Size Of Si Nanocrystals Research Articles

Improvement of blue response of black Si solar cells due to graded band structure

We have investigated characteristics of energy band structure of a nanocrystalline Si (nc-Si) layer produced by the surface structure chemical transfer (SSCT) method, especially its effects on quantum efficiency of crystalline Si solar cells. In the nc-Si layer, the size of Si nanocrystals decreases toward the surface because Si dissolves nonuniformly from the surface during the SSCT reaction. The valence band maximum of the outermost surface of the nc-Si layer is located at 0.1–0.3 eV lower than that of Si bulk, and the conduction band minimum increases by 0.17–0.25 eV toward the surface. These results indicate that the nc-Si layer has a graded band structure in which the band-gap energy gradually increases toward the surface. By optimizing the SSCT reaction conditions, thinner thickness (∼100 nm) of the nc-Si layer and smaller size of Si nanocrystals can be obtained, resulting in a steep graded band structure. Because of reduction of carrier recombination rate in the surface region by the steep graded band structure, a high internal quantum efficiency over 80% in the 300–400 nm wavelength region is obtained.

Removal of Ag remanence and improvement in structural attributes of silicon nanowires array via sintering

Metal-assisted chemical etching (MACE) is popular due to the large-area fabrication of silicon nanowires (SiNWs) exhibiting a high aspect ratio at a low cost. The remanence of metal, i.e., silver nanoparticles (AgNPs) used in the MACE, deteriorates the device (especially solar cell) performance by acting as a defect center. The superhydrophobic behavior of nanowires (NWs) array prohibits any liquid-based solution (i.e., thorough cleaning with HNO3 solution) from removing the AgNPs. Thermal treatment of NWs is an alternative approach to reduce the Ag remanence. Sintering temperature variation is chosen between the melting temperature of bulk-Ag (962 °C) and bulk-Si (1412 °C) to reduce the Ag particles and improve the crystallinity of the NWs. The melting point of NWs decreases due to surface melting that restricts the sintering temperature to 1200 °C. The minimum sintering temperature is set to 1000 °C to eradicate the Ag remanence. The SEM–EDS analysis is carried out to quantify the reduction in Ag remanence in the sintered NWs array. The XRD analysis is performed to study the oxides (SiO and Ag2O) formed in the NWs array due to the trace oxygen level in the furnace. The TG-DSC characterization is carried out to know the critical sintering temperature at which remanence of AgNPs removes without forming any oxides. The Raman analysis is studied to determine the crystallinity, strain, and size of Si nanocrystals (SiNCs) formed on the NWs surface due to sidewalls etching. An optimized polynomial equation is derived to find the SiNCs size for various sintering temperatures.

Determination of crystallite size of nanodiamond by Raman spectroscopy

Although the phonon confinement model (PCM) was claimed to be successfully used to accurately calculate the size of larger Si nanocrystals, quantitative size characterization by Raman spectra still remains a challenge in the case of nanodiamonds due to its complexity. Here, we find that a local-mode model of Raman spectra developed recently can be employed to determine the bond number of the ordered diamond core in nanodiamonds, and then furtherly determine the size of nanodiamonds. The Raman lines of nanodiamonds of 3.0 nm, 2.0 nm, 2.2 nm, 3.3 nm, 3.7 nm 4.42 nm and 6.3 nm are calculated. Results are in good agreement with the measured Raman spectra. It not only provides a new approach to predict the size of nanodiamonds accurately by Raman spectra, but also helps to clarify issues in Raman spectra of nanodiamond and other carbon nanomaterials.

Indukovana olovom krystalizaciya amorfnoho kremniyu pry impul"snomu lazernomu oprominenni

Tin-induced crystallization of amorphous silicon in thin-film Si–Sn–Si structures under the influence of laser irradiation of various types has been studied, by using Raman scattering. The size and concentration dependences of Si nanocrystals on the power of 10-ns and 150-мs laser pulses with a wavelength of 535 or 1070 nm are experimentally measured and analyzed. A possibility of effective tin-induced transformation of silicon in a-Si layers 200 nm in thickness from the amorphous to crystalline phase within 10 ns time interval under the action of laser light pulses is demonstrated. The theoretical calculation of the spatial temperature distribution and its time evolution in the area of the laser beam action is used to interpret the experimental results.

Accurate analysis of the size distribution and crystallinity of boron doped Si nanocrystals via Raman and PL spectra

Through fitting of Raman and PL spectra, Si nanocrystal size distributions and Si crystalline fractions are obtained.

