M. Miritello

Electroluminescence at 1.54 μm in Er-doped Si nanocluster-based devices

The electroluminescence (EL) properties of Er-doped Si nanoclusters (NC) embedded in metal–oxide–semiconductor devices are investigated. Due to the presence of Si NC dispersed in the SiO2 matrix, an efficient carrier injection occurs and Er is excited, producing an intense 1.54 μm room temperature EL. The EL properties as a function of the current density, temperature, and time have been studied in detail. We have also estimated the excitation cross section for Er under electrical pumping, finding a value of ∼1×10−14 cm2. This value is two orders of magnitude higher than the effective excitation cross section of Er ions through Si NC under optical pumping. In fact, quantum efficiencies of ∼1% are obtained at room temperature in these devices.

Excitation and de-excitation properties of silicon quantum dots under electrical pumping

In this work, the stationary and time-resolved electroluminescence (EL) properties of Si quantum dots embedded within a metal–oxide–semiconductor device are investigated. In particular, we measured the excitation cross section of Si nanocrystals under electrical pumping, finding a value of 4.7×10−14 cm2 which is two orders of magnitude higher with respect to the excitation cross section under 488 nm optical pumping. We also studied the radiative and nonradiative decay processes occurring in these devices by measuring the time evolution of the EL signal. We demonstrate that the mechanism responsible for the emission is the same under both electrical and optical pumping. The overall quantum efficiency of the electrical pumping is estimated to be two orders of magnitude higher than the quantum efficiency for optical pumping in all the studied temperature ranges.

Optical and structural properties of Er2O3 films grown by magnetron sputtering

The structural properties and the room temperature luminescence of Er2O3 thin films deposited by magnetron sputtering have been studied. In spite of the well-known high reactivity of rare earth oxides towards silicon, films characterized by good morphological properties have been obtained by using a SiO2 interlayer between the film and the silicon substrate. The evolution of the properties of the Er2O3 films due to thermal annealing processes in oxygen ambient performed at temperatures in the range of 800–1200°C has been investigated in detail. The existence of well defined annealing conditions (rapid treatments at a temperature of 1100°C or higher) allowing to avoid the occurrence of extensive chemical reactions with the oxidized substrate has been demonstrated; under these conditions, the thermal process has a beneficial effect on both structural and optical properties of the film, and an increase of the photoluminescence (PL) intensity by about a factor of 40 with respect to the as-deposited material has been observed. The enhanced efficiency of the photon emission process has been correlated with the longer lifetime of the PL signal. Finally, the conditions leading to a reaction of Er2O3 with the substrate have been also identified, and evidences about the formation of silicate-like phases have been collected.The structural properties and the room temperature luminescence of Er2O3 thin films deposited by magnetron sputtering have been studied. In spite of the well-known high reactivity of rare earth oxides towards silicon, films characterized by good morphological properties have been obtained by using a SiO2 interlayer between the film and the silicon substrate. The evolution of the properties of the Er2O3 films due to thermal annealing processes in oxygen ambient performed at temperatures in the range of 800–1200°C has been investigated in detail. The existence of well defined annealing conditions (rapid treatments at a temperature of 1100°C or higher) allowing to avoid the occurrence of extensive chemical reactions with the oxidized substrate has been demonstrated; under these conditions, the thermal process has a beneficial effect on both structural and optical properties of the film, and an increase of the photoluminescence (PL) intensity by about a factor of 40 with respect to the as-deposited material h...

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...

Direct evidence of light confinement and emission enhancement in active silicon-on-insulator slot waveguides

The authors experimentally demonstrate strong light confinement and enhancement of emission at 1.54μm in planar silicon-on-insulator waveguides containing a thin layer (slot) of SiO2 with Er3+ doped Si nanoclusters. Angle-resolved attenuated total reflectance is used to excite the slab guided modes, giving a direct evidence of the strong confinement of the electric field in the low-index active material for the fundamental transverse-magnetic mode. By measuring the guided photoluminescence from the cleaved-edge of the sample, the authors observe a more than fivefold enhancement of emission for the transverse-magnetic mode over the transverse-electric one. These results show that Si-based slot waveguides could be important as starting templates for the realization of Si-compatible active optical devices.

