F. Simone

Light absorption in silicon quantum dots embedded in silica

The photon absorption in Si quantum dots (QDs) embedded in SiO2 has been systematically investigated by varying several parameters of the QD synthesis. Plasma-enhanced chemical vapor deposition (PECVD) or magnetron cosputtering (MS) have been used to deposit, upon quartz substrates, single layer, or multilayer structures of Si-rich-SiO2 (SRO) with different Si content (43–46 at. %). SRO samples have been annealed for 1 h in the 450–1250 °C range and characterized by optical absorption measurements, photoluminescence analysis, Rutherford backscattering spectrometry and x-ray Photoelectron Spectroscopy. After annealing up to 900 °C SRO films grown by MS show a higher absorption coefficient and a lower optical bandgap (∼2.0 eV) in comparison with that of PECVD samples, due to the lower density of Si–Si bonds and to the presence of nitrogen in PECVD materials. By increasing the Si content a reduction in the optical bandgap has been recorded, pointing out the role of Si–Si bonds density in the absorption proce...

Matrix role in Ge nanoclusters embedded in Si3N4 or SiO2

Ge nanoclusters (NCs), synthesized by ion implantation and annealing up to 900 °C, result small (∼2 nm) and amorphous in Si3N4, crystalline and much larger in SiO2. The NCs ripening and crystallization kinetics in Si3N4 is retarded by larger interfacial energy and lower diffusivity of Ge in comparison to SiO2. Ge NCs absorb light more efficiently when embedded in Si3N4 than in SiO2. A significant effect of the barrier height on absorption was evidenced, in agreement with effective mass theory predictions. The smaller bandgap of Ge NCs embedded in Si3N4 and their closeness is promising features for light harvesting applications.

Light absorption enhancement in closely packed Ge quantum dots

Multilayers of Ge quantum dots (QDs, 3 nm in diameter) embedded in SiO2, separated by SiO2 barrier layer (3, 9, or 20 nm thick), have been synthesized by sputter deposition and characterized by transmission electron microscopy and light absorption spectroscopy. Quantum confinement affects the optical bandgap energy (1.9 eV for QDs, 0.8 eV for bulk Ge); moreover, the absorption probability greatly depends on the QD-QD distance. A strong electronic coupling among Ge QDs is evidenced, with a significant increase of the light absorption efficiency when the QD-QD distance is reduced. These data unveil promising aspects for light harvesting with nanostructures.

Light absorption and electrical transport in Si:O alloys for photovoltaics

Thin films (100–500 nm) of the Si:O alloy have been systematically characterized in the optical absorption and electrical transport behavior, by varying the Si content from 43 up to 100 at. %. Magnetron sputtering or plasma enhanced chemical vapor deposition have been used for the Si:O alloy deposition, followed by annealing up to 1250 °C. Boron implantation (30 keV, 3–30×1014 B/cm2) on selected samples was performed to vary the electrical sheet resistance measured by the four-point collinear probe method. Transmittance and reflectance spectra have been extracted and combined to estimate the absorption spectra and the optical band gap, by means of the Tauc analysis. Raman spectroscopy was also employed to follow the amorphous-crystalline (a-c) transition of the Si domains contained in the Si:O films. The optical absorption and the electrical transport of Si:O films can be continuously and independently modulated by acting on different parameters. The light absorption increases (by one decade) with the Si ...

Light absorption and conversion in solar cell based on Si:O alloy

Thin film Si:O alloys have been grown by plasma enhanced chemical vapor deposition, as intrinsic or highly doped (1 to 5 at. % of B or P dopant) layers. UV-visible/near-infrared spectroscopy revealed a great dependence of the absorption coefficient and of the optical gap (Eg) on the dopant type and concentration, as Eg decreases from 2.1 to 1.9 eV, for the intrinsic or highly p-doped sample, respectively. Thermal annealing up to 400 °C induces a huge H out-diffusion which causes a dramatic absorption increase and a reduction of Eg, down to less than 1.8 eV. A prototypal solar cell has been fabricated using a 400 nm thick, p-i-n structure made of Si:O alloy embedded within flat transparent conductive oxides. Preliminary electrical analyses show a photovoltaic (PV) effect with an open circuit voltage of 0.75 V and a spectral conversion efficiency blue-shifted in comparison to a-Si:H based cell, as expected since the higher Eg in Si:O alloy. These data are presented and discussed, suggesting Si:O alloy as pr...

Enhanced light scattering in Si nanostructures produced by pulsed laser irradiation

An innovative method for Si nanostructures (NS) fabrication is proposed, through nanosecond laser irradiation (λ = 532 nm) of thin Si film (120 nm) on quartz. Varying the laser energy fluences (425–1130 mJ/cm2) distinct morphologies of Si NS appear, going from interconnected structures to isolated clusters. Film breaking occurs through a laser-induced dewetting process. Raman scattering is enhanced in all the obtained Si NS, with the largest enhancement in interconnected Si structures, pointing out an increased trapping of light due to multiple scattering. The reported method is fast, scalable and cheap, and can be applied for light management in photovoltaics.