Visible photoluminescence from silicon nanoclusters embedded in silicon nitride films prepared by remote plasma-enhanced chemical vapor deposition

The photoluminescence (PL) of silicon nanoclusters embedded in silicon nitride films grown by remote plasma-enhanced chemical vapor deposition at 200 °C, using mixtures of SiCl₄/H₂/Ar/NH₃ is investigated. It was found that the color and the intensity of the PL of the as-grown samples depend on the H₂ flow rate, and there is an optimum flow for which a maximum luminescence is obtained. A strong improvement of the PL intensity and change in color was obtained with annealing treatments in the range of 500–1000 °C. The changes in the composition, structure and optical properties of the films, as a function of H₂ flow rate and thermal treatments, were studied by means of Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, ellipsometry and ultraviolet–visible transmission measurements. We conclude that the PL can be attributed to quantum confinement effect in silicon nanoclusters embedded in silicon nitride matrix, which is improved when a better passivation of the nanoclusters surface is obtained.     

Specific heat of amorphous silicon at low temperatures

The heat capacity of amorphous silicon has been measured between 2 and 50 K using an a.c. calorimetry technique. A comparison with heat capacity measurements of crystalline silicon shows a reduction of the Debye temperature by a factor 0.82 and of the position of the maximum in $\text{c}\text{T}{}^3$ by a factor of 0.77. This behaviour indicates a general reduction in frequency of the transversal-acoustic modes with respect to the corresponding modes of crystalline silicon.     

Interaction of germanium with silicon dioxide

The interaction of germanium (Ge) adatoms with SiO₂ (silica) plays an important role in selective, heteroepitaxial growth of Ge(100) through windows created in silica on Si(100) and in the selective growth of Ge nanoparticles on hafnia, located at the bottom of pores etched through silica. Both processes rely on the inability of Ge to accumulate on silica. In hot wire chemical vapor deposition of Ge nanoparticles from GeH₄, etching of the silica has been invoked as one path to prevent accumulation of Ge on silica; whereas dense silica is not etched when Ge atoms are incident on the surface in molecular beam processes. Surface studies were conducted to determine the nature of oxidized Ge on SiO₂, to reconcile the etching claim with GeH₄, and to look for the additional etching product that must accompany GeO, namely SiO. Etching of silica is not found with GeH₄ or GeH_x fragments. A more complete examination of the Ge isotopes reveals instead the m/e 90 signal, previously attributed to GeO, originates from interactions between iron oxide impurities in the molybdenum holder, and hydrogen and GeH_x fragments. Coating the Mo with gold eliminates m/e 90 from Ge TPD spectra. The high temperature m/e 74 and m/e 2 peaks observed from 800 to 900 K are attributed to GeH_x decomposition to Ge and H followed by their desorption, while the appearance of GeO_x is attributed to possible reactions between GeH_x species with hydroxyl groups and/or oxidation of Ge clusters by background oxidants.     

Pressure-induced phase transition of crystalline and amorphous silicon and germanium at low temperatures

Abstract

X-ray diffraction has been measured for crystalline silicon, crystalline germanium, amorphous silicon and amorphous germanium at temperatures down to 100 K and pressures up to 20 GPa using a diamond anvil cell and synchrotron radiation. The structural phase transitions, including amorphization, take place in the pressure-temperature range. It has been found that the structures after the phase transitions strongly depend on the path in the pressure-temperature diagram through which the system undergoes the phase transitions. For any of the aforementioned four materials, the high-pressure phase with the p-Sn structure is quenched during a release of pressure at 100 K, and transforms into an amorphous state when heated up to around 2 GPa. The path dependence of the states is discussed in relation to the pressure dependence of the heights of the energy barriers which have to be overcome when phase transitions occur. The effect of a structural disorder on the phase transition is also discussed by comparing the experimental results for the crystalline and amorphous materials.     

Fatigue effect in luminescence of glow discharge amorphous silicon at low temperatures

Fatigue in the luminescence was observed in glow discharge amorphous silicon at 4.2 K and 77 K. This fatigue was not recovered by infra-red illumination, but by heating the sample at higher temperatures. These results are interpreted in terms of enhancement of non-radiative recombination associated with dangling bonds created by high optical excitation.     

Metastable surface alloys produced by ion implantation,laser and electron beam treatment

Ion Implantation, Laser and Electron-beam Treatment (LET) of metals have been employed extensively to produce metastable surface alloys. Recent published work on implanted alloys is reviewed first. The dilute implanted alloys (solute concentration <10 at. %) are shown to lead to crystalline metastable solid solutions. At higher solute concentrations, an amorphous phase has been observed for several binary systems and recently for a ternary system. The physical mechanisms at play, are discussed in detail. A review of the surface alloys produced by LET of metals is then presented—with an emphasis on the mechanisms involved. In particular, general criteria governing formation of metastable solid solutions under LET are proposed and shown to have excellent agreement with available data on metals and Si.     