Erbium-doped Si nanocrystals: optical properties and electroluminescent devices

Abstract In the last decade, a strong effort has been devoted towards the achievement of efficient light emission from silicon. Among the different approaches, rare-earth doping and quantum confinement in Si nanostructures have shown great potentialities. In the present work, the synthesis and properties of low-dimensional silicon structures in SiO 2 will be analyzed. All of these structures present a strong room temperature optical emission, tunable in the visible by changing the crystal size. Moreover, Si nanocrystals (nc) embedded in SiO 2 together with Er ions show a strong coupling with the rare earth. Indeed each Si nc absorbs energy which is then preferentially transferred to the nearby Er ions. The signature of this interaction is the strong increase of the excitation cross section for an Er ion in the presence of Si nc with respect to a pure oxide host. We will show the properties of Er-doped Si nc embedded within Si/SiO 2 Fabry–Perot microcavities. Very narrow, intense and highly directional luminescence peaks can be obtained. Moreover, the electroluminescence (EL) properties of Si nc and Er-doped Si nc in MOS devices are investigated. It is shown that an efficient carrier injection at low voltages and quite intense room temperature EL signals can be achieved, due to the sensitizing action of Si nc for the rare earth. These data will be presented and the impact on future applications discussed.

Electroluminescence properties of light emitting devices based on silicon nanocrystals

Abstract We have fabricated MOS devices where the dielectric layer consists of a substoichiometric SiO x (x thin film, annealed at 1100°C for 1 h to induce the separation of the Si and SiO 2 phases, with the formation of silicon nanocrystals (nc) embedded in the insulating matrix. We have studied the electroluminescence (EL) properties of such devices as a function of the current density and of the temperature. We have evaluated the excitation cross section of Si nc under electrical pumping at room temperature and at low temperature (12 K ) . Moreover, we have used the experimental EL intensities and decay times to evaluate the radiative rate as a function of the temperature.

High-efficiency silicon-compatible photodetectors based on Ge quantum dots

We report on high responsivity, broadband metal/insulator/semiconductor photodetectors with amorphous Ge quantum dots (a-Ge QDs) as the active absorbers embedded in a silicon dioxide matrix. Spectral responsivities between 1–4 A/W are achieved in the 500–900 nm wavelength range with internal quantum efficiencies (IQEs) as high as ∼700%. We investigate the role of a-Ge QDs in the photocurrent generation and explain the high IQE as a result of transport mechanisms via photoexcited QDs. These results suggest that a-Ge QDs are promising for high-performance integrated optoelectronic devices that are fully compatible with silicon technology in terms of fabrication and thermal budget.

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

Microstructural evolution of SiOx films and its effect on the luminescence of Si nanoclusters

In this paper we demonstrate that the structural and optical properties of Si nanoclusters (Si ncs) formed by thermal annealing of SiOx films prepared by plasma enhanced chemical vapor deposition (PECVD) and magnetron sputtering are very different. In fact, at a fixed Si excess and annealing temperature, photoluminescence (PL) spectra of sputtered samples are redshifted with respect to PECVD samples, denoting a larger Si ncs size. In addition, PL intensity reaches a maximum in sputtered films at annealing temperatures much lower than those needed in PECVD films. These data are correlated with structural properties obtained by energy filtered transmission electron microscopy and electron energy loss spectroscopy. It is shown that in PECVD films only around 30% of the Si excess agglomerates in clusters while an almost complete agglomeration occurs in sputtered films. These data are explained on the basis of the different initial structural properties of the as-deposited films that become crucial for the sub...