Dense Ge nanocrystals embedded in TiO2 with exponentially increased photoconduction by field effect

What is it about?

In this work, dense Ge nanocrystals (NCs) were fabricated from 60% Ge in TiO2 amorphous layers (deposited by magnetron sputtering) by low temperature rapid thermal annealing at 550 °C. An exponential increase of photocurrent with the applied voltage was observed in coplanar structure of Ge NCs composite films deposited on oxidized Si wafers. The behaviour was explained by field effect control of the Fermi level at the Ge NCs-TiO2 layer/substrate interfaces. The blue-shift of the absorption gap from bulk Ge value to 1.14 eV was evidenced in both photocurrent spectra and optical reflection-transmission experiments, in good agreement with quantum confinement induced bandgap broadening in Ge NC with sizes of about 5 nm as found from HRTEM and XRD investigations. A nonmonotonic spectral dependence of the refractive index is associated to the Ge NCs formation. The NC morphology is also in good agreement with the Coulomb gap hopping mechanism of T–1/2-type explaining the temperature dependence of the dark conduction.

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Why is it important?

This work brings new developments and understanding of the physics behind the properties of Ge NCs in TiO2 matrix with high concentration of 60% Ge by investigating the structure, optical, electrical and photoconductive properties in correlation with the crystalline structure and morphology and in turn with the annealing temperature: - HRTEM showing how the formation of Ge NCs is dependent on annealing. - Spectral optical constants n and k in VIS-NIR range correlated to the Ge NCs formation at different annealing temperatures (for the first time reported in literature). The blue-shift of the optical bandgap due to quantum confinement in ~5 nm size Ge NCs (as HRTEM image shows) is evidenced. - The temperature dependence of the dark current reveals Efros-Shklovskii Coulomb-gap variable range hopping in Ge NCs-TiO2 composite for the first time in this paper. - Spectral photocurrent is improved by Ge NC formation and enhanced by field effect in coplanar structures of Ge NC-TiO2 layer/ SiO2/c-Si substrate; the exponential enhancement of photocurrent is obtained by charging the layer with the aim to create a carrier (hole) depleted zone, thus extending the photo-effect applications of Ge-TiO2 films.

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