360-nm Photoluminescence from Silicon Oxide Films Embedded with Silicon Nanocrystals

Si-rich silicon oxide films were deposited by RF magnetron sputtering onto composite Si/SiO2 targets. After annealed at different temperature, the silicon oxide films embedded with silicon nanocrystals were obtained. The photoluminescenee（PL） from the silicon oxide films embedded with silicon nanocrystals was observed at room temperature. The strong peak is at 360 nm, its position is independent of the annealing temperature. The origin of the 360-nm PL in the silicon oxide films embedded with silicon nanoerystals was discussed.     

Kinetics of exciton photoluminescence in low-dimensional silicon structures

Time-resolved photoluminescence (PL) spectra have been measured at 90–300 K in the visible spectral range for porous nanocrystalline silicon films fabricated by laser deposition. The energy and time ranges in which the spectra were taken were 1.4–3.2 eV and 50 ns–10 µs, respectively. The correlation between PL characteristics (intensity, emission spectrum, relaxation times and their temperature dependence), structure, and dielectric properties (size and shape of Si nanocrystals, oxide phase of their coatings, porosity of films) has been studied. A model of photoluminescence is adopted, in which the absorption and emission of photons occur in quantum-size nanocrystals, and in which kinetically coupled subsystems of electron-hole pairs and excitons are involved in the radiative recombination. Possible mechanisms of the exciton Auger recombination in low-dimensional silicon structures are proposed.     

Light emission from crystalline silicon and amorphous silicon oxide (SiOx) nanoparticles

Bright orange-red light emission was observed from single crystal silicon nanoparticles and silicon oxide (SiO_x) nanoparticles. The emission peak was recorded at about 1.5 eV both at room temperature and 77K. Varying the mean silicon particle size, we observed no effect of particle diameter on the emission wavelength. Amorphous silicon oxide (SiO_x) nanoparticles also showed essentially the same emission spectrum as the crystalline particles. The absence of change in the photoluminescence (PL) spectrum with variations in particle size and crystallinity indicates that quantum confinement is not the controlling PL mechanism. An examination of the hydrogen content with relation to the PL intensity showed no direct correlation; however, all samples did contain some hydrogen, so its effect on PL cannot be ruled out. To test for the presence of photoluminescent siloxene on the surface of the particles, nitric acid was applied; a violent reaction occurred with the silicon particles, while the SiO_x particles showed no reaction. Taken in conjunction with the emission data, these experiments demonstrate that the PL of the SiO_x is also not dependant on siloxene. Evidence points to an amorphous coating as the source of photoluminescence.     

Photoluminescence of silicon nanocrystals under the effect of an electric field

The effect of an electric field on photoluminescence (PL) of silicon nanocrystals formed in silicon dioxide by ion implantation with subsequent annealing has been studied. Application of an electric field leads to an increase in PL intensity by ～10% at low temperatures and an electric field strength of 12 kV/cm and to its decrease at temperatures above 20 K. The increase in exciton PL intensity in an electric field is inconsistent with the model of recombination of quantum-confined excitons in nanocrystals. The effect can be described in terms of a model of recombination of self-trapped excitons formed at the interface between a Si nanocrystal and SiO₂.     

Quantum-confinement effect in silicon-on-insulator films implanted with high doses of hydrogen ions

280-nm-thick silicon-on-insulator films are implanted with high doses of hydrogen with the energy 24 keV and the dose 5 × 10¹⁷ cm^?2. Peaks corresponding to optical phonons localized in the silicon nanocrystals 1.9?C2.5 nm in size are observed in the Raman spectra. The fraction of the nanocrystal phase is ??10%. A photoluminescence band with a peak at about 1.62 eV is detected. The intensity of the 1.62 eV band nonmonotonically depends on the measurement temperature in the range from 88 to 300 K. An increase in the radiative recombination intensity at temperatures <150 K is interpreted in the context of a two-level model for the energy of strongly localized electrons and holes. The activation energy of photoluminescence enhancement is 12.4 meV and corresponds to the energy of splitting of the excited state of charge carriers localized in the silicon nanocrystals.     

