General Physics Department, Faculty of Physics

Effects of tin doping on crystallization of amorphous silicon were studied using Raman scattering, Auger spectroscopy, scanning electron microscopy, and X-ray fluorescence techniques. Formation of silicon nanocrystals (2-4 nm in size) in the amorphous matrix of Si 1Àx Sn x , obtained by physical vapor deposition of the components in vacuum, was observed at temperatures around 300 C. The aggregate volume of nanocrystals in the deposited film of Si 1Àx Sn x exceeded 60% of the total film volume and correlated well with the tin content. Formation of structures with $80% partial volume of the nanocrystalline phase was also demonstrated. Tin-induced crystallization of amorphous silicon occurred only around the clusters of metallic tin, which suggested the crystallization mechanism involving an interfacial molten Si:Sn layer. V C 2013 AIP Publishing LLC.

Experimental evidences for a recently proposed mechanism of tin-induced crystallization of amorphous silicon are presented. The mechanism discusses a crystalline phase growth through cyclic processes of formation and decay of a super-saturated solution of silicon in molten tin at the interface with the amorphous silicon. The suggested mechanism is validated using a nonlinear dynamical model that takes into account the mass diffusion of the components of the system, heat transfer caused by latent (crystallization) heat release and amorphous silicon dissolution events, and concentration nonuniformities created by silicon crystallization. The analysis of a stationary-state solution of the model confirms the existence of periodic solutions for the partial volume of the crystalline phase and other system's variables. Possible applications of the proposed mechanism in manufacturing of cost-effective nanocrystalline silicon films for the third-generation solar cell technology are discussed.

In this paper photothermal transformation in semiconductor structures with modified properties of subsurface layer under its irradiation by pulse laser (~10 ns) radiation was analyzed. It was show that the presence of this surface modified layer leads to increasing of surface temperature in comparison with homogeneous case. Moreover, this increasing could even compensate the temperature decreasing induced by thermal source redistribution caused by charge carrier diffusion.

Photoacoustic effect in nanocomposite structure 'porous silicon-liquid' has been investigated. Main mechanisms involved in the formation of photoacoustic signal in such structures have been experimentally studied. Liquids with different viscosity (ethanol and acetone) filling the nanopores have been used. A proposed mathematical model describing the photoacoustic signal formation was found to be in good agreement with the experimental results. The role of thermally induced pressures provoked by the liquids confined inside the nanopores in the photoacoustic process has been analyzed.

The analysis of photo-thermo-acoustic transformation in materials with the modified properties of a surface layer has been made in this work. Formation of a photoacoustic response in a layered structure of the type "implanted layer + crystalline Si substrate" as a result of its irradiation by one laser pulse with duration of 20 ns is analyzed.

In the article the results of analyze of laser ultrasound generation in inhomogeneous semiconductors plate is presented. The case of strong absorption of electromagnetic fields is considered. It is shown that in this case submicron surface layer thermal properties influence on thermal distribution evolution. The case of thin infinite plate considered. Peculiarities of excitation and propagation of elastic waves in such media regarding gives the possibility to obtain dispersive equation for laser ultrasound wave-vectors. It is shown that such ...

In the article the results of practical implementation of the PA methods in the study of materials of different structure and dimensions are presented. The features of PA conversion in inhomogeneous semiconductor structures such as porous silicon and doped semiconductor materials are analyzed. To investigate the thermal and elastic properties of porous silicon's based composites gas-microphone and piezoelectric methods registration of PA signal were used. The results of theoretical and experimental investigation of PA conversion in inhomogeneous silicon-based semiconductor structures under their irradiation by short (∼ 10 ns) laser pulse are presented. The influence of thermal parameters distribution of the material structure on the PA signal parameters is shown. Principal possibility of using pulsed radiation for investigation of the modified submicron surface layer is demonstrated.

Based on bending vibrations model method and also construction of multilayer transducer, which allow the measurements of thermal conductivity for porous layer on Si wafer is proposed. Time dependence of photoacoustic signal under rectangular modulation of exciting light was numerical modeling. The experimental tests were done on the samples of porous silicon on Si wafers as well as for the porous silicon free standing layers, obtained under the same anodization regime. The value of thermal diffusivity of the porous silicon layer correlates well with the value of thermal diffusivity of the porous silicon free layer, determined by TDC method.

A new efficient model describing photoacoustic (PA) signal formation with piezoelectric detection is reported. Multilayer sandwich-like systems: heterogeneous studied structure-buffer layer-piezoelectric transducers are considered. In these systems, the buffer layer is used for spatial redistribution of thermoelastic force moments generated in the investigated structure. Thus, mechanical properties of this layer play a crucial role to ensure perfect control of the detected voltage formed on a piezoelectric transducer by contribution of different regions of the studied structure. In particular, formation of the voltage signal strongly depends on the point at which the thermoelastic source is applied. Therefore, use of relatively simple linear Green's functions introduced in frames of the Kirchhoff-Love theory is chosen as an efficient approach for the PA signal description. Moreover, excellent agreement between the theoretical model and measured results obtained on a heterogeneous "porous silicon-bulk Si substrate" structure is stated. Furthermore, resolving of the inverse problem with fitting of the experimental curves by the developed model allows reliable evaluation of the thermal conductivity of the nanostructured porous silicon layer.

This paper is devoted to the study of various carbon-based nanomaterials as photoacoustic contrast agents. The research work was performed on agarose-based tissue phantom containing inclusions with and without carbon-based nanomaterials. The inclusion was created with the higher density compared to phantom in order to simulate a tumor. A specially designed photoacoustic probe was introduced for measuring a level of photoacoustic signal and its enhancement caused by the nanoinclusions presence. The probe consists of a buffer for time separation of the signal coming from the excitation source, piezoelectric transducer, and amplifier. A point-by-point measurement of the signal was performed to obtain a two-dimensional map from magnitude of photoacoustic signal and phase delay of the signal registration. From phase delay the 3D photoacoustic images were reconstructed by evaluation of the depth coordinate based on the tissue sound velocity. As an excitation source the light radiation from Nd:YAG laser with a 16 ns pulse duration and a 1064 nm wavelength was used. Firstly, we considered tissue phantom with a tumor covered by graphene oxide as a reference one. It has been shown that the use of graphene oxide leads to significant improvement of the image contrast. Further, the tumors labelled with nanodiamonds (NDs) and carbon fluoroxide (CFO) nanoparticles (NPs) were studied systematically. Amplitude of the photoacoustic signals registered from such tumor phantoms are one order of magnitude lower than the signal ensured by graphene oxide. All three types of the studied carbon-based nanomaterials (GO, NDs, CFO) give stable photoacoustic signal, this allows to consider them as good candidates for further in-vitro experiments in photoacoustic imaging for biological applications. The dependences of the signal level as a function of the NPs concentration were measured for types of NPs. Considering much more efficient penetration of NDs and CFO NPs inside the cells as well as their extremely low cytotoxicity, these both types of carbon nanomaterials could be used for further in-vivo experiments.