CHARGE-TRANSPORT IN ULTRATHIN SILICON NITRIDES

The conduction current in silicon nitride increases even at constant electric field as the nitride thickness is reduced to less than 5 nm in oxide equivalent thickness (t(eq)). In order to analyze the charge transport in the ultrathin nitrides less than 5 nm t(eq), we measured the thickness and temperature dependence of conduction current through nitrides of 3.4 to 10.2 nm t(eq), in the temperature range from 77 to 398 K. Current increase was observed in both the tunnel emission component, which is thickness dependent, and in the temperature-dependent component. The temperature-dependent current component was dominant at high temperatures and low fields in the ultrathin nitride. The method of separating the electron and hole currents was used for both n- and p-channel metal-nitride-silicon transistors, to study the charge transport in nitrides from 3.8 to 8.6 nm t(eq), at 296 and 398 K. The increase in the number of electrons injected into the nitride was larger than the increase in the number of holes injected into the nitride when the nitride thickness was reduced. The increase in electron current flowing out of the nitride was also large compared with the increase in hole current flowing out of the nitride. We claim that the contribution of electrons to the total charge transport is increased with the reduction in nitride thickness. Finally, we discussed the dependence of the breakdown field on nitride thickness in oxide/nitride/oxide structures. We claim that top and bottom oxides should be as thin as possible to obtain the high breakdown field.