Size Of Si Nanocrystals Research Articles

Optical and structural investigation of Si nanoclusters in amorphous hydrogenated silicon

This paper presents the results of PL and X-ray diffraction investigations for amorphous hydrogenated silicon films with and without Si nanocrystals. Hydrogenated amorphous Si layers were prepared by the hot-wire CVD method on glass substrates at the permanent mixture flow of silane /hydrogen [SiH4]/[H2] gas sources with the 7/15 flow ratio and with different oxygen flows. The joint analysis of PL and X-ray diffraction results in dependence on technological conditions and on different sizes of Si nanocrystals has been done. The correlation of optical parameters with the amorphous Si:H and crystal Si phase volumes in the films has been discussed.

Photoluminescence induced by Si implantation into Si3N4 matrix

Up to the present, photoluminescence (PL) was obtained from near stoichiometric or amorphous Si nitride films (SiNx) after annealing at high temperatures. As a consequence, the existence of PL bands has been reported in the 400–900nm range. In the present contribution, we report the first PL results obtained by Si implantation into a stoichiometric 380nm Si3N4 film. The Si excess is obtained by a 170keV Si implantation at different temperatures with a fluence of Φ=1017 Si/cm2. Further, we have annealed the samples in a temperature range between 350 and 900°C in order to form the Si precipitates. PL measurements were done using an Ar laser as an excitation source, and a broad PL band basically centered at 910nm was obtained. We show that the best annealing condition is obtained at Ta=475°C for the samples implanted at 200°C, with a PL yield 20% higher than the obtained at room temperature implantation. Finally, we have varied the implantation fluence and, consequently, the Si nanocrystals size. However, no variation was observed nor in the position neither in the intensity of the PL band. We concluded that the PL emission is due to radiative states at the matrix and the Si nanocrystals interface, as previously suggested in the literature.

Influence of Si crystallization evolution on 1.54 μm luminescence in Er-doped Si/Al2O3 multilayer

The crystallization evolution of the nanostructured Si （ns-Si） in the Er-doped Si/Al2O3 multilayer fabricated by using pulsed laser deposition technique and its effects on the Er3+ luminescence at 1.54 μm are investigated. Raman scattering and transmission electron microscopy measurements are used to characterize the microstructure evolution of the ns-Si during annealing treatment processes. The maximum photoluminescence intensity is obtained in the sample with ultrathin ns-Si sublayers annealed at 600—700 ℃, where the density, the size of Si nanocrystals, the interaction distance, and the optimized local environment for effectively activating the Er3+ are well controlled. From the analysis of the decay process of time-dependent luminescence, two decay channels are considered, the fast and slow decay channels. The bulk-like Si is responsible for the fast process and the Si nanocrystals are responsible for the slow decay process.

Annealing temperature dependence of Raman scattering in Si/SiO 2 superlattice prepared by magnetron sputtering

Si/SiO 2 superlattices were prepared by magnetron sputtering, and the deposition temperature and annealing temperature had a great influence on the superlattice structure. In terms of SEM images, the mean size of Si nanocrystals annealed at 1100 °C is larger than that of nanocrystals annealed at 850 °C. It was found that the films deposited at room temperature are amorphous. With increasing deposition temperature, the amorphous and crystalline phases coexist. With increasing annealing temperature, the Raman intensity of the peak near 470 cm −1 decreases, and the intensity of that at 520 cm −1 increases. Also, on increasing the annealing temperature, the Raman peak near 520 cm −1 shifts and narrows, and asymmetry emerges. A spherical cluster is used to model the nanocrystals in Si/SiO 2 superlattices, and the observed Raman spectra are analyzed by combining the effects of confinement on the phonon frequencies. Raman spectra from a variety of nanocrystalline silicon structures were successfully explained in terms of the phonon confinement effect. The fitted results agreed well with the experimental observations from SEM images.