Adiabatic invariance treatment of reactive collisions between ions and polar molecules at low temperatures

A new semiclassical adiabatic invariance treatment of ion-molecule reactive collisions is proposed to investigate the influence of the molecular rotation on the cross-sections and rate constants at very low temperatures. Within the domain of validity of the adiabatic separation of the ion-molecule radial motion and the molecular rotation, the method is applicable to linear or symmetric-top molecules, for which the system is integrable. The correspondence principle is then used to partition the space of the classical action space into quantum bins, each of which corresponds to a specific quantum state. The procedure differs from the more usual Einstein-Brillouin-Keller (EBK) semiclassical quantization, where each quantum state is represented by a single point of action space. The results for the linear rigid rotor case, obtained using this modified semiclassical adiabatic invariance model, are in excellent agreement with the quantum mechanical methods, even for low rotational levels of the molecule, where the EBK semiclassical quantization fails.Received: 26 June 2003, Published online: 26 August 2003PACS: 34.50.Lf Chemical reactions, energy disposal, and angular distribution, as studied by atomic and molecular beams - 34.50.Pi State-to-state scattering analyses     

Si-nanostructures formation in amorphous silicon nitride SiNx:H deposited by remote PECVD

The formation of silicon nanoclusters embedded in amorphous silicon nitride (SiN_x:H) can be of great interest for optoelectronic devices such as solar cells. Here amorphous SiN_x:H layers have been deposited by remote microwave-assisted chemical vapor deposition at 300 °C substrate temperature and with different ammonia [NH₃]/silane [SiH₄] gas flow ratios (R=0.5−5). Post-thermal annealing was carried out at 700 °C during 30 min to form the silicon nanoclusters. The composition of the layers was determined by Rutherford back scattering (RBS) and elastic recoil detection analysis (ERDA). Fourier transform infrared spectroscopy (FTIR) showed that the densities of SiH (2160 cm⁻¹) and NH (3330 cm⁻¹) molecules are reduced after thermal annealing for SiN:H films deposited at flow gas ratio R>1.5. Breaking the SiH bonding provide Si atoms in excess in the bulk of the layer, which can nucleate and form Si nanostructures. The analysis of the photoluminescence (PL) spectra for different stoichiometric layers showed a strong dependence of the peak characteristics (position, intensity, etc.) on the gas flow ratio. On the other hand, transmission electron microscopy (TEM) analysis proves the presence of silicon nanoclusters embedded in the films deposited at a gas flow ratio of R=2 and annealed at 700 °C (30 min).     

An infra-red study of defects produced in n-type silicon by electron irradiation at low temperatures

The infra-red local mode absorption produced by irradiation of n-type silicon by 2 MeV electrons at temperatures in the range 100–140°K has been investigated. A new band at 884 cm^-1 has been observed and interpreted as due to a vacancy— oxygen complex (A-centre) with a trapped electron.     

Structure and optical features of silicon carbide nanocrystals confined in alumina matrices

Thin films based on silicon carbide and alumina were synthesized by means of rf-sputtering using a co-deposition process. Several nanostructures were created which consist of thin films (∼200 nm thick) with homogeneous distribution of SiC nanocrystals (∼5 nm mean diameter) in the host alumina matrices. Characterization methods including X-ray photoelectrons spectroscopy (XPS), UV-vis absorption and photoluminescence (PL) were used to identify the involved structures, compositions and optical features of these nanostructures. Thus, XPS investigations were relevant to point out the involved chemical bonding in the core SiC nanocrystals and in the host alumina environments. Additionally, mixed bonding such as Si-O-C was also shown and seems to correlate with the SiC-alumina interfaces. Optical properties of the nanostructures films such as UV-vis absorption and photoluminescence (PL) were measured in representative samples and compared to simulated PL responses obtained by a theoretical model.     

Efficient light emission from crystalline and amorphous silicon nanostructures

Optical and electronic properties of crystalline silicon (c-Si) and amorphous silicon (a-Si) nanostructures are reviewed. The photoluminescence (PL) peak energies of c-Si and a-Si nanostructures are blueshifted from those of bulk c-Si and a-Si. The temperature dependence of the PL intensity is drastically improved in c-Si and a-Si nanostructures, and efficient luminescence from c-Si and a-Si nanostructures is observed at room temperature. The quantum confinement, spatial confinement, and surface effects on luminescence properties are summarized and the PL mechanism of silicon nanostructures is discussed.     

