Charge transport in MNOS structures with metal grains at the silica–silicon nitride interface

The charge transport in MNOS-structures with molybdenum deposited on an oxide surface before deposition of silicon nitride is studied. It is shown that metal grains of molybdenum carbide and oxycarbide with typical sizes of 16 to 20 Å are formed at the interface. The deposition of grains considerably accelerates the transport of a negative charge from the dielectric bulk into the substrate and the storage of a positive charge under the action of a negative voltage pulse on the field electrode. A quantitative model is suggested for these processes. If the metal-oxide barrier height is 3.4 eV, good agreement between experimentally measured and calculated tunnel currents is achieved. A calculation of the changes in the tunneling rate is made by varying the charge state of a grain on the basis of electron-microscopic data of the grain sizes. The comparison of this calculation with the experimental data indicates that in the case of a storage of positive charges in the structure its considerable part is stored in the nitride bulk. The transport of electrons from the substrate into the structure with no metal grains is assumed to occur by tunneling through the complicated nitride-oxide barrier. Experimental data agree with theory. The experimentally found depth of the trap in silicon nitride is about 1.7 eV.