S. Mirabella

Self-assembled silver nanoparticles for plasmon-enhanced solar cell back reflectors: correlation between structural and optical properties

The spectra of localized surface plasmon resonances (LSPRs) in self-assembled silver nanoparticles (NPs), prepared by solid-state dewetting of thin films, are discussed in terms of their structural properties. We summarize the dependences of size and shape of NPs on the fabrication conditions with a proposed structural-phase diagram. It was found that the surface coverage distribution and the mean surface coverage (SC) size were the most appropriate statistical parameters to describe the correlation between the morphology and the optical properties of the nanostructures. The results are interpreted with theoretical predictions based on Mie theory. The broadband scattering efficiency of LSPRs in the nanostructures is discussed towards application as plasmon-enhanced back reflectors in thin-film solar cells.

Activation and carrier mobility in high fluence B implanted germanium

High doping regimes of B implanted Ge have been accurately characterized combining Hall effect technique and nuclear reaction analysis. Preamorphized Ge was implanted with B at 35keV (spanning the 0.25–25×1020B∕cm3 concentration range) and recrystallized by solid phase epitaxy at 360°C. The Hall scattering factor and the maximum concentration of active B resulted rH=1.21 and ∼5.7×1020B∕cm3, respectively. The room-temperature carrier mobility was accurately measured, decreasing from ∼300to50cm2∕Vs in the investigated dopant density, and a fitting empirical law is given. These results allow reliable evaluation for Ge application in future microelectronic devices.

Role of fluorine in suppressing boron transient enhanced diffusion in preamorphized Si

We have explained the role of fluorine in the reduction of the self-interstitial population in a preamorphized Si layer under thermal treatment. For this purpose, we have employed a B spike layer grown by molecular-beam epitaxy as a marker for the self-interstitial local concentration. The amorphized samples were implanted with 7×1012, 7×1013, or 4×1014 F/cm2 at 100 keV, and afterwards recrystallized by solid phase epitaxy. Thermal anneals at 750 or 850 °C were performed in order to induce the release of self-interstitials from the end-of-range (EOR) defects and thus provoke the transient enhanced diffusion of B atoms. We have shown that the incorporation of F reduces the B enhanced diffusion in a controlled way, up to its complete suppression. It is seen that no direct interaction between B and F occurs, whereas the suppression of B enhanced diffusion is related to the F ability in reducing the excess of silicon self-interstitials emitted by the EOR source. These results are reported and discussed.

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

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

Complete suppression of the transient enhanced diffusion of B implanted in preamorphized Si by interstitial trapping in a spatially separated C-rich layer

A method for completely suppressing the transient enhanced diffusion (TED) of boron implanted in preamorphized silicon is demonstrated. Boron is implanted in a molecular beam epitaxy (MBE) grown silicon sample that has been previously amorphized by silicon implantation. The sample is then annealed in order to epitaxially regrow the amorphous layer and electrically activate the dopant. The backflow of silicon interstitials released by the preamorphization end-of-range (EOR) damage is completely trapped by a carbon-rich silicon layer interposed by MBE between the damage and the implanted boron. No appreciable TED is observed in the samples up to complete dissolution of the EOR damage, and complete electrical activation is obtained. The method might be considered for the realization of ultrashallow junctions for the far future complementary metal–oxide–semiconductor technology nodes.

Evolution of boron-interstitial clusters in crystalline Si studied by transmission electron microscopy

The thermal evolution of large boron-interstitials clusters (BICs) in crystalline Si has been studied by transmission electron microscopy (TEM). After ion implantation (20keV and 1×1014Si∕cm2) and annealing (815°C and 5min), large clusters (6–8nm) have been observed in correspondence of a narrow, highly doped Si:B layer (2×1020B∕cm3). Under prolonged annealing, such clusters dissolve, progressively shrinking their mean size below the TEM detection limit. The time evolution of such a BIC shrinking is fully compatible with the slow path dissolution kinetics recently published. These data suggest the identification of the slow dissolving BICs with the large observed clusters.

B activation and clustering in ion-implanted Ge

Experimental studies about electrical activation and clustering of B implanted in crystalline Ge (c-Ge) are reported. To this aim, we structurally and electrically investigated c-Ge samples implanted at different temperatures with B at 35 keV in the high-concentration dopant regime (0.67–25×1020 B/cm3). We elucidated that a high level of damage, in the form of amorphous pockets, favors the electrical activation of the dopant, and a complete activation was achieved for properly chosen implant conditions. We found, by joining channeling measurements with the electrical ones, that the reason for incomplete B activation is the formation of B-Ge complexes with a well-defined stoichiometry of 1:8. The thermal stability of the B-doped samples, up to 550 °C, was also investigated. The tested stability demonstrates that the B clustering, responsible of B inactivity, is characterized by high binding energies and higher thermal budgets are needed to make them to dissolve. These studies, besides clarify the physical ...

Fluorine in preamorphized Si : Point defect engineering and control of dopant diffusion

While it is known that F modifies dopant diffusion in crystalline Si, the physical mechanisms behind this process are still unclear. In this work we report experimental studies about the F control of the point defect density in preamorphized Si layers. These studies put the basis for the understanding of the F behavior and for the realization of ultra-shallow junctions. We first investigated the F incorporation process during the solid phase epitaxy (SPE) of amorphous Si layers. We elucidated the role of the SPE temperature on the F incorporation and suggested a new route towards a F profile engineering. Moreover, we explained the role of F in modifying the point defect population (self-interstitials, Is, and vacancies, Vs), employing B and Sb spike layers as markers for Is and Vs, respectively. We clearly showed that F decreases the B diffusion while enhances the Sb one, pointing out the capacity to induce an Is undersaturation or a Vs supersaturation. These data rule out the hypothesis of a chemical bon...

Transient enhanced diffusion of B mediated by self-interstitials in preamorphized Ge

The dissolution of interstitial-type end-of-range (EOR) damage in preamorphized Ge is shown to induce a transient enhanced diffusion of an epitaxially grown boron delta at temperatures above 350 °C that saturates above 420 °C. The B diffusion events are quantitatively correlated with the measured positive strain associated with the EOR damage as a function of the annealing temperature with an energy barrier for the EOR damage dissolution of 2.1±0.3 eV. These results unambiguously demonstrate that B diffuses in Ge through a mechanism assisted by self-interstitials, and impose considering the interstitial implantation damage for the modeling of impurity diffusion in Ge.

Effect of O:Er concentration ratio on the structural, electrical, and optical properties of Si:Er:O layers grown by molecular beam epitaxy

The structural, electrical, and optical properties of crystalline Si codoped with Er and O by molecular beam epitaxy (MBE) have been investigated in detail. Si:Er:O layers (∼250 nm thick) have been grown by MBE, realizing uniform dopant concentrations in the range 8×1018–1.5×1020 cm−3 for Er and up to 5×1020 cm−3 for O. The O:Er ratio was varied between 0 and ∼20. Samples have been subsequently annealed at 900 °C for 1 h. We observed that clear constraints to the Er and O contents exist in order to incorporate them in a good quality single crystal. We also found that the O:Er ratio represents the main parameter in determining the properties of this system. For instance, Er is observed to behave as a donor in MBE grown samples and the donor concentration increases with the O:Er ratio until a saturation regime is achieved for a ratio higher than 6–8. All the samples emit light at 1.54 μm and similar behavior is also found for the optical activation of the Er ions. The thermal process usually increases the n...