Tuning Ferroelectric Nanodomain Stability
 
Investigators: N. Balke, P. Yu, L-P. Wang, R. Ramesh
 
Ferroelectric Pb(Zr,Ti)O3 (PZT) thin films are discussed as material for new memory devices. Besides the often mentioned Ferroelectric Random Access Memories (FeRAM), probe based memory devices are also discussed as new possibility for information storage. In this case the information is stored in form of small nanodomains with a Piezoresponse Force Microscopy (PFM) tip, which is also used to read the information. PFM is a widely distributed method to characterize ferroelectric domain structures. For a high storage density the nanodomains have to be as small as possible. However, the smaller the domains the more instable they become which is the main problem for future devices. The decrease of domain size as function of time without external influences is called retention. The nanodomain stability is influenced by the preferred polarization direction of the as-grown PZT film. This direction can be influenced by the material of the bottom electrode. SrRuO3 (SRO) and La(Sr,Mn)O3 (LSMO) are typically used oxide electrodes and form different space charge regions at the Schottky contact with the PZT film and therefore leading to differently oriented internal electrical fields and as-grown polarization directions. Nanodomains switched against the as-grown polarization direction are very unstable and vanish within 100 – 300 minutes. These domains are switched with positive and negative voltage pulses applied to the PFM tip for PZT/SRO and PZT/LSMO, respectively. Nanodomains switched in the as- grown direction are stabilized by the internal electrical field and therefore stable over the same period of time. The different retention properties for differently oriented nanodomains one of the biggest problem for future memory devices.

To tune the retention properties of nanodomains in PZT, SRO and LSMO are combined as bottom electrode material. Therefore x nm LSMO was grown on 50 nm SRO before PZT was grown on top. If the thickness of LSMO is equal or below 3 nm the SRO underneath still dominated the as-grown polarization direction as SRO as single bottom electrode. A thickness of 4 nm LSMO on top of SRO is necessary to change the preferred polarization direction to the one PZT/LSMO (see Fig. 1 a). The retention properties of nanodomains switched with positive voltages are shown in Fig. 1 b, and switched with negative voltages in Fig. 1 c.

With increasing LSMO thickness the size of the switched nanodomains increases if switched with the same positive voltage pulse (Fig. 1 b). At the same time the stability of these domains is gradually increased. On the other hand, the size and stability of domains switched with negative voltage pulses is not decreasing (Fig. 1 c). This shows the tunability of retention properties for differently oriented nanodomains if SRO and LMSO are combined as bottom electrode and can help to improve the quality of probe based memory devices.
 
Fig. 1: Retention properties of nanodomains for PZT/x nm LSMO/SRO with 1 ≤ x ≤ 6. Shown are the different as-grown polarization states for x = 3 and x = 4 (a), and the retention properties of PZT/SRO (b) and PZT/LSMO (c).