Growth Of Silicon Nanocrystals Research Articles

(Invited) Characterization of Novel Nanostructured Terbium Doped Oxygen Rich Silicon Oxide for Photonic Applications

Rare earth doped silicon-based thin films can be used for light emitting applications, where the control of dopant type and concentration plays a significant role in the enhancement of optical functionalities [1]. In this work, nanostructured terbium (Tb) doped oxygen-rich silicon oxide (ORSO) samples were fabricated using a novel fabrication technique: integrated sputtering and plasma enhanced chemical vapor deposition (PECVD) [2]. The Tb dopant concentration in the ORSO host matrix was varied from 0.5 at. % to 17 at. % and post-deposition thermal annealing was performed in a wide range of temperatures in a N2 atmosphere. In addition, the presence of defects, one of the general physical problems limiting the emission efficiency of rare earth doped silicon-based materials, was investigated using a combination of photoluminescence measurements and positron annihilation spectroscopy (PAS).We discuss the influence of the Tb dopant concentration on the silicon nanocrystal growth and the formation of Tb silicate nanocrystals. A precipitation mechanism as a function Tb content and annealing temperature is proposed and tested using X-ray diffraction, photoluminescence, and microscopy techniques. Following post-deposition annealing at 1200 °C, all Tb ions agglomerate and form large nanocrystals with diameters reaching 50 nm (Fig. 1). Scanning transmission electron microscopy (STEM) analysis shows that these large nanocrystals are composed of Tb, silicon, and oxygen, however, no Tb remains in the amorphous regions of the silicon oxide matrix. Despite the oxygen-rich content, small silicon nanocrystals (Si-ncs) are formed in the matrix with a size distribution between 2 and 5 nm.The influence of hydrogen passivation and the contribution of trap states to the luminescence mechanism is discussed using the interdependency of the photoluminescence emission for the dominant Tb excited state (5D4 to 7F5) and the values of the S-parameter obtained from PAS. The values of the S- and W-parameters identify the traps at the interface between the amorphous silicon oxide host matrix and embedded nanocrystals.Finally, the solubility level of Tb ions is extended using this novel processing method and an optimized Tb concentration for the most luminescent matrix is suggested. High-temperature annealing promoted the formation of a different Tb silicate crystal phase than the phases observed in samples with lower Tb content. A fiber texture with no preferred orientation in the plane but growth direction of <200> is observed [Fig. 2]; however, samples with lower Tb content are fully random and show no distinct texture. The correlation between the nanostructure and emission properties is an important step to fabricating Tb-doped silicon oxide materials which can be potentially used in a variety of light emitting applications such as displays, solar cells, optical communications, and other silicon optoelectronic devices.Fig. 1 High-resolution transmission electron microscopy (HR-TEM) image of a focused ion beam (FIB) prepared Tb-doped ORSO thin film annealed at 1200 ℃ for 1 hour in N2 atmosphere.Fig. 2 A fiber texture is evident by the ring of intensity shown in the pole figure and no preferred orientation in plane is observed.

Silicon nitride-capped silicon nanocrystals via a nonthermal dual-plasma synthesis approach

Improving the photoluminescence quantum yields and air-stability of silicon nanocrystals is crucial to expanding their influence in optoelectronic devices and other burgeoning application areas. Here, a dual-plasma approach for the synthesis of silicon nanocrystals capped with silicon nitride is reported. The reactor consists of two plasma stages in series: a primary radiofrequency (rf) plasma for silicon nanocrystal growth from silane and argon gas followed by a secondary rf plasma for silicon nitride growth using nitrogen gas as the reactant. The core-shell nanocrystals were characterized using optical and structural analyses, and the plasma was characterized using optical emission spectroscopy. The resulting core-shell nanocrystals show a reduced susceptibility to ambient air oxidation as compared to bare silicon nanocrystals alone. This result is a step toward achieving highly efficient and air-stable photoluminescence from silicon nanocrystals while avoiding organic functionalization.

