Embedded Silicon Nanocrystals Research Articles

Hydrogenation of silicon-nanocrystals-embedded silicon oxide passivating contacts

We investigate the effect of hydrogen passivation of dangling bonds in silicon oxide passivating contacts with embedded silicon nanocrystals (NAnocrystalline Transport path in Ultra-thin dielectrics for REinforced passivation contact, NATURE contact). We first investigated the differences in electrical properties of the samples after hydrogen gas annealing and hydrogen plasma treatment (HPT). The results show that the NATURE contact was efficiently passivated by hydrogen after HPT owing to the introduction of hydrogen radicals into the structure. Furthermore, we examined the dependence of process parameters such as HPT temperature, duration, and H2 pressure, on the electrical properties and hydrogen depth profiles. As a result, HPT at 500 °C, 15 min, and 0.5 Torr resulted in a large amount of hydrogen inside the NATURE contact and the highest implied open-circuit voltage of 724 mV. Contact resistivity and surface roughness hardly increased when HPT was performed under the optimized condition, which only improved the passivation performance without deteriorating the electron transport properties of the NATURE contact.

Comparison of the Effects of Environmental Treatments on Hydrogen Concentration and Energy Gap Variations of Hydrogenated Amorphous and Polymorphous Silicon Films Prepared by PECVD Technique

Hydrogen to silicon (Si–H) bond concentration and strength play important roles in high quality hydrogenated amorphous silicon layers prepared by PECVD techniques. In this paper, a number of amorphous and polymorphous Si layers have been deposited at different plasma conditions where a wide range of hydrogen concentration in the films are obtained. Some of the samples were stored in free air and the others in nitrogen for eight days. The layers were analyzed using AFM, FTIR, Raman, UV–Visible, and TEM immediately after deposition and after treatments. The results indicate that in the amorphous films with appreciable amount of embedded silicon nanocrystals, the variation of hydrogen content behaves differently than that of the amorphous films. It has been observed that treatment in the air increases the energy gap of the nanocrystals surrounded by oxide shells, formed around the surface nanocrystals, due to the quantum confinement effects.

Red-shifted photoluminescence and gamma irradiation stability of “micromorph” (nc-Si/SiOx)/DLC down-converter anti-reflection coatings

Down-converting “micromorph” silicon dioxide films with embedded silicon nanocrystals (nc-Si) covered by diamond-like carbon (DLC) are fabricated. DLC used as a radiation-protective, stabilizing and anti-reflection coating is found to modify optical and structural properties of nc-Si. The films are in detail characterized employing scanning electron microscopies, energy dispersive X-ray, Raman and FTIR spectroscopies. The existence of amorphous and nanocrystals phases in nc-Si/SiOx films has been confirmed using Raman spectroscopy. The impact of γ-irradiation on the DLC film was also shown. The main benefit is that photoluminescence (PL) spectrum of nc-Si/SiOx after deposition of DLC shifts in the infrared region close to the maximum of Si-based solar cells photoresponse with simultaneous PL enhancement by about 1.2 times. This effect has been explained by the passivation of non-radiative recombination centers in nc-Si/SiOx by H atoms. Studying the influence of γ-irradiation on the DLC coatings, the decrease of integral PL intensity of the structures due to the radiation damages of C-Hn bonds has been found. Moreover, γ-irradiation increases the disorder of C-Hn bonds in DLC as well as decreases the optical transmittance by ~20% in the range of 500 to 1100 nm, possibly, due to a weak graphitization of the films. The results show that nc-Si films with DLC as an antireflection, protection and down-converting coating might be considered as a perspective combination to improve the efficiency and degradation stability of solar cells.

Size and Surface Effects of Silicon Nanocrystals in Graphene Aerogel Composite Anodes for Lithium Ion Batteries

Silicon is recognized as a promising anode material for high-performance lithium ion batteries due to its high theoretical specific capacity and elemental abundance. Challenges related to the low electrical conductivity of Si and large volume changes during the lithiation/delithiation cycles, as well as the low rate of lithium diffusion in silicon anodes, hinder practical applications. To provide fundamental insights into these issues, silicon nanocrystal/graphene aerogel nanocomposites were synthesized by combining undecanoic acid-functionalized silicon nanocrystals of various sizes (SiX-COOH, where X represents the nanocrystal diameter of 3, 5, 8, and 15 nm) with conductive mesoporous graphene aerogels (GAs). The silicon nanocrystals are evenly dispersed throughout the graphene aerogel as shown by energy-dispersive X-ray (EDX) mapping. In terms of electrochemical performance, SiX-COOH/GA nanocomposites demonstrated a clear dependence on the size of the embedded silicon nanocrystals, with the composites ...

