Andreas Schüler

Structural and optical properties of titanium aluminum nitride films (Ti1−xAlxN)

Titanium aluminum nitride films (Ti1−xAlxN) have been deposited by reactive magnetron cosputtering. Elemental compositions of these films have been determined by core level photoelectron spectroscopy. Scanning electron microscopy reveals a columnar film growth. This is also reflected by the topography of film surfaces as studied by atomic force microscopy. By x-ray diffraction a crystalline atomic structure is revealed. Single phase samples can be obtained, consisting of the substitutional solid solution (Ti, Al)N. Crystallites show preferential orientation. The optical properties of these films have been investigated by spectrophotometry in the UV-VIS-NIR wavelength range. Depending on the elemental composition, the optical constants vary from metallic to dielectric behavior. For film compositions with x<0.5 typical features are a tunable transmission maximum and reflection minimum in the visible spectral range, a high infrared reflection, and a low infrared absorption. Due to these optical properties, T...

Application of titanium containing amorphous hydrogenated carbon films (a-C:H/Ti) as optical selective solar absorber coatings

A combined PVD/PECVD process for the vacuum deposition of titaniumcontainingamorphoushydrogenatedcarbonfilms is described. Elemental compositions of the deposited films have been determined by in situ core level photoelectron spectroscopy (XPS). The long-term stability of the plasma process has been demonstrated. Target poisening has not been observed. We have fabricated optical selective surfaces by the deposition of a-C:H/Ti multilayers onto aluminum substrates. Eventhough we have not optimized layer thicknesses and stoichiometries so far, the experimental results are promising: solar absorptance αS of 0.876 and thermal emittance e100°C of 0.061 have been achieved yielding an optical selectivity s≔αS/e100°C of 14.4. Accelerated aging tests of these coatings have demonstrated their aging stability: the service lifetime is predicted to amount to more than 25 years. Raman spectroscopy has been used to monitor changes in the structure of the aged coatings. Degradation mechanisms are being discussed.

Optical properties of titanium containing amorphous hydrogenated carbon films (a-C:H/Ti)

Titanium containing amorphous hydrogenated carbon films have been deposited by reactive magnetron sputtering. Real time laser reflectometry and ex situ spectrophotometry have served as a means for the determination of the optical constants index of refraction n and extinction coefficient k. The experimental results are being compared to effective medium theories, modeling the optical constants of the composite material from the ones of amorphous hydrogenated carbon and TiC. We have used the effective medium theories to estimate the bulk stoichiometries which agree with Rutherford backscattering experiments but deviate from the surface stoichiometries determined by in situ photoelectron spectroscopy. These deviations can be explained by surface diffusion and three dimensional cluster growth.

Steep slope VO 2 switches for wide-band (DC-40 GHz) reconfigurable electronics

This work proves the feasibility of electrically actuated, CMOS compatible, microwave VO₂ switches on SiO₂/Si substrates with low variability, 100% yield, better than 109 cycles lifetime, ultra-steep OFF-ON transition and better RF performance than previously reported VO₂ switches on Al₂O₃ substrates (flat -0.6 dB S_21-ON with -10 dB S_21-OFF at 40 GHz). The extensive characterization of the fabricated switches has led to an optimum design with maximized S_21-ON/S_21-OFF ratio and validation as a promising solution for wideband reconfigurable electronics.

Principles of Monte-Carlo Ray-Tracing Simulations of Quantum Dot Solar Concentrators

A tool for ray tracing simulations of quantum dot solar concentrators is developed on the basis of Monte-Carlo methods that are applied to polarization-dependent reflection/transmission at interfaces, photon absorption by the semiconductor nanocrystals and photoluminescent reemission. A real time three-dimensional representation of the beam trajectories provides the user with an immediate visual control of the optical behavior of the simulated system. The software allows importing measured or theoretical absorption/reemission spectra describing the photoluminescent properties of the quantum dots. Hereby the properties of photoluminescent reemission are described by a set of emission spectra depending on the energy of the incoming photon. The inverse function method is then used to simulate the photoluminescent emission according to the corresponding photon energy distribution. For quantum dot containing systems, the simulated and measured transmission spectra are matching closely. Simulated spectra of the concentrated radiation exhibit the redshift which is typical for planar photoluminescent concentrators.

Steep-slope Metal-Insulator-Transition VO2 Switches with Temperature-Stable High ION

This letter reports a detailed experimental investigation of the slope of the current switching between OFF and ON states exploiting the metal-insulator-transition (MIT) in vanadium dioxide devices. The reported devices are CMOS compatible two-terminal switches. We experimentally demonstrate for the first time the very little dependence on temperature of the steep slope of these switches, ranging from 0.24 mV/decade at room temperature, to 0.38 mV/decade at 50 °C. The fabricated devices show excellent ON-state conduction, with ION > 1.8 mA/μm or RON <; 3 mΩ/μm, for the whole range of investigated temperatures (from room temperature to the MIT transition temperature), which recommends them as future candidates for steep-slope, highly conductive, and temperature-stable switches.

