An a-SiN,x/nanocrystalline silicon [(nc-Si)/a-SiN,x] sandwiched structure is fabricated in a plasma enhanced chemical vapour deposition (PECVD) system at low temperature (250℃). The nc-Si layer is fabricated from a hydrogen-diluted silane mixture gas by using a layer-by-layer deposition technique. Atom force microscopy measurement shows that the density of nc-Si is about 2×1011cm-2. By the pretreatment of plasma nitridation, low density of interface states and high-quality interface between the Si substrate and a-SiNx insulator layer are obtained. The density of interface state at the midgap is calculated to be 1×1010cm-2eV-1 from the quasistatic and high frequency C-V data. The charging and discharging property of nc-Si quantum dots is studied by capacitance-voltage (C-V) measurement at room temperature. An ultra-large hysteresis is observed in the C-V characteristics, which is attributed to storage of the electrons and holes into the nc-Si dots. The long-term charge-loss process is studied and ascribed to low density of interface states at SiN,x/Si substrate.