Pulsed plasma ion source to create Si nanocrystals in SiO 2 substrates

A pulsed KrF excimer laser of irradiance of about 10 8 W/cm 2 was utilized to synthesize Si nanocrystals on SiO 2/Si substrates. The results were compared with that ones obtained by applying low bias voltage to Si(1 0 0) target in order to control the kinetic energy of plasma ions. Glancing incidence X-ray diffraction spectra indicate the presence of silicon crystalline phases, i.e. (1 1 1) and (2 2 0), on SiO 2/Si substrates. The average Si nanocrystal size was estimated to be about 45 nm by using the Debye–Scherrer formula. Scanning electron microscopy and atomic force microscopy images showed the presence of nanoparticles of different size and shape. Their distribution exhibits a maximum concentration at 49 nm and a fraction of 14% at 15 nm.

Photoluminescence of Si nanocrystals formed by the photosynthesis

Photosynthesis, which could control the size and position of Si nanocrystals being formed, as a novel nanofabrication technique making the best use of the strong interaction between photons and nanoparticles, is discussed in great detail. A nanocrystal growth is self-limited to the laser power and the laser-exposure time. The model is proposed to explain the self-limited growth and luminescence from the Si-rich oxide which was exposed to the laser. When the balance between the formation and loss of small amorphous Si clusters on the nanocrystal surface is maintained at a certain size, the nanocrystal growth will stop, and the final average size of the Si nanocrystals is achieved for each laser wavelength. The photoluminescence (PL) is observed when the Si nanocrystals are formed. The origin of the PL is associated with the small amorphous Si clusters, and its intensity increases with the increasing density of the Si clusters or photosynthesized Si nanocrystals. These small amorphous Si clusters remain particularly in the SiO nanopowder, which was made by thermal CVD using SiH 4 and O 2, even when the final average size is reached. The PL peak wavelength is well determined by the laser wavelength, which affects the structure of the small amorphous Si clusters and their light-emission energy. The photosynthesis is found to not only selectively form Si nanocrystals at low temperature, but also controls their size and even light-emission energy.

Linear and nonlinear optical properties of Si nanocrystals in SiO2 deposited by plasma-enhanced chemical-vapor deposition

Linear and nonlinear optical properties of silicon suboxide SiOx films deposited by plasma-enhanced chemical-vapor deposition have been studied for different Si excesses up to 24at.%. The layers have been fully characterized with respect to their atomic composition and the structure of the Si precipitates. Linear refractive index and extinction coefficient have been determined in the whole visible range, enabling to estimate the optical bandgap as a function of the Si nanocrystal size. Nonlinear optical properties have been evaluated by the z-scan technique for two different excitations: at 0.80eV in the nanosecond regime and at 1.50eV in the femtosecond regime. Under nanosecond excitation conditions, the nonlinear process is ruled by thermal effects, showing large values of both nonlinear refractive index (n2∼−10−8cm2∕W) and nonlinear absorption coefficient (β∼10−6cm∕W). Under femtosecond excitation conditions, a smaller nonlinear refractive index is found (n2∼10−12cm2∕W), typical of nonlinearities arising from electronic response. The contribution per nanocrystal to the electronic third-order nonlinear susceptibility increases as the size of the Si nanoparticles is reduced, due to the appearance of electronic transitions between discrete levels induced by quantum confinement.

Chemical composition and light emission properties of Si-rich-SiOx layers prepared by magnetron sputtering

The process of thermal decomposition of SiOx layers prepared by magnetron sputtering is studied with the use of photoluminescence and Auger and SIMS spectroscopies. From these measurements, we obtained the distributions of the emission properties and the chemical composition over the depth. The effect of the redistribution of silicon and oxygen over the depth is found after the high-temperature annealing which results in the formation of a Si nanocrystal. These redistributions result in the appearance of a Si-depleted region near the layer-substrate interface. The formation of a depletion layer is dependent on the excess of Si. A decrease of the silicon content over the depth of annealed layers is accompanied by a decrease of the Si nanocrystal size, as it is evidenced by the blue shift of the photoluminescence maximum. The mechanism of decomposition of SiOx and the possible reasons for the appearance of a Si-depleted region are discussed.

