Influence of trapped charges on the electric field distribution within nanocapacitors.

10 janvier 2025

Operando off-axis electron holography was used to quantify the charges trapped at the dielectric/dielectric interfaces and metal/dielectric interfaces in HfO₂– and Al₂O₃-based nanocapacitors. By mapping the electrostatic potential at sub-nanometer spatial resolution while applying a bias, we demonstrate that these interfaces present a high density of trapped charges, which strongly influences the electric field distribution within the device.

Interfaces in heterostructures play a major role in the functionality of electronic devices. Phenomena such as charge trapping/detrapping at interfaces under electric field affect the dynamics of metal/oxide/metal capacitors and metal/oxide/semiconductor transistors used for memory and logic applications. However, electric-field induced charging phenomena remain unclear even in conventional dielectric heterostructures due to a lack of direct measurement methods.

CEMES researchers have studied twin trilayer nanocapacitors, HfO2/Al2O3/HfO2 and Al2O3/HfO2/Al2O3 with TiN electrodes using operando electron holography experiments. Thanks to a setup and a methodology improving the sensitivity, associated with a high spatial resolution and in combination with FEM simulation, they evidenced the presence of negative or positive trapped charges at different interfaces (dielectric in contact with a metal and dielectric in contact with a dielectric) upon biasing and further quantitatively clarify the influence of these interfacial layers on the electric field distribution.

In contrast to the theoretical calculation for perfect capacitors composed of dielectrics with different permittivities, an equivalent electric field distribution throughout the whole dielectric stack is observed. The presence of trapped charges at the dielectric/dielectric interfaces leads to the uniformization of the electric field distribution inside the trilayer dielectric stack. The charges at the metal/dielectric interfaces screen the applied field so that the effective field across the dielectrics is lower. The trapped charge densities at the metal/dielectric interfaces depend on the stacking sequence (leading to different interfacial layers in terms of defects due to different processing), the nature of the dielectric and the sign of the bias. The amount of charge trapped at these dielectric/dielectric interfaces is related to the difference in permittivity to homogenize the effective field. This may be the driving force for explaining the presence of the charges trapped on the dielectric/dielectric interfacial layers. It would be interesting to study other systems of the same type to investigate if this result could be generalised to all interfaces between dielectrics of different permittivities.

Contacts:
Christophe Gatel | christophe.gatel[chez]cemes.fr
Martin Hÿtch | martin.hytch[chez]cemes.fr

Publication:
Quantification of interfacial charges in based HfO2/Al2O3 nanocapacitors by operando electron holography
L. Zhang, M.H. Raza, K. Gruel, R. Wu, C. Dubourdieu, M.J. Hÿtch, and C. Gatel
Advanced Materials, 2413691 (2024)
DOI: https://doi.org/10.1002/adma.202413691