Changing TiN film morphology by “plasma biasing”

The influence of the substrate potential with respect to the plasma on the morphology of reactively sputtered TiN thin films on Si(100) has been investigated. It is well known that the film quality with respect to grain size and distribution can be improved by applying a negative substrate bias to increase energetic ion bombardment. For large-area applications, however, a grounded substrate is very much desirable. Therefore, a technique has been developed to deposit films with comparably improved morphology on grounded substrates by means of a so-called “plasma electrode.” Grain size and distribution have been analyzed by top- and side-view scanning electron microscopy. To adjust the parameters for the TiN deposition we have used in situ photoelectron spectroscopy as the process control.

Embedded microstructures for daylighting and seasonal thermal control

A novel concept for an advanced fenestration system was studied and samples were produced to demonstrate the feasibility. The resulting novel glazing will combine the functions of daylighting, glare protection, and seasonal thermal control. Coated microstructures provide redirection of the incident solar radiation, thus simultaneously reducing glare and projecting daylight deep into the room in the same manner as an anidolic mirror-based system.The solar gains are reduced for chosen angles corresponding to a estival elevations of the sun, thereby minimising heating loads in winter and cooling loads in summer. A ray-tracing program developed especially for the study of laminar structures was used for the optimisation of structures with the above mentioned goals. The chosen solution is based on reflective surfaces embedded in a polymer film that can be combined with a standard doubled glazed window. The fabrication of such structures required several steps. The fabrication of a metallic mould with a relative high aspect ratio and mirror polished surfaces is followed by the production of an intermediate Polydimethylsiloxane moulds that was subsequently used to replicate the structure with a UV curable polymer. Selected facets of these samples were then coated with a thin film of highly reflective material in a physical vapour deposition process. Finally, the structures were filled with the same polymer to integrated the mirrors. The samples were characterised using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), confocal microscopy and laser profilometry. A miniature goniophotometer was built to assess the performance of the structured glazing. The daylighting behaviour was successfully demonstrated.

A Steep-Slope Transistor Combining Phase-Change and Band-to-Band-Tunneling to Achieve a sub-Unity Body Factor

Steep-slope transistors allow to scale down the supply voltage and the energy per computed bit of information as compared to conventional field-effect transistors (FETs), due to their sub-60 mV/decade subthreshold swing at room temperature. Currently pursued approaches to achieve such a subthermionic subthreshold swing consist in alternative carrier injection mechanisms, like quantum mechanical band-to-band tunneling (BTBT) in Tunnel FETs or abrupt phase-change in metal-insulator transition (MIT) devices. The strengths of the BTBT and MIT have been combined in a hybrid device architecture called phase-change tunnel FET (PC-TFET), in which the abrupt MIT in vanadium dioxide (VO2) lowers the subthreshold swing of strained-silicon nanowire TFETs. In this work, we demonstrate that the principle underlying the low swing in the PC-TFET relates to a sub-unity body factor achieved by an internal differential gate voltage amplification. We study the effect of temperature on the switching ratio and the swing of the PC-TFET, reporting values as low as 4.0 mV/decade at 25 °C, 7.8 mV/decade at 45 °C. We discuss how the unique characteristics of the PC-TFET open new perspectives, beyond FETs and other steep-slope transistors, for low power electronics, analog circuits and neuromorphic computing.

Monte Carlo simulations of quantum dot solar concentrators: ray tracing based on fluorescence mapping

One promising application of semiconductor nanostructures in the field of photovoltaics might be quantum dot solar concentrators. Quantum dot containing nanocomposite thin films are synthesized at EPFL-LESO by a low cost sol-gel process. In order to study the potential of the novel planar photoluminescent concentrators, reliable computer simulations are needed. A computer code for ray tracing simulations of quantum dot solar concentrators has been developed at EPFL-LESO on the basis of Monte Carlo methods that are applied to polarization-dependent reflection/transmission at interfaces, photon absorption by the semiconductor nanocrystals and photoluminescent reemission. The software allows importing measured or theoretical absorption/reemission spectra describing the photoluminescent properties of the quantum dots. Hereby the properties of photoluminescent reemission are described by a set of emission spectra depending on the energy of the incoming photon, allowing to simulate the photoluminescent emission using the inverse function method. By our simulations, the importance of two main factors is revealed, an emission spectrum matched to the spectral efficiency curve of the photovoltaic cell, and a large Stokes shift, which is advantageous for the lateral energy transport. No significant energy losses are implied when the quantum dots are contained within a nanocomposite coating instead of being dispersed in the entire volume of the pane. Together with the knowledge on the optoelectronical properties of suitable photovoltaic cells, the simulations allow to predict the total efficiency of the envisaged concentrating PV systems, and to optimize photoluminescent emission frequencies, optical densities, and pane dimensions.