Enhancement of the luminescence by the controlled growth of silicon nanocrystals in SRO/SiO2 superlattices

This work reports the study of highly-luminescent silicon nanocrystals (SiNCs) in silicon rich oxide (SRO)/SiO2 multilayers (MLs). Parameters such as silicon excess (Si-excess) and SRO-thickness were modified to evaluate the structure and composition and their effect on the photoluminescence (PL) response of the different superlattices. SRO monolayers with the same silicon excess were also deposited for comparison. Both, monolayers and MLs, emit a broad emission band in the red-orange region (1.45–2.1 eV). The PL of SRO monolayers strongly increases as Si-excess decreases from 10.2 to 5.2 at.%. Nevertheless, SRO/SiO2 MLs allow up to 14-fold PL enhancement as compared to SRO monolayers. A silicon diffusion from SRO nano-layers towards the SiO2 ones reduces the Si content within the SRO allowing the SiNC size reduction (thus increasing the SiNC density) as compared to SRO monolayers. Therefore, the high luminescence is correlated with the SiNCs formation with a mean size below 3 nm where the surface defects (SiO bonds) are strongly active.

Direct Insight into Ce-Silicates/Si-Nanoclusters Snowman-Like Janus Nanoparticles Formation in Ce-Doped SiOx Thin Layers

The present work reports a nanoscale chemical and structural study on the influence of Ce content in Ce-doped SiO1.5 thin films via atom probe tomography (APT). Using this technique, we can explore 3D mapping of the atomic distribution inside a material. Such an investigation is crucial to optimize the optical properties of Ce-doped SiOx films. As a result, we clearly identify the influence of cerium on the phase separation process, on the silicon nanocrystal growth, and on the formation of cerium silicate nanoparticles occurring during annealing treatments. The observed nanoscale structure is correlated with the optical properties measured by room temperature photoluminescence spectroscopy thus leading to a comprehensive understanding of the properties of Ce-doped SiO1.5 thin films.

Structural and optoelectronic properties of p-type SiO:H films deposited in transition zone

P-type hydrogenated silicon oxide (p-SiOx:H) films are prepared by radio frequency plasma enhanced chemical deposition with various CO2 flow rates. We use gas mixtures of carbon dioxide (CO2), hydrogen (H2), silane (SiH4) and diborane (B2H6) as reaction source gases. For all experiments the substrate temperature, pressure and power density are fixed at 200 oC, 200 Pa and 200 mW/cm2, respectively. The films are deposited on Corning Eagle 2000 glass substrates for optoelectronic measurements and on crystalline Si wafers for Fourier transform infrared (FTIR) measurement. The structural, optical and electronic properties of the films are systematically studied as a function of CO2 flow rate. The CO2 flow rate is varied from 0 to 1.2 cm3 min-1, with all other parameters kept constant. It is shown that with the CO2 flow rate increasing from 0 to 1.2 cm3 min-1, the Raman peak shifts from 520 cm-1 to 480 cm-1 and corresponding crystalline volume fraction decreases from 70% to 0. In addition, the FTIR spectrum shows that the oxygen content increases from 0 to 17% and the hydrogen bond configuration gradually shifts from mono-hydrogen (Si-H) to di-hydrogen (Si-H2) and (Si-H2)n complexes in the film. What is more, with the incorporation of oxygen, the optical band gap of each of all p-type SiO:H films increases from 1.8 eV to 2.13 eV, while the dark conductivity decreases from 3 S/cm (nc-Si:H phase) to 8.310-6 S/cm (a-SiOx:H phase). Furthermore, the oxygen incorporation tends to disrupt the growth of silicon nanocrystals due to the created dangling bonds that arises from an increased structural disorder. This leads to microstructural evolution of SiO:H film from a single nanocrystalline phase into first a mixed amorphous-nanocrystalline and subsequently into an amorphous phase. At a certain threshold of CO2 flow rate, a transition from nanocrystalline to amorphous growth takes place. The transition from nanocrystalline to amorphous silicon is confirmed by Raman and FTIR spectra. In the transition region or crystalline volume fraction of about 45%, Raman spectrum also reveals that the a mixture of nanocrystalline silicon and amorphous silicon oxide (a-SiOx:H) phase exists in the film. This means that nanocrystalline silicon oxide (nc-SiO:H) is a two-phase structural material consisting of a dispersion of silicon nanocrystals (nc-Si) embedded in the amorphous SiOx network. As is well known, the oxygen-rich amorphous phase can help enhance the optical band gap, while the nc-Si phase contributes to high conductivity. Finally, it is the SiO:H film deposited at phase transition that can realize a relatively high dark conductivity (about S/cm) with a wide optical band gap of 2.01 eV in the film. By using the transition p-layer as the window layer in conjunction with a suitable buffer thickness, we obtain a thin film solar cell with an open-circuit voltage of 890 mV, a short-circuit current density of 12.77 mAcm-2, fill factor of 0.73, and efficiency of 8.27% without using any back reflector.

