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A suitable description of the photoluminescence dynamics in a complex system such as an ensemble of semiconductor nanocrystals can bring invaluable insight into its carrier dynamics. In this contribution, we study a system of silicon nanocrystals sensitized by light-harvesting diphenylanthracene molecules enhancing their absorption. The emission-wavelength-resolved photoluminescence decay of this system can be well-described by the Becquerel (compressed hyperbola) function, featuring a characteristic power-law-like tail. This shape of the photoluminescence decay function is linked to a model based on trapping and releasing of excited carriers, which are the cause of the longer tail. Our model allows us to estimate the value of the trap capture cross-section of σt ≈ 1.5 × 10-16 cm2.

As we have mentioned above, the uppermost valence band of simple semiconductors with zincblende structure is mainly built from atomic p-orbitals that are three-fold degenerate. In addition, electrons are spin (1/2)-particles such that each p-orbital can accept two electrons with different spins.

We have seen in the foregoing chapters that the development of an effective Hamiltonian, which is adapted to the crystal symmetry, is an efficient tool to describe electronic excitations in semiconductors. The development procedure becomes, however, quite tedious and inefficient if a large basis of electron states is considered when describing a system. On the contrary, this technique is interesting if only a couple of almost degenerate states, which are well separated in energy from other states, has to be analyzed. This is the case for example in the exciton problem that has been discussed in the previous chapter. Here, the split-off exciton states (resulting from electrons and holes that transform as \( \Gamma _6\) and \(\Gamma _7\), respectively) are separated by the spin-orbit coupling from the exciton block obtained from electrons and holes that transform as \( \Gamma _6\) and \(\Gamma _8\), respectively. As we have already discussed, the two exciton blocks are coupled by the electron-hole exchange interaction, which is, however, small compared to the spin-orbit coupling. In this case, the exciton blocks are approximately decoupled and different effective Hamiltonians that are independent from each other can be developed for the two exciton blocks. This will be done in the present chapter. We understand this as a “Pseudo-Spin Development” of excitons in a subspace of almost degenerated states.

We are now interested in the invariant expansion of the electron, hole, and exciton states in direct semiconductors with wurtzite structure, i.e. with \(C_{6v}\) point-group symmetry. The electronic structure of many binary semiconductors with wurtzite structure is very similar to the case discussed above for zincblende structure. Often, materials can even realize both structures or can easily undergo phase transitions from one structure to the other if some parameter (as temperature or pressure) is varied, for instance CdS. Other important semiconductors crystallizing in wurtzite structure are GaN, InN and ZnO. It is, however, important to recall that the symmetry properties and the crystal structure of both systems are quite different, giving rise to their different physical properties.

As in Chap. 4, let us first consider the symmetry of exciton states at the \(\Gamma \)-point. One should mention at this stage that the total symmetry of exciton states \(\Gamma _{ex}\) is given by the symmetry of the envelope function (labeled \( \Gamma _{env}\)) multiplied by the symmetry of the valence-band states “w” (\( \Gamma _w\)) and multiplied by the symmetry of the conduction-band states (in our case \( \Gamma _7\)).

We are now interested in the following problem: Let us consider the ground state of an ideal direct gap semiconductor at low temperature, i.e. at T = 0 K, containing N electrons. As indicated in Fig. 2.2 the semiconductor is characterized by the presence of an energy gap between the lowest lying conduction band (noted “c”) and the uppermost valence band (noted “v”). In addition the semiconductor is supposed to possess only one conduction and one valence band and the states are defined by spin-orbitals. The N electrons occupy all electronic valence-band states and all conduction-band states are unoccupied. This system shows no electric conductivity (or it behaves as an insulator) since, when applying a small electric field to it, no electric current can flow. The sample can be polarized by the field but no free electric charges can move in the material. This situation characterizes the crystal-ground state whose energy and charge distribution is supposed to be known.