The role of nitrogen in the formation of luminescent silicon nanoprecipitates during heat treatment of SiO2 layers implanted with Si+ ions

Twenty-five kiloelectronvolt Si⁺ ions with doses of (1–4)×10¹⁶ cm^?2 and 13-keV N⁺ ions with doses of (0.2–2)×10¹⁶ cm^?2 were implanted into SiO₂ layers, which were then annealed at 900–1100°C to form luminescent silicon nanoprecipitates. The effect of nitrogen on this process was deduced from the behavior of the photoluminescence spectra. It was found, for a certain ratio between the concentrations of implanted silicon and nitrogen, that the photoluminescence intensity increases significantly, and its peak shifts to shorter wavelengths. It is concluded that the number of precipitation nuclei increases owing to the interaction of nitrogen with excess silicon. Eventually, this results in an increase in the number of nanocrystals and in a decrease in their average sizes. In spite of introducing additional precipitation nuclei, the minimal concentrations of excess Si on the order of 10²¹ cm^?3 and heat treatments at temperatures higher than 1000°C were still required for the formation of nanocrystals.     

Silicon nanocrystal formation upon annealing of SiO2 layers implanted with Si ions

The formation of silicon nanocrystals in SiO₂ layers implanted with Si ions was investigated by Raman scattering, X-ray photoelectron spectroscopy, and photoluminescence. The excess Si concentration was varied between 3 and 14 at. %. It was found that Si clusters are formed immediately after implantation. As the temperature of the subsequent annealing was raised, the segregation of Si accompanied by the formation of Si-Si₄ bonds was enhanced but the scattering by clusters was reduced. This effect is attributed to the transformation of loosely packed clusters into compact, separate-phase nanoscale Si precipitates, with the Raman peak observed at 490 cm^?1 being related to surface scattering. The process of Si segregation was completed at 1000°C. Nevertheless, characteristic nanocrystal photoluminescence was observed only after annealing at 1100°C. Simultaneously, scattering in the range 495–520 cm^?1, typical of nanocrystals, appeared; however, the “surface-related” peak at 490 cm^?1 persisted. It is argued that nanocrystals are composed of an inside region and a surface layer, which is responsible for their increased formation temperature.     

Strained Interface Defects in Silicon Nanocrystals

The surface of silicon nanocrystals embedded in an oxide matrix can contain numerous interface defects. These defects strongly affect the nanocrystals’ photoluminescence efficiency and optical absorption. Dangling‐bond defects are nearly eliminated by H₂ passivation, thus decreasing absorption below the quantum‐confined bandgap and enhancing PL efficiency by an order of magnitude. However, there remain numerous other defects seen in absorption by photothermal deflection spectroscopy; these defects cause non‐radiative recombination that limits the PL efficiency to <15%. Using atomistic pseudopotential simulations, we attribute these defects to two specific types of distorted bonds: Si‐Si and bridging Si‐O‐Si bonds between two Si atoms at the nanocrystal surface.     

Coulomb blockade of the conductivity of SiO<Subscript>x</Subscript> films due to one-electron charging of a silicon quantum dot in a chain of electronic states

The electrical characteristics of metal-oxide-semiconductor (MOS) structures with silicon nanoparticles embedded in silicon oxide have been studied. The nanocrystals are formed by decomposition of an oversaturated solid solution of implanted silicon during thermal annealing at a temperature of ～1000°C. At liquid-nitrogen temperature, a stepped current-voltage characteristic is observed in a MOS structure consisting of Si nanocrystals in a SiO₂ film. The stepped current-voltage characteristic is, for the first time, quantitatively described using a model in which charge transport occurs via a chain of local states containing a silicon nanocrystal. The presence of steps is found to be associated with one-electron charging of the silicon nanocrystal and Coulomb blockade of the probability of a hop from the nearest local state to the conducting chain. The local states in silicon dioxide are assumed to be related to an excess of silicon atoms. The presence of such states is confirmed by measurements of the differential conductance and capacitance. For MOS structures implanted with silicon, the differential capacitance and conductance are found to be higher, compared to the reference structures, in the range of biases exceeding 0.2 V. In the same bias range, the conductance is observed to decrease under ultraviolet irradiation due to a change in the population of the states in the conductivity chains.     

Light emission from Si/SiO2 superlattices fabricated by RPECVD

Si/SiO₂ superlattices that exhibit intense luminescence properties were fabricated by remote plasma enhanced chemical vapor deposition. (RPECVD) and subsequent rapid thermal annealing for silicon crystallization. The effects of charge carrier confinement like blue shifting of the PL spectra and intensity increase with decreasing Silicon quantum well thickness are observed in low temperature photoluminescence experiments. The Si/SiO₂ interface quality is calculated from capacitance voltage (CV) measurements on metal oxide semiconductor teststructures showing excellent layer and Si/SiO₂ interface properties. The Si crystallization process is investigated and analyzed by Raman and transmission electron microscopy. Decreasing the Si quantum well thickness to 2 nm leads to light emission at room temperature.     