Thermodynamic properties of small amorphous and crystalline Silica particles at low temperatures

We have measured the low-temperature (K) specific heat and heat release of small amorphous and crystalline SiO₂ particles embedded in Teflon and of Vycor. The temperature and time dependence of these properties have been interpreted in terms of the tunneling model. We found that the particle size influences the density of states of tunneling systems of the composite. The smaller the size of the particles the larger is the density of states of tunneling systems P₀. Quartz grains with dimensions in the micrometer range show similar glass-like properties as vitreous silica. In comparison with bulk vitreous silica, Vycor shows a much larger P₀ in agreement with the behavior we found for small SiO₂ particles. We discuss the implication of our results on the origin of the universal low-temperature properties of glasses. Received 9 April 1998     

Investigation of the interaction between electrical discharges and low resistivity silicon substrates

In this work the impact of single discharge pulses in air on single-crystalline, p-type silicon having a low bulk resistivity of 0.009-0.012 Ω cm is investigated. Compared to platinum specimens, the craters in silicon have lateral dimensions which are about one order of magnitude larger despite comparable values for the melting point and the melting energy. This finding is attributed to the substantially higher bulk resistivity of silicon leading a higher energy input into the substrate when spark loaded. The energy generated by joule heating is, however, distributed across a larger area due to a current spreading effect. To study the impact of different surface properties on the sparking behaviour, the crater formation on the silicon substrate is investigated applying coatings with different material properties, such as sputter-deposited aluminium layers and thermally-grown silicon dioxide. In general, the crater characteristics formed on unmodified silicon is not influenced when a thin aluminium layer of 24 nm is deposited. At higher film thickness above 170 nm, the sparking energy is almost completely absorbed in the top layer with low influence on the underlying silicon substrate. In the case of a dielectric top layer with a thickness of 155 nm, the formation of many small distinct craters is supported in contrast to a 500 nm-thick SiO₂ film layer where the generation of a single crater with a large area is energetically favoured. A surface roughness of several nm on the silicon probes has no measurable effect on crater formation when compared to an original surface characteristic with values in the sub-nm range.     

Structure and mobility on amorphous silicon surfaces

The structure and dynamics of amorphous surfaces are poorly understood. The present work develops methods employing classical molecular dynamics (MD) simulations to elucidate these phenomena on amorphous silicon. Careful relaxation of the initial ensemble and taking account of exchange with the bulk yield surface diffusion coefficients in good agreement with experiment. Randomly oriented dimer pairs dominate the surface structure. Diffusion proceeds by several pathways, which all differ in basic character from those typically observed on crystalline silicon. The primary pathways involve single atoms and dimer pairs, which typically move only one or two atomic diameters before reincorporating into the surface. Frequent vertical migration takes place between the first two atomic layers.     

Interaction between silver nanoparticle and bovine hemoglobin at different temperatures

The binding of silver nanoparticles to bovine hemoglobin (BHb) was studied by fluorescence, UV–Visible, and circular dichroism (CD) spectroscopic techniques at different temperatures of 20, 37, and 42 °C. The absorption spectrum of soret band, in the presence of silver nanoparticle, showed a significant spectral change, which indicated the heme groups of BHb were directly attacked and degraded by silver nanoparticle. The fluorescence data explained that the nanoparticle binding to BHb occurred at a single binding site, which demonstrated a dynamic quenching procedure. Nanoparticles could reduce the fluorescence of tryptophanyl residues of BHb to a lesser extent. Circular dichroism studies demonstrated a conformational change of BHb in the presence of silver nanoparticles. The helicity of BHb was reduced by increasing silver nanoparticle concentration at different temperatures. Thermodynamic analysis of the protein interaction by silver nanoparticles suggested that the binding process is only entropy driven.     

Efficiency characteristics of a silicon oxide passivation layer on p-type crystalline silicon solar cell at low illumination

Silicon dioxide (SiO₂) is widely used to improve the surface passivation properties of silicon solar cells. To minimize solar cell potential-induced degradation when the PV module is installed outdoors, a silicon oxide film is widely used as an insulator. However, experiments have confirmed that solar cells with a silicon oxide (SiO₂) film have a lower efficiency than solar cells without a silicon oxide (SiO₂) film at low illumination (<0.4 sun). Actually, the efficiency in the low illumination condition affects the average power output per day because the PV module mostly operates when the solar irradiation dose is less than 1 sun. To maximize the performance of the PV module, the output at a low light intensity level should also be considered. Shunt resistance (R_shunt) is known to cause a decrease in solar cell efficiency under low illumination conditions. PC1D simulation was used to analyze parameters, such as the series resistance, parallel resistance, and surface recombination, that affect the characteristics of the solar cell at low light intensity. In this study, we confirmed how the SiO₂ layer affected the low illumination properties of solar cells, even though these cells were more efficient at 1 sun. Silicon solar cells with a SiN_x/SiO₂ bilayer or a SiN_x single film were fabricated, and their characteristics were evaluated. Passivation characteristics were measured using the quasi-steady-state photoconductance (QSSPC) technique to evaluate the minority carrier lifetime and the implied open-circuit voltage (V_OC), and capacitance-voltage measurements were used to analyze the fixed charges. The values of the shunt resistance and series resistance in solar cells with different passivation layers were compared, and the cause of the decrease in the efficiency under low illumination was also analyzed via fill factor calculation.