Anna Kaźmierczak-Bałata

Correlation between morphology and local thermal properties of iron (II) phthalocyanine thin layers

The effect of substrate temperature and post-deposition annealing temperature on the surface morphology, topography and local thermal properties of iron(II) phthalocyanine (FePc) 500 nm thick films was investigated. The investigations were conducted by a combination of scanning probe microscopies (atomic force microscopy, scanning thermal microscopy) and scanning electron microscopy with structural (x-ray diffraction) and chemical (energy-dispersive x-ray spectroscopy) analysis. FePc is always obtained in its α-form. While for heated substrates, FePc crystallites lie flat on the surface, so post-deposition annealing leads to vertically oriented crystallites. The total surface area obtained does not depend significantly on the substrate temperature, while it is strongly dependent on the annealing temperature. For both groups of samples, the increase of roughness was accompanied by a decrease of the samples thermal resistivity, which is in agreement with the picture of a decreased number of grain boundaries resulting in higher heat transport. The results obtained here show that the exposed surface area of an organic film strongly depends on the thermal treatments; hence surface morphology can be tailored to particular needs.

Photothermal methods for determination of thermal properties of bulk materials and thin films

Information on the thermal properties of materials is very important both in fundamental physical research and in engineering applications. The development of materials with desirable heat transport properties requires methods for their experimental determination. In this paper basic concepts of the measurement of parameters describing the heat transport in solids are discussed. Attention is paid to methods utilizing nonstationary temperature fields, especially to photothermal methods in which the temperature disturbance in the investigated sample is generated through light absorption. Exemplary photothermal measuring techniques, which can be realized using common experimental equipment, are described in detail. It is shown that using these techniques it is possible to determine the thermal diffusivity of bulk transparent samples, opaque and semi-transparent plate-form samples, and the thermal conductivity of thin films deposited on thick substrates. Results of the investigation of thermal diffusivity of the ground in the polar region, which is based on the analysis of the propagation of the thermal wave generated by sun-light, are also presented. Based on chosen examples one can state that photothermal techniques can be used for determination of the thermal properties of very different materials.

Influence of doping on thermal diffusivity of single crystals used in photonics: measurements based on thermal wave methods

Three crystals used in solid-state lasers, namely, yttrium aluminum garnet (YAG), yttrium orthovanadate (YVO(4)), and gadolinium calcium oxoborate (GdCOB), were investigated to determine the influence of dopants on their thermal diffusivity. The thermal diffusivity was measured by thermal wave method with a signal detection based on mirage effect. The YAG crystals were doped with Yb or V, the YVO(4) with Nd or Ca and Tm, and the GdCOB crystals contained Nd or Yb. In all cases, the doping caused a decrease in thermal diffusivity. The analysis of complementary measurements of ultrasound velocity changes caused by dopants leads to the conclusion that impurities create phonon scattering centers. This additional scattering reduces the phonon mean free path and accordingly results in the decrease of the thermal diffusivity of the crystal. The influence of doping on lattice parameters was investigated, additionally.

Determination of thermal conductivity of thin layers used as transparent contacts and antireflection coatings with a photothermal method

A photothermal experiment with mirage detection was used to determine the thermal conductivity of various thin films deposited on semiconductor substrates. The first type consisted of conducting oxide films: ZnO and CdO deposited on GaSb:Te, while the other contained high dielectric constant HfO(2) layers on Si. All films were fabricated using a magnetron sputtering technique. Experimental results showed that the value of the thermal conductivity of ZnO and CdO films is lower than the value obtained for HfO(2). Thermal conductivities of investigated thin films are about 2 orders of magnitude lower than those corresponding to bulk materials.

Measuring thermal conductivity of thin films by Scanning Thermal Microscopy combined with thermal spreading resistance analysis

While measuring the thermal properties of a thin film, one of the most often encountered problems is the influence of the substrate thermal properties on measured signal and the need for its separation. In this work an approach for determining the thermal conductivity κ of a thin layer is presented. It bases on Scanning Thermal Microscopy (SThM) measurement combined with thermal spreading resistance analysis for a system consisting of a single layer on a substrate. Presented approach allows to take into account the influence of the substrate thermal properties on SThM signal and to estimate the true value of a thin film κ. It is based on analytical solution of the problem being a function of dimensionless parameters and requires numerical solution of relatively simple integral equation. As the analysis utilizes a solution in dimensionless parameters it can be used for any substrate-layer system. As an example, the method was applied for determination of the thermal conductivities of 4 different thin layers of thicknesses from 12 to 100nm. The impact of model parameters on the uncertainty of the estimated final κ value was analyzed.

Influence of probe-sample temperature difference on thermal mapping contrast in scanning thermal microscopy imaging

The purpose of this work is to investigate the influence of a temperature difference through a probe-sample contact on thermal contrast in Scanning Thermal Microscopy imaging. A variety of combinations of temperature differences in the probe-sample system were first analyzed based on an electro-thermal finite element model. The numerical analysis included cooling the sample, as well as heating the sample and the probe. Due to the simplicity in the implementation, experimental verification involved modifying the standard imaging technique by heating the sample. Experiments were carried out in the temperature range between 298 K and 328 K. Contrast in thermal mapping was improved for a low probe current with a heated sample.

Correlation between the thermal diffusivity and the velocity of ultrasound in YVO4 single crystals

Four single crystals of YVO4 were examined to determine their thermal and elastic properties. The thermal diffusivity was investigated by photothermal method using the mirage effect. The velocity of ultrasound in crystals was measured using the pulse echo method. Two of investigated samples were doped with neodymium (1 at.% and 2 at.% of Nd), one with calcium and thulium (0.4 at.% of Ca and 5 at.% of Tm), and one was pure crystal. Experimental results showed anisotropy of the thermal diffusivity and the velocity of ultrasound. The thermal diffusivity as well as the sound velocity is lower in (001) crystallographic plane than in [001] direction (c-axis). Both quantities decrease with growing concentration of dopants for all crystallographic direction, but the influence of dopants is more distinct in the case of the thermal diffusivity. This fact allows to draw the conclusion that impurity atoms create scattering centres in crystal structure which result in shortening of phonon mean free paths and lowering of the thermal conductivity.Abstract.Four single crystals of YVO4 were examined to determine their thermal and elastic properties. The thermal diffusivity was investigated by photothermal method using the mirage effect. The velocity of ultrasound in crystals was measured using the pulse echo method. Two of investigated samples were doped with neodymium (1 at.% and 2 at.% of Nd), one with calcium and thulium (0.4 at.% of Ca and 5 at.% of Tm), and one was pure crystal. Experimental results showed anisotropy of the thermal diffusivity and the velocity of ultrasound. The thermal diffusivity as well as the sound velocity is lower in (001) crystallographic plane than in [001] direction (c-axis). Both quantities decrease with growing concentration of dopants for all crystallographic direction, but the influence of dopants is more distinct in the case of the thermal diffusivity. This fact allows to draw the conclusion that impurity atoms create scattering centres in crystal structure which result in shortening of phonon mean free paths and lowering of the thermal conductivity.