Studying electric fields in devices at the nanoscale by operando electron holography

Work package:
Summary: 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. Through a new methodology to directly measure polarisation and charge densities in thin-layer devices by mapping the electric fields at the nanoscale, the objectives of this internship are to determine local polarisation in dielectric and ferroelectric materials as a function of applied bias across layers and interfaces. Operando studies using in situ biasing electron holography will be performed on dedicated nanostructures combining dielectric materials and ferroelectric materials, and compared to advanced macroscopic electrical measurements. Numerical modeling of the systems with finite element method (FEM) is mandatory to correctly interpret the experimental measurements. Subject : Dielectric and ferroelectric materials are widely used in microelectronic devices because of their ability to polarize in response to an applied electric field. These materials have spurred much fundamental and applied research over the years. Materials with high dielectric constants, so-called high-K dielectrics, were of special interest to replace silicon dioxide in microelectronic devices when the benefits of miniaturisation began to fade [1]. Very recently, ferroelectric materials which have a spontaneous polarisation even in the absence of an applied field, have excited tremendous interest to produce devices exploiting negative capacitance [2–4] and to offer a solution to power-dissipation and overheating occurring in transistor miniaturisation [5]. The local measurement of the polarisation in dielectric and ferroelectric materials is therefore essential to furthering understanding and to develop a new generation of devices. Whilst ferroelectric and dielectric properties can be readily measured macroscopically using electrical methods, the task is difficult at the nanoscale and for thin-film materials. Cross-sectional samples, required to visualize phenomena across a device or sequence of layers, are the domain of transmission electron microscopy (TEM) which enables measurements with a very high spatial resolution. TEM techniques currently rely on structural analysis to identify changes in polarisation. At the atomic-scale for example, the displacements of atoms within the lattice are analyzed in detail to deduce the local polarisation [6-8]. Measurements are usually carried out on unbiased samples, relaxed from a previous polarisation state with uncertain history. What is lacking is a local and direct measurement of the polarisation whilst applying bias. Electron holography is a powerful TEM technique for measuring local fields in materials, from electric and magnetic [9] to crystalline strain [10]. Indeed, the phase of the electron hologram can be directly related to the electrostatic potential encountered by the fast electron along its trajectory. Whilst it was shown early on that electric fields could be measured in semiconductor devices in such way [11], the development of operando experiments has however been a long one. Rare have been the studies of biased devices [12.13] due to bottlenecks to solve such as sample preparation, surface damage layers, stray fields, electron radiation and low signal-to-noise ratio. For several years the I3EM team of CEMES has developed state-of-the-art expertise on these different problems and has demonstrated that an electric field can now be measured in different nanodevices using in situ biasing electron holography with unprecedented sensitivity [14.15]. Our developments are now mature enough to propose a new methodology required to measure electrical polarisation at the nanoscale. Work The internship will participate at developing the direct measurement of polarisation and charge densities in thin-layer devices by mapping the electric fields at the nanoscale. Studies will be performed using operando electron holography on dedicated nanostructures combining dielectric materials and ferroelectric materials. The TEM experiments will be accompanied by modeling to understand the influence of experimental parameters and construct a realistic model of the nano-object under observation. Depending on the candidate, the internship can either be focused on the experimental or modelling aspects. Collaboration: the study is in collaboration with C2N (Centre de Nanosciences et de Nanotechnologies) , LNO-SPEC at CEA-Saclay, and Freie Universität Berlin.