In situ inclusion of Au nanoparticles in porous silicon structure

The aim of this work was to study the structural modification in the porous silicon layer (PSL), when they are obtained from electrodeposition using a metal salt of Au (HAuCl4) in the electrolyte. The deposition of Au nanoparticles and the formation of the PSL were performed simultaneously. The structural and optical properties of the gold/porous-Si were analyzed by scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), photoluminescence (PL) and Raman scattering. Through the methodology implemented, it was obtained gold/porous-Si nanocomposites. The size of the gold nanoparticles was above 15 nm, and the pore size was 18 nm. The PL intensity showed an increase with the incorporation of gold nanoparticles due to the enhancement of a surface plasmon effect. The size of Si nanocrystals in the PSL structure was estimated through PL and Raman measures and it was ~3 nm.

Study of the correlation properties of the surface structure of nc-Si/a-Si:H films with different fractions of the crystalline phase

The correlation properties of the structure of nc-Si/a-Si:H films with different volume fractions of the crystalline phase are studied using 2D detrended fluctuation analysis. Study of the surface relief of experimental samples showed that with increasing in volume fraction of the crystalline phase in the nc-Si/a-Si:H films, the size and number of nanoclusters on their surface grow. The size of Si nanocrystals in the a-Si:H matrix (6–8 nm) indicates the formation of coarse nanoclusters due to the self-organization of Si nanocrystals in groups under laser radiation. According to 2D detrended fluctuation analysis data, the number of correlation vectors (harmonic components) in the nc-Si/a-Si:H film structure increased with an increase in the nanocrystal fraction in the films.

High-efficiency near-infrared emission from Bismuth-doped SiO0.73 thin films fabricated by ion implantation technology.

Over the past decade, the possibility of near-infrared light generation and amplification on chip has attracted great interest for future monolithic integrated optical components. In this Letter, we demonstrated a CMOS-compatible method to fabricate amorphous SiO0.73 thin films doped with Bi ions. It exhibited highly improved σ(em)×τ of up to 4.2×10(-23) cm2 s and greatly enhanced near-infrared characteristic emission originated from Bi ions by nearly 60 times via Si nanocrystal size control. We anticipated that this Bi-doped near-infrared light emitter would be a new starting point for future research in the field of optoelectronic integration.

Effects of Si-Layer-Thickness Ratio on UV-Light-Emission Intensity from Si/SiO<SUB>2</SUB> Multilayered Thin Films Prepared Using Radio-Frequency Sputtering

We investigated the effects of Si-layer-thickness ratios on ultraviolet (UV) peak intensities of Si/ SiO2 multilayered films produced by alternately stacking several-nanometer-thick Si and SiO2 layers using radio-frequency sputtering for the first time. The Si-layer-thickness ratio of the Si/SiO2 film is a very important parameter for enhancing the peak intensity because the ratio is concerned with the size of Si nanocrystals in the film, which might affect the intensity of the UV light emission from the film. We prepared seven samples with various estimated Si-layer-thickness ratios, and measured the photoluminescence spectra of the samples after annealing at 1150°C, 1200°C, or 1250°C for 25 min. From our experiments, we estimate that the proper Si-layer-thickness ratio to obtain the strongest UV peaks from the Si/SiO2 multilayered films is around 0.29. Such a UV-lightemitting thin film is expected to be used in future higher-density optical-disk systems.

Enhancement of luminescence quantum yield of 1.5 µm emission from Er-doped SiO2 sensitized with Si nanocrystals

ABSTRACTExcitation of multiple Er3+ ions upon absorption of a single high-energy photon increases Er-related emission at 1.5 μm, and therefore enhances UV/visible-to-IR photon conversion efficiency. Here we investigate this effect for layers of Er-doped SiO2 sensitized with silicon nanocrystals by measuring the quantum yield of 1.5 µm Er-related emission. We demonstrate dramatic increase of the emission commencing for excitation energies above a certain threshold value, as the number of Er3+ ions excited upon absorption of a single incoming photon increases. By comparing differently prepared materials, we show that the actual value of this threshold energy and the rate of the observed increase of the quantum yield depend on sample characteristics – the size of Si nanocrystals and the ratio of Er3+ ions and nanocrystals concentrations.

