Polarisation mapping by electron holography studies – POLARYS

Dielectric materials are ubiquitous in microelectronic devices because of their ability to polarize. Ferroelectric materials, which exhibit high dielectric constant and spontaneous polarisation, are under intense investigation to improve the local capacitance or to create so-called negative capacitance devices. The ability to measure polarisation locally is therefore essential to furthering understanding.
The POLARYS project aims at developing a new methodology to directly measure polarisation and charge densities in thin-layer devices by mapping the electric fields at the nanoscale. The objectives are to determine local polarisation in ferroelectric materials as a function of applied bias across layers and interfaces, and to measure the local effective dielectric constant of thin layers of dielectric materials. The key concept is to fabricate a special geometry of specimen-device incorporating a floating electrode connecting the materials of interest to a reference capacitor of nanometric dimensions and to use the capability of operando electron holography to quantitatively map the local potential when applying a bias. Studies will be performed on dedicated nanostructures combining dielectric materials and ferroelectric materials, and compared to advanced macroscopic electrical measurements.
The consortium involves three laboratories: CEMES (Toulouse), C2N (Palaiseau) and LNO-SPEC-CEA (Saclay). The project is divided into four work packages (WP):
- WP0 (Resp. C. Gatel - CEMES) concerns the coordination of the project.
- WP1 (Resp. J.-B. Moussy, LNO-SPEC) is dedicated to the growth of different dielectrics (DE) and epitaxial ferroelectric (FE) oxide layers, which will be incorporated between electrodes (M) in model and complex systems for electrical measurements and electron holography analysis in the following WPs. We will consider two systems in order to consolidate the POLARYS approach: dielectric constant matched amorphous dielectric systems M/DE1/M/DE2/M and fully epitaxial ferroelectric systems M/DE/M/FE/M. For all samples iso-thickness and iso-capacitance configurations will be grown. Following the results, systems to be elaborated could then be adapted to more complex or original investigations, such as negative capacitance.
- WP2 (Resp. S. Matzen, C2N) is focused on the advanced electrical characterization of the films and the patterned devices elaborated in WP1 to be investigated by operando electron holography in WP3. The as-grown films will be first electrically characterized at the local scale by atomic force microscopy (AFM) in order to study the local homogeneity of the electrical resistivity (AFM in conductive mode - CAFM), and using piezoresponse force microscopy (PFM) to investigate the local arrangement of the electrical polarization in ferroelectric layers. The structures elaborated in WP1 will be patterned into microcapacitors and the electrical response of the devices, including capacitance and polarization measurements, will be measured at the macroscale. The local picture and the global dielectric study of the devices will be important inputs for the TEM studies (WP3).
- WP3 (Resp. M. Hÿtch, CEMES) focuses on the local measurements of electrical properties by operando electron holography. Using the state-of-the-art methods developed at CEMES The nanodevices elaborated and macroscopically characterized in the previous WP will be prepared for electron transparency and electrically connected before being studied by operando electron holography and complementary TEM methods. The analysis of the sub-nanometric electrostatic potential maps associated to numerical simulations by finite element modeling and comparison with WP2 measurements will allow the uncertainties and limits of this new method of metrology to be evaluated.