Thin films with ca. 200 nm nc-Si particles embedded in sol-gel SiO_x thin films (x<2) were prepared. The films exhibit a sizeable electro-optical effect, for converter devices. Owing to the marked differences in bandgap and band-structure between the nanocrystalline grains and the amorphous matrix, heterojunction like structures are formed in the interface regions, where the band offset effects reduce dramatically the activation energy and the grains act like quantum dots. In the presence of an externally applied field, the activated electrons in the quantum dots participate to conduction through the interface barriers via quantum tunneling. For localizing electrons at or near the Si/SiO_x interface, the SiO_x film needs to have a large bandgap (insulator like). It is the localized electrons and their motion under applied electric field that determine the hysteresis/memory properties and refractive index change in the films.

Ellipsometric measurements performed under electric field (3V/15 mm) displayed a refractive index change Δn = 0.02 - 0.03 (maxing out in UV and IR). Scanning tunneling microscopy (STM) on the films (deposited on conductive p-type silicon) showed for low positive bias (0 V) scans the non-conductive state having an average roughness of R_a = 4.39 nm. Increasing the positive bias to 3V the average roughness diminishes to R_a = 4.34 nm – in good agreement with the ellipsometric result of a higher refraction index under electric field. This fluctuation can be attributed to the motion induced by the injected electrons. Once electric field is removed, the surface characteristics gradually regain their initial values.

    The sizeable refraction index change (ca. 20-30% that of organics, but x20-30 larger than other inorganics) makes such films suited for electro-optical modulators.

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