Switchable Chemical-Bond Reorganization for the Stable Charge Trapping in Amorphous Silicon Nitride.

Despite of the widespread use of charge-trap flash (CTF) memory, the atomistic mechanism behind the exceptionally stable charge-storage at the localized trap sites is still controversial. Herein, by combining first-principles calculations and orbital interaction analysis, we elucidate a charge-dependent switchable chemical-bond reorganization as the underpinning chemistry in the working mechanism of CTF. Especially, positively charged fourfold-coordinated nitrogen (dubbed N + center), unappreciated until now, is the decisive component of the entire process; once an electron occupies this site, the N + center disappears by breaking one N-Si bond, simultaneously forming a new Si-Si bond with a nearby Si atom which, in turn, creates five-fold coordinated Si. As a result, the electron is stored in a multi-center orbital belonging to multiple atoms including the newly formed Si-Si bond. We also observe that hole trapping accompanies the creation of an N + center by forming a new N-Si bond, which represents the reverse process. To further support and validate this model by means of core-level calculations, we also show that an N + center's 1s core level is 1.0-2.5 eV deeper in energy than those of the three-fold coordinated N atoms, in harmony with experimental XPS data. This article is protected by copyright. All rights reserved.