Connection of silicon nanocrystals (Si-nc) with multi-walled carbon nanotubes

During the last decade silicon nanocrystals (Si-nc) have received widespread interest because of their high quantum efficiency for light emission at room temperature. However, the challenge still ahead is to study and apply these to single Si-ncoptoelectronics, i.e., solving problems linked with connection and manipulation. In this letter we report on connecting (wiring) single Si-nc with conducting multi-walled carbon nanotubes (MWNTs). We have been able to establish a strong mechanical connection by direct growth of MWNTs on Si-nc used as support of iron nanoparticles, by catalytic chemical vapor deposition (CCVD). To monitor the initial stage of the MWNTs growth process, we used a tapered element oscillating microbalance (TEOM). We compared the growth process on Si-nc coated by iron (Fe/Si-nc) to the standard process of growing MWNTs on alumina as support for iron (Fe/Al). The results showed that in the case of Fe/Si-nc catalyst, we obtained three times larger diameter of multi-walled CNTs compared to Fe/Al. This was mainly due to the Si-nc size. The diameter of the CNTs only depended on the size of the Si-nc particles that rested stuck on the tip of the MWNTs. The connected Si-nc kept their photoluminescence properties at room temperature. The present findings open new opportunities in the development of nanodevices for the optoelectronic application field. PACS 81.07.Bc; 81.07.Lk; 81.07.De     

Photoluminescence studies from silicon nanocrystals embedded in spin on glass thin films

Photoluminescence (PL) from silicon nanocrystals (Si-nc) prepared from pulverised porous silicon and embedded in undoped (SOG) or phosphorus-doped spin-on-glass (SOD) solutions was studied. Effects of rapid thermal annealing on the PL was also investigated. A strong room temperature PL signal was observed at 710 nm due to the recombination of electron-hole pairs in Si-nc and the PL maximum shifts to the blue region as the phosphorus concentration in the spin on glass increases. However, the rapid thermal annealing process (30 s, 900°C) quenches the PL response. These results suggest that for Si-nc/SOG (SOD) the surface termination is efficient but high phosphorus doping of Si-nc is detrimental to the PL.     

Correlations of a bound interface over a random substrate

The effects of Si nanocluster (Si-nc) size on the energy transfer rate to Er ions were investigated through studies made on appropriate configurations of mutilayers (MLs) consisting in about 20 periods of Er-doped Si-rich SiO₂/SiO₂. These MLs were deposited by reactive magnetron sputtering at 650 °C and subsequently annealed at 900 °C. For Si-rich layer thickness or Si-nc larger than about 4 nm, the sensitizing effect of Si-nc towards rare earth ions is highly lowered because of the weak confinement of carriers and the loss of resonant excitation of Er through the upper levels (second, third, ...). The latter is liable to prevent the energy back transfer process, while the weak confinement reduces strongly the probability of no phonon radiative recombination necessary for the energy transfer from Si-nc to Er ions.     

Effects of Si nanocluster size and carrier-Er interaction distance on the efficiency of energy transfer

The effects of Si nanocluster (Si-nc) size and spacing from Er³⁺ ions were investigated through studies made on appropriate configurations of multilayers obtained by reactive magnetron sputtering at 650 °C and subsequently annealed at 900 °C. Si-nc larger than about 5 nm appear ineffective for resonant excitation of Er because of the resulting weak confinement responsible for negligible direct radiative recombination. This direct no-phonon transition probability is closely correlated to the energy transfer rate, both decreasing when Si-nc size increases. For large Si-nc having a bandgap lower than 1.26 eV, the energy transfer to the upper levels (second, third, etc.) of Er³⁺ is no more possible, leading to the observed abrupt decrease of the 1.54-μm emission. The latter is governed by the distance separating Er ions from their Si-nc sensitizers, whose behavior was well described by an exponentially decreasing exchange interaction. The characteristic interaction distance was found to be dependent on the amorphous or crystalline nature of Si-nc, and it appears as small as 0.4±0.1 nm for the former (amorphous) and of some nanometers for the crystallized Si-nc.     

