Limits...
The role of the surfaces in the photon absorption in Ge nanoclusters embedded in silica.

Cosentino S, Mirabella S, Miritello M, Nicotra G, Lo Savio R, Simone F, Spinella C, Terrasi A - Nanoscale Res Lett (2011)

Bottom Line: The optical absorption of Ge nanoclusters has been measured by spectrophotometry analyses, evidencing an optical bandgap of 1.6 eV, unexpectedly independent of the QDs size or of the solid phase (amorphous or crystalline).A simple modeling, based on the Tauc law, shows that the photon absorption has a much larger extent in smaller Ge QDs, being related to the surface extent rather than to the volume.These data are presented and discussed also considering the outcomes for application of Ge nanostructures in photovoltaics.PACS: 81.07.Ta; 78.67.Hc; 68.65.-k.

View Article: PubMed Central - HTML - PubMed

Affiliation: MATIS-IMM-CNR and Dipartimento di Fisica e Astronomia, Università di Catania, Via Santa Sofia 64, 95123 Catania, Italy. mirabella@ct.infn.it.

ABSTRACT
The usage of semiconductor nanostructures is highly promising for boosting the energy conversion efficiency in photovoltaics technology, but still some of the underlying mechanisms are not well understood at the nanoscale length. Ge quantum dots (QDs) should have a larger absorption and a more efficient quantum confinement effect than Si ones, thus they are good candidate for third-generation solar cells. In this work, Ge QDs embedded in silica matrix have been synthesized through magnetron sputtering deposition and annealing up to 800°C. The thermal evolution of the QD size (2 to 10 nm) has been followed by transmission electron microscopy and X-ray diffraction techniques, evidencing an Ostwald ripening mechanism with a concomitant amorphous-crystalline transition. The optical absorption of Ge nanoclusters has been measured by spectrophotometry analyses, evidencing an optical bandgap of 1.6 eV, unexpectedly independent of the QDs size or of the solid phase (amorphous or crystalline). A simple modeling, based on the Tauc law, shows that the photon absorption has a much larger extent in smaller Ge QDs, being related to the surface extent rather than to the volume. These data are presented and discussed also considering the outcomes for application of Ge nanostructures in photovoltaics.PACS: 81.07.Ta; 78.67.Hc; 68.65.-k.

No MeSH data available.


Absorption spectra, Tauc plots, and relative linear fits. (a) Absorption spectra of SiGeO samples annealed at various temperatures (1 h, N2 ambient), together with the spectrum of crystalline Ge [34]. Ion implantation (1.3 × 1014 Ge/cm2, 600 keV, max Ge density lower than 0.01 at.%) was performed to induce the amorphization of Ge QDs. (b) Tauc plots (symbols) and relative linear fits (according to the reported law, lines) for the same samples and for a thin (120 nm) amorphous Ge film (color online).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC3211182&req=5

Figure 4: Absorption spectra, Tauc plots, and relative linear fits. (a) Absorption spectra of SiGeO samples annealed at various temperatures (1 h, N2 ambient), together with the spectrum of crystalline Ge [34]. Ion implantation (1.3 × 1014 Ge/cm2, 600 keV, max Ge density lower than 0.01 at.%) was performed to induce the amorphization of Ge QDs. (b) Tauc plots (symbols) and relative linear fits (according to the reported law, lines) for the same samples and for a thin (120 nm) amorphous Ge film (color online).

Mentions: Selected α spectra are reported in Figure 4a for the as-deposited sample (squares) or after annealing at 600°C (circles) and 800°C (open triangles). The absorption spectrum of crystalline Ge (c-Ge, continuous line) is also reported for comparison [34]. The difference of about one order of magnitude between bulk Ge and our sample is not surprising since the main part of the SiGeO film is a transparent matrix (SiO2 and GeO2), while the Ge involved in QD formation is about 10 at.%. Thus, the reported α spectra can be associated to the photon absorption by Ge QDs. Annealing at 600°C does not significantly modify the absorption of Ge QDs, while the change of α at 800°C is inferred to the presence of crystalline QDs (evidenced by TEM already at 750°C). In fact, at 800°C, two broad peaks (dashed vertical lines) at about 2.6 and 5 eV appear in the spectrum, recalling the E1 and E2 direct transitions (at 2.1 and 4.3 eV) of the bulk c-Ge spectrum, but at a slightly larger energy. Such broad peaks in the 800°C-annealed sample can be related to direct transitions within the c-Ge QDs having an energy band structure modified by the confinement.


