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Electrooptic effect studies  in nano-silicon doped silica sol-gel thin films

Antonela Dima 1Craig J. Williams 1Massimo Gagliardi 2Mihai O. Dima 3Stuart P. Edwardson 1Joseph Croft 1Ivo Rendina 2Geoff Dearden 1Ken G. Watkins 1

1. The University of Liverpool (UOL), Oxford Street, Liverpool L697ZD, United Kingdom
2. Istituto per la Microelettronica e Microsistemi (IMM-CNR), via P. Castellino 111, Napoli 80131, Italy
3. Institute for Physics and Nuclear Engineering, Str. Atomistilor 407, Bucharest R-76900, Romania

Abstract

    Thin films with ca. 200 nm nc-Si particles embedded in sol-gel SiOx 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/SiOx interface, the SiOx 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 Ra = 4.39 nm. Increasing the positive bias to 3V the average roughness diminishes to Ra = 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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Related papers

Presentation: Poster at E-MRS Fall Meeting 2009, Symposium I, by Antonela Dima
See On-line Journal of E-MRS Fall Meeting 2009

Submitted: 2009-05-06 13:06
Revised:   2009-08-01 23:28