Sensitisation of erbium emission by silicon nanocrystals-doped SnO2

SnO₂ thin films undoped and doped with antimony (Sb), erbium (Er) and Si nanocrystals (Si-nc) have been grown on silicon (Si) substrate using sol-gel method. Room-temperature photoluminescence (PL) measurement of undoped SnO₂, under excitation at 280 nm, shows only one broad emission at 395 nm, which is related to oxygen vacancies. The PL of Er³⁺ ions was found to be enhanced after doping SnO₂ with Sb and Si-nc. The excitation process of Er is studied and discussed. The calculation of cross-section suggests a sensitisation of Er PL by Si-nc.     

Doping in silicon nanocrystals: An ab initio study of the structural, electronic and optical properties

There are experimental evidences that doping control at the nanoscale can significantly modify the optical properties with respect to the pure systems. This is the case of silicon nanocrystals (Si-nc), for which it has been shown that the photoluminescence (PL) peak can be tuned also below the bulk Si band gap by properly controlling the impurities, for example by boron (B) and phosphorus (P) codoping. In this work, we report on an ab initio study of impurity states in Si-nc. We consider B and P substitutional impurities for Si-nc with a diameter up to 2.2 nm. Formation energies (FEs), electronic, optical and structural properties have been determined as a function of the cluster dimension. For both B-doped and P-doped Si-nc the FE increases on decreasing the dimension, showing that the substitutional doping gets progressively more difficult for the smaller nanocrystals. Moreover, subsurface impurity positions result to be the most stable ones. The codoping reduces the FE strongly favoring this process with respect to the simple n-doping or p-doping. Such an effect can be attributed to charge compensation between the donor and the acceptor atoms. Moreover, smaller structural deformations, with respect to n-doped and p-doped cases, localized only around the impurity sites are observed. The band gap and the optical threshold are largely reduced with respect to the undoped Si-nc showing the possibility of an impurity-based engineering of the Si-nc PL properties.     

Laser-induced thermal effects on Si/SiO<Subscript>2</Subscript> free-standing superlattices

The thermal effects produced by continuous-wave laser radiation on free-standing Si/SiO₂ superlattices are studied. We compare two samples with different SiO₂ layer thicknesses (2 and 6 nm) and the same Si layer thickness (2 nm). The as-prepared free-standing superlattices contain some amount of Si nanocrystals (Si-nc). Intense laser irradiation at 488 nm of the as-prepared samples enhances the Raman scattering of Si-nc by two orders of magnitude. This laser-induced crystallization originates from melting of Si nanostructures in silica, which makes Si-nc better ordered and better isolated from the oxide surrounding. Continuous-wave laser control of Si-nc stress was achieved in these samples. In the proposed model, intense laser radiation melts Si-nc, and Si crystallization upon cooling down from the liquid phase in a silica matrix leads to compressive stress. The Si-nc stress can be tuned in the ∼3 GPa range using laser annealing below the Si melting temperature. The high laser-induced temperatures were verified with Raman spectroscopy. The laser-induced heat leads to a strongly nonlinear rise of light emission. The light emission is also observed in the anti-Stokes region, and its temperature dependence is practically the same for the two studied samples. The laser-induced temperature is essentially controlled by the absorbed laser power. PACS 78.55.-m; 78.20.-e; 68.55.-a; 78.30.-j     

Silicon nanocrystal synthesis by implantation of natural Si isotopes

Implantations of pure , , and into SiO₂ can provide significant insight into the formation of silicon nanocrystals (Si-nc) and their light emission properties. Si-nc produced with different fractions of the heavier Si isotopes have been characterized by Raman and photoluminescence spectroscopy. Weak Stokes shifts of the Si-nc phonon peaks indicate that both the implanted Si and the native Si from the SiO₂ substrate contribute to Si-nc nucleation. The Raman measurements also indicate that the Si isotopic composition of the Si-nc is similar to the Si isotopic fraction of the implanted SiO₂. The Si-nc photoluminescence (PL) spectra are shifted towards the blue with increasing Si isotope mass, an indication that the increase of the Si-nc effective mass enhances the excitonic bandgap. Measurements from samples implanted with heavy isotopes at high Si excess concentrations indicate that the Si-nc isotope fraction evolves with annealing time such that the heaviest Si isotope are more concentrated in the vicinity of the Si-nc/SiO₂ interface, which can modify the energy states involved in the radiative transitions associated with Si-nc.     