Unexpected features of the formation of Si and Ge nanocrystals during annealing of implanted SiO2 layers: Low frequency Raman spectroscopic characterization

The present paper reports a study of the influence of heat treatments on the ion-beam synthesis of Si and Ge nanocrystals in SiO2 layers by low-frequency Raman scattering. Low-frequency Raman scattering is used just because the appearance in the glass matrix of crystal nuclei leads to an additional contribution to the density of only low-frequency acoustic vibrational states due to surface vibration modes of the nuclei. Electron microscopy, contrary to expectations, revealed a decrease rather than increase in the size of the crystal nucleus during annealing. Additionally, low-frequency Raman spectra show that the samples do not have a smooth distribution of nanoparticle sizes, as expected, but two different sizes of Si and Ge nanocrystals. This similarity is surprising because Si and Ge have different diffusion coefficients, temperatures of crystallization, meltings, and binding energies. Despite this, in both cases the same mechanism operates during the growth of Si and Ge nanocrystals.

Low-temperature formation of nanocrystalline SiC particles and composite from three-layer Si/C/Si film for the novel enhanced white photoluminescence

In this article, nanocrystalline silicon carbide (nc-SiC) and composite have been synthesized at an annealing temperature as low as 750 °C through the thermal reaction of Si/C/Si multilayers deposited on the Si(100) substrate by ultra-high-vacuum ion beam sputtering (UHV IBS) compared with the conventional thermal formation of crystalline SiC (c-SiC) nanostructures above 1,000 °C. The evolution of microstructure and reaction between C and Si was examined by Raman spectroscopy, Fourier transform infrared spectrometer (FTIR), high-resolution field emission scanning electron microscope (HR-FESEM), and high-resolution transmission electron microscopy. The c-SiC nanoparticles (np-SiC) of around 20–120 nm in diameter appeared on the top and bottom of the three-layer film with a particle density of around 2.63 × 1010 cm−2 after 750 °C annealing. The composite of nc-SiC and Si nanocrystals (nc-Si) size below 5 nm embedded in an amorphous SiC (a-SiC) matrix appeared at the interface between the Si and C layers. Efficient thermal energy is the driving force for the formation of nc-SiC and composite through interdiffusion between C and Si. The broad visible photoluminescence (PL) spectrum of 350–750 nm can be obtained from the annealed composite Si/C/Si multilayer and deconvoluted into four bands of blue (~430 nm), green (~500 nm), green–yellow (~550 nm), and orange (~640 nm) emission, corresponding to the emission origins from nc-SiC, sp2 carbon clusters, np-SiC, and nc-Si, respectively.

The effect of Si-nanocrystal size distribution on Raman spectrum

The effect of Si-nanocrystal (Si-nc) size distribution on Raman spectrum is studied in detail within the framework of a phonon confinement model. It is found that size distribution has little effect on Raman frequency shift, but greatly affects the width and shape of Raman spectrum. Si-nc Raman spectrum can be well explained by considering the size distribution. Furthermore, a set of simple relationships between Raman frequency shift/full width at half maximum (FWHM) and size distribution is set up based on the framework of a modified phonon confinement model, which can be applied to calculate Si-nc size distribution from the Raman spectrum.

Double Junction Tunnel Using Si Nanocrystalline Layer for Nonvolatile Memory Devices

A novel nonvolatile memory tunnel layer structure is proposed in which a Si nanocrystalline layer lies between double tunnel oxides. By Si nanocrystal size downscaling to 2 nm, the new double junction tunnel attains a remarkable 3×106 times retention improvement while keeping high-speed write/erase compared to single tunnel oxide. Based on Si nanocrystal size confirmation by transmission electron microscope (TEM), we show quantitatively that the advantage is due to Coulomb blockade and quantum confinement, and smaller Si nanocrystal will lead to greater improvement. We clarify a characteristic effect in double junction tunnel, tunnel penetration disappearance, which is extremely advantageous for nonvolatile memory applications and never occurs in other multilayer dielectrics structures. The double tunnel junction using Si nanocrystalline layer is very promising for future memory.

Double Junction Tunnel Using Si Nanocrystalline Layer for Nonvolatile Memory Devices

A novel nonvolatile memory tunnel layer structure is proposed in which a Si nanocrystalline layer lies between double tunnel oxides. By Si nanocrystal size downscaling to 2 nm, the new double junction tunnel attains a remarkable 3×106 times retention improvement while keeping high-speed write/erase compared to single tunnel oxide. Based on Si nanocrystal size confirmation by transmission electron microscope (TEM), we show quantitatively that the advantage is due to Coulomb blockade and quantum confinement, and smaller Si nanocrystal will lead to greater improvement. We clarify a characteristic effect in double junction tunnel, tunnel penetration disappearance, which is extremely advantageous for nonvolatile memory applications and never occurs in other multilayer dielectrics structures. The double tunnel junction using Si nanocrystalline layer is very promising for future memory.