Optical and Compositional Properties of SiOx Films Deposited by HFCVD: Effect of the Hydrogen Flow

In this work, the effect of hydrogen flow and thermal annealing on the compositional and optical properties of non-stoichiometric silicon oxide (SiOx) films with embedded silicon nanocrystals is reported. The SiOx films are obtained by hot filament chemical vapor deposition technique at three different hydrogen flow levels, namely, 50 sccm, 100 sccm, and 150 sccm. The SiOx films are characterized by different techniques. It is found by x-ray photoelectron spectroscopy (XPS) that with increasing hydrogen flow, the SiOx films contain higher silicon (Si) concentration. When the hydrogen flow decreases, the absorption edge of the as-grown SiOx films, as obtained from the transmittance spectra, shifts from 300 nm to 500 nm, and this opens the possibility of band gap tuning. Increasing the hydrogen flow level in turn means that the SiOx films contain higher Si concentration, as confirmed by the XPS profile composition measured in the SiOx films. After thermal annealing, the SiOx films transmittance spectra showed a further shift of the absorption edge toward larger wavelengths. The Fourier transform infrared (FTIR) spectroscopy reveals film composition changes induced by the hydrogen flow variations. In addition, the FTIR spectra reveal the bands attributed to the hydrogen presence in the as-grown SiOx films. The bands become more intense with increasing hydrogen flow, but they rapidly disappear after the thermal annealing. The as-grown SiOx films exhibit wide band photoluminescence (PL) spectra with the main components at 688 nm, 750 nm, and 825 nm. The SiOx film deposited at 100 sccm hydrogen flow level shows the strongest PL intensity. According to PL results, the thermal annealing of the SiOx films generates the PL quenching in all samples due to hydrogen evaporation. The defects such as OH and Si–H groups in the as-grown SiOx films not only modify the optical band gap structure, but they also play the role of passivating non-radiative defects, which enhances the PL emission.

Radiation Induced Enhancement of Hydrogen Influence on Luminescent Properties of nc-Si/SiO2 Structures.

Using photo-luminescence, infrared spectroscopy, and electron spin resonance technique, the silicon dioxide films with embedded silicon nanocrystals (nc-Si/SiO2 structures) have been investigated after γ-irradiation with the dose 2 × 107 rad and subsequent annealing at 450 °C in hydrogen ambient. For the first time, it was shown that such a radiation-thermal treatment results in significant increase of the luminescence intensity, in a red shift of the photoluminescence spectra, and in disappearance of the electron-spin resonance signal related to silicon broken bonds. This effect has been explained by passivation of silicon broken bonds at the nc-Si–SiO2 interface with hydrogen and by generation of new luminescence centers, these centers being created at elevated temperatures due to transformation of radiation-induced defects.

Study of the photon down-conversion effect produced by thin silicon-rich oxide films on silicon solar cells

In the present work, we studied the photon down-conversion effect produced by thin films of silicon oxide with embedded silicon nanocrystals also called silicon-rich oxide (SRO). These films have been used to absorb high energy light and the re-emission of two or more low energy photons (~1.1 eV) with the goal of improving the external quantum efficiency and consequently the conversion efficiency of silicon solar cells. According to our results, the incorporation of a thin SRO film on the solar cell surface increases the short circuit current and the FF of the silicon solar cells; the enhancement of spectral response is due to the high photoluminescence intensity of the SRO in the visible region when irradiated with UV light. An improvement of 38% in the solar cell efficiency has been observed in our particular solar cell fabrication process by the use of an SRO film with high photoluminescence intensity, which replaces the conventional silicon dioxide film. Copyright © 2016 John Wiley & Sons, Ltd.

Effect of the reflection by the silicon aggregates on the photoluminescence from silicon nitride film embedded silicon nanocrystals

In this work, the effect of the high silicon nanocrystals density as well as the radiated waves interference on the photoluminescence (PL) spectrum of silicon nanocrystal (Si-nc) embedded in silicon nitride film is studied. The film is prepared using low pressure chemical vapor deposition (LPCVD) following by a high temperature annealing. It was found that for silicon nitride of a high silicon content sample, the interference effect based on only the thickness of the film is unable to satisfactory explain the distortion on the PL spectrum. Using Monte Carlo analysis, it was shown that the simulated and the experimental PL spectra accurately superpose when taking into account the contribution of the reflection from silicon nanocrystals as well as the distance traveled by the radiated waves.