Optimized O/Si Composition Ratio for Enhancing Si Nanocrystal Based Luminescence in Si-rich SiOx Grown by PECVD with Argon Diluted SiH4

ABSTRACTEffect of O/Si composition ratio on near-infrared photoluminescence (PL) of PECVD grown Si-rich SiOx after 1100°C annealing are analyzed by Rutherford backscattering (RBS) and Fourier-transformed infrared spectroscopy (FTIR) to show nonlinear relationship with strongest PL at 760 nm at optimized O/Si = 1.24, total Si concentration of 44.6 atom.%, and N2O/SiH4 fluence ratio of 4.5. A nearly Gaussian function of the normalized PL intensity vs. O/Si composition ratio has been observed due to the significant variation on the Si nanocrystals size with the density of the excessive Si atoms.

Doping in silicon nanostructures

AbstractWe report on an ab initio study of the structural, electronic and optical properties of boron and phosphorous doped silicon nanocrystals. The scaling with the Si‐nanocrystal size is investigated for both the neutral formation energies (FE) and the impurity activation energies. Both these energies scale with the nanocrystal inverse radius. The optical properties reveal the existence of new absorption peaks in the low energy region related to the presence of the impurity. The effects of B and P co‐doping show that the formation energies are always smaller than those of the corresponding single‐doped cases due to both carriers compensation and minor structural distortion. Moreover in the case of co‐doping the electronic and optical properties show a strong reduction of the band gap with respect to the pure silicon nanocrystals that makes possible to engineer the photoluminescence properties of silicon nanocrystals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence and Raman Studies on Boron-Doped Nanocrystalline Si:H Thin Films

We report on room-temperature visible photoluminescence (PL) of B-doped hydrogenated nanocrystalline Si (nc-Si:H) thin films grown by plasma enhanced chemical vapor deposition. It is found that with increasing the boron doing ratio, the PL peak energy blue shifts while the PL intensity first increases and then decreases. The PL profiles can be well reproduced by using the model of Islam and Kumar [J. Appl. Phys. 93, 1753 (2003)] which incorporates the effects of quantum confinement and localized surface states, together with a log-normal rather than normal crystallite size distribution. The yielded microstructural information is in good agreement with the Raman analysis, revealing that B doping tends to reduce the size of Si nanocrystals and the PL intensity is jointly determined by the amount of amorphous Si:H phase and the fraction of B-doped Si nanocrystals. These results also provide implications to realize control of PL properties of nc-Si:H by B doping under optimized growth conditions.

Effect of implanted Si concentration on the Si nanocrystal size and emitted PL spectrum

Implantation of Si+ in excess into SiO2 followed by annealing produces Si nanocrystals (Si-nc) embedded in the SiO2 layer, which can emit a strong photoluminescence (PL) signal. Several samples have been characterized by means of ellipsometry and transmission electron microscopy (TEM). For local Si concentrations in excess of ∼2.4×1022 Si+/cm3, the Si-nc diameter ranges from ∼2 to ∼22nm in the whole sample, the Si-nc in the middle region of the implanted layer being bigger than those near the surface or the bottom of the layer. The depth distribution of the Si-nc agrees relatively well with the SRIM simulation as well as with the depth distribution of the n and k components of the complex refractive index. For SiO2 layers thermally grown on a Si wafer, the PL spectrum is modulated by optical interference of the pump laser and of the light emitted by the Si-nc in this layer. The good agreement between the results of the model calculations and experimental measurements indicates that for low and moderate Si concentration in excess (<8×1021cm−3) the PL light emitters are localized in a layer situated at the same depth as the Si-nc depth distribution. However, for a Si concentration in excess of ∼2.3×1022cm−3, the depth distribution of light emitters is narrow and situated mostly in the first half (relative to the surface) of the Si-nc depth distribution. This observation indicates that the recombination of the electron–hole pair at the interfaces could be responsible for the emitted PL spectrum.