X-ray absorption study of light emitting silicon nanocrystals

X-ray absorption spectra obtained by total electron yield (TEY) at the Si absorption K-edge have been measured to have chemical and structural information about Si nanocrystals (Si-nc) produced by plasma-enhanced chemical vapour deposition (PECVD). The TEY technique has been employed to investigate the formation of Si-nc and the modification of the silica matrix as a function of annealing temperature (500–1250°C) and of silicon content in the film (35–46 at%). The amount of silicon present in the Si-nc has been evaluated by TEY. Thanks to Rutherford backscattering spectrometry measurements, the amount of Si atoms bonded to oxygen and to nitrogen, incorporated by PECVD, has been assessed. A compositional model that interprets the experimental findings is presented.     

One step towards the fabrication of a nanoscale Si-nc based laser cavity

In this paper, we report on the design of two major components of a laser architecture using Si-nc embedded in SiO₂ as the optical gain medium and sub-wavelength periodic structures to form the resonant cavity. Dimensions of the structures have been matched to near-infrared wavelengths (∼850 nm) of the maximum photoluminescent emission where optical gain has been observed from Si-nc. Both the front (FM) and rear (RM) mirrors have been fabricated by the implantation of Si ions (50 keV, 2×10¹⁷ Si⁺/cm²) through a mask, in order to produce a Bragg reflector by optical index contrast between the implanted and the non-implanted zones. Two closely spaced Bragg reflectors are used in the FM structure to allow a narrow bandpass (partial transmission) centered at 850 nm. The implanted structures have been annealed to produce Si-nc and passivation. Scanning electron microscopy (SEM) images show that the design dimensions of the structure have been obtained. Characterization of the structures by laser excitation reveals an optical gap in both mirrors between 825 and 870 nm, as per the design parameters. A quality factor Q∼95 and a reflectivity R∼0.2 have been measured for the FM. These results support the concept that a complete Si-nc based laser cavity can be built to emit coherent light.     

Optical losses and gain in silicon-rich silica waveguides containing Er ions

Rib-loaded waveguides containing Er³⁺ coupled to Si-nc have been produced by magnetron sputtering and successive thermal annealing to investigate optical gain at 1535 nm. It has been shown that all Er ions are optically active, whereas the fraction that can be excited at high pump rates under non-resonant excitation is strongly limited by confined carrier absorption (CA), up-conversion processes, and mainly by the lack of coupling to the Si-nc. Er³⁺ absorption cross-section is found comparable to that of Er³⁺ in SiO₂, but a dependence with the effective refractive index has been found. Although the presence of Si-nc strongly improves the efficiency of Er³⁺ excitation, it introduces additional optical loss mechanisms, such as CA. These Si-nc losses affect the possibility of obtaining net optical gain. In the present study, they have been minimized by lowering the annealing time of the Er-doped Si-rich oxide. In pump-probe measurements it is shown that signal enhancement of the transmitted signal can be achieved at low pumping rate when the detrimental role of confined CA is attenuated by reducing the annealing time. A maximum signal enhancement of about 1.30 at 1535 nm was observed.     

Free-carrier absorption modulation in silicon nanocrystal slot waveguides

Free-carrier absorption (FCA) has proven to be an important obstacle in the development of a silicon-based laser; however, FCA may serve as a potential advantage in active silicon-based switches or modulators. In this work, we present FCA modulation in slot waveguides with silicon nanocrystals (Si-ncs) embedded in SiO(2) as the low-index slot material. Slot waveguides were fabricated with and without Si-ncs, and the presence of Si-ncs was shown to increase the pump-induced FCA loss in the waveguides by a factor of 4.5. We modeled the Si-nc material using a four-level rate equation analysis to estimate the excited population of Si-ncs, allowing us to extract a value of 2.6 × 10(-17) cm(2) for the FCA cross section of the Si-nc material.     

