We demonstrate experimentally all-optical switching on a silicon chip at telecom wavelengths. The switching device comprises a compact ring resonator formed by horizontal silicon slot waveguides filled with highly nonlinear silicon nanocrystals in silica. When pumping at power levels about 100 mW using 10 ps pulses, more than 50% modulation depth is observed at the switch output. The switch performs about 1 order of magnitude faster than previous approaches on silicon and is fully fabricated using complementary metal oxide semiconductor technologies.
Precipitation and crystallization of Si nanocrystals have been monitored by means of Raman spectroscopy. SiOx films with different compositions have been deposited by low-pressure chemical-vapor deposition technique onto silica substrates and treated to temperatures exceeding 800 °C. The evolution of the Raman signal with the thermal budget reveals that the silicon transition from amorphous to crystalline state shifts to higher temperatures as the Si content in the layers is lowered. A rather complete crystallization of the nanoparticles is achieved after annealing at 1250 °C for a Si excess lower than 20%, while for higher excesses the crystalline fraction reaches only 40%, suggesting the formation of a crystalline core surrounded by an amorphous shell. The Raman spectra have been analyzed by a phonon confinement model that takes into account stress effects. An increasing nanocrystal size, from 2.5 to 3.4 nm, has been estimated when the Si excess varies from 16 to 29 at. %. For small Si nanocrystals a strong hydrostatic stress has been observed, induced by a very abrupt transition with the surrounding SiO2. Its magnitude correlates with the increase in thermal budget required for the crystallization of the amorphous clusters. This study underlines the fundamental role of hydrostatic stress in retarding the crystallization of Si nanoclusters.
Linear and nonlinear optical properties of silicon suboxide SiO x films deposited by plasma-enhanced chemical-vapor deposition have been studied for different Si excesses up to 24 at. %. The layers have been fully characterized with respect to their atomic composition and the structure of the Si precipitates. Linear refractive index and extinction coefficient have been determined in the whole visible range, enabling to estimate the optical bandgap as a function of the Si nanocrystal size. Nonlinear optical properties have been evaluated by the z-scan technique for two different excitations: at 0.80 eV in the nanosecond regime and at 1.50 eV in the femtosecond regime. Under nanosecond excitation conditions, the nonlinear process is ruled by thermal effects, showing large values of both nonlinear refractive index ͑n 2 ϳ −10 −8 cm 2 / W͒ and nonlinear absorption coefficient ͑ ϳ 10 −6 cm/ W͒. Under femtosecond excitation conditions, a smaller nonlinear refractive index is found ͑n 2 ϳ 10 −12 cm 2 / W͒, typical of nonlinearities arising from electronic response. The contribution per nanocrystal to the electronic third-order nonlinear susceptibility increases as the size of the Si nanoparticles is reduced, due to the appearance of electronic transitions between discrete levels induced by quantum confinement.
Silica layers implanted with Si and Er ions to various doses and annealed at 950°C have been investigated by means of energy-filtered transmission electron microscopy (EFTEM) and high annular angle dark field (HAADF). EFTEM analysis reveals Si nanoclusters (Si-nc) with an average size around 3nm for high Si content (15at.%) whereas no clusters can be imaged for the lowest Si excess (5at.%). Raman scattering supports that amorphous Si precipitates are present in all the samples. Moreover, the filtered images show that Er ions appear preferentially located outside the Si-nc. HAADF analysis confirms that the Er atoms form agglomerations of 5–10nm size when the Er concentration exceeds 1×1020cm−3. This observation correlates well with the reduction of the Er population excitable by Si nanoclusters, in the best case corresponding to 10% of the total. A suitable tuning of the annealing drastically reduces this deleterious effect.
