New results on electron injection, hole injection, and trapping in MONOS nonvolatile memory devices

Charge injection and trapping in silicon nitride layers are studied with the three-terminal metal-oxide-nitride-oxide-semiconductor (MONOS) gated-diode structure. A new experimental technique based on the linear voltage ramp is developed which measures electron and hole currents separately in the semiconductor during the actual charge injection (nonsteady-state measurement as opposed to the steady-state method) across the tunneling oxide. In addition, the technique measures the flat-band voltage shift and minimizes the back tunneling of the injected charge (a problem with the pulse measurement). The blocking oxide between the gate electrode and the nitride layer prevents any injection from the gate electrode. The main conclusion from these studies is that the semiconductor injects electrons and holes into the nitride layer for positive and negative polarities of the gate bias, respectively. This result is in sharp contrast with the existing interpretations based on a single-carrier type. It is speculated that the recombination of electrons and holes takes place in the nitride layer via an "amphoteric" trap. At small levels of charge injection, centroids of the trapped charge (measured from the tunneling oxide-nitride interface) for both electron and hole injection conditions are found to be located at 75-80 Å at room temperature and 15-20 Å at 100 K.