Corrado Bongiorno

Bio: Corrado Bongiorno is an academic researcher from National Research Council. The author has contributed to research in topics: Annealing (metallurgy) & Silicon. The author has an hindex of 36, co-authored 223 publications receiving 4217 citations. Previous affiliations of Corrado Bongiorno include University of Catania.

Formation and evolution of luminescent Si nanoclusters produced by thermal annealing of SiOx films

Abstract: Si nanoclusters embedded in SiO2 have been produced by thermal annealing of SiOx films prepared by plasma enhanced chemical vapor deposition. The structural properties of the system have been investigated by energy filtered transmission electron microscopy (EFTEM). EFTEM has evidenced the presence of a relevant contribution of amorphous nanostructures, not detectable by using the more conventional dark field transmission electron microscopy technique. By also taking into account this contribution, an accurate quantitative description of the evolution of the samples upon thermal annealing has been accomplished. In particular, the temperatures at which the nucleation of amorphous and crystalline Si nanoclusters starts have been determined. Furthermore, the nanocluster mean radius and density have been determined as a function of the annealing temperature. Finally, the optical and the structural properties of the system have been compared, to demonstrate that the photoluminescence properties of the system depend on both the amorphous and crystalline clusters.

Sensitizing properties of amorphous Si clusters on the 1.54-μm luminescence of Er in Si-rich SiO2

Abstract: In this letter, the role of amorphous Si clusters in the excitation of Er implanted in substoichiometric SiOx films will be elucidated. It will be shown that the temperature of the SiOx thermal process prior to Er implantation is crucial in determining the luminescence properties of the samples. In particular, the luminescence intensity at 1.54 μm is almost constant for SiOx samples not annealed or pre-annealed at temperatures lower than 800 °C, reaches the maximum at 800 °C, and decreases at higher temperatures. The structural properties of these samples have been studied by energy filtered transmission electron microscopy. It will be shown that for annealing temperatures lower than 1000 °C, only amorphous Si nanoclusters are present. We demonstrate that a large density of small amorphous Si clusters produces the best luminescence performance and enhances the fraction of optically active Er.

Efficient Luminescence and Energy Transfer in Erbium Silicate Thin Films

Abstract: Currently, electrical interconnections based on metal lines represent the most important limitation on the performances of silicon-based microelectronic devices. The parasitic capacities generated at the metal/insulator/metal capacitors present in the complex multilevel metallization schemes actually used, the intrinsic resistivity of the metal lines, and the contact resistance at the metal/metal interfaces constitute the main contributions to the delay in the signal propagation. Recently, a reduction of the delay times was achieved by replacing the traditional metallization schemes based on Al and SiO2 with new materials, such as copper-based alloys and low-dielectricconstant insulating layers, but as soon as the minimum feature size of the devices will be further reduced, the delay resulting from metal interconnections will again represent an unacceptable bottleneck for device performances. A definitive solution to this problem could be the use of optical interconnections for the transfer of information inside a chip or for chip-to-chip communications. To develop this strategy, siliconcompatible materials and devices able to generate, guide, amplify, switch, modulate, and detect light are needed. Recent major breakthroughs in this field have been the observation of optical gain in Si nanocrystals, the development of a Si Raman laser, the realization of a high-speed Si electro-optic modulator, and the observation of electroluminescence from ultrapure Si diodes and Si nanocrystal field-effect transistors. A primary requirement for the materials proposed for the above applications is compatibility with current Si technology. However, because Si is intrinsically unable to efficiently emit light, owing to its indirect bandgap, it is evident that the main limitation to the approach described above is the lack of an efficient silicon-based light source. Among the efforts of the scientific community to efficiently produce photons from silicon, the introduction of light-emitting impurities, such as erbium ions, has a leading role. A relevant advantage of this approach is that standard silicon technology can be used to introduce erbium as a dopant and to process the material. Furthermore, Er ions emit at 1.54 lm, which is a strategic wavelength for telecommunication because it corresponds to a minimum in the loss spectrum of the silica optical fibers. Incorporation of Er in crystalline silicon (c-Si) emerged as the first promising method to turn silicon into a luminescent material, but doping concentration was limited (ca. 1 × 10 cm) by the low solid solubility of Er. A co-implantation of Er and O allowed to limit Er segregation and precipitation, owing to the formation of Er-O complexes. However, at room temperature a relatively low luminescence efficiency was obtained as a result of the strong nonradiative processes competing with the radiative Er de-excitation in c-Si. More recently, it was shown that by using a SiO2 matrix containing Er-doped Si nanoclusters, an intense room-temperature Er luminescence can be obtained. Indeed, it has been demonstrated that Si nanoclusters in presence of Er act as efficient sensitizers for the rare earth owing to the effective Er excitation cross section, which is more than two orders of magnitude higher compared with the Er resonant absorption of a photon. Optical gain from waveguides based on Er-doped Si nanoclusters has been also reported, and lightemitting devices have been fabricated. However, the optical gain that can be obtained from this system is critically dependent on its Si content, and it is limited by the confined carrier absorption resulting from the presence of Si nanoclusters. Even if gain can prevail over absorption through a careful balance of Er and Si concentrations, absorption could represent a limit for obtaining high gain values from Er-doped Si nanoclusters. A new and different approach, which may allow to overcome some of the limits of the above-described systems in which erbium ions are present as dopants, may be represented by erbium compounds, such as oxides or silicates. In these compounds Er is a major component, and therefore its concentration (about 10 cm) can be several orders of magnitude greater than those typically obtained by ion implantation, allowing access to a huge amount of emitting centers. Very few reports are present in literature that discuss the optical properties of Er2O3 [19–21] and Er silicate. In particular, in the case of Er silicate the few available reports refer to sysC O M M U N IC A TI O N

