7.2.1 Effect of a top electrode - X-ray study
In perspective of future dielectric measurements for which top platinum (Pt) electrodes are deposited to define capacitors, it is interesting to com-pare the domain pattern with and without the top Pt layer. To do so, we have measured the (001)θ-2θscans and the rocking-curves for as-grown films and, after fully covering their surfaces with 40-nm-thick Pt layers, we have repeated the measurements. Fig.7.10 shows XRD data for two PST films on different bottom electrodes: (a) LaNiO3and (b) Nb:SrTiO3.
Figure 7.10: XRD measurements comparing two samples with and without top Pt electrode. The left panel areθ-2θscans and right are rocking curves around the (001) PST reflection. Measurements show that the top Pt has a much stronger effect on the sample with bottom LNO electrode.
Let us first discuss the sample with the LaNiO3electrode (Fig. 7.10(a)).
We see that the top Pt layer changes the PSTc-axis and modifies the do-main configuration. In the RC, the loss of the distinct satellite peaks could
7.2 PST in a capacitor: XRD and PFM
suggest that the long range order of the domains is lost (meaning some inhomogeneities), that their average size is too large to distinguish their contribution from the main film reflection, or that the proportion of up to down domains has changed (the satellite intensity is maximum for a 50/50 ratio of up to down domains [40]). We note that when comparing the room temperature blue curve to the paraelectric state (gray curve in Fig. 7.10 taken atT > TC where no 180◦ domains are present) there is still some domain contribution. For films on Nb:STO substrates with or without a top Pt electrode, thec-axis is unchanged (Fig. 7.10(b)). The satellites in the RC with top Pt are slightly shifted toward the main peak, indicating a slightly larger domain period.
The strong difference observed between the ferroelectric samples grown on Nb:STO and LNO after the addition of the top Pt electrode reveals the role played by the bottom electrode in defining the electrostatics of these capacitors. This can be explained by the fact that since the Nb:STO offers by itself such a poor screening the top electrode has almost no influence on the polarization state. In the case of LNO, since the screening is rather good to start with but maybe not enough to allow a monodomain configu-ration, the presence of the top electrode might induce a built-in field in the system fighting against the depolarizing field and favoring a monodomain polarization with a largerc-axis. One way to test the possible presence of a built-in field (imprint) is to measure the ferroelectric response sweep-ing the electric field. To this purpose, we have measured piezoresponse hysteresis loops by AFM on samples withx= 0.8 (for those samples, the polarization is large and therefore the PFM signal is bigger).
7.2.2 Characterization of the polarization configuration using PFM
In this section we present a PFM study of the polarization configuration of the PST thin films on Nb:STO and LNO with and without a top elec-trode. The idea here is to look at the configuration of the local polar-ization, whether it is monodomain or polydomain for different applied voltages.
Without a top electrode
Fig. 7.11 shows the amplitude and the phase as a function of the voltage for the as-grown state of two samples via switching spectroscopy piezore-sponse force microscopy [117]. Fig. 7.11(a) corresponds to a 63-nm-thick
sample ofx= 0.8with bottom Nb:STO electrode and (b) 58-nm-thick of x = 0.8 and bottom LNO electrode. The measurements are local mea-surements cycled four times and are confirmed to be representative of the whole area of the sample. A change of180◦is observed in the phase at the switching coercive voltage accompanied by a drop of the amplitude. The main observation that can be drawn is that an imprint, which is defined by a shift of the center of the loop away from zero, is observed for the sample with Nb:STO. Note that the sign of the imprint has to be carefully treated and no straight comparison can be done with other type of mea-surements such as the dielectric ones presented below.3 Such conclusion is valid independently of the composition and thickness.
Figure 7.11: Switching spectroscopy piezoresponse force microscopy mea-surements. The measurements are local measurements cycled four times and are confirmed to be representative of the whole area of the sample.
A change of180◦ is observed in the phase at the switching coercive volt-age accompanied by a drop of the amplitude. (a) 63 nm thick sample of x= 0.8with bottom Nb:STO electrode and (b) 58 nm thick ofx= 0.8and bottom LNO electrode. The black dashed lines indicate the centers of the loops which are determined by statistical averaging.
With a top electrode
In order to follow what happens when the voltage is applied macroscopi-cally to a platinum (Pt) electrode, we use an AFM tip to lomacroscopi-cally trace the switching of the polarization. To avoid any electrostatic interactions
be-3The dielectric measurements are performed with a top electrode susceptible to cause changes in the resulting built-in field (amplitude and sign). Additionally, one can only com-pare data taken at the same frequency and under the same boundary conditions.
7.2 PST in a capacitor: XRD and PFM
tween the tip and the top electrode, the voltage is simultaneously applied to both [118–120]. Results are shown in Fig. 7.12 on both Nb:STO and LNO with top Pt electrode. Figs. 7.12(c) and (d) show PFM phase and amplitude images for different applied voltages. By binarizing the phase images and counting the proportion of up and down domains we recon-structed loops of down domain proportion versus applied voltage in (a) and (b).
For the Nb:STO-sample, at zero voltage, the PFM signal indicates the presence of domains, in line with the XRD results. For the LNO-sample, the PFM signal is characteristic of a monodomain behavior though XRD measurements suggest the presence of domains. It is worth mentioning that the PFM resolution through the 40-nm-Pt electrode is rather low and that therefore we might not be capturing the smaller domains.
For both samples, while the voltage is applied negatively to the top (tip and electrode), partial switching is observed with the growth of do-mains in the field direction. The sample eventually completely reverses its polarization state. Once the voltage is increased, domains of the initial polarization state reappear, until a final re-switching close to zero voltage.
For each domain a drop in the amplitude is observed at the domain wall, characteristic of180◦ferroelectric domains.
The resulting polarization loops appear to be shifted toward negative voltages. This indicated that the top Pt electrode might induce a built-in-field which modifies the as-grown domain configuration. The comparison with the loops in Fig. 7.11 is delicate since the time scales of the measure-ments are not at all comparable. However, combining these results with the XRD data allows us to confirm that the top Pt electrode induces an internal field in the films.4
4Notice that switching spectroscopy PFM measurements can be performed even for leaky samples, for which macroscopic switching measurements are not always easy or possible.
Figure 7.12: PFM measurements through a top Pt electrode. The insert in (a) is an optical microscope image of the set-up used to perform the PFM through a Pt electrode. A wire is connected to the Pt contact ensuring that the tip and the electrode are at the same voltage. Two samples with x= 0.8are measured: one 42-nm-thick on Nb:STO and one 58-nm-thick on LNO. The different color dots along the polarization loop (a) and (b) represent the voltage values at which the images shown in (c) and (d) are taken. (c) and (d) are phase (top) and amplitude (bottom) for different voltages for the two samples. When applying negative voltages, regions of opposite polarization grow until eventually full switching is achieved.
For each domain a drop in the amplitude is observed at the domain walls characteristic of180◦ferroelectric domains.