T. Nychyporuk

Effect of total pressure on the formation and size evolution of silicon quantum dots in silicon nitride films

The size of silicon quantum dots (Si QDs) embedded in silicon nitride (SiN(x)) has been controlled by varying the total pressure in the plasma-enhanced chemical vapor deposition (PECVD) reactor. This is evidenced by transmission electron microscopy and results in a shift in the light emission peak of the quantum dots. We show that the luminescence in our structures is attributed to the quantum confinement effect. These findings give a strong indication that the quality (density and size distribution) of Si QDs can be improved by optimizing the deposition parameters which opens a route to the fabrication of an all-Si tandem solar cell.

Thermal conductivity of meso-porous germanium

Thermal conductivity value of sponge-like meso-porous germanium (meso-PGe) layers measured by means of photoacoustic technique is reported. The room temperature thermal conductivity value is found to be equal to 0.6 W/(m K). The experimental results are in excellent agreement with molecular dynamic and Monte Carlo simulations. Both experiments and simulations show an important thermal conductivity reduction of the meso-PGe layers compared to the bulk Ge. The obtained results reveal meso-PGe as an interesting candidate for both thermoelectric and photovoltaic applications in which thermal transport is a really crucial issue.

Luminescence mechanisms in Si quantum dots-SiNx nanocomposite structures

Silicon quantum dots have been grown in situ in amorphous silicon nitride (SiNx) films on silicon substrate by plasma-enhanced chemical vapor deposition using ammonia (NH3) and silane (SiH4) as reactant gases. After deposition, films were annealed in N2 atmosphere at temperatures ranging from 700 to 1000 °C for 1 min using rapid thermal annealing system. The evolution of the Si quantum dots (QDs) and the chemical composition of the films have been investigated by Raman scattering and infrared absorption spectroscopy, and the correlation with the optical properties is presented. From the photoluminescence measurements, the position and the intensity of the photoluminescence peak were studied as a function of the annealing temperature (Ta), and the luminescence mechanism in Si QDs-SiNx composite structures is attributed to the quantum confinement in silicon clusters. These results lead to further understanding the physical and optical properties of Si QDs embedded in silicon nitride films and thus open up t...

Might Silicon Surface Be Used for Electronic Tongue Application

An electronic tongue concept based on 2D mapping of photogenerated charge carrier lifetimes in silicon put in contact with different liquids is reported. Such method based on intrinsic sensitivity of the silicon surface states to the surrounding studied liquids allows creation of their characteristic electronic fingerprints. To increase recognition reliability, a set of characteristic fingerprints for a given liquid/silicon interface is proposed to be recorded at different bias voltages. The applicative potential of our sensing concept was demonstrated for different spirits and water samples.

Anisotropic heat conduction in silicon nanowire network revealed by Raman scattering

Anisotropic nanomaterials possess interesting thermal transport properties because they allow orientation of heat fluxes along preferential directions due to a high ratio (up to three orders of magnitude) between their in-plane and cross-plane thermal conductivities. Among different techniques allowing thermal conductivity evaluation, micro-Raman scattering is known to be one of the most efficient contactless measurement approaches. In this letter, an experimental approach based on Raman scattering measurements with variable laser spot sizes is reported. Correlation between experimental and calculated thermal resistances of one-dimensional nanocrystalline solids allows a simultaneous estimation of their in-plane and cross-plane thermal conductivities. In particular, our measurement approach is illustrated to be applied for anisotropic thermal conductivity evaluation of silicon nanowire arrays.

Increase in the lifetime of a photon and in the efficiency of raman scattering and second-harmonic generation processes in porous silicon carbide

Raman scattering and second-harmonic generation processes in porous layers obtained by the electrochemical etching of polycrystalline silicon carbide, which contain nanocrystals with dimensions from several to hundreds of nanometers, have been studied. It has been found that the efficiencies of Raman scattering and second-harmonic generation in layers of porous silicon carbide increase by several times and more than two orders of magnitude, respectively, compared to the values in the initial sample. The efficiency of transformation to the second harmonic reaches 0.1% at pumping by femtosecond pulses with a wavelength of 1240 nm. The lifetime of a photon in layers of porous silicon carbide has been estimated as more than 2 ps from the measurement of the cross-correlation functions; this value indicates the deceleration of light in this optically inhomogeneous medium owing to multiple scattering. This effect in layers of porous silicon carbide explains the observed increase in the efficiencies of Raman scattering and second-harmonic generation.

Organization of silicon nanocrystals by localized electrochemical etching

An approach to form a monolayer of organized silicon nanocrystals on a monocrystalline Si wafer is reported. Ordered arrays of nanoholes in a silicon nitride layer were obtained by combining electron beam lithography and plasma etching. Then, a short electrochemical etching current pulse led to formation of a single Si nanocrystal per each nanohole. As a result, high quality silicon nanocrystal arrays were formed with well controlled and reproducible morphologies. In future, this approach can be used to fabricate single electron devices.

Porous silicon Bragg reflectors on multi-crystalline silicon wafer with p-n junction

Bragg reflectors consisting of the sequence of dielectric layers are considered to create p-n junction solar cells (SC) with improved efficiency in the longwave spectral range. Bragg mirrors (BM) based on porous silicon (PS) mutilayers at the backside of single crystalline and multicrystalline silicon wafer were formed by electrochemically etching. Maximal experimental reflectivity for BM on multicrystalline substrate achieves 62% due to the natural crystallites disorientation of multicrystalline substrate, whereas for single crystalline silicon the reflectivity in maximum is 87%. BM was formed also on rear side of multicrystalline silicon wafer with p-n junction.

Plasmon-induced enhancement of optical absorption in silicon thin films due to embedded silver nano-pillars

In the literature, plasmonic nanostructure localization outside the silicon-based solar cells is mainly considered. In our paper, an array of rear-side?located silver nano-cylinders incorporated into a Si thin film is shown to ensure a significant enhancement of the absorbed power density in the near-infrared spectral range. Several different geometrical configurations ensuring high extinction enhancement factors are numerically investigated and compared in view of their application in silicon-based solar cells.