Abstract. An in-depth study of the physical and electrical properties of Sinanocrystals embedded in silicon dioxide is presented. These layers were fabricated with different Si concentrations by both ion implantation and plasma-enhanced chemical vapour deposition. Subsequently, LEDs devices based on a metal-oxide-silicon configuration with a ∼350 nm polycrystalline Si top electrode and an active layer of about 45-50 nm, were fabricated in conventional lithography process. In order to optimize the device performances, prior to the top electrode deposition, the structural and photoluminescent properties of the active layers were exhaustively studied.Devices fabricated by ion implantation exhibit a combination of direct current and field-effect luminescence under a bipolar pulsed voltages excitation. The onset of the emission decreases with the Si excess from 6 to 3 V. The direct current emission is attributed to impact ionization, and is associated with the reasonably high current levels observed in current-voltage measurements. This behaviour is in good agreement with transmission electron microscopy images that revealed a continuous and uniform Si-nanocrystals distribution. The emission power efficiency is relatively low, ∼10 −3 %, and the emission intensity exhibits fast degradation rates, as revealed from accelerated aging experiments.Devices fabricated by chemical deposition only exhibit field-effect luminescence which onset decreases with the Si excess from 20 to 6 V. The absence of the continuous emission is explained by the observation of a 5-nm region free of nanocrystals, which strongly reduces the direct current through the gate. The main benefit of having this nanocrystal-free region is that tunnelling current flow assisted by nanocrystals is blocked by the SiO 2 stack so that power consumption is strongly reduced, which in return increases the device power efficiency up to 0.1 % . In addition, the accelerated aging studies reveal a 50% degradation rate reduction as compared to implanted structures.PACS numbers: 73.63. Bd, 78.67.Bf, 85.60.Jb Submitted to: Nanotechnology Si nanocrystal-based LEDs fabricated by ion implantation and PECVD 2
Optical properties of directly excited erbium ͑Er 3+ ͒ ions have been studied in silicon rich silicon oxide materials codoped with Er 3+ . The spectral dependence of the direct excitation cross section ͑ dir ͒ of the Er 3+ atomic 4 I 15/2 → 4 I 11/2 transition ͑around 0.98 m͒ has been measured by time resolved -photoluminescence measurements. We have determined that dir is 9.0Ϯ 1.5 ϫ 10 −21 cm 2 at 983 nm, at least twice larger than the value determined on a stoichiometric SiO 2 matrix. This result, in combination with a measurement of the population of excited Er 3+ as a function of the pumping flux, has allowed quantifying accurately the amount of optically active Er 3+ . This concentration is, in the best of the cases, 26% of the total Er population measured by secondary ion mass spectrometry, which means that only this percentage could provide optical gain in an eventual optical amplifier based on this material.
The nanoparticle volume fraction employed in the figures and text was 3.4%, rather than the erroneously quoted 0.8%. The abscissa in Figures 1 and 2 should say nanoparticle radius and not nanoparticle diameter. We also note that the experimentally adjusted value of δ 3 employed in eq 5 is 38.8 Å 3 , rather than the actual geometrical volume. None of the conclusions in the paper is affected by the previous corrections. We thank Shidong Wang and Ivana Savic ´for pointing out these errata.
Slot and sandwiched waveguides with silicon nanocrystals were fabricated by means of industrial microelectronic tools, including DUV lithography. Low loss of 4 dB/cm will pave the way to compact all-optical XOR logic gates. IntroductionUsing silicon, oxides, nitrides and other Si-based materials as active means for photonic functionalities is of great interest since it would allow a potential monolithic integration of electrical and optical circuits in a fully compatible CMOS processing. For example, optical interconnects, optical amplifiers and switches integrated in CMOS photonic chips would make it possible to develop low cost and all-optical communication networks without bottlenecks induced by electrical/optical converters. Passive devices (filters, couplers, multiplexers...) that make use of silicon-based waveguides and materials exhibiting non-linear optical properties have already been demonstrated. Nevertheless they use Si thermo-optic effects or refractive index variation due to the free carrier concentration in Si, as bulk Si is a very poor material for non-linear optics. Notwithstanding, nanostructured Si, due to quantum confinement effects, and particularly silicon nanoclusters (Si-nc) embedded in SiO2 (Si-nc/SiO2) are considered very promising materials due to extraordinarily enhanced nonlinear properties in comparison to bulk Si, as its Kerr coefficient is reported in the range
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