A. Kuzmich

Photoacoustic effects in nanocomposite structure ‘porous silicon-liquid’

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.

Features of Photoacoustic Transformation in Microporous Nanocrystalline Silicon

Results of a study of the photoacoustic transformation in microporous nanocrystalline silicon are described. The amplitude-frequency and phase-frequency characteristics of the photoacoustic signal from microporous silicon samples on a monocrystalline substrate exposed to illumination at various wavelengths are experimentally determined. Informative response was measured by the gas-microphone and piezoelectric detection methods. In terms of the proposed mathematical model, it is shown that the difference in the parameters of the photoacoustic signal for different wavelengths of exciting radiation is attributed to a shift of the fundamental absorption edge in nanocrystalline silicon. It is pointed out that the piezoelectric detection method is more sensitive to changes in the thermophysical and optical parameters of the porous layer.

Photoacoustic thermal conductivity determination of layered structures PS-Si: piezoelectric detection

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.

Nanocrystals Growth Control during Laser Annealing of Sn

An efficient technique for low temperature metal-induced nanocrystalline silicon fabrication is presented. The technique is based on laser annealing of thin films of “amorphous silicon-tin” composites combined with in situ control and monitoring with Raman technique. Laser annealing was shown to provide the possibility of fine-tuning the nanocrystals size and concentration, which is important in photovoltaic and thermoelectric devices fabrication.

Photothermoacoustic conversion in porous matrix-liquid filler systems

Features of the photothermoacoustic conversion in systems of the porous silicon-liquid filler type on a single crystal silicon substrate have been experimentally studied. It is established that the presence of liquid in the pores leads to a significant increase in the photoacoustic signal amplitude and front slope, which is explained by a significant contribution of the liquid pressure to thermoinduced stresses in the material. The extrema observed in time series of the photoacoustic signal in the case of a low-viscosity liquid are related to the process of pressure relaxation in the pores.

Role of Laser Power, Wavelength, and Pulse Duration in Laser Assisted Tin-Induced Crystallization of Amorphous Silicon

This work describes tin-induced crystallization of amorphous silicon studied with Raman spectroscopy in thin-film structures Si-Sn-Si irradiated with pulsed laser light. We have found and analyzed dependencies of the nanocrystals’ size and concentration on the laser pulse intensity for 10 ns and 150 μm duration laser pulses at the wavelengths of 535 nm and 1070 nm. Efficient transformation of the amorphous silicon into a crystalline phase during the 10 ns time interval of the acting laser pulse in the 200 nm thickness films of the amorphous silicon was demonstrated. The results were analyzed theoretically by modeling the spatial and temporal distribution of temperature in the amorphous silicon sample within the laser spot location. Simulations confirmed importance of light absorption depth (irradiation wavelength) in formation and evolution of the temperature profile that affects the crystallization processes in irradiated structures.

Interfacial thermal resistance between porous layers: Impact on thermal conductivity of a multilayered porous structure

Abstract Features of thermal transport in multilayered porous silicon nanostructures are considered. Such nanostructures were fabricated by electrochemical etching of monocrystalline Si substrates by applying periodically changed current density. Hereby, the multilayered structures with specific phononic properties were formed. Photoacoustic (PA) technique in gas-microphone configuration was applied for thermal conductivity evaluation. Experimental amplitude-frequency dependencies were adjusted by temperature distribution simulation with thermal conductivity of the multilayered porous structure as a fitting parameter. The experimentally determined values of thermal conductivity were found to be significantly lower than theoretically calculated ones. Such difference was associated with the presence of thermal resistance at the interfaces between porous layers with different porosities arising because of elastic parameters mismatch (acoustical mismatch). Accordingly, the magnitude of this interfacial thermal resistance was experimentally evaluated for the first time. Furthermore, crucial impact of the resistance on thermal transport perturbation in a multilayered porous silicon structure was revealed.

Nonlinear laser ultrasound formation in silicon

In this paper, the mechanisms of laser ultrasound response formation in monocrystalline silicon are discussed. The ultrasound waves in the test specimen were generated with laser pulses of two different wavelengths and registered with a piezoelectric transducer. The amplitude of the measured signal was found to be a nonlinear function of the laser radiation intensity. It was shown, that the observed nonlinearity is related to the features of optical absorption and thermoelastic sources formation in the material. A simple model taking into account temperature dependencies of the thermal conductivity and thermal expansion coefficient was developed. An excellent agreement between experimental and simulation for different wavelengths was demonstrated.