Light corresponds to a transverse electromagnetic radiation field. It is characterized by its electric- \(\varvec{e}\) and magnetic-polarization vectors \(\varvec{h}\). \(\varvec{e}\) and \(\varvec{h}\) are both oscillating in time with frequency \(\omega \) and propagating in space, characterized by the wave-vector \(\varvec{q}\). The fields obey the wave equation and will be described below by harmonic plane waves. The electric- \(\varvec{e}\) and magnetic-polarization vectors \(\varvec{h}\) are special forms of electric field \(\varvec{E}\) and magnetic induction vectors \(\varvec{B}\) whose evolution in space and time is governed by Maxwell’s equations.

Light extraction from a thin planar layer can be increased by introducing a two-dimensional periodic pattern on its surface. This structure, the so-called photonic crystal (PhC) slab, then not only enhances the extraction efficiency of light but can direct the extracted emission into desired angles. Careful design of the structures is important in order to have a spectral overlap of the emission with extraction (leaky) modes. We show that by fabricating PhC slabs with optimized dimensions from silicon nanocrystals (SiNCs) active layers, the extraction efficiency of vertical light emission from SiNCs at a particular wavelength can be enhanced ∼ 11 times compared to that of uncorrugated SiNCs-rich layer. More importantly, increased light emission can be obtained in a broad spectral range and, simultaneously, the extracted light can stay confined within relatively narrow angle around the normal to the sample plane. We demonstrate experimentally and theoretically that the physical origin of the enhancement is such that light originating from SiNCs first couples to leaky modes of the PhCs and is then efficiently extracted into the surrounding.

Silicon nanostructures are synthesized with a DC atmospheric pressure microplasma jet using an Ar/SiH4/H-2 gas mixture. The plasma is characterized by OES and imaged using an EMCCD camera. The effect of hydrogen admixture to the formed structures is studied by transmission electron microscopy. Under specific conditions, crystalline silicon nanoparticles grow in an amorphous matrix investigated by electron energy loss spectroscopy. As-grown silicon nanoparticles are collected in ethanol for dynamic light scattering and photoluminescence measurements. The size distribution peaks at 4nm. The silicon nanocrystals exhibit roomtemperature photoluminescence that peaks at approximate to 415 and approximate to 465 nm.

The number of newly developed nanomaterials is steadily increasing but only a few are suitable for applications in biology. These nanomaterials are often made from harmful compounds and there is no convenient technique to remove them from the body after application. This study is focused on silicon nanocrystals and nanodiamonds, which are two promising nanomaterials for bio-applications. Silicon nanocrystals (Si-NCs) are exceptional since they provide several desired properties: low cytotoxicity, suitability for chemical activation, efficient photoluminescence, and bio-degradability. All these parameters promote application of Si-NCs in living organisms and even human medicine. Nanodiamonds (NDs), on the other hand, are non-biodegradable which limits their use to mostly long term in vitro studies. However, when using these nanomaterials one needs to address the effect of accumulation and aggregation of such materials in cells and how it contributes to the overall cytotoxicity. Here, we present studies on the interaction of two promising nanomaterials Si-NCs and NDs with actin structure of mammalian cells, evaluation of their cytotoxicity by various methods, and observation of single nanoparticle luminescence spectra within living cells. According to our results Si-NCs are more promising for application in nanomedicine compared to NDs.

Various theoretical as well as empirical considerations about how to achieve lasing between the conduction and valence bands in indirect band gap semiconductors (germanium and silicon) are reviewed, starting from the dawn of the laser epoch in the beginning of the sixties. While in Ge the room-temperature lasing under electrical pumping has recently been achieved, in Si this objective remains still illusory. The necessity of applying a slightly different approach in Si as opposed to Ge is stressed. Recent advances in the field are discussed, based in particular on light-emitting Si quantum dots.

This paper reports on the observation of an electronic energy transfer channel between silicon nanocrystals and the SiO2 matrix they are embedded in. This energy transfer manifests itself by photoluminescence at 650 nm, i.e. above the indirect band gap, in superlattices formed by alternating layers of P- or B-doped as well as undoped silicon nanocrystals (NCs) and stoichiometric SiO2 or SiOxNy. This band is only observable under strong (above ~13 mJ/cm2) UV femtosecond excitation. A detailed investigation of the excitation intensity dependence together with luminescence dynamics enabled us to ascribe this band to a non-bridging oxygen hole center located in the SiO2 (or SiOxNy) matrix, to study the influence of NC treatment on its photoluminescence and to determine the energy transfer mechanism.