Size dependence of phosphorus doping in silicon nanocrystals

Doping of silicon nanocrystals (Si-NCs) is one of the major challenges for silicon nanoscale devices. In this work, phosphorus (P) doping in Si-NCs which are embedded within an amorphous silicon matrix is realized together with the growth of Si-NCs by plasma-enhanced chemical vapor deposition under a tunable substrate direct current (DC) bias. The variation of phosphorus concentration with substrate bias can be explained by the competition of bonding processes of Si–Si and P–Si bonds. The formation of Si–Si and P–Si bonds is differently influenced by the ion bombardment controlled by the substrate bias, due to their bonding energy difference. We have studied the influences of grain size on P doping in Si-NCs. Free carrier concentration, which is provided by activated P atoms, decreases with decreasing grain size due to increasing formation energy and activation energy of P atoms incorporated in Si-NCs. Furthermore, we have studied the P locations inside Si-NCs and hydrogen passivation of P in the form of P–Si–H complexes using the first-principles method. Hydrogen passivation of P can also contribute to the reduced free carrier concentration in smaller Si-NCs. These results provide valuable understanding of P doping in Si-NCs.

In situ growth of fluorescent silicon nanocrystals in a monolithic microcapsule as a photostable, versatile platform.

A facile, one-step method was developed for the in situ formation of fluorescent silicon nanocrystals (SiNC) in a microspherical encapsulating matrix. The obtained SiNC encapsulated polymeric microcapsules (SiPM) possess uniform size (0.1-2.0 μm), strong fluorescence, and nanoporous structure. A unique two stage, time dependent reaction was developed, as the growth of SiNC was slower than the formation of polymeric microcapsules. The resulting SiPM with increasing reaction time exhibited two levels of stability, and correspondingly, the release of SiNC in aqueous media showed different behavior. With reaction time <1 h, the obtained low-density SiPM (LD-SiPM) as matrix microcapsules, would release encapsulated SiNC on demand. With >1 h reaction time, resulting high-density SiPM (HD-SiPM) became stable SiNC reservoirs. SiPM exhibit stable photoluminescence. The porous structure and fluorescence quenching effects make SiPM suitable for bioimaging, drug loading and sorption of heavy metals (Hg(2+) as shown) as an intrinsic indicator. SiPM were able to reduce metal ions, forming SiPM/metal oxide and SiPM/metal hybrids, and their applications in bio-sensing and catalysis were also demonstrated.

Silicon nanocrystal growth under irradiation of electron beam

In our experiment, it was observed that silicon nanocrystal rapidly grows with irradiation of electron beam on amorphous silicon film prepared by pulsed laser deposition, and shape of silicon nanocrystal is usually sphere in smaller nanoscale with less exposure time under electron beam, in which the quantum dots are prepared in nanoscale near 3 nm. In the electron interaction process, it was investigated that the various crystals structures in different orientations occur in the same time and the condensed structures of silicon nanocrystal are changed with different impurity atoms in silicon film.

