Anderson’s negative- <i>U</i> chemistry in amorphous silicon nitride: A complex system approach

P. W. Anderson introduced a negative-<i>U</i> (i.e., attractive electron-electron interaction) to describe the scarcity of singly occupied spin states in amorphous materials. To uncover the underlying chemistry behind this phenomenon, we combined structural sampling with quantum-chemical analysis of amorphous silicon nitride (a-SiN<i>_x</i>) based on DFT. Our analysis demonstrates that coordination defects act as charge traps, with a density on the order of 10²¹ per cubic centimeter. These defects render singly occupied electronic states energetically less favorable than paired or fully emptied states, primarily due to electron donor-acceptor interactions. Furthermore, excess charge trapping occurs through chemical bond reorganization, which disrupts the balance of existing electron and hole traps. In addition, we found that in Si-rich a-SiN<i>_x</i>, various Si─Si bonding networks exhibit power-law-like size distribution, where larger networks are associated with deeper trap levels. These findings explain why a-SiN<i>_x</i> has been used as a charge storage layer in the charge trap flash memory.