Thin films and nanostructures

Prof. Dr. Michael Huth

Goethe University, Frankfurt am Main

Ferroelectric organic charge transfer systems

Unit cell of TTF-CA
Fig.1: Crystallographic unit cell of the organic ferroelectric TTF-CA. C: grey, S: yellow, O: red, Cl: green.

It is thin films and, in particular, their interfaces that define the electronic functionality of devices. The ferroelectric (FE) state is very attractive in this regard, due to the possibility to switch the electric polarization by an electric field, which can be used in FE random access memories or field effect transistors. The paraelectric (PE) to FE phase transition at the Curie temperature TC is equally important in basic research, since it is one of the most representative phase transitions in solids.

An important question in analyzing the FE state close to interfaces is: how can the influence of thin-film specific effects, such as clamping and interfacial charge transfer, be monitored, ideally in a way that does not interfere with the FE state itself?

We have introduced a novel concept for monitoring the FE state in thin films with large sensitivity to the near-interface region. We have shown that strong modulations in the electrical conductance of nano-granular metals occur, if this is in close proximity to a FE thin film. For our experiments we have used the organic FE tetrathiafulvalene-p-chloranil (TTF-CA), whose crystal structure is shown in Fig. 1.

We demonstrated the monitoring effect for a bilayer thin film structure (see schematic in upper left of Fig. 2) consisting of the organic FE and a Pt-based nano-granular metal (Pt(C)) prepared by focused electron beam induced deposition

Conductance modulation of
                    ferroelectric-granular metal heterostructure
Fig.2: Simulated modulation of electrical conductivity of a nano-granular metal at the ferroelectric phase transition of TTF-CA which is part of the heterostructure indicated in the upper left.
In Fig. 2 we show results of a model calculation of the temperature-dependent conductance of a nano-granular metal within a three-layer structure (see schematic). The temperature dependence of the dielectric constant of the TTF-CA top layer is assumed to follow a Lorentzian with a half-width at half maximum of 2 K and peak values of 500. The dashed line indicates TC which is set to 56 K for the TTF-CA thin film. Blue line: Pt(C) thickness t is 20 nm, Pt grain diameter D is 1.5nm. Green line: Pt(C) t = 20nm, D = 3.0nm. Red line: t = 10nm, D = 3.0 nm. The dashed line represents the calculated Pt(C) conductance without TTF-CA top layer. Inset: Coulomb chargig energy EC for spherical Pt particle with D = 3 nm diameter at different positions below the TTF-CA top layer, as indicated. The different curves were calculated for different dielectric constants as indicated.