Since the 1940’s, phenomena in ferroelectric materials have been successfully modelled based on the Landau theory of phase transitions, which was first applied to ferroelectrics by Ginzburg and Devonshire. Phenomenological models based on this Landau–Ginzburg–Devonshire (LGD) approach have been an essential tool in understanding the basic physics of ferroelectricity. In LGD theory, a ferroelectric below its transition temperature is described by a double-well free energy landscape F as a function of the polarization P (Fig. 1a). The two degenerate energy minima define two stable spontaneous polarization states in the material, which can be reversed by the application of an electric field. By differentiating F with respect to P, one obtains the ‘S’-shaped P(–E), where E is the electric field in the ferroelectric (Fig. 1b). This ‘S’-shape of the P(-E) curve implies that in a certain region around P≈0 the ferroelectric possesses a negative differential capacitance (NC), because the capacitance C is proportional to the slope dP/dE, which is a direct consequence of the energy barrier in the double-well free energy landscape.
In theory, NC can be used to increase the capacitance of a dielectric layer by adding a ferroelectric layer and as such promises the realization of highly power efficient high-performance devices and supercapacitors (Fig. 2). NaMLab’s research in this field focusses on the verification and physical understanding of the NC effects in ferroelectric HfO2. Thereby, all the physical and electrical boundary conditions are considered, such as the influence of electrodes, additional layers in multi-layer stacks or the domain dynamics in the ferroelectric layer itself. The work performed at NaMLab includes the fabrication, physical and electrical characterization as well as modeling of ferroelectric capacitor-based devices.
The major goal of NaMLab’s research is the development and optimization of novel device concepts and operation principles based on the NC effect. Due to the strong expertise in the field of ferroelectric HfO2, NaMLab was able to directly measure the NC effect predicted by LGD theory for the first time in this material. This groundbreaking result was recently published in the prestigious journal Nature (Fig. 3), positioning NaMLab at the forefront of the international research on this topic. In a joint project with GLOBALFOUNDRIES negative capacitance transistors are being realized based on the already established FeFET integration flow. Future work will explore further applications of NC e.g. in energy storage supercapacitors.
Dr. Stefan Slesazeck