Charging effects in silicon nanocrystals embedded in SiO2 films

Structures with Si nanocrystals embedded in SiO2 were fabricated by growing thin oxides and depositing Si films by low pressure chemical vapor deposition (LPCVD), followed by 900 °C oxidation and anneal at 900 °C or 1100 °C, forming nanocrystal layers of three expected thickness between 1 and 6 nm. The charge trapping, determined from the hysteresis ΔV of capacitance–voltage (C–V) curves from inversion to accumulation and back, depended on the nanocrystal size. For structures with the largest nanocrystals annealed at 900 °C, C–V sweeps increasing into accumulation showed abrupt ΔV increase and forward C–V curve translation at fields above 2.5 MV cm−1, indicating charging mainly above this field with partial charge retention. Similar structures annealed at 1100 °C showed gradual charging with increasing field; with increasing sweeps ΔV now increased linearly with end voltage, possibly because of increased contribution of oxide defects to trapping. Very small ΔV of 0.1–0.2 V was measured for the two smaller nanocrystal sizes. The current–voltage (I–V) curves show N-shaped behavior indicating screening effects due to charging; an initial current spike, attributed to transient current charging the nanocrystals, occurs at the voltage causing abrupt forward C–V curve shift and ΔV increase, with Fowler–Nordheim current rising at higher voltages. These results support the conclusion that charge trapping occurs primarily in Si nanocrystals, with an increasing contribution of trapping in oxide defects in structures processed at higher temperature.