UV-Vis Photocurrent in SiOx Films with Silicon Nanocrystals Obtained by HFCVD

Actually, optical and electrical characteristics of the SiOx films need to be understood in order to improve and propose optoelectronics devices. Non-stoichiometric silicon oxide (SiOx) films with embedded silicon nanocrystals (Si-ncs) were obtained by hot filament chemical vapor deposition (HFCVD) technique. The authors report high photocurrent of two-terminal metal-oxide-semiconductor (MOS)-like where light is absorbed in the SiOx films with Si-ncs on n–type silicon substrates. Operated at both bias (reverse and direct), where enhanced photocurrent and increased when applied white light, short UV and large UV were observed. The optical properties as photoluminescence and absorption spectra were obtained. Current-Voltage (I–V) measurements in dark and under illumination conditions were realized. The Si-ncs and defects in the SiOx films should have an important role in the high photocurrent. We assume that a high response of the SiOx/Si junction is achieved probably due to a combined effect of the optical down conversion and photo excited electrons in the SiOx films.

Nanocrystal size distribution analysis from transmission electron microscopy images.

We propose a method, with minimal bias caused by user input, to quickly detect and measure the nanocrystal size distribution from transmission electron microscopy (TEM) images using a combination of Laplacian of Gaussian filters and non-maximum suppression. We demonstrate the proposed method on bright-field TEM images of an a-SiC:H sample containing embedded silicon nanocrystals with varying magnifications and we compare the accuracy and speed with size distributions obtained by manual measurements, a thresholding method and PEBBLES. Finally, we analytically consider the error induced by slicing nanocrystals during TEM sample preparation on the measured nanocrystal size distribution and formulate an equation to correct this effect.

Thermo-optical bistability with Si nanocrystals in a whispering gallery mode resonator

We report on the observation of optical bistability in an integrated planar microresonator with embedded silicon nanocrystals (Si-ncs). The phenomenon originates from the thermo-optical modulation of the silica-embedded Si-ncs refractive index, which in turn alters the spectral position of the resonator mode. The estimated thermo-optical coefficient of the Si nanocrystalline material, dn/dT≈2.92×10(-5)> K(-1), is an order of magnitude lower than that of bulk silicon. Both time-resolved pump-and-probe experiments and numerical simulations confirm that the silica host is responsible for the heat dissipation from the resonator. Moreover, a negligible Q-factor degradation at pump powers as high as 100 mW, along with the absence of a fast component in time-resolved measurements, confirm the minute contribution from excited carriers effects. These observations, combined with the already published large third-order nonlinearities of Si-ncs (an order of magnitude larger than in bulk Si), make this system an outstanding candidate for low-power on-chip nonlinear comb generation.

Two-dimensional photonic crystal slab with embedded silicon nanocrystals: Efficient photoluminescence extraction

A two-dimensional photonic crystal (PhC) slab was fabricated from a luminescent planar waveguide, formed by a (800 nm thick) layer of silicon nanocrystals (SiNCs) embedded in a polished silica plate. Dimensions of the PhC were designed so that light emitted by SiNCs under excitation with an external UV source can, during its propagation in the layer, interact with the periodicity and be Bragg-diffracted into air. This approach leads to up to 8-fold vertical extraction enhancement of SiNCs luminescence from the PhC slab compared to the bare planar layer. Results of the experiment are supported by the computer simulation.

Photoluminescence in dense arrays of silicon nanocrystals: the role of the concentration and average size

The computer-assisted Monte Carlo simulation of the photoluminescence kinetics in planar insulator structures with embedded silicon nanocrystals is carried out. In the simulation, the number of nanocrystals in the array was >106. It is shown that, in rather dense arrays, a significant role is played by the processes of energy exchange between neighboring nanocrystals, resulting in the elimination of smaller nanocrystals from the luminescence process and in a noticeable decrease in the integrated luminescence intensity. Nonexponential decay of the luminescence intensity is established. Such character of the decay is attributed to different relaxation processes prevailing at different stages of system’s evolution.

Performance Enhancement of Thin-Film Transistors With Suspended Poly-Si Nanowire Channels by Embedding Silicon Nanocrystals in Gate Nitride

In this letter, we fabricated and characterized thin-film transistors with a suspended poly-Si nanowire (NW) channel and gate nitride with embedded silicon nanocrystals (Si NCs). The embedded Si NCs increase the surface roughness, thus reducing the adhesive force as the nitride is in contact with the poly-Si NW channel during the operation. Such a feature results in a reduction in pull-in voltage and sharper pull-out behavior. Moreover, this approach also greatly improves the endurance characteristics of the devices.

