dark‐field electron holography and in‐situ biasing
Authors: Denneulin Thibaud , Wollschläger Nicole , Hÿtch Martin 
Published: December 2016

Ferroelectric and piezoelectric oxides have a large number of applications in memories and micro‐electro‐mechanical systems (MEMS). Recently, a piezoelectronic transistor (PET) has been proposed as an alternative to the conventional metal‐oxide‐semiconductor (CMOS) transistor [1]. Therefore, there is a growing need for the characterisation of piezoelectric thin films at the nanoscale.

We investigated Pb(Zr,Ti)O
3
(PZT) and Pb(Mg
1/3
Nb
2/3
)O
3
−PbTiO
3
(PMN‐PT) epitaxial thin films containing
c‐
and
a
‐domains (or 90° domains) using transmission electron microscopy (TEM) techniques. Experiments were carried out using the I2TEM‐Toulouse microscope (Hitachi HF‐3300) equipped with a cold field emission gun, an image corrector (B‐COR) and multiple electron biprisms. Firstly, dark‐field electron holography [2] was carried out to map the residual deformations due to the epitaxial constraints. Secondly, in‐situ biasing was conducted using a picoindenter PI 95 (Hysitron) to observe directly the switching of ferroelectric domains.

We found that the residual deformation and rotation fields depend on the inclination of neighbouring
a
‐domains. Fig. 1(a) shows an example of dark‐field electron hologram obtained on a tetragonal Pb(Zr
0.2
,Ti
0.8
)O
3
thin film grown on SrTiO
3
. The investigated region contains two main
a
‐domains with opposite inclination, forming a hat shape. The rotation map in Fig. 1(b), retrieved from the hologram, reveals two rotation gradients in the direction perpendicular to the
a
‐domains, increasing towards the interface. The rotation field is guided by partial
a
‐domains located in the central region. These gradients are related to the lattice mismatch at the domain walls and the growth constraints onto the substrate [3].

Fig. 2(a) shows an in‐situ biasing experiment conducted on a Pb(Zr
0.2
,Ti
0.8
)O
3
thin film using a diamond probe (doped) placed in contact with the surface of the film. The SrTiO
3
substrate (Nb doped) is connected to the ground. The application of the electric field favours the formation of
c
‐domains (vertical polarisation) at the expense of the two
a
‐domains (horizontal polarisation) located just below the probe. When applying ‐15V (Fig. 2(b)), the
a
‐domains become narrower and then they are completely erased at ‐25V (Fig. 2(c)). New
a
‐domains are created on the left side of the probe which can be due to a redistribution of the deformation or a deviation of the electric field from the vertical direction (stronger in‐plane component) in this region.

Finally, Fig. 3(a) shows an experiment conducted on the same film but with a gold top electrode. The region showed here contains two
a
‐domains indicated by dashed lines. When applying +10V (Fig. 3(b)), it was observed that
c/c
domain walls move laterally through the film until they form close pairs. The two domain walls on the right side moved quickly towards each other and finally disappeared (Fig. 3(c)). The
c/c
domain walls on the left side were slower (still exist after 60s in Fig. 3(c)) which might be related to defects or pre‐existing domain walls.