Photo-modification and synthesis of semiconductor nanocrystals

Both photo-oxidation and photosynthesis manifest a strong interaction between nanoparticles and photons due to the large surface area-to-volume ratio. The final sizes of the semiconductor nanocrystals are determined by the photon energy during these phenomena. The photosynthesis is demonstrated in a Si-rich oxide and is similar to thermal synthesis, which involves the decomposition of SiO x into Si and SiO 2, that is well known and often employed to form Si or Ge nanocrystals embedded in SiO 2 by annealing SiO x at high temperature. However, photosynthesis is much faster, and allows the low-temperature growth of Si nanocrystals and is found to be pronounced in the SiO nanopowder, which is made by thermal CVD using SiH 4 and O 2. The minimum laser power required for the photosynthesis in the SiO nanopowder is much lower than in the Si-rich oxide formed by the co-sputtering of Si and SiO 2. This is attributed to the weak bond strength of Si–Si and Si–O in the SiO nanopowder. Photosynthesis, which can control the size and position of Si nanocrystals, is a novel nanofabrication technique making the best use of the strong interaction between photons and nanoparticles.

Effect of Implantation Dose and Annealing Time on the Formation of Si Nanocrystals Embedded in Thermal Oxide

AbstractPhotoluminescence (PL) and X-ray Photoelectron Spectroscopy (XPS) are employed to study the Si nanocrystals formed in the thermal oxide by Si+ implantation. PL results estimate the size of nanocrystals and the concentration of Pb centers in the Si-SiO2 nanocrystal-matrix interfaces. It is shown that the size of Si nanocrystals increase with implantation dose. Increasing the dose from 1×1016 to 1×1017 Si+/cm2 shifts the size of nanocrystals from ~2 nm to ~3.5nm, while prolonging the annealing time from 1h to 2h has no effect on the position of PL peak. P and Ar implantations into the SiO2 films are also investigated to suggested that the PL peak is due to implant induced chemical changes rather than implant induced damage. XPS analysis shows that the concentration of Si nanocrystals increases with Si implantation dose. Research on the annealing dependence of the forming of Si nanocrystals suggests that 1000°C annealing produces larger amount of Si nanocrystals than 1100°C annealing.

One- and two-phonon Raman scattering from hydrogenated nanocrystalline silicon films

One- and two-phonon Raman scattering from intrinsic and boron as well as phosphorus doped hydrogenated nanocrystalline silicon films prepared by plasma enhanced chemical vapor deposition technique were investigated. With regard to one-phonon Raman measurements of intrinsic films, redshifts attending by asymmetrical broadening of one-phonon transverse optical (TO) mode with diminishing mean dimension of Si nanocrystals can be ascribed to incorporating effects of phonon confinement and tensile strain. Photoluminescence behavior of these intrinsic specimens can be interpreted by a consistent way when recombination of quantum-confined excitons in Si nanocrystals is assumed. As to one-phonon Raman signals of doped nc-Si:H materials, besides joint effects of phonon confinement and tensile strain, additional redshifts accompanying with asymmetrical broadening of one-phonon TO band with increasing doping level can be assigned to carrier effect and disorder from doping. With diminishing average size of Si nanocrystals or increasing dopants, the decay of two-phonon Raman amplitudes of intrinsic and doped samples can be attributed to disorder. Raman experiments indicate that all the energies of the two-phonon TO branches are different from twice the energies of the one-phonon TO active bands, which reveal that the two-phonon TO modes do not come from the Raman active phonons at wavevector k=0. The peak shift of two-phonon transverse optical (2TO) modes relates to phonon confinement and disorder. Negligible peak shift in TO (2TO) modes of intrinsic and doped films on temperature indicates that the interface strain in nc-Si:H/c-Si can be ignored.

Nucleation and growth of Si nanocrystals in an amorphousSiO2matrix

This paper discusses the physical mechanisms governing the nucleation and growth of Si nanocrystals embedded in an amorphous $\mathrm{Si}{\mathrm{O}}_{2}$ matrix. Reactive pulsed laser deposition combined with a postannealing treatment is shown to be a flexible approach to synthesize Si nanocrystals. This technique ensures an excellent control of Si nanocrystal size by varying the oxygen pressure. By correlating nanocrystal size (measured by x-ray diffraction and transmission electron microscopy) with the nonoxidized Si volume fraction (determined by x-ray photoemission spectroscopy), it is found that the formation of nanocrystals follows classical nucleation theory, whereby the average distance between nuclei centers remains constant.