Lifetime of excitons localized in Si nanocrystals in amorphous silicon

The introduction of nanocrystals plays an important role in improving the stability of the amorphous silicon films and increasing the carrier mobility. Here we report results of the study on the photoluminescence and its dynamics in the films of amorphous hydrogenated silicon containing less than 10% of silicon nanocrystals. The comparing of the obtained experimental results with the calculated probability of the resonant tunneling of the excitons localized in silicon nanocrystals is presented. Thus, it has been estimated that the short lifetime of excitons localized in Si nanocrystal is controlled by the resonant tunneling to the nearest tail state of the amorphous matrix.     

Photoluminescent and electronic properties of nanocrystalline silicon doped with gold

Doping nanocrystalline silicon (nc-Si) films grown by laser ablation with gold leads to a considerable suppression of the nonradiative recombination of the charge carriers and excitons, an increase in the intensity and stability of the visible photoluminescence, and enhancement of the low-energy (1.5–1.6 eV) photoluminescence band. In Au-doped samples, the magnitude of the photovoltage and the rate of electron capture by traps in the film are reduced, and the density of boundary electron states and the concentration of deep electron traps at the single-crystal silicon (c-Si) substrate are decreased as well. The observed effect of doping on the photoluminescent and electronic properties of nc-Si films and nc-Si/c-Si structures is caused by the passivation of dangling Si bonds with Au and by the further oxidation of silicon at the surface of nanocrystals, which results in the formation of high-barrier SiO₂ layers.     

Dramatic Enhancement of Photoluminescence Quantum Yields for Surface‐Engineered Si Nanocrystals within the Solar Spectrum

Substantial improvements of the absolute photoluminescence quantum yield (QY) for surfactant‐free silicon nanocrystals (Si‐ncs) by atmospheric pressure microplasma 3‐dimensional surface engineering are reported. The effect of surface characteristics on carrier multiplication mechanisms is explored using transient induced absorption and photoluminescence QY. Surface engineering of Si‐ncs is demonstrated to lead to more than 120 times increase in the absolute QY (from 0.1% up to 12%) within an important spectral range of the solar emission (2.3–3 eV). The Si‐ncs QY is shown to be stable when Si‐ncs are stored in ethanol at ambient conditions for three months.     

Photoluminescence of nanocrystalline silicon films formed by pulsed laser-assisted deposition with the introduction of carbon

The effect of carbon on the photoluminescent properties of films consisting of quantum-dimensional Si nanocrystals in the SiO_x (x → 2) matrix is studied. The spectra of time-resolved photoluminescence in the photon-energy range of 1.4–3.2 eV and the infrared-absorption spectra in the wave-number range of 650–1500 cm^?1 were measured. It is established that the introduction of carbon in the presence of oxygen in the course of the pulsed laser-assisted deposition of the films brings about the white-blue emission spectrum and also an increase in the intensity and stability of photoluminescence. The effect of carbon on the photoluminescent properties of the films is related to the formation of the SiO₂ barrier phase instead of SiO_x (1 < x < 2), saturation of silicon dangling bonds at the surface of Si nanocrystals with larger sizes, and mechanical strengthening of Si nanocrystals with smaller sizes.     

ESR studies of nanocrystalline silicon films obtained by pulsed laser ablation of silicon targets

Nanocrystalline silicon films formed using laser ablation of silicon targets were studied using electron spin resonance. The measurements were performed in the X band with modulation of the magnetic field at a frequency of ～100 kHz at temperatures of 300 and 77 K. Two types of spectra were observed. The first type of spectra is related to the high concentration of dangling silicon bonds in Si nanocrystals and SiO_x sheaths of nanocrystals and are inherent in nanocrystalline silicon (nc-Si) films that do not exhibit photoluminescence in the visible region of the spectrum. The second type of spectra is related to the presence of E′ centers, nonbridging oxygen hole centers (NBOHC), and peroxide radicals and is characteristic of films with photoluminescence in the visible region of the spectrum, which indicates that high-barrier SiO₂ layers exist in these films. An increase in the photoluminescence intensity and a decrease in the signal of electron spin resonance were observed in porous nc-Si films exposed to atmospheric air for a long time.     

