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

A.-M Lepadatu,T Stoica,V S Teodorescu,A Slav,S Lazanu,C Palade,M L Ciurea,I Dascalescu,S Iftimie,M Enculescu

doi:10.1038/s41598-018-23316-3

A.-M Lepadatu, T Stoica + Show 8 more

Open Access

https://doi.org/10.1038/s41598-018-23316-3

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Abstract

Si and Ge nanocrystals in oxides are of a large interest for photo-effect applications due to the fine-tuning of the optical bandgap by quantum confinement in nanocrystals. In this work, dense Ge nanocrystals suitable for enhanced photoconduction were fabricated from 60% Ge in TiO2 amorphous layers by low temperature rapid thermal annealing at 550 °C. An exponential increase of the photocurrent with the applied voltage was observed in coplanar structure of Ge nanocrystals composite films deposited on oxidized Si wafers. The behaviour was explained by field effect control of the Fermi level at the Ge nanocrystals-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 nanocrystal 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 nanocrystals formation. The nanocrystal 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.

Highlights

TiO2 is a promising material for a broad area of applications such as photocatalysts[1,2,3,4,5,6], dye sensitized and perovskite based solar cells[7,8,9,10,11,12], rechargeable batteries[13], gas sensors[14,15,16] and biomedical devices[17]
Beside the expected quantum confinement effect of the blue shift of the optical bandgap in Ge NCs, other Ge NCs-related phenomena are evidenced on this composite material for the first time: a deep minimum in the spectral dependence of refractive index; Efros-Shklovskii T −1/2 hopping conduction in the presence of a Coulomb gap in the electronic state distribution; spectral photocurrent enhanced by Ge NCs formation and carrier depletion induced by field effect
The layers of 60% Ge in TiO2 were prepared on SiO2/Si substrates by magnetron sputtering deposition and subsequent rapid thermal annealing (RTA)

Summary

Introduction

TiO2 is a promising material for a broad area of applications such as photocatalysts[1,2,3,4,5,6], dye sensitized and perovskite based solar cells[7,8,9,10,11,12], rechargeable batteries[13], gas sensors[14,15,16] and biomedical devices[17]. The influence of annealing temperature on the morphology and structure of Ge-TiO2 films deposited by magnetron sputtering with 50:50 Ge:TiO2 composition was previously studied by our group[37,38]. It was shown by annealing in a furnace that the amorphous state is preserved up to 500 °C, while for 600 °C and higher temperatures NCs of Ge and TiO2 are formed. Beside the expected quantum confinement effect of the blue shift of the optical bandgap in Ge NCs, other Ge NCs-related phenomena are evidenced on this composite material for the first time: a deep minimum in the spectral dependence of refractive index; Efros-Shklovskii T −1/2 hopping conduction in the presence of a Coulomb gap in the electronic state distribution; spectral photocurrent enhanced by Ge NCs formation and carrier depletion induced by field effect

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Conclusion