Gas phase synthesis of two ensembles of silicon nanoparticles

Dusty plasmas provide a very favorable environment for the growth of silicon nanocrystals. For application of silicon nanocrystals in a solar cell, the fabrication of monodisperse silicon quantum dots has been challenging. We report a single step method to synthesize silicon (Si) nanoparticles in a custom designed dedicated plasma reactor. The nanoparticles produced in the gas phase belong to two different phases exhibiting different structural and optical properties. Particles made in the bulk of the plasma are aggregates of crystalline particles with a mean size of 100 nm. Particles made in locally enhanced plasma regions produced at holes present in the grounded electrode contain free-standing quantum sized particles with crystallites (with mean size of 2.95 nm) embedded within an amorphous matrix. We provide insight on different plasma processes leading to the formation of aggregates and free-standing particles. We hypothesize that the free standing particles are formed due to the excess energetic electrons present in locally enhanced discharges.

Formation of size controlled silicon nanocrystals in nitrogen free silicon dioxide matrix prepared by plasma enhanced chemical vapor deposition

This paper reports the growth of silicon nanocrystals (SiNCs) from SiH4–O2 plasma chemistry. The formation of an oxynitride was avoided by using O2 instead of the widely used N2O as precursor. X-ray photoelectron spectroscopy is used to prove the absence of nitrogen in the layers and determine the film stoichiometry. It is shown that the Si rich film growth is achieved via non-equilibrium deposition that resembles a interphase clusters mixture model. Photoluminescence and Fourier transformed infrared spectroscopy are used to monitor the formation process of the SiNCs, to reveal that the phase separation is completed at lower temperatures as for SiNCs based on oxynitrides. Additionally, transmission electron microscopy proves that the SiNC sizes are well controllable by superlattice configuration, and as a result, the optical emission band of the Si nanocrystal can be tuned over a wide range.

Application of Metal-Oxide-Semiconductor structures containing silicon nanocrystals in radiation dosimetry

Abstract This article makes a brief review of the most important results obtained by the authors and their collaborators during the last four years in the field of the development of metal-insulator-silicon structures with dielectric film containing silicon nanocrystals, which are suitable for applications in radiation dosimetry. The preparation of SiOx films is briefly discussed and the annealing conditions used for the growth of silicon nanocrystals are presented. A two-step annealing process for preparation of metal-oxide-semiconductor structures with three-layer gate dielectrics is described. Electron Microscopy investigations prove the Si nanocrystals growth, reveal the crystal spatial distribution in the gate dielectric and provide evidences for the formation of a top SiO2 layerwhen applying the two-step annealing. Two types of MOS structures with three region gate dielectricswere fabricated and characterized by high frequency capacitance/conductancevoltage (C/G-V) measurements. The effect of gamma and ultraviolet radiation on the flatband voltage of preliminary charged metal-oxide-semiconductor structures is investigated and discussed.

Density improvement of silicon nanocrystals embedded in silicon carbide matrix deposited by hot-wire CVD

The thin films of silicon nanocrystals (Si-NC) embedded in silicon carbide (SiC) matrix (Si-NC:SiC) were developed by using hot-wire chemical vapor deposition (HWCVD) from silane (SiH4) and methane (CH4) mixture gases diluted by hydrogen (H2). This method avoids the co-precipitation of Si-NCs and SiC-NCs from a high-temperature annealing process. According to the classical theory of nucleation, this paper experimentally investigates the possibilities to increase the density of Si-NCs by optimizing two processing parameters (ratio of SiH4 to CH4 and working gas pressure). Using Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infra-red spectroscopy (FTIR), we characterized the grain size, crystal volume fraction, topography and bond configurations of as-deposited films. The experimental results demonstrate that increasing the working gas pressure can lead to higher density of Si-NCs, while increasing the ratio of SiH4 to CH4 can only increase the grain size, which is consistent with the mechanism of nucleation and growth of Si-NCs. This method can be used to improve the density of Si-NCs embedded in SiC matrix deposited by CVD without high-temperature annealing process.