Blinking dynamics of strain-engineered direct-band-gap Si nanocrystals is measured over a broad interval of time scales and excitation intensities. The measured data are at first described using the autocorrelation function approach. Then, the relationship between the shape of the autocorrelation function and the distributions of on and off times is discussed. An original numerical approach allowing us to deduce and quantify the corresponding on- and off-times distributions is proposed. Both the autocorrelation function and the distribution of on and off times are observed to follow a broken-power-law function, for which we introduce an analytical formula. The broken-power-law function signifies a change in the power-law dynamics at short time scales (the power-law exponent changes on the time scale of 1--20 ms). Under the lowest excitation intensities, both the autocorrelation function and the distribution of on times are flat, implying the existence of a nonblinking regime. In addition to the broken-power-law dynamics, the end of the on-times distribution is also cut off by an exponential tail. All these temporal regimes are observed to be independent of excitation intensity for the lowest excitation intensities, whereas at higher excitation intensities, excitation intensity dependence appears. The distribution of off times is observed to be more robust, with changes occurring at higher excitation intensity than in the case of the on-times distribution. The different observed regimes are discussed in detail and a qualitative model including the emission of a trion is proposed.

Porous silicon and similar materials, like micro- and nanocrystalline silicon, are nowadays studied not only in physical research (e.g. optical gain studies, electro-optical devices, solar energy conversion), but they are very promising also in biological research as fluorescent labels, biological sensors, drug delivery systems or scaffold for various tissues. We are giving an overview of various approaches of preparation of micro- and nanocrystalline silicon and current studies of applications with main focus on biology and medicine. In contrast to other nanomaterials used in biological studies (e.g. carbon nanotubes, fullerenes, cadmium containing quantum dots) silicon based nanomaterials show very good biocompatibility and low cytotoxicity. Therefore, these materials have potential to become powerful tools for in vivo investigation of life processes on subcellular and molecular level. Our group concentrates on developing of gentle florescent label based on porous silicon for single molecule detection in the cell. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on ultrafast photoluminescence (PL) spectroscopy of blue and red-emitting silicon nanostructures. We prepared samples of porous Si with different dominating PL bands in time-integrated PL spectrum. These samples show also striking differences in ultrafast PL dynamics. For the red-emitting samples we observe rapid PL onset followed by a two-exponential decay, which is spectrally dependent. For the blue-emitting porous Si the PL builds up several picoseconds and decays mono-exponentially with spectrally independent PL rise and decay times. Our results indicate that a dominant part of the blue and red emission comes from two distinct species with different energy states structures. This is furthermore supported by two-photon absorption induced PL mea-surements. The two-photon and one-photon absorption induced PL spectra, which are identical for the red-emitting samples, differ substantially for the blue-emitting ones. However, polarization properties of PL suggest that the blue PL band originates from several overlapping PL bands with sample-dependent importance. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The article presents a short overview of the worldwide experimental activity directed towards vindicating the occurrence of positive optical gain in dense assemblies of luminescent silicon nanocrystals. In spite of an intense interest in the field, the achieved results are non-uniform till now. Hints how to proceed further are given.

The first attempts on introducing luminescent porous silicon agglomerates into various TiO2 thin-film matrices (amorphous, crystalline and inverse opal) are reported. We describe preparation methods towards such matrices and how to impregnate them with luminescent Si nanocrystals (Si-ncs). Si-ncs keep their high luminescent capability in these matrices. Moreover, Si-ncs embedded in the inverse TiO2 (anatase) opal matrix show marked modifications both in their emission spectrum and in luminescence dynamics. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the spectral and dynamical properties of photoluminescence (PL) under ultrashort (femtosecond and picosecond) laser pulse excitation in Si nanocrystals prepared by Si+-ion implantation into a silica matrix. We concentrate on the luminescence properties that make it possible to distinguish several luminescence bands within a broad luminescence spectrum. In time-integrated luminescence, saturation-related changes suggest a presence of two different recombination channels connected to the nanocrystal volume states and the nanocrystal–SiO2 interface. The saturation of the luminescence can be explained in terms of Auger recombination. We investigated in detail fast sub-nanosecond luminescence, where significant differences were observed under UV excitation (linear pump-intensity dependence, PL maximum at 450 nm) and for the excitation wavelengths in the visible part of the spectrum (superlinear pump-intensity dependence, broad luminescence band including wavelengths as short as 300 nm). We propose a rate-equations model based on the Auger and radiative/nonradiative recombination to interpret the experimental data.

