We have investigated electrical switching behavior of sol-gel derived SiO2 films in c-Si(p)/SiO2/metal structures. The SiO2 film is fabricated from silicafilm (a soluble Si polymer in dissolved in denatured alcohol) using spin-coating technique. The thickness of the film is in the range of 300~2000 Å which depends on both spinning speed and the degree of dilution. We find that, with Ag as the top contact, when the applied voltage reaches a critical value of 1.5 ~ 2.5 V, current increases rapidly many orders of magnitude higher, and an irreversible switching occurs. The switching is also found to be polarity-dependent. The switching occurs only when the top contact (Ag) is biased positively, indicating the switching involves diffusion or electromigration of Ag. Both switching voltage and current are found to scale with the top contact area of the device. The switching dynamics is studied using voltage-pulse over a duration of 300 ns to 500 ms and amplitude of 2 to 20 V. We find that the switching electrical field is strongly dependent of the delay time prior to switching, and not related to the film thickness.
Thin film nanocrystalline silicon (nc-Si), a promising material for photovoltaic and optoelectronic applications, is comprised of nanometer-scale crystals of silicon embedded in a matrix of hydrogenated amorphous silicon. The degree of crystallinity of the material can be controlled by varying the deposition conditions, yielding materials that span the transition from the amorphous to the nanocrystalline state, and yielding variable grain size and crystalline fraction. Pump-probe measurements using optical pulses 35 fs in duration in the near-infrared were carried out on a series of nc-Si films of varying composition. Photoexcitation results in an induced absorbance signal with a nonexponential time dependence that is strongly dependent on excitation density. The response can be understood in terms of a multicomponent model that includes distinct contributions from each phase of the heterogeneous material. We observe a 240-fs exponential relaxation process associated with intraband relaxation in the silicon crystallites, a response characteristic of bimolecular recombination in the amorphous silicon matrix, and a long-lived component assigned to grain boundary states.
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