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An efficient way for minimizing phonon thermal conductivity in solids is to nanostructure them by means of reduced phonon mean free path, phonon scattering and phonon reflection at interfaces. A sophisticated approach toward this lies in the fabrication of thin multilayer films of different materials. In this paper, we show by femtosecond-pump-probe reflectivity measurements that in different multilayer systems with varying acoustic mismatch (consisting of metals, semiconductors, oxides and polymers), oscillations due to phonon localization can be observed. For the growth of multilayer films with well-defined layer thicknesses, we used magnetron sputtering, evaporation and pulsed laser deposition. By altering the material combinations and reducing the layer thicknesses down to 3 nm, we observed different mechanisms of phonon blocking, reaching in the frequency regime up to 360 GHz.

During growth of multilayers by pulsed laser deposition (PLD), often both intermixing and resputtering occur due to the high kinetic energy of the particles transferred from the target to the substrate surface. In order to obtain a fundamental understanding of the underlying processes, W/Si multilayers have been studied by the complementary methods of transmission electron microscopy (TEM), X-ray reflectivity (XRR) and in-situ rate monitoring. For the experiments, deposition conditions were chosen that result in high energetic Si ions and mainly low energetic W atoms for the multilayer growth. Under these conditions, interface mixing of up to 3 nm occurs at the W/Si interfaces, while the Si/W interfaces remain sharp. Furthermore, enhanced resputtering of Si leads to a Si thickness deficit of up to 2 nm at the W/Si interfaces. The presented results can be understood by a combination of theoretical calculations as well as SRIM and TRIDYN simulations, which match perfectly to the experimentally obtained intermixing and enhanced resputtering of Si at the W/Si interfaces. peerReviewed

Nanoscale thin-film multilayer materials are of great research interest since their large number of interfaces can strongly hinder phonon propagation and lead to a minimized thermal conductivity. When such materials provide a sufficiently small thermal conductivity and feature in addition also a high thermal stability, they would be possible candidates for high-temperature applications such as thermal barrier coatings. For this article, we have used pulsed laser deposition in order to fabricate thin multilayers out of the thermal barrier material ZrO2 in combination with W, which has both a high melting point and high density. Layer thicknesses were designed such that bulk thermal conductivity is governed by the low value of ZrO2, while ultrathin W blocking layers provide a high number of interfaces. By this phonon scattering, reflection and shortening of mean free path lead to a significant reduction in overall thermal conductivity even below the already low value of ZrO2. In addition to this, X-ray reflectivity measurements were taken showing strong Bragg peaks even after annealing such multilayers at 1300 K. Those results identify W/ZrO2 multilayers as desired thermally stable, low-conductivity materials.

One of the major advantages of pulsed laser deposition (PLD) is the ability to transfer complex multicomponent materials stoichiometrically from a target to the growing film on a substrate. Nevertheless, in some systems small deviations from stoichiometry were found. In this paper, we show that it is possible to grow homogeneous alloy films with concentrations over an unprecedented large range of 0–97 wt% W from a single W80Cu20 target by just varying the laser fluence and additionally tilting the substrate during deposition. This is probably the largest deviation from stoichiometry transfer possible in a binary system.

In common, one of the most characteristic properties of pulsed laser deposition is the stoichiometry transfer between target and substrate, which has been used heavily for many complex systems. In this paper we show that it is yet possible to obtain drastic deviations from stoichiometry transfer in a binary system by just varying the fluence during laser deposition. In the W–Cu system, the W concentration of films grown from a composite W60Cu40 target (60 wt% W) was indeed continuously changed over an unprecedented large range of 0–70 wt% W. Close to the deposition threshold, pure Cu films are formed due to the much higher vapor pressure of Cu. At higher laser fluences, more and more W-rich W–Cu alloy samples are obtained, since ion implantation and intermixing processes occur. These alloys can reach W contents even higher than that of the target because of enhanced resputtering and reflection of the lighter Cu atoms at the film surface. Stoichiometric films with 60 wt% of W are only obtained at laser fluences around 2.7 J/cm2, when the strong Cu evaporation from the target and reflection and resputtering effects of Cu at the film surface are in balance.

The focusing properties of a one-dimensional multilayer Laue lens (MLL) were investigated using monochromatic soft X-ray radiation from a table-top, laser-produced plasma source. The MLL was fabricated by a focused ion beam (FIB) structuring of pulsed laser deposited ZrO2/Ti multilayers. This novel method offers the potential to overcome limitations encountered in electron lithographic processes. Utilizing this multilayer Laue lens, a line focus of XUV radiation from a laser-induced plasma in a nitrogen gas puff target could be generated. The evaluated focal length is close to the designed value of 220 μm for the measurement wavelength of 2.88 nm. Divergence angle and beam waist diameter are measured by a moving knife edge and a far-field experiment, determining all relevant second-order moments based beam parameters. The waist diameter has been found to be approximately 370 nm (FWHM). peerReviewed