Fabrication and simulation of glass micromachining using CO2 laser processing with PDMS protection

An efficient synthesis route for highly luminescent silicon nanocrystal (Si-nc) films is presented. Si-ncs in the films are synthesized in the gas phase by using an argon-silane radio-frequency dielectric-barrier discharge (RF-DBD) plasma. The size of Si-ncs is well tunable by changing the resident time. The resulting Si-nc films with different oxidation degree exhibit emission across the full visible spectrum. Structural and optical characterization indicates that the red-to-green luminescence from big particles show quantum confinement effect (QCE), while this effect disappears in blue luminescence from small ones. A model is presented to explain this result. In this model, the radiative process in big particles is Band-to-Band recombination, in which surface states have a negligible impact on the QCE, while the blue emission from small Si-ncs is due to the Band-to-Bound recombination, in which surface state plays an important role, resulting in the disappearance of QCE. Additionally, obvious double-exponential decay from midsize particles is observed, in which the two kinds of recombination may coexist.     

Photoluminescence of Si from Si nanocrystal-doped SiO2/Si multilayered sample

A multilayered Si nanocrystal-doped SiO₂/Si (or Si-nc:SiO₂/Si) sample structure is studied to acquire strong photoluminescence (PL) emission of Si via modulating excess Si concentration. The Si-nc:SiO₂ results from SiO thin film after thermal annealing. The total thickness of SiO layer remains 150 nm, and is partitioned equally into a number of sublayers (N = 3, 5, 10, or 30) by Si interlayers. For each N-layered sample, a maximal PL intensity of Si can be obtained via optimizing the thickness of Si interlayer (or d_Si). This maximal PL intensity varies with N, but the ratio of Si to O is nearly a constant. The brightest sample is found to be that of N = 10 and d_Si = 1 nm, whose PL intensity is ∼5 times that of N = 1 without additional Si doping, and ∼2.5 times that of Si-nc:SiO₂ prepared by co-evaporating of SiO and Si at the same optimized ratio of Si to O. Discussions are made based on PL, TEM, EDX and reflectance measurements.     

Emission properties of a distributed feedback laser cavity containing silicon nanocrystals

We report on light emission from silicon nanocrystals (Si-nc) in a laser cavity. Using modified electrochemical etching of Si wafers we prepare Si-nc with blue-shifted photoluminescence spectrum down to 580-620 nm, embedded at high-volume fractions in a SiO₂-based solid matrix. We insert this active medium into an optically pumped resonator. Since our samples are only partially homogeneous, we cannot use external mirrors in order to achieve optical feedback: we induced optically an internal distributed cavity by intense, spatially periodical excitation. Mode selection was simulated by a simplified theoretical model, based on an approach of multiple reflections. In the framework of the model we discuss the experimentally observed spectral emission changes induced by the distributed cavity.     

A combined approach to fabricating Si nanocrystals with high photoluminescence intensity

This work demonstrates that by combining three methods with different mechanisms to enhance the photoluminescence (PL) intensity of Si nanocrystals embedded in SiO₂ (or Si-nc:SiO₂), a promising material for developing Si light sources, a very high PL intensity can be achieved. A 30-layered sample of Si-nc:SiO₂/SiO₂ was prepared by alternatively evaporating SiO and SiO₂ onto a Si(1 0 0) substrate followed by thermal annealing at 1100 °C. This multilayered sample possessed a fairly high PL efficiency of 14% as measured by Greenham's method, which was 44 times that of a single-layered one for the same amount of excess Si content. Based on this multilayered sample, treatments of CeF₃ doping and hydrogen passivation were subsequently applied, and a high PL intensity which was 167 times that of a single-layered one for the same amount of excess Si content was achieved.     