The role of the surfaces in the photon absorption in Ge nanoclusters embedded in silica.

Cosentino S, Mirabella S, Miritello M, Nicotra G, Lo Savio R, Simone F, Spinella C, Terrasi A - Nanoscale Res Lett (2011)

Absorption spectra, Tauc plots, and relative linear fits. (a) Absorption spectra of SiGeO samples annealed at various temperatures (1 h, N2 ambient), together with the spectrum of crystalline Ge [34]. Ion implantation (1.3 × 1014 Ge/cm2, 600 keV, max Ge density lower than 0.01 at.%) was performed to induce the amorphization of Ge QDs. (b) Tauc plots (symbols) and relative linear fits (according to the reported law, lines) for the same samples and for a thin (120 nm) amorphous Ge film (color online).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211182&req=5

Figure 4: Absorption spectra, Tauc plots, and relative linear fits. (a) Absorption spectra of SiGeO samples annealed at various temperatures (1 h, N2 ambient), together with the spectrum of crystalline Ge [34]. Ion implantation (1.3 × 1014 Ge/cm2, 600 keV, max Ge density lower than 0.01 at.%) was performed to induce the amorphization of Ge QDs. (b) Tauc plots (symbols) and relative linear fits (according to the reported law, lines) for the same samples and for a thin (120 nm) amorphous Ge film (color online).
Mentions: Selected α spectra are reported in Figure 4a for the as-deposited sample (squares) or after annealing at 600°C (circles) and 800°C (open triangles). The absorption spectrum of crystalline Ge (c-Ge, continuous line) is also reported for comparison [34]. The difference of about one order of magnitude between bulk Ge and our sample is not surprising since the main part of the SiGeO film is a transparent matrix (SiO2 and GeO2), while the Ge involved in QD formation is about 10 at.%. Thus, the reported α spectra can be associated to the photon absorption by Ge QDs. Annealing at 600°C does not significantly modify the absorption of Ge QDs, while the change of α at 800°C is inferred to the presence of crystalline QDs (evidenced by TEM already at 750°C). In fact, at 800°C, two broad peaks (dashed vertical lines) at about 2.6 and 5 eV appear in the spectrum, recalling the E1 and E2 direct transitions (at 2.1 and 4.3 eV) of the bulk c-Ge spectrum, but at a slightly larger energy. Such broad peaks in the 800°C-annealed sample can be related to direct transitions within the c-Ge QDs having an energy band structure modified by the confinement.

Bottom Line: The optical absorption of Ge nanoclusters has been measured by spectrophotometry analyses, evidencing an optical bandgap of 1.6 eV, unexpectedly independent of the QDs size or of the solid phase (amorphous or crystalline).A simple modeling, based on the Tauc law, shows that the photon absorption has a much larger extent in smaller Ge QDs, being related to the surface extent rather than to the volume.These data are presented and discussed also considering the outcomes for application of Ge nanostructures in photovoltaics.PACS: 81.07.Ta; 78.67.Hc; 68.65.-k.

View Article: PubMed Central - HTML - PubMed

Affiliation: MATIS-IMM-CNR and Dipartimento di Fisica e Astronomia, Università di Catania, Via Santa Sofia 64, 95123 Catania, Italy. mirabella@ct.infn.it.

ABSTRACT
The usage of semiconductor nanostructures is highly promising for boosting the energy conversion efficiency in photovoltaics technology, but still some of the underlying mechanisms are not well understood at the nanoscale length. Ge quantum dots (QDs) should have a larger absorption and a more efficient quantum confinement effect than Si ones, thus they are good candidate for third-generation solar cells. In this work, Ge QDs embedded in silica matrix have been synthesized through magnetron sputtering deposition and annealing up to 800°C. The thermal evolution of the QD size (2 to 10 nm) has been followed by transmission electron microscopy and X-ray diffraction techniques, evidencing an Ostwald ripening mechanism with a concomitant amorphous-crystalline transition. The optical absorption of Ge nanoclusters has been measured by spectrophotometry analyses, evidencing an optical bandgap of 1.6 eV, unexpectedly independent of the QDs size or of the solid phase (amorphous or crystalline). A simple modeling, based on the Tauc law, shows that the photon absorption has a much larger extent in smaller Ge QDs, being related to the surface extent rather than to the volume. These data are presented and discussed also considering the outcomes for application of Ge nanostructures in photovoltaics.PACS: 81.07.Ta; 78.67.Hc; 68.65.-k.

No MeSH data available.