EFFICIENT LIGHT ABSORPTION MODULATION IN GERMANIUM NANOSTRUCTURES
antonio terrasi a, salvatore mirabella a, maria miritello a, isodiana crupi a, eric barbagiovanni a, rosario raciti a, salvatore cosentino a, atilla aydinli b, emel s. ozen b, giuseppe nicotra c, corrado spinella c, domenico pacifici d
a University of Catania and IMM-CNR, via S.Sofia 64, Catania, 95123
b Bilkent University, Turkey, Ankara, Turkey
c IMM-CNR, VIII Strada,5 Zona Industriale, Catania, 95121, Italy
d School of Engineering, Brown University, Providence, Rhode Island 02912, EE. UU., Providence, United States
Poster, salvatore cosentino, 032
Publication date: 27th June 2014

Ge nanostructures (NS) are gaining a renewed interest because of their lower synthesis temperature, larger optical absorption and stronger quantum confinement effect (QCE) compared to Si NS. Ge NS are now valuable candidate for applications in optoelectronics and photovoltaics. In this work we report on the fundamental mechanisms of light absorption in Ge NS and how to efficiently control the light harvesting by acting on the fabrication process.  

In particular, structural and optical characterizations are presented for single amorphous Ge quantum well (QW) grown at room temperature by magnetron sputtering deposition. We used transmission electron microscopy (TEM) and rutherford backscattering spectrometry (RBS) to measure the QW thickness (2-30 nm) and atomic density. The optical absorption of single amorphous Ge QW evidences a marked blue-shift (from 0.8 to 2.1 eV) and an enhanced oscillator strength with reducing the QW thickness. The experimental values of the bandgap and oscillator strength have been modelled in agreement with the effective mass approximation (EMA) model, demonstrating a strong QCE of excitons occurring at room temperature for amorphous Ge QWs [1].           

Though size-dependent bandgap is typically advocated for tuning the light absorption in NS, it is essential to disentangle the role of size from other effects related to the hosting matrix or synthesis techniques. For this reason, we performed a detailed investigation on Ge quantum dots (QDs) synthesized by annealing of Ge-rich SiO2 or Si3N4 thin films produced by magnetron sputtering and plasma enhanced chemical vapor deposition. Si3N4 matrix hosts QDs at higher density and lower size than SiO2, with Raman spectroscopy revealing a higher threshold for the amorphous-to-crystalline transition [2]. Light absorption spectroscopy shows a clear size-dependent shift in the optical bandgap, between 1.4 – 2.5 eV, that was modeled using EMA or a spatially dependent effective mass (SPDEM) approximation [3, 4]. The reported QCE was exploited to enhance the light harvesting efficiency of Ge QDs-based devices. We demonstrate a tunable, and high photoconductive gain (up to 15x) in metal-insulating-semiconductor photodetectors based on a preferential trapping of photo-generated holes by QDs that enhance the charge carrier collection [4, 5]. These results demonstrate the potentiality of Ge QDs as active material for the development of efficient light harvester and solar cells. 

[1] Cosentino et al., NRL 8, 128 (2013)

[2] Cosentino et al. JAP 115, 043103 (2014)

[3] Barbagiovanni et al. Physica E, 63, 14 (2014)

[4] Cosentino et al., submitted to Soral energy material & solar cells

[5] Cosentino et al., APL 98, 221107 (2011)



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