Amorphous Silicon Nanoclusters Research Articles

Composition and optical properties of amorphous plasma-chemical silicon oxynitride of variable composition a-SiO-=SUB=-x-=/SUB=-N-=SUB=-y-=/SUB=- : H

The a-SiOxNy : H films of various compositions were obtained by plasma chemical deposition from a gas mixture of 10% monosilane (diluted with argon) and nitrogen in the presence of residual oxygen in the working gas mixture. The nitrogen flow rate varied in the range from 4 to 6 cm3/min, the power of the high-frequency generator (13.56 MHz) varied in the range of 50-150 Watts. The electronic structure and optical properties of the films were studied using X-ray photoelectron spectroscopy, vibrational spectroscopy, transmission and reflection spectroscopy, and spectral ellipsometry. It is shown that, as the generator power decreases, the content of excess silicon in the films increases and amorphous silicon nanoclusters appear. As the generator power increases, the oxygen concentration in the films decreases. Apparently, this is due to the greater dissociation of molecular nitrogen with an increase in the power of the plasma discharge and an increase in the concentration of active nitrogen. Thus, it is possible to control the composition of aa-SiOxNy : H films not only by changing the nitrogen flow, but also by varying the generator power. Keywords: silicon oxynitride, plasma-chemical deposition, stoichiometric composition, Si nanoclusters.

Состав и оптические свойства аморфного плазмохимического оксинитрида кремния переменного состава a-SiO-=SUB=-x-=/SUB=-N-=SUB=-y-=/SUB=- : H

The SiOxNy:H films of various compositions were obtained by plasma chemical deposition from a gas mixture of 10% monosilane (diluted with argon) and nitrogen in the presence of residual oxygen in the working gas mixture. The nitrogen flow rate varied in the range from 4 to 6 cm3/min, the power of the high-frequency generator (13.56 MHz) varied in the range of 50-150 Watts. The electronic structure and optical properties of the films were studied using X-ray photoelectron spectroscopy, vibrational spectroscopy, transmission and reflection spectroscopy, and spectral ellipsometry. It is shown that, as the generator power decreases, the content of excess silicon in the films increases and amorphous silicon nanoclusters appear. As the generator power increases, the oxygen concentration in the films decreases. Apparently, this is due to the greater dissociation of molecular nitrogen with an increase in the power of the plasma discharge and an increase in the concentration of active nitrogen. Thus, it is possible to control the composition of a SiOxNy:H films not only by changing the nitrogen flow, but also by varying the generator power.

Effect of the Temporal Characteristics of Modulated DC Plasma with the (SiH4–Ar–O2) Gas Phase on ncl-Si Growth in an a-SiOx:H matrix ( $${{C}_{{{{{\text{O}}}_{{\text{2}}}}}}}$$ = 15.5 mol %)

The effect of various operating conditions of time-modulated DC (direct current) plasma on the formation of an amorphous a-SiOx:H matrix and silicon nanoclusters is studied using IR (infrared) and photoluminescence spectra. DC-plasma modulation consists in repeated (n = 180) switching off and on of the magnetron magnet coil with various time combinations, toff = 1, 2, 5, 10, 15 s and ton = 5, 10, 15 s, respectively, at a fixed oxygen concentration ( $${{C}_{{{{{\text{O}}}_{{\text{2}}}}}}}$$ = 15.5 mol %) in a (SiH4 + Ar + О2) gas mixture. The positive effect of self-induction on the formation of both the amorphous matrix and silicon nanoclusters is confirmed. The largest values of x in a-SiOx:H and photoluminescence intensity are observed in the case of the combination of prolonged plasma residence in the working state (ton = 10–15 s) and the maximum magnetic-field strength. The effect of toff on the processes of the formation of both the a-SiOx:H matrix and silicon nanoclusters is also noted.

Influence of 20 MeV electron irradiation on the optical properties and phase composition of SiOx thin films

Homogeneous films from SiO1.3 (250 nm thick) were deposited on crystalline Si substrates by thermal evaporation of silicon monoxide. A part of the films was further annealed at 700 °C to grow amorphous Si (a-Si) nanoclusters in an oxide matrix, thus producing composite a-Si-SiO1.8 films. Homogeneous as well as composite films were irradiated by 20-MeV electrons at fluences of 7.2 × 1014 and 1.44 × 1015 el/cm2. The film thicknesses and optical constants were explored by spectroscopic ellipsometry. The development of the phase composition of the films caused by the electron-beam irradiation was studied by transmission electron microscopy. The ellipsometric and electron microscopy results have shown that the SiOx films are optically homogeneous and the electron irradiation with a fluence of 7.2 × 1014 el/cm2 has led to small changes in the optical constants and the formation of very small a-Si nanoclusters. The irradiation of the a-Si-SiOx composite films caused a decrease in the effective refractive index and, at the same time, an increase in the refractive index of the oxide matrix. Irradiation induced increase in the optical band gap and decrease in the absorption coefficient of the thermally grown amorphous Si nanoclusters have also been observed. The obtained results are discussed in terms of the formation of small amorphous silicon nanoclusters in the homogeneous layers and electron irradiation induced reduction in the nanocluster size in the composite films. The conclusion for the nanoparticle size reduction is supported by infrared transmittance results.

Effect of annealing in oxidizing atmosphere on optical and structural properties of silicon suboxide thin films obtained by gas-jet electron beam plasma chemical vapor deposition method

The effect of the annealing temperature on the optical and structural properties of the a-SiOx:H thin films prepared by gas-jet electron beam plasma chemical vapor deposition method was studied. Annealing was carried out at 600, 700, 800, 900 and 1000 °C for 4 h in oxidizing atmosphere. According to FTIR spectroscopy measurements, the oxygen and hydrogen concentration in the as-deposited films was 25 at.% and 2 at.%, respectively. The SEM image showed that the as-deposited material had column structure with a large number of vertical voids. As a result of annealing, the thickness of the films decreased by approximately 1.5 times for all samples. The value of refractive index at 650 nm decreased from 2.5 to 2.0 with the increase of the annealing temperature. The E04 optical gap decreased in comparison with the value of the as-deposited thin films for 600 °C and 700 °C, and increased for 800–1000 °C. For annealing at a temperature of 600 °C, the structure of the material changes insignificantly. A rearrangement in the structure of the matrix with the formation of amorphous silicon nanoclusters occurs at 700 °C and 800 °C. Annealing at the higher temperatures leads to transition from a material with amorphous nanoclusters to a material with nanocrystallites.

Изучение влияния временных характеристик модулированной DC-плазмы с (SiH-=SUB=-4-=/SUB=--Ar-O-=SUB=-2-=/SUB=-)-газовой фазой на рост ncl-Si в матрице a-SiO-=SUB=-x-=/SUB=-:H (C-=SUB=-O-=SUB=-2-=/SUB=--=/SUB=-=15.5 мол%)

AbstractThe effect of various operating conditions of time-modulated DC (direct current) plasma on the formation of an amorphous a -SiO_ x :H matrix and silicon nanoclusters is studied using IR (infrared) and photoluminescence spectra. DC-plasma modulation consists in repeated ( n = 180) switching off and on of the magnetron magnet coil with various time combinations, t _off = 1, 2, 5, 10, 15 s and t _on = 5, 10, 15 s, respectively, at a fixed oxygen concentration ( $${{C}_{{{{{\text{O}}}_{{\text{2}}}}}}}$$ = 15.5 mol %) in a (SiH_4 + Ar + О_2) gas mixture. The positive effect of self-induction on the formation of both the amorphous matrix and silicon nanoclusters is confirmed. The largest values of x in a -SiO_ x :H and photoluminescence intensity are observed in the case of the combination of prolonged plasma residence in the working state ( t _on = 10–15 s) and the maximum magnetic-field strength. The effect of t _off on the processes of the formation of both the a -SiO_ x :H matrix and silicon nanoclusters is also noted.

Lithiation of Silicon Nanoclusters

Engineering better batteries for mobile electronics is a major endeavor in energy research. Silicon is potentially a great anode material for lithium-ion systems, but unfortunately it undergoes dramatic volume changes during lithiation and delithiation, leading to rapidly fading ability to hold a charge. The authors show that lithiation of amorphous silicon nanoclusters is a two-stage process governed by Coulomb repulsion, and that at high loading the Li atoms switch where they like to reside in the clusters, causing expansion. Knowing the physics of this threshold allows for optimized battery design.

Photoluminescence of amorphous and crystalline silicon nanoclusters in silicon nitride and oxide superlattices

The photoluminescence properties of silicon nitride and oxide superlattices fabricated by plasmaenhanced chemical vapor deposition are studied. In the structures annealed at a temperature of 1150°C, photoluminescence peaks at about 1.45 eV are recorded. The peaks are defined by exciton recombination in silicon nanocrystals formed upon annealing. Along with the 1.45-eV peaks, a number of peaks defined by recombination at defects at the interface between the nanocrystals and silicon-nitride matrix are detected. The structures annealed at 900°C exhibit a number of photoluminescence peaks in the range 1.3–2.0 eV. These peaks are defined by both the recombination at defects and exciton recombination in amorphous silicon nanoclusters formed at an annealing temperature of 900°C. The observed features of all of the photoluminescence spectra are confirmed by the nature of the photoluminescence kinetics.

Sensitization of the Blue-green Electroluminescence by Gadolinium Coupled to Si Nanocluster Embedded in a SiO2 Matrix

In this work an enhancement of the blue and green electroluminescence (EL) related to silicon inclusions into SiO 2 layer by gadolinium co-doping was investigated. The blue (460 nm) and green (550 nm) EL corresponds to the oxygen-deficient-centres (ODC) and Si nanoclusters with average diameter below 1 nm, respectively. After gadolinium co-doping a fourfold increase of the total EL intensity was observed. It was found that the increase of the blue and green EL is due to the energy transfer from Gd 3+ ions to the ODC defects and amorphous silicon nanoclusters. Moreover, the gadolinium co-doping increases the concentration of the small silicon nanoclusters. DOI: http://dx.doi.org/10.5755/j01.ms.19.2.1789

Photocurrent and photovoltage spectroscopy of amorphous silicon nanoclusters

Photoelectric properties (photocurrent efficiency and photovoltage) of the silicon suboxide films containing amorphous silicon nanoclusters were investigated in the spectral range of 300–1100nm. A strongly pronounced increase of the photocurrent efficiency of the sandwich-like structures with such films on c-Si substrates was observed in the short-wavelength region. The possible mechanisms of the increase were discussed, and carrier multiplication due to impact ionization of the defect states was considered to be the most probable. Impact ionization of the defect states involves two main steps: (i) trapping of the photogenerated electron in a defect state, and (ii) impact ionization of this state by another photoexcited electron that has got sufficient energy due to absorption of a high-energy photon.

TEM and Spectroscopic Ellipsometry studies of multilayer gate dielectrics containing crystalline and amorphous Si nanoclusters

Gate dielectrics containing crystalline or amorphous silicon nanoclusters are fabricated by thermal oxidation, followed by deposition of SiOx films (x=1.15) using thermal evaporation of SiO in vacuum and a two-step N2/N2+O2 annealing process. Cross-sectional Transmission Electron Microscopy proves the formation of two regions in the SiOx film: a region free of nanoclusters close to the top surface and a second region with nanoclusters underneath the first one. Spectroscopic Ellipsometry was used to obtain the volume fraction of the pure Si phase in the oxide matrix, ∼31 and 28vol% for the films with nanocrystals and amorphous nanoparticles, respectively. The dependencies of the index of refraction and extinction coefficient on wavelength obtained using Bruggeman Effective Medium Approximation are close to those of silicon monoxide. A correlation is found between the thicknesses of the three regions determined by Transmission Electron Microscopy and Spectroscopic Ellipsometry.

Crystallization of Amorphous Si Nanoclusters in SiOx Films Using Femtosecond Laser Pulse Annealings

The SiO(x) films of various stoichiometries deposited on Si substrates with the use of the co-sputtering from two separate Si and SiO2 targets were annealed by femtosecond laser pulses. Femtosecond laser treatments were applied for crystallization of amorphous silicon nanoclusters in the silicon-rich oxide films. The treatments were carried out with the use of Ti-Sapphire laser with wavelength 800 nm and pulse duration about 30 fs. Regimes of crystallization of amorphous Si nanoclusters in the initial films were found. Ablation thresholds for SiO(x) films of various stoichiometries were discovered. The effect of laser assisted formation of a-Si nanoclusters in the non-stoichiometric dielectric films with relatively low concentration of additional Si atoms was also observed. This approach is applicable for the creation of dielectric films with semiconductor nanoclusters on non-refractory substrates.

Evolution of silicon nanoclusters and hydrogen in SiNx:H films: Influence of high hydrostatic pressure under annealing

Hydrogenated silicon-rich nitride (SRN) films of various stoichiometry (SiNx:H, 0.7<x≤1.3) were deposited on single-crystalline Si substrates with the use of plasma enhanced chemical vapor deposition at a temperature of 100°С. Furnace annealing for 5h in ambient Ar at 1130°С under atmospheric and enhanced hydrostatic pressure (HP — 11kbar, 1.1GPa) was applied to modify the structure of the films. The properties of as-deposited and annealed films were studied using ellipsometry, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, photoluminescence (PL), as well as high-resolution transmittance electron microscopy (HRTEM). According to the Raman data, the as-deposited film, in which the stoichiometry parameter x is below 1.0, contains amorphous silicon nanoclusters. Furnace annealing leads to crystallization of the nanoclusters. From the HRTEM and Raman data, the average size of the Si nanocrystals in the annealed films was 6–7nm. No silicon nanoclusters were observed in the as-deposited films with relatively low concentration of excessive silicon atoms (the case of SiNx:H, x>1); furnace annealing leads to segregation of the Si and Si3N4 phases, so, the amorphous Si clusters were observed in annealed films according to Raman data. Surprisingly, after annealing with such high thermal budget, according to the FTIR data, the SRN film with parameter x close to that of the stoichiometric silicon nitride contains hydrogen in the form of SiH bonds. From analysis of the FTIR data of the SiN bond vibrations one can conclude that silicon nitride is partly crystallized in the films with x>1 after annealing for 5h. No influence of HP on the structure of Si nanoclusters was observed in the case of SRN films with x≤1.1. Dramatic changes in the PL spectra of the SRN films with the x parameter close to that of the stoichiometric silicon nitride (x≈1.3), annealed under atmospheric pressure and HP, were observed. HP stimulates the formation of very small hydrogenated amorphous nanoclusters. The size of amorphous Si nanoclusters determined from the quantum size effect model describing the PL spectra, should be 2–4nm in this case.

Investigation of the dependence of the photoluminescence properties of silicon nanoclusters on their volume fraction in a silicon oxide matrix

The photoluminescence (PL) spectra and kinetics of amorphous and crystalline silicon nanoclusters are investigated. The given nanoclusters are formed by thermal annealing of thin suboxide silicon films with different volume fractions of silicon. It is demonstrated that the PL intensity and lifetime of the ensembles of silicon nanocrystals have a steplike dependence on the silicon volume fraction in the film. The influence of the percolation effect on the photoluminescence properties of the structures under study is discussed.

Silicon nanocluster sensitization of erbium ions under low-energy optical excitation

The sensitizing action of amorphous silicon nanoclusters on erbium ions in thin silica films has been studied under low-energy (long wavelength) optical excitation. Profound differences in fast visible and infrared emission dynamics have been found with respect to the high-energy (short wavelength) case. These findings point out to a strong dependence of the energy transfer process on the optical excitation energy. Total inhibition of energy transfer to erbium states higher than the first excited state (4I13/2) has been demonstrated for excitation energy below 1.82 eV (excitation wavelength longer than 680 nm). Direct excitation of erbium ions to the first excited state (4I13/2) has been confirmed to be the dominant energy transfer mechanism over the whole spectral range of optical excitation used (540 nm–680 nm).

Percolation effect in structures with amorphous and crystalline silicon nanoclusters

AbstractIn this paper the correlations between optical, electrical and structural properties of ensembles of amorphous and crystalline silicon nanoclusters in silicon oxide matrix are explained in terms of percolation theory. SiOx (1 ≤ x &lt; 2) thin films were deposited on silicon or quartz substrates by reactive evaporation of SiO powder and were subsequently thermally annealed at temperatures in the range from 450 to 1200 °C. Five sets of samples with different stoichiometry parameters x = 1.0, 1.1, 1.29, 1.56, 1.68 of initial SiOx were fabricated. The volume fraction of total excess silicon as well as the volume fraction of silicon nanocrystals in high temperature annealed structures are calculated for each sample set. It is found that samples with excess Si volume fraction of more than 16% (which is critical value in percolation theory) demonstrate a local maximum at about 500‐600 °C in the dependence of integrated photoluminescence intensity on annealing temperature, which is attributed to the first percolation threshold in the system of amorphous Si nanoclusters. In turn, the second percolation threshold is detected in the system of crystalline Si nanoclusters, when their volume fraction exceeds 16%. Such effect, resulted in dramatic increase of the electrical conductance of SiOx film, was registered experimentally for the set of samples with x = 1.0. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Influence of oxygen on light emission from amorphous silicon nanoclusters

AbstractSurface effects, in particular oxygen passivation, are known to have drastic influence on emission spectra of semiconductor nanoclusters. The goal of this work was to obtain light emission from amorphous silicon nanoclusters by means of oxygen passivation. Granulated films of amorphous silicon nanoclusters were fabricated by the laser electrodispersion technique, which allows obtaining nanoclusters of uniform sizes. Two methods were applied in order to incorporate oxygen into the films, and both result in improvement of the luminescent properties of the films. The role of oxygen passivation as well as the effect of thermal annealing is considered. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Photoluminescence study of the structural evolution of amorphous and crystalline silicon nanoclusters during the thermal annealing of silicon suboxide films with different stoichiometry

The effect of the stoichiometry of thin silicon suboxide films on the processes of the formation and evolution of silicon nanoclusters during thermal annealing is studied by photoluminescence measurements. The samples are produced by the thermal sputtering of a SiO powder in an oxygen atmosphere, with the subsequent deposition of a 500 nm-thick SiOx layer onto a Si substrate. The morphological properties and size of Si nanoclusters are explored by analyzing the photoluminescence spectra and kinetics. A comparative study of the luminescence properties of thin SiOx layers with different stoichiometric parameters, x = 1.10, 1.29, 1.56, and 1.68, is accomplished for samples annealed at different temperatures in the range 850 to 1200°C. The dependences of the photoluminescence decay time on the annealing temperature, the stoichiometric parameter of the initial silicon suboxide film, and the nanocluster size are studied.

Luminescence of amorphous silicon nanoclusters

Films of amorphous silicon suboxide α-SiO x containing amorphous silicon nanoclusters have been grown by direct current magnetron sputtering. It has been found that two radiation bands are observed in the photoluminescence spectra of relatively large amorphous nanoclusters (a size of ∼2 nm) unlike one photoluminescence band of silicon nanocrystals of the same size. The form of the spectra upon the change in the nanocluster sizes agrees with that predicted in theoretical works, in which the energy spectrum of amorphous nanoclusters has been calculated taking into account quantum confinement of delocalized, weakly localized, and strongly localized states.

Photocurrent and Photoluminescence of Amorphous Silicon Nanoclusters Embedded in Silicon Suboxide Matrix

We have presented a technique, based on magnetron sputtering of silicon target in the mixture of argon, silane, and oxygen. Addition of oxygen gas was shown to cause formation of silicon suboxide layers with amorphous silicon nanoclusters without subsequent annealing. The layers exhibit significant photoluminescence at room temperature. Their photoluminescence spectra reveal special features predicted in the preceding well-known theoretical works. Heterostructures, fabricated with such layers, show high photocurrent efficiency in short-wavelength spectral region. Our results demonstrate that the investigated structures are promising for photoelectric and photovoltaic applications.