From models to mechanisms: defects and charge trapping in amorphous silicon nitride

Doctoral Thesis

Amorphous silicon nitride (a-Si₃N₄) is widely used in nanoelectronics due to its favorable chemical properties and its unique ability to trap charge carriers, a key feature in non-volatile charge-trap flash memory. However, the structural response and the mechanisms of charge trapping still lack fundamental understanding at the atomic scale. This thesis establishes a hierarchical workflow combining atomistic simulations at both the force field (FF) and the density functional theory (DFT) level to generate structural models and systematically build an ensemble matching observable distributions of a-Si₃N₄. This enables investigation of the interplay between charge localization, defects, and structural response based on a realistic structural ensemble. The results reveal previously unknown intrinsic trapping mechanisms: Beyond undercoordinated Si atoms (K-centers), electrons can localize on distorted SiN₄ units or induce Si-N...Show moreAmorphous silicon nitride (a-Si₃N₄) is widely used in nanoelectronics due to its favorable chemical properties and its unique ability to trap charge carriers, a key feature in non-volatile charge-trap flash memory. However, the structural response and the mechanisms of charge trapping still lack fundamental understanding at the atomic scale. This thesis establishes a hierarchical workflow combining atomistic simulations at both the force field (FF) and the density functional theory (DFT) level to generate structural models and systematically build an ensemble matching observable distributions of a-Si₃N₄. This enables investigation of the interplay between charge localization, defects, and structural response based on a realistic structural ensemble. The results reveal previously unknown intrinsic trapping mechanisms: Beyond undercoordinated Si atoms (K-centers), electrons can localize on distorted SiN₄ units or induce Si-N bond breaking, forming new defect sites. At the same time, discharging restores the amorphous network’s original structure. Introducing hydrogen into the network reveals its dual role: It can repair coordination defects and heal the network, yet it also promotes Si-N bond breaking in strained regions, compromising structural integrity. Oxygen defects, in contrast, tend to incorporate into the Si-N network, causing local structural reorganization. This process creates shallow traps while deeper traps become occupied, providing a mechanism consistent with experimental observations of a shift in the trap density toward shallower levels. Finally, a-Si₃N₄ plays an active role when employed as a battery anode: Upon lithium incorporation, electrons localize at intrinsic trap sites, and increasing Li concentration gradually leads to the formation of bipolaronic states and Si-rich domains. Overall, this thesis provides a unified mechanistic picture of charge trapping and defect interactions in a-Si₃N₄, offering key insights for optimizing its performance in nanoelectronic and energy storage applications.
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Amorphous silicon nitride

Computational modeling

Lithium intercalation

Statistical sampling

Intrinsic charge trapping

Oxidation mechanism

Nitrogen defects