Pulverized porous silicon, fabricated by the electrochemical etching of silicon wafers, is used to prepare colloidal suspensions of silicon nanocrystals (ncSi) in various organic solvents including xylene. The porous silicon powder aggregates into large clumps, making the suspensions scatter light strongly. The suspensions are subsequently gently stirred in optical cuvettes for a period of weeks, in air and at room temperature. The temporal development of spatially resolved photoluminescence emission spectra in the cuvette with xylene+ncSi is monitored at regular intervals. The spectra show a slow evolution from the original orange color through blue to a yellow one. After approximately 5 weeks, a yellow emitting (∼560nm) transparent colloidal solution can be extracted from the cuvette, with a high and stable photoluminescence yield (∼20%).

Spectra of individual silicon nanocrystals within porous Si grains are studied by the wide-field imaging microspectroscopy and their ON-OFF, blinking is detected by the confocal single-photon-counting microscopy. Observed spectral and blinking properties comprise all features reported before in differently prepared single Si nanocrystals (SiNCs). Former apparently contradictory results are shown to be due to different experimental conditions. When the effect of dark periods (OFF switching) is removed the common ultimate photoluminescence properties Of SiO2 passivated SiNCs are found, namely the quantum efficiency (QE) of about 10-20% up to the pumping rate corresponding to one exciton average excitation per quantum dot. At higher pump rates the QE is slowly decreasing as the 0.7th power of excitation. This is most likely due to Auger recombination which, however, seems to be weakened compared with measurements of nanocrystal assemblies. We conclude that SiNCs may be pumped above one exciton occupancy to yield a higher light emission, being advantageous for applications.

This paper deals with light emission induced by a scanning tunneling microscope using a silver nanoparticles film as a sample. We measured both two photon maps of different spectral ranges along with a topography and light emission spectra corresponding to different spots of the samples. The spectra revealed two distinct peaks, corresponding to different-order tip-induced plasmon modes of the tunneling junction.

To satisfy the need for CPU clock frequencies continuous increase, optical data transmission process integrated within silicon chips is required. Practical optical amplification in silicon is therefore very desirable. Unfortunately, bulk silicon is an indirect bandgap semiconductor with very low light-emission efficiency. To overcome this problem, the stimulated Raman scattering has been successfully employed for light amplification and lasing in silicon. Due to the two-photon absorption nonlinear optical loss, this scheme was initially working only in a pulsed operation mode. Very recently, an improved design has been allowed also for the continuous operation. However, this encouraging development still contains a major drawback—a need for an external optical pumping. It seems that this problem might be overcome by switching from the bulk silicon to its properly prepared nanocrystalline form provided a reasonably good electroluminescence can be achieved. An intensive research is under way with some first promising results already in the pipeline.

We study the influence of the wavelength of picosecond excitation pulses on the properties of photoluminescence (PL) in a series of samples of silicon nanocrystals prepared by ion implantation into silica matrix. We observed a gradual change in the behaviour of the PL fast component (spectral shape, decay times, pump-intensity dependence) when tuning the excitation wavelength from 355 to 532 nm. We interpret the results in terms of an interplay between the PL originating from volume states of nanocrystals containing two photoexcited carrier pairs, and the PL due to the silicon oxide states. We discuss also the role of the implant fluence on the PL properties of samples.