Direct observation of quantum confinement of Si nanocrystals in Si-rich nitrides

The band gap of the active layers in stacked Si-based tandem solar cells can be fine tuned by changing the size of embedded silicon nanocrystals (Si NCs). Although SiO${}_{2}$ matrices have been predominantly used for such applications, nitride phases have recently emerged as a promising alternative. In this paper, we use high-resolution scanning transmission electron microscopy and energy-loss spectroscopy to report on the electronic structure of individual Si NCs embedded in silicon nitride films. Si NCs were produced by rf sputtering and exhibited controllable crystallite size and quality via different thermal annealing conditions. Quantum confinement effects were observed through a blue shift in both conduction band edges and volume plasmon energies as a function of particle size and structure. We show that, in good agreement with theoretical models, the volume plasmon energy ${E}_{p}$ (eV) is related to the size $d$ (nm) of Si NCs by ${E}_{p}$ $=$ 16.89$+$ 23.90/${d}^{2}$. Lattice distortion in twinned Si NCs and dangling bonds at defect centers are shown to be the cause of a weakening in quantum size effects and a reduction in the light emission efficiency of the films. Both electron spectroscopy and optical results are consistent in explaining the correlation between structure and optoelectronic properties of Si NCs.

Ultra-high four wave mixing efficiency in slot waveguides with silicon nanocrystals

We report on wavelength conversion through four-wave-mixing in silicon slot-waveguides with embedded silicon nanocrystals. The combination of strong optical confinement and Si:nc nonlinearity provides a huge waveguide nonlinear coefficient γ = 1100 W−1m−1. Moreover, the improvement in the fabrication procedure allowed a loss reduction with respect to previous reported structures, enabling the achievement of an extreme value for the conversion efficiency which represents the best result ever reported in the scientific literature.

Charge spectroscopy of SiO2 layers with embedded silicon nanocrystals modified by irradiation with high-energy ions

The method of charge deep-level transient spectroscopy (Q-DLTS) is used to study and compare the ejection of charge carriers from silicon nanocrystals (NCs) located in an ordered or random way in the SiO2 matrix. It is shown that, in all cases, this ejection is a thermally activated process. The parameters of energy barriers characterizing the processes of ejection of charge carriers from the levels of nanocrystals in the layers NCs:SiO2 before (random distribution) and after their modification by irradiation with high-energy ions (ordered distribution of nanocrystals) are determined. It is found that the activation energies for ejection of charge carriers from nanocrystals and the size of nanocrystals estimated from the difference between energies of two levels observed by the Q-DLTS method decrease as the ion fluence is increased. The density of nanocrystals observed by the Q-DLTS method decreases by approximately an order of magnitude as a result of irradiation with fluence of 1012–1013 cm−2 in comparison with an initial unirradiated structure; this decrease is due to formation of conducting chains of nanocrystals in tracks.

Formation of silicon nanocrystals embedded in high-κ dielectric HfO2 and their application for charge storage

Annealing thin films of silicon containing HfO2 films deposited by an electron-beam coevaporation produces silicon nanocrystals embedded in high-κ dielectric HfO2. Such films can be used to fabricate nonvolatile memory devices. By changing the Si content in the precursor HfSixO2 (x=1, 2, 3, or 4) film, the size and density of silicon nanocrystal could be controlled and high-density of silicon nanocrystals could be obtained. Transmission electron microscopy observations showed that the maximum density of silicon nanocrystals was as high as 1.3×1013 cm−2 for HfSi4O2 and the average nanocrystal diameter was 4.3 nm. The metal-oxide semiconductor capacitor memory structure with embedded silicon nanocrystals in HfSi4O2 exhibited the largest memory window, 3.94 V under ±5 V sweep voltage.

A Novel p-n-Diode Structure of SONOS-Type TFT NVM With Embedded Silicon Nanocrystals

In this letter, for the first time, a novel p-n-diode (PND) structure of SONOS-type thin-film transistor (TFT) nonvolatile memory (NVM) with embedded silicon nanocrystals (Si-NCs) in the silicon nitride layer using an in situ method is successfully demonstrated. This novel structure has many advantages, including high density and suitability for 3-D circuit integration. Hot-electron injection and hot-hole injection are used as the program and erase methods, respectively. The sensing current of the three-terminal PND-TFT NVM is 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> A by the band-to-band tunneling current. A much larger memory window (> 12 V) and good data retention time (> 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> s for 12% charge loss) are exhibited. The device appears to have great potential for system-on-panel applications.

Embedded Silicon Nanocrystals Studied by Photoluminescence and Raman Spectroscopies: Exciton and Phonon Confinement Effects

The optical and vibrational properties of silicon nanocrystals are studied in two systems elaborated by evaporation. The first one is constituted by a thick SiO layers. The second one is a multilayered sample made by successive evaporations of SiO and SiO2 layers with controlled thicknesses. The luminescence and Raman spectra are fitted by phenomenological exciton and phonon confinement models accounting for the size distribution of the embedded nanocrystals. The coherence between the two models and experimental data is demonstrated and gives support to the notion of exciton and phonon confinement effect in silicon nanocrystals embedded within silica matrix.