Optical and structural properties of Er2O3 films grown by magnetron sputtering

Abstract: 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...

Si-based materials and devices for light emission in silicon

Abstract: We report on the fabrication and performances of extremely efficient Si-based light sources. The devices consist of MOS structures with erbium (Er) implanted in the thin gate oxide. The devices exhibit strong 1.54 μm electroluminescence (EL) at 300 K with a 10% external quantum efficiency, comparable to that of standard light-emitting diodes using III–V semiconductors. Er excitation is caused by hot electrons impact and oxide wearout limits the reliability of the devices. Much more stable light-emitting MOS devices have been fabricated using Er-doped silicon rich oxide (SRO) films as gate dielectric. These devices show a high stability, with an external quantum efficiency reduced to 1%. In these devices, Er pumping occurs by energy transfer from the Si nanostructures to the rare-earth ions. Finally, we have also fabricated MOS structures with Tb- and Yb-doped SiO2 which show room temperature EL at 540 nm (Tb) and 980 nm (Yb) with an external quantum efficiency of a 10% and 0.1%, respectively.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Perovskite solar cells with atomically coherent interlayers on SnO2 electrodes.

Abstract: In perovskite solar cells, the interfaces between the perovskite and charge-transporting layers contain high concentrations of defects (about 100 times that within the perovskite layer), specifically, deep-level defects, which substantially reduce the power conversion efficiency of the devices1–3. Recent efforts to reduce these interfacial defects have focused mainly on surface passivation4–6. However, passivating the perovskite surface that interfaces with the electron-transporting layer is difficult, because the surface-treatment agents on the electron-transporting layer may dissolve while coating the perovskite thin film. Alternatively, interfacial defects may not be a concern if a coherent interface could be formed between the electron-transporting and perovskite layers. Here we report the formation of an interlayer between a SnO2 electron-transporting layer and a halide perovskite light-absorbing layer, achieved by coupling Cl-bonded SnO2 with a Cl-containing perovskite precursor. This interlayer has atomically coherent features, which enhance charge extraction and transport from the perovskite layer, and fewer interfacial defects. The existence of such a coherent interlayer allowed us to fabricate perovskite solar cells with a power conversion efficiency of 25.8 per cent (certified 25.5 per cent)under standard illumination. Furthermore, unencapsulated devices maintained about 90 per cent of their initial efficiency even after continuous light exposure for 500 hours. Our findings provide guidelines for designing defect-minimizing interfaces between metal halide perovskites and electron-transporting layers. An atomically coherent interlayer between the electron-transporting and perovskite layers in perovskite solar cells enhances charge extraction and transport from the perovskite, enabling high power conversion efficiency.

Understanding the phase-change mechanism of rewritable optical media.

Abstract: Present-day multimedia strongly rely on rewritable phase-change optical memories. We demonstrate that, different from the current consensus, Ge(2)Sb(2)Te(5), the material of choice in DVD-RAM, does not possess the rocksalt structure but more likely consists of well-defined rigid building blocks that are randomly oriented in space consistent with cubic symmetry. Laser-induced amorphization results in drastic shortening of covalent bonds and a decrease in the mean-square relative displacement, demonstrating a substantial increase in the degree of short-range ordering, in sharp contrast to the amorphization of typical covalently bonded solids. This novel order-disorder transition is due to an umbrella-flip of Ge atoms from an octahedral position into a tetrahedral position without rupture of strong covalent bonds. It is this unique two-state nature of the transformation that ensures fast DVD performance and repeatable switching over ten million cycles.