Markus Winkler

Electrical and structural properties of Bi2Te3 and Sb2Te3 thin films grown by the nanoalloying method with different deposition patterns and compositions

Thin films of Bi2Te3 and Sb2Te3 were synthesized by the nanoalloying approach, which has recently been proven to yield V–VI compounds with good thermoelectric properties and has several advantages over “conventional” growth on hot substrates. Firstly, repeating layers of the elements Bi, Sb and Te with a thickness in the range between 0.2 nm and 2.4 nm were deposited on BaF2 (111) substrates in an MBE system at room temperature with different deposition patterns for different samples, i.e. in bilayer and quintuple stacks, with different starting layer thicknesses and different Te contents. Subsequently, the element layer stacks were annealed in order to induce crystallization and compound formation of Bi2Te3 and Sb2Te3 thin films. The annealed thin films were characterized using X-ray diffractometry (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The transport properties, i.e. electrical conductivities, carrier concentrations, carrier mobilities, Seebeck coefficients and thermal conductivities, were determined at room temperature for several sets of starting layer thicknesses and deposition patterns depending on the Te content. The texture was found to be strongly influenced by the starting thicknesses of the elemental layers in the deposition pattern. Results of temperature dependent measurements of the Seebeck coefficient and electrical conductivity on one sample of nanoalloyed Bi2Te3 and Sb2Te3 together with results from temperature dependent in situ XRD investigations are presented.

Thermoelectric transport in Bi2Te3/Sb2Te3 superlattices

The thermoelectric transport properties of

A Simple Sb2Te3 Back-Contact Process for CdTe Solar Cells

\text{Bi}_2\text{Te}_3/\text{Sb}_2\text{Te}_3

Pulsed laser deposition of ferroelectric potassium tantalate-niobate optical waveguiding thin films

superlattices are analyzed on the basis of first-principles calculations and semi-classical Boltzmann theory. The anisotropy of the thermoelectric transport under electron and hole-doping was studied in detail for different superlattice periods at changing temperature and charge carrier concentrations. A clear preference for thermoelectric transport under hole-doping, as well as for the in-plane transport direction was found for all superlattice periods. At hole-doping the electrical transport anisotropies remain bulk-like for all investigated systems, while under electron-doping quantum confinement leads to strong suppression of the cross-plane thermoelectric transport at several superlattice periods. In addition, insights on the Lorenz function, the electronic contribution to the thermal conductivity and the resulting figure of merit are given.

Potassium-tantalate-niobate mixed crystal thin films for applications in nonlinear integrated optics

CdTe solar technology has proved to be a cost-efficient solution for energy production. Formation of the back contact is an important and critical step in preparing high-efficiency, stable CdTe solar cells. In this paper we report a simple CdTe solar cell (Sb2Te3) back contact-formation process. The CdS and CdTe layers were deposited by close-space sublimation. After CdCl2 annealing treatment, the CdTe surface was etched by use of a mixture of nitric and phosphoric acids to obtain a Te-rich surface. Elemental Sb was sputtered on the etched surface and successive post-annealing treatment induced Sb2Te3 alloy formation. Structural characterization by x-ray diffraction analysis confirmed formation of the Sb2Te3 phase. The performance of solar cells with nanoalloyed Sb2Te3 back contacts was comparable with that of reference solar cells prepared with sputtered Sb2Te3 back contact from a compound sputter target.

Potassium tantalate-niobate mixed crystal thin films for applications in nonlinear integrated optics

We present the epitaxial growth of ferroelectric potassium tantalate-niobate (KTN) thin films by pulsed laser deposition. As a result of the optimization of the deposition and the surface finishing processes, a c-axis oriented KTa0.5Nb0.5O3 thin film with homogeneous polarization phase grown on KTaO3 and an efficient KTa0.5Nb0.5O3 waveguiding thin film grown on MgO are demonstrated. The highly improved crystalline and optical quality of KTN layers grown in this work reveal the great potential of such films for integrated nonlinear optics.

New Concept for High-Efficient Cooling Systems Based on Solid-State Caloric Materials as Refrigerant

Perovskite potassium tantalate-niobate mixed crystals (KTa 1−x Nb x O 3 with 0 ≤ x ≤1, KTN) undergo a phase transition from a paraelectric cubic to a ferroelectric tetragonal structure with decreasing temperature [1]. By adjusting the Ta/Nb content one can tune the phase-transition temperature of KTN and thus also its main properties at a given temperature. For example, the phase transition occurs around room temperature for x = 0.4, accompanied by changes in the dielectric and electro-optic (EO) properties [2]. This extremely promising material is of great interest because of its large EO effect and excellent nonlinear optical performance [2, 3]. However, due to the fact that the crystals grown have compositions being different from those of the molten ingredients, high-quality and homogeneous single-crystalline KTN is difficult to produce, which is limiting the application of this material [4, 5].

Real Structure and Structural Changes of Functional Tellurides

Perovskite potassium tantalate-niobate mixed crystals (KTa1−xNbxO3 with 0 ≤ x ≤1, KTN) undergo a phase transition from a paraelectric cubic to a ferroelectric tetragonal structure with decreasing temperature [1]. By adjusting the Ta/Nb content one can tune the phase-transition temperature of KTN and thus also its main properties at a given temperature. For example, the phase transition occurs around room temperature for x = 0.4, accompanied by changes in the dielectric and electro-optic (EO) properties [2]. This extremely promising material is of great interest because of its large EO effect and excellent nonlinear optical performance [2, 3]. However, due to the fact that the crystals grown have compositions being different from those of the molten ingredients, high-quality and homogeneous single-crystalline KTN is difficult to produce, which is limiting the application of this material [4, 5].

Thermoelectric transport in

Caloric materials – in particular magneto-, electro- and elastocaloric materials – show a strong reversible thermal response close to a ferroic phase transition when they are exposed to their respective fields. By cyclic operation of these materials and their alternating thermal coupling to heat sink and source, efficient heat pumps can be realized where no harmful fluids are involved. In the last few years several different groups worldwide have worked on the improvement of the properties of caloric materials as well as on the development of caloric cooling systems with larger temperature span, cooling power and efficiency. Basically, all of these systems are based on a concept using a heat transfer fluid which is actively pumped through a bed of caloric material in order to transfer thermal energy from a heat source to a heat sink. Hereby, especially for magnetocalorics, several powerful systems were built, generating large temperature spans of more than 50 K while others provide large cooling capacities of several kW. However, up to now no caloric system has been built which provides large temperature span and cooling capacity while having a coefficient-of-performance (COP) better than standard compressor-based cooling systems.

\text{Bi}_2\text{Te}_3/\text{Sb}_2\text{Te}_3

Nowadays, functional tellurides are widely used as bulkand nanomaterials for thermoelectric power generators and phase-change based applications, e.g. optical data storage. The function and performance of the materials strongly depend on their unique nanostructural properties and their structural evolution upon operation. Both features can be fully characterized in-situ, ex-situ and on a broad range of length scales by combining diverse characterization techniques with transmission electron microscopy. Consequently, essential information about real-structure property relations and fatigue mechanisms can be determined enabling first steps to a knowledge-based tailoring of the materials.