Smooth poly(methyl methacrylate) (PMMA) films without any droplets were pulsed laser deposited at a wavelength of 248 nm and a laser fluence of 125 mJ/cm2. After deposition at room temperature, the films possess low universal hardness of only 3 N/mm2. Thermal treatments up to 200°C, either during deposition or afterwards, lead to film hardening up to values of 200 N/mm2. Using a combination of complementary methods, two main mechanisms could be made responsible for this temperature induced hardening effect well above the glass transition temperature of 102°C. The first process is induced by the evaporation of chain fragments and low molecular mass material, which are present in the film due to the ablation process, leading to an increase of the average molecular mass and thus to hardening. The second mechanism can be seen in partial cross-linking of the polymer film as soon as chain scission occurs at higher temperatures and the mobility and reactivity of the polymer material is high enough. peerReviewed

Pulsed laser deposition was used to grow complex polymer–metal nanocomposites consisting of Ag, Cu and Nb clusters embedded in a poly(methyl-methacrylate) (PMMA) matrix. During deposition at room temperature, the size and amount of the metal clusters can be tuned in different ways. First, it is controlled by the number of laser pulses hitting the respective targets. In the case of Ag, a bimodal size distribution of the clusters is observed, induced by total coalescence and secondary nucleation processes. For Cu and Nb, much smaller clusters and higher cluster densities are obtained due to a stronger reactivity with the polymer and thus a lower diffusivity in the PMMA. Additionally, the microstructure of the Ag clusters is affected by the degree of cross linking of the polymer and can be influenced by pre-deposition of nucleation seeds in the form of small Cu clusters.

The structural and mechanical properties of polymethyl methacrylate (PMMA) thin films produced by pulsed laser deposition in ultra-high vacuum were investigated by infrared spectroscopy, electron microscopy, size-exclusion chromatography and microhardness measurements. After preparation, the film hardness and elastic modulus are found to be increased. This is caused by a reduced chain length of the polymer film to values of 5800 g/mol and by cross links of an amount of about 20% of the polymer, while the chemical structure of the deposited films is comparable to the target material. During annealing, relaxation processes lead to a reduction of the film hardness, without changing the cross-linked part in the samples.

Ceramic–metal (MgO combined with Fe, Ti and Ni80Nb20) and polymer–metal (polycarbonate combined with Ag and Pd) nanocomposite multilayers were deposited at room temperature by laser ablation (at 248 nm). The multilayers were characterized by X-ray reflectometry, infrared spectroscopy and transmission electron microscopy. In the case of MgO/metal multilayers, well-layered structures are produced down to layer periodicities of 1.2 nm, necessary for tunneling magnetoresistance devices and X-ray mirrors in the ‘water window’. The interface roughness in the case of polymer/metal multilayers is found to be a strong function of the metal layer thickness and also the nature of the metal.

The changes in the properties of laser deposited metal thin films were investigated in different inert gas atmospheres (He, Ne, Ar and Xe). With increasing inert gas pressure, the reduction of particle energy is accompanied by a strong increase of the deposition rate (especially in He atmosphere), a transition from compressive to tensile stress, and changes in structure and texture. This is explained by a reduction of surface mobility of the deposited particles, a decrease of implantation, resputtering and shot-peening effects. At high gas pressures, deposition conditions similar to sputtering or even thermal deposition are obtained.

Triangular and quadratic Cu pyramids were epitaxially grown on Si(111) and Si(100) substrates, respectively, by pulsed laser deposition at elevated substrate temperatures above 200°C as well as by post-annealing of closed Cu layers prepared at room temperature. In both cases, three-dimensional pyramids with edge lengths of up to 9 μm were obtained, as observed by scanning electron microscopy and atomic force microscopy. Although the macroscopic shape is a pyramid, microscopically the islands consist of columnar grains (with lateral sizes of only about 50 nm at 260°C). The size and shape of the pyramids can be controlled by the substrate used, the amount of material deposited, and the temperature during deposition or annealing. Additionally, first hints were found that the pyramids can be aligned by structuring the substrate. The formation of such large pyramids is explained by a fast diffusion of Cu atoms on Si over distances of some μm and a high jump probability to higher pyramid layers. peerReviewed

The development of the surface roughness of amorphous ZrO2 layers with different thicknesses, which were laser deposited on nanocrystalline Ag surfaces, was analyzed by atomic force microscopy. With increasing ZrO2 layer thickness first the surface slightly roughens due to island growth, but above a layer thickness of about 30 nm it continuously smoothens. From the power spectral densities it is clear that the smoothing processes are frequency dependent. First the high-frequency surface features vanish before lower roughness frequencies are decreased. Although the starting roughness is only 1 nm, a ZrO2 layer thickness of about 4 μm is necessary to smooth the long-wavelength surface features. From the decrease of the roughness with ZrO2 layer thickness and the observed final spectral densities, the dominant smoothing mechanisms were identified as downhill currents, probably induced by the energetic ions impinging on the substrate surface during deposition. peerReviewed