Exoelectron emission studies of trap spectrum in ultrathin amorphous Si₃N₄ films

We applied thermally stimulated exoelectron emission (TSEE) and thermally stimulated luminescence (TSL) methods for trap spectroscopy studies of ultrathin amorphous Si₃N₄ films used in ONO (oxide/nitride/oxide) structure-based microFlash memory devices. The temperature spectra of TSEE of the films demonstrate two groups of TSEE peaks: three low temperature peaks (T₁=383 K, T₂=393 K, T ₃=413 K) and a high temperature maximum at T₄=813 K. The developed Auger model of TSEE allowed calculating trap energy spectrum responsible for the TSEE peaks generation. TSEE results are shown to be consistent with trap energy activation Φ estimates obtained from microFLASH^® two bit per cell memory transistor measurements where electrons stored at deep traps are responsible for high temperature TSEE peak with Φ₄=1.73 eV. We believe that deep traps in silicon nitride are hydrogen containing centers. TSEE studies of the high temperature decay process of these traps explain excellent retention properties of microFlash memory devices.