Nanophotonic structures combining electronic confinement in nanocrystals with photon confinement in photonic structures are potential building blocks of future Si-based photonic devices. Here, we present a detailed optical investigation of active planar waveguides fabricated by Si + -ion implantation (400 keV, fluences from 3 to 6×10 17 cm −2 ) of fused silica and thermally oxidized Si wafers. Si nanocrystals formed after annealing emit red-IR photoluminescence (PL) (under UV-blue excitation) and define a layer of high refractive index that guides part of the PL emission. Light from external sources can also be coupled into the waveguides (directly to the polished edge facet or from the surface by applying a quartz prism coupler). In both cases the optical emission from the sample facet exhibits narrow polarization-resolved transverse electric and transverse magnetic modes instead of the usual broad spectra characteristic of Si nanocrystals. This effect is explained by a theoretical model which identifies the microcavity-like peaks as leaking modes propagating below the waveguide/substrate boundary. We present also permanent changes induced by intense femtosecond laser exposure, which can be applied to write structures like gratings into the Si-nanocrystalline waveguides. Finally, we discuss the potential for application of these unconventional and relatively simple all-silicon nanostructures in future photonic devices.

We report on the ultrafast photoluminescence dynamics in silicon nanocrystals measured by femtosecond up-conversion technique. The samples were prepared by embedding porous silicon grains in a sol-gel derived SiO2 matrix. Efficient initial relaxation of the excess energy of photoexcited carriers with the effective rate ≥4 eV/ps was observed. The ultrafast photoluminescence decay (about 400 fs) was interpreted in terms of quenching the interior exciton radiative recombination by fast carrier trapping on the nanocrystal surface. We also present the observation of the photoluminescence excited by two-photon absorption at 830 nm and open aperture Z-scan measurements to demonstrate a strong optical nonlinearity in silicon nanocrystals. We have found the value of two-photon absorption coefficient β = 1.7 m/GW which corresponds to the imaginary part of the third-order nonlinear susceptibility Im χ(3) = – 1 × 10–10 esu. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thin layers of silicon nanocrystals prepared by silicon-ion implantation into silica substrate form active planar optical waveguides. Testing experiments of optical gain have been performed on a sample implanted to a dose of 4 × 1017 cm–2 by using the variable stripe length (VSL) technique. The photoluminescence collected from the sample facet shows spectrally narrow, polarization-resolved transverse electric (TE) and transverse magnetic (TM) substrate modes. Continuous wave VSL revealed a reduction of optical losses in both modes only. On the contrary, a fast decay component due to amplified spontaneous emission is observed in time-resolved VSL experiments. This fast component shows positive net modal optical gain of ∼12 cm–1. The observed superlinear emission as a function of pump intensity, accompanied by shortening of the fast component lifetime, supports further the observation of stimulated emission. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Samples with the structure of asymmetric planar waveguides are fabricated by implanting Si + ions with energy of 400 keV and doses from 3 to 6 × 10 17 cm −2 into synthetic silica slabs. Broad photoluminescence spectrum is observed when collecting photoluminescence (PL) signal in the direction perpendicular to the plane of waveguides while PL collected from the edge of the sample reveals narrow (FWHM of 10–20 nm) polarization-resolved transverse electric (TE) and transverse magnetic (TM) peaks. This peculiar observation is explained using a theoretical model, developed within the framework of wave optics. The TE and TM modes are identified as radiative leaky modes of the planar waveguide. Conditions under which the narrow modes can be seen in the edge PL spectra are described.

Stimulated emission from nanocrystalline silicon in the visible has become a hot topic during the past years. Various forms of silicon nanostructures are being exploited, among them planar optical waveguides made of silicon nanocrystals, silicon superlattices and tiny silicon nanoparticles. We report on optical gain measurements using the variable-stripe-length and the shifting-excitationspot methods in two different types of nanocrystalline samples: a planar nanocrystalline waveguide prepared by silicon-ion implantation (400 keV, 4 · 10 17 cm 2 ) into a silica substrate and a layer of porous silicon grains embedded in a sol–gel derived SiO2 matrix. The latter does not exhibit any waveguiding. At a first sight one would expect much more favorable conditions for the stimulated emission occurrence in the former type of samples. We observed in fact small optical gain in both types (modal gain of 12 cm 1 at 760 nm in ion implanted sample and of 25 cm 1 at 650 nm in sol–gel embedded sample, respectively), however, under different pumping conditions. We discuss advantages and disadvantages of both nanostructures, referring in particular to leaky-mode emission that may occur in planar waveguides on transparent substrates. � 2004 Elsevier B.V. All rights reserved.

The propagation of light emitted from silicon nanocrystals forming planar waveguides buried in SiO2 is studied both experimentally and theoretically. Experiments reveal that photoluminescence spect ...

