Minimizing thermal conductivity in laser deposited multilayers

Heat, light and electricity are three foundation pillars not only of physics, but also of our daily lives. Therefore, these have been studied by scientists for centuries and an always better understanding of the underlying exchange particles, phonons, photons and electrons, was gained. In physics, core disciplines opened up in the fields of phononics, optics and electronics. While the latter had great technological impact on our society, the field of phononics is rather new and currently seen in the ambitious light of potentially following up on the impact of electronics almost a century ago. The field of phononics deals with the propagation of mechanical waves in condensed matter and is studied in different bulk materials, as well as nanostructures, special metamaterials, phononic crystals and multilayers with possible applications in fields of acoustics, vibration, communication as well as thermal storage and transport. Especially the aspects of thermophysics are of great interest today, since by far most of the energy we consume results from thermal processes. Thus, a better understanding of thermal processes could help with current and prevailing energy issues worldwide. Therefore, the reduction of thermal conductivity in modern materials is an exciting question. This issue is not only important for thermal insulation and storage and could then find applications in cryogenics, aerospace industry or damping, but also for energy conversion for instance in the field of thermoelectrics, solid state refrigeration or as thermal barrier coatings that allow thermal engines, like gas turbines, to run at higher temperatures and consequently with enhanced efficiencies. In order to answer this question, one has to study thermal transport processes and thus the propagation of phonons. Phonons are elemental excitations of a quantized elastic lattice and belong to the family of bosonic quasi-particles. They can be best understood in terms of lattice vibrations in a solid state object that carry wave energy through interatomic forces. In terms of long wavelengths this is known as sound, but as the frequencies get higher and therefore the wavelengths shorten, heat is conducted by lattice waves. For minimizing thermal conductivity, it is essential to hinder phonon propagation through the material. This can be achieved by different approaches and will only be described briefly here in the introduction since more details will follow in chapter 2. The thermal conductivity scales with the velocity and the mean free path of the phonons. Since those values are comparably high in a periodic lattice with strong atomic bonds, crystalline materials have a much higher thermal conductivity than amorphous ones. In amorphous materials, phonon propagation is hindered due to the absence of the ordered lattice making them better thermal insulators. But heat transport can be suppressed further by an increase of phonon scattering and thus a reduction of mean free path. Phonon scattering is a typical process in heat conduction, but the number of scattering centers can be increased artificially, for instance by increasing the amount of boundaries which have to be passed. This is the case for thin film multilayers, where stacking of nanoscale individual layers allows for a great interface density. At each interface, phonons can be scattered or even reflected, which can lead to interference and thus, to a change in phonon dispersion and the formation of band gaps. At this point, phonon modes can be confined and localized so that they do not contribute to thermal transport anymore. Furthermore, a reduction in phonon group velocity and enhanced scattering rates can be seen for some frequencies. In this way, a reduction of phonon mean free path and group velocity can be achieved, which results at the same time in a minimization of thermal conductivity. For fabricating such thin film multilayers, a variety of methods exist, for instance sputtering, atomic layer deposition, molecular beam epitaxy or pulsed laser deposition. From all these methods, the pulsed laser deposition technique is outstanding, because it allows not only the fabrication of nanoscale thin films and multilayers with high quality, but is beyond that a very versatile method. This means, it enables preparation of multilayers consisting of very different components such as metals, semiconductors, oxides and even polymers, because almost every solid material can be pulsed laser deposited. The advantage for phonon blocking materials lies in the ability of the method to combine materials with very different properties, such as hard and soft or heavy and light materials, for instance metals and polymers. Such multilayers have the potential for a high number of interfaces with very different material properties at each side of the interface. Since the interface resistance gets stronger with a greater difference in density and sound velocity at each side, pulsed laser deposition enables to fabricate multilayers with a high density of strongly resisting interfaces. In order to get information about phononic transport in such structures, one has to measure the elastic dynamics on very short timescales. Therefore, a close cooperation with the working groups of Henning Ulrichs from the 1st Institute of Physics in Göttingen and Markus Münzenberg from the Institute of Physics in Greifswald has been established in order to perform fs-pump-probe measurements of the pulsed laser deposited samples. Here, an ultrashort laser pulse is split up in a pump and a probe beam that both hit the sample with a very short delay in time. Thereby, the sample gets excited by the pump pulse and its reflectivity dynamic is measured with the probe pulse repeatedly with different delays. In that way, a time-dependent reflectivity signal can be obtained from which information about phonon modes can be deduced. For the measurement of thermal conductivities, different methods come into consideration, like pyrometry, the 3-omega technique, photodisplacement, laser flash or thermoreflectance methods. Since pulsed laser deposition allows the preparation of a great variety of thin film multilayers, the measuring method should also be highly flexible and applicable to a large group of samples with low preparation effort. Therefore, the method of transient thermoreflectance (TTR) is prominent, which also works as a pump-probe scheme, comparable to the phonon dynamics measurements. Since thermal conductivity is a much slower, diffusive process, a measurement on the µs timescale without the need of ultrafast lasers is applicable. This method works by heating up the sample surface with a laser pulse (this time with a pulse length in the ns regime) and utilizing at the same time a second, continuous wave laser in order to measure the reflectivity of the sample surface, which is tracked with a fast Si-photodiode and an oscilloscope. From the temporal evolution of the reflectivity after pulsed laser heating, the temperature at the surface can be obtained. Taking these measured information into account and fitting them to the solution of the heat conduction equation, one can get information about the thermal conductivity. Aim of the present thesis is to fabricate high-quality, nanoscale multilayers of dissimilar materials with pulsed laser deposition and to minimize the cross-plane thermal conductivity of these structures. In order to achieve this final goal, first, the deposition of metal/polymer (mainly W/polycarbonate) and metal/oxide (e.g. W/ZrO2) multilayers must be perfected and phonon transport will be studied in such structures. Secondly, a measurement method capable of obtaining information about thermal conductivities has to be installed and optimized for bulk materials and especially thin films. With this, information about heat transport in nanoscale multilayers will be obtained systematically. From the combination of all those efforts, this thesis could be developed