Donor ionization in size controlled silicon nanocrystals: The transition from defect passivation to free electron generation

We studied the photoluminescence spectra of silicon and phosphorus co-implanted silica thin films on (100) silicon substrates as a function of isothermal annealing time. The rapid phase segregation, formation, and growth dynamics of intrinsic silicon nanocrystals are observed, in the first 600 s of rapid thermal processing, using dark field mode X-TEM. For short annealing times, when the nanocrystal size distribution exhibits a relatively small mean diameter, formation in the presence of phosphorus yields an increase in the luminescence intensity and a blue shift in the emission peak compared with intrinsic nanocrystals. As the mean size increases with annealing time, this enhancement rapidly diminishes and the peak energy shifts further to the red than the intrinsic nanocrystals. These results indicate the existence of competing pathways for the donor electron, which depends strongly on the nanocrystal size. In samples containing a large density of relatively small nanocrystals, the tendency of phosphorus to accumulate at the nanocrystal-oxide interface means that ionization results in a passivation of dangling bond (Pb-centre) type defects, through a charge compensation mechanism. As the size distribution evolves with isothermal annealing, the density of large nanocrystals increases at the expense of smaller nanocrystals, through an Ostwald ripening mechanism, and the majority of phosphorus atoms occupy substitutional lattice sites within the nanocrystals. As a consequence of the smaller band-gap, ionization of phosphorus donors at these sites increases the free carrier concentration and opens up an efficient, non-radiative de-excitation route for photo-generated electrons via Auger recombination. This effect is exacerbated by an enhanced diffusion in phosphorus doped glasses, which accelerates silicon nanocrystal growth.

Silicon nanocrystals on amorphous silicon carbide alloy thin films: Control of film properties and nanocrystals growth

The present study demonstrates the growth of silicon nanocrystals on amorphous silicon carbide alloy thin films. Amorphous silicon carbide films [a-Si1−xCx:H (with x<0.3)] were obtained by plasma enhanced chemical vapor deposition from a mixture of silane and methane diluted in hydrogen. The effect of varying the precursor gas-flow ratio on the film properties was investigated. In particular, a wide optical band gap (2.3eV) was reached by using a high methane-to-silane flow ratio during the deposition of the a-Si1−xCx:H layer. The effect of short-time annealing at 700°C on the composition and properties of the layer was studied by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. It was observed that the silicon-to-carbon ratio in the layer remains unchanged after short-time annealing, but the reorganization of the film due to a large dehydrogenation leads to a higher density of SiC bonds. Moreover, the film remains amorphous after the performed short-time annealing. In a second part, it was shown that a high density (1×1012cm−2) of silicon nanocrystals can be grown by low pressure chemical vapor deposition on a-Si0.8C0.2 surfaces at 700°C, from silane diluted in hydrogen. The influence of growth time and silane partial pressure on nanocrystals size and density was studied. It was also found that amorphous silicon carbide surfaces enhance silicon nanocrystal nucleation with respect to SiO2, due to the differences in surface chemical properties.

Impact of AFM-induced nano-pits in a-Si:H films on silicon crystal growth

Conductive tips in atomic force microscopy (AFM) can be used to localize field-enhanced metal-induced solid-phase crystallization (FE-MISPC) of amorphous silicon (a-Si:H) at room temperature down to nanoscale dimensions. In this article, the authors show that such local modifications can be used to selectively induce further localized growth of silicon nanocrystals. First, a-Si:H films by plasma-enhanced chemical vapor deposition on nickel/glass substrates are prepared. After the FE-MISPC process, yielding both conductive and non-conductive nano-pits in the films, the second silicon layer at the boundary condition of amorphous and microcrystalline growth is deposited. Comparing AFM morphology and current-sensing AFM data on the first and second layers, it is observed that the second deposition changes the morphology and increases the local conductivity of FE-MISPC-induced pits by up to an order of magnitude irrespective of their prior conductivity. This is attributed to the silicon nanocrystals (<100 nm) that tend to nucleate and grow inside the pits. This is also supported by micro-Raman spectroscopy.

Interfacial Polar‐Bonding‐Induced Multifunctionality of Nano‐Silicon in Mesoporous Silica

AbstractThe optoelectronic response of a material governs its suitability for a wide range of applications, from photon detection to photovoltaic conversion. To conquer the material limitations and achieve improved optoelectronic responses, nanotechnology has been employed to arrange subunits with specific size‐dependent quantum mechanical properties in a hierarchically organized structure. However, building a functional optoelectronic system from nano‐objects remains a formidable challenge. In this paper, the fabrication of a new artificially engineered optoelectronic material by the preferential growth of silicon nanocrystals on the bottom of the pore‐channels of mesoporous silica is reported. The nanocrystals form highly stable interface structures bonded on one side; these structure show strong electron–phonon coupling and a ferroelectric‐like hysteretic switching property. A new class of multifunctional materials is realized by invoking a concept that employs semiconductor nanocrystals for optical sensing and utilizes interfacial polar layers to facilitate carrier transport and emulate ferroelectric‐like switching.