Recent reports on experimental observation of optical gain in silicon nanostructures in the visible region, performed at several laboratories all over the world, have triggered an extraordinary surge of interest in silicon lasing. However, attempts aimed at reproducing the red stimulated emission from „standard“silicon nanocrystals (sized 3-5 nm) at some other laboratories either failed, or. did not come to definite conclusions. Therefore, more detailed measurements of optical gain in a wider variety of samples containing Si nanocrystals are required in order to unravel whether or not the observation of optical gain is an intrinsic property of Si nanocrystals. We have performed a detailed study of optical gain in layers of densely packed Si nanocrystals in SiO2, prepared on the basis of porous Si, using the variable-stripe-length (VSL) method in combination with the shifted-excitation-spot (SES) method. In selected samples we have observed a distinct difference in behaviour between VSL and SES curves, indicating the occurrence of positive optical gain of ~ 24 cm-1. Preliminary reports on transport and electroluminescence measurements in thin films of SiO2 doped with porous silicon grains, prepared by spin-coating technique, are also discussed.

A description of mixed phase hydrogenated microcrystalline silicon (μc-Si:H) geometry by a Voronoi network based on the growth in the grain neighborhoods is used as a basis for modelling electronic transport in μc-Si:H. The experimentally observed evolution of the grain shape during growth is used to discuss enhancement of the internal electric field at the grain tip. A possibility of light focussing by a spherical grain cap is suggested. Coincidence of the focussed photogeneration with the high electric field region could lead to solar cells with photogeneration of carriers spatially separated according to the photon energy.

Changes in the growth of thin silicon films at very low substrate temperatures (35 °C

Silicon thin films grown near the boundary between the amorphous/microcrystalline growth offer superior properties for industrial applications. Series of silicon samples, in which crossing of this transition region was achieved by changing a single technological parameter (dilution of silane in hydrogen, deposition temperature, sample thickness) were used to test our model of transport, connecting the macroscopically observed transport properties and the crystallinity, hydrogen content, grain size and grain boundaries. Microscopic study by AFM led to the formulation of the geometrical model of growth of mixed phase Si. The demand for research of microcrystalline or polycrystalline silicon prepared at low substrate temperatures is stimulated by the use of cheap plastic substrates. In addition to a direct deposition an alternative technology, such as metal-induced crystallization supported by the electric field is discussed. Possible future application of thin silicon films, for example in a “nanolithography”, is also shown. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Charge transport in microcrystalline silicon is strongly influenced by its heterogeneous microstructure composed of crystalline grains and amorphous tissue. An even bigger effect on transport is their arrangement in grain aggregates or possibly columns, separated by grain boundaries, causing transport anisotropy and/or depth profile of transport properties. We review special experimental methods developed to study the resulting transport features: local electronic studies by combined atomic force microscopy, anisotropy of conductivity and diffusion length and also their thickness dependence. A simple model based on the concept of changes of transport path for description of the observed phenomena is reviewed and its consequences for charge collection in microcrystalline based solar cells are discussed.

Photon emission from polycrystalline silver induced by scanning tunneling microscopy is studied for three different tip materials (Au, PtIr and W). Photon emission intensity curves as a function of the tip voltage are observed to be almost identical for platinum–iridium alloy and gold tips (and more than 10 times enhanced as compared with the tungsten tip). An evolution in topography and photon map for different applied voltages is investigated along with the study of the spatial distribution of photon emission in dependence upon the surface local differential height. It turns out that no clear correlation between a local curvature and enhancement of light emission can be found. Simultaneous measurements of tunneling current and photon intensity as a function of vertical tip displacement confirm the earlier observation, namely, that similar apparent barrier heights exist for both elastic and inelastic tunneling channels. The role of the tip material as well as its shape is discussed.

In this contribution we present a new type of optoelectronic silicon nanocrystal (Si-nc) based material, namely, Si-nc embedded into solidified pure or doped spin-on-glasses. The resulting self-supporting samples contain thin layers with high Si-nc concentrations. The visible photoluminescence (PL) maximum at room temperature is blue-shifted when the concentration of phosphorus in the spin-on-glass is increased.