Si/SiO<Subscript>2</Subscript> superlattice based optical planar microcavity

Si/SiO₂ Fabry–Pérot microcavities with a silicon nanocrystal (Si-nc) active spacer have been realized using a novel process based on a reactive magnetron sputtering of a pure silica target. Spectral, spatial and temporal behaviours of the quantum dots confined inside the resonator are detailed. Compared with a reference sample, the spectral and spatial emission distributions are significantly narrowed and the forward emission intensity is enhanced. Time resolved photoluminescence measurements also revealed an increase of the spontaneous emission rate. PACS 42.70.Qs; 78.55.-m; 78.66.-w     

Ab-initio calculations of luminescence and optical gain properties in silicon nanostructures

Density-functional and many body perturbation theory calculations have been carried out in order to study the optical properties both in the ground and excited state configurations, of silicon nanocrystals in different conditions of surface passivation. Starting from hydrogenated clusters, we have considered different Si/O bonding geometries at the interface. We provide strong evidence that not only the quantum confinement effect but also the chemistry at the interface has to be taken into account in order to understand the physical properties of these systems. In particular, we show that only the presence of a surface Si–O–Si bridge bond induces an excitonic peak in the emission-related spectra, redshifted with respect to the absorption onset, able to provide an explanation for both the observed Stokes shift and the near-visible PL experimentally observed in Si-nc. For the silicon nanocrystals embedded in a SiO₂ matrix, the optical properties are discussed in detail. The strong interplay between the nanocrystal and the surrounding host environment and the active role of the interface region between them is pointed out, in very good agreement with the experimental results. For each system considered, optical gain calculations have been carried out giving some insights on the system characteristics necessary to optimize the gain performance of Si-nc. To cite this article: E. Degoli et al., C. R. Physique 10 (2009).     

Nd photoluminescence study of Nd-doped Si-rich silica films obtained by reactive magnetron sputtering

Nd³⁺-doped silicon-rich silicon oxide (SRSO) thin films have been fabricated by reactive magnetron sputtering of a pure silica target topped with Nd₂O₃ chips. The concentration of Nd ions in the deposited layers is controlled by the number of Nd₂O₃ chips, whereas the incorporation of silicon excess is monitored by the hydrogen partial pressure, P_H2, introduced in the Ar plasma, owing to the ability of hydrogen to reduce the oxygen released by the sputtering of the silica target. Photoluminescence (PL) experiments were made at room temperature using a nonresonant excitation line from an Ar laser. The influences of Nd³⁺ content and P_H2 have been studied to optimize the Nd³⁺ emission. PL spectra reveal a two order of magnitude enhancement of the Nd³⁺ emission around both 0.9 and 1.1 μm, when Si nanoclusters (Si-nc) are formed in the same Nd³⁺-doped matrix. The dependence of the Nd³⁺ PL with P_H2 and Nd concentration is indicative of the occurrence of an efficient energy transfer from the Si-nc to the rare earth ions. The radiative lifetime is also deduced and commented in the light of Nd³⁺-Si-nc coupling.     

Aging effect on blue luminescent silicon nanocrystals prepared by pulsed laser ablation of silicon wafer in de-ionized water

Blue luminescent colloidal silicon nanocrystals (Si-ncs) were synthesized at room temperature by nanosecond pulsed laser ablation of a single-crystal silicon target in de-ionized water. Irregular Si-nc fragments obtained by laser ablation are stabilized into regularly shaped, spherical, and well-separated aggregates during the aging process in water. Aging in de-ionized water for several weeks improved the photoluminescence (PL) intensity. At least two weeks of aging are necessary for observation of broad blue room temperature PL with a maximum centered at 420 nm. Detailed structural analysis revealed that agglomerates after aging for several months contain Si-ncs with irregular shape smaller than the quantum confinement limit (<5 nm). These blue luminescent Si-ncs dispersed in de-ionized water exhibited a PL decay time of 6 ns, which is much faster than that of Si-ncs prepared in traditional ways (usually on the order of microseconds). The oxidized Si-ncs with quantum confinement effects are responsible for a PL band around 400 nm visible to the naked eye at room temperature.