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 degreesC oxidation and anneal at 900 degreesC or 1100 degreesC, forming nanocrystal layers of three expected thickness between 1 and 6 nm. The charge trapping, determined from the hysteresis DeltaV 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 degreesC, C-V sweeps increasing into accumulation showed abrupt DeltaV 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 degreesC showed gradual charging with increasing field; with increasing sweeps DeltaV now increased linearly with end voltage, possibly because of increased contribution of oxide defects to trapping. Very small DeltaV 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 DeltaV 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. (C) 2003 Elsevier Science B.V. All rights reserved.