A way in which thin films of hydrogenated amorphous silicon (a-Si: H) can be instantaneously crystallized at room temperature is reported. The metal-induced solid-phase crystallization (MISPC) method with nickel surface coverage is used. In comparison with previous reports on the MISPC of a-Si: H, the crystallization temperature is reduced by more than 350°C. This is achieved by introducing two novel technological steps: firstly, we use hydrogen-rich a-Si: H films (hydrogen content between 20 and 45at.% H) and, secondly, we apply a high transverse electric field. Polycrystalline silicon islands as large as 3 mm across appear instantaneously after having reached a threshold electric field of about 105Vcm−1. We report macroscopic visualization of the crystallization process as well as microscopic investigation (micro-Raman measurements and scanning electron microphotography) of the crystallized films. We have found that appropriate patterning of the nickel electrode helps to increase homogeneity of...

We discuss results of surface photovoltage (U) measurements for d=10 μm thick layers of undoped hydrogenated microcrystalline silicon (μc-Si:H). By applying excitation with low energetic photons (down to 1.1 eV), i.e., with small absorption coefficient α and a large penetration depth α−1, a photovoltage peak appears on a curve U=U(α). This peak is located at α≳d−1 and its occurrence depends critically on the substrate material. The peak is present in a μc-Si:H film grown on p+ crystalline silicon (c-Si), on the other hand it is missing in the μc-Si:H samples grown on n+ c-Si or ZnO film. We present a mathematical model that enables us to link this peak to photocharge separation in the bottom space charge region at the interface μc-Si:H/substrate. Besides the magnitude of the ambipolar carrier diffusion length L, a parameter particularly critical for the occurrence of the peak turns out to be the ratio n of reverse saturation current densities of the two diodes representing surface and bottom space charge ...

Large complexity of microstructure in hydrogenated microcrystalline silicon and existence of at least two different sizes of crystallites is demonstrated by combined atomic force microscope topography/local current map. We correlate activation energy and prefactor of the simplest transport property – dark conductivity, measured parallel to the substrate – with the crystallinity and roughness in wide range of microcrystalline silicon samples. This allowed us to formulate a simple model of transport based on the idea that, contrary to small grains, the formation of their aggregates (large grains/columns) dramatically changes the mechanism of transport from band like to hopping.

The influence of the substrate temperature during μc-Si:H deposition on the material structure and optoelectronic properties was explored in the range from 150 to 350 °C. The low temperature material is especially interesting with regard to the suppressed oxygen-related donor formation and high efficiency of the resulting solar cells. Surprisingly, in this material, conductivity is the same parallel and perpendicular to the substrate (as measured by dc and ac techniques). The same is true for the ambipolar diffusion length which was measured by steady-state photocarrier grating (SSPG) (L∥) and by surface photovoltage (SPV) (L⊥) methods. Finally, the relevance of the SPV method extended to the measurement in complete thin film solar cell structures is demonstrated.

A novel simple method of crystallization of hydrogenated amorphous silicon (a-Si:H) thin films is described. Namely, we studied a metal-induced crystallization enhanced by a dc electric field in sandwich p+–i–n+structures. The samples were fabricated from wide-bandgap a-Si:H with high hydrogen content (13–51 at. % H). Macroscopic islands of a-Si:H (up to ∼1 mm in diameter) in the region between upper (CrNi) and lower (ITO) contacts crystallize instantaneously when a sufficiently high dc electric field (≳105 V cm-1) is applied. The crystallization sets in at room temperature and ambient atmosphere and is spatially selective. A proposed microscopic mechanism of such an easy macroscopic crystallization consists in easy diffusion of Ni and/or Ni silicides (representing nucleation sites) through a dense network of voids in hydrogen-rich a-Si:H.

We investigate the blue photoluminescence of Si+-implanted SiO2 films under picosecond UV excitation. The emission intensity exhibits a nonlinear increase with increasing excitation intensities, accompanied by pulse shortening. The photoluminescence decays nonmonoexponentially in time. However, the nonlinearities are not associated with significant spectral narrowing. To explain the results, we propose and numerically investigate a kinetic model based on competition between radiative (both spontaneous and stimulated) and nonradiative recombination in isolated luminescence centers in the SiO2 matrix. Good agreement between theoretical and experimental data seems to confirm the existence of stimulated emission in the films, however, under extremely high excitation densities only (approximately 100 MW/cm2).