Hot wire CVD deposition of nanocrystalline silicon solar cells on rough substrates

In silicon thin film solar cell technology, frequently rough or textured substrates are used to scatter the light and enhance its absorption. The important issue of the influence of substrate roughness on silicon nanocrystal growth has been investigated through a series of nc-Si:H single junction p-i-n solar cells containing i-layers deposited with Hot-wire CVD. It is shown that silicon grown on the surface of an unoptimized rough substrate contains structural defects, which deteriorate solar cell performance. By introducing parameter v, voids/substrate area ratio, we could define a criterion for the morphology of light trapping substrates for thin film silicon solar cells: a preferred substrate should have a v value of less than around 1 × 10 - 6 , correlated to a substrate surface rms value of lower than around 50 nm. Our Ag/ZnO substrates with rms roughness less than this value typically do not contain microvalleys with opening angles smaller than ~ 110°, resulting in solar cells with improved output performance. We suggest a void-formation model based on selective etching of strained Si-Si atoms due to the collision of growing silicon film surface near the valleys of the substrate.

Effect of the line tension at the vapor-liquid-solid boundary on the growth of silicon nanocrystals

We examine the thermodynamic aspects of the effect of the line tension at a bent vapor-liquid-solid boundary on quasi-one-dimensional growth of silicon nanowires. Experimental data demonstrate that there is a limiting size of solvent nanodrops below which they cannot exist in equilibrium and that there is a limiting nanowire diameter starting at which the increase in line tension with decreasing diameter prevents nanocrystal growth at a given supersaturation.

Effect of Hydrogen Dilution on Growth of Silicon Nanocrystals Embedded in Silicon Nitride Thin Film by Plasma-Enhanced CVD

An investigation was conducted into the effect of hydrogen dilutionon the microstructure and optical properties of silicon nanograinsembedded in silicon nitride (Si/SiNx) thin film depositedby the helicon wave plasma-enhanced chemical vapour depositiontechnique. With Ar-diluted SiH4 and N2 as the reactant gassources in the fabrication of thin film, the film was formed at ahigh deposition rate. There was a high density of defect at theamorphous silicon (a-Si)/SiNx interface and a relative lowoptical gap in the film. An addition of hydrogen into the reactantgas reduced the film deposition rate sharply. The silicon nanograinsin the SiNx matrix were in a crystalline state, and thedensity of defects at the silicon nanocrystals (nc-Si)/SiNxinterface decreased significantly and the optical gap of the filmswidened. These results suggested that hydrogen activated by theplasma could not only eliminate in the defects between the interfaceof silicon nanograins and SiNx matrix, but also helped thenanograins transform from the amorphous into crystalline state. Bychanging the hydrogen dilution ratio in the reactant gas sources, atunable band gap from 1.87 eV to 3.32 eV was obtained in theSi/SiNx film.

Study of radial growth rate and size control of silicon nanocrystals in square-wave-modulated silane plasmas

The growth of silicon nanocrystals in high pressure and high dilution silane plasmas is investigated by using the temporal evolution of the self-bias on the radio frequency electrode and transmission electron microscopy. A square-wave-modulated plasma was used in order to control the growth of monodispersed nanoparticles with sizes smaller than 12nm. To this end, the plasma on time was kept below 1s. The radial growth rate of nanoparticles was varied in the range from 7.5to75nm∕s by changing silane partial pressure. Nanoparticles grown in silane-helium discharges have been found amorphous while they are crystalline in silane-hydrogen-argon discharges. Surprisingly, the crystallization in the gaseous phase does not depend on how slow or fast the particles grow but on the presence of atomic hydrogen.