Influence of deposition pressure on structural, optical and electrical properties of nc-Si:H films deposited by HW-CVD

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films were deposited using HW-CVD technique at various deposition pressures. Characterisation of these films from Raman spectroscopy revealed that nc-Si:H thin films consist of a mixture of two phases, crystalline phase and amorphous phase containing small Si crystals embedded therein. We observed increase in crystallinity in the films with increase in deposition pressure whereas the size of Si nanocrystals was found ∼2 nm over the entire range of deposition pressure studied. The FTIR spectroscopic analysis showed that with increasing deposition pressure the predominant hydrogen bonding in the films shifts from, Si–H to Si–H 2 and (Si–H 2) n complexes and the hydrogen content in the films was found in the range 6.2–9.3 at% over the entire range of deposition pressure studied. The photo and dark conductivities results also indicate that the films deposited with increasing deposition pressure get structurally modified. It has been found that the optical energy gap range was between 1.72 and 2.1 eV with static refractive index between 2.85 and 3.24. From the present study it has been concluded that the deposition pressure is a key process parameter to induce the crystallinity in the Si:H thin films using HW-CVD.

Si nanocrystals embedded in SiO2: Optical studies in the vacuum ultraviolet range

Photoluminescence excitation and transmission spectra of Si nanocrystals of different diameters embedded in a SiO${}_{2}$ matrix have been investigated in the broad visible-vacuum ultraviolet spectral range using synchrotron radiation. The dependence of the photoluminescence excitation spectra on the nanocrystals size was experimentally established. It is shown that the photoluminescence excitation and absorption spectra are significantly blueshifted with decreasing Si nanocrystal size. A detailed comparison of photoluminescence excitation and absorption spectra with data from theoretical modeling has been done. It is demonstrated that the experimentally determined blueshift of the photoluminescence excitation and absorption spectra is larger than the theoretical predictions. The influence of point defects in the SiO${}_{2}$ matrix on the optical and luminescence properties of the embedded Si nanocrystals is discussed. Moreover, it is demonstrated that no energy transfer takes place between the SiO${}_{2}$ and Si nanocrystals when the excitation energy is higher than the band-to-band transition energy in SiO${}_{2}$.

Annealing effects on the size of Si-nanocrystals embedded in bulk SiO

In the process of synthesizing Si-nanocrystals (Si-nc) from bulk SiO, the relationship between Si-nc size distribution and annealing condition (temperature from 800 to 1150 °C and time from 1 to 16 h) is experimentally investigated by X-ray diffraction and high resolution transmission electron microscopy. It is found that the average size of Si-ncs can be tuned through annealing condition from less than 3 nm to ∼10 nm, while the size distribution follows a lognormal function with an almost unchanged standard deviation of 0.2. After annealing at even higher temperature (1150 °C), two groups of Si-ncs with very different average sizes exist simultaneously and a double lognormal function should be applied to describe the size distribution.

Size control of Si nanocrystals by two-step rapid thermal annealing of sputtered Si-rich oxide/SiO2 superlattice

Controllable size of silicon (Si) nanocrystals can be achieved by a two-step rapid thermal annealing technique consisting of rapid annealing at 1000°C in nitrogen ambient and rapid oxidation at 600–800°C of a radio frequency magnetron co-sputtered Si-rich oxide/SiO2 superlattice structure. The photoluminescence (PL) spectra related to Si nanocrystals were observed in the visible range (600–900 nm). After rapid oxidation, the size of the nanocrystals was reduced and the quality of the Si nanocrystal/SiO2 interface was improved, resulting in a blue shift and an increase of the PL peak intensity. Finally, annealing in air increases the PL intensity further.

Enhanced electroluminescence from SiN-based multilayer structure by laser crystallization of ultrathin amorphous Si-rich SiN layers

Luminescent SiN-based multilayers were prepared in a plasma enhanced chemical vapor deposition system followed by subsequently laser crystallization of ultrathin amorphous Si-rich SiN sublayers. The cross-sectional TEM analysis reveals that grain size of Si nanocrystals embedded in the Si-rich SiN sublayers is independent of the laser fluence, while the grain density can be well controlled by the laser fluence. The devices containing the laser crystallized multilayers show a low turn-on voltage of 5 V and exhibit strong green light emission under both optical and electrical excitations. Moreover, the device after laser-irradiated at 554 mJ/cm(2) shows a significantly enhanced EL intensity as well as external quantum efficiency compared with the device without laser irradiation. The EL mechanism is suggested from the bipolar recombination of electron-hole pairs at Si nanocrystals. The improved performance of the devices was discussed.