We have studied ultra-fast carrier dynamics of photo-excited carriers in hydrogenated microcrystalline silicon prepared by a very high frequency glow-discharge technique. We report on direct observation of two types of dynamics using selective photo-excitation in picosecond pump and probe measurements. One type of the observed dynamics has been found to be independent of the sample preparation, while the other reflects the relative weights of crystalline and amorphous fractions. We propose a simple rate-equation model that describes the carrier dynamics in microcrystalline silicon in terms of the composition of those in Si microcrystallites and in the a-Si:H tissue which surrounds the microcrystallites. The model without any fitting parameters reproduces the experimental data very well when the dynamics are scaled with relative volume fractions as obtained from Raman spectra.

We report on photoluminescence dynamics in CdSe nanocrystalline films on silicon substrate prepared by chemical bath deposition. The photoluminescence spectra and dynamics are influenced strongly by carrier trapping. The short (5 ps) effective decay time of the band edge photoluminescence in CdSe nanocrystalline films on silicon substrate is interpreted in terms of carrier trapping in combination with the transfer of hot carriers from the silicon substrate to the film. We have demonstrated room-temperature electroluminescence of the samples and measured the diode-like current–voltage and electroluminescence intensity–voltage characteristics.

Photoluminescence (PL) properties of as-prepared and surface derivatized porous silicon (PS) in the presence of organic compounds in gas phase were studied. Surface derivatization, aimed at increasing stability of porous silicon properties, was performed by Lewis acid mediated hydrosilylation with methyl 10-undecenoate. We have systematically measured changes in photoluminescence intensity for a set of alcohols from C 1 to C 6 . From the variation of the photoluminescence quenching response as a function of alcohol concentration, we determine the sensitivities and detection limits of our porous silicon sensors and these correlate with physical and chemical properties of studied species. For methyl 10-undecenoate derivatized PS samples, we have observed a remarkable enhancement of the selectivity for C 4 -C 6 alcohols as compared with C 1 -C 3 alcohols.

We proposed and studied a new light emitting material based on silicon nanocrystals (Si-nc). The new material was fabricated using a low cost way by incorporating Si-nc into a sol–gel derived SiO2 matrix. The Si-nc were prepared (i) by electrochemical etching of monocrystalline Si wafers and (ii) pulverizing the obtained porous silicon film. The porous silicon powder was then dispersed in the SiO2 sol. After solidification, we obtained transparent and self-supporting SiO2 layers of about 1 mm thickness containing Si nanocrystals. The sol–gel layers exhibit bright red photoluminescence (PL) under UV lamp excitation at room temperature. This novel method circumvents the usual Si+-implantation step. We present first basic experimental studies of the PL properties of these sol–gel derived SiO2 layers and compare them with that of as-prepared porous silicon. We mention also observation of a non-linear behavior in the PL spectra. We emphasize potential advantages of this technology compared to the standard Si+-implanted SiO2 layers.

Silicon nanocrystals (Si-ncs) are promising for biological studies due to their supposed low cytotoxicity, good biocompatibility and biodegradability in living organisms. However, the bioresearchers' focus on Si-ncs has lasted only for a few recent years, and detailed studies of the interaction of various types of Si-ncs with biological environment are still rare. Suitable size and solubility of the Si-ncs in water-based isotonic solutions are important towards bringing the nanocrystals inside the living cells. We have prepared colloidal solutions of luminescent porous silicon of different cluster sizes in methanol, water and phosphate-buffered saline (PBS). By combination of ultrasonic treatment with filtration, we have obtained two different silicon cluster sizes in methanol (120 and 525 nm) and three different cluster sizes (85, 210 and 1,500 nm) in PBS. Nanoclusters of heavily oxidized porous silicon are hydrophilic and well soluble in water and/or PBS. They can be further used for studies on the biocompatibility of these materials and may be potentially employed as luminescent markers in living cells in biological research. PACS 78.67.Rb; 78.67.-n; 87.85.Qr; 87.85.Rs; 81.07.-b