Study of the oxygen role in the photoluminescence of erbium doped nanocrystalline silicon embedded in a silicon amorphous matrix

We have produced and studied erbium doped nanocrystalline silicon thin films with different oxygen and hydrogen content in order to evaluate the influence of the matrix on the Er3+ emission and on the 0.89eV and 1.17eV bands. Films were grown by reactive magnetron sputtering on glass substrates under several different conditions (RF power, Er content and gas mixture composition) in order to obtain different microstructures. The structural parameters and the chemical composition of the samples were obtained by X-ray in the grazing incidence geometry, Raman spectroscopy and Rutherford back scattering analysis. Using X-ray technique combined with Raman spectroscopy information on the crystalline fraction and the average crystallite size of Si nanocrystals was obtained. Dependence of the 0.89eV and 1.17eV peaks in Si heterogeneous matrixes on the films crystallinity and O/H ratio has been analyzed.

Size and density control of Si-nanocrystals realized by SiO x/SiO 2 superlattice

A process which allows the preparation of size and density-controlled Si nanocrystals (Si-NCs) with a crystal diameter down to 2 nm and a nanocrystal density up to 10 19/cm 3 has been developed. The process is based on the growth of amorphous SiO x /SiO 2 superlattice, and produces Si-NCs due to the thermally induced phase separation of SiO x layers within the films and the crystallization of the resulting Si-nanoclusters. Amorphous SiO x films with adjustable oxygen content have been deposited on Si wafers by reactive evaporation of SiO powder in different O 2 atmospheres. The oxygen content of the deposited films can be controlled by changing the oxygen pressure during evaporation. Photoluminescence spectra and transmission electron microscopy images of the superlattice samples indicate that the size of Si-NCs is determined by the initial SiO x layer thickness and, the density of Si-NCs within the layer varies with the stoichiometry of the SiO x layer in the superlattice. In this way, the size and density of Si-NCs within a layer can be controlled separately.

A mechanism of charge transport in electroluminescent structures consisting of porous silicon and single-crystal silicon

Electroluminescent structures that emit in the visible region of the spectrum and are based on porous silicon (por-Si) formed on the p-Si substrate electrolytically using an internal current source are fabricated. The photoluminescent and electroluminescent properties, as well as the current-and capacitance-voltage characteristics of the structures are studied. Electroluminescence is observed only if the forward bias voltage is applied to the structure; the electroluminescence mechanism is based on the injection and is related to the radiative recombination of electrons and holes in quantum-dimensional Si nanocrystals. The injection of holes is controlled by the condition of their accumulation in the space-charge region of p-Si and by a comparatively low concentration of electronic states at the por-Si/p-Si interface. The charge transport in por-Si is caused by the direct tunneling of charge carriers between the quantum-mechanical levels, which is ensured by an appreciable number of quantum-dimensional Si nanocrystals. The leakage currents are low as a result of a small variance in the sizes of Si nanocrystals and the absence of comparatively large nanocrystals.

A study of the correlation between ultraviolet and infrared emission in Ni-doped SiOx films

The correlation between ultraviolet and infrared emission in Ni-doped SiOx (1 < x < 2) films has been investigated. Due to the presence of nickel, the maximum intensity of UV emission was obtained after annealing at 800 °C, which was explained by the formation of oxygen excess defects in the oxireduction reaction between Ni and SiOx. However, the intensity of infrared emission increases with the increase of annealing temperature and so does the mean size of SiO2 clusters characterized by scanning electron microscopy. An oxireduction reaction model was proposed to explain the correlation between ultraviolet and infrared emission in this system. The mean size of Si nanocrystals annealed at 1000 and 1100 °C was obtained from Raman spectra, which is in good agreement with the estimation by the PL peak shifts, based on quantum confinement effects.