Effect of high-power nanosecond and femtosecond laser pulses on silicon nanostructures

The effect of high-power nanosecond (20 ns) and femtosecond (120 fs) laser pulses on silicon nanostructures produced by ion-beam-assisted synthesis in SiO₂ layers or by deposition onto glassy substrates is studied. Nanosecond annealing brings about a photoluminescence band at about 500 mn, with the intensity increasing with the energy and number of laser pulses. The source of the emission is thought to be the clusters of Si atoms segregated from the oxide. In addition, the nanosecond pulses allow crystallization of amorphous silicon nanoprecipitates in SiO₂. Heavy doping promotes crystallization. The duration of femtosecond pulses is too short for excess Si to be segregated from SiO₂. At the same time, such short pulses induce crystallization of Thin a-Si films on glassy substrates. The energy region in which crystallization is observed for both types of pulses allows short-term melting of the surface layer.     

Study of the layer-substrate interface in <Emphasis Type="Italic">nc</Emphasis>-Si-SiO<Subscript>2</Subscript>-<Emphasis Type="Italic">p</Emphasis>-Si structures with silicon quantum dots by the method of temperature dependences of photovoltage

Layers grown by magnetron deposition of Si and SiO₂ on a p-type silicon substrate and containing silicon nanocrystals in the oxide matrix have been studied by the method of temperature dependences of the capacitive photovoltage. The effect of the substrate orientation and natural oxidation preceding high-temperature annealing that results in the formation of Si nanocrystals in the SiO₂ matrix on the layer-substrate interface characteristics is studied. The density of fast interface states trapping majority carriers was estimated. It is found that structural changes occur at the layer-substrate interface in the case of a (111) substrate and are caused by stresses appearing upon cooling. It was shown that natural oxidation of the deposited layer, preceding high-temperature annealing, causes an increase in the charge trapped in the oxide.     

Low‐Cost Post‐Growth Treatments of Crystalline Silicon Nanoparticles Improving Surface and Electronic Properties

Freestanding silicon nanocrystals (Si‐ncs) offer unique optical and electronic properties for new photovoltaic, thermoelectric, and other electronic devices. A method to fabricate Si‐ncs which is scalable to industrial usage has been developed in recent years. However, barriers to the widespread utilization of these nanocrystals are the presence of charge‐trapping defects and an oxide shell formed upon ambient atmosphere exposure hindering the charge transport. Here, we exploit low‐cost post‐growth treatment routes based on wet‐etching in hydrofluoric acid plus surface hydrosilylation or annealing enabling a complete native oxide removal and a reduction of the defect density by up to two orders of magnitude. Moreover, when compared with only H‐terminated Si‐ncs we report an enhancement of the conductivity by up to a factor of 400 for films of HF etched and annealed Si‐ncs, which retain a defect density below that of untreated Si‐ncs even after several months of air exposure. Further, we demonstrate that HF etched and hydrosilylated Si‐ncs are extremely stable against oxidation and maintain a very low defect density after a long‐term storage in air, opening the possibility of device processing in ambient atmosphere.     

Luminescence properties of thin nanocrystalline silicon-carbide films fabricated by direct-beam ion deposition

Nanocomposite films containing nanocrystals of silicon and silicon carbide are fabricated by direct ion-beam deposition onto silicon substrates. The films obtained are by Raman spectroscopy and the photoluminescence method. It is found that the samples under study exhibit two photoluminescence bands, in the “red” and near-infrared (IR) (600–1000 nm) and “blue” (400–550 nm) spectral ranges, which are accounted for by, respectively, the radiative recombination of excitons in silicon nanocrystals and radiative transitions between levels related to local centers (defects) on the surface of silicon nanocrystals and in the surrounding matrix. Short-duration treatment of the films in a solution based on hydrofluoric acid leads to modification of the emission spectrum and to an increase in the intensity of the exciton band due to the passivation of defects on the surface of silicon nanocrystals.     

Optical and electrical properties of Al2O3 films containing silicon nanocrystals

Optical, electrical, and structural properties of Al₂O₃ films subjected to silicon-ion implantation and annealing were investigated by means of photoluminescence measurements, current-voltage measurements and transmission electron microscopy. Transmission electron microscopy revealed that silicon nanocrystals were epitaxially formed in ϑ-Al₂O₃. Visible photolum inescence was observed, for the first time, from Al₂O₃ films containing silicon nanocrystals. Observed visible photoluminescence seems to be related to quantum size effects in silicon nanocrystals as well as localized radiative recombination centers located at the interface between silicon nanocrystals and matrix, similar to porous Si and other Si nanostructures. The conduction mechanism in the samples was studied by using dc current-voltage measurements. The conduction properties depend on temperature and applied electric fields. The conduction behavior in low electric fields consists of thermally activated region dominated by the Schottky conduction and nonthermally activated region in which carrier transport is controlled by space-charge-limited currents. The conduction behavior under relatively high electric fields is almost independent of temperature and well fitted by space-charge-limited conduction.