Operando physics of novel ferroelectric devices by soft-X-ray spectroscopy

Progress of the electronic devices based on the conventional Si-based technology is presently slowing down, calling for alternative device concepts. One of them is heterostructures embedding a ferroelectric (FE) layer such as doped HfO2. Using FE-HfO2 as a gate dielectric in field-effect transistors, one can realize multifunctional  devices combining the conventional field effect on the channel conductivity (logic functionality) with the effect of FE-polarization reversal (non-volatile memory functionality). The channel materials may vary from the conventional Si and A3B5 semiconductors (SCs) to transition-metal oxides (TMOs), where FE polarization can affect the subtle balance between the spin, charge, orbital and lattice degrees of freedom and thus the device functionality. For example, a recent study on the BaTiO3/La1-xSrxMnO3 multiferroic interface has revealed a dramatic FE effect on the polaronic coupling and thus mobility of the interfacial charge carriers. However, the fundamental physics of FE interfaces still remains far from detailed understanding.

We propose a collaborative research between teams from the Swiss Light Source (SLS), Paul Scherrer Institute, Switzerland and Moscow Institute of Physics and Technology (MIPT), Russia aiming at fundamental physics of novel FE-based electronic devices. Specifically, we will perform operando spectroscopic investigations of FE/SC heterostructures (HfO2/Si and HfO2/GaAs) and FE/TMO heterostructures (HfO2/La1-xSrxMnO3 and HfO2/CaMnO3) under variation of the FE-polarization. The scientific exchange between the Swiss and Russian scientists will be fostered by a joint workshop in Moscow.

The samples embedding FE-HfO2 layers will be fabricated at MIPT using a combination of atomic layer deposition and pulsed laser deposition techniques. Both polycrystalline and heteroepitaxial samples will be grown. The multilayered structures will be optimized and patterned in prototype devices to enable operando measurements at ADRESS beamline. MIPT team holds unique expertise in the operando synchrotron studies of electronic and structural vs. functional properties of prototype electronic devices.

The operando spectroscopic experiments at SLS will aim the electron-momentum-resolved electronic structure of the FE/SC and FE/oxide heterostructures, underlying all their physical properties. We will use angle-resolved photoelectron spectroscopy (ARPES) with synchrotron radiation in the soft-X-ray photon energy range ~1 keV, where the photoelectron escape depth becomes sufficient to probe electron states in the channel through the few-nm thick FE layer. These experiments will use the ADRESS beamline of SLS, which is presently the most advanced soft-X-ray ARPES facility worldwide in terms of energy resolution and photon flux. Its unique scientific potential has been demonstrated by discoveries of polaronic charge carriers at LaAlO3/SrTiO3 the paradigm oxide interface, electronic-structure anisotropy in GaN-based high-electron-mobility transistors, magnetic impurity states in the diluted magnetic semiconductor Ga(Mn)As, etc.

We envisage that our project will deliver FE-polarization dependent (1) concentration and distribution of electrons/holes between different energy bands, defining their effective masses; (2) polaronic coupling and the corresponding effective-mass renormalization; (3) shape of the interfacial potential barrier. Whereas the first two properties define the mobility of the charge carriers, the third one defines their spatial localization. This fundamental information about FE heterostructures is essential development of novel multifunctional electronic devices.


Paul Scherrer Institute:

Moscow Institute of Physics and Technology: