Amorphous Si Nanoclusters Research Articles

Ultraviolet, visible and near infrared photoresponse of SiO2/Si/SiO2 multilayer system into a MOS capacitor

In this paper, we report on the photoresponse properties of a SiO2/Si/SiO2 multilayer (ML) system deposited at 500 °C and integrated into a metal-oxide-semiconductor structure (Al/ML/p-Si) without any further thermal annealing. High contents of elemental Si (Si0) revealed by the XPS measurements suggests the formation of amorphous Si-nanoclusters (a-Si ncls) within the ML. Dark and illuminated I–V curves of the device indicate its sensitivity to ultraviolet, visible and near infrared light under applied bias and at 0 V. In addition, we observed variation of the spectral response and the external quantum efficiency as a function of the electrode size. At 0 V, the device illuminated with a wavelength at 550 nm exhibits a maximum external quantum efficiency of 0.95% and a spectral response of 4.1 mA/W. This photoresponse has been ascribed to photon absorption in SiO2 sub-band states created by the a-Si ncls and defects formed by the excess Si inside the SiO2/Si/SiO2 multilayers.

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.

Structural transformation of Si-rich SiNx film on Si via swift heavy ions irradiation

The effects of 200 MeV-Xe+ irradiation with fluencies of (109–1014) cm−2 on the phase-structural transformation of Si-rich SiNx film deposited on Si substrate by low-pressure chemical vapor deposition have been reported. It has been shown from Raman scattering data that the swift heavy ions irradiation results in the dissolution of amorphous Si nanoclusters in nitride matrix. It has been shown, too, that the swift heavy ion irradiation leads to quenching a visual photoluminescence from nitride films.

Optical properties of Si nanocrystals formed with laser pulse annealing

Nano- and femtosecond laser pulse annealings were applied for crystallization of thin amorphous Si films and amorphous Si nanoclusters in silicon-rich nitride and oxide films. Laser assisted formation of amorphous Si nanoclusters in the non-stoichiometric dielectric films was observed. Non-thermal effects in crystallization of amorphous Si by femtosecond annealing were supposed. This approach is applicable for the creation of dielectric films with Si nanoclusters on non-refractory substrates.

Generation of silicon nanocrystals by damage free continuous wave laser annealing of substrate-bound SiOx films

Silicon nanocrystals have been generated by laser induced phase separation in SiOx films. A continuous wave laser emitting at 405 nm is focused to a 6 μm diameter spot on 530 nm thick SiOx films deposited on fused silica substrates. Irradiation of lines is accomplished by focus scanning. The samples are investigated by atomic force microscopy, TEM, Raman spectroscopy, and photoluminescence measurements. At a laser power of 35 mW corresponding to an irradiance of about 1.2 × 105 W/cm2, the formation of Si-nanocrystals in the film without any deterioration of the surface is observed. At higher laser power, the central irradiated region is oxidized to SiO2 and exhibits some porous character, while the surface remains optically smooth, and nanocrystals are observed beside and beneath this oxidized region. Amorphous Si-nanoclusters are formed at lower laser power and around the lines written at high power.

Synthesis and Light Absorption Mechanism in Si or Ge Nanoclusters for Photovoltaics Applications

Photon absorption in the solar energy range has been investigated in semiconductor nanostructures. Different synthesis techniques (magnetron sputtering, plasma enhanced chemical vapor deposition, ion implantation) followed by thermal annealing, have been employed to fabricate Si or Ge nanoclusters (1-25 nm in size) embedded in SiO2or Si3N4matrices. The thermal evolution in the formation of Si nanoclusters (NCs) in SiO2was shown to depend on the synthesis technique and to significantly affect the light absorption. Experimentally measured values of optical bandgap (EgOPT) in Si NCs evidence the quantum confinement effect which significantly increases the value ofEgOPTin comparison to bulk Si.EgOPTspans over a large range (1.6-2.6 eV) depending on the Si content, on the deposition technique and, in a most significant way, on the structural phase of NC. Amorphous Si NCs have a lowerEgOPTin comparison to crystalline ones. The matrix effect on the synthesis and light absorption in semiconductor NCs was investigated for Ge NCs. Large difference in the Ge NCs synthesis occurred when using SiO2or Si3N4matrices, essentially due to a much lower Ge diffusivity in the latter, which slows down the formation and growth of Ge NCs in comparison to silica matrix. Light absorption in NCs is also shown to be largely affected by the host matrix. Actually, Ge NCs embedded in Si3N4material absorb photons in the solar energy range with a higher efficiency than in silica, due to the different confinement effect. In fact, Si3N4host offers a lower potential barrier to photogenerated carriers in comparison to silica, thus a lower confinement effect is expected, leading to slightly smaller optical bandgap. These effects have been presented and discussed for potential application in light harvesting purposes.

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.

Atomic scale observation of phase separation and formation of silicon clusters in Hf higk-κ silicates

Hafnium silicate films were fabricated by RF reactive magnetron sputtering technique. Fine microstructural analyses of the films were performed by means of high-resolution transmission electron microscopy and atom probe tomography. A thermal treatment of as-grown homogeneous films leads to a phase separation process. The formation of SiO2 and HfO2 phases as well as pure Si one was revealed. This latter was found to be amorphous Si nanoclusters, distributed uniformly in the film volume. Their mean diameter and density were estimated to be about 2.8 nm and (2.9 ± 0.4) × 1017 Si-ncs/cm3, respectively. The mechanism of the decomposition process was proposed. The obtained results pave the way for future microelectronic and photonic applications of Hf-based high-κ dielectrics with embedded Si nanoclusters.

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)

Precipitation of silicon nanoclusters by laser direct-write

The ability to use a laser to direct-write tracks of localized emission enhancement in PECVD-deposited Silicon rich oxide (SRO) films is demonstrated. For this purpose, 400 nm thick SRO films with varying excess Si content were irradiated with loosely focused 355 nm, 12 ps pulses at 80 MHz while being translated at 2mm/s. Mapping of areas irradiated with energies between 4.7 nJ and 5.5 nJ/pulse exhibits regions with the largest emission enhancement. Raman and photoluminescence (PL) measurements suggest precipitation of amorphous and crystalline Si nanoclusters. In the most emissive regions, the PL efficiency of the laser-annealed films was ~70% of that obtained by standard oven-annealing processes. Stress in Si crystals in some areas is identified as leading to quenching of the PL and is hypothesized to be caused by the densification of SRO matrix.

Quantum emission efficiency of nanocrystalline and amorphous Si quantum dots

The paper presents the comparison of emission efficiencies for crystalline Si quantum dots (QDs) and amorphous Si nanoclusters (QDs) embedded in hydrogenated amorphous (a-Si:H) films grown by the hot wire-CVD method (HW-CVD) at the variation of technological parameters. The correlations between the intensities of different PL bands and the volumes of Si nanocrystals (nc-Si:H) and/or an amorphous (a-Si:H) phase have been revealed using X-ray diffraction (XRD) and photoluminescence (PL) methods. These correlations permit to discuss the PL mechanisms in a-Si:H films with embedded nc-Si QDs. The QD parameters of nc-Si:H and a-Si:H QDs have been estimated from PL results and have been compared (for nc-Si QDs) with the parameters obtained by the XRD method. Using PL and XRD results the relations between quantum emission efficiencies for crystalline ( η cr ) and amorphous ( η am ) QDs have been estimated and discussed for all studied QD samples. It is revealed that a-Si:H films prepared by HW-CVD with the variation of wire temperatures are characterized by better passivation of nonradiative recombination centers in comparison with the films prepared at the variation of substrate temperatures or oxygen flows.

Microstructure of Magnetron Sputtered Amorphous SiOx Films: Formation of Amorphous Si Core−Shell Nanoclusters

The microstructures of the as-sputtered amorphous silicon-rich oxide (SiOx) films were investigated by using a combination of X-ray photoelectron spectroscopy (XPS) and Raman microscopy. XPS analysis reveals that the as-sputtered amorphous SiOx films are chemically inhomogeneous. Raman spectra and valence band XPS spectra suggest the formation of amorphous Si nanoclusters in the as-sputtered amorphous SiOx films. The size of these nanoclusters depends on the Si concentration in the films. The formation of amorphous Si nanoclusters was attributed to the high kinetic energy and surface migration of the sputtered Si atoms. It is found that the as-sputtered amorphous SiOx films have a complex structure consisting nanoscale cluster of amorphous Si core with suboxides shell embedded in the amorphous SiO2 matrix.

Femtosecond laser induced formation of Si nanocrystals and amorphous Si clusters in silicon-rich nitride films

Femtosecond laser treatments (Ti-sapphire laser, 800 nm wavelength, < 30 fs pulse duration) were applied for crystallization of amorphous silicon nanoclusters in silicon-rich nitride films. The laser fluences needed for crystallization of the silicon nanoclusters were found for films of various non-stoichiometric parameters (SiN x :H, 0.6 ≤ x < 1.33). The effect of laser assisted formation of amorphous Si nanoclusters in SRN films with relatively low concentration of additional silicon atoms was observed. The developed approach can be used for the creation of dielectric films with semiconductor nanoclusters on non-refractory substrates.

Electroluminescence of as-sputtered silicon-rich SiO x films

Si-rich oxide films (SiO x, 0 < x < 2) were synthesized by reactive magnetron sputtering of a single Si target in a gas mixture of argon and oxygen. Intense visible electroluminescence was observed from the as-deposited SiO x film. The microstructure of the as-sputtered SiO x films was characterized by Raman and X-ray photoelectron spectroscopy techniques. Nanoscale amorphous Si clusters formed in the as-sputtered films. The electroluminescence was attributed to the oxygen-deficient defect luminescent centres and the formation of the amorphous Si nanoclusters.

Temperature Dependence of the Photoluminescence Intensity in Si3+xN4:H Films with Amorphous Si Nanoclusters: Evidence for Two Processes Involved in the Nonradiative Relaxation of Photoexcitations

The temperature dependence of the photoluminescence intensity in Si3+xN4:H films grown on Si substrates in a PECVD process held at temperatures 100 and 380oC was analyzed versus the film stoichiomery. The film stoichiometry parameter x was varied in the range x = 0.08 to 3.7 via varying the NH3/SiH4 flow ratio (0.5 to 5). According to Raman scattering data, the films contained amorphous Si nanoclusters, whose sizes increased with x and substrate temperature. For the first time, the involvement of two different processes in the nonradiative relaxation of excitations optically generated in the system under study has been demonstrated. These processes were tentatively identified as the thermal dissociation of nanocluster-bound excitons and their nondissociative migration to nonradiative sites in neighboring Si nanoclusters. The activation energies and the frequency factors of the two processes were shown to be consistent with the quantum confinement model for the amorphous Si nanocluster related photoluminescence.

Temperature Dependence of the Photoluminescence Intensity in Si3+xN4:H Films with Amorphous Si Nanoclusters: Evidence for Two Processes Involved in the Nonradiative Relaxation of Photoexcitations

not Available.

Energy transfer between amorphous Si nanoclusters andEr3+ions in aSiO2matrix

We report on the energy-transfer mechanism between amorphous Si nanoclusters and erbium ions in a SiO2 matrix. We have studied a set of optimized samples which show maximum Er3+ to Si-nc coupling ratio. We demonstrate that the transfer mainly occurs to the 4I112 level in less than 100 ns and that higher Er3+ energetic levels are not involved. Furthermore, we show that there are no traces of Auger back-transfer, excited state absorption, or pair-induced quenching mechanisms in our samples, leading us to propose a model in which the short interaction distance between Si-nc and Er3+ ions is the limiting factor for the noncomplete excitation of the whole Er3+ active population.

Sensitization efficiencies in Er‐doped SiOx films containing amorphous or crystalline silicon nanoclusters

AbstractA suite of SiOx films with oxygen concentrations ranging from x = 1 to 1.8 (25 at% to 3.6 at% excess silicon) were synthesized by physical vapour deposition. Erbium was incorporated by co‐evaporation of erbium metal to a concentration of about 0.2 at%. A subsequent annealing step was performed at temperatures ranging from 400 °C to 1100 °C to induce phase separation and cluster growth, and to optically activate erbium ions. This range of compositions and annealing temperatures was used to generate a fluorescence map for nanocluster‐sensitized Er3+. Photoluminescence (PL) spectra were measured in the visible and near‐infrared wavelength regions to explore the nature of the energy transfer between Si‐NCs and Er3+ ions. The highest PL intensities for undoped films occurred for samples annealed above 1000 °C, containing silicon nanocrystals. In contrast, in Er‐doped films the strongest Er3+ emission at 1.54 μm was observed from films annealed at temperatures below 1000 °C, demonstrating that amorphous Si nanoclusters support effective energy transfer to the Er3+ ions. The sensitization was most efficient when the NC peak emission wavelength was near 660 nm (1.88 eV) matching the energy of the 4F9